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mm. This increased wall thickness design option was studied
as Design 2 and results are discussed below.
Figure 6(a) shows flow stresses in the part with the
increased wall thickness of 0.125 mm. Gate location is the
same as Gate 3. As Figure 6(a) shows, the flow stresses in
the NEEDLE HOUSING are much lower than those
obtained with the current design in Figure 4(b) (Design 1). It
indicates that the increase of wall thickness considerably
reduced the fluidic resistance to the advance of flow front in
the cavity. The percentage increase in material utilization for
Design 2 (relative to Design 1) is 5 %. This may be
acceptable in terms of quality and cost for the parts because
the increase of 5 % does not significantly affect entire
production cost of the NEEDLE COVER.
Figure 6(b) represents the flow stresses in the part with a
melt temperature of 220 o
C for Design 1 in order to show the
effect of a decrease in melt temperature on flow stresses. In
the comparison of the flow stresses shown in Figure 6(b) and
4(b), the flow stresses shown in Figure 6(b) along the entire
NEEDLE HOUSING area significantly increase to levels
above the degradation level of polypropylene.
The decrease in melt temperature induces higher flow
stresses because the lower melt temperature accelerated the
solidification of the polymers at the mold walls. This led to
degradation of replication of the parts as the temperature was
decreased. The analysis result in Figure 6(b) notices that
how a 20 o
C decrease in melt temperature increase flow
stresses.
In case of press limitations for some machinery there is a
possibility that the molten polymer may not be able to fill a
multiple cavity production mold at a 0.6 second injection
time. In order to demonstrate the effects of longer fill times,
Figure 6(c) shows flow stresses in the part (Design 1) with
an injection time of 1.2 second. As the figure shows, the
flow stresses in half of the NEEDLE HOUSING area
increase above the degradation level of polypropylene. This
can be explained by the increase of fluidic resistance of the
molten polymer due to the temperature drop with longer
injection time along the long and slender area in particular.
The increase of injection time decreased the injection speed.
The lower injection speed induced a lower injection pressure
to push the melt into the cavities. The melt experienced
longer cooling time and the viscosity of the melt increased
while the longer injection time is applied.
From the analyses for Design 1 and 2, the maximum
injection pressures required to fill the cavity for all four gate
locations, are shown in Table 1. These injection pressures
are approximately equal, with gate 1 requiring the lowest
injection pressure of 26.4 MPa. Table 1 also shows the
minimum and maximum temperatures of molten polymer at
the end of the fill cycle for the different gate locations. The
minimum temperatures, shown in Table 1 for all four gate
locations, are well above the No-Flow temperature ofpolypropylene (152 o
C) and should not cause a problem.
All of the options studied and discussed above should help
to determine the appropriate combination of processing
conditions, gate location and material usage for molding a
high quality part.
Conclusion
Cavity filling analysis of an injection-molded needle cover
was performed. Four different gating options and three
different designs were studied to the identification of
appropriate processing conditions, gate location, and material
usage for the replication of high quality parts.
Gate 1 leads to very high flow stresses in the NEEDLE
HOUSING area which may cause the part to bow. Gating
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