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:
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
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.