Plastics Engineering - May 2014 - (Page 40)
Using One-Sided Constraints to Improve
Prediction of Core Shift During Mold-Filling
Simulation
By Alexander Bakharev, Zhiliang Fan, and David Astbury
Autodesk Australia, Kilsyth, Australia
c
ore shift can be defined as the spatial deviation of the
position of the mold cores and inserts caused by the
imbalance of polymer pressure during injection molding. the resulting change of the boundary conditions in
turn affects the filling pattern and packing pressure distribution and may create residual pressure at the end of
cooling (over-packing) and deviations of the molding thickness. the final result may be inferior quality of the molded
parts. it is a pervasive problem in the manufacturing of
thin-walled containers.1-8
in previous work,9 we presented an algorithm for predicting the core shift effects by mold filling simulation. the
algorithm assumes simple (two-sided) geometrical constraints for the cores that fully remove some translational
degrees of freedom of the cores. it means that if a constraint prevents displacements in some direction, then this
constraint also prevents displacements in the opposite direction. still, many injection molds include a special boundary
condition of "constraint in one direction, but free movement in the opposite direction" that we will call a "one-sided
constraint."
A few typical cases from customer supports are shown in
Figures 1 and 2. in Figure 1, the insert is supported by several pins and therefore the constraints at the pin-supported
points are one-sided. in Figure 2, the blue insert can move into
the plastic part (green), but it is blocked from the opposite
direction by the mold surface. to properly predict core shift
on such molds, we need to handle one-sided constraints,
which is the aim of the present work.
Figure 2: The insert (blue) can move up into the plastic part
but is blocked from downward movement by the mold surface. One-sided constraints are needed for such parts.
Theory
We predict melt pressure through the cavity and runners
using a numerical solution of the low-reynolds number
momentum equation:
(1)
together with a rheological equation:
(2)
energy equation:
Figure 1: Pin supports of cores require one-sided constraints
for accurate core-shift analysis.
40
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Plastics Engineering - May 2014
Table of Contents for the Digital Edition of Plastics Engineering - May 2014
Contents
Plastics Engineering - May 2014 - Cover1
Plastics Engineering - May 2014 - Cover2
Plastics Engineering - May 2014 - Contents
Plastics Engineering - May 2014 - 2
Plastics Engineering - May 2014 - 3
Plastics Engineering - May 2014 - 4
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Plastics Engineering - May 2014 - Cover3
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