Reaction Injection Molding (RIM)

Process Definition & Working Principle

Reaction Injection Molding (RIM) is a specialized manufacturing process used to produce thermoset polymer components by mixing two low-viscosity liquid reactants under high pressure and injecting them into a closed mold where they rapidly polymerize and cure. Unlike conventional thermoplastic injection molding, which melts solid polymer pellets and forces them into a cold mold under high pressures (50–150 MPa), RIM utilizes reactive monomeric or oligomeric liquid systems: typically a polyol (Component A, containing catalysts, surfactants, blowing agents, and chain extenders) and an isocyanate (Component B, usually polymeric methylene diphenyl diisocyanate, MDI, or toluene diisocyanate, TDI).

Kinematics, Axes, & Machine Configuration

A standard RIM machinery setup consists of four core assemblies: the chemical dosing unit, the mixing head, the mold carrier (press), and the thermal control system. The dosing unit utilizes high-pressure axial or radial piston pumps driven by variable-frequency motors. For fiber-reinforced systems (RRIM), where abrasive wear is high, hydraulic dosing cylinders with hardened plungers are preferred. The day tanks are kept under a nitrogen blanket to prevent isocyanate reaction with atmospheric moisture.

The mixing head is the central kinematic organ, incorporating a hydraulically driven, self-cleaning plunger. The mixing head operates in two distinct kinematic modes:

1. Recirculation Mode: The cleaning plunger is in its forward position, sealing the mixing chamber. Components A and B flow through recirculation grooves back to their respective day tanks under low pressure (10–20 bar), maintaining constant temperature and preventing stagnation.
2. Injection Mode: The plunger is hydraulically retracted at high velocity (up to 2 m/s), exposing the mixing chamber and nozzles. Components impinge and mix. At the end of the shot, the plunger moves forward, mechanically purging any residual reacting mix into the mold sprue, ensuring a solvent-free operation.

The mold carriers (presses) provide clamping and geometric positioning. Since mold cavity pressure is low, clamping forces are modest, typically ranging from 100 to 3000 kN (10 to 300 tons). To optimize flow fronts and ensure complete venting of trapped gases, modern mold carriers feature multi-axis kinematics: