The RIM process allows moulded articles to be produced at rates comparable to conventional injection moulding. The process produces a range of self skinned mouldings from both flexible and rigid PUs.
Depending on the reactivity of the PU system and part dimensions, the part can be removed from the mould within 1 to 10 minutes. The moulding pressures are low, typically less than 100 psi, which means lower tooling costs.
RIM machinery can allow fillers to be mixed into the reacting system, yielding products with improved physical properties called Reinforced Reaction Injection Moulding (RRIM).
Cast PU Elastomers are processed in liquid form and, for best results, should be degassed to remove any moisture or air. This is best done by heating to the processing temperature and applying a vacuum.
Mixing may be either batch or continuous followed immediately by pouring or dispensing into a cold or heated moulds. The moulds are left at the appropriate temperature until the part cures to a solid end product which is discharged and trimmed as necessary.
The flow of the reactive chemical exerts very little pressure on the mould so they can be made from inexpensive materials e.g. glass reinforced plastic (GRP). Usually after 3 to 8 minutes the moulding can be removed. However a post moulding cure is often required to develop optimum levels of physical properties. A typical cycle is 40 to 50 minutes at 110°C.
If a cast PU is not sufficiently cured, lower impact, flexural strength and heat distortion temperature may result.
The casting of PUs is a batch process. Small and large numbers can be readily accommodated and the castings from one gram to one hundred kilos may be made by the same process. The solid end products can range from about a 10 Shore A to 90 Shore D hardness.
PU millable gums are processed like conventional rubbers using a Banbury mixer and mill and are cured with peroxide or sulphur.
Both polyether and polyester derived PUs are available, their products are formed by the usual elastomer techniques of extrusion, compression and transfer moulding.
Thermoplastic Polyurethanes - TPUs
TPUs are supplied in granules or pellets for processing by all of the usual thermoplastic processing techniques such as injection moulding, extrusion (for tubing, film and profiles) and vacuum forming.
TPUs are usually supplied in a sealed moisture proof bag since they are, to some degree, hygroscopic and will deteriorate if left exposed to the atmosphere. Ideally, dried TPUs should have a maximum level of 0.02 wt% water, and must be kept below 0.05 wt% water, at which level the reaction with the water starts. TPUs that have been processed whilst wet are total scrap and cannot be recovered.
Of the two major types of TPUs available, polyesters and polyethers. A polyether based TPU is more sensitive to overheating during processing. Typical injection and extrusion temperatures are between 180 to 230°C.