Abstract
Reaction injection molding includes aspects of polymerization reactor analysis as well as melt injection molding. Processing can be broken down into several unit operations: metering and machine performance, impingement mixing, mold filling, curing and mold design. Commercial RIM machines have been used successfully for non-filler systems. The results are generally satisfactory. Reactants are usually allowed to circulate through the mixhead, or even through impingement nozzles, such that uniform temperature control and appropriate agitation of reactants can be obtained. Most heads can switch from the recycle to injection mode under high pressure operation, to minimize the lead/lag problem. The mixing chamber is self-cleaned at the end of each shot by a hydraulically driven piston which pushes out all the residue mixture from the mixhead after mold filling. Flow rate/flow area can be adjusted externally by the nozzle size adjuster. Flow rate also can be controlled by the pressure setting. Two or four streams impingement mixing is the common mixing technology used in the present RIM machines. Static or impingement type after-mixers have been used extensively to improve the mixing. Mold filling seems not to be a problem for conventional RIM operation if mold design is appropriate. The typical pressure inside the mold cavity during filling is less than 350 kPa. With the help of slight foaming during curing, RIM polyurethanes usually have excellent, depression-free surfaces. A number of qualitative descriptions of automotive type RIM have appeared, and some basic studies of the process are being carried out. However, as yet there does not appear to be a complete understanding of how the process influences the structure and properties of the polymer formed. The majority of RIM parts have been made in the 140–300 MPa flexural modulus range for fascia applications, where appearance, weight-reduction, and impact resistance are the most important criteria. A representative formulation used to produce automotive fascia by reaction injection molding consists of a polyether/polyester polyol with molecular weight in the range of 1000–3000, a short chain extender like ethylene glycol, 1,4-butane diol or a diamine and a modified derivative of 1,4-diphenylmethane diisocyanate, MDI. The need for further weight reduction to meet government mileage requirements, and for improved corrosion and impact resistance, makes the extension of RIM materials to other external automotive body components increasingly attractive. For these applications, there will be several new requirements: flexural moduli in the 700–4000 MPa range to provide dimensional stability, but still with the desirable impact strength; a low thermal coefficient of linear expansion to allow satisfactory mating with sheet metal surfaces. An increased thermal stability would also be required for parts that would be painted and baked on the car, or for applications such as hoods with higher in-use thermal exposure.
