The polyurethane casting process (also called the urethane casting process or hot-cast PU process) turns liquid polyurethane elastomer into solid, load-bearing parts — wheels, rollers, screens, scrapers, seals and bushings — by metering, mixing and pouring a two-component system into heated molds where it cures. This guide walks through each step of the process and the equipment that controls it, so buyers can specify a line that produces consistent, void-free parts.
What is polyurethane casting?
Polyurethane casting is a thermoset elastomer process: a liquid prepolymer (the isocyanate-terminated component) is combined with a curative (an amine such as MOCA/MCDEA, or a polyol) in an exact ratio, poured into a mold, and cured into a dense solid part. Unlike foaming, no blowing agent is used — the goal is a full-density elastomer with high abrasion resistance and load capacity. Cast polyurethane parts routinely range from about 60 Shore A to 75 Shore D, covering everything from soft printing rollers to rigid mining screens. For a materials-science overview of polyurethane chemistry, see the polyurethane topic library on ScienceDirect.
The polyurethane casting process, step by step
1. Conditioning and degassing the components
Prepolymer and curative are held in temperature-controlled jacketed tanks (typically 70–120 °C for hot-cast systems, held to about ±2 °C). The prepolymer is vacuum-degassed to remove dissolved air — the single most important step for avoiding bubbles in the finished part. Skipping or under-sizing degassing is the most common cause of scrap.
2. Metering at an exact ratio
Servo-driven metering pumps dose the two components at a fixed ratio (often measured to about ±1%). Ratio accuracy directly controls crosslink density: a few percent of drift changes hardness, resilience and cure time, and the defect may not appear until the part is under load.
3. Mixing and pouring
The components meet in a mixing head — dynamic (motor-driven, self-cleaning) for most elastomer work — and are poured into the mold. A programmable shot control sets pour weight and time per cavity; a manipulator or fixed head delivers an identical fill to every mold.
4. Curing and demolding
Molds are held in a heated station (or carousel) so the part gels and cures uniformly. After demolding, most cast PU parts are post-cured in an oven (commonly several hours at 100–110 °C) to reach full mechanical properties.
Hot cast vs. cold cast vs. DIY resin kits
| Method | Typical hardness | Best for |
|---|---|---|
| Hot cast (machine, thermoset) | ~60 Shore A – 75 Shore D | Production of wheels, rollers, screens, seals — high abrasion and load |
| Cold cast / two-part resin | Soft to medium | Prototypes, low-load parts, small batches |
| DIY resin kits | Low | Hobby / one-off, not production |
For production of load-bearing parts, hot-cast on a metering-and-mixing machine is the only method that delivers repeatable properties and throughput.
Equipment that controls the process
A polyurethane casting machine integrates the four process stages above: jacketed heated tanks with agitation, vacuum degassing, servo metering pumps, a mixing head and programmable shot control. Tank capacity, number of pour heads, ratio range and automation are configured to the part and the production volume. Because handling hot isocyanate-based systems requires proper ventilation and PPE, review the diisocyanate safety and mandatory-training guidance from the European Chemicals Agency (ECHA) and the OSHA isocyanates page when planning the casting area.
Common casting defects and their process cause
- Bubbles / voids — insufficient vacuum degassing or moisture in the system.
- Soft spots or inconsistent hardness — ratio drift or poor mixing.
- Short shots / incomplete fill — pour weight or pot-life mismatch with mold geometry.
- Surface tackiness — under-cure; extend post-cure time or temperature.
Every one of these traces back to a controllable process parameter — which is why a machine with stable temperature, degassing and metering out-earns hand-pouring on both quality and scrap rate.
Frequently asked questions
What is the polyurethane casting process?
It is the thermoset process of metering and mixing a liquid PU prepolymer with a curative, pouring it into a heated mold and curing it into a solid elastomer part. It differs from foaming, which uses a blowing agent to make cellular foam.
What is hot cast polyurethane?
Hot cast polyurethane uses heated prepolymer and curative (typically 70–120 °C) metered and mixed by a casting machine, then cured in heated molds and post-cured. It gives the high abrasion resistance and load capacity needed for wheels, rollers and industrial parts.
Why do bubbles appear in cast polyurethane?
Almost always from dissolved air or moisture that was not removed by vacuum degassing, or from moisture in the curative. Proper degassing and dry, temperature-controlled components eliminate most voids.
Do I need a machine, or can I hand-pour?
Hand-pouring works for prototypes and very small batches. For consistent production of load-bearing parts, a metering-and-mixing casting machine controls ratio, temperature and degassing far more reliably and at much higher output.
What hardness range can be cast?
Roughly 60 Shore A to 75 Shore D, depending on the prepolymer/curative system. Tell us your target hardness and part and we will confirm the material and machine configuration.