Injection molding definition

Injection molding requires the barrel temperature to be about 350°C with a barrel pressure in excess of 138 MPa. The mold is maintained at 110°C to ensure uniform flow and high definition, and to discourage an uneven index of refraction, birefringence. The CD is about four one-hundredths of an inch (0.5 mm) thick. For prerecorded CDs, the PC is compression-molded on a stamper imprinted with the recorder information. This takes about 4 sec. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc. Then, a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic surface and the CD is complete. This process is described later in greater detail. 

The compression-molded part, by definition, does not have flow-induced orientation. Comparison of compression-molded part properties with those of an injection-molded part can show the effect of melt temperature on properties. In the compression-molded article without flow-induced orientation, the impact strength remains constant until a certain melt temperature is surpassed and then decreases. This thermal degradation effect can be attributed to the polybutadiene component, which acts as an initiation site for oxidative degradation of the matrices. 

In summary, the future of the Reaction Injection Molding process in the USA definitely appears to be extremely positive. Fueled by growing market awareness and interest, especially in the Inudstrial Consumer area, new material and process developments should bring forth a variety of new applications. The automotive market for RIM continues to be strong, and for the near term will be the major driving force for new material developments. 

In terms of design, injection molded cups make for easier and more flexible designing. Especially undercuts, and so-called cup designs, are not possible with thermoforming. Customers sometimes prefer the transparency of a molded PET or PS cup or the higher form definition. 

A simple definition for a composite is two or more materials, that when combined, will retain their original identities but do interface chemically and physically when subjected to specific conditions of either time, temperature, pressure, or chemical catalyzation. Composites are widely available with different resin systems and reinforcement combinations. They are produced in several physical forms and can be shaped with conventional molding methods as well as a variety of other less conventional methods such as resin transfer molding (RTM), resin injection molding (RIM), structural resin injection molding (SRIM), hand-layup, and autoclave molding(i.e., vacuum bag technique). 

Different properties of plastics are preferred for different fabrication methods. The viscosity of the resin melt is typically measured against a standard test referred to as the melt flow index (MFI). The MFI and resin density characteristics of some of the primary applications are shown in Table 1.4. As can be seen, the MFI and density properties vary with fabrication method. To allow for high tolerances and precise definition, injection molding utilizes a resin melt which is runny (MFI=5-400) in comparison to blow mold grade resin (MFI=0.01-0.2) or extrusion grade resin (MFI=0.3-2). Blow mold resin tends to be more taffy like so that it will retain thickness while pressure is applied inside a mold. This viscosity difference limits the type of recycling production methods possible because different melt viscosity resins do not mix homogeneously. If different melt viscosity resins are not separated pror to reuse, the resin properties will not be uniform in production. 

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