Urethane Casting

What is Urethane Casting?

Urethane casting production, also known as cast urethane molding, is best used for small to large prototype parts and end-use production parts. Urethane casting is one of the oldest manufacturing processes using a master pattern and temporary rubber mold to cast thermoset polyurethane materials.

Urethane molding is excellent for large batches of prototype parts and low volume production from one to hundreds of pieces. Polyurethane molding is an excellent alternative to injection molding, RIM molding, structural foam molding, thermoforming, and machining. Urethane casting production is commonly used to create large plastic housings and covers in low volumes to save on tooling costs. There are hundreds of materials available depending on your application. Parts can be made to match specific colors and textures by molding in or with post-processing with finish and paint.

Urethane Casting Process

A master pattern is created through 3D printing or CNC machining from the provided 3D CAD data. The master is then finished, lightly painted with primer, and attached to a parting line board. Any required cores or side actions are added, and one side of the master is covered with a mold box. This box is filled with an RTV silicone rubber. After curing, the parting line board is removed, and the other half of the mold box is attached, then the process is repeated. When complete, the master pattern is removed from the mold and stored as an engineering model or to create additional molds as needed. Molds are typically good for 20-25 parts depending on the size, part geometry and material.

Now the mold is ready for the urethane molding process. A conventional polyurethane resin is injected at low pressure into the RTV mold and cured for 2 to 12 hours in pressurized tanks maintained at 65 PSI for proper material density.

Mack Proto’s precision meter mixing system allows for rapid part production using a UL94-VO rated, 80D polyurethane colored either black or white. Each material exhibits a heat deflection of 200º F. If another material is required, a manual mix and gravity pour process is used as an alternative. Typical urethane material properties are similar to thermoplastics at about 80-90% of the strength by comparison.

After completely curing and setting in the mold, the finished piece is removed. All parts will require some finishing work to remove excess flash and standard part cleanup. Most parts will then be finished and painted to match aesthetic requirements. This is especially recommended for any elements with customer-facing surfaces. If there are fine features and/or tight tolerance dimensions, those can be machined in post-processing.