Injection molding two-shot

Two-Shot Injection Molding. Two-shot or oveimolding refers to a process whereby either different colors or different materials are molded into one part. In this process, the first material or color is injected, then the mold is rotated, and the second shot is injected as depicted in Fig. 1.27. An alternative method is to use a retractable core. In this case, the first material is injected, cooled to solidify, and then the core is retracted to allow injection of the second material as shown in Fig. 1.28. Bonding is accomplished through either strictly mechanical means or by adhesion between the two components through diffusion of the chains. This can result in parts with two materials combined with-... 

To ensure consistency, the two critical areas for control specific to injection molding are shot size control and temperature control on the injection system, i.e., screw portion for the reciprocating screw machines, and both the screw and ram sections for the screw/ram machines. As mentioned previously, for temperature and cure control, the use of microprocessor controls has significantly improved the performance of injection molding machines, including control of the shot size. 

In sandwich injection molding, two polymerizates in separate injection units are successively injected into a mold such that the second shot is fully enclosed in the first. The process is suitable, for example, for the enclosing... 

Overmolding is the process by which two different materials are joined into one assembly without using secondary operations like gluing or welding. In case the materials are chemically compatible, chemical bonds may form between them and so mechanical interlocks are not required. There are two common techniques of overmolding—insert molding and multiple-shot injection molding. 

Fig. 13.23 Two-color, PP injection, high-rate mold-filling short shots documenting fountain flow instability. [Reprinted by permission from A. C. B. Bogaerds, G. W. M. Peters, and F. P. T. Baaijens, Tiger Stripes Instabilities in Injection Molding, in Polymer Processing Instabilities, S. G. Hatzikiriakos and K. B. Migler, Eds., Marcel Dekker, New York, 2005.]...

Co-injection molding involves injecting two or more different materials into a mold, generally in sequence. To accomplish this, molding machines are fitted with multiple injection units (Figure 14-53). Two-shot molding, the most common version, often utilizes an indexing mold. In this process, the mold is rotated 180° after the first shot fills a portion of the cavity, then the second shot finishes the part... 

In the injection molding process, setting the temperature involves optimization of the temperature profile of the plasticating unit (extruder barrel), temperatures of the mnners and gates, (aU these determine the molten polymer temperature) as well as the mold temperature. The temperature setpoints depend on the material type (viscosity profile, thermal and shear stability, thermal properties) as well as machine or process considerations (machine capacity to shot size ratio, screw design, mold and part design, cycle time, etc.). Temperatures of the two basic units, the injection system and the mold, should be discussed separately since their selection stems from very different considerations. 

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