Page 32 - Plastics News December 2017

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How to Address Uneven Wall Thickness in Stretch-Blow Molding

OTTMAR BRANDAU
There are a number of conditions that must be maintained in order to achieve the described result so as
to ensure wall thickness is not uneven

the preform gate wall thickness. This will ensure that
the preform cannot slip during high-pressure blow. As
preforms have become thinner, these gaps have to
become smaller.
• Preblow pressure is too high and/or too early. The
stretch rod has to connect with the preform before
preblow pressure has a chance to blow the preform
off-center. When it is connected, the preblow cannot
be so high as to blow the preform off the rod. The
amount of preblow pressure required depends on the
preform thickness and temperature.
Therefore, operators may have to experiment with
different pressures to test their effect. The same is true
for when to engage the preblow. In two-stage (reheat)
mong the beneficial properties of PET is its self- stretch-blow molding this is most often controlled by
Aleveling ability. This means that as one part of a position rather than time, and often delaying the onset
preform starts to stretch first, the very act of stretching of pressure keeps the gate in the center.
induces strain-hardening, making it tougher to stretch as a • High pressure is too early. High pressure may not
result. This forces adjacent parts that were maybe a little be energized before the stretch rod has firmly
cooler to stretch, and in this back-and-forth of stretching pressed the preform against the blow mold bottom.
and pausing, the preform develops into a bottle that may Otherwise, even small differences in temperature or
show as little as 0.001 in. difference in wall thickness preform wall thickness will invariably move the gate
around the circumference of a round bottle. off-center. Again, in most machines this is controlled

However, many bottles show much greater differences by position, and the operator has to make sure
than that. As with all plastic processes, there are a number this is adjusted correctly. Rotary machines do this
of conditions that must be maintained in order to achieve automatically but many linear machines do not.
the described result. Following are the various issues that • Blow mold bottom is not machined correctly. In
can skew wall thickness, in order of relevance: order to assist the stretch rod in pinning down the
OFF-CENTER GATE preform, a small well is machined into the blow mold
bottom. This allows room for the small injection
This is by far the most common cause of wall-thickness vestige at the end of the preform and prevents the
problems. The vestige of the injection molding process preform from slipping.
must be centered to the blow mold and held there firmly
when high-pressure air inflates the pre-inflated preform • Bent stretch rod. Stretch rods are typically very
bubble into a bottle. Otherwise, wall thickness will go sturdy at diameters of around 0.5 in. and do not easily
where the gate goes and no amount of self-leveling can bend. But lightweight water bottles require stretching
prevent this from happening. out the entire base, and often rods are slimmed
down at the ends to allow for a smaller contact area
Here are the most common causes for this defect: and cooling effect on the preform. Or, in the case of
• Stretch rod does not pin the preform down enough. bottles with necks as small as 20 mm for cosmetic
Typically, the distance between stretch rod and blow applications, the thick rods just won’t fit into them.
mold bottom should be adjusted to 0.040 in. less than Thin rods can bend much more easily, and even thicker

Plastics News December 2017 32