Injection molding Coinjection

Plastic products are made by a variety of basic manufacturing processes. As an example a major method such as extrusion has subdivisions that include profile, pipe, tube, film, sheet, coating, post forming, etc. equipment In injection molding there are subdivisions such as coinjection, gas assist, foam, inmold decorating, etc. equipment. There are literally hundreds of processes used with only about the dozen, as reviewed in this chapter, that are principally used (2). 

Coinjection Molding A hybrid technology, also known as multimaterial injection molding, that creates composite pieces made of plastic and metal parts locked together via injection molding or extrusion. The process combines the strength of steel with the corrosion-resistant properties of plastic. Overall weight is reduced, the need for exterior paint is eliminated and assembly and labor costs are slashed due to more efficient use of machinery. 

Coinjection n. A process similar in its results to coextrusion but accomplished by modifications of the injection-molding process. By means of various nozzle and valving arrangement, two or more materials, can be injected either simultaneously or sequentially to form an article with an outer shell of one material with certain desired properties, the shell filled with another material to attain other desired properties such as reduced cost. Coinjection, like coextrusion, basically means that two or more different plastics are formed into a composite or laminated structure. 

As the part increases in size beyond 1 ffi, other processing options beyond thermoforming, such as structural foam and coinjection molding, come into the pecking order. These processes can compete with injection molding to hi er volumes as the size of the part grows because the cost differential with the injection mold grows disproportionately with size. 

Liquid silicone rubbers (LSRs) are two-part injection-molded compounds that can be cured rapidly. They cure in 20 s at 175°C, and new grades can cure at 95°C. They have low compression set, low durometer, and outstanding adhesion and are biocompatible. The LSRs have been coinjection molded with thermoplastics to produce vent flaps and door locking systems. LSRs cost 50 percent more than conventional moldable silicone rubber and the initial cost of a mold can be three times higher, but the cure time is three to four times faster with the LSRs. 

Ethylene-vinyl acetates (EVA) copolymers are in the polyolefin family of thermoplastics. They are used in all processes, particularly for extrusion, injection molding, and blow molding. They are used either alone or are coextruded or coinjected and used in compounds to provide unique properties. They approach elastomeric materials in their softness and flexibility. 

The review in coextrusion also applies with coinjection providing similar advantages. Two or more injection molding barrels are basically joined together by a common manifold and nozzle through which melts flow before entering the mold cavity by a controlled device such... 

Coinjection molding. Coinjection molding refers to a process whereby two materials are injected into the same cavity. The first material is injected into the cavity and then followed by the second material as depicted in Fig. 1.66. In this process, the first material goes to the outside of the mold and forms the skin, and the second material forms... 

Both roughness mechanisms are controlled in coinjection and gas counterpressure molding. The smooth surface and spherical cells are made by both methods. However, both methods have less weight reduction and high cost of hardware. The best surface quality is achieved by co-injection molding. The gas counterpressure tested for smooth surface of microcellular foam with lowest gas pressure 1.73 MPa successfully. In addition, a new overlapped smooth surface on the foamed bottom layer has been developed. 

The parts were made by setting up a 2-stage decoupled molding process in which velocity and position were used for the filling phase, and hold pressure and time were used for the packing and compensation phases. All parts were produced using a Boy 35A injection molding machine. A full part was achieved and the coinjection portion of the experiment followed. 

Fig. 13.30 Four stages of coinjection molding, (a) Short shot of skin polymer melt (shown in dark shade) is injected into the mold, (b) Injection of core polymer melt until cavity is nearly filled, as shown in (c). (d) Skin polymer is injected again, to purge the core polymer away from the sprue. [Reprinted by permission from Design Center, School of Engineering, Santa Clara University, Santa Clara, CA.]...

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