In
this blog post I give you a “cheating” guide to tackle some of the most
frequent challenges in injection moulding: venting,
mould release, mould shrinkage, hot runner systems and energy consumption.
Let’s
get straight to it:
- How to improve your venting?
Burn
marks or short shots are often related to inefficient venting caused by air entrapment
in the mould’s cavity. The first action to take is to lower the injection speed.
This will reduce the amount of air which needs to leave the cavity within the
injection time frame. If the latter does not help, venting can also be enhanced
by placing a thin copper foil (~ 0.01 mm) on the closing surfaces of the mould.
By trial and error the foil thickness may be adjusted to enhance venting while
keeping flash phenomena to a minimum.
- How to have a better mould release?
One
way to go is chemically by using silicone-based releasing agents, however,
this is no option in case the parts need to be painted afterwards. In such
cases silicone-free products can also be utilized. When you have a mould which
produces cups or (soup) bowls, it is helpful to have the core (forming the
positive of the cup/bowl) at a lower temperature to prevent it from sticking in
the cavity. The lower temperature leads to shrinkage of the part on the core
and ejection is easier due to the availability of ejector pins there. Regarding
the set-up of the cooling system, cooling lines connected in parallel (instead
of in series) are more suitable to keep the mould temperature constant. Besides
mould temperature fluctuations (too high mould temperature leads to warpage),
overpacking (too high injection pressures) and overfilling are the main causes
of mould releasing problems. In the latter cases, it is useful to control the
part’s weight during production.
- How to have a better hand on mould shrinkage?
Part’s shrinkage can
be to a great extent minimized by applying the right pressures during the
filling and packing processes. Choosing the optimal process pressures can be
supported by consulting the pvT-diagram (see figure below) of the material
system you are working with. For most material groups, e.g. commodities and engineering
thermoplastics, multipoint pvT data is available free of charge on online
databases such as CAMPUSPlastics.
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In the
pvT diagram we can follow what happens to the material during the injection
moulding cycle: Injection of the melt happens fast, thus, there is no
significant variation in temperature (vertical line from 1 to 2). The melt can
only fill the mould cavities under high pressures. This results in a
compression of the melt (2). Once filled in the cavities, the melt is allowed
to cool down so that the part(s) can be ejected. During cooling, if no more
melt is added, the specific volume of the part stays constant until atmospheric pressure (1 bar) is reached (horizontal
line from 2 to 3). At 1 bar, no further relaxation of the melt can take place,
however, it can happen that at 1 bar the melt has not yet reached room temperature (3). In
this case, the melt will follow the 1 bar line until room temperature or
ejection temperature (4). As it can be
seen in the pvT diagram the isobaric trajectory of the melt from 3 to 4 is
associated with a volume decrease or shrinkage. To keep shrinkage to a minimum
you need to control your process in such a way that point 3 in the pvT diagram
is as close as possible to point 4, or it does not exist at all. This can be
reached by holding onto the right packing pressure in point 2. The ideal
scenario is drawn in orange on the above diagram.
Mould
temperature, melt temperature, injection speed, injection pressure, packing
pressure level and time set, and dimensions of gate- and runner systems are all
crucial parameters to be controlled in order to attain the right cavity
pressures during your process.
Controlling
process parameters play a key role in steering the resulting part’s
shrinkage. Nevertheless, depending on the material system you are working with,
e.g. semi-crystalline or amorphous polymer-based systems, post-processing
shrinkage may additionally occur to different extents. Conversely, parts based
on hygroscopic polymers such as polyamide may undergo swelling in
post-processing stages due to moisture absorption.
- How to handle hot runner systems?
One of
major issues which can occur when using hot runners is material degradation.
This has mainly two causes: longer residence time at higher temperatures on one
hand and often not completely balanced flow/temperature gradients in the hot
runner system on the other. It is helpful to lower the temperature of the hot
runner whenever moulding is interrupted. Additionally, after moulding, not only
purging of the cylinder should be done, but also purging of the hot runner
system to be sure that all the remaining melt is out. Beneficial is to have a hot
runner system which allows a separate heating zone by using different control
units as well as using an electrical circuit which enables gradual heating
(less risk of short circuiting due to moisture present in the heater
cartridges).
- How to save some energy?
Here
are some tips which help reducing energy as well as material consumption: the
machine nozzle should not always be docked to the mould. Once the gate is
frozen, pull the cylinder back. When you use a shut-off nozzle you could place
insulated plates between the mould and the machine. Keep an eye on the mould
temperature and on the difference in the coolant temperature when entering and
exiting the mould. The difference should be 1-2°C for good quality moulding and
maximum 3-5°C for economic reasons. The plasticization can be at lower r.p.m
and it should be slightly shorter than the cooling time. And regarding the
cylinder: do not use a too big capacity cylinder for your shot weight.
I hope that my 5 how’s can
help you in your moulding operations.
Thank you for reading my post.
Greetings and #findoutaboutplastics
Herwig
Literature: GE Plastics injection moulding guide
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