Page 633 - Carrahers_Polymer_Chemistry,_Eighth_Edition

596 Carraher’s Polymer Chemistry

called the dwell. The step is referred to as breathe and dwell. Timing and duration of the breath and
dwell steps are important.
While curing reactions occur at room pressures, it is important for good mold contact to employ
high pressures, generally on the order of 20–70 MPa though some molding processes can get by
with low pressure (0.7–7 MPa). Under pressure, the molecules behave as non-Newtonian fl uids and
some ordering occurs. Further, forced flow of polymers and prepolymers into the mold causes some

aligning of the molecules. Thus, the fine structure, and associated properties, can be somewhat con-

trolled by flow rates, heating/cooling, curing rate and extent, and pressure with speed associated
with many of these factors. For instance, if a homogeneous, isotropic behaving material is wanted

the flow rate into the mold should be slow and flow pathway short. Further, for thermosetting mate-
rials, the time that the material is preheated should be low and the rate rapid. Thus, there exist many
balances where the end result is reached, not surprisingly, through a mix of science and practice
(trial and error). Each machine, mold, and material will present a new opportunity for determining
the optimum set of conditions.
In solvent molding a mold is immersed in a solution and withdrawn, or a mold is fi lled with
a polymer and evaporation or cooling occurs producing an article such as a bathing cap. Solvent
molding and casting are closely related.

18.9.5 THERMOFORMING

Thermoforming involves heating a sheet or thick film just above its T or T , stretching it against a

g m
rigid mold, cooling, and trimming the formed part. Inexpensive aluminum, wood, epoxy, and steel
molds are often employed. This allows the construction of inexpensive molds that allow the pro-
duction of low-volume articles. All thermoplastic materials that can be formed into sheets can be
thermoformed provided that the heating does not exceed the ability of the sheet to support itself.
Thermoforming is employed to convert extruded sheets into smaller items such as packaging
containers, plates, trays, bath tubs, pick-up truck liners, freezer liners, cabinetry, and cups. The

skin packaging that involves a flexible plastic skin drawn tightly over an article on a card back-
ing is made by thermoforming. Thermoforming permits the production of small to large articles,
including those with thin walls such as drinking cups. Thus, thermoforming is employed to produce
articles with a relatively high surface to thickness ratio. Figure 18.14 illustrates the operation of a
simple plug-assisted vacuum thermoforming assembly.
Multilayered materials can be readily formed using thermoforming, including food packaging
that may involve inclusion of layers of ethylene–vinyl alcohol copolymers, PS, polyolefi ns, and/or
copolymers of vinylene dichloride and vinyl chloride. Microwavable food trays from (crystallized)
poly(ethylene terephthalate) are manufactured using thermoforming.
Polystyrene is the most widely used resin material for thermoforming. High-impact PS (HIPS)
is the most widely used being employed largely in the packaging areas, including disposables

Plug

+

Sheet
Product
Vacuum

FIGURE 18.14 Steps in plug-assisted vacuum thermoforming. Initially vacuum is applied and the plug is
pulled into the sheet pushing it into the mold. Further vacuum pulls the sheet against the walls of the mold.
Finally, vacuum is turned off and the plume and formed product removed.

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