4" to 6" dia pvc coupling

[gao7857]

This coupling is connecting a 100mm pipe to a 150mm pipe. The contract molder was using a 280ton JSW (max rate 32oz/s) to mold the single cavity mold, with the circular gate (2mm at part) at the larger outlet surface. The thickness is about 5mm for both outlets except the joint a little thicker.

The molder uses room temperature mold, 185C barral temp for the mid-flow pvc.

At the beginning, the part looks good but dimension of smaller outlet is out of the tolerance. The molder thought it does not flow well at the joint thus not packed well thereafter. So they cut new insert so there are dents in the joint section, making the cross section smaller, though I later feels that cross section may become too small. The part still have the tolerance problem, and at the same time there are short shots at the dents area. The melt freezes before it start to pack. The part also become more brittle that it starts to fail the impact test.

So now we ask the molder to send the moulds to us to play in house. It has not arrived yet. But I am thinking of what should I do and what to get some inputs, as it is new to me in practical. Thanks in advance. Any input/comment is welcome.
What I think first is to go back to the part/insert without the dents, which creates more problm that it solves.
Second is to use our 450T machine to add more power. However I am not sure about what should I try first: I would like to enlarge the gate size to 2.5mm. Then I can shorten the fill time from 9s to maybe 7s with the hope the shear does not go up too much. One big issue is that the part is quite hot when ejected and will continue shrink, as we cannot increase the cycle time so that it cools down. How should I make sure it shrink correctly after ejection to within tolerance?
Any other places that I should pay attention to? Thanks!

 

[MikeHalloran]

ISTR that parts of that size and shape are typically gated into the most massive section, e.g. the central diaphragm between the two cylinders.

You might be able to get there, sort of, by adding a heavy axial rib from the gate to the periphery of the central diaphragm.

If that works, you might then remove the rib feature by putting an insert in the mold. ... and also make the insert a submarine gate.







Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

I don't quite see the problem from your description. How long is the part? I would have thought a 5mm wall thickness in PVC would be no problem, although as gates should be at least 75% of max wall thickness, the gate at 2mm is a little small (3.75mm optimum).

Your suggested fill time seems very long to me - or are you including the hold time in the 7-9s?

Increasing tool temperature will also increase gate freeze-off time. Try up to 40º-50ºC. Check the actual melt temperature - 185ºC is on the low side and barrel set temp is not a good indication of melt condition.

Not sure how your 450t machine is going to give you more "power". Has it got a relatively small diameter barrel on it?

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[gao7857]

Mike,

The gate is at the large periphery near to the joint region to try to balance the mass. It is not at the joint as we think it may weaken the mechanical strength at joint. There are ribs on the small outlet that make it mass'ier than the large outlet side of the gate. Are you saying to add ribs to the large outlet to balance the weight?

Thanks,
G

 

[gao7857]

Pud,

Thanks for the comments.
I agree the gate size is small, maybe 2.5mm is still too small...
It is totally about 200mm long, 22oz part that filled at 3.4oz/s by the molder. I have to increase the gate size (and perhaps the runner and sprue too), so that I can reduce fill time without cause it shear too much. Actually I just had some mold flow analysis done, the molder's setup gives 26000 /s shear while the material ask for max 20000/s. The sim also tells me it will take 280s to cool to ejection temperature... the molder did 9s fill and 70s cycle time... I am worrying if increase both temperature would take it even longer to cool down and shrink more? And now I am thinking if the mold maker include the correct shrinkage rate when cut the mold...
I should say our plant 450T has a larger max rate of fill. Its screw is larger (3.15" than 2.6").

Have a good day
G