Thermoforming comer

TPEs in comparison to thermoset are also suited for thermoforming, which is a fast, low-cost production. The maximum draw ratio decreases with decrease in hardness. Draw ratio up to 0.1 can be attained with TPEs in the hardness range of 40-50 Shore D. The major limitations are to avoid undercut sharp comers, and adequate taper angle for rapid ejection of parts. 

Fourth, molded-in metal parts should be avoided whenever alternate methods will accomplish the desired objectives. If it is essential to incorporate such inserts, they should be shaped so that they will present no sharp inside comers to the plastic. The effect of the sharp edges of a metal insert would be the same as explained in the first point above, namely, brittleness and stress concentration. The cross-section that surrounds a metal insert should be heavy enough that it will not crack upon cooling. A method of minimizing cracking around the insert is to heat the metal insert prior to mold insertion to a temperature of 250 to SOOT so that it will tend to thermoform the plastic into its finished shape. The thickness of the plastic enclosure will vary from material to material. A reasonable guide is to have the thickness 1.75 to 2 times the size of the insert diameter. 

Parts have relatively good wall thickness distribution compared to processes, such as blow-molding and thermoforming. External comers tend to thicken, which can be an advantage in applications where wear is critical. 

As noted, plastic is stretched last into horizontal two- and three-dimensional corners. The sheet that is drawn into a three-dimensional corner decreases in thickness in direct proportion to the corner radius. In many applications, comers are most vulnerable to impact. In thermoforming, corners of large radii are always recommended. Chamfers, being flattened areas with 45-degree angles to all intersecting walls, are substantially stronger than radii and are always preferred for three-dimensional thermoformed corners [21]. 

In addition to gauge distribution, four additional measurements are made on the formed cavities. They are retained volnme, crush resistance, bottom puncture, and comer puncture. These measurements are done on cavities thermoformed nnder optimum conditions. All of these thermoformed properties are summarized in Table 4. 

Puncture resistance of the thermoformed cavities can be characterized in two locations, bottom and comer. The cavity bottom is the center of the tray. The comer is the one opposite of the peel tab. To maintain consistent test geometry with flat film, the formed cavity is mounted and secured on mandrel. The mandrel is moimded on a fixture in such a way to present the outer smface of the comer perpendicular to the puncture probe. 

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