Despite making plastic prototypes via CNC machining without using molds, there are molding techniques involved in prototype plastic molding. They are vacuum casting and reaction injection molding (RIM). They use different molds to manufacture prototype parts.

1. Vacuum casting

It is a process in which a model is used to create a silicone mold in a vacuum environment. Then polyurethane resin is poured into it to clone a replica that is the same as the original. The silicone materials of the molds for vacuum casting are domestic silicone, imported silicone, transparent silicone, and special silicone. 

The materials for prototypes by plastic molding via vacuum casting are slightly different from that of plastic injection molding. Though the product performance is inferior to that of mass production, it can meet production requirements in most cases. Also, parts made by vacuum casting are majorly for prototyping purposes but not for end-use.

Its molding process is less complicated than that of plastic injection molding too. Generally speaking, it can be summarized as three steps.

Step 1: make the prototype for the silicone mold

Before making the silicone mold, we need a prototype. It is usually produced via CNC machining in ABS material or SLA 3D printing in resin material.

Step 2: make the silicone mold

The silicone mold is manufactured based on the prototype. After eight hours of drying down, we take the prototype out and the silicone mold is ready.

Step 3: have the parts injected

Injecting the liquid PU material into the silicone mold. After it dries, the part is done. 

One silicone mold normally lasts for 15-25 shots. The properties of the materials can be similar to ABS, PP, PC, PMMA, POM, Nylon, etc.

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2. Reaction injection molding

RIM produces plastic prototypes via injection molding under low pressure efficiently. Prototypes made by reaction injection molding enjoy better strength, tenacity, and heat resistance than vacuum casting. 

Molds for reaction injection molding are different from vacuum casting. The most commonly applied ones are resin molds (20~50 shots), ABS molds (50~500 shots), and aluminum molds (500~5,000 shots). The parts’ materials can be foamed PU, micro-foaming PU, and non-foaming PU. Components produced by RIM are not only applied for rapid prototyping but also for end-use products.

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Its plastic prototype injection molding process is more complicated than vacuum casting. Because it requires two materials to be mixed. The whole manufacturing can be summed up as storage of raw materials → metering → mixing → mold filling → curing → demolding. 

Prototypes produced by reaction injection molding can be with uneven wall thicknesses. As there are fewer limitations of mold sizes, RIM is widely applied in the automotive and medical industries. 

Prototypes via plastic molding are in low volumes. If the quantity of plastic prototypes needed is less than 5 pieces, CNC machining and 3D printing would be more economic without the costs of making molds. By making short-run prototypes by plastic molding through vacuum casting and RIM, product development costs are minimized, time is saved, and risks are reduced. 

With over 10 years of experience in the industry of prototype plastic molding or prototype injection molding, X Rapid Technologies offers high-quality rapid prototyping and low volume production services. Contact us now to get your 1-on-1 project manager!