injection molding
Injection Molding is a manufacturing process for producing parts in large volume. It is most typically used in mass-production processes where the same part is being created thousands or even millions of times in succession.
The mold consists of two primary components, the injection mold (A plate) and the ejector mold (B plate). These components are also referred to as molder and mold maker. Plastic resin enters the mold through a sprue or gate in the injection mold;
Reaction injection molding (RIM) is similar to injection molding except thermosetting polymers are used, which requires a curing reaction to occur within the mold.
The two parts of the polymer are mixed together, usually by injecting them under high pressure into an impinging mixer. Then the mixture is injected under lower pressure into a mold. The mixture is allowed to sit in the mold long enough for it to expand and cure.[1]
If reinforcing agents are added to the mixture then the process is known as reinforced reaction injection molding (RRIM). Common reinforcing agents include glass fibers and mica.
Reaction injection molding can produce strong, flexible, lightweight parts which can easily be painted. It also has the advantage of quick cycle times compared to typical vacuum cast materials. The bi-component mixture injected into the mold has a much lower viscosity than molten thermoplastic polymers, therefore large, light-weight, and thin-walled items can be successfully RIM processed. This thinner mixture also requires less clamping forces, which leads to smaller equipment and ultimately lower capital expenditures. Another advantage of RIM processed foam is that a high-density skin is formed with a low-density core.
Polyurethane
Polyurethane reaction injection molding (RIM) technology was developed in the late 1960s. Since then, the technology has evolved dramatically as more and more product designers and manufacturers have learned to tap the unique capabilities and benefits of RIM for an ever-growing range of products. The universal physical characteristics of polyurethane RIM parts are high strength and low weight. It’s helpful to view RIM not as a specific resin with narrowly defined properties, but as a process capable of achieving a broad range of properties. As its name implies, the polyurethane RIM process uses polyurethanes to produce molded parts. The polyurethanes begin as two liquid components, compared with the pellet form of most thermoplastics. These liquid components – an isocyanate and a polyol – are developed in two-part formulations, which are often called polyurethane RIM systems.
Depending on how the polyurethane RIM system is formulated, the parts molded with it can be a foam or a solid, and they can vary from flexible to extremely rigid. Thus, polyurethane RIM processing can produce virtually anything from a very flexible foam-core part to a rigid solid part. Part density can vary widely, too, with specific gravities ranging from 0.2 to 1.6.
