Thermoforming, part processing methods

Laminating lends itself to the manufacture of resilient products with a large flat surface area and excellent surface properties. These products can be converted to their final form by thermoforming and insert molding. This method avoids the use of paint with its negative side effects, that is, the influence of solvents on the physical properties of the injection-molded part (tension and plasticizing effect). The lamination process also circumvents the optical problem of flow and weld lines. 

Blow molded parts demonstrate that, from technical and cost standpoints, BM offers a promising alternative to other processes, particularly injection molding (IM) and thermoforming. The technical evolution of BM, plus accompanying improvements and new developments in plastics, has led to new BM parts. With the coextrusion technology now established and the hardware in place, the variety of achievable properties can readily be extended by the correct combination of different materials (see Chapter 3). The potential for BM products includes much more than the simple bottles that have been made for many decades. Now the expertise and economics of the method are such that many ideas once deemed futuristic are much closer to realization (2, 96, 186-212). 

With regard to methods of fabrication, all processes in Table 1.1 that are applicable to unfilled, unmodified thermoplastics can also be used for discontinuous systems (with the exception of expandable bead molding). In addition to thermoforming, hot stamping of reinforced thermoplastic sheets mostly containing randomly oriented continuous or discontinuous fibers is used for the production of large semistructural parts. Fillers can also be used in the thermoset processes in Table 1.1, often in combination with the primary continuous fiber reinforcement. The content and inherent properties of the additive, as well as its physical/chemical interactions with the matrix, are important parameters controlling the processability of the composite. 

These three processes use closed molds and low pressures. The closed molds are more costly than the open molds used in thermoforming because there are two halves instead of one. However, the low pressure keeps the tool cost significantly lower than traditional injection molding. Unfortunately, it is that high pressure associated with injection molding that permits its fast cycles. Thus, piece part prices are higher for these methods than they are for injection. 

The thinnest nominal walls are created by the stretching processes thermoforming and blow molding. In each case, the material is first processed by some other method before it is shaped. Thermoforming uses extruded sheet which is then heated and stretched over a tool. Blow molding uses an extruded tube or injection moldment which is then blown into a mold. Plastic thins as it is stretched, and therefore, the effect of this thinning must be carefully considered in the part design. 

Thermoformed decorative inserts and labels have been molded into cold-press molded parts however, threaded metal inserts cannot be molded-in. Threaded inserts can be glued or tapped in place, but not into bosses, which are impractical. Molded-in holes, core pulls, slides, undercuts, and ribs are, likewise, imfeasible. Holes need to be drilled and their location can be held to 0.030 in. The same assembly methods used for parts made firom compression molding can be used for those made from this process. 

There are a number of advantages to the thermoforming process. One of the most significant is that the tooling, in comparison to other methods of making plastics parts, is relatively inexpensive. Because the technique is particularly suited to the production of very large parts, cost savings can be impressive. Many parts with very thin walls can be made only by this method. The process is also ideal for the production of a small number of parts (samples, prototypes, etc.)... 

Table 9-1 lists a number of methods used to manufacture plastic parts and the general types of shapes that are made by the process. The most important processes are molding methods that include injection, blow, extrusion, rotational, thermoforming, compression, and transfer molding. In each of these processes, as well as some of the others, the part is produced in a mold (an enclosure that is the boundary surface of the part), and the use of the mold introduces a series of limitations on the shape of the molded part. 

Thick-sheet thermoforming can be used to produce hollow parts that are more difficult to fabricate using a blow molding process. In this method, referred to as twin-sheet thermoforming, two sheets are fed to the open mold so that the sheets remain separated from each other with a blow-pin placed between the two sheets. The mold is closed and the polyethylene is forced into the mold above and below the sheets with a combination of air pressure introduced through the blow-pin between the sheets, while at the same time drawing a vacuum between the sheets and the molding wall to remove air from the mold above and below the two sheets. 

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