CRP Technology’s Windform PS is a polystyrene-based powder formulated for selective laser sintering (SLS) platforms using CO₂ laser energy at 10.6 µm. The product is designated Windform PS and belongs to the Windform family of laser-sintering powders supplied by CRP Technology. Unlike structural Windform grades based on glass-filled or carbon-fiber-reinforced polyamide 6/12, Windform PS is an unfilled polystyrene system intended mainly for investment casting patterns, form-and-fit prototypes, and low-load models in which post-processing burnout or low ash residue is more important than tensile strength. The material is typically supplied as a white, free-flowing powder with a nominal particle size distribution in the 40–100 µm range, although lot-specific certificates supersede any general statement. The sintering mechanism depends on partial melting of the polystyrene particles in a heated powder bed; resistance to part distortion requires close control of the powder-bed surface temperature and laser energy input. No attempt should be made to use Windform PS as a load-bearing end-use polymer where polyamide 12, polyamide 11, or filament-based thermoplastics are specified.

What Differentiates Windform PS from Glass-Filled Polyamide Powders?

Unlike glass-filled polyamide powders such as Windform GT and carbon fiber-reinforced Windform LX 3.0, Windform PS is an unfilled amorphous thermoplastic with a comparatively narrow sintering window. The principal differentiation is not impact strength or flexural modulus but burnout behavior, because the polystyrene matrix can be removed from a ceramic shell with lower residual ash than most reinforced polyamide grades. Polyamide 12 and composite Windform grades are semi-crystalline and tend to retain mechanical properties at elevated temperatures, whereas Windform PS loses dimensional stability as the part approaches the glass transition temperature, typically reported below 100 °C. Published data for Windform PS-specific comparative values are limited; however, the general class of unfilled polystyrene SLS powders exhibits tensile strength values an order of magnitude lower than glass-filled polyamide powders. This difference imposes handling constraints because thin-wall areas below 1.0 mm may fracture during powder removal or shell dipping unless gussets or temporary ribs are incorporated into the pattern design.

For investment casting pattern production, the principal process risk is not green-part tensile failure but differential thermal expansion during shell burnout. In a typical sequence, the Windform PS pattern is removed from the build cake, cleaned with bead blasting at 2–3 bar using spherical glass or sodium bicarbonate media, and then coated with successive layers of yttria-stabilized zirconia or alumina-silicate ceramic slurry. The shell is dried at 22–25 °C and 40–60 % RH, and the pattern is then melted or burned out in a furnace. Because polystyrene expands before softening, a rapid ramp through the 100–200 °C interval can raise shell stress sufficiently to cause cracking. Investment casting foundries frequently specify a heating ramp no faster than 2 °C/min between 100 °C and 250 °C, followed by a dwell at 600–800 °C to consume organic residue. Ash residue after burnout is typically cited below 0.1 % by weight; total burnout behavior depends on shell permeability, part thickness, and furnace oxygen flow. Published data for Windform PS-specific burnout curves are limited, so each foundry should run a sacrificial lot to establish shell compatibility before production release.

Nominal Sintered-State Properties Reported Against ASTM and ISO Methods

The sintered-state data below reflect representative nominal ranges reported for unfilled polystyrene SLS materials; CRP Technology lot-specific certificates should be consulted for production qualification.

The low tensile and flexural values are deliberate trade-offs for the burnout requirement. Because Windform PS is not a structural material, it should not be compared directly with polyamide 12 powder properties such as tensile strength above 40 MPa; the comparison is inappropriate unless the accepted part must be sacrificial or temporary. Reporting against ASTM D638-14 requires Type IV specimens produced in the XY orientation; Z-oriented tensile data are typically lower and should be separately assessed where build orientation is fixed. Heat deflection temperature under load is not a creep resistance indicator for PS and should be read only as a short-term thermal softening threshold.

Dimensional compensation for Windform PS patterns is usually established by building a calibration artifact with known features such as bore diameters, wall thicknesses, and step corners. Shrinkage in unfilled polystyrene SLS is typically in the range of 0.8–1.2 % in the XY plane and 1.0–1.8 % in the Z direction, although published data for Windform PS-specific shrinkage are limited and interaction with powder bed temperature can shift those values. Build orientation affects both mechanical strength and pattern accuracy; features parallel to the build plane show better dimensional stability than vertical walls, while vertical thin-wall sections are more susceptible to stair-stepping and to fracture during powder removal. For investment casting patterns, the sprue assembly often benefits from orienting the part so that critical aerofoil edges or thin trailing edges do not lie horizontally at the bottom of the build, where contact with partially sintered powder can cause distortion. Supports are not used in SLS, but anchors and stabilizers may be modeled into the part layout to prevent curling. Curling in unfilled polystyrene is typically caused by a rapid temperature drop when the build chamber is opened too early; the part should remain in the cake until the surface temperature falls below 45 °C.

Within the SLS build chamber, temperature control at the powder bed surface is the dominant parameter, because polystyrene lacks the broad semi-crystalline plateau available in PA12. Typical processing envelopes for unfilled polystyrene powders on 30 W CO₂ platforms include a feed bed temperature of 70–90 °C, a build bed surface temperature of 85–105 °C, layer thickness of 100–120 µm, laser power of 20–30 W, and scan speed of 3–6 m/s. The actual setpoint depends on the machine’s thermal correction, the part packing density in the build volume, and the fraction of reused powder. A critical threshold is the melt viscosity collapse region; if the bed surface drifts more than ±3 °C above the qualified setpoint, adjacent powder particles can fuse unintentionally and produce “orange peel” or crust on downward-facing surfaces. If the surface temperature is more than ±5 °C below the setpoint, interlayer adhesion drops and delamination occurs during breakout. The operator should therefore warm the machine for at least 2 h before first scan and verify the surface temperature with a calibrated pyrometer before each build. Published data for Windform PS-specific process windows are limited; these values are representative of clinical SLS process guidance for unfilled polystyrene, and users should confirm the machine-specific parameter set with CRP Technology or the equipment manufacturer.

When Powder Bed Temperature Control Deviates from the Sintering Window

Because the processing window is narrow, a thermal excursion above the specified bed setpoint can lead not only to dimensional loss but also to localized crosslinking or molecular weight degradation in the powder bed. Recycled powder from the overflow containers is more prone to surface oxidation and yellowing than virgin Windform PS. Process technicians often restrict the used-powder fraction to 30–50 % by weight with virgin powder make-up for pattern applications, but that ratio must be validated by melt flow rate testing in accordance with ISO 1133-1:2022. A drop in melt flow rate below a lot-specific control limit indicates that the recycled powder has undergone excessive thermal history; continued use may create brittle patterns and increased ash residue. Furthermore, the low thermal conductivity of polystyrene powder means that dense packing of multiple patterns can create hot spots in the center of the build. Build layout should maintain at least 10–15 mm spacing between large solid volumes and distribute cross-sectional area uniformly across the build platform. When hot spots occur, parts may exhibit “coring,” in which interior regions remain partially fused while exterior surfaces appear smooth. Coring is a known failure mode on production-scale SLS lines, and it is best addressed by reducing scan power density or by increasing the number of evenly distributed patterns rather than by lowering bed setpoint alone.

The melt flow behavior of Windform PS under SLS conditions differs from that of PA12 in that the amorphous polystyrene does not exhibit a sharp recrystallization exotherm during cooling. This reduces part warping during the cool-down phase but also lowers the maximum allowable build chamber temperature before the powder cake becomes sticky. In production-scale powder-bed fusion equipment, the feed and build cartridges should be kept sealed and dry; if the powder has been exposed to relative humidity above 60 % for more than 4 h, pre-drying at 60 °C for 12 h in a dehumidifying oven is recommended before sieving. Sieving through a 150 µm mesh removes fused agglomerates and debris from previous builds. Powder batches that have been sieved more than 3–5 cycles may show reduced apparent density and should be blended with virgin material at the qualified refresh ratio. These powder handling controls are not optional when the printed part is intended for investment casting, because any agglomerate or contamination can form a local low-density region that collapses during shell firing or leaves a void in the ceramic shell.

Solvent exposure data for Windform PS are limited, but polystyrene compatibility tables indicate dissolution or stress crazing in aromatic hydrocarbons, ketones, chlorinated solvents, and some acrylic adhesives. For pattern assembly, hot-melt adhesives or cyanoacrylates are therefore preferred over solvent-based cements; toluene-based glues can create localized dimensional swelling at the bond line. In post-processing, solvent smoothing with acetone vapor is not recommended because the process attacks the surface and may reduce the dimensional accuracy required for investment casting shells. The unreinforced material also exhibits low resistance to impact and abrasion; operators should avoid vibratory finishing with hard ceramic media and instead use low-pressure bead blasting with spherical glass at 2–3 bar. Because Windform PS is not a food-contact material, no FDA 21 CFR clearance statement is published; compliance with RoHS Directive 2011/65/EU and REACH must be verified from the lot-specific safety data sheet. The product is supplied for industrial use only, and waste powder should be managed in accordance with local regulations for polystyrene particulates.