Thermoset molding using the RIM process

Polymers resulting from polycondensation have been the subject of numerous studies, but one of the most important discoveries was undoubtedly the synthesis of polyurethanes by Otto Bayer in 1937.

Polyurethanes, which result from the reaction of a diisocyanate with a bifunctional compound containing mobile hydrogen, can react more or less rapidly depending on the base products and the quantities of catalysts used.

These materials were first used on machines that allowed the two components to be dosed separately into a mixing head fitted with an agitator. In the early 1950s, this meant that the products had to pass through the pump body, and then, once the casting was complete, the operator had to clean or rinse the mixing head with a solvent.

Around 1970, the first publications describing so-called "high-pressure" mixing heads for polyurethane processing appeared. Their main advantage was the elimination of the need to clean the mixing head after casting, hence the name self-cleaning heads.

Studies were mainly carried out in the Federal Republic of Germany. The development of the RIM ("reaction injection molding") process using this type of mixing head underwent a major industrial boom from the mid-1970s onwards. As the name suggests, this process involves injection into a mold at the same time as the chemical reaction takes place.

Any polymer resulting from the polycondensation of liquid or easily liquefiable components can be processed by the RIM process.

Reinforcing or non-reinforcing fillers of a size compatible with the R-RIM (reinforced RIM) process can be added to these polymers, i.e. fillers that can pass through the injectors of the mixing heads.

It is also possible to reinforce parts with long, unidirectional woven or randomly distributed fibers, by pre-positioning these reinforcements in the mold prior to polymer injection. This process is known as S-RIM ("structural RIM").

Polyurethanes are the polymers most widely used industrially in the RIM process. Polyurethanes are currently being progressively replaced by polyureas or other polymers, which should enable the RIM process to fully meet the needs of manufacturers, particularly European carmakers, for the production of painted bodywork parts, within the framework of truly industrial and automatable production.

The price/performance ratio of these new RIM materials is promising for the future. Polymerizable materials based on two liquid components, which can be filled with plant or mineral fibers, offer a wide range of possibilities.

In addition to polyureas, other materials such as polydicyclopentadiene...