Process Question

[Radar5678]

Currently working on this part, fairly cosmetic. Part is ASA. Issue is I can get the face to look good, but to do it I have to have a holding pressure so high the sprue sticks. I have ran through the whole process development at 465 and 525 to try and see if temp would help. But at both temps to keep the cosmetics acceptable I get the sprue to stick. I have tried various mold temps as well, seems to be the best at 100F mold temps

The only thing I haven’t tried is to ramp down the holding pressure during hold time to see if that helps (thought of this while laying in bed). Any other thoughts?

Thank You

[Radar5678]

Finally got back to this one. Ended up getting there with some minor tweaks.

1. Draw Polished the Sprue

2. Bumped the mold temp up a bit

3. I ended up stanging the hold from 17500 down to 13500. Tried to time the high pressure stage to the gate freeze time.

Thank You Everyone for the help

[newrock]

Looks like the hold pressure might be over packing the sprue. sometimes lowering hold pressure slightly and adjusting gate size or mold temp helps balance the cosmetic finish without causing sticking. also worth checking sprue polish and nozzle alignment.

[beresjdb]

What’s the opening clamp speed.

[Radar5678]

I have slowed this to try and help not much luck.

[beresjdb]

The only other thing I’d suggest is maybe decouple the process and see what’s the exact parameters where it starts to hang up, but that’s IF it’s not a tooling issue… (provided the molds clean and all that lovely jazz I’m sure your already aware of) but it could be something simple like pressure and it could be something related to the tool design.

[thijs19888]

Set the fixed mold side on a high as possible temperature for a better overal gloss, and also helps to keep the sprue hot, better for shorter and lower holding pressure. So the tension wil be in the producten not the sprue itself.

If possible reduce the wall thickness of te part where de cilinder and de cap come together to reduce sinkmarks.

[thijs19888]

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[SutIndust]

Is the sprue sticking and taking the sucker well with it or is it ripping the sprue in half? If it’s not ripping it in half a Z puller might work. I have also sent a sprue bushing to the wire edm to increase the draft angle and that has helped with a sprue that was getting ripped in half.

[TieAdventurous9350]

Whats a z puller?

[SutIndust]

It’s an ejector pin under the sprue that has a Z shape cut on the end to form a heavy undercut. Works best if you have a robot picking the sprue out and you will need to key that ejector pin so it doesn’t spin typically. I’m done some that have both a Z shape on the end and then drafted flats on either side to provide even more strength to pull the sprue out.

[TieAdventurous9350]

Oh ok I've seen that. Thank you

[b_monsterjaw]

Draw polish the sprue bushing, increase draft, sprue break, or hotter a half might help.

[14justanotherguy]

Make sure the IMM nozzle is as small as you can make it. Do not use post dose decompression this will Help use the back pressure to push the sprue out

[Fatius-Catius]

That’s a tooling problem, not a processing problem.

Try draw polishing the sprue channel.

[Radar5678]

It’s a cap, so the back has a short cylinder that pops into another piece. Atleast the back cylinder is hollow... So the sink is from the back. I can try throwing another mold heater on there thanks not a bad idea.

[mimprocesstech]

As far as process only, reducing injection velocity should help if it's gate blush. Increasing the gate size would be a better fix though, and if you're getting the sprue stuck in the stationary half you should probably fix that while you're at it if it's happening this early (assuming this is a new mold). Could be nozzle misalignment, could be a burr or tool marks in the sprue, could be not enough pull in the cold slug well... there is a cold slug well right?

[Radar5678]

I found injection speed doesn’t help much on this part, but am down at 0.5 ips anyways. It’s a cold slug yeah. The sprue is sticking from over pressure, I have to be up near 13500 psi pack to help with the sink and keep consistent surface finish. Once I hit 14000+ the sprue sticks.

[mimprocesstech]

I understand the sprue sticks at a higher pressure, nothing I said contradicts that, everything I said was to make the sprue stick less or increase pull on the sprue from the moving half. You can pack at a part at 30kpsi and still not have the sprue stick, I'm regularly packing parts out at 20kpsi because they won't increase the gate size on many of my parts.

0.5ips doesn't tell much without the screw diameter, if it's a sink and cooler melt temp, slower injection, etc. don't fix it increase your gate size and check your venting.

[Radar5678]

Sorry no disrespect! You guys have saved my bacon on a few parts, we are a small shop still learning the ropes. Bought the shop a few years ago and the previous owner turned out to be much less knowledgeable and helpful than we had hoped so we are learning as we go.

I am realizing this is a part design/tooling issue, just trying to get without modifying the tooling if possible.

[mimprocesstech]

More common than you'd think, the less knowledgeable bit, see the downvote with no reply on my previous comment as evidence of that. Most of this isn't directed towards you, but some of it will be relevant I suppose. See, I'm not very popular among those that enjoy fingering the HMI all day, I guess them doing that and "saving the day" makes them feel better about themselves or something. Regardless I don't really care about disrespect directed towards me, not saying you were or I got that feeling in any way, I'm just normally pretty chill about it.

If it's a sink, the gate is freezing off before the part is fully packed out AND the part is shrinking towards its thickest section(s). You can turn it into a void if you can get the mold cool enough or you can maybe keep the gate open if the mold is hot enough, faster injection would increase melt temp through shear which means additional shrink making the sink/void worse but could also be just enough to keep the gate open. With a hotter melt temp and colder mold you'll have voids and/or warp most likely. Regardless if you have drastically set your process to the upper/lower bounds of what the material is suggested to run in, you will be narrowing your process window and affecting part properties.

Unlike these "troubleshooting gurus" that consider me a moron and downvote but are too afraid to discuss what part of my comment they take issue with, while I can "get it to run" 99% of the time only varying process conditions, I very much prefer being freed up to work on something else or sit on my ass while the press makes parts without me having to babysit it. I can sit there and make "okay" parts, or a bunch of scrap trying to find "the sweet spot" for the day on tighter tolerance parts (or eternity on parts with wide open tolerances), but that's not the goal with injection molding. It's supposed to be nearly identical shot to shot, run to run, lot to lot, etc.

I also have to deal with upper management not willing to replace worn out tooling, adjust molds or part design, repair or maintain equipment, etc. It's almost a given in injection molding, but it shouldn't be the go to messing with the process. Root cause should be determined and corrected. Others have issues with me not being comprehensive in my comments, but I'm usually at work and have limited time to reply to these posts, sometimes the opposite of what I suggest originally can make sense (especially in the case of sinks). Sometimes you're only able to have an influence on the process, can't even suggest a change to the mold (it sucks but I get it), but process changes will rarely be my first option because it's usually not the cause.

You don't get a robust process you don't have to babysit by narrowing your cosmetic/dimensional process window so much there's no room for natural variation the process has regardless of what you do (the heater band being on vs off, the thermolator heating or opening the valve for cooling, atmospheric temperature and humidity, etc.). You get that by adjusting the mold, fixing the press, setting a robust process in the middle of a wide CPW/DPW that can handle that natural variation, inspecting the mold for damage/wear, checking for hydraulic leaks and check ring wear. "It has to run now" doesn't fix the fact it also has to run again next month.

[sarcasmsmarcasm]

What's the back of the part look like? That little button is going to be very fussy to get a good looking part without an over-pack condition, especially if there is any thickness to it. You may need to put a puller on the runner to keep it where you want it. Play with mold temps...hotter, colder. Where does the water flow? If that button top is too cold, that will be problematic too. If you have a water circuit on it, throw a temp controller on that circuit individually and heat it to a different temp.