Biomedical Engineering Reference
In-Depth Information
3.4.3 Compression molding
In the compression molding process, a thermoplastic or a partially polymerized
thermosetting polymer is placed in a heated cavity. The material is usually
preheated and preshaped with a form roughly similar to that of the cavity.
The mold is closed and pressure is applied in order to force the material
to fill the mold cavity. In the process, the polymer undergoes complete
polymerization or crosslinking. At the end, the mold is opened, the part is
ejected and the cycle can start again. This process wastes very little material.
However, it is difficult to produce parts with close tolerances because the
final size of compression molded products depends on the exact amount
of the preform. Moreover, it is not possible to obtain parts with a complex
shape, for example with deep undercuts (Osswald and Hernández-Ortiz,
2006, Tadmor and Gogos, 2006).
3.4.4 Rotational molding
rotational molding is used to produce hollow objects. rotational molding
can produce large parts with uniform thickness at low costs compared with
injection molding. It can be seen as a sequence of six different steps:
∑ induction or initial temperature rise,
∑ melting and sintering,
∑ bubble removal and densification,
∑ precooling,
∑ crystallization of the polymer melt, and
∑ final cooling.
To begin with, a certain amount of a powdered polymer is loaded in the
mold. The mold is closed and placed in an oven where it turns around two
axes while the polymeric powder melts. During the heating and melting
phases, the polymer is deposited uniformly on the mold walls. at the end
of the process, the mold is cooled and the final product is removed. The
cooling rate is particularly important because it affects residual stresses in
the molded object. if cooling is too fast, objects can be warped. The mold
undergoes a slow initial cooling using air, followed by a faster quenching
using a water spray. The main disadvantage of rotational molding is the long
time required to heat the powder and the mold, and then to cool the mold.
induction time can be reduced by preheating the polymeric powder, whereas
bubble removal and cooling stage times can be decreased by pressurizing
the material in the mold. Melting and sintering depend on the rheology and
geometry of the particles (Osswald and Hernández-Ortiz, 2006).
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