Urethane Casting vs. Plastic Injection Molding
Urethane casting and injection molding are manufacturing processes used to create plastic parts. Both processes have their own advantages and disadvantages that will ultimately determine project feasibility and cost.
Similarities
Urethane casting and injection molding both use a mold to create the parts. The mold used for urethane casting is made out of a soft silicone. Molds for injection molding are made out of metal, typically steel or aluminum. These types of molds will have an A-side (cavity) that comes together with the B-side (core) to form a void. Melted plastic resin is then forced into that void to form the plastic part.
Urethane casting molds are made with poured and cured silicone. Pouring the liquid silicone around a master pattern forms the cavity in the silicone mold. Masters are made by hand, with 3D printing, CNC machining, or from an already existing part. The silicone mold is cut in half once it has cured around the master pattern, creating the A-side and B-side of the mold.
Differences
Durability is the major difference in urethane casting vs. injection molding. The silicone mold used for urethane casting will only produce 20 to 25 parts. Urethane casting is ideal for low volume or prototype manufacturing. Injection molding use a metal tool that can run thousands of parts per day and last for hundreds of thousands, even millions, of cycles. Injection mold tooling 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 price-per-piece for mass production.
The difference in mold type will translate to overall cost for each project. Silicone molds for urethane casting can cost as little as several hundreds dollars. Injection mold tooling starts at several thousand dollars and increases in cost with design complexity. Turnaround time should also be considered. Silicone molds can be made quickly, often in days. Metal molds for injection molding typically take weeks to manufacture with conventional tooling. Direct Metal Laser Sintering, a metal 3D printing technology, can be used to make metal tooling much quicker than conventional machining.
Urethane casting and injection molding will both produce robust, high-quality parts. Urethane casting can even produce parts with more strength than injected-molded parts. The silicone molds have the ability to reproduce very fine details in the surface finish.
The deciding factor when considering urethane casting vs. injection is usually the volume of parts needed. Injection molding should be used for high-volume projects. Urethane casting is faster and less expensive when producing low volumes or prototypes.
Can Urethane be Injection Molded?
Casting urethane involves pouring the heated material into a silicone mold. The material will then cool and solidify before being removed from the mold. Polyurethane is a plastic material made from a chemical reaction that joins together multiple units of urethane through the process of polymerization. Urethane and polyurethane are essentially the same material with different physical properties. Polyurethane can be used for injection molding. Urethane is more suited to casting. Polyurethane injection molding can be used to make a variety of plastic products.
