A little late to add anything about the difference between extrusion and plastic injection molding, blow molding, but I want to add a practical market view including the factors of how much the molds cost and how many items are wanted.Extrusion is a continuous process used to make sheet, pipe and profiles, film and coating, wire covering, filaments and fibers, feeding blow molders, and mixing/making pellets for other processes, including plastic injection molding. Continuous extrusion dominates these markets -- there are ways to extrude into a closed mold, perhaps useful for low-volume applications. Almost all injection molding really does this, too, using a screw to melt the material, and then as a piston to force the melt into the mold. However, these processes are always called "injection," and the use of the phrase "extrusion molding" is confusing without further explanation. Basically, injection FILLS molds, blow molding (and sheet thermoforming) draws material to the mold surfaces. As a result, injection molds need to take much more pressure and are therefore much more expensive. Injection-blow molding is a process that always existed for small containers but has grown because of PET beverage containers; in this process, preforms are injected, then usually reheated, stretched for greater strength, and blown out into molds. Most of these preforms are made by straight injection molding in very large and very expensive multiple-cavity molds, which become economical because of the huge volumes of these products on the market. Preforms can also be made by a combination of partial injection and compression (first seen as Dynaplast Co-Blow at the 1978 K show), which allows lower melt temperatures and corresponding advantages for PET food/beverage packaging. It also may allow lower mold costs, and thus be preferable in low-volume applications which don't run 24/7 for weeks or months at a time with the same product.
All the above relate to thermoplastics. Thermosets (phenolics, epoxies, some urethanes, certain polyesters) are another world, may be compression molded, transfer molded, injected, even processed continuously (pultrusion). They use different materials and usually serve different markets, seldom competitive with thermoplastics.
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