Polymers - Orthopaedic Biomaterials in Research and Practice - page 142

There are a variety of special processes used to fabricate polymeric
parts and devices that take advantage of basic resin properties.
Molding . Polymers may be machined, as indicated above, but there
is a general difficulty in maintaining tolerances and producing
smooth surfaces. Thus, molding is a very popular way of produc-
ing either partially or fully finished parts. There are several varia-
tions of the generic process.
Injection molding (also called die casting ). Thermoplastic resins are
melted and heated to slightly higher temperatures (to reduce their
viscosity) and then forced into closed water-cooled molds. Molds
are usually made from metal alloys, but the low melting point of
some resins permits them to be cast in molds made from high-
melting-point polymers. The resin must possess a reasonably low
viscosity to permit mold filling, so this technique is not easily
applicable to high-molecular-weight polymers. A version of this
method, called reaction in mold or RIM molding, uses compo-
nents of thermoset resins combined and immediately injected into
molds. This technique is especially well suited for making porous
polymers, such as sponges, with the pores produced by the addi-
tion of a volatile material called a blowing agent.
Compression molding . Precise amounts of finely powdered thermo-
plastic (or some thermoset) resins are introduced into a split heated
mold. The mold is closed, and a combination of heat and pressure
produces the finished part.
Blow molding . Preformed sheets of thermoplastic resin are placed in
a closed heated mold and formed to shape by air pressure. This
technique is well adapted to making bottles and other containers.
The material may be drawn into the mold by a vacuum on the other
side; then, the process is called vacuum forming .
3D printing . During prototyping, it is often desirable to quickly create a
3D form of an object in either very small batches or on its own. 3D
printing can create a solid part of any object from just a digital model,
and without the traditional sourcing and design effort required of other
fabrication techniques. Printing is typically carried out using an addi-
tive manufacturing process that will deposit liquid or powder material
onto a substrate in successive layers, until a full object is hardened and
rendered. The use of 3D printing has continued to accelerate as the
equipment continues to become less expensive. Beyond traditional
prototyping utility, there is some interest in utilizing 3D printing for
custom implants, potentially on site at the point of care.
Other fabrication techniques include the following:
Direct casting . Parts are fabricated in open molds from either molten
thermoplastic resin or freshly compounded and unreacted thermo-
set resin. A variant of this process is to dissolve the polymer resin
in a suitable solvent, pour it into the mold, and then drive the sol-
vent off in a vacuum or by heating.