Buzz - I had an opportunity to be thinking about injection molding, specifically using 'gas assist'. Anyway I was looking at some scrap and thinking about gassing and voiding in general. What had originally attracted me to the scrap was that there were some cut up sections where the center of the thicker rib areas were almost all void - as though they had been start up or R&D run in as 'gas assisted' parts - where a void is intentionally the outcome in the middle of thick sections. Does this have any additional affect on thinking during the design phase?
Skippy - the experts in the industry say that the overall effect of gas assist is the same as running a thinner wall part - a faster cycle with less shrink and less sink on the part; important to know when working with tooling people. Remember that recommendations concerning raw material shrinkage from the raw materials guys are just that - recommendations; as processors, we have to smarten ourselves up too. Anyway, when you get ready for gas assist, you should set up some trials on known parts from known tools dimensions and should intentionally shoot parts at the same cycle and barrel heats before gas assist and after to observe and measure the changes in SHRINKAGE which should occur reductions in overall shot/part weights - aka the shrinkage should be less. These tests could then be augmented with adjusting barrel temps (stock temp) and cycle time to pull hotter parts in an effort to 'shrink' them some more if too much larger.
Buzz - so if the mold isn't right on the first hit, we're sunk?
Skippy - When there is a question about shrinkage, the typical recommendation is to ask to 'section' molds where possible and put core pins in so that they can be replaced or at least moved around a little bit on an elliptical of some kind. The hotter the material, the thicker the wall, and the shorter the cycle - the more the material is going to shrink under normal circumstances - but you can only makes so much adjustment to these (plus perhaps annealing can be incorporated to 'shrink' an oversize part) processing parameters. The suggestion is that when ready to incorporate gas-assist, do some testing with various materials to see what changes in both 'the direction of flow' and 'cross flow direction' shrinkages that may occur.