Page 28 - Plastics News April 2019

FeAtures

polymer. This orientation is at a maximum in the layer of and the end of the flow path. Higher mold temperatures
polymer that is just below the mold surface. This is also reduce the rate of frozen-layer development and allow
the layer of material that cools most rapidly. While some for achievement of a more even pressure distribution in
level of orientation can be beneficial, if it is excessive the cavity. These principles of polymer flow and cooling
it will represent a source of internal stress. The level are behind the observations that were discussed in the
of retained orientation and the difference in cooling previous column. It is interesting that the same attention
rate between the surface of the part and the core can to mold temperature that ensures optimal crystallization
be reduced by operating at a higher mold temperature. in semi-crystalline polymers also produces optimal
This reduces the rate of cooling, allowing the polymer properties in amorphous polymers. If mold temperature
to relax to a greater extent as it develops the structure is thought of as the sole factor in determining cycle
in the part. Another secondary benefit to running higher time, then there is a great reluctance on the part of the
mold temperatures is that it slows down the rate at which processor to increase it. However, if it is understood that
the frozen layer develops. mold temperature and
The instant that the molten There are three possibilities of inducing moulded-in melt temperature work
polymer enters the mold, stress in a plastic part:- together, then it can be
the material at the surface When melt is over packed (while moulding) it can cause seen that an appropriate
begins to freeze. The flow moulded-in stress. This can be avoided by applying follow balance between the
of the polymer through up pressure just enough to avoid sink marks. two parameters is
the cavity is continued by the key to optimizing
molten material moving Even unbalanced melt flow in the mould can cause material properties
through the interior layers overpack in some regions. It is not possible to avoid while maintaining a
that have not yet solidified, overpacking with unbalanced melt flow while processing. competitive cycle time.
a phenomenon known as This problem should have been considered while designing While the focus of
the part itself.
fountain flow. The shearing these articles has been
forces that develop between Un-equal freezing of melt through out the part, can also on processing, it is
the exterior layers that cause moulded-in stress. Cooling system, which does important to note that
have stopped flowing and not provide uniform mould surface temperature can be the ability to achieve
the interior layers that are responsible for this cause. This is the responsibility of a uniform cooling rate
still moving are a source of mould design. and an optimal pressure
internal stress. So, to the distribution in the cavity
extent that the rate of frozen-layer development can also depends upon good part design and mold design.
be reduced, the internal stress can be reduced also. Designing a part with large variations in wall thickness,
Many cosmetic defects, such as tiger stripes, flow lines or selecting a gate location that results in filling a thicker
and orange peel, are symptoms of a frozen layer that section through a thinner section, will create problems
develops too early in the mold-filling process.
that the processor will have difficulty compensating for.
In addition, recall that the pressure drop is governed to Traditionally, poor design decisions have simply been
a significant degree by the size of the flow path. In a passed along and it has been left to the processor to figure
round cross-section, a reduction from 0.050" (1.25 mm) it out. Frequently the result is a set of process conditions
to 0.040" in. (1 mm) results in doubling the pressure loss. that represent an attempt to correct for the poor design.
We tend to think that if a part has a wall thickness of These often include lower mold temperatures and high
0.080" (2 mm), this represents the available path through packing pressures, the very things that create elevated
which the polymer flows, regardless of location in the levels of internal stress. Good concurrent engineering
cavity. But in reality, the size of the available flow path practices call for a thorough discussion between all
becomes smaller as the material moves farther into the interested parties during the early stages of product
cavity, away from the gate or gates. This is another factor development so that these problems can be solved before
in the pressure drop that we observe between the gate the mold is built.

Plastics News April 2019 28