Varying thickness in PET preforms 2

[Tomoharu98]

Greetings, my company produces PET preforms and also blow them, but we have been having some problems related to preform thickness, there is a difference as high as 0.2mm from one side to the other. And, because we use these preforms to make carbonated beverages bottles, once blown and applied under internal pressure, the bottles tends to incline because of the difference in thickness of their walls. Any advice about how can I solve this problem?

 

[btrueblood]

You say you produce the preforms, presumably by injection molding? Have you inspected the mold(s)? Reviewed mold temperatures and pressures, and other variables regarding the molding process? What is the base material, any regrind?

Basically, start digging into the process that produces the preforms.

 

[Tomoharu98]

Yes, it is injection molding. Could this kind of problem be related to a defective mold? We started using a new mold and this problem appeared and then progressively became worse. We checked temperatures and pressures, trying new parameters but nothing seems to work. I recently saw that there is a fine "strip" that goes from the top to the bottom of the preforms(shown in the picture), and ends with a white strip that is connected to the gate of the preform. This strip has a different thickness but I don´t have any idea of what is causing this, being the first time I saw this problem. (I´m relatively new to plastics engineering)

 

[btrueblood]

Certainly. A poor design of the core (the pin that forms the hollow) could allow it to flex, or it may be wearing out on the feature that holds it centered in the cavity. The "strip" you mention - possibly this is the parting line of the mold showing?

 

[Tomoharu98]

It could certainly be a poor designed core or mechanical failures. This strip is not a parting line, it´s difficult to explain and see, you could imagine it as if 2 flows of material encounter and unite, interrupting the "flowing" aspect of the preform walls when observed under polarized light.

 

[3DDave]

The stripe looks like someone drew it with a graphics program before saving as a jpeg.

If not, maybe it is a reflection on the surface. Take another picture without the polarization.

 

[btrueblood]

"This strip is not a parting line, it´s difficult to explain and see, you could imagine it as if 2 flows of material encounter and unite, interrupting the "flowing" aspect of the preform walls when observed under polarized light."

Ok, that sounds like a knit line. Does the preform mold have more than one gate, or something else that interrupts the flow of material into the cavity? Where is/are the gate(s) located relative to the part and parting line? Where is/were gate(s) located on the old mold?

You say this is a new mold, do you mean the design of the mold has changed, or just that it is a newly-machined/produced mold? If the design has changed, was the new design proven, and if so was it done on your machines using your standard resin, or...?

Better: do you have the old mold lying around, and could you replace the new mold with it, and run some samples to compare. This would either tell you that the new mold is the problem (old mold produces good final parts on the same machine and resin as the new mold) or not. I'd at least get the inspection papers and any test data taken from the new mold and start measuring the "is" to the "was" condition of the mold (especially the bearing area of the core pin and the mold halves).

 

[Pud]

Is it a single cavity pre-form tool?

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[Tomoharu98]

The mold has 24 cavities, each one has only 1 gate. I checked preforms made in the older mold and they didnt have this problem, also, we produce other type of preforms and they don´t have this problem. The mold design is supposed to be the same as the old one, but new modl preforms are slightly smaller (0.2mm on every dimension). I thought it could be a gate position problem, where it is not perfectly centered and the flow of material is not uniform. What do you think?

 

[Pud]

I am thinking the same as you: that the hot tip is out of line to the gate. Tip might be damaged or a piece of debris (swarf?) Stuck in.
I'm assuming it's only one cavity is a problem?
Try some short air shots and see if the flow is going off square to tool. Cover the tie bars and moving half with cardboard! Slow injection rate!

Politicians like to panic, they need activity. It is their substitute for achievement.