Too much shrinkage

Hi all,

I'm running some parts that we are having tolerance issues with. Comparing to the last time we ran these (last run was in 2015), the parts are running .01" shorter than previous, which is at the low end of our tolerance. We also have a bore in the part that is .002" undersize as well. I've changed my process multiple times including cooling, packing, holding, and mold temperature and haven't had much as far as a dimensional change. I'm wondering if I might have a material issue? Material is DuPont ST-801, material is dried 175*F for 2-4 Hours. Barrel temps are (Noz to Rear) 550, 540, 535, 530. Mold Temp is 150.

Any suggestions?

*Side thought, we did not run these parts in this machine before, we just bought this machine used so I'm unsure of the condition of the check ring, could that make that much of a difference?*

Picture attached isn't the exact part, but as close as I can make a drawing for without breaking NDA.

The check ring or worn barrel will make a big difference in controlling the size of the part. Especially with Nylon 6/6. Your barrel temps, dryer temps and mold temps. are inline with Dupont recommendations. First thing is to make sure your check ring is holding a cushion and not allowing the screw to bottom out.

100% Agreed with Bryan! Check Ring and barrel. Check if the cushion is changing with increasing pack and hold pressure.
Suhas

I recommend you to ensure that mold has a good maintenance, specially in cooling circuits, if the mold doesn't run so often, its a service parts only, its very common that cooling circuits get rust and water flow get affected, then mold run Hot, then you could get shrinkage issues,,

let us know if you fix it ..

BR.

So I put it in another machine to narrow down if it's a machine issue, seems to still be the same. I think we're just not packing out the parts enough. Cushion is around .15 mm variance (according to the machine) from shot-to-shot. I packed the mold out today with higher than usual pressures and the tolerances are close, I'm surprised this little part is taking so much pressure to get it in tolerance...