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of analyses were represented as flow front with a time scale
and flow stresses, showing the fluidic resistance in advance.
Figure 2(a) shows the flow fronts in the part gated on the
COLLAR, Gate 1. On close inspection of this figure, it is
clear that after the PORT HOUSING fills, the polymer
accelerates suddenly in the NEEDLE HOUSING (see the
wide pink bands in Figure 2(a)). If possible, this type of
uneven filling pattern should be avoided because it induces
high flow stresses along the NEEDLE HOUSING as shown
in Figure 2(b).
Light to dark pink areas, as shown in Figure 2(b), denote
high flow stresses that are acceptable in small areas, but may
tend to cause warpage in large areas. Areas represented in
black have flow stresses that are above the degradation level
of polypropylene (250,000 Pa) and are generally unacceptable
except in confined regions such as the gate. The developed
flow stresses in the NEEDLE HOUSING area are very high,
compared to the PORT HOUSING and may cause this long
and slender area to bow.
Figure 3(b) shows the flow stresses with the part gated at
Gate 2. As the figure shows that the flow stresses in the
NEEDLE HOUSING area are much lower than those for all
other gating options studied. This gating scheme also leads
to a very uniform filling pattern as shown in Figure 3(a).
However, based on the experience with earlier designs of
Needle Covers, molds gated at the tip of the NEEDLE
HOUSING are prone to short shots during start-up, leading
to damaged core pins [13].
Gate location 3 provides a uniform filling with the advance
Figure 2. Cavity filling analysis of the part with gate 1: (a) flow
fronts and (b) flow stresses.
Figure 3. Cavity filling analysis of the part with gate 2: (a) flow
fronts and (b) flow stresses.PORT HOUSING.of the molten polymer as shown in Figure 4(a). Figure 4(b)
indicates that the flow stresses in the NEEDLE HOUSING
area are slightly lower than those obtained with Gate 1, but
slightly higher than those obtained with Gate 2. Although
the stresses in the internal ribs are not shown in this figure,
they are well within acceptable limits. This gating scheme
leads to a very uniform filling pattern and would allow
relatively tight dimensional tolerances in the PORT HOUSING
area.
The flow stresses shown in Figure 5(b) indicate that the
gating option 4 developed the stresses along the bottom
portion of the PORT HOUSING and COLLAR area. The
stresses are at or near the degradation level of polypropylene
(250,000 Pa) and the stresses in the NEEDLE HOUSING
area are essentially the same as those obtained with Gate 3.
The flow pattern shown in Figure 5(a) is not very uniform.
Due to the high stresses and non-uniform flow pattern, this
gating scheme is the least favored.
Based on the four different gate location studied, Gate 2
and 3 are probably optimum due to the uniform fill patterns
and minimization of flow stresses. As discussed earlier, Gate
3 may be preferable to Gate 2 due to the tendency of
excessive short shot formation during start-up with parts
gated at the tip of the needle housing.
Filling Analysis for the Wall Thickness, Lower Melt
Temperature, and Longer Injection Time
Although Gate 3 might be the best alternative, this gating
option still leads to high stresses in the NEEDLE HOUSING
area as shown in Figure 4(b). In order to reduce the high
stresses and possibility of bowing or twisting, it may be
necessary to increase the wall thickness in the NEEDLE
Figure 4. Cavity filling analysis of the part with gate 3: (a) flow
fronts and (b) flow stresses.
Figure 5. Cavity filling analysis of the part with gate 4: (a) flow
fronts and (b) flow stresses.HOUSING by 0.125 mm with its original thickness of 1.25
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