Mould wear in the age of high-speed injection moulding In the Good Old
Days, say 20 years ago, a 28 gram preform for a CSD package may have
run at a cycle time of 24 seconds, wrapping up just over one million
cycles per year, even if the tool was running all year round. At that
time mould wear was not much of an issue.
Years ago, running at a cycle time of 24 seconds, it is more likely
that the package had become obsolete before the mould had worn out.
Today, a super-lightweight preform for a 500 ml water package may run
as fast as 6.5 seconds, accumulating well over 4 million cycles per
year. Tools may run for about 8 million cycles on horizontal machines
and up to 15 million cycles without qualification as to what
maintenance is required and what criteria are used to determine how
long the tool can be used.In an incomrehensive survey we are given
numbers between 5 and 12 million cycles but in the end it all depends
hhow mould wear is measured. We can say for sure that some moulders
are forced to buy new tools or major refurbishment every two years and
this bi-annual capital expense has certainly caught their attention.s
two-part article we will examine all of the issues surrounding this
increasingly hot topic.
What is mould wear?
There are a number of components in preform moulds that lose all or
part of their functionality over time. Valve-gate cylinders and stems,
heaters, and nozzle tip insulators can be a nuisance to repair during
a tight production schedule but nevertheless can be put right in a
reasonable amount of time, and by processors themselves. The type of
mould wear we will discuss in this article however concerns the
parting line in the neck area. This parting line is formed by the two
neck insert halves which form the outside of the threads. The neck
inserts slide open during ejection, releasing the thread-form that
would otherwise create an undercut. They are held together by two
matching tapers. One (the male part) is an integral part of the neck
inserts, the other, female part is integrated in the cavities. This
so-called taper lock transmits the clamping force onto the mould
plates, counteracting the opposing injection force that tries to open
the mould during the injection and hold phase. There is considerable
pressure on each taper lock and the tapers wear off over a certain
number of cycles, depending on a variety of factors to be discussed
later. When they do wear they fail to close tightly around the
thread-form allowing plastic to flow into the gaps thus created. This
is known as neck flash. As time goes on the increasing gap allows neck
flash to grow until the tool has to be renewed or refurbished. There
are considerable differences between users with respect to the time at
which the size of the neck flash necessitates tool renewal. The PCO
neck finish drawing limits the amount of flash to 0.13 mm (0.005”) per
side and the larger brand owners may well expect this limit from their
suppliers. On the other hand an in-house operation may tolerate much
more flash, and regional differences certainly play a role. Practical
limits are what the capping machines can tolerate and how much
consumers who drink directly from bottle may find the flash
uncomfortable.
Why do taper locks wear?

In a perfect world we should expect negligible wear on these parts.
Made from hardened and precision-ground tool steel the compressive
force exerted during the moulding cycle is well below what the
material can bear. In the real world things are not quite as perfect.
For one thing, there are manufacturing tolerances in the area of +/-
0.005 mm (0.0013”) on the tapers as well as all the positioning bores
in the plates. Add to those tool positioning tolerances as well as
machine plate offsets in two directions and the result is that not all
tapers in a multi-cavity tool match perfectly. A further aggravation
is that the clamping force may not be evenly distributed across the
entire tool because of small differences in the castings or an offset
in the centre location. As a result of these imperfections some tapers
will be more engaged than others, leading to uneven wear over the
millions of cycles that the tool is operating. Another factor may be
dirt that has accumulated on the tapers, forcing the metal to shift
slightly and once again pressurising certain parts more than others.
On horizontal machines gravity is another detriment to proper
alignment. Only very rigid tools set on perfectly straight guide
bearings can hold the tool in place during the closing of the clamp as
gravity exerts a force 90º to the closing action causing a sagging
effect on less-than-perfect moulds. Vertical machines have an
advantage in this respect, as they experience no side load during
clamp movement. In summary, there are a number of imperfections that
locally exert excessive stress on some parts of the tapers causing
more wear than would be expected if everything was perfect. There is
another stress factor that is more arbitrary, but should nevertheless
be mentioned. The main culprit is failure of the mould protection
system. To understand mould protection we need to look at how machines
control clamp movement. On some machines one or two cylinders that are
just strong enough to move the platen facilitate movement of the clamp
from uully open to almost closed. On toggle-type clamps a measuring
device senses the position of the clamping cylinder and engages mould
protection at a certain point. The function of mould protection is to
provide the necessary speed and force to close the mould to the point
where the mould faces are in direct contact with each other and
prevent the high force clamp pressure to be applied prematurely. Once
the mould protection is engaged the machine now senses any resistance
exceeding a preset limit aborting the closing sequence if thta is the
case. The mould protection device ensures that no obstacle, such as a
partial preform stuck in the cavity, is present when the clamping
cylinder engages. This prevents damage to the tool but operators can
make mistakes when setting the system up and significant tool damage
may ensue. It is also paramount that sensors check the presence of
preforms after ejection to avoid preforms being stuck in cavities or
on cores that would cause trouble during the next cycle.