Page 526 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

0 Chapter 19 Plastics and Composite Materials: Forming and Shaping
l9.l0 Foam Molding

Products such as styrofoam cups, food containers, insulating blocks, and shaped
packaging materials (such as for shipping appliances, computers, and electronics)
are made by foam molding, using expandable polystyrene beads as the raw
material. As is readily seen upon close inspection, these products have a cellular
structure. The structure may have open and interconnected porosity (for polymers
with low viscosity) or have closed cells (for polymers with high viscosity).
There are several techniques that can be used in foam molding. In the basic
operation, polystyrene beads obtained by polymerization of styrene monomer are
placed in a mold with a blowing agent-typically pentane (a volatile hydrocarbon)
or inert gas (nitrogen)-and exposed to heat, usually by steam. As a result, the
beads expand to as much as 50 times their original size and take the shape of the
mold cavity. The amount of expansion can be controlled by varying the tempera-
ture and time. Various other particles, including hollow glass beads or plastic
spheres, may be added to impart specific structural characteristics to the foam
produced.
Poiystyrene beads are available in three sizes: (a) small, for cups with a fin-
ished part density of about 50 kg/m3, (b) medium, for molded shapes; and (c)
large, for molding insulating blocks with a finished part density of about 15 to
30 kg/m3 (which can then be cut to size). The bead size selected also depends on the
minimum wall thickness of the product: The smaller the size, the thinner the part.
The beads can be colored prior to expansion; thus, the part becomes integrally col-
ored. Both thermoplastics and thermosets can be used for foam molding, but ther-
mosets are in a liquid-processing form and hence are in a condition similar to that of
polymers in reaction-injection molding.
A common method of foam molding is to use preexpanded polystyrene beads,
in which the beads are expanded partially by steam (hot air, hot water, or an oven
also can be used) in an open-top chamber. The beads then are placed in a storage bin
and allowed to stabilize for a period of 3 to 12 hours. They then can be molded into
desired shapes in the manner described previously.

Structural Foam Molding. This is a molding process used to make plastic products
with a solid outer slain and a cellular core structure. Typical products made are fur-
niture components, computer and business-machine housings, and moldings (re-
placing more expensive wood moldings). In this process, thermoplastics are mixed
with a blowing agent (usually an inert gas such as nitrogen) and injection molded
into cold molds of desired shapes. The rapid cooling against the cold-mold surfaces
produces a skin that is rigid [which can be as much as 2 mm thick] and a core of the
part that is cellular in structure. The overall part density can be as low as 40% of the
density of the solid plastic. Thus, with a rigid skin and a less dense bulk, molded
parts have a high stiffness-to-weight ratio (see also Fig. 3.2).

Polyurethane Foam Processing. Products such as furniture cushions and insulat-
ing blocks are made by this process. Basically, the operation starts with the mixing
of two or more components; chemical reactions then take place after the mixture is
(a) poured into molds of various shapes or (b) sprayed over surfaces with a spray
gun to provide sound and thermal insulation. Various low-pressure and high-
pressure machines are available, having computer controls for proper mixing. The
mixture solidifies with a cellular structure, the characteristics of which depend on
the type and proportion of the components used.