Gate mark on the IML of a PP Lid 2

[prceng123]

Product: PP round flat Lid, dyed dark brown. Injection gate on the male mould side to allow for the label to be applied on the female mould.
Label material: Multilayer PP white 65 microns

Since PP material is dyed dark brown and being injected opposite the white label, it fuses into the label to the label's full thickness with adverse aesthetical result on the label - it produces a brown circle of 2-3 mm in diameter.
Tried to keep the label cooler by replacing the mould part with a piece made in phosphorous bronze (cheap alternative to BeCu since no special mechanical strength is required. This reduced the mark diameter but did not solve the problem completely. Any suggestions please?

 

[Demon3]

Process at lower temperature if possible. Perhaps by using a higher MFI PP or a flow aid.

Reduce cycle time by using a nucleated PP or a nucleating agent in your existing PP (polymer hardens faster due to better crystallization).

Inject from another position i.e. from the side to avoid the flow causing a round spot. Maybe a wider gate would do the same (note I am not an injection molding expert so if e.g. Pat or Pud contradict me then listen to them instead).



Chris DeArmitt PhD FRSC CChem

 

[prceng123]

Dear Demon3,
Thank you for your suggestions.
Lowering the temperature does not eliminate the spot - already tested.
Reducing cycle time will not help either, and injecting from a different position is not feasile.
Melt flow rate is already high enough abt.40
However, I have seen similar lids with IML, with the gate at the centre without the spot!! That is, it has been done...........but how?

 

[Compositepro]

It sounds like the jet of hot plastic at the gate is melting through your label. I think a larger diameter gate will lower the the jet velocity and perhaps solve your problem.

 

[prceng123]

Dear Compositepro,
Thank you for your suggestion. Indeed the problem is the jet of plastic melting through the label.
I missed to say that the gate was modified not in diameter as you suggest but a part of sphere was introduced to give material filling route other than direct to the label surface. It did not eliminate the problem! I will try your suggestion, although I do not believe the jet velocity is the remedy, but the filling time-something I cannot increase due to machine limitations.

 

[patprimmer]

An increased gate diameter will reduce shear through the gate.

Also despite the thicker bump under the gate, at high fill speeds the jet will stream straight across and hit the opposite side with high velocity.

Increase gate diameter to reduce material velocity and sheer. Inject at considerably lower speed for first 10 ti 20% of the filling of the cavity (note to not count degree of fill until the runner is filled and the cavity is actually being filled).

They do make PP in up to 100 MFI grades. That might help.

PP copolymer runs at lower melt temperatures. That might also help.

Look to use the absolute minimum amount of colour you can get away with.

Talk with your pigment or masterbatch supplier as you might be getting some colour migration as well as some melting.

Can you get a thicker film or multilayer film with a higher melting temp layer in the middle or a barrier layer if the colourant is sublimating or diffusing by other method.

What s your cycle rate. What is your reject rate. For a true cycle rate, only count good parts per hour, not shots times cavities per hour.

If possible, decrease cavity temperature, increase core temperature and reduce melt temperature.

If you slow cycle time by 30% and reduce rejects from 50% to 20% you have the same actual production rate and a considerable cost saving on materials.




Regards
Pat
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[prceng123]

Dear Patprimmer,
Thank you for your attention and suggestions.
Well, I fully understand and appreciate what you say about the gate and the ball, etc. I already decided to increase the gate diameter to reduce shear heating at the gate which might be overheating the melt, remove the gate internal tip and leave the hole alone. These steps will leave larger gate mark, but being on the inside of the lid who cares!
The material used is indeed PP Copo, the temperatures employed are already at the low side, and the number of rejects is ZERO.
I already tried to reduce the injection speed at the initial filling stage but this induced other problems.

 

[patprimmer]

Did you increase the temperature of the core as you reduced injection speed.

If you have zero rejects, why is it a problem?

Regards
Pat
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[prceng123]

Sorry fo the misunderstanding.
The customer does not mind the brown spot on the label, but it is my concern to solve the issue. All lids come with the spot - so in that sence it is 100% rejects.

 

[patprimmer]

Of course the old old trick of highlighting a fault you cannot rectify and calling it a feature is always possible.

Regards
Pat
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[prceng123]

I like your way of thinking!!!!!!!