Pack Phase – Judge, Jury and Executioner?
Background
You may have heard the old phrase where someone was
described as being the “judge, jury and executioner.” Its definition per
Wikipedia:
judge, jury and executioner
1.
(idiomatic) Someone with the roles of judge, jury and executioner;
someone with full power to judge and punish others unilaterally.
This phrase is used to describe
one person or entity as being completely in control over another’s fate – a sole
body yielding the power and authority to determine another’s outcome. In
injection molding, the PACK portion of plastic injection is often overlooked,
but it has the power to control so much – typically more than is often
realized.
It has been decades since the
injection molding industry underwent a significant and very positive growth in
machine and process capabilities. Anymore, at a minimum, two injection stages
should be used when molding properly. This 2-stage filling of the mold begins
using a constant and typically fast fill rate (FILL) followed by the remainder
of fill being completed under constant pressure for a period of time – the very
definition of the V►P (velocity to pressure) transfer on
your injection molding machines (IMM). Processing this way is called by many
names with the most popular being referred to as scientific
injection molding or decoupled molding.
Under 2-stage molding, this stage of injection, aside from
being called 2nd stage, is commonly referred to as HOLD, PACK, or
PACK and HOLD. Even though multiple things are being accomplished, this PACK
and HOLD stage is typically a shared set of machine parameters. After V►P,
the 2nd stage begins with the final filling of the mold cavity and
then the pressurization (or compression) of plastic into the mold cavity –
these two things equal the PACK only portion of 2nd stage injection.
Next, and without any machine setpoint transition, the true HOLD phase begins.
HOLD maintains this 2nd stage plastic pressure setting to help
compensate for some of the plastic shrinkage that occurs from the molten
plastic solidifying and cooling in the cavity. This continues for a set period
of time until the gate has solidified at which point the screw may then be
programmed to rotate to prepare the next shot of plastic. It is important to
note that as a single setpoint, the 2nd stage pressure is only set
to what gives the best balance for many items – filling the part without
flashing, yielding the best dimensional results, and more.
3-Stage Injection for Immediate Improvements
Flash, short shots, and dimensional repeatability are some
of the most common injection molded part quality issues regularly faced by an
injection molder and mold maker. From the tooling side, flash can be the result
of poor tooling condition, design and construction (i.e., where flash is
present even when intentionally molding a short shot); however, the machine
process settings play a very important role with regards to flash as well. So
how can we better control our process to help improve upon these everyday
issues?
PACK is arguably the most critical stage in an injection
molding process, yet it is often combined and “along for the ride” with the
HOLD stage. As it would be under any 2-stage injection process, PACK and HOLD
are set as one occurring after the V►P transfer. With constant
pressure (P) being the control for any injection occurring after this transfer,
it is performed with far less control what any injection occurring under
constant velocity (V) prior to transfer.
Creating a 3-stage injection process from a 2-stage one
means separating PACK into its own stage allowing for the three separate stages
(FILL, PACK and HOLD) to each have their own distinct and individually
optimized machine settings. The PACK stage now accurately performs the final
filling of the mold cavity and establishes the amount of plastic to be
compressed into the cavity through a slower, more controlled 2nd
injection stage. With PACK moved prior to V►P, its speed
(V) and positioning are able to be managed properly. The third stage, HOLD, is
now alone after V►P – after the critical quality concerns
have been established and managed in process. HOLD is now only a set pressure
(P) for a set period of time starting very near to where the final screw
forward position will be (i.e., the cushion) doing its job to help maintain the
degree of PACK until gate freeze is reached. When implemented properly,
significant improvements to scrap, quality, and overall manufacturing
efficiencies will be realized throughout your operation.
What’s the Catch?
This change does require another level of processing
competency, but it is a much easier jump than making the leap to using
in-cavity pressure sensors for control and monitoring of your processes using
expensive RJG or Kistler types of technologies. And this change is relatively
free. We are heavily invested in the use of in-cavity sensors at Matrix; their
merits are well-proven for the high-precision tight-tolerance products we make,
but please know you can still make large gains by only investing in the
training of your people. This is a “no-brainer” ROI if ever there was one in
injection molding, since no large capital expenditure is required.
Matrix received an older, worn transfer mold that had always
used a 2-stage injection process. We elected to have this mold serve as our
test subject – an imperfect mold to serve as an example to clearly illustrate
the differences between 2-stage injection and 3-stage injection. Our experiment
on this worn 8-cavity electrical connector mold was performed in our oldest –
but still quite capable – 120-ton injection molding machine. I asked our lead
process engineer to establish his best 2-stage injection process; as an experienced,
degreed plastics engineer who excelled at processing, he set a fundamentally
sound process for this tool. We saved shots for evaluation. His process was
then adjusted only to separate PACK from HOLD – optimizing each one
individually – all of which took between 5-10 minutes. Shots from this 3-stage
advanced injection molding process were also saved for evaluation.
Results
The results in our example showed significant improvements
from the 3-stage injection process in every measured category. Critical dimensions
between circuits in the X and Y directions as well as part flatness were
improved by 50%. A limiting factor was that this part had worn steel resulting
in excessive flash in the mold’s non-critical void coring areas, something we
later rectified in tooling for our customer. As-is, however, parts from the
improved process with the highly controlled PACK phase exhibited a flash
reduction of nearly 50%. Part weight increased by 0.5%, a significant increase
for a thin-walled connector part already exhibiting excessive flash. The
increased control allowed for improved pack resulting in a heavier part than
before. The denser part shrunk less, and therefore warped less, providing
greater dimensional accuracy and stability. Process monitoring over time would
have showed the 3-stage process to have less variability and therefore more
consistent part quality as well.
Often overlooked, this real-world example shows how the PACK
portion of injection holds so much importance and relevance to the quality of
molded parts. PACK has the ability to significantly affect the molded product’s
outcome with regards to flash, dimensional accuracy and repeatability, and
more. Moving forward, this ideological change may be the low hanging fruit that
provides immediate and relatively pain-free improvements to your metrics while
reducing unnecessary waste in your operation. Your machines are capable, and it
isn’t quite the leap you may think it is to implement. I’ve been establishing
all processes in this manner for nearly 30 years, and I have yet to find a
reason to not start with a 3-stage process and make adjustments from there. Why
settle when better is free for the taking and best is a slight process change
away?
Tom Moyak
Director of Business and Engineering Development
Matrix Tool, Inc.
4976 Franklin Road
Fairview, PA 16415
USA
www.matrixtoolinc.com
814-474-5531
About The Author: Tom Moyak is a
plastics professional with over 30 years of diverse engineering and
manufacturing experience who takes pride in achieving significant increases in
quality, efficiency, and profitability through the development of innovative
tooling and molding solutions rooted in fundamentals.