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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].
260 S.H. Tang et al. / Journal of Materials Processing Technology 171 (2006) 259–267
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
experience differential shrinkage causing thermal stresses.
Significant thermal stress can cause warpage problem. There-
fore, it is important to simulate the thermal residual stress field
of the injection-molded part during the cooling stage [8]. By
understanding the characteristics of thermal stress distribu-
tion, deformation caused by the thermal residual stress can
be predicted.
In this paper the design of a plastic injection mould for
producing warpage testing specimen and for performing ther-
mal analysis for the mould to access on the effect of thermal
residual stress in the mould is presented.
2. Methodology
2.1. Design of warpage testing specimen
This section illustrates the design of the warpage testing
specimen to be used in plastic injection mould. It is clear
that warpage is the main problem that exists in product with
thin shell feature. Therefore, the main purpose of the prod-
uct development is to design a plastic part for determining
the effective factors in the warpage problem of an injection-
moulded part with a thin shell.
The warpage testing specimen is developed from thin
shell plastics. The overall dimensions of the specimen were
120 mm in length, 50 mm in width and 1 mm in thickness. The
material used for producing the warpage testing specimen
was acrylonitrile butadiene stylene (ABS) and the injection
temperature, time and pressure were 210 ◦C, 3 s and 60 MPa,
respectively. Fig. 1 shows the warpage testing specimen pro-
duced.
2.2. Design of plastic injection mould for warpage
testing specimen
This section describes the design aspects and other consid-
erations involved in designing the mould to produce warpage
testing specimen. The material used for producing the plastic
Fig. 1. Warpage testing specimen produced.
injection mould for warpage testing specimen was AISI 1050
carbon steel.
Four design concepts had been considered in designing of
the mould including:
i. Three-plate mould (Concept 1) having two parting line
with single cavity. Not applicable due to high cost.
ii. Two-plate mould (Concept 2) having one parting line with
single cavity without gating system. Not applicable due
to low production quantity per injection.
iii. Two-plate mould (Concept 3) having one parting line
with double cavities with gating and ejection system. Not
applicable as ejector pins might damage the product as
the product is too thin.
iv. Two-plate mould (Concept 4) having one parting line with
double cavities with gating system, only used sprue puller
act as ejector to avoid product damage during ejection.
In designing of the mould for the warpage testing spec-