Thermoforming Variations

One of the most common problems In thermoforming is nonuniform walls. If a draw is too deep, a corner too sharp, etc., nonuniform thinning may result. A number of thermoforming variations are directed towards alleviating problems associated with nonuniformity of wall thickness. Other thermoforming variations are designed primarily to increase production speeds. Some do both. 

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A variation of thermoforming which does not involve gas pressure or vacuum is matched die forming. The concept is very simple and is illustrated in Fig. 4.53. The plastic sheet is heated as described previously and is then sandwiched between two halves of a mould. Very precise detail can be reproduced using this thermoforming method but the moulds need to be more robust than for the more conventional process involving gas pressure or vacuum. 

Products are affected dimensionally by the difference between their forming temperature and their product-use temperature. Thus, a plastic s coefficient of thermal expansion and contraction has a significant effect on service conditions. The thermoforming pressure, time, and temperature variations that can exist will affect the final dimensions. Of these factors, evenness in heating throughout the sheet thickness before forming is usually the most important control. Type of heater has a direct effect on obtaining uniform heat... 

Vinyl chloride has been known for over a hundred years and its polymerization to polyvinyl chloride (PVC) was achieved in 1912. Industrial-scale production of this plastic began in 1927. PVC is still the most versatile plastic. One of the reasons for this is the numerous variations made possible by the method of manufacture of the polymer, namely by copolymerization with other monomers and their processing. Thus, PVC can be thermoformed on all conventional processing machines if the slight thermal damage is taken into consideration. Machining is easy and the material can be bonded, bent, welded, printed and thermoformed. 

Other variations of film decoration are, for example, the rocker moulding for General Motors Chevrolet Trailblazer North Face Edition, a 2-metre-long part thermoformed over injection moulding because of the relatively low volumes (7000 to 8000 vehicles per year) and the short development time required. 

Figure 7.83 Schematic illustration of (a) vacuum-forming and (b) plug-forming variations of thermoforming. From Z. Tadmor and C. G. Gogos, Principles of Polymer Processing. Copyright 1979 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Which thermoforming process variation is closest to providing the benefits of injection molding Why ... 

Pumps are very helpful to sheet extruders who also do in-house thermoforming, as they often run up to 50 percent regrind mixes. This normally variable-particle-size mix promotes surging and up to 2 percent gauge variation. Pumps practically eliminate the problem and make cross-web gauge adjustments much easier. Pumps are recommended in (1) most two-stage... 

Skin packaging n. A variation of the thermoforming process in which the article to be packaged serves as the mold. The article is usually placed on a printed card prepared with an adhesive coating or mecharucal surface treatment to seal the plastic film to the card. 

Many of the same types of dairy and deli containers as produced by thermoforming are produced by injection molding. For PP, injection molding came first. Which technique is better can be argued at length. Arguments are related to cost, speed, efficiency, part appearance, and thickness uniformity. The latter is important not only in terms of top to bottom variation for rigidity and stackability considerations but in terms of the circumference, where minor variations can cause printability problems. Thickness uniformity has been reputed to be one of the virtues of injection molded containers, and another is part appearance. 

Thermoforming is one of the simplest, most economical plastic-forming processes. There are numerous variations of the thermoforming process, but most involve heating... 

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