Core Shift During Mold-Filling Simulation ______________________
similarly for the one-sided constraints of type (B):
(11)
And for the one-sided constraints of type (c):
(12)
We solve Equations 5-9 using three-dimensional models
for mold cores represented by 4- and 10-node tetrahedral
elements.
As described in our previous work,9 we discretize elasticity Equations 6-8 using three-dimensional models for mold
cores represented by 4- and 10-node tetrahedral elements,
assemble them together with equations describing support
9-12, and solve the system simultaneously with flow Equations 1-5. the resulting software allows prediction of core shift
with one-sided constraints during injection molding of thermoplastics.
We consider three very similar types of support for the core:
*
Fully bonded to the mold (fixed support): this support will be difficult to implement.
*
One-sided supports limited by the mold surface:
they are much easier to implement, but they are less
stiff. We assume that there are no gaps between the
cores and the mold surface at the point of support.
*
One-sided supports limited by the mold surface
but with a gap of 0.5 mm in the Z direction on the
left constraints. small gaps between the core and
mold can be expected. the gap makes the mold
even less stiff as it makes rotation even easier.
since the injection is on a side of the tube, there is an imbalance in flow fronts and pressures between the injection
and the opposite sides of the cavity (see Figures 4 and 5); these
are causing deformation of the core away from the injection
side that additionally amplify the imbalances. the result is significant deformation of the cores and a strong difference
between the wall thickness on the injection and the opposite
sides of the moldings.
Case Study
As an example of the simulation results, we use the core shift
of a square polypropylene 5×5×80-mm tube of 1-mm thickness injected from the middle (shown by Figure 3a). the
molding is modeled using mid-plane triangular elements. the
core for this molding is simply a rectangular 4×4×100-mm
steel piece supported from both sides (see Figure 3b).
Figure 4: Flow front propagation; fixed support of the core
a.
b.
Figure 3: Case study (from top): a) molding, b) core
42
Figure 5: Pressure in the middle of the packing phase; fixed
support of the core
Fixed core support
Predictions for the fixed core are shown on Figure 6. there
are significant changes of the wall thickness going from the
nominal 1 mm up to 1.5 on the injection side and down to 0.3
mm on the opposite side, which causes a warpage of 1.4 mm.
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Plastics Engineering - May 2014
Table of Contents for the Digital Edition of Plastics Engineering - May 2014