Negative thermoforming

Example 4.8 A small flower pot as shown in Fig. 4.56 is to be thermoformed using negative forming from a flat plastic sheet 2.5 mm thick. If the diameter of the top of the pot is 70 mm, the diameter of the base is 45 mm and the depth is 67 mm estimate the wall thickness of the pot at a point 40 mm from the top. Calculate also the draw ratio for this moulding. 

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. 

In matched mold thermoforming (Fig. 10.9), matched sets of negative and positive molds are used to form the part from the softened sheet. Vacuum is generally applied through the negative cavity, as well. The result is excellent dimensional control, and the ability to form very complex shapes. This method is most often used in thermoforming of foams, due to their tendency to deform if not restrained. 

Thermoforming with matching moulds (a) the heated sheet in position over the negative (female) mould (b) the male is rammed down forcing the sheet into the female mould and (c) the form is cooled and removed from the mould (after Modem Plastics Encyclopedia). 

If the sheet is formed over the male or positive mold and the inner surface of the thermoformed part touches and replicates, the mold is called positive forming (Figure 10.2a). In other way, if the thermoformed part is in the cavities of a female or negative mold and the outer surface of the part replicates, the mold is called negative forming (Figure 10.2b)... 

Figure 10.2 Two basic types of thermoforming processes (a) positive and (b) negative forming...

The tool can be negative (as for the bath) or positive. A negative mould gives an accurate outer shape a positive tool gives an accurate inner shape. Thermoforming is now performed also with matched moulds (see Fig. 7.38). Excellent reproduction of mould detail and dimensional accuracy can be obtained. The moulds are cheap and can be made of wood, metal, plaster, or epoxy. 

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