Processing in a part today and ran across something that didn't make sense. It is a small encoder cover running in a 75 ton electric Sumitomo. It has a heated sprue in an H-Frame carrier. Mold is sub-gated. Part is 20% glass filled PC. I got the part to 98% full with hold pressure and time set to zero. Went to add hold time and pressure on to finish up a process study and the part came out short. Thought it was weird. So I let it run some more parts thinking it might be an oddity. Well they were still short. Thought well maybe I forgot to add time to my recovery delay. Nope 7 seconds of delay time. Ran them without hold time and pressure and back to 98% full. Ran the recovery delay up to 25 seconds and it still did the same thing. Part with zero hold time and pressure comes out to 7.54 grams and holds a cushion of .315" (which is the VPT point). The part with 5 seconds of hold time and zero hold pressure weighs in at 6.49 grams and the cushion is .295". All other process parameters are the same. Seems a little weird to me. I emailed Sumitomo and hope they answer soon, but track record of them answering back in a timely manner isn't good so I figured I would see if anyone else had a good idea on this one.
Maybe press has something in software to stop screw bounce with NOholding psi/time but when you run hold psi the pressure is too low allowing the screw to bounce back?!
Rick.
Rick, thanks. I was thinking something like that as well. I am going to test that today. I was thinking with zero hold time and then going into recovery delay maybe it is locking up the screw and acting like hold pressure. When I toss time onto hold pressure it is letting the screw drift back. I haven't heard anything from Sumi yet other than send them pictures....
Is there a maximum velocity setting for pack/hold phase? My guess would be that with hold time set, the screw is trying to move forward, but with no force behind it the check ring is floating and letting plastic by. Per Rick's comment - is the cushion holding steady at .295 or does it get there and then drift backwards?
Went and ran it again this morning. With zero hold time & pressure the screw is stopping at VPT (.315") and drifting back to .443" consistently. With 5 seconds of hold time and zero hold pressure the screw goes to .295" and drifts back to between .486" and .507". So guess the question from here would be which way is the right way to set the switchover position? I am noodling that one over right now.
JayDub, the max velocity on hold is set at 2 in/sec right now.
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