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Abstract This paper presents the design of a plastic injection mould for producing warpage testing specimen and performing thermal analysis for the mould to access on the effect of thermal residual stress in the mould. The technique, theory, methods as well as consideration needed in designing of plastic injection mould are presented. Design of mould was carried out using commercial computer aided design software Unigraphics, Version 13.0. The model for thermal residual stress analysis due to uneven cooling of the specimen was developed and solved using a commercial finite element analysis software called LUSAS Analyst, Version 13.5. The software provides contour plot of temperature distribution for the model and also temperature variation through the plastic injection molding cycle by plotting time response curves. The results show that shrinkage is likely to occur in the region near the cooling channels as compared to other regions. This uneven cooling effect at different regions of mould contributed to warpage.© 2005 Elsevier B.V. All rights reserved.Keywords: Plastic Injection mould; Design; Thermal analysis 22833
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1. Introduction
Plastic industry is one of the world’s fastest growing
industries, ranked as one of the few billion-dollar industries.
Almost every product that is used in daily life involves the
usage of plastic and most of these products can be produced
by plastic injection molding method [1]. Plastic injection
molding process is well known as the manufacturing process
to create productswith various shapes and complex geometry
at low cost [2].
The plastic injection molding process is a cyclic process.
There are four significant stages in the process. These stages
are filling, packing, cooling and ejection. The plastic injec-
tion molding process begins with feeding the resin and the
appropriate additives fromthe hopper to the heating/injection
systemof the injection plastic injectionmoldingmachine [3].
This is the “filling stage” in which the mould cavity is filled
with hot polymermelt at injection temperature.After the cav-
ity is filled, in the “packing stage”, additional polymermelt is
packed into the cavity at a higher pressure to compensate the
expected shrinkage as the polymer solidifies. This is followed by “cooling stage” where the mould is cooled until the part is
sufficiently rigid to be ejected. The last step is the “ejection
stage” in which the mould is opened and the part is ejected,
after which the mould is closed again to begin the next cycle
[4].
The design and manufacture of injection molded poly-
meric parts with desired properties is a costly process domi-
nated by empiricism, including the repeated modification of
actual tooling. Among the task of mould design, designing
the mould specific supplementary geometry, usually on the
core side, is quite complicated by the inclusion of projection
and depression [5].
In order to design a mould, many important designing
factors must be taken into consideration. These factors are
mould size, number of cavity, cavity layouts, runner systems,
gating systems, shrinkage and ejection system [6].
In thermal analysis of the mould, the main objective is
to analyze the effect of thermal residual stress or molded-in
stresses on product dimension. Thermally induced stresses
develop principally during the cooling stage of an injection
molded part, mainly as a consequence of its low thermal
conductivity and the difference in temperature between the
molten resin and the mould. An uneven temperature field
exists around product cavity during cooling [7].During cooling, location near the cooling channel experi-
ences more cooling than location far away from the cooling
channel. This different temperature causes the material to