Injection molding large surface production parts

Table 5.9 RRIM and injection molding processing conditions compared for large surface production parts...

Laminating lends itself to the manufacture of resilient products with a large flat surface area and excellent surface properties. These products can be converted to their final form by thermoforming and insert molding. This method avoids the use of paint with its negative side effects, that is, the influence of solvents on the physical properties of the injection-molded part (tension and plasticizing effect). The lamination process also circumvents the optical problem of flow and weld lines. 

Because of the low pressure (0.3 MPa) in the mold as compared to those encountered in thermoplastic injection molding (150 MPa) this process is suitable for the production of thin part with large surface areas. A significant application for this is in panel formation in the automotive industry, including fascias, door panels, spoilers, grills, and bumpers. While a rigid foam part with a flexural strength of 700 MPa would require a thickness of 7 mm, a RRIM part, because of the reinforcement, would allow a much smaller thickness (< 3 mm) by virtue of the increase in flexural modulus. 

Table 8-1 compares PUR-RIM and injection molding of unreinforced and glass fiber reinforced thermoplastics, in the production of parts with large surface areas. RIM also is comparable to resin transfer molding (RTM ... 

Table 8-1. Comparison of RIM and Injection Molding of Unreinforced and Reinforced Plastics in the Production of Parts with Large Surface Areas.

The machining of press tools of large sheet metal parts such as car parts or injection molding tools of large plastic products needs the removal of a large quantity of material to produce high quality 3D surfaces. Productivity is enhanced by reducing the control of... 

Molded plastics parts in many ways seem analogous to castings since the mold can be any shape. There are a number of limitations on the part because of the fact that the mold is filled with plastics material and it represents, in addition to the form for the product, the flow path for the plastics material. One factor that is affected is the wall thickness of the part. The viscous melts produced by many plastics require that a minimum wall thickness be used based on the ability to fill the mold rather than on the function of the part. For example, rarely is any injection molded part of any size designed with a wall thickness less than 0.025 to 0.030 inch. This is particularly true if the part has a fairly large surface area because the plastics will not flow to properly fill the part. In addition, as will be seen in Chapter 10, flow conditions can severely affect the properties of the material. 

Injection molding Rapid production rates low per part cost large complex shapes possible good dimensional accuracy good surface finish High tool and die costs not for small runs... 

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