Reaction Injection Molding - RIM
In the polyurethane industry, Reaction Injection Molding (RIM) is a process where two or more reactants, typically isocyanate (iso) and polyol are pumped under high pressure into a high pressure mixing head. Inside the RIM mixing head the iso and polyol collide at extreme velocities inside the mixing chamber, this is referred to as impingent mixing. The resulting high speed collision creates turbulence inside the mixing chamber, this turbulence mixes the reactants. The mixed reacting fluid exits the mixing head and enters the mold.
The image below illustrates the impingement mixing process used in RIM mixing heads.
RIM - L-Type Mixing Head
Below is a cross section of the 3D model of PU-RIM.com ‘s MXL L-Type mixing head.
Below is a cross section of the lower portion of our MXL L-Type RIM mixing head.
L-Type Mixing Head Piston Positions
High pressure RIM mixing heads have essentially three states: Circulation, Injection and Cleaning.
While the PU metering system is waiting for the next shot or cycle the mixing chamber piston is in the closed position.
The mixing chamber piston allows the poly and iso to pass through the mixing head through groves cut into the piston.
Circulation is necessary to maintain flow through the system so as to maintain material process parameters such as temperature and pressure.
The cleanout piston can be in either the cleanout or ready piston while in circulation mode
When the machine is ready to cycle the mixing head to pour the next shot the cleanout piston is checked to ensure the cleanout plunger is retracted. With the cleanout plunger retracted the mixing chamber piston is energized retracting the mixing piston allowing material to flow through the nozzles into the mixing chamber. The reactants flow into the mixing chamber which results in a high velocity collision and extreme turbulence which effectively mixes the PU system this is referred to as impingement mixing. Once the shot is complete the mixing chamber piston is returned to the circulation position.
Once a shot is complete and the mixing chamber piston is returned to circulation, there are still reactive materials inside the outlet nozzle. If not cleaned from the outlet nozzle the reactive polyurethane will setup inside the head. To clean the outlet nozzle the cleanout piston is energized which pushes the cleanout plunger to through the outlet nozzle cleaning the residual material form the nozzle. The tight tolerance between the nozzle and the cleanout plunger facilitate removal of the residual reactive PU.