Urethane Casting Comparison to Plastic Injection Molding

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By Jeremy Losek, Jan 25th, 2021

Comparison to Plastic Injection Molding

Urethane casting and injection molding are both manufacturing processes used to create plastic parts. There are some similarities between the two processes, and also some very important differences.


One of the most obvious similarities between the two processes is that they both use a mold to create the parts. In injection molding, the mold consists of a two-part “hard” mold typically made out of steel or aluminum. The A-side (cavity) comes together with the B-side (core) to form a void into which melted plastic is injected to form the plastic part.

Urethane casting, on the other hand, uses a poured and cured silicone “soft” mold. The cavity in the silicone mold is formed by pouring the liquid silicone around a “master pattern,” which is a model for the end parts to be made from, and is created by 3D printing, CNC machining, or an actual part. There is still an A-side and a B-side, which are created by cutting the silicone mold in half once it has cured with the master pattern in it. Those are the major similarities.


The first difference between the two processes is durability. A hard tool used in injection molding can run thousands of parts per day and last for hundreds of thousands of cycles. It is built to withstand the high heat and stress of the process, and is therefore designed for producing large volumes of parts with speed, consistency and a low piece price for mass production. A silicone mold, on the other hand, is not durable enough to last for more than about 20-25 parts. It is therefore used as a low-volume or prototype manufacturing method.

Another difference is cost. Silicone molds for urethane casting can cost as little as hundreds of dollars, versus the thousands (or even hundreds of thousands) of dollars for injection mold tooling. In addition, there is the speed factor – soft silicone molds can be produced much more quickly than the weeks it typically takes to machine injection mold tooling.

When it comes to part quality, both methods produce parts that are robust, and generally have more toughness and impact resistance than parts produced by additive manufacturing (3D printing). Urethane casting can even produce parts with more strength than injected molded parts, and the silicone molds have the ability to reproduce very fine details in surface finish.

Considering all these similarities and differences, the deciding factor usually comes down to volume. For high volumes, injection molding will be advantageous. For low volumes and prototypes, urethane casting is faster and less expensive.

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