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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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