Embedded Bubbles

Hello,

We are seeing what our team calls embedded bubbles and were wondering if anyone has any insight or experience.

Cavities: 64
Material: PET
Delivery: Direct Valve Gates


If you cannot view the images attached I will try to explain what we are seeing.

Essentially there is a bubble with an opaque tail following it. Many of our quality associates refer to this defect as an icecream cone or a flaming comet.

Normally we see these defects when we initially start up the machines, but occasionally we will find one in a process that has been running for hours. If there are stoppages (mold protections) the press might be down for 10-15 minutes and the operators will immediately start saving parts without allowing the process to settle back in. We will find them here quite often.

So my question is - what is causing this and is it possible to prevent it?

Thanks,

Rook

Hi Rook,

If you are running with no problems, then shut down and startup in say 15 mins and you see the problem, I would think there is a additive that is degrading and causing a gas that will cause the bubble.

Do you purge out the manifold also before restart?

Suhas

I could see an additive or even the FR or UV package breaking down. One question: What do you use to purge and shut down with? Kinda looks like PE or PP contamination.
Rick

Sorry all, it has been a crazy month with furloughs at the plant and I am just now getting back into work!

Suhas, we do not purge through the manifold before starting. After the press shuts down we run the barrel empty and before start up we purge several times to get fresh material. We do use a wax additive to assist with flow and de-molding (so they say). I wonder if what you are suggesting with degradation could be playing a role.

In my experience with running clear resin , you never run the screw empty and you never shut the barrel heats off. You ramp them down low enough the resin won’t degrade and leave them on; say 350*F. When restarting, you set heats at run temp, wait on them to reach running range and cold screw prevention to turn off, and purge/start.
Now I think your defect may be an unmelted pellet or a gel in the melt. What are your barrel temperatures and do they cycle off and on? If not your settings are too low and raising them will make the defects go away.
Rick.

I would also throw in that before starting the mold up it might be worth purging out the manifold with fresh material just to make sure the material left in the manifold and drops are not degraded, this would help get you to have better startups.

If this is from a degradation issue it might be worth reviewing the residence time of your material, ensuring you are using the correct barrel size and not one too small, and ensure that your melt temperature is higher than the recommended range for the resin. Looking at the actual melt temp of the melt while it is running would be better than basing everything off of barrel temperatures.