How Important is Minimizing Residence Time in Injection Molding?

I’ve recently spent some time reviewing shot capacities and a term that kept getting repeated was “minimum residence time.” I had never heard residence time referenced in relation to an upper or lower limit until then. Residence time is of course a very common concept in molding that refers to the time the plastic remains in the barrel or hot runner. 

I have always based my maximum residence time on part quality, and as long as that quality of was not impacted, I would target a screw recovery speed that produced a repeatable recovery time. I’d also aim for one that minimized the time between achieving my overall shot stroke and mold open.

But as for a minimum residence time, I’m not sure what the value of that would be, if you consider that the longer the material stays in the barrel, the hotter it will get, resulting in a more uniform melt. On a basic level, that concept makes sense to me because hotter material ensures that more potential non-melts are melted. That said, couldn’t I also accomplish this via any means that increase the temperature of the material?

Also: how much heat is really being added to the material due to residence time, and wouldn’t that heat be very much impacted by what happens to the melt prior to the actual dwell time of the material?

“Dwell time?” you might interject, “I thought we were talking about residence time.” Well, dwell time is the phase after the shot stroke has been achieved and before the next injection starts. As such, it represents a piece of the overall residence time. Frankly, in my experience, when most people are talking about residence time, they actually mean dwell time.  

Temperature and Time

To truly come up with a minimum residence time would require running countless trials to determine the temperature and the time needed to actually reduce the likelihood of non-melts present in the melt.

Since the heat of the melt is a product of both temperature and time, wouldn’t the amount of time change based on the temperature, and wouldn’t the temperature needed change in turn based on the time it’s exposed to that temperature? The answer, of course, is yes. This correlation makes it a struggle for me to hold my tongue when machine manufacturers continually bring up the concept of a minimum residence time. 

Considering this, I decided to undertake some thought experiments to try to better understand how the melt sitting in front of the screw actually behaves during dwell time.

The first struggle I had was thinking about how does the plastic insulate itself from either losing or gaining heat during this time. Theoretically, losing heat isn’t as much of a problem, but it does bear consideration and could potentially have more of an impact with large-diameter barrels as opposed to smaller barrels.

In larger diameter barrels, I could imagine scenarios in which the center of the melt starts losing heat since the plastic surrounding it keeps it insulated from the heated steel of the barrel. That said, this is one of those fun theories to discuss rather than more of a practical issue to resolve. Let’s focus more on the concept of increasing the melt temperatures to promote a more uniform melt.

Uniformity of the melt is absolutely critical to a repeatable process, but arbitrary limits on things like residence time do nothing but complicate an already complicated process.

As the material sits there waiting to be injected, it is forming different melt-temperature layers. I compare this to the layers within the earth’s interior. In this instance, there are two distinctive sections of the melt as it dwells in front of the screw.

There is the area nearest the inner diameter of the barrel and most influenced by barrel temperatures — let’s call that the shell — and then the molten layer in the center of the barrel. Here, the resin was melted by heat generated from the friction of the screw conveying the plastic forward. Now despite the shell layer sitting against the heated steel of the barrel, odds are that it is cooler than the molten layer near the center of the barrel.

Barrel Heat Versus Friction

We’re all familiar with the old saying that once a machine is up and running, you could turn the barrel heats off and the press would still run because, at that point, almost all the melting of the plastic is a byproduct of the friction of the screw.

Now this is a slight exaggeration, but it can be the case with polyolefins, if less so with engineering-grade resins. I myself have actually kept machines running without heaters functioning while molding polyethylene, so there is definitely some truth to this concept.

Functionally, this phenomenon means that friction is raising the melt temperature beyond the barrel-temperature sittings, and it explains why that molten layer is hotter than the shell layer, which is technically cooling to the temperature of the inner diameter of the barrel steel.

Given this, if most of the heat being generated comes from friction, how is the amount of time the material sits in the barrel increasing the uniformity of the melt itself? It seems to me that it doesn’t necessarily provide a more uniform melt, but it essentially can increase the melt’s overall temperature and thereby potentially reduce some non-melts in the shot.

That said, the more effective way of ensuring a uniform melt targets the screw-recovery phase of the cycle. During the dwell time, the melt is just sitting idle — there is no movement, no friction and no mixing occurring. In addition, the temperature differences in the layers of the melt could actually increase — the opposite of a uniform melt.

Screw Considerations

Ultimately, in order to achieve uniform melt and an optimal melt temperature, you need to first identify the best screw design available for the specific material that is being run. Different screw designs are available to increase mixing of the melt and to increase the friction generated by the screw rotation.

Given this, we understand that screw design is going to play the biggest role in ensuring a uniform melt. Next, making sure that screw is run in a machine that will allow for a shot size using no less than 20% and no more than 60% of the shot capacity is key. Keeping the shot size within this range helps guarantee that the screw rotates enough to generate sufficient friction and mixing of the melt. Lastly, process conditions are also critical, so that using screw rpm, backpressure and, yes, even barrel temperatures is going to play a huge role in guaranteeing that uniform material melting occurs.

Uniformity of the melt is absolutely critical to a repeatable process, but arbitrary limits on things like residence time do nothing but complicate an already complex process. These limits can have a negative impact on troubleshooting issues as well. For instance, you could have process technicians chasing residence time when they should be focused on identifying a root cause that is impacting the temperature or mixing of the melt.

When things like “minimum residence time” pop up during your next machine specification review don’t hesitate to ask why and even how is a limit like this being set. Ask to see the data — there is nothing wrong with educating yourself and challenging industry norms. Learning something new is a daily occurrence in our industry and is what makes the plastic industry interesting and challenging for us all.

ABOUT THE AUTHOR: Robert Gattshall has more than 22 years’ 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 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. Gattshall is also a member of the Plastics Industry Association’s Public Policy Committee. In January 2018, he joined IPL Plastics as process engineering manager. Contact: (262) 909-5648; rgattshall@gmail.com.