Page 27 - Plastics News April 2019

FeAtures

A Processor’s Most Important Job, Part 9: Avoid Molded-In Stress

Michael Sepe

Poor molding practices can result in high stress levels reducing impact resistance hence the objective is
establishing molding conditions that minimize internal stress in a part

n the previous column, we discussed the fundamental mechanisms such as environmental stress cracking (ESC).
Iconnection between variations in the condition of the The objective should be to minimize internal stress while
polymer at different locations in a molded part and the still making the part cost-effectively.
level of internal stress in that part. These variations Mold Filling & Packing
are influenced to a significant extent by processing
conditions. Differences in pressure within the mold Mold filling and packing have a significant influence on
cavity will produce variations in the way the material the pressure distribution in the cavity. The pressure
shrinks. This is one source of internal stress. The other drop in a flowing polymer system is proportional to the
consideration is differences in the rate at which the viscosity of the material. Viscosity is influenced by two
processing conditions: The melt temperature and the
shear rate applied to the material. Higher shear
rates result in lower viscosities and can frequently
achieve this objective as an alternative to raising
melt temperatures. This is the reason that the
principles of scientific molding dictate that most of
the cavity should be filled at a relatively rapid rate
during what is typically referred to as the first stage.
The switchover to pack, or second stage, should take
place when the cavity is almost full by volume. The
velocity of the polymer during the second stage is
usually much slower than on the first stage, and this
will result in a much higher viscosity.

For some polymers, the viscosity on the second stage
can be more than 10 times greater than on the first
stage. Therefore, switching over too early means
that it is more difficult to pressurize the polymer
uniformly. Switching over too late will result in an
overpressure condition that can cause flash and may
even damage the mold.
polymer cools. Knowledge of these factors allows a
processor to establish molding conditions that reduce The relationship between the mold-filling strategy and
internal stress in a part. It is not possible to mold a part the pressure distribution across the cavity can be readily
with zero internal stress. demonstrated if the cavity contains pressure transducers
at the beginning and end of the flow path. The pressure
Even under the best of circumstances, a well-molded will never be exactly the same at these two locations,
part will still have a stress level of 500-600 psi. But but good mold-filling practices can reduce the difference.
poor molding practices can result in stress levels that The more uniform the pressure in the cavity, the more
exceed 3,000 psi. This will result in reduced impact uniform the shrinkage of the material as the polymer
resistance, dimensional instability upon exposure to cools. The cooling rate is the other critical factor.
elevated temperatures and greater sensitivity to failure Rapid filling introduces a high level of orientation in the

27 April 2019 Plastics News