RRIM process

The reinforced reaction injection moulding (RRIM) process is a development of RIM in which reinforcing fillers such as glass fibres are incorporated into the polymer. One advantage of such a system is to reduce the coefficient of thermal expansion, and with a 40-50% glass fibre content the coefficient is brought into line with those of metals. 

In the reinforced RIM (RRIM) process a dry reinforcement preform is placed in a closed mold. Next a reactive plastic system is mixed under high pressure in a specially designed mixing head. Upon mixing, the reacting liquid flows at low pressure through a runner system to fill the mold cavity, impregnating the reinforcement in the process. Once the mold cavity is filled, the plastic quickly completes its reaction. The complete cycle time required to produce a molded thick product can be as little as one minute. 

The main advantages and drawbacks of the low-pressure RIM or RRIM processes are ... 

The basic princ les of the RIM and RRIM processes are shown in Figure 7.41. The mixing chamber is mounted directly on the mould. Micro-mixing of the reactants is achieved through high-pressure impingement of the two liquid jets. It is essential that ... 

In the RRIM process two fast-reacting liquid streams are precisely metered and mixed using high-pressure impingement mixing. A schematic of a typical process is shown in Fig. 12.1. 

Long Fiber Technology Gains Greater Acceptance in RRIM Processing, Modem Plastics, April 2001, p. 24. 

Applications for hard RIM products are mainly large automotive parts, such as front and rear bumpers, and rocker panels (see Figure 1.135). The most frequently used materials are polyurethanes reinforced with milled short fibers, which are processed in the so-called RRIM process (Reinforced Reaction Injection Molding). The short fibers increase the stiffness and decrease the thermal expansion in the case of temperature changes. They are abrasive, therefore the molds are made from steel (in small series also aluminum). 

The RRIM process is limited by the problems associated with dispersing the glass evenly into all areas of the mold. Because of this, it may be difficult to develop adequate strength in key performance areas of the parts such as ribs and bosses. Small changes in processing conditions also open the door to significant part-to-part variation in physical performance. 

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