Cushion Changes with Pack Time

[The Rookie]

Hello all!

As I mentioned in my first post, I am very green to injection molding, so bare with me :)

This week I was working on a press to see if we could reduce cosmetic defects. The technician that I was working with increased the pack time from 1.5 seconds to 2.0 seconds and our cushion went from 0.601 to 0.051 inches. When he saw this, he changed the shot size such that the new cushion was around 0.400 inches and started to inspect parts. The technician explained that while there is a "big" change in minimum injection position, there are 96 cavities, so realistically its a very small change for each part.

Is this normal for high cavitation molds?

[iautry1973]

I don't usually run high cavitation, but 1.5 seconds is a pretty fast gate freeze so my guess is that you have not reached gate freeze and therefore you are able to push more plastic into the cavities hence the lower cushion. Do you know if a gate freeze study was done on this part?

[Joel JS]

Plastic is compressible, so you will often see cushion vary with more pack time, especially in a hot runner tool. (The parts however will get to maximum weight that will not further increase at gate freeze off time ....... 1.5-2 seconds does sound low, but may be ok, as iautry spoke about above)

Joel

[iautry1973]

Yep, being compressible if it reaches the gate freeze at that lower time only wasting energy running a longer hold and possibly inducing some thermal degradation in the material by compressing it. Best to probably check the gate freeze and then move on from there.

[chrisprocess]

Generally, if your cushion is too small and at risk of bottoming screw out, you can increase cushion by increasing shot size AND transfer position at same increments.
By just increasing shot size you're effectively filling the cavities with more plastic, possibly over filling them which can cause stress in your parts and mold steel.
You should optimize your shot size and transfer position to make parts 98% full w/o any pack pressure or pack time. Then adjust shot size and transfer position (up or down) to make your desired cushion.

And what others have said plastic is compressible (especially when it's hot, like in a hot-runner mold) so by adding more pressure/time it will likely compress further.

[jnewmanco1]

Like others have said, 2 seconds is pretty short for parts other than in micro-molding applications or very thin walled stuff where it's mostly just fill speed involved with really little packing.
It could be something simple like a worn or damaged check ring or seat or worn barrel. Increase the pack time to something larger like 10 seconds, if it's safe to do so, and see if the screw's forward motion does actually stop and hold a consistent cushion
or whether it keeps moving forward and possibly bottoming out in that amount of time.

[The Rookie]

Thanks for all the comments! Sorry I have been away at a machine FAT all week.

The product we make is a relatively thin test tube if that helps anyone get more ideas about the gate freeze. At 1.5 seconds pack time, we are maintaining a constant cushion. Shot to shot it is 0.601 inches. Once I increase the pack time to 2 seconds the cushion drops immediately to a lower value, say 0.130 inches. It also is not very consistent at all with +/- 0.15 inches. I did follow the advice for setting the shot size larger and changing transfer position, but the result was the same. I even changed back pressure to a lower value and decompression (still trying to learn what they really impact and how they are set).

I have yet to get into machine trouble shooting, as I am still trying to wrap my head around processing and what I should be seeing when I change one variable. I would like to challenge the validated process and ensure that the gate freeze time is correct. My engineering supervisor insists that a gate freeze study can not be done with a hot runner system, but I would like to change his mind!

Thanks all for the knowledge! I appreciate it.

[JayDub]

Taken literally, your supervisor is correct – the whole point of a hot runner system is that the material between the nozzle tip and the part – including the gate – doesn’t freeze at all. If you over-compress the material in the hot runner system, you are likely to see drool/angel hair.

What cosmetic defects are you trying to fix?
What’s the material?
Have you checked manufacturer’s recommended processing conditions to see if you’re in the ballpark?
Have you read the manufacturer’s processing guide to see if there’s a troubleshooting section?
Does the validated process have a validation report on file? Have you read it to understand how the process was established?

[The Rookie]

Taken literally, yes he is correct that a gate freeze study isn’t needed. But I thought that even for hot runners we should perform the study. We won’t look for the weight to level off, but the % weight change to be minimized.

The defect we are trying to get rid of is a “bubble” in the “gate area.” It’s too small for me to tell if it’s trapped air by using a heat gun or if it’s a void from differential cooling. I will try to get pictures tonight or tomorrow to upload as I am out for a doctors appointment.

I have checked the material suppliers processing guide for PET and trouble shooting recommendations. But the the only thing that I have seen thus far to work is increasing pack time.

I read the validation report and I understand how the process was established. If I load the benchmark process settings for this mold into the press, almost every tube has a bubble in it. But if I simply increase the pack time to 2 seconds they are gone.

What I would like to do is check the fill By removing pack/hold to see where the parts are before we pack.

[Joel JS]

Hey Rookie,

You are on the right track. And it's perfectly fine to do a "gate freeze" on a hot runner, although the terminology is admittedly not correct.. So, in this case, just take your standard process, start with a fill only, (No pack time), then add 0.5 seconds pack at a time, preserve your parts for inspection, and weight too!, and see what you can determine. (Pack pressure should be moderate, not low, not excessive, and probably ok where you are now. FYI - Having little plastic baggies ahead of time with pack times written on them can be very useful for saving parts )

Verify bubbles are more present at low/no pack times. Do you have cracks at higher pack times?.... less sink? other cosmetics? etc.... Then you have the info/answers to your question.

Note: Verify you still have a cushion at higher pack times, you may have to increase ALL screw positions, including transfer position, to achieve cushion. Try to keep everything else the same, so for every increase in pack time is an equal lowering of cool time for the same cycle time....(so you would add more cool time to your first fill only parts to start)

Joel

[chrisprocess]

almost every tube has a bubble in it. But if I simply increase the pack time to 2 seconds they are gone.

So less pack time results in bubbles, more pack time results in no bubbles?
Sounds like they could be voids. Other conditions that may help are increasing back pressure and/or reducing fill speed

[JayDub]

As I understand it, the established process used to produce good parts, but now leads to under-packing. Additional packing gives good parts but inconsistent cushion.

It might be worth checking with maintenance to see when the screw was last pulled & cleaned. There could be gunk on the check ring or wear/damage to the seating surface.

[The Rookie]

Thanks Joel! I already made my bags this morning to start the process - once I get an okay from management.

[The Rookie]

Yes JayDub, the established process did produce good parts. I am in the same boat as you to check the machine. When running, I thought it sounded different than the other identical presses.

Attached are the images with 1.5 seconds pack and a "Void in the Gate Area" and 2.3 seconds and no void.