Warping issues

[moldingppm]

I have a job running PA66 making a stem. No matter what I do I keep getting bent pins (coming out almost straight, being bad after cooling). Could a bad check ring be the culprit? If not, any ideas? I've had pressures up and down, heats up and down and cooling up and down.

[pjhall]

More information needed here. Could you post a picture of the part? What pins?

[moldingppm]

I'll post a pic later. It's a pin 3 inch x 3/8. Heats 475-515. What else do you need to know?

[rickbatey]

Well hold time and psi would be nice. Part gating, resin filled, steel/water temps and cycle time would be nice. Cycle time and shot utilization plus back psi would help.
Have you tried dropping them in tub of water out of press? Internally the part is probably still molten. Otherwise I think the whole issue is post mold stress, but you might need a cooling rack/fixture.
Rick.

[brentb]

Did the part ever run well, or is it new?

KOM

[moldingppm]

Well hold time and psi would be nice. Part gating, resin filled, steel/water temps and cycle time would be nice. Cycle time and shot utilization plus back psi would help.
Have you tried dropping them in tub of water out of press? Internally the part is probably still molten. Otherwise I think the whole issue is post mold stress, but you might need a cooling rack/fixture.
Rick.

I've been out of my mind lately with how busy I am. I keep forgetting to post up the specs. Sub-Gate, non filled running ST-801. Running a ~21-22 second cycle time, making one at a time. 1.5 seconds of hold time, this is being run in an H-Frame with another stem.

I've never had to drop them in water before. When I get to work tomorrow I'll post up the injection psi and BP along with a picture of what we're talking about.


Did the part ever run well, or is it new?

KOM

I've been running this part for about 15 years now only with this problem coming up now. The issue could be the fact I've never ran this part in this machine before as my other one is swamped with work.

[Ashley]

I have several questions, you said this was on a different press?
I guess the first and easiest (if you have a stable process from the "good press" ) is to do a melt temp study. not barrel but probe the material from the "good press" with the heats set to the good process. Then probe the material from the "suspect press" and compare. if this checks out to within a few degrees, then check the water flow on the in and out of the mold from each press and assure its the same gpm's. Not water temp make sure its flow. Both of these could take a little time away from the "good" press that is swamped. but these would be an easy non teardown study.

You can also contact the supplier which I think is DuPont for ST801 nylon and see if they have any documented issues or changes that they may divulge.

next I would check the process/Press. What sort of process are you running, if its a pack and hold or boost and hold how do you transfer from high pressure to hold (time,pressure,position <---I hope, cavity pressure<---I hope more :)). If position look at the cushion history or graph if the press is capable of generating this and make sure it is fairly stable. for example if you have a cushion that ranges from .3 to .36 then every other shot or even every ten shots you hit .0000 or whatever your known screw bottom is then you could have a check ring problem.

Also which I think is the actual first question is this a random problem or consistent in this press. If it is a random issue on this press then maybe its a random press problem. Check all actual histories if you can, fill time<---Biggie,cushion <---biggie,temps<---biggie, cycle time, pressure at transfer.

on lower volume jobs when I have had operators come to me with issues I could look at the history and tell them everytime they had a bad part because there would be noticeable differences.

Depending on the use or customer I would not advise quenching them in water as this could do several things. It can first of all absorb the water resulting in a "tougher" but more flexible part which I am guessing would not be a terrible thing since this is a modified nylon to be tough, also it could "freeze" in stress and you may have a straight part that fails very quickly.

Please advise on any findings.
Also I did not join to step on toes. if this is a cosmetic part and the only thing that is important is straightness and quenching works then do it :)

[Ashley]

Are you sure that the gate is sealed at 1.5 seconds?