EVA processing help

[TechQA]

Dear all,

My customer is facing some streaking problems with injection moulding of this EVA material for a shoe sole. The molded part is somewhat curvy and with thick and thin areas, this part is used for medical purposes to help flat footed people. The EVA used is from Taiwan Formosa Taisox injection/foaming grade 7340M eva. At 1st, i thought it was due to insufficent flow of the material that cause the streaking lines which appears somewhat caused by high shear. I introduced amide wax and eva wax and still nothing changed.

But i observed that, by reducing the injection speed the streaking defect improved tremendously, however there are still much improvement to be done. The injection speed is already at the minimum but still couldn't solve the problem. Do anyone have any idea of the exact defect for this case or suggestion/solutions to my defect. At this moment, i suspect the material is not compatiable in the 1st place but this is spec-ed in by customer. Any possible cases ? Improve gating ?

 

[patprimmer]

You already gave the answer. Reduce the shear.

This could involve many things from increasing mould temperature, increasing melt temperature, reducing material velocity (by either reducing injection speed OR by increasing cross sectional area of the restrictions in the flow path). The worst restriction is often the gate.

Also removing sharp edges that create turbulence or localised hot spots on the mould surface at a high shear point.

Without seeing the part it is difficult to really be specific.

Regards

eng-tips, by professional engineers for professional engineers
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[Pud]

Hi,

In addition to Pat's comments:

Reduce screw back pressure
Reduce screw surface speed
Check for sealing of the check ring - material bypassing this will degrade.
Check material residence time in the barrel - ideally shot weight should be 50 - 70% of maximum.
Check actual melt temperature - compare to manufacturer's reccomendations - do not rely on controller setting. Do 5 - 10 production shots, air purge material and check with thermocouple and meter.

Rgds


Harry

 

[Pud]

Hi TechQA,

Have just re-read your post - I had assumed that you were experiencing brown streaks but I now see you do not mention the colour...

What colour are they?

Rgds


Harry

 

[TechQA]

Hi Pud,

There is no colour of the streaks. To put it plainly, i would see it as silver streaks alike. We are suspecting it's due to the tooling of design of the part that result in this. But again, in the commerical world users would always go for modification of the tooling as last resort.

 

[Pud]

Hi TechQA,

It's not just the blowing (foaming)agent is it? Blowing agents are sometimes used to give a streaky, "wood" effect...


Rgds


Harry