Injection Molding: Is There a ‘Most-Important’ Process Parameter?

I was recently asked a question that provoked a serious emotional reaction from me due to its utter disregard for the scientific principles that are the foundation of a molding process. For me, the question—“What is the most important process parameter of a molding process?”—is akin to asking which one of my children I loved more. My first reaction, which I was (fortunately) able to keep from verbalizing: “Are you kidding me?”

A little background: I sat down with a group from a major medical manufacturer to discuss injection molding medical process validations and the principles of Scientific Injection Molding. So, to be asked which parameter is the most important, especially by someone from the medical side of our industry, was surprising, to say the least.

Considering that in the medical market, the molding process itself is essentially a customer specification, as critical to shipping product as the part dimensions, I was not expecting this type of question.

If you can’t fill the parts the same, you can’t pack the parts the same, and thus Cpk values can become extremely low.

Whether it is medical or pharma, the customer typically requires that you show evidence that the product you ran was molded within the process-control limits established during the validation. I would argue that the OQ or Operation Qualification is the stage of the IQ/OQ/PQ protocol where the molder’s process is developed, while the PQ (Performance Qualification) sets your upper and lower control limits, but that is a different article.

This conversation was the motivation for this column, where I figured I would argue the points for several process parameters as “most important,” and with that maybe help identify whether there is in fact a “most important” process parameter. What better place to start than the answer I reluctantly gave at the time, after making it very clear that all outputs of an injection molding process are important.

• Fill Balance: I made it abundantly clear that fill balance on its own is not technically a parameter but actually an output of many parameters. That said, if I had to pick something, fill balance would be one of the starting points of process development.

I almost lost my composure when the follow-up question was, “Okay, so what’s the most important parameter in controlling fill balance?” My response: “Really?” Literally hundreds of variables can affect fill balance. Whether it is thermodynamics, percentage of regrind, mold dimensions or shear, it is impossible to pick one of these as being more important than the others.

That said, a debate can be had about the importance of having a balanced fill and, more importantly, the effects unbalanced filling will have on part quality and dimensional repeatability, specifically in regard to the medical requirements to hit Cpk and Ppk numbers of 1.33 and 1.67, respectively. The Cpk of a dimensional specification in the medical molding industry is simply the comparison of one cavity to another. So it helps determine the variation from Cavity One to, let’s say, Cavity 32 on a 32-cavity mold and so on. Fill balance is absolutely critical from this perspective. If you can’t fill the parts the same, you can’t pack the parts the same, and thus Cpk values can become extremely low.

Cavities that fill on first stage will be packed out less because the viscosity in these cavities will become extremely high. Don’t forget the basic rule of plastic flow: Plastic flows in the direction of least resistance. If we have a two-cavity mold and one cavity is full on first stage and the other is 80% full, the path of least resistance becomes the cavity that isn’t completely full.

A balanced fill ensures that each cavity fills and packs at the same point in time; it ensures that each cavity is experiencing similar process conditions. It is a fundamental on which a process is built, so it’s importance is self-evident, but calling it the “most important” parameter is still difficult.

• Fill Time: Maybe the most important process parameter is fill time. Let me be clear again—this is an output not a setpoint. The word parameter is defined as “a quantity whose value is selected for the particular circumstances.” We do select a fill time, but we do so by adjusting machine parameters. The fundamentals of molding that many of us have adopted are focused on outputs of the process.

The entire premise of “what is the most important parameter” is questionable, considering that many parameters contribute to the process outputs. There are many molders that would say fill time is the foundation of your process and there are many experiments used to establish it.

I typically run a study on rheology, viscosity linearity, fill balance, and machine variation before identifying a fill time for a process. Did I say fill balance? I did. Fill balance is greatly affected by the fill rate and will change based on how fast or slow we inject.

An injection molding process is a connected process, meaning that multiple variables and parameters will have a direct or indirect impact on one another. Very little about an injection molding process is isolated and won’t have an impact on something else.

With fill time, some processors may complete fewer experiments and some might even add to the list. Regardless of how fill times are identified, it is critical to ensure that we maintain that same fill time throughout the life of that mold or part. If we maintain a fill time, we ensure molecular-orientation equivalency, which results in filling consistently run to run. If we fill consistently run to run, this will contribute to a consistent shrinkage, shear rate, and fill balance.

Again, there are several variables that affect how a part shrinks, and I am only pointing to one. There are many experts in our industry that would say the fill time is the foundation of a robust process, but is it really the most important?

Now if we ask, “What do I need to ensure a repeatable fill time?” the answer is the right injection pressure. So. does that mean injection pressure is the most important parameter?

• Injection Pressure: Injection pressure is critical to a repeatable process, and although you do expect some variation in this output, if you have insufficient pressure, it can be catastrophic to part quality.

Pressure-limited processes lack the ability to adjust themselves based on variation in the material viscosity. If material viscosity increases beyond the available machine pressure, our pressure curve will abruptly flatten out and our fill time will increase. It will take longer for the machine to hit the V-to-P transfer point, if at all. There are several defects this can cause, most notably short shots or underfills.

The entire premise of “what is the most important parameter” is questionable, considering that many parameters contribute to the process outputs.

When we refer to Scientific Molding or Decoupled Molding, we are really referring to a velocity-controlled first stage, but we are completely unable to control that velocity if we have a limit on the amount of pressure we have to achieve it.

I know of several processors that kind of ignore injection pressure when it comes to process monitoring, reasoning that “as long as it is not limited, it is not a critical process output,” and that we expect to see variation.

I agree that we expect to see variation, but how much is too much and why is that important? I would say that too much variation is anything outside established “normal” variation during validation runs. Depending on the product and customer specifications, it is important to challenge the process during validation and mimic viscosity changes. For example, if your specification allows for regrind to be added in production, then you need to include a run with regrind during your process validation.

Once you have a baseline and have identified normal variation for your process, when the peak injection-pressure variation exceeds that baseline, it is important to understand why. I am not saying you should immediately reject this product, I am saying that this product needs to be reviewed and more importantly, the molder must identify where the added variation is coming from.

• Cavity Pressure: Given the millions of dollars molders spend every year adding cavity-pressure transducers and equipment to monitor and control their process, cavity pressure must be a critical output. Having the ability to pack out each cavity based on the cavity pressure, or even controlling the V-to-P transfer by cavity pressure, provides dimensional stability from part to part and shot to shot. Although the investment can be substantial, the benefits are proven. That said, my position is not that every product needs this technology.

If you’re an automotive molder and you’re molding class “A” surface products, even parts that are run within your control limits for cavity pressure may not be acceptable due to visual defects.

Ensuring that you have a skilled, motivated, and properly trained workforce is ultimately what will determine the quality of the product being sent to your customer.

Cavity-pressure transducers aren’t going to let you know whether the parts have a scratch or a spot of rust on your texture. If set up properly, they can monitor for shorts and flash; but cavity-pressure transducers might not catch visual defects that aren’t related to material flow.

If the cavity-pressure transducers are installed in the correct locations, peak cavity pressure can provide a high level of control and monitoring. However, I would say the biggest benefit of cavity-pressure transducers isn’t even the cavity-pressure output. The cooling-rate output from a cavity-pressure transducer provides the rate at which the plastic shrinks away from the transducer. This can reassure molders that the water is cooling at the same rate on each run. So maybe the argument for cavity pressure is actually an argument for cavity-pressure transducers.

That would be a difficult stance to take, however, since there are so many successful molders out there that do not invest in this technology but are still data-driven scientific molders.

• People as a Parameter: Trying to identify a “most important parameter” for a connected process like ours is absolutely impossible. There is not a most important parameter nor is there a most important piece of equipment. There is however one critical resource that is “the most important” and that is your team members.

Your people, from technicians to operators to everyone else, are the most important. Whether we are discussing process development, process controls or technologies, the person setting up that job has the most influence on its success. Ensuring that you have a skilled, motivated and properly trained workforce is ultimately what will determine the quality of the product being sent to your customer.

Investing in people as much if not more than you invest in equipment is critical to a safe and successful injection molding process. Think of it this way: If you decide to say that fill time is the most important process parameter and you apply tight control limits, someone still needs to turn those controls on.

Someone needs to understand not only how to turn on the controls, but how to troubleshoot them when the equipment doesn’t function properly. Much of our industry’s training is focused on engineering and process-development skills, yet we still do not provide training apprenticeships for mold and setup technicians or even operators.

We need standards similar to other skilled trades and a clear career path for our technical team members. I have spent many years in injection molding, working with all types of equipment and developing processes and systems to monitor them. I can tell you that these systems can never replace a well-trained technician. Instead, they give them the tools needed to deliver best-in-class efficiencies and part quality to our customers. One is not independent of the other. Like an injection molding process, our resources (equipment and team members) are a connected process and molders cannot be successful isolating them.

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ABOUT THE AUTHOR: Robert Gattshall has more than 22 years of experience in the injection molding industry and holds multiple certifications in Scientific Injection Molding and the tools of Lean Six Sigma. He has contributed articles for Plastics Technology and other magazines on multiple topics, such as scientific process development, process monitoring and the effects of variation. Gattshall has developed several “Best in Class” Poka Yoke systems with third-party production and process monitoring such as Intouch Production Monitoring and RJG. He has held multiple management and engineering positions throughout the industry in automotive, medical, electrical and packaging production. Gattshall is also a member of the Plastics Industry Association’s Public Policy Committee. As of January 2018, he joined the IPL Plastics team as process engineering manager. Contact: (262) 909-5648; rgattshall@gmail.com.