Urethane Casting & Silicone Casting vs. Plastic Injection Molding

Posted on Sep 11, 2019 by Cullen Hikene

At 3Diligent we work in a variety of manufacturing processes. That’s because they all offer different strengths and weaknesses, and it’s our intention to always provide the best solution possible to our customers. Which brings us to today’s installment of our manufacturing process smackdown series: the key ways urethane casting and silicone casting outperform plastic injection molding.

Lower Fixed Costs

It’s well documented that injection molding offers highly cost-effective options for large volumes of parts, but what about when you only need a few dozen or a few hundred parts for your application? In relatively low volume circumstances, urethane casting can be a much better way to go. Since the cost of tooling involved with creating a mold for silicone or urethane casting is so much lower than engineering and constructing an aluminum or steel injection molding tool, you can typically bring your first few dozen or hundred units to market at a fraction of the cost. This, in turn, provides you with strategic flexibility when it comes to products that may not have high enough market demand to justify an expensive tool. This circumstance might exist when you’re bringing a product to market and unsure of how fast it will fly off the shelves, or when annual demand has been established as relatively small and casting nets out to a lower cost than building and maintaining an injection molding tool.

Faster Turnarounds

Castings can be turned around at a much faster rate for the first several parts than injection molding can. With urethane and silicone castings, the creation of a tool can be done in a matter of hours or days. This is especially true of urethane cast parts, where the tool is typically constructed by putting a pattern part into a container and surrounding it with the mold material, which proceeds to set around the object (the object is removed after curing to create the void into which urethane is poured for your part). Creating resin tools is often done by 3D printing them, which may take days at most, but still quite quick. The actual part creation process is moderately fast, as the silicone or urethane may require hours to set once poured into the mold. In contrast, the creation of an injection-molding tool is a significant mental exercise as well as a physical one. This means both mold creation time and labor components are higher than they are for casting molds. The higher labor component also contributes to the overseas production of injection molding tools, which also can increase shipping time for parts. For these reasons, urethane casting can deliver much faster turnarounds so long as quantities are relatively small.

Design Flexibility

With urethane casting, you have a higher degree of design flexibility than you do with injection molds. For starters, injection molding requires consideration of a draft angle throughout the design. In brief, surfaces of an injection-molded part need to have a slight angle to them so that they can be readily ejected by the injection molding tool. In contrast, urethane cast parts can have straight lines because the nature of the process does not require a draft angle. The mold itself is flexible and a mold-release agent can be applied to it so parts can be removed from the casting mold without the same issues you encounter with a metal tool. This also means undercut features can be incorporated into cast parts in a way not plausible for injection molding. Additionally, having a consistent wall thickness ratio throughout your part is less relevant to urethane casting than it is to injection molding. Due to the heat involved in the injection-molding process, careful attention to heat transfer must be considered when designing the part. If very thin walls are connected to too thick ones, the likelihood of warpage due to heat transfer that take the part out of its tolerance range is a real threat. In contrast, the urethane-casting process is not impacted significantly by heat, mitigating this concern.


While sometimes overlooked due to injection molding’s popularity for scale production, urethane and silicone casting can provide an extremely valuable solution to businesses bringing products to market or sustaining production of low volume parts. By mitigating the up-front costs of bringing production parts to the market, it can provide a level of strategic flexibility and cost savings if the circumstances of your program are right. Additionally, the speed with which new casting molds and parts can be fabricated can provide you great strategic flexibility to test different designs before scaling production. Lastly, the design flexibility that casting grants you can be powerful in delivering you the exact part design you want. For all these reasons, urethane and silicone casting may be a great solution for you.

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