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.

Applications of the molding in plastic rapid prototyping

1. Scenarios

In plastic rapid prototyping, molding is often used for small-batch runs of prototypes, typically ranging from dozens to hundreds of quantities. These prototypes are used for testing or trial production of newly designed products, allowing businesses to refine their designs and quickly launch them into the market, even ahead of their competitors. 

2. Industries

The automotive and medical device industries are two representative industries that often use molding in rapid prototyping. For example, the automotive industry commonly uses small batch prototypes for testing dozens or even hundreds of the same part, while the medical device industry often uses rapid prototyping to produce low-volume medical devices at an affordable cost.

Molding techniques involved in plastic rapid prototyping

Molding techniques involved in plastic rapid prototyping vary between Western and Eastern countries. Western countries have more developed molding techniques and rapid prototyping capabilities, which include bridge molding, over molding, insert molding, and more. Chinese counterparts often utilize vacuum casting and reaction injection molding (RIM) in prototype plastic molding, although they also produce plastic prototypes via CNC machining without using molds. In the following sections, we will delve into these molding techniques in detail.

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.


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.


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. 

Advantages of using molding in plastic rapid prototyping

1. Lower unit cost

The cost of molds for rapid injection molding is much lower than those for mass production, as they have shorter mold life cycles. Also, businesses do not need to meet the high MOQ (Minimum Order Quantity) requirements of conventional plastic injection molding. They can stick to their production volume plan. Therefore, there is a significant reduction in the cost of each product.

2. Speed up the time to the market

The turnaround time (TAT) of rapid injection molding is much shorter than that of conventional plastic injection molding. Through rapid injection molding, we can create a mold in weeks. But it would be months for plastic injection molding. Therefore, the former is ideal for prototyping and low-volume production. In this way, enterprises can verify and produce their new products within a short time. It enables them to bring new products to the market quickly, resulting in remaining one step ahead of the market.

3. High flexibility for new product development

Rapid injection molding offers high flexibility for new product development. As the production process is small-scale, changing the design of a product has little impact on the entire order. It allows for the refinement of the part before it goes to the consumer, and companies can improve the product based on consumer feedback. Additionally, small-batch manufacturing via rapid injection molding saves manufacturers money on immovable inventory. 

However, prototypes made 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 economical without the costs of making molds. By making short-run prototypes by plastic molding, 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!