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S.H. Tang et al. / Journal of Materials Processing Technology 171 (2006) 259–267 261
the surface of the product. The product and the runner were
released in a plane through the parting line during mould
opening.
Standard or side gate was designed for this mould. The gate
is located between the runner and the product. The bottom
land of the gate was designed to have 20◦ slanting and has
only 0.5 mm thickness for easy de-gating purpose. The gate
was also designed to have 4 mm width and 0.5 mm thickness
for the entrance of molten plastic.
In the mould design, the parabolic cross section type of
runner was selected as it has the advantage of simpler machin-
ing in one mould half only, which is the core plate in this
case. However, this type of runner has disadvantages such as
more heat loss and scrap compared with circular cross section
type. This might cause the molten plastic to solidify faster.
This problem was reduced by designing in such a way that
the runner is short and has larger diameter, which is 6 mm in
diameter.
It is important that the runner designed distributes material
or molten plastic into cavities at the same time under the
same pressure and with the same temperature. Due to this,
the cavity layout had been designed in symmetrical form.
Another design aspect that is taken into consideration was
air vent design. The mating surface between the core plate
and the cavity plate has very fine finishing in order to prevent
flashing from taking place. However, this can cause air to trap
in the cavity when the mould is closed and cause short shot
or incomplete part. Sufficient air vent was designed to ensure
that air trap can be released to avoid incomplete part from
occurring.
The cooling system was drilled along the length of the
cavities and was located horizontally to the mould to allow
even cooling. These cooling channels were drilled on both
cavity and core plates. The cooling channels provided suffi-
cient cooling of the mould in the case of turbulent flow. Fig. 2
shows cavity layout with air vents and cooling channels on
core plate.
In this mould design, the ejection system only consists of
the ejector retainer plate, sprue puller and also the ejector
Fig. 2. Cavity layout with air vents and cooling channels.
plate. The sprue puller located at the center of core plate not
only functions as the puller to hold the product in position
when the mould is opened but it also acts as ejector to push
the product out of the mould during ejection stage. No addi-
tional ejector is used or located at product cavities because
the product produced is very thin, i.e. 1 mm. Additional ejec-
tor in the product cavity area might create hole and damage
to the product during ejection.
Finally, enough tolerance of dimensions is given consid-
eration to compensate for shrinkage of materials.
Fig. 3 shows 3D solid modeling as well as the wireframe
modeling of the mould developed using Unigraphics.
3. Results and discussion
3.1. Results of product production and modification
From the mould designed and fabricated, the warpage
testing specimens produced have some defects during trial
run. The defects are short shot, flashing and warpage. The
short shot is subsequently eliminated by milling of additional
air vents at corners of the cavities to allow air trapped to
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