Further Considerations During the RIM process
During the RIM process, a light pressure builds inside the mould. To enable this pressure to release, and for air to expel, all bosses and up- stands must be connected by webs to the split line and at least one part of the split line must be at the very top of the mould. The mould can be tilted to assist this. With a component comprising both thick and thin sections, it is important that the material flows through the thin section first, to avoid air traps. Optimum surface finish is achieved on the lower face of the moulding.
Where possible, components should be designed to have exterior faces at the bottom of the mould and the split line above. Inserts can be moulded in with a high degree of accuracy and repeatability, and can dramatically reduce assembly costs at later production stages. Inserts can be used either for fastening or reinforcement purposes. Sharp corners can trap air bubbles during the moulding processes, therefore components should be designed with a minimum of a 1mm radius on any corners or edges if possible.
Rigid Self-skinning Polyurethane -Brief
Polyurethane (PU) foams are produced by mixing two liquid chemicals and injecting a controlled quantity of the mixture into a mould. The heat developed by the reaction causes a non-CFC blowing agent, present in one of the chemicals, to expand the mixture and fill the mould cavity. The foam structure collapses in contact with the mould surface forming a dense skin, and leaving a fine cell foam core