Page 634 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Technology 597

(foam drinking cups, lids, lunch trays, and food service containers), packaging for medical
devices, and food packaging (meat and poultry trays, egg cartons, dairy and delicatessen contain-
ers, and barrier packages). Acrylonitrile–butadiene–styrene (ABS) is thermoformed to produce
more durable articles such as refrigerator inner door liners, recreational vehicle and boat parts,
automotive panels, picnic coolers, and luggage exteriors. HDPE is used in the manufacture of
pick-up truck liners, golf cart tops, and sleds. Crystallized PET is used in the thermoforming of
food trays that can be heated in a microwave or regular oven. Frozen-food oven-friendly trays
are made from coextruded polycarbonate–polyetherimide. Polycarbonates and poly(methyl meth-
acrylate) (PMMA) are thermoformed to produce skylights, windows, tub and shower stalls, and
outdoor display signs. PVC is used in the production of blister packaging of pharmaceuticals,
foods, cosmetics, and hardware.
Thermoforming is easily carried out for materials where the “sag” temperature is broad.
Amorphous resins such as foamed, oriented, and HIPS, acrylics, and PVC are primary resins
employed in thermoforming. Semicrystalline thermoformable resins include both HDPE and LDPE.
Control of conditions is more important where the “sag” temperature range is narrower such as for
semicrystalline resins like PP, some polyesters, and fluoropolymers. The range for effective sag can

be increased through the introduction of appropriate branching, copolymerization, and addition of
selected additives, including crystallization modifi ers.
While some molding techniques are adaptable to the production of thermoset materials, thermo-
forming is carried out using thermoplastic materials.
There are a wide variety of thermoforming techniques in use today.

Vacuum can be used in a process called basic vacuum forming. The sheet is fixed to a frame,
heated, and vacuum applied that implodes the sheet to conform to the mold contour. The vacuum
site is generally at the base of the male mold. It cools as it comes into contact with the cold mold.
For thick sheets, extra cooling is supplied by means of forced air- or mist-sprayed water. Articles
formed using vacuum forming typically have thinner walls the further the sheet must travel to the
mold location. The excess plastic material is trimmed and reused. At times vacuum forming is run
in line with a sheeting extruder. It is similar to the plug-assisted system described in Figure 18.14
except no plug is used.
In drape forming the thermoplastic sheet is clamped and heated and the assembly then sealed
over a male mold. The mold may be forced into the sheet or the sheet may be pulled into the mold
by introduction of a vacuum between the sealed sheet and the mold. By draping the sheet over the
mold, the part of the sheet touching the mold remains close to the original thickness. Foamed PS
and polyolefins are generally used in this procedure.

In pressure forming, positive pressure is employed to assist the sheet-contents into the mold. The
major advantage is a decreased cooling time for pressure forming. Crystalline PS, HDPE, and ori-
ented PP are used in this procedure.
After the heated sheet is sealed across the mold, a shaped plug is pushed into the sheet, stretching
it as it enters into the mold cavity in plug-assisted forming. The plug is generally of such a size and
shape as to assist in the formation of the final mold shape and generally occupies about 90% of the

mold volume. As it gets near the bottom of the mold, full vacuum is applied.
A variety of the plug-assisted process are the prestretching-bubble techniques. In pressure-bubble
plug-assisted forming a heated sheet is sealed across a female cavity and pressure is blown through
the cavity forcing the sheet from the mold. An “assist-plug” is then forced against the blown bub-
ble with the heated sheet beginning to form about the plug as it forces the sheet against the female
mold. As it nears contact with the female mold bottom, vacuum is applied through the mold causing
the material to collapse onto to mold. Alternately, positive pressure can be applied on the “plug”
side forcing the sheet against the mold walls. In the reverse of the pressure-bubble technique, called
vacuum snapback forming, the heated sheet is sealed against a vacuum female cavity and controlled
vacuum draws the concave shaped sheet away from the entering male mold. The male mold is then
pressed against the sheet and vacuum applied through the male mold and/or pressure applied from

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