Shot size shifting during the first few shots.

[Valentin Leung]

Hi All,
I mainly do thin packaging tools (PP, <0.9 mm thickness, multi cavities, hot runner). On some tools, when the cycle stops because the robot arm is not able to pick up the part, the first few shots made upon resuming the process are usually a bigger shot size even though the cushion is not reaching historical value (it's more ex: 5 mm instead of 3). I had to set a procedure to my staff to have them move the VP setting out before resuming the process in full auto. If they don't do so, the plastic will flash all over the place, into lifters etc... and it's a nightmare.
I know that it's not happening on all the tools I have, some are not really affected by that, but when it does, some needs the VP setting to be moved as far as 3 mm (on a 60mm screw) to reach the same shot size I usually have in a steady process.
So far, the only reason I can think of is the time the plastic wait inside the hot runner + barrel that heat it up. But... as I said earlier, I have some tools that never show this issue and has the reverse behavior... First few shorts are short and I need to semi-auto cycle a few times before the shot size works its way in.

So, my question is do you encounter this kind of issues? I think it's tool construction and heat related, but I can't still pinpoint my finger on what exactly.

Thanks.

[rickbatey]

These issues can be due to the resin your running, the residence time, barrel capacity used, heat profile or settings, screw speed when charging, suck back stroke and velocity, initial fill rate, over all fill rate, how long the press has set, how quickly the back psi comes on when the screw starts charging, or a million other little things.
Rick

[Valentin Leung]

These issues can be due to the resin your running, the residence time, barrel capacity used, heat profile or settings, screw speed when charging, suck back stroke and velocity, initial fill rate, over all fill rate, how long the press has set, how quickly the back psi comes on when the screw starts charging, or a million other little things.
Rick
So in other words, just live with it... or investigate myself more thoroughly and bring an answer to the community.
On a lot of parts, I am ok, but some has the end of fill area very close to vacuum cups position for the take away and it's jamming tool restart as the first few shots are inconsistent and without the robot, it's inconsistent... chicken and egg issue.

[JayDub]

Sounds like a residence time issue. I wouldn’t expect problems with propylene – it’s fairly indestructible – but there may be an additive that is degrading.

I used to mold a medical part (urethane) where a change of less than 10% in cycle time would have a measurable effect on part dimensions. I put a cycle time alarm on the press, so if the cycle was delayed for more than a few seconds, the operators would purge 5 shots before re-starting.

Keep in mind that a change in viscosity (which is what is causing your shot size variation) also means a change in material properties, so it might be better to just discard (purge) the material in front of the screw rather than change the shot parameters to compensate.

[rickbatey]

What I'm saying is that all of these could be the root cause but again maybe not. Your issue isn't so much that it happens but that the press is difficult to restart because of the issue. We have tools you MUST remove the first 1-3 parts by hand because they stick in lifters or are too short for the robot to pick. This happens on machines from 60-4000t. I doubt that you can fix the issue unless the press has start up switchover settings that would make this an easy fix.
Rick.

[Jorge Villegas]

Is it an electric or hidraulic machine?

I saw the same effect, and it was trouble with the screw check valve. We solved the problem with a feature that the electric machine offered. When the machine finishes decompresiong, it pushed again the screw forward a little bit so the check valve closes, and I assume it stops more material from going into the chamber (now depressurized), thus injectiing more material. It alse makes more stable the cushion.