Reinforced reaction injection molding

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Reinforced Reaction Injection Molding or RRIM - is a process used to produce polyurethane and polyurea thermoset polymers. When short fibers (1.6 mm), carbon or mineral fillers are incorporated into one of the liquids to increase modulus and reduce coefficient of expansion.

Processing Conditions

RIM equipment has conditioned vessels with agitators and temperature controllers for proper material temperature (usually between 90°– 150°F [32°– 65°C]). Nitrogen is used as a blanket on both A- and B-side day tanks because it is free of oxygen and water. In the case of the resin B-side slurry, nitrogen is used as an aid for improved surface quality, efficient component mixing, and enhanced polymer flowability in the RIM equipment. The nitrogen concentration is usually 30 to 40 percent by volume and is typically called “nucleation” because of the small bubble formation.

Both the resin slurry and the isocyanate prepolymer in the day tanks are usually circulated (by pump) through the RIM dispensing equipment and the mixhead before returning to the tank. This circulation, in combination with the in-tank agitation, ensures homogeneity of the components. Where necessary, nitrogen can be added to the A side as well, improving the flowability even further.

The RIM dispensing operation parameters are preset prior to the actual RIM “shot” (injection of components through the mixhead and into the mold initiating a polymerization reaction). These parameters include injection rate, shot size, the polymer curing time and the A/B ratio (predetermined weight ratio of prepolymer to slurry for complete reaction of components in the polymerization reaction). Additionally, the desired injection pressures are set by adjusting mixhead orifices and setting the prepressurization points. When the operator is ready to fire a “shot,” he/she applies an external mold release (EMR), if needed, to the mold and then initializes the automatic equipment for the dispensing operation. The RIM dispensing operation is started once the mold is closed. The recirculation lines are blocked automatically and the lines between the dispensing cylinder, or pump, are close-looped to the RIM mixhead. The A- and B-side materials in the lines are prepressured to a setpoint between 1800-2200 (12.4 –15.1 MPa) psi, in preparation for the shot.

Once the prepressure point is reached, the mixhead pin is opened allowing the B-side slurry and the A-side prepolymer to impinge in the mixhead chamber (which is the space provided in the mixhead when the mixhead pin retracts) and flow through an aftermixer, a gate and runner, and into the heated mold. Once the correct amount of material has been injected, the mixhead pin closes, sweeping the mixing chamber clean. The recirculation lines are then opened and circulation returns to normal.

The typical press used to hold the mold closed during the filling and curing process is a booked hydraulic clamp which provides a clamping pressure of 50-400 psi of projected pressure; flash is reduced which, in turn, reduces trimming. After reaching the preset curing time of 20-30 seconds, at 150°-180°F (66°– 82°C), the mold is automatically opened and the part can be demolded by the operator. The operator will clean any excess polymer flash from the mold, apply external mold release (EMR) if necessary, and start the RIM operation again. For a 30-second polymer cure time, an overall cycle time of between 90 and 120 seconds is normal with conventional RIM equipment. The cycle time will be contingent on the complexity and size of the mold. Generally, parts are randomly selected from a day’s production for mechanical property testing. This gives the feedback required to determine whether the process is staying in control.