Fast or Slow: What’s Better for Injection Molding Fill Times?

Halloween is here and, as you’d expect, there is a lot of horror movie talk going on. The other day I stumbled across a series about the history of horror films. In the first episode there was a debate over fast vs. slow zombies, and this made me think about the growing debate in our industry around slow vs. fast fill times.

Before you ask, I know how that sounds, and, yes, I do spend a great deal time thinking about plastics. I recently read an article on Plastics Technology about a low-pressure/slow-fill-speed system being integrated into machine controllers. This system uses slow fill speeds with consistent low pressure to control the injection molding process. This made me think about how just few years ago, I had a conversation I had with RJG regarding its mantra of “shoot as fast as you can.” Talk about polar-opposite approaches. 

Fill Time as a Process Parameter

When I first started in the industry, it was unheard of to set your injection pressure high and allow the machine to use what injection pressure it needed to achieve your desired fill time. Frankly, we had no idea what was really meant by fill time. In fact many machines had a boost time or first-stage time, and this time setting meant your machine would continue to fill until the set-time had elapsed—there really was no attention to an actual fill time.

If the entire fill stage is essentially the machine trying to achieve the max fill speed, the linearity is going to go as flat as a zombies EKG. 

So before we could see inside the mold, or even before machines provided output data to the processor, most of us would fill our molds conservatively. Even if we had the injection pressure set low enough to easily pressure limit a machine, we still had the mindset of, “We don’t want to flash the mold,” so fill speeds would be set rather slow compared to the RJG and SIM methodologies.

Think about this: many machines didn’t have digital readouts or even numbers on the dials for pressure settings. Some of the machines I first started molding on had colored rings on the valve so we would say the hold pressure is set at “yellow”. Of course there were molders on the cutting edge of technology already using instrumented molds, but for the majority, this was not even on our radar yet.

So when I think slow fill speed I immediately flash back to my early years in this industry, but there is a significant difference here. I was actually able to spend some time getting introduced to this system at a supplier open house last year, and I am looking forward to see how it performs on multiple cavity molds in the caps and closure industry. Either way, it is extremely important for companies to push the envelope and explore new technologies in our industry.    

Is Faster Better?

As far as “Shoot as fast as you can,” I have never agreed with this approach, and I have never seen consistent part quality data when fill speed is set at the maximum setpoint.  The performance and repeatability of machines filling as fast as they can, in my experience, is decreased compared to machines that are not.

The No. 1 contributor to this is acceleration. If the entire fill stage is essentially the machine trying to achieve the max fill speed, the linearity is going to go as flat as a zombies EKG. Fill times are going to vary shot to shot, and your process will be out of control due to losing any control over the velocity.  There are several methods of identifying a fill time for your process but at the end of the day there is one critical rule that should be followed:

Can my machine consistently achieve this fill time?

If the answer is no, and the velocity is not linear on your process than there is some work to do and decisions to make.

I am definitely not writing this column to argue whether one is better than the other. Depending on the application, you may have only one option so what works for one product may not be possible on another. In any case, I must say that it is exciting to see all the new technologies that are being developed for our industry and having innovators question, “Is there a better way of doing this?”.