How to control void/ bubble with PP injection molded part?
How to control void / bubble with PP injection molded part using valve gate multi cavity mold venting provided, melt temperate minimized, small tonnage machine used, injection speed minimized, preheating though not required still carried out for surface moisture, back pressure zero and max utilized
Still we get voids / bubble which is very tiny cannot see with naked eye but when cut the part section it is visible identified after assembly because it leave minor crack mark at void / bubble area
Anyone experience such issue kindly share
Thanks in advance
For an optimum transfer of the holding pressure, the moulded part must be gated at the largest croos-section. If this is not possible , a flow promoter between the gate and melt accumulations may improve the holding pressure transfer. To prevent the gate from freezing prematurely, sufficient dimensioning is necessary.
Make a small inquiry:
Is the residual melt cushion too small ( min. 2-5 mm) or does the residual melt cushion fluctuate?
if yes: Increase metering stroke; check non-return valve and/or cylinder; change dynamic pressure ( increase); check material feed.
Do the voids appear near the gate or in the thick-wall areas of the moulded part?
If yes: Optimize holding pressure time;Increase holding pressure; change mould wall temperature( decrease); change melt temperature ( decrease); change injection rate ( decrease).
Do the voids appear away from the gate or in thin-wall areas of the moulded part?
If Yes: Optimize holding pressure time;Increase holding pressure;Change injection rate (increase);change melt temperature ( increase);change mould wall temperature( increase).
If Not: Check gate dimension; introduce flow promoter;eliminate melt accumulations;consider wall thickness/rib ratios.
Voids and bubbles are of course two different problems. Voids are caused by shrinkage. Bubbles are trapped gas. From your description i.e. seeing the voids in a cross section of the part it sounds like a shrink void. There is an easy way to tell however. If you increase the steel temperature significantly and do not change any other parameter the void should show as a sink mark. What happens is that the plastic is shrinking internally. If the wall is strong enough a shrink void will be formed instead of a sink mark. If the wall is weak enough i.e. hot enough the internal shrinkage will pull on the outside wall and create a sink mark.
Now I also notice that you say you are using 0 back pressure. This will often result in bubbles in the melt. One of the reasons we use back pressure is to assure a consistent volume of plastic shot to shot. The back pressure also helps push out trapped gases in the melt. For PP I generally suggest a minimum of 50 Psi hyd. pressure for BP. Sometimes I will even use as high as 150 psi depending on the application and screw RPM.
Because your bubble/voids are microscopic I suspect they are bubbles and increasing BP will mitigate them.
When adding back pressure keep in mind the volume of the shot will increase some as will the melt temperature. Also be sure to decompress, since you are valve gated you will want to decompress the manifold before screw rotation and after if you can.
Wishing you a prosperous and pleasant day!
Thanks for the detailed info, I agree with you on shrink void in this case as the wall is strong internal and external diameter area of the sleeve profile and no deformation observed
When I fine tune with back pressure until 150psi let the part leaves some flashes in the parting surface, still we observe this issue
Part profile does not allow to provide flow leaders
Thanks again for your inputs