Page 33 - Plastics News December 2017

FEATURES
FEATURES

rods have been known to bend occasionally. In any in. or more. This usually indicates that one side of the
case, a bent rod will easily be recognized as it will preform is cooler than the other. The warmer side
skew the gate always in the same direction while stretches more and so thins out. In single-stage processes,
other defects occur more randomly. this is quite common, as noted above, but can also happen

• Preform is bent before entering the blow mold. in two-stage stretch-blow. It could be that air is blowing on
This is an issue that occurs more frequently in single- the preforms after they stopped rotating. I have also seen
stage stretch-blow molding and has a different cause that when preforms do not spin while in the oven
there. In two-stage it may happen when the preform system, heat from the oven metal heats up the side of the
wall thickness is uneven by more than 0.004 in. This preform that is turned towards it. A thermal camera is
leads to uneven heating—i.e., the thinner side gets helpful here to detect heat differences and locate their
hotter, and this side may then shrink more than the sources.
cooler side between the preforms leaving the ovens SMALL STRETCH RATIOS
and the blow mold. In that case, the stretch rod hits
the preform off-center and transports it to the blow For the self-leveling effect described above to work,
mold in the same way. preforms must be stretched in both the vertical and
hoop direction. Minimum ratios are 2:1 in the vertical
In single–stage stretch-blow there may be another problem and 4: 1 in the hoop plane. But design limitations,
besides possible wall-thickness differences—non-uniform especially for small bottles below 12 oz, or the process
heat distribution in the preform. This is because viscous itself (these numbers are already at the realistic
heating creates a ring of hotter material inside the maximum for single-stage) may prevent designers from
molten plastic. When the runner is typically divided into implementing large enough stretch ratios. As a result the
two streams, hotter material is pushed more to the back material cannot fully stretch out the cooler parts and
than the front and this can often be measured in uneven they stay thicker.
wall thickness.
In many cases preforms are purchased that have the right
UNEVEN HEATING OR COOLING neck finish and weight but are not necessarily designed for

It often baffles processors when the gate is in the center the particular application they are used for. This can also
of a round bottle but the walls show differences of 0.004 lead to improper stretch ratios in all or part of the bottle.

Bad Power is the Root of Many Plastics Production Problems

Undetected plant power problems cost plastics processors $9.6 billion a year in preventable equipment
failure, downtime and excessive energy costs. This inexpensive power monitoring and analysis system
can find problems before the damage is done.

f you run a plastics processing operation, you know
Ithat motors burn out, transformers fail, controls suffer
memory loss. Sure it’s disruptive, and expensive, but you
fix the equipment and move on. It’s just part of doing
business, right?
But what if you could identify and correct many of the
root causes of premature equipment failure? And what if
you could lower your utility bill in the bargain?
You can. You just have to pay attention to something
most plastics processors rarely think about; your plant

33 December 2017 Plastics News