A novel formwork technology based on wax foam 3D printing and CNC milling

1 Introduction

The precast concrete industry achieves high production efficiency through factory-based serial production, supported by standardized processes and increasing levels of automation (Kosse et al., 2023; Reichenbach and Kromoser, 2021). In industrial precast concrete production, steel formwork systems are widely employed due to their high durability, high reusability, high demoulded concrete surface quality and compatibility with automated production lines (Gappmaier et al., 2024). Circulating pallet systems, commonly applied in large-scale precast production of wall and ceiling elements, offer some adaptability through placement of embedded components and minor geometric variations, but remain fundamentally limited to standardized designs (Bock, 2008).

In contrast, individualized freeform formwork for small production series, typically manufactured from CNC milled polystyrene, plywood, or timber generates substantial waste, and is commonly designed for single use and formwork manufactured by plywood or timber, is labour-intensive (Li et al., 2022). Although these materials can theoretically be downcycled, effective recycling is often prevented by contamination from concrete residues, release agents, and coatings. Recent advances in digital fabrication have introduced manufacturing approaches that combine additive and subtractive processes for concrete formwork production (Jipa et al., 2023; Jipa and Dillenburger, 2022). These methods seek to exploit the material efficiency of 3D printing while maintaining the dimensional precision achievable with CNC machining (Kovaleva et al., 2026). While recycling into new formwork is frequently stated as an objective, published work often provides limited or no evidence of repeated, closed-loop material recycling under practical production conditions, indicating a gap between intended recyclability and demonstrated implementation. The remaining challenge is to identify a formwork material that is simultaneously printable, machinable, thermally recyclable, robust and retains its shape for repeated concrete casting.

Wax-based formwork systems have a long tradition in lost-wax casting, the metal casting used for jewellery and sculpture where sacrificial wax formworks are melted away. In construction, wax formwork technology has been investigated at the Institute of Structural Design (ITE), TU Braunschweig, for more than 10 years, focusing on CNC milled solid large-scale wax blocks for complex concrete geometries in the precast industry. While these systems demonstrated successful recycling over more than 200 remelting cycles, they suffered from high material consumption and long machining times due to subtractive manufacturing from solid blocks. In addition, volumetric contraction during cooling remains a critical challenge for large-scale wax blocks, as it can induce deformation. This motivates the exploration of combined additive-subtractive approaches using wax-based materials to reduce the amount of melted wax already in the stage of wax block production. Current strategies for wax-based formwork production can be categorized into four manufacturing approaches (Figure 1) and will be described in detail in Section 2.

Approaches 1–3 each exhibit specific limitations and disadvantages and differ in their levels of material efficiency, dimensional accuracy, scalability, and automation potential. Category four near-net-shape 3D printing combined with CNC milling has the potential to overcome the main limitations of category 1–3. In this first feasibility study a wax foam formwork technology is developed that combines extrusion-based 3D printing with CNC milling. The key innovation is the wax foam material, a porous wax–air composite that enables both additive manufacturing and subtractive finishing while maintaining thermal recyclability. The research objectives are: (1) characterisation of two wax foam formulations with respect to material properties relevant for the first prototype production for the use as formwork; (2) calibration of the custom WF3DP extrusion system for high-throughput wax foam deposition; (3) establishing process parameters for reliable near-net-shape manufacturing; (4) validation of the proposed digital process chain through the full-scale fabrication of a geometrically complex concrete formwork.

The study was conducted as an industry–academia partnership between the Institute of Structural Design (ITE), Technische Universität Braunschweig, and Herrhammer GmbH (Ochsenfurt, Germany). It combined ITE’s long-standing research expertise in wax-based formwork technology with Herrhammer’s industrial know-how in wax-processing machinery, enabling the development of a custom extrusion system for wax foam deposition. The 3D printing investigations were carried out at Herrhammer’s production facilities, final contour milling and hybrid formwork assembly at the Digital Building Fabrication Laboratory (DBFL) of TU Braunschweig, and concrete casting under industrial conditions with a cooperating precast manufacturer (Kloft et al., 2022).

This study presents the initial results of a novel wax foam-based 3D printing technology and constitutes a first step in the investigation of this approach. It is therefore positioned as a feasibility demonstration, covering part of the characterisation of wax foam materials and the process calibration required to enable a first industrial full-scale case study. The results indicate a broad range of potential improvements in wax formwork technology while also revealing several aspects related to 3D printing with wax foam that require further investigation, which are addressed in the Discussion. The study thus identifies the limitations of the current state of research on WF3DP and demonstrates an approach with the potential for largely automated formwork production, addressing the gap in customized series production of precast concrete components.

2 State of the art: digitally manufactured wax formwork for concrete casting

2.1 Wax-formwork net shape casting

The net-shape casting method for wax formwork production is based on off-site fabrication of wax formwork elements. In 1969 a patent by (Scott, 1969) described a method for wax formwork casting for concrete where wax formwork is cast into a form to transfer the surface structure to the wax mould which then serves as formwork for concrete casting. Later the approach of net-shape casting for wax formwork production was investigated at ETH Zurich under the leadership of Gramazio Kohler Research, where hot liquid wax is poured directly onto a reconfigurable mould surface using a flexible, actuated moulding system (Gramazio et al., 2012; Oesterle et al., 2011). The target geometry is transferred from the actuated mould to the wax. After cooling, the wax elements are demoulded and in order to save weight and material, the wax formwork elements are conceived with a hollow backside and orthogonal rib structures to reduce formwork weight (Gramazio et al., 2012). The wax formwork surface is subsequently transferred to the concrete without intermediate machining operations. To obtain seamless concrete surfaces between adjacent modules, hot wax joint filler may be applied at the interfaces.

Despite the advantages of recyclability and reuse, several technical limitations have been reported. Volumetric contraction during cooling can lead to dimensional deviations between the mould surface and the final wax geometry, reducing geometric precision and surface tolerances. Heat generated by cement hydration and solar radiation can significantly increase formwork temperatures, requiring mitigation measures such as shading, reflective coatings, or protective covers. Reported studies do not specify the melting ranges or thermomechanical properties of the wax formulations, limiting quantitative assessment of thermal stability and safety margins. Although wax typically exhibits low polarity, the use of release agents is recommended to facilitate separation and reduce surface defects such as air voids; detailed performance evaluations are not reported.

From a production perspective, the moulding process is highly automated, with limited manual intervention beyond assembly. Geometric freedom is constrained by the kinematics and size of the actuated mould, and scalability is limited by the available mould dimensions. While net-shape casting enables rapid and material-efficient formwork production, the lack of post-processing restricts achievable dimensional accuracy.

2.2 Wax formwork solid block manufacturing + milling

An alternative wax-based approach to concrete formwork fabrication is the solid wax block plus milling method, in which large solid wax blanks are produced first and subsequently shaped into precise rectangular prisms with straight edges to compensate for volumetric contraction deformation and then subtractively shaped to high-precision formwork. In 1989, Klaus Dittmann patented a ‘Method for the automatic manufacture of special parts from concrete or reinforced concrete’ (Dittmann, 1989), in which a machinable wax-based modelling compound is shaped into a negative mould for concrete by a computer-controlled cutting head. Building on this general concept, research at ITE has investigated CNC machining as a manufacturing route that enables automated fabrication of planar surfaces, fine details, and highly complex geometries, making the approach suitable for mass customization and small-batch precast components (Herrmann et al., 2018; Mainka, 2019; 2022; Mainka et al., 2016; Mainka et al., 2017; Tran et al., 2024). In contrast to net-shape casting strategies, machining is performed after volumetric contraction and complete solidification, reducing geometric inaccuracies and enabling high-precision formwork surfaces. The method is particularly suited to off-site prefabrication of formwork elements. This method has been investigated in detail and applied in industrial practice using CNC machining for final shaping (Mainka, 2019; Wirth, 2020; Jonischkies et al., 2022b).

A key milestone in the research at ITE was the identification of ConFormWax (CFW) (Mainka, 2019; Mainka et al., 2015) as a suitable wax formulation combining good machinability, sufficient compressive strength, and recyclability. Recyclability was demonstrated through remelting, with 200 documented melting cycles and up to 500 cycles considered feasible. Investigations into coatings on CFW wax surfaces for architectural concrete applications produced high-quality finishes; at the same time, the low polarity and low surface energy of wax limited coating adhesion, making repeated casting on coated wax surfaces impractical. Mineral-oil-based release agents enabled glossy surface finishes, but full-scale curing tests (1.5 t) with self-compacting concrete (Jonischkies, Mainka, et al., 2022) and hydration temperatures exceeding 55 °C revealed limitations in thermal tolerance: the wax softened and partially melted, and the formwork could no longer be reused for repeated casting.

To obtain crack- and stress-free wax blocks, several wax-block production techniques have been investigated and optimized (Jonischkies et al., 2026; Jonischkies, Mainka, et al., 2022; Mainka, 2019). Subsequent multi-axis CNC milling converts the solid blocks into high-precision formwork geometries and compensates for deformation resulting from volumetric contraction during cooling. Milling chips, together with discarded formwork elements, can be collected, remelted, and reused, enabling a closed material cycle.

As an alternative to recycling by remelting, another approach processes collected wax chips through mechanical compression using large-scale hydraulic presses, producing cylindrical wax blocks suitable for subsequent CNC milling (Wirth, 2020). Compared with complete remelting, this chip-to-block approach reduces energy demand and processing time and supports a largely automated production chain in which block fabrication and shaping are digitally integrated, and which also addresses the thermal shrinkage of cast liquid wax. Maximum block size remains constrained by the dimensions of the hydraulic press. In addition, blocks are typically produced as cylinders and must be machined into prismatic forms. Larger or more complex formwork geometries therefore require modular assembly of multiple milled elements, introducing joints, increasing assembly effort, limiting geometric continuity, and constraining scalability.

Recent research projects, such as Robotcrete (Jonischkies and Hack, 2024), aim to further automate and scale wax formwork fabrication by integrating automated block production with improved CNC milling processes (Jonischkies et al., 2026). One low-cost, labour-reduced approach involves casting large-scale wax blocks in a single step under optimized cooling conditions (Jonischkies et al., 2026). While this method enables reliable and repeatable wax block production and has been applied in industrial practice, it remains constrained by long cooling times. To reduce cooling time while maintaining scalable block dimensions, Robotcrete investigated an automatable process in which pre-cast wax blocks are placed in moulds and the remaining gaps are filled with molten wax. Although technically promising, the approach requires substantial automation effort and did not reach industrial readiness. In addition, the resulting large and heavy wax blocks remain difficult to handle within typical precast workflow chains.

Within the Robotcrete project, it was shown that the CO₂ advantage of wax formwork compared with conventional single-use materials depends strongly on the achieved reuse rate (Jonischkies et al., 2026). Because wax has a high density relative to foamed polymers such as EPS, a clearly improved CO₂ balance is obtained mainly at high cycle numbers, when the initial material impact is distributed over many remelting cycles. Reported comparisons indicate that CFW wax can achieve an approximately eightfold better CO₂ balance than EPS 100 when 200 reuse cycles are assumed (Jonischkies et al., 2026). This comparison refers to single use of the conventional formwork and the wax formwork. A comparison against conventional formwork used for multiple casting cycles was not carried out and would reduce this ratio, since the environmental impact of the conventional system is then distributed over several castings. Further improvement potential lies in reducing the density of the wax formwork material and minimizing the amount of wax removed during rough machining, since this material requires additional handling and reprocessing despite not contributing directly to the formwork function during use.

Overall, while the solid block plus milling approach enables high-precision wax formwork and indicates strong potential as an alternative to conventional systems, key challenges remain. In particular, fully digital and automated wax-block production to complete the manufacturing process chain, the high material removal volumes required during rough machining, the relatively low melting range of CFW wax, and the handling of heavy solid wax blocks continue to limit scalability and broader industrial application.

2.3 Wax formwork net-shape printing

The net-shape 3D printing method for wax formwork fabrication employs additive manufacturing to produce formwork geometries in fine resolution ranging from approximately 0.1 mm–1 mm layer height directly, without subsequent subtractive finishing. It primarily comprises fused deposition modelling (FDM) using wax-based filaments, wax pellets, or liquid wax feedstocks and enables a fully digital process. Typical workflow chains integrate 3D CAD modelling, tool path generation via slicing algorithms or robotic motion planning, layer-by-layer deposition, concrete casting, and wax recovery through remelting and reuse as formwork material.

In FDM-based systems, wax feedstock is heated and extruded through a nozzle to build formwork geometries in successive layers. Implementations range from desktop-scale FDM printers for small geometries, widely used to produce sacrificial patterns for jewellery manufacturing (Alami et al., 2023), to robotic or industrial systems for medium-scale prototypes. The 3D Systems ProJet MJP 2500 IC enables high-precision additive manufacturing of wax components using MultiJet Printing (MJP). However, its build volume (294 × 211 × 144 mm) and material throughput are limited compared with large-scale extrusion-based processes. Recent work includes the 3DWaxForm research project at SDU CREATE, which investigates the printability of CFW wax combined with fillers for concrete formwork applications in collaboration with industrial partners, including Odico A/S, within a practice-oriented and interdisciplinary research framework following a direct-casting strategy without intermediate post-processing (Naboni and Ramirez, 2026). Beyond these academic and industrial efforts, Rolab Studio’s Formwax (Behram and Yılmaz, 2020) proposes a multi-part strategy in which (i) an outer shell is 3D printed from a wax–cellulose mixture (Formwax | Rolab Studio, 2020), (ii) an inner shell defines the casting cavity, (iii) paraffin wax is cast between both shells to reinforce the assembly against hydrostatic pressure, and (iv) concrete is cast into the inner shell.

Formwork surface quality remains a key limitation of 3D printed wax formwork when no post-processing step such as CNC milling is applied. The layer-wise nature of FDM produces visible layer lines and surface roughness that are highly parameter-dependent and are transferred to the demoulded concrete surface. Printing at fine resolution can improve surface quality, but it substantially increases production time, reduces economic efficiency, and typically results in thinner, more delicate wall structures that are more susceptible to deformation under the hydrostatic pressure of fresh concrete. Coarse-resolution printing increases productivity, but at the expense of surface finish and usually yields rougher concrete surfaces. In addition, the low viscosity of molten wax at processing temperatures can reduce extruded strand stability and further impair surface fidelity. Dimensional accuracy is also affected by cooling-induced volumetric contraction.

Relative to CNC milled wax formwork and net-shape casting approaches, direct 3D printing offers material-efficiency advantages by eliminating machining waste and depositing material only where required. It also provides high geometric freedom for double-curved and customized geometries without tool-access constraints. Overall, net-shape 3D printed wax formwork remains largely at a demonstration stage; however, its fully digital process enables rapid iteration, mass customization, and substantially reduced labour input, indicating a promising development direction for fast and material-efficient wax formwork production. Consequently, direct wax printing alone does not currently provide the surface quality required for high-quality architectural concrete applications.

2.4 Wax formwork near-net-shape printing + milling

The near-net-shape printing plus milling approach for wax formwork fabrication combines the material efficiency and geometric freedom of wax 3D printing with the dimensional accuracy and surface quality achievable through CNC milling. Using FDM-based deposition, wax is applied layer by layer only where material is required, typically at a coarse resolution to maximize deposition rate and enable rapid production of near-net-shape formwork elements. In a subsequent step, the printed geometry is finish-milled to generate the final contour and a high-precision casting surface.

Because the component is printed close to its final geometry, machining can be largely limited to finishing operations. Compared with conventional solid-block milling, extensive roughing, associated with high material removal volumes, additional machining time, and increased recycling effort, is reduced. This decreases machining waste, lowers the amount of material that must be remelted after removal, and improves overall process efficiency. In addition, near-net-shape deposition and subtractive finishing can be integrated into a continuous digital process chain, supporting a highly automated production chain with reduced manual labour.

The concept has been demonstrated at mid-scale within the FreeFAB (Gardiner et al., 2016; Gardiner and Janssen, 2017; Gardiner et al., 2016; Gardiner and Janssen, 2017) context, where wax 3D printing was combined with CNC milling to improve surface quality. The invention developed by Gardiner and Janssen and patented by Laing O'Rourke Australia Pty Ltd. In 2014 under the title “Method for casting a construction element” (Gardiner and Janssen, 2014; Gardiner and Janssen, 2014) describes, in broader terms, a process chain for casting concrete construction elements and includes technical details on the production of printed and subsequently milled wax formwork, as well as further potential applications. At construction scale, the FreeFAB approach implements FDM-based 3D printing and subsequent milling of recyclable wax formwork elements for complex architectural and industrial-scale geometries. Published results indicate that the 3D printed wax formworks were effectively single-use, as the elements were damaged during demoulding, limiting the approach primarily to one-off casting applications. Cooling-induced wax shrinkage was also reported as a relevant challenge affecting dimensional accuracy and formwork precision. It can be assumed that research and development of FreeFab within Laing O'Rourke Australia was discontinued, as there have been no known publications since 2018. Despite significant progress in digital wax formwork fabrication, no existing approach simultaneously combines material efficiency, geometric accuracy, thermal recyclability, scalability, and a highly automated process chain. Furthermore, the potential of wax foam as a recyclable feedstock for near-net-shape additive manufacturing followed by precision CNC milling has not yet been systematically investigated.

To address this research gap, a novel manufacturing approach was developed through an industry–academia partnership between ITE of TU Braunschweig, and Herrhammer GmbH. The approach combines extrusion-based wax foam 3D printing with precision CNC milling within a robotic manufacturing unit, termed Wax Foam 3D Printing (WF3DP), to enable the production of recyclable formwork. The underlying process chain has been described in a patent application entitled “Wax formwork manufacturing device and method for manufacturing a wax formwork” (Jonischkies, Hack, et al., 2022; Jonischkies, Hack, et al., 2022), which forms the technological basis of the present study.

3 Materials and methods

The methodology is structured into five interconnected areas (3.1) hardware, WF3DP-Unit (3.2) wax foam characterisation (3.3) digital process chain (3.4) process calibration and (3.5) CNC milling parameters.

The wax-air composition based on Tudastar wax, together with the calibrated additive and subtractive process parameters, provides the basis for the case study presented in Section 4.

3.1 Wax foam 3D printing unit

The WF3DP robotic unit consists of a heated material reservoir (temperature-controlled 90 °C), an extruder with controlled gas injection, and a temperature-regulated extrusion nozzle (diameter 12 mm). The extrusion pressure and gas injection rate can be adjusted to control foam density and extrusion consistency. The system is mounted on an industrial 6-axis robot KUKA KR210 R2700 (predefined working volume length 1600 mm x width 1200 mm x height 2000 mm) for automated deposition.

The unit integrates seven components that form the WF3DP-Unit (Figure 2):

• Wax melting and storage. Solid wax is liquefied and maintained at approximately 90 °C. The molten material is subsequently pumped through a filtration system to remove residual contaminants, such as concrete particles from previous formwork applications.
• Wax foaming unit. Filtered wax is mixed with air to produce a stable wax–air composite in a mechanical foaming process. During foaming, the mixture is pre-cooled using temperature-controlled water, resulting in a homogeneous, semi-solid foam suitable for pumping. The wax is conveyed at a constant mass flow rate 28.5 kg/h to ensure consistent foam characteristics. Since foaming is achieved mechanically and does not alter the chemical composition of the wax, the material can be repeatedly re-foamed during reprocessing.
• Robotic positioning system. Material deposition is executed using a five-axis industrial robot manufactured by KUKA, enabling precise three-dimensional positioning of the extrusion tool.
• Heated delivery hose. A temperature-controlled hose transports the wax foam from the foaming unit to the print head while preventing premature solidification.
• Near-nozzle cooling unit. Immediately prior to extrusion, the foam passes through a localized cooling stage that reduces the temperature to slightly below the freezing point. This controlled two-phase state produces a partially crystallized microstructure with favourable rheological properties, enabling dimensionally stable extrusion and clean layer formation. Temperature variations are limited to ±1 °C.
• Extrusion nozzle. The conditioned foam is deposited through the nozzle according to the calibrated process parameters.
• Printing bed. A level substrate provides the base for layerwise near-net-shape fabrication.

3.2 Wax foam characterisation tests for WF3DP

For the investigations presented here, Tudastar wax (H&R Wax & Specialties GmbH, Germany) was characterized to evaluate its suitability for extrusion-based 3D printing and compared with CFW wax (Tromm GmbH, Germany), which was used in previous wax formwork research at ITE. Both formulations are based on paraffin-wax matrices with proprietary additive packages.

The investigation of a new wax formulation was necessary because of the limited thermal stability and release behaviour of CFW wax. CFW wax has a comparatively low freezing range of 62 °C–66 °C, and as a multi-component paraffin blend it begins to soften well below this range. This restricts its suitability for large-scale concrete applications: during casting of massive components (up to 1.5 t), hydration heat led to thermal softening and local melting of the formwork surface, preventing repeated use. Tudastar wax was therefore selected as an alternative formulation designed for higher thermal loads. It exhibits an increased freezing point of approximately 81 °C, which improves dimensional stability during concrete curing and reduces the risk of thermally induced deformation.

Tudastar wax (solid and foamed) was evaluated for WF3DP by (i) surface-energy polarity measurements (OWRK contact-angle method) to indicate demoulding behaviour and the likely need for release agents or coatings, (ii) compressive strength testing of solid and 20% air foam specimens to benchmark load-bearing capacity against fresh-concrete pressure, and (iii) micro-CT and microscopy to quantify the pore-size distribution and assess its impact on CNC milled surface quality. Most tests were conducted for both Tudastar and CFW wax to ensure comparability with previous investigations on wax formwork technology. The characterisation tests provide first indicators of suitability for formwork application.

The air content of the wax–air composite investigated in this feasibility study was set to 0% and 20%. The 20% value was selected on the basis of preliminary experiments conducted during the development of the WF3DP process, in which the air content of the Tudastar wax–air mixture was increased stepwise in 10% increments up to 30%. An air content of 30% appeared to represent the upper limit, as irregular bubble formation indicated a constraint on the amount of air the mixture could absorb; for stable wax foam extrusion, the air content was therefore set to 20%. This represents a practical process condition rather than a proven optimum. The actual air content was determined from density measurements: the density of the wax foam was calculated from its measured mass and volume and compared with that of the corresponding solid wax formulation, and the volumetric air content was derived from the relative density reduction.

The recyclability of the wax and its reuse as formwork material, which underlie the proposed closed-loop material concept, are supported by earlier investigations at ITE. There, CFW wax was cyclically remelted more than 200 times without noticeable changes in its mechanical properties, thereby closing the cycle at material level. The removal of concrete and other residues was confirmed both by manual filtration using sieves and by automated filtration, in which the liquid wax was pumped through filters; no deterioration of the material was observed after repeated melting cycles or as a result of residue contamination. For Tudastar wax, comparable remelting behaviour is expected, provided that the material is not heated above the temperature at which oxidation begins and that the melt is stirred so that individual components cannot separate and settle. Mechanical foaming does not affect recyclability, as the process induces no chemical changes; the decisive factor is that the chemical composition remains unchanged, which requires oxidation to be avoided as far as possible by heating the wax gently. Thermal decomposition is not expected as long as the processing temperature is kept within approximately 10 °C–15 °C above the melting range. Within the present study, both the CFW and the Tudastar wax underwent several foaming cycles during the calibration tests. Following the case study described in Section 4, the formwork produced with Tudastar wax was completely remelted, re-foamed, and reused as formwork material in the WF3DP process, thereby closing the material cycle at process level.

3.2.1 Surface-energy and polarity comparison

Release agents are commonly applied to achieve damage-free demoulding. In wax-based formwork systems, however, wax-based release agents can cause discoloration of the demoulded concrete surface, while mineral-oil-based agents may be absorbed by the lipophilic wax. Two material routes are therefore relevant: (i) selecting a wax of sufficiently low polarity, so that the wax surface itself provides favourable release behaviour, or (ii) applying a coating that allows reliable use of mineral-oil-based release agents. Since coatings typically show limited adhesion on low-surface-energy wax surfaces, this study focuses on route (i), evaluating wax formulations with intrinsically favourable release behaviour.

Surface energy and its dispersive and polar components are indicators of release behaviour from cured concrete. A lower total surface energy, and in particular a smaller polar fraction, generally supports damage-free demoulding while simultaneously reducing the adhesion potential of coatings on the wax surface. To assess release behaviour for direct concrete casting and coating adhesion on wax surfaces, surface energy and wettability were characterized using the Owens–Wendt–Rabel–Kaelble (OWRK) method with a contact-angle measurement system (OCA, DataPhysics Instruments). Contact angles were measured with water (γ = 72.8 mN/m; γ^d = 21.8 mN/m, γ^p = 51.0 mN/m) and diiodomethane (γ = 50.8 mN/m; entirely dispersive) on milled wax samples (200 × 200 × 40 mm) at four positions to account for local variation, and the polar and dispersive components of the surface energy were calculated from these measurements. The results were compared with previously reported CFW wax measurements from ITE and evaluated with respect to the expected release behaviour in the case study (Table 1).

TABLE 1. Surface energy of CFW and Tudastar wax determined by the OWRK method with water and diiodomethane as test liquids (n = 4 positions per formulation). CFW wax values from Jonischkies et al. (2026).

Tudastar wax exhibits a lower total surface energy than CFW wax, dominated by dispersive interactions, with a polar component that is negligible and not statistically distinguishable from zero. The difference is most evident in the water contact angle, which is approximately 15° higher on Tudastar than on CFW wax, whereas the diiodomethane contact angles differ by only about 2°. This indicates that the two formulations behave similarly towards nonpolar liquids but differ markedly in their interaction with polar ones. The predominantly dispersive, near nonpolar character of Tudastar wax therefore suggests improved demoulding behaviour and reduced adhesion to concrete. Since surface energy alone cannot predict demoulding force or formwork durability, these values serve as a screening indicator rather than as proof of release performance.

3.2.2 Compressive strength testing

Compressive testing quantified the influence of foaming (nominal 20% air), wax formulation, 3D printing, and temperature representing the hydration heat of curing concrete. Standardized specimens (40 × 40 × 80 mm) were CNC milled with ±1 mm accuracy from cast solid wax and 20% air foam of each formulation and tested at room temperature 22 °C (±1 °C), with three specimens per condition. In addition, Tudastar 20% foam was 3D printed (layers 6 mm high, 20 mm wide) and milled specimens were tested at 22, 35, and 45 °C (±1 °C) after 24 h of temperature conditioning, with four specimens per temperature and the printed layers parallel to the loading axis so that the interlayer bonds formed the weak planes. The 45 °C level corresponds to the formwork temperature measured during casting (Section 4.4). Tests were performed on a 50 kN universal testing machine ZwickRoell Z050 (50 N preload, way-controlled, feed 200 mm/min, to 10% strain). Peak stresses are reported as mean ± standard deviation. Compressive strength was adopted as the primary mechanical indicator, as it constitutes the reference property in the preceding ITE investigations of wax formwork (Mainka, 2019; Wirth, 2020). Determining the same property for Tudastar wax and for the printed foam thus establishes direct comparability with these earlier results.

Compressive strength alone, however, does not fully characterise the mechanical behaviour of a formwork element under service conditions, which additionally comprises bending, shear, stresses at connections and printed-layer interfaces, and creep under sustained loading at elevated temperature. The present tests are consequently of an indicative nature: they quantify the strength level and its temperature dependence for the material employed in the case study, whereas interlayer strength, long-term behaviour, and the influence of build orientation and loading rate remain to be determined (Section 5.5).

As shown in the stress–strain curves (Figure 3, diagram 1), all four cast material states show an initial quasi-linear elastic response followed by yielding and plastic deformation. Solid CFW wax (0% air) reached a stable plateau of 3.16 ± 0.23 N/mm², which dropped to 1.64 ± 0.26 N/mm² at 20% air, a reduction of 48%. Tudastar wax was markedly stronger and far less affected by foaming, reaching 4.69 ± 0.14 N/mm² solid and 4.40 ± 0.07 N/mm² at 20% air, a reduction of only 6%. Foamed Tudastar wax therefore carried 2.7 times the load of foamed CFW wax.

Printing the same Tudastar 20% foam lowered the mean peak stress at room temperature from 4.40 ± 0.07 to 3.59 ± 0.65 N/mm² (Figure 3, diagram 2), a difference of about 18%. With rising temperature the printed foam softened considerably, from 3.59 ± 0.65 N/mm² at 22 °C to 3.03 ± 0.53 N/mm² at 35 °C and 1.72 ± 0.19 N/mm² at 45 °C, less than half the room-temperature value. The decline is evident across the full temperature range, although the difference between 22 °C and 35 °C lies within the scatter of the measurements. The failure mode changed accordingly: at 22 °C, specimens failed partly by abrupt brittle cracking, with cracks running partly through the bulk material and partly along the layer interfaces, whereas at 45 °C deformation was ductile and no sharp load drop occurred.

Notably, the printed Tudastar foam retained at 45 °C (1.72 ± 0.19 N/mm²) the room-temperature strength of foamed CFW wax (1.64 ± 0.26 N/mm²). With a softening range below the freezing range of 62 °C–66 °C, CFW wax approaches softening at the temperatures recorded during casting in Section 4.4, whereas Tudastar wax at 45 °C remains well below.

3.2.3 Pore-size distribution

Pore size and distribution in the wax foam can affect CNC milled surface integrity (pits, local defects) and thus formwork surface quality; the pore structure of both formulations at nominal 20% air was therefore quantified by micro-CT and correlated with microscopy. Three specimens (10 × 10 × 10 mm) per formulation were analysed to assess reproducibility. Scans were performed on an RX Solutions EasyTom system (Hamamatsu L10711-03 source, Varian 2520DX CsI detector) at 79 kV and 126 µA without filtration, with 1440 projections over 360° (two radiographs averaged per angular position) reconstructed by filtered back-projection at a voxel size of 10 µm (the first specimen at ≈11 µm). In Avizo (Thermo Fisher Scientific), the air phase was segmented using a greyscale threshold and isolated voxels were removed; pores at or below ≈10–11 µm could not be reliably resolved. Individual pore volumes were classified by size to obtain the pore-size distribution and the cumulative pore volume. CNC milled surfaces were additionally examined by microscopy at ×10 magnification (Figures 4A–D).

Figures 4E,F shows formulation-dependent differences consistent with the microscopy. CFW wax foam is dominated by fine, predominantly closed pores, the large majority by number below ≈58 µm equivalent spherical diameter (ESD; the <0.0001 mm³ class), indicating a homogeneous, stable foam. Tudastar wax foam is coarser: ≈94% of the pore volume lies in classes above 0.001 mm³ (ESD >124 µm), including ≈31% above 0.1 mm³ (ESD >576 µm), again with predominantly closed pores. Tudastar wax therefore incorporates the gas phase less finely, indicating stronger bubble growth and coalescence and producing coarser pores that are more likely to be intersected during CNC milling, thus raising the risk of local pits, consistent with the microscopic observations. The pore-size distribution and the total resolved porosity were reproducible across the three specimens of each formulation; for Tudastar wax, the material used in the case study, the total resolved porosity was 16.4%–18.0%. These CT values lie below the density-based nominal air content of 20% because pores at or below the ≈10–11 µm resolution limit are not captured, most notably for the finer CFW foam. As the analysed specimens are small relative to the printed component, the distributions are representative of the tested volumes rather than statistically representative of the entire part, where pore morphology may vary with local thermal history.

3.3 Digital process chain and WF3DP-process parameter calibration

To enable 3D printing parameter calibration, a dedicated, parametrically configurable software environment was developed based on established fused deposition modelling (FDM) principles. The process was implemented in Rhinoceros 3D using the visual programming plugin Grasshopper, with KUKA|prc for robot-specific tool path generation and G-code export. The environment supports rapid adjustment of geometric inputs and process parameters for experiment-driven calibration.

Tool paths are generated as sequences of spatially oriented tool-centre-point (TCP) planes that define the position and orientation of the extrusion nozzle in the global machine coordinate system. The TCP follows the geometric centreline of each deposited strand and serves as the reference trajectory for accurate material placement. A continuous deposition strategy without stopping or restarting is applied since start/stop automation is not yet available for the wax foam 3D printing hardware.

The TCP data are converted into machine coordinates and exported as G-code via KUKA|prc to the robot controller. During fabrication, the motion speed can be adjusted in real time within approximately 0.5–1.5× of the nominal value in ±0.01 m/s increments. This enables tuning of the extrusion-volume-to-travel-speed ratio without interrupting the print or regenerating G-code and allows compensation for minor process variations. Calibration outcomes, such as suitable layer height, extruded strand width, extrusion volume flow, and robot travel speed, are fed back into the software environment and incorporated into subsequent tool path generation, forming an iterative closed-loop calibration process. The parametric environment provides the rapid software modifications required for the parameter calibration described in Section 3.4.

The Rhinoceros 3D model of the target geometry, together with the offset geometry generated for printing, served as the digital reference throughout the manufacturing process. Following printing, the wax component was 3D scanned to capture the as-built geometry and to generate the digital model required for subsequent CNC machining. The scan data were used for process control and tool path generation; however, a quantitative scan-to-CAD deviation analysis was not performed within the scope of the present study. CNC tool paths were generated in Easystone CAM software based on the scanned geometry and its reconstructed CAD representation. This established a continuous digital process chain (Figure 5) comprising: (1) 3D modelling and offset generation, (2) wax foam 3D printing, (3) 3D scanning, and (4) CNC milling to the target geometry.

3.4 3D printing parameter calibration

Due to the novelty of the process and the exploratory nature of the feasibility study, a sequential one-factor-at-a-time approach was selected to establish a stable process window prior to more advanced parameter optimization.

Process calibration focused on deposition stability and layer quality of 3D printed Tudastar wax. The air content of the wax–air composite was fixed at 0% and 20% at constant wax mass flow rates 28.5 kg/h. Layer formation was investigated by varying the vertical layer height in 2 mm increments to 4, 6 and 8 mm (Figure 6A) while keeping all other parameters constant. The TCP feed rate was then varied in 0.01 m/s steps from 0.08 m/s to 0.24 m/s to identify travel speeds that maintained consistent material deposition without over- or under-extrusion. Deposition quality was assessed through visual inspection and quantitative dimensional measurements, including strand continuity, surface uniformity, deviations attributable to under- or over-extrusion, and measured layer height and extruded strand width.

In the last calibration step, the horizontal spacing between adjacent tool paths was reduced incrementally to identify the threshold at which neighbouring strands fused into a continuous, monolithic layer (Figure 6B). Successful interlayer bonding without visible separation was used as an indicator of sufficient bonding and structural integrity. Parameter validation was conducted by printing monolithic test blocks with continuous tool paths to assess interlayer bonding quality (Figure 6C). The tests were performed sequentially and iteratively with the goal to calibrate 3D Printing parameters for the case study, which involved printing large-scale near-net-shape wax preforms as continuous prints.

For the case study, a layer height of 6 mm and an extruded strand width of 20 mm were selected, with a horizontal tool path spacing of 19.5 mm.

3.5 CNC milling parameter

Before the final CNC milling operation, the 3D printed module is digitized using a 3D scanner (Shining 3D Einstar). The resulting point cloud is converted into a volumetric model and imported into Rhinoceros v7.1. The point cloud is then aligned with the design geometry and transferred as abstract volumetric geometry to Easystone CAM Software v. 6.0 to generate tool paths for subtractive high precision finishing.

Milling of the case study in Section 4.3 was conducted at the Digital Building Fabrication Laboratory (DBFL) of Technische Universität Braunschweig using a five-axis CNC milling machine (Digital Building Fabrication Laboratory DBFL, 2025).

A 40 × 80 mm end mill was used for planar areas, and a 300 mm long, 20 mm diameter ball-nose cutter for curved geometries. Machining parameters for the case study in Section 4 were set to a feed rate of 5000 mm/min and a spindle speed of 3500 rpm, based on previous wax CNC milling studies at ITE.

4 Geometrically complex radial concrete stairs - first WF3DP case study

The application of wax foam formwork technology is particularly advantageous for geometrically complex and double-curved concrete components, especially in small-batch production or the fabrication of one-off architectural elements where conventional formwork becomes labour-intensive and uneconomical. Within this context, a cooperating precast concrete manufacturer commissioned the production of a customized radial seating step (Figures 7A–C).

The element corresponds to a bespoke seating step designed as a precast concrete component in accordance with DIN EN 13198 (“Betonfertigteile - Straßenmöbel Und Gartengestaltungselemente,” 2002), (“Betonfertigteile - Straßenmöbel Und Gartengestaltungselemente,” 2002) and manufactured from concrete grade C35/45. The component comprises a two-step radial geometry with a constant curvature, an inner radius of 115.0 cm and an outer radius of 155 cm. The design incorporates an inclined front surface that transitions into a rounded upper step edge, forming a continuous sculptural profile. Edge detailing includes 3/3 mm chamfers on exposed edges, an unchamfered lower edge, and a rounded upper longitudinal front edge with a radius of 3 cm. An integrally formed mowing edge (B/H = 26/15 cm) is included. The plan view and two elevations were specified as formwork-smooth surfaces.

Due to the high architectural and visual quality requirements, the concrete was specified as fair-faced concrete class SB3 (German classification: Sichtbetonklasse 3) in standard grey. This classification demands high dimensional accuracy, precise edge definition, and defect-free surface finishes, thereby imposing strict requirements on formwork quality and making the element well suited for evaluating the performance of wax foam formwork technology in the feasibility demonstration.

The conventional formwork was fabricated manually by a single carpenter within approximately 1.5 days. The hybrid formwork system comprised a CNC milled, sanded, and sealed radial edge manufactured from solid modelling foam, while the remaining Sections were constructed from film-coated plywood (Figures 7D–F).

The conventional formwork was designed for up to 15 casting cycles; however, due to project-specific demand, it was reused for only seven cycles. Following its use in concrete casting, the formwork was stored temporarily and subsequently discarded after several months. Owing to material degradation, contamination, and the composite construction of foam and plywood, the individual elements were not suitable for recycling or reuse in subsequent formwork applications, resulting in a predominantly non-recyclable downcycled system.

In parallel with the conventionally manufactured reference formwork, the case study was reproduced as the first large-scale test of WF3DP formwork production. The case study implements the digital and largely automated process chain up to manual formwork assembly (Figure 8). Used 3D printed Tudastar wax foam elements and milling chips are remelted and pumped through fine filtration units to remove concrete particles and other contaminants. The purified melt is then processed in the WF3DP unit described in Section 3.1.

Automated fabrication of the wax formwork is carried out in two dependent steps, both based on the three-dimensional model developed in the Rhinoceros/Grasshopper environment (Section 3.3). In Step 1, tool paths are generated as the basis for additive wax foam deposition. The printing step follows the calibration methodology described in Section 3.4 and the corresponding evaluation, which define the parameter set applied in the case study (Section 4.1). In Step 2, the printed near-net-shape wax preform is 3D scanned and finish-machined by CNC milling using tool paths derived from the scan data and the milling parameters specified in Section 3.5 (results in Section 4.2), thereby closing the digital process chain.

In Step 3, the milled wax components are assembled with pre-cut multilayer timber panels to form a hybrid formwork system using two materials (results in Section 4.3). Wax elements are used for double-curved and horizontal surfaces, while timber components are applied to planar vertical regions. In Step 4, reinforcement is placed and concrete is cast into the assembled formwork (results in Section 4.4). In Step 5, after approximately 23 h of curing, the formwork is disassembled and the concrete element is demoulded (described and evaluated in Section 4.5). Research into wax formwork technology currently pursues two approaches to reusing wax formwork: direct reuse of the formwork for a subsequent concrete pour, which presently represents the greater challenge, or recycling of the wax to 3D print a new formwork.

4.1 Case study step 1 | wax foam 3D printing parameter & printing

For the case study Tudastar wax was processed as wax foam with an wax-air composite of 20 vol% air and 80 vol% wax. The density of the solid wax (without air) was ρsolid = 928 kg/m³, resulting in an apparent wax foam density of ρfoam ≈0.8⋅ρsolid = 742 kg/m³ (air mass neglected). The robot TCP feed rate was set to 0.15 m/s. The extrusion mass flow rate was set to 28.5 kg/h, and the extrusion temperature was set to 64 °C. For wax melting, an 8 kW melting pot with a 150 L capacity was used. The melting unit was operated for 18 h for premelting, resulting in an estimated electricity consumption of 100–150 kWh for 350 kg of wax. All electricity consumption was covered by renewable energy generated by the solar panel field owned and operated by Herrhammer GmbH. The WF3DP printing setup (foaming unit and industrial robot) operated continuously for 8 h 45 min with an electricity consumption of approximately 46 kW h, during which roughly 250 kg of wax foam were deposited (Figures 9A–C). The printed block measured 1050 mm × 1550 mm × 500 mm including 20 mm offset for CNC milling (Longest dimensions, not rectangular but close to the final contour) and was produced in a continuous, long-duration print using the selected parameter set. The total electricity consumption for the printing campaign is estimated at 150–200 kWh. Based on the measured mass and printing time, the average mass throughput was m˙≈250/8.783 = 28.5 kg/h.

To track the cooling behaviour of the formwork module, temperature sensors were embedded in the core of the 3D printed part. Cooling from 65 °C to 25 °C required 3 days and 21 h. Throughout printing, thermal conditions, material flow, and structural formation were continuously monitored.

During the build, several characteristic behaviours were observed: warpage occurred at the outer perimeter, where edges warped upward by approximately maximum of 50 mm, resulting in a bowl-shaped deformation. In addition, cracks developed along the second outer row of deposited strands. This behaviour is likely related to insufficient interlayer bonding between adjacent tool paths, particularly in regions where strands did not merge into a monolithic layer. The effect appears to be exacerbated when robot travel speed exceeds the available extrusion rate, causing the extrudate to stretch and reduce effective extruded strand overlap and consolidation, which in turn lowers horizontal cohesion.

Producing a formwork element of comparable size by the solid wax block route would have required a block of 1200 mm × 1700 mm × 600 mm, including tolerances to account for volumetric contraction during cooling. This corresponds to a volume of approximately 1.2 m³ and a mass of approximately 1.13 t of wax. Most of this material would have been removed during roughing without contributing to the formwork surface, and would subsequently have required collection, remelting and reprocessing. With approximately 250 kg of wax foam deposited, the near-net-shape printing route consumed approximately 80% less wax in the formwork production.

4.2 Case study step 2 | CNC milling to final contour

Prior to milling, warpage-induced deformation was compensated to obtain a stable, planar reference surface for secure fixation on the milling table using vacuum pods. To reduce warping and prevent edge detachment from the printing bed, the component was embedded in a base layer of molten wax, which solidified and provided additional support during subsequent processing. Cracks attributed to insufficient horizontal bonding were reheated locally using a hot-air gun and filled with molten Tudastar wax. As described in Section 3.5, the component was 3D scanned prior to machining and transferred into a 3D CAD model that served as the basis for tool path generation in Easystone CAM software, using the milling parameters specified in Section 3.5.

Machining of all sides required approximately 1.5 h, including automatic tool changes, and consisted primarily of finishing operations due to the near-net-shape printing strategy (Figures 10A–C). During machining, limited vertical cohesion of the printed structure became apparent. While bonding within horizontal layers was strong in regions without defects, delamination occurred along layer interfaces, leading to partial or complete layer separation. This behaviour is attributed to insufficient interlayer bonding, as previously deposited material cooled and was not sufficiently reheated by subsequent layers. To improve interlayer bonding in the build direction, the printed body was pretreated prior to final finishing. Vertical holes were drilled across critical interlayer regions and filled with molten wax to reconnect layers exhibiting insufficient cohesion. A low-viscosity repair wax (molten Tudastar wax mixed with wax milling chips) was then applied manually to fill remaining gaps. This repair resulted in an inhomogeneous surface, with locally solidified repair wax adjacent to milled wax foam regions. The formwork surface was therefore reheated using infrared heating to allow near-surface air to escape and to locally densify the surface layer toward more solid-wax-like characteristics. In this way, the coarse surface texture associated with the pore structure observed in the micro-CT analysis of Tudastar wax foam was reduced, yielding a more homogeneous and denser formwork surface. The reheated and repaired regions were then remachined by applying an overall reduction of the −0.5 mm Z-offset, resulting in a smoother and uniform dense surface (Figures 10D,E).

4.3 Case study step 3 | assembling hybrid formwork

The final formwork system was constructed as a hybrid assembly, combining CNC milled, 3D printed wax foam elements for curved and geometrically complex regions (including the connected horizontal planar sections), while standard plywood panels were used for the vertical planar areas.

The wax foam elements were assembled with 21 mm thick multilayer timber panels using screw joints (Figure 11A). These connections followed investigations of previous research on hybrid formwork systems and the assembly of wax formwork modules (Wirth, 2020; Wirth, 2020), in which wood screws proved effective for joining formwork modules. Pilot holes were predrilled to reduce local stress concentrations during assembly and thereby prevent cracking.

During assembly, an advantage of the wax foam structure compared to solid-cast wax became apparent. The porous internal structure allowed slight local compression around the screw threads, resulting in secure, crack-free strong connections. In contrast, dense and brittle monolithic wax tends to crack during screw insertion, particularly when pilot holes are not provided.

To ensure watertightness at the interfaces between wax and timber, all joints were sealed with silicone (Figure 11B). For the curved vertical outer formwork section, a 21 mm thick multilayer panel was used. This panel was incised along its inner radius to enable bending alongside the milled wax contour. After assembly, the hybrid formwork was additionally stabilized using ratchet straps to resist outward forces caused by the hydrostatic pressure of the freshly cast concrete (Figure 11C).

4.4 Case study step 4 | concrete casting

Following the assembly of the hybrid formwork combining wax foam and timber elements, the reinforcement cage was integrated into the system. Prior to casting, temperature sensors were installed in both the concrete and the wax foam formwork to monitor the thermal behaviour during curing. The measured temperature profiles indicate that the wax foam provided thermal insulation sufficient to maintain a relatively uniform temperature distribution.

For concreting, a self-compacting concrete (SCC) C35/45 was used. The SCC was based on a proprietary mix design, and its detailed composition is not publicly available. The wax–plywood hybrid formwork remained structurally stable throughout casting. The silicone-sealed interfaces prevented leakage of water or cement paste, and the mechanical screw connections withstood the internal pressure generated by the hydrostatic load of the fresh concrete. The wax foam elements were not treated with any release agent, as this was considered unnecessary due to the non-polar surface properties of Tudastar wax. The inherent surface chemistry provided sufficient natural release behaviour. Only the timber panels were treated with a standard release agent. During and after casting, the formwork system remained fully sealed and dimensionally stable.

Figure 12 shows the temperature evolution of the concrete element and the adjacent wax formwork during curing. Temperatures were measured at four locations: in the concrete core, 1 cm above the wax surface within the concrete, 1 cm below the wax surface within the wax foam, and 2 cm below the wax surface. Following casting, temperatures increased from approximately 20 °C–25 °C due to the heat of hydration and reached a maximum after 12–16 h. The highest temperature was recorded in the concrete core at approximately 46 °C–47 °C, while temperatures near the wax–concrete interface reached approximately 40 °C–44 °C. The small temperature gradient between the core and the interface indicates efficient heat transfer, demonstrating that the wax formwork is exposed to a thermal load close to that of the concrete itself. The recorded peak values lie within the range in which CFW wax loses stiffness, well below its freezing range of 62 °C–66 °C, as previously observed in the 1.5 t pillar application. In contrast, Tudastar wax, with a freezing point of approximately 81 °C, retains its stiffness and dimensional stability under the measured curing conditions. Consequently, Tudastar wax maintains higher stiffness and dimensional stability, reducing deformation, improving demoulding behaviour, and enhancing the durability of the formwork. This thermal robustness supports the goal of achieving multiple reuse cycles in concrete casting applications.

4.5 Case study step 5 | demoulding and concrete surface quality

After approximately 23 h of curing, the concrete element was demoulded. The process began with the removal of all mechanical connections, followed by the disassembly of the timber formwork components (Figures 13A,B). The plywood panels detached cleanly with normal resistance. The wax formwork was subsequently separated by lifting the concrete element using an overhead crane and inserting wooden wedges between the wax formwork surface and the concrete to introduce air and initiate separation. In contrast to the timber elements, this step required mechanical force and resulted in localized surface damage to the wax formwork caused by wedge insertion. Despite the observed local surface damage, which prevented direct reuse of the formwork, the demoulding behaviour confirmed the expected release characteristics of Tudastar wax. No coatings or release agents were required, and no adhesion, residue, or surface contamination was detected on the concrete. Visual inspection showed clean separation overall, with only minor defects at sharp wax-formwork edges. The research steps necessary to achieve repeated use of WF3DP manufactured formwork for casting concrete will be discussed in Section 5. After demoulding, the wax formwork was melted in the melting tank and concrete residues were removed by pumping the liquid wax through filters (as described in Section 3.1; (1) wax melting and storage). The recovered wax was thereby returned to the production cycle, was re-foamed and used again for 3D printing of formwork elements for ongoing research. The goal of reusing the formwork for repeated concrete casting could not be achieved. In addition, insufficient vertical bonding between printed layers led to further post-demoulding issues. Fine gaps and crevices between adjacent horizontal layers enabled cement paste and mixing water to penetrate the interlayer interfaces by capillary action. This ingress promoted local bleeding and paste migration at the formwork boundary: fines and water were drawn into the crevices, locally reducing the amount of paste available at the concrete surface for uniform setting and for adequately binding the aggregate. As a result, the demoulded surface exhibited localized discoloration and a heterogeneous texture with alternating rough and smooth regions.

The demoulded staircase element, weighing approximately 650 kg, represents the first prototype produced using the hybrid formwork system, which consists of 3D printed wax foam with an air content of 20% and timber. Horizontal layer-related gaps and slight milling marks from the finishing process remained visible on the concrete surface, leading to minor surface irregularities (Figure 13C). Assessed against the requirements of fair-faced concrete class SB3, the element therefore met these requirements only in part. The geometric criteria were satisfied: the complex radial geometry was reproduced accurately, edges and transitions were sharply defined and no concrete residues adhered to the formwork, so the surface was free of the tearing and spalling associated with demoulding damage. The surface-quality criteria were not fully met. SB3 requires a largely uniform colour and an even closed texture, whereas the demoulded element showed localized discoloration, alternating rough and smooth regions, and visible layer-related marks originating from the printed formwork structure. The element would therefore not be accepted as SB3 without remedial surface treatment.

Nevertheless, the case study demonstrates a significant technical milestone in wax-based formwork fabrication: in the feasibility study both the near-net-shape production of the wax preform and the subtractive finishing were successfully realized within a largely digital and robotic process chain with future potential of integration in a fully automated production chain.

5 Discussion

This study demonstrates the technical feasibility of Wax Foam 3D Printing (WF3DP) as an additive–subtractive approach for producing geometrically complex concrete formwork. The combination of near-net-shape wax foam extrusion and subsequent CNC milling was implemented in a largely digital process chain and validated in a first full-scale case study.

The case study revealed limitations that were not apparent from small-scale prints and characterisation tests alone: warpage during cooling, cracks between adjacent deposited strands, delamination along layer interfaces during machining, the need for local repair and infrared surface treatment, and damage to the formwork during demoulding. The principal development challenges are therefore associated with the interfaces between deposited layers, dimensional stability during cooling, surface quality, and demoulding durability.

Compared with casting and subtractively machining a solid wax block, near-net-shape deposition reduced the wax mass required by approximately 80% when the air fraction is taken into account, and cooling of the printed module from 65 °C to 25 °C required 3 days and 21 h rather than the more than 2 weeks expected for a comparable solid block. These results should not be read as a material, economic, or environmental comparison with the conventional plywood/modelling foam formwork used as the reference.

5.1 Material behaviour and formulation selection

Tudastar wax was used for the first time as a formwork material because its higher melting and freezing range provided a larger thermal margin under hydration heat, which is decisive because CFW wax had softened at elevated temperatures in earlier applications.

Foaming affected the two formulations very differently: 20 vol% air substantially reduced the compressive strength of CFW wax, whereas the reduction for Tudastar wax was comparatively small. The temperature-dependent tests show, however, that the printed material behaves markedly differently from the cast material. Peak compressive stress decreased from approximately 3.9 N/mm² at 22 °C ± 1 °C to 2.6 N/mm² at 35 °C and 1.7 N/mm² at 45 °C, while the response changed from partly abrupt cracking towards increasingly ductile deformation. A high strength at 22 °C ± 1 °C is therefore necessary precisely because so much of it is lost as the wax approaches its melting range. Even at 45 °C the strength remains well above the hydrostatic pressure of fresh concrete. Delamination during machining and separation between deposited layers indicate that interlayer bonding, not bulk compressive strength, is the critical mechanical limitation for WF3DP, consistent with the process–structure–property relationships reported for additive manufacturing (Andreacola et al., 2021; Brando et al., 2023).

Tudastar wax also showed a lower total surface energy and an almost negligible polar component, and no concrete adhesion or residue was observed despite the absence of a release agent. Favourable surface energy should nevertheless not be equated with low demoulding force or formwork durability, since mechanical force and wooden wedges were required to initiate separation and caused local damage.

The pore analysis reveals a formulation-dependent trade-off. CFW wax foam forms a fine, predominantly closed pore structure, whereas Tudastar foam is substantially coarser. The larger pores are more frequently intersected during milling and generate local pits, which is why local repair, infrared reheating, and remachining were required. Measurements on three specimens per formulation showed reproducible pore structures and, for Tudastar wax, a total resolved porosity of 16.4%–18.0%.

5.2 Process calibration, geometric accuracy and surface quality

The sequential one-factor-at-a-time calibration was appropriate for establishing a stable parameter set, but it does not capture interactions between parameters and therefore does not define an optimized process window yet (Andreacola et al., 2023; Capasso et al., 2024). The 20 vol% air fraction is a practical process condition rather than a demonstrated optimum; preliminary trials showed increasingly unstable bubble formation towards 30 vol%, and a systematic study of intermediate air contents remains necessary.

Although the printed component was scanned before machining, no quantitative CAD-to-scan deviation analysis was performed, so the intended dimensional accuracy is not yet proven. This is significant in view of the warpage of up to approximately 50 mm observed during printing, which results from volumetric contraction and is expected to increase with component size. A complete geometric validation should compare the target CAD geometry, the as-printed component, the milled formwork, and the demoulded concrete element using mean and RMS deviation, maximum deviations, and colour-coded deviation maps.

The element reproduced the complex radial geometry with clearly defined edges and no tearing or spalling from adhesion, but showed localized discoloration, alternating rough and smooth regions, layer-related defects, and visible milling marks. It therefore met the specified SB3 requirements only in part and would require remedial surface treatment to comply fully.

5.3 Material cycle, direct formwork reuse and environmental potential

Earlier investigations at ITE demonstrated that CFW wax withstands repeated remelting without noticeable property changes. The present study extends this principle to the process level: after the case study, the used Tudastar formwork was remelted, filtered, re-foamed, reprinted, machined, and used for a further casting. This demonstrates one complete process-level material cycle, although a single pass does not establish long-term stability over many cycles.

Direct reuse of the same formwork is a distinct objective and was tested here. Local damage sustained during demoulding prevented a second casting at comparable surface quality, so the formwork was returned to the material loop instead. Recycling retains the wax within the production cycle but requires remelting, printing, and machining before each further casting, whereas direct reuse would avoid these steps and depends primarily on damage-free demoulding and formwork durability.

The approximately 80% reduction in wax demand is a clear measure of resource efficiency relative to the solid wax block route, but reduced material demand alone does not establish superior environmental performance. The balance also depends on the energy for melting, foaming, printing, cooling, scanning, machining, repair, and filtration, and on the number of recycling and casting cycles achieved. Electricity from the on-site photovoltaic installation reduces operational emissions but cannot be equated with zero life-cycle emissions. A dedicated life-cycle assessment is required to quantify the environmental benefits of WF3DP formwork.

5.4 Limitations of the present study

Several boundaries follow from the design of a first feasibility study. The mechanical characterisation used short-term loading, so creep and long-term deformation under sustained concrete pressure remain undetermined. Specimen-scale values describe the material rather than a full-scale component, in which layers cool substantially before the next is deposited. The material cycle covers a single pass, and the 200-cycle evidence concerns remelting of CFW wax rather than repeated foaming and printing of Tudastar wax. Labour input remained high, comprising tool path preparation, continuous supervision during printing, and manual repair, infrared treatment, assembly, and demoulding intervention. The process chain is therefore at prototype and process-development level, mirroring the constraints reported for additive manufacturing in construction (Capasso et al., 2024).

5.5 Outlook

Future research should address the limitations identified in the case study. The main technical priorities are interlayer bonding, warpage, surface quality, demoulding durability, and geometric accuracy. In addition, the ecological and economic efficiency of WF3DP-produced formwork remains to be assessed in detail.

Interlayer bonding requires dedicated characterisation, as it proved to be the governing mechanical limitation. Interlayer tensile and shear strength should be determined as a function of printing direction, path arrangement, and deposition temperature. The decisive variable is the interval between consecutive layers, which governs how far the preceding layer cools before the next is deposited. A parameter study should relate this interval to the resulting bond quality, combining shear and tensile tests with micro-CT analysis to correlate interface defects and bonded area with the measured strength. As the admissible interval depends on how quickly the deposited layer cools, the ambient temperature of the printing environment should be included as a variable, since it governs the cooling rate of the previously deposited layer and therefore the surface temperature encountered by the following one. Where the admissible interval is exceeded, whether through long deposition paths or low ambient temperatures, local remelting of the preceding layer by infrared radiation, hot air, or comparable thermal activation should be investigated.

Warpage should be addressed through both material development and printing strategy, combining formulations with reduced volumetric contraction with thermal-management and path-planning strategies that minimize differential cooling. Material development should also target a finer, more homogeneous pore distribution, which could reduce milling-induced defects and increase the effective interlayer bonding area, alongside optimized milling parameters that minimize visible tool marks.

Two complementary routes should be pursued towards repeatable fair-faced surfaces. The first is an uncoated wax surface in which sufficiently fine porosity and optimized machining strategies together yield a closed, milling-mark-free finish, retaining the release behaviour observed for the near-nonpolar Tudastar wax, which permitted demoulding without release agents. The second is a durable coating that closes the residual milling grooves and provides a high-gloss surface. Such a coating would additionally permit the use of conventional release agents, which are absorbed by the bare wax surface. This route is constrained by a material conflict, however: the low polarity and surface energy responsible for the favourable release behaviour are the same properties that limit the adhesion of coatings. Demoulding should be quantified rather than only observed, comparing uncoated and coated surfaces and release-agent combinations, and considering aids such as controlled compressed-air injection to initiate separation without local damage (Mainka, 2019).

A quantitative geometric quality-control workflow should be established across the process chain by 3D scanning the formwork at each production stage together with the demoulded concrete element. Repeated recycling of Tudastar wax should be investigated over more than 200 cycles, monitoring foamability, pore morphology, mechanical properties, printability and surface characteristics, to verify for the foamed and printed material the recyclability already established for CFW wax. A life-cycle assessment should then cover all steps from melting to recycling, comparing defined scenarios such as one, ten and twenty casting cycles per formwork with conventional and solid wax alternatives in terms of emissions, energy, material, time, labour, cost, precision and surface quality.

Table 2 indicates the order of magnitude of the time saving rather than a validated comparison, as it covers the core production steps only. Tool path generation currently requires approximately 8 h of manual preparation, and the insufficient interlayer bonding a further 8 h of local repair, which is why WF3DP presently demands considerably more labour than the solid wax block route (17 h against 4 h), even though it reduces the time until the formwork is available from approximately 14 days to about 5 days. Dedicated slicing software should reduce tool path generation to below 30 min, and improved interlayer bonding would largely eliminate the repair effort, bringing both routes to a comparable labour input of about 2 h. Replacing fully filled blocks with structurally optimized infill patterns reduces the weight of the formwork, the deposited and the thermal mass. Assuming that infill reduces the required wax volume to approximately one-third, printing time would fall proportionally from 8 h 45 min to approximately 2 h 30 min, and the reduced thermal mass is expected to shorten cooling from 3 days to 21 h to approximately 1 day. Whereas the reduction in printing time follows from the deposited mass at constant throughput, the corresponding reduction in cooling time remains to be confirmed experimentally, since cooling depends also on the surface-to-volume ratio and the internal geometry of the infill pattern. The advantage of WF3DP therefore lies not in labour input, which would become comparable after largely automated manufacturing steps, but in the waiting time, which has the potential to fall to approximately one-10th of that required for the solid wax block route, whereas the corresponding potential of that route is largely exhausted.

TABLE 2. Production steps, labour input, and waiting time for formwork elements of comparable size, manufactured by solid wax block casting and by WF3DP.

Black values were measured during the case study; grey values are not validated but reasoned expectations, comprising the solid wax block reference times and the future values anticipated from dedicated slicing software, higher throughput printing and printed infill patterns. In addition, all figures have been rounded up and are intended to be indicative.

6 Conclusion

This study establishes the technical feasibility of Wax Foam 3D Printing (WF3DP), an additive–subtractive approach for geometrically complex hybrid concrete formwork. A custom wax foam extrusion system was calibrated and combined with CNC finishing in a largely digital process chain, validated through the fabrication of a full-scale hybrid wax–timber formwork and the casting of a precast concrete staircase element.

In comparison to CFW wax, Tudastar wax offers a substantially higher freezing range (around 81 °C, compared with roughly 62 °C–66 °C for CFW wax) together with greater compressive strength, but forms a coarser pore structure. Foamed CFW wax instead yields a finer pore structure and a superior milled surface finish, at the cost of softening at markedly lower temperatures. The selection of Tudastar wax therefore reflects a prioritization of thermal stability under hydration heat and release behaviour over the finer pore structure and superior surface quality of CFW wax. With formwork temperatures of 40 °C–47 °C measured during curing, the thermal margin provided by the freezing range of Tudastar wax was the feasible material choice, as CFW wax had softened and partially melted under comparable conditions in earlier applications. A formulation combining the thermal range of Tudastar wax with the pore fineness of CFW wax with reduced volume contraction would resolve this conflict of objectives and is identified as a target for further material development.

The temperature-dependent tests further show that the behaviour of the printed material is governed not only by the bulk wax properties but also by temperature and the interfaces generated during deposition, the response shifting from abrupt interlayer cracking towards ductile deformation as temperature increases. Although the measured strengths remain well above the hydrostatic pressure of fresh concrete, the case study showed that interlayer bonding is the critical limitation, identifying interface quality and warpage control as the principal targets for further development.

Relative to subtractive machining of a solid wax block, near-net-shape WF3DP reduced the initial wax demand by approximately 80%, and cooling of the printed module required 3 days and 21 h rather than more than 2 weeks. These results demonstrate increased material efficiency relative to the solid wax block route.

A complete process-level material cycle was also demonstrated: the used formwork was remelted, filtered, re-foamed, reprinted, machined, and used for a further casting. Direct reuse of the same formwork, a distinct objective tested here, was not achieved because local damage during demoulding prevented a second casting at comparable quality; long-term recycling stability and repeated direct reuse therefore remain separate research objectives.

The case study reproduced the intended complex geometry, but geometric accuracy was not quantified. The concrete element met the specified SB3 surface requirements only in part, as localized discoloration, heterogeneous texture, layer-related defects, and milling marks remained visible. Together with the manual formwork repair, process supervision, and intervention during demoulding, these observations indicate that WF3DP remains at the prototype and process-development stage.

Overall, the study demonstrates a technically feasible additive–subtractive formwork process, a substantial reduction in wax demand relative to solid wax block manufacturing, and a complete process-level material-recycling cycle. Interlayer bonding, warpage control, geometric accuracy, surface quality, damage-free demoulding, repeated material cycling, and environmental and economic assessment are identified as the principal requirements for progressing towards a robust, industrial-ready technology.

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Footnotes

Edited by: Binrong Zhu, Nanjing Forestry University, China

Reviewed by: Giuseppe Brando, University of Studies G. d'Annunzio Chieti and Pescara, Italy

Ahmad Hammoud, Texas A and M University, United States