Reaction injection molding and plastic injection molding have almost the same name, so even experienced engineers and designers may think they are the same. But the ‘reaction’ creates a big difference.
The Reaction Injection Molding Process: Pressure & Temperature
Step 1: Polymer Liquids
The RIM molding 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.
Step 2: Mixing
When each part is made, a piston or plunger inside the mix-head retracts, breaking the continuous loop, and the polymers mix at a high velocity (approximately 1200 psi) to ensure the proper combination and 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, a much lower pressure.
Step 3: Molding
Since these liquid polymers require less pressure and lower temperatures than typical ingredients, they can be injected into cost-efficient aluminum molds, lowering tooling costs. The molds are then moderately heated (~190°), but the resulting exothermic reaction quickly brings the materials to (~325°) and promptly cures the part inside the mold.
Step 4: Curing & Cooling
Cure times vary from less than a minute to several minutes, depending on the part’s size, geometry, function, and thickness. Polymers and RIM processes can be chosen to produce solid, elastomeric, rigid-foam, or flexible-foam finished polyurethane parts.
Differences Between Reaction Injection Molding & Plastic Injection Molding
Unlike plastic injection molding, reaction injection molding (or RIM molding techniques) utilizes low-viscosity liquid polymers in thermoset – not thermoplastic or thermoforming – processes.
Through a variety of chemical reactions, these polymers expand, thicken, and harden only after they’re injected into a heated mold, accommodating much more intricate designs than ordinary plastic injection molding.
Raw materials and polyurethane reaction techniques can be selected and even customized to precisely deliver desired weight, strength, density, and hardness characteristics. The result is large polyurethane parts with a much lighter weight than those created by more commonplace processes.
Learn the Advantages of the Reaction Injection Molding Process from RIM Manufacturing
Reaction injection molding not only offers certain 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 colors.