Rapid Injection Molding is done by injecting thermoplastic resins into a mold, just as is done in production injection molding. What makes the process “rapid” is the technology used to produce the mold, which is often made from aluminum instead of the traditional steel used in production molds. Molded parts are strong and can have excellent finishes. It is also the industry standard production process for plastic parts, so there are inherent advantages to prototyping in the same process if the situation allows. Almost any engineering grade resin can be used, so the designer is not constrained by the material limitations of the prototyping process.
There is an initial tooling cost associated with RIM that does not occur with any of the additive processes or with CNC machining. So in most cases it makes sense to do one or two rounds of rapid prototypes (subtractive or additive) to check fit and function before moving to injection molding.
Pros: Molded parts are made from a wide array of engineering grade resins, have excellent surface finish and are an excellent predictor of manufacturability during the production phase.
Cons: Front-end costs can be higher due to tooling costs.