Coinjection molding

Coinjection molding Coinjection molding produces products that can help one... 

Coinjection Molding. Coinjection molding refers to a process whereby two ma-... 

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... 

A similar problem is presented by vehicle tires and certain blow molded bottles, which must be virtually impermeable to air and other gases. An example of the use of a very impermeable elastomers is butyl rubber. Because of its impermeability to gases, butyl rubber is used as a roof coating. With plastic bottles, different layers of both coinjected and coextruded plastics (Chapter 8) can be used to fabricate the bottle to make it impermeable to different vapors and gases depending on the barrier plastic included. 

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). 

The basics observed in molded products are always the same only the extent of the features varies depending on the process variables, material properties, and cavity contour. That is the inherent hydrodynamic skin-core structure characteristic of all IM products. However, the ratio of skin thickness to core thickness will vary basically with process conditions and material characteristics, flow rate, and melt-mold temperature difference. These inherent features have given rise to an increase in novel commercial products and applications via coinjection, gas-assisted, low pressure, fusible-core, in-mold decorating, etc. 

Multilayered articles can be made by coinjection blow-molding or coextrusion methods. A three-layer system generally contains a barrier layer sandwiched between two exterior layers. These are actually laminar products. In the coextrusion sequence, several extruders can be used to place the material into the mold. The multilayer container is then produced from blowing air into the preform. 

In 1967, Imperial Chemical Industries (ICI) developed the sandwich or coinjection molding process for producing structural foam products. In this process, the first melt is... 

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.]...

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. 

Using the coinjection procedure, a solid melt is injected to form the solid, smooth skin against the cavity surface. Simultaneously a second short shot melt with blowing agent is injected to form the foamed core. With a full second shot, the mold can incorporate pins or a mold that opens similar to high-pressure foam molding. 

The molding of two or more different types of plastics in a single product may be accomplished to combine their specific properties and/or a better performing or lower-cost product. This process, called corotation, is similar to coinjection or coextrusion in terms of the performance of the designed product (Chapters 4 and 5). 

Technology to successfully produce multilayer injection blow molded bottles is much newer than that to produce multilayer extrusion blow molded bottles. Heinz Inc. s ketchup bottle was the first U.S. example (Fig. 12.22). The use of PET bottles made by coinjection blow molding has grown rapidly. As is the case for coextrusion blow molding, the key is production of the parison. Once the multilayer parison is produced, the remainder of the process is essentially the same as for single layer materials. Even stretch blow molding can be used. 

Figure 12.22 Coinjection stretch blow molded bottle (reprinted with permission from [1])...

As illustrated in Pig. 12.22, the coinjection stretch blow molded PET bottles actually have a five-layer structure, containing two layers of EVOH and three layers of PET. Presumably, the reason is that two thin EVOH layers provide better oxygen barrier than one thick EVOH layer, because the effects of any defects in one layer are diminished by the presence of the second. No tie layers are used in the structure, to facilitate recycling of the containers in existing PET recycling facilities. When the bottle is granulated and washed, most of the EVOH is removed. The small amount that remains does not significantly impact the quality of the recyclate. 

As was the case for ketchup bottles, most uses of coinjection blow molding have the aim of improving the barrier characteristics of the container, to extend the shelf life, and to better preserve the flavor, aroma, or other characteristics of the product. Reheat stretch blow molding is the most common process, with three to five layers of resin forming the parison. The skin layers, both inside and outside. 

One development for coinjection equipment was the inclusion of separate hot-runner temperature control systems for the different resins, for example, in order to allow an EVOH core layer to be processed at a temperature 70 C lower than the PET skin layers. Another development was the use of computer simulation to design tooling that manipulates the barrier resin distribution to fortify critical areas such as wall sections. Both wide mouth and narrow neck coinjection stretch blow molded bottles are available. 

---