Is Faster Always Better When it Comes to Injection Molding Fill Times?

For years I have been using part quality to determine hold-pressure settings, and I have often wondered why haven’t I been using this same standard for parameters such as fill time and transfer position? I was recently working on process development standards and testing those protocols on a new stack mold. Fill-time optimization was a point of discussion. We reviewed many options, and I started challenging the faster-is-better mentality.

I wrote an article a few years back about a time I was asked, “What is the most important process parameter?” Knowing that the injection molding process is connected, and that all process changes impact all outputs, the question itself troubled me. Despite those reservations, I did list out a few, which I believe are more impactful than others, with the first being fill balance.

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With fill balance arguably being the foundation on which a process is built, using fill balance to help us identify a fill time seems more than reasonable. The fill speed will impact the fill balance of a mold, but it isn’t as simple as: The faster we fill, the better the balance.

While developing the process for the aforementioned stack mold, we discovered that the fill balance improved as we increased injection speed — until it didn’t. Increasing fill speed beyond that point caused the fill balance to decay and get worse as we filled the part faster. This was a little surprising. I thought that a short-shot progression with a fill balance done at each transfer position would help point to a potential issue with the mold steel, but the progression provided us data that when the part was filling evenly, it was doing so throughout the entirety of the part. This fact, along with measurements, proved wall thickness throughout the part were consistent.

Performing another short-shot progression at the faster fill time resulted in the flow front starting to lag behind around the diameter of the core. With the faster fill times and when hold pressure was added, a burn started to appear on the rim of the cup and progressively got worse as it ran. The mold’s venting was verified, and they were designed as aggressively as we could without flashing.

The fill speed/fill time with the best fill balance was producing a high-quality part consistently with very little variation in the fill time. This was the case even with the machine’s fill speed setting well below 50% of what was available and slower than what had been modeled in the simulations.

Why Fill Faster?

At this point I asked the team, “Why do we want to fill faster?” We were under standard cycle time and producing a part that meets our customer requirements. The first response was, “Well, at slower fill speeds, changes in fill time can impact material viscosity.” I said, “Sure, fill speed absolutely impacts viscosity, but why do we think the fill time will change?” We have a process that is well below the injection pressure limit, the machine is within .02 second on fill-time variation, and we have the ability to set upper and lower control limits on the machine to monitor fill time, injection pressure and overall shot size. We aren’t physically able to fill faster because air can’t vent from the mold fast enough, which is resulting in fill balance issues, and, when packed out, burns at the end of fill. The vents are as deep as the material manufacturer and tool maker recommend, so increasing them is a risk of flash that isn’t easy to undo.

Consistency vs. Speed

After that, we couldn’t provide a reason to fill faster other than potentially a faster cycle time to support a faster fill time. The velocity linearity study showed a significant decline after the machine hit about 50% of the maximum available injection speed. That’s not uncommon since most machine manufacturers still use air shots to determine the injection speed.

A faster fill time wasn’t better, it was just faster. Don’t get me wrong — there are some merits to going faster, but it is relative to the standard cycle time. A consistent process produces more parts than a fast inconsistent process thanks to reduced downtime and lower scrap. That might sound obvious, but I continuously see unnecessarily fast fills that compromise quality. Moving forward, our team has decided that velocity linearity, fill balance and a short-shot progression are standard tools to help with identifying a fill time but none is more important than the part quality itself.

ABOUT THE AUTHOR: Robert Gattshall has more than 31 years of experience in the injection molding industry and holds multiple certifications in Scientific Injection Molding and the tools of Lean Six Sigma. Gattshall has developed several “Best in Class” Poka-Yoke systems with third-party production and process monitoring suppliers such as Intouch Monitoring Ltd. and RJG Inc. He has held multiple management and engineering positions throughout the industry in automotive, medical, electrical and packaging production. He is currently the molding engineering manager at Amcor. Contact: 262-909-5648; rgattshall@gmail.com.