Urethane Casting vs. Injection Molding

In the prototyping of plastic components, urethane casting and injection moulding are widely used. However, there are some similarities between the processes.

Our customers are frequently torn between injection moulding — a common process in which molten material is injected into a metal mould — and vacuum casting, which uses less expensive silicone moulds to create smaller quantities of plastic parts.

RpProto has extensive experience in these fields and can provide you with professional advice on which manufacturing process is best suited for a specific project.

Urethane Casting

vacuum casting part

Urethane Casting is suitable for the trial production of low volume (10-100 pieces) samples, especially in the new product development stage. It can be used to make small quantities of a part at a low cost. The mold is made of soft silicone, which has good toughness and can be produced with complex structures, fine patterns, and no drafting. Parts with angles or even inverted drafts have a short production cycle and high product quality.

Before mass production of products, many manufacturers need to produce some products in small batches for market feedback testing. At this time, if mass production processes, such as injection molding, are used, the production cycle is long and the cost of mold opening is high. It requires structural adjustments, and the cost of changing the mold is high, and it is even impossible to change the mold. Therefore, the process of Urethane Casting in the early stage is a very good choice.

The vacuum casting process begins with the creation of a master model, which can be done with CNC machinery or 3D printing. After that, the master model is immersed in liquid silicone, which is then cured, cut, and separated from the master model.

After the silicone has cured, it can be used as a mould. To ensure a smooth finish, casting resin is poured into the silicone mould while a vacuum removes bubbles and air pockets. The resin part cures and is removed from the silicone mould, which can be reused approximately 20 times.

Injection Molding

Injection moulding is one of the most widely used processes for mass production of plastic products today. It entails heating and mixing material — sometimes several materials — in a barrel. The material is melted and forced into a mould, where it cools and takes the shape of the mold’s interior.

Injection moulding is used to produce large volumes of parts by repeatedly repeating this process. It is used in a variety of industries, including medical and consumer products. The most notable characteristics of injection moulding are its extensive material selection and high production efficiency.

While injection moulding is commonly thought of as a manufacturing process, it can also be used to create prototypes. This is largely due to rapid tooling, which is a method of quickly and cheaply creating prototype moulds for the injection moulding process. Injection moulding, on the other hand, is not ideal for small quantities of parts because is relatively expensive.

Urethane Casting vs. Injection Molding

Before the products are mass-produced and marketed, some products need to be produced in small batches for design verification and market testing. At present, it is generally produced by vacuum casting. So what is the difference between silicone casting processing and injection molding products?

The lead time of the silicone mold is very short, and dozens of parts can be manufactured in a few days. The lead time of injection molding is very long, and complex parts need 1-2 months to manufacture and test injection mold.

Polyurethane casting can quickly manufacture a small number of parts at a very low cost. While injection molding costs thousands to tens of thousands of dollars to manufacture. After the injection mould has been manufactured and tested, it is very cheap and quick to use it to produce large quantities of products.

Vacuum casting is very flexible. If the casted product is not as expected, the cost of making adjustments is very low and very quickly. So it is very suitable for the verification design and test market. Injection molds, on the other hand, are almost impossible to change once manufactured. So it is suitable for the production of the final product.

The material of vacuum casting is the fusion of two different liquids, A and B. Cures through chemical and physical reactions. The hardness and flexibility of casted products are compared with those of injection-molded engineering plastics( PU, ABS, PC, PP, PMMA, PA etc.). Casted products generally have performance similar to injection molded products, but they are still slightly inferior.

Vacuum Casting is to pour the material into the silicone mold cavity by hand or machine, and then bake and solidify it by vacuuming. If the vacuuming time is not accurately grasped, the surface of the product will produce air bubbles or even be incomplete. The mass production is formed by high-pressure injection molding, so the surface finish is significantly better than that of silicone mold casting; therefore, casted products require later grinding and repairing.
Products produced by vacuum casting are still different in appearance and performance from open mold injection products, and are only suitable for some products with low performance requirements.

Although vacuum casting can make a variety of different colors, its color is relatively dull. Since the casted parts are poured out one by one, the molding time of the product usually takes more than 2 hours, and the storage time of the A-group materials that have been adjusted each time should not exceed 24 hours, otherwise it will deteriorate, so if the reproduction exceeds more than a dozen with The color of the product is difficult to achieve consistency in color (except black). The color of injection molding is not deteriorated in a short period of time through the intercalation of toner and rubber particles, and the consistency of color is better ensured;
Therefore, casted parts are generally processed by post-painting to achieve the smooth and bright surface of the model.

Item Urethane Casting Injection Molding
Lead Time 1 Week 1~2 Months
Materials PU, ABS, PC, PP, PMMA, PA etc. ABS, PC, PMMA, PA, PP, PEEK, PETE etc.
Material characteristics Good anti-aging and corrosion resistance Wide selection and various properties
Mold life 20~30 Times 10000+ Times
Product surface characteristics Usually need to be painted to achieve various effects Usually can be used directly as appearance parts without painting

When is vacuum casting a better option?

Small Quantity

Vacuum casting may be a preferable alternative when smaller number of parts are needed — from a few to a hundred — because the setup costs are substantially lower.


Vacuum casting may be better for simpler parts, especially those that only need to be produced in small quantities. However, vacuum casting isn’t only a low-cost option: it can provide an extraordinarily smooth surface finish.

Short Lead Time & Flexibility

Due to the low setup costs of vacuum casting, it is significantly easier and less expensive to modify vacuum casting moulds than it is to remake injection moulding tooling. As a result, vacuum casting is frequently the preferable choice in the early stages of development. Vacuum casting is frequently used to create early prototypes before injection moulding is used to create end-use parts.

Importantly, developing moulds for vacuum casting is a faster process than creating tooling for injection moulding, so vacuum casting is typically a better fit for time-sensitive prototype work for small volumes. 

When is injection molding a better option?

Large Quantity

Injection moulding is always the best option for big numbers of plastic parts. Although injection moulding has large setup costs (especially when using quick tooling), the unit costs are quite low. This means that if a company needs hundreds or thousands of copies of a product, the early startup costs are quickly recouped through lower unit costs.


Injection moulding enables for the manufacture of more detailed parts than vacuum casting since its tooling is often constructed of high-quality metals. For more intricate products, injection moulding may be the superior alternative.

Material Performance

For those parts that will be subjected to physical testing, they must closely resemble the end-use part. If a part will ultimately be manufactured using injection molding, it may be beneficial to create the prototype using that same process, even if the outlay is greater.

RpProto has extensive experience in both vacuum casting and injection molding. Contact us for your project.

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