Even if most of the engineers and designers think that RIM and the injection molding is the same method, there are several differences between them. Unlike injection molding, RIM techniques utilize low-viscosity liquid polymers in thermoset. These polymers expand, thicken and harden because of the chemical reactions only after they’re injected into the heated mold. Raw material and molding techniques can be selected and specialized according to desired weight, strength, density and stiffness.
Since these liquid polymers require less pressure (~100 psi) and lower temperatures (~90°) than typical ingredients, they can be injected into cost-efficient aluminum molds, lowering tooling costs. The molds are moderately heated (~190°), but the resulting exothermic reaction quickly brings the materials to (~325°) and promptly cures the part inside the mold. Curing process time can change from 1 minute to several minutes according to the part geometry, dimensions, functions and wall thickness.
The RIM Manufacturing process begins with polymer liquids (polyol and isocyanate) stored in large storage tanks and dispensed by large, high-pressure industrial pumps. The polymers are recirculated from the storage tanks to a multi-stream mix-head on the machined aluminum mold and back to the storage tanks in a continuous loop.
When each part is made, a piston or plunger inside the mix-head retracts, breaking the continuous loop, and the polymers then mix or impinge at a high velocity – approximately 1200 psi – to ensure the proper mixture of the polymers. The resulting polyurethane enters the mold through the after-mixer, which maintains the mixture’s properties while reducing its velocity to 95-100 psi.
Reaction Injection Molding not only offers significant advantages over injection molding but also vacuum-forming, pressure-forming, and cast molding.
The admittedly longer production time of reaction injection molding is more than offset by its benefits to design, flexibility and cost-efficiency, not to mention the wide ranges of part size, design uniqueness, and overall superiority.