Vacuum Casting Materials: What Polyurethane Can and Cannot Replace

"ABS-like" is a description of behaviour, not a material equivalence. Knowing where the resemblance ends is what keeps a bridge batch from becoming a field failure.

The misunderstanding that starts most projects

A buyer needs thirty housings before the mold is ready. The drawing says ABS. The vacuum casting quote says "ABS-like PU". The parts arrive, look right, assemble correctly, and pass a drop test. Everyone relaxes.

Six months later the same parts are on units in the field, and two of them have crept out of shape under a screw boss, or yellowed on a windowsill. Nothing was done wrong in manufacturing. What was wrong was the assumption that "ABS-like" means "behaves like ABS for the life of the product".

Polyurethane is a family of castable systems, not a thermoplastic. It can match a thermoplastic's stiffness, surface and color at room temperature. It does not automatically match its long-term creep, its UV stability, or its behaviour at 90 °C.

1. What the material actually is

Vacuum casting uses two reactive components - a polyol and an isocyanate - mixed, degassed and poured into a silicone mold under vacuum. The mix cures at room or slightly elevated temperature. The result is a thermoset: once cured, it does not remelt.

That single fact explains most of the differences buyers notice:

Good: fine surface detail, low tooling cost, room-temperature processing, easy color and hardness variation, inserts potted in place.

Different: thermoset behaviour under sustained load, limited heat resistance, and no recycling path.

2. The grades that cover most work

The list is a starting point, not a catalogue. Most projects are solved by selecting a grade for the dominant requirement and accepting a compromise on the others - which is exactly what happens in thermoplastic selection too, only with different failure modes.

3. Where the resemblance ends

Four differences cause most of the trouble:

Creep. A thermoset under a constant load at elevated temperature will relax and deform over months. Screw bosses, snap fits and pre-loaded seals are the usual victims.

UV. Many PU systems yellow and chalk outdoors. If the part lives in sunlight, specify a UV-stabilised grade or plan for paint.

Heat. Continuous service temperature is lower than ABS or PC. A part that survives a 70 °C test may still creep at 70 °C over a year.

Chemicals. Fuels, ketones and some cleaning agents attack PU. A "solvent wipe" in the assembly line is a real risk.

None of these disqualify vacuum casting. They disqualify using it as a drop-in substitute without checking the duty cycle.

4. Color, texture and feel

Vacuum casting is unusually good at matching the look of a production part:

Color is pigmented in the mix, so it goes through the part rather than sitting on it. Batch-to-batch matching is done against a master chip, not a Pantone number.

Texture is inherited from the master. A bead-blasted or etched master produces the same texture in every casting, which is why master preparation matters more than any post-process.

Gloss can be tuned by mold finish and by a clear coat, but a highly polished silicone mold degrades faster.

The usual warning: color matching is a visual and instrumental process, and a "close enough" match under one light source can look wrong under another.

5. Inserts, overmolding and assembly features

Because casting happens at low pressure and room temperature, inserts are easy:

Threaded inserts are placed in the mold and potted in place, giving metal threads in a plastic part.

Metal brackets and bushings can be encapsulated without the thermal stress of injection molding.

Multi-durometer parts can be built by casting a rigid shell and a soft grip in sequence, though the bond line needs design attention.

6. Finishing

Cast parts accept the same finishing as molded ones: sanding, primer, paint, laser marking, and masking for two-tone work. Two practical notes:

Paint adhesion depends on mold release removal; a part pulled from a silicone mold carries residue that must be cleaned before painting.

Build-up from primer and paint changes fits; specify whether dimensions apply before or after finishing.

7. What this means for design

The design rules follow from the process, not from thermoplastic habit:

Draft is still needed, though silicone flexes further than steel.

Wall thickness should be uniform; thick sections cure unevenly and can sink.

Sharp internal corners concentrate stress in a material with lower elongation than ABS.

Deep, narrow features trap air and are hard to fill; they are the most common cause of short shots.

Parting line should be planned on a non-cosmetic edge, because the mold is cut open by hand.

8. How to specify it in an RFQ

Give the supplier the drawing, the master model (or the machining/printing route to it), the quantity per mold, the color target with a physical chip, the surface texture, the inserts, and - most importantly - the duty cycle: load, temperature, UV exposure and chemical contact over the intended life.

That last item is what separates a successful bridge batch from a batch that fails in the field six months later. "ABS-like" is not a specification. "ABS-like, 30 units, indoor use, no sustained load above 40 °C, color matched to chip" is.

About the manufacturer

Shenzhen Strongd Model Technology Co., Ltd. produces vacuum cast parts from silicone molds at its 40,000 m² facility in Bao'an District, Shenzhen, alongside CNC machining, 3D printing, tooling, injection molding, die casting and sheet metal. The company holds ISO 9001:2015 and IATF 16949:2016 certification and has served more than 10,000 enterprises over 14 years of export manufacturing.

Website: https://www.strongdcnc.com/