Proper Short Shot - What do you think

Hey all!

We have been divided!

Today our team was having a discussion about the proper short shot of our 100mm test tube (and various other lengths). Team Wavy Tube vs Team Cosmetically Perfect.

Team Wavy Tube
Parts should be filled out completely. Thin walls of the tube require more fill on first stage and the last part to actually fill is the closed end of the tube due to fountain flow. The tube will be full in length but the appearance is wavy due to shrinkage.

Team Cosmetically Perfect
Parts should look as if we are able to send them to assemble, minus the fact that they are a little short. We shoot for 90-95% for first stage fill. At times we see wavy-ness at the open end of the tube which might require us to fill a little more during first stage, but the tube itself up to the open end of the tube should be perfect.




The samples collected were during a viscosity study performed by our interns. With pack and hold pressure set to zero, they collected shots with velocities that ranged from 100-1000 mm/sec in intervals of 100 mm/sec. They also collected a shot at 50 mm/sec to have the data point and see the tube length. They noticed that at 200 mm/sec the tubes began to look very wavy, but were filled out completely. When they showed one of our more senior members, they said "ah perfect short shots" and this is how it all began.

Tell us what you think and help us get resolution!

From right to left we have a short shot comparison, 100 mm/sec, 200 mm/sec, and lastly a 900 mm/sec tube

Attachment 394Attachment 396Attachment 397Attachment 398

96 Cavity Valve Gated Hot Runners
Processing PET at around 545F melt temp
Mold Temps at 70F

95% - 98% fill by weight comes to mind, regardless of cosmetics

I am with Chris. Should be 95-98% by weight. Now in the finished product maybe the switchover needs to be adjusted for cosmetics, but the viscosity curve shouldn't be boxed in by cosmetics (other than obviously noting speeds that produce bad parts due to sheer, flash, etc).

All I would add to the above advice is to weigh the parts to see if fill speed has any effect on cavity balance. I'd be looking at within-shot (cavity to cavity) weight consistency, shot to shot weight consistency, and what the viscosity curve tells me before I'd worry about what the 'first-stage only' parts look like.