PS cracking

[Louis]

For me, it is duifficult to tell where the crack is from the photo.

Can you tell us the material? I am assuming it is something that does not take well to fast cooling cycles that cause built in stress. Of course it could be a tool design problem where there is insufficient clearance to remove a brittle part, which is why the warmer tool doesn't cause a crack.

Did the part ever run well?

[Will Call]

I woulld check for small under cut in that area of the mold. crystall styrene is extremely brittle and it doesn't take much for it to crack.

If possible see if you can add between 5% and 10% k-resin. Do this and you can cool it as fast as you want without problems. (at least i n my experience)

[rickbatey]

You need to check and confirm draft and tool finish in the area. Too little draft and the part will be cracked while opening the mold.
You need to check and confirm steel temp in this area. Poor or too much cooling could cause this as molded in stress from differntial cooling.
You could have a poor knit line or porosity. Improve venting in this area (cut an overflow puddle on the bottom of the part and have them clip/trim it off), and you may even need a flow helper (localized thickening of the wall stock) to move the melt fronts to a better vented/less stressed area of the part.
How much radius is on the inside and outside of the part? I would guess that there is no radius on the part walls, and this (crack) is a result of molded in stress. You should have a 25% of the nominal wallstock as a radius on the part. Without such, the sharp edges become stress raisers for the part and cause early failure. Sharp corners in a brittle resin like this cause the part to want to stick (broken ejector pin), and could explain why it's better when you eject it off before it finishes shrinking onto the core. Radius on the part walls will stop the issue on the next shots.
What are your holding time and pressures? If you are fighting flash, then you may be running the hold pressure or time too low.
Issues with trying to run the mold too cold along with a melt temp that is too low can cause molede in stress or poor/porous knitlines.

[colourwei]

I agree with Rickbatey.
So you may check the part structure and the mould design,
The thickness and the radius of the part is important.
The draft and the position of ejection pins is important too.
And you must check the area ,are there undercut on the draft direct.
some polish work cause the undercut happen.

For "This little crack or whatever it is there before ejecting the part. So we can say that it happens while opening the clamps. But slowing down the clamps did not work."
You must check the cavity side ,are there undercut on the draft direct.
and do the part rise when the clamps opening.

Maybe hlep you.

[rickbatey]

I missed one point?! If it is there when the mold is opened, BEFORE ejection, it could be from sticking in the stationary half of the mold.
Best Regards,
Rick.

[rickbatey]

Jonturka,
The issue may be the timing of the inserts as they are not moving correctly each cycle. With plastic in the mold or up to heat molding parts, the timing may be slow on the insert clearing the part for release. Perhaps you need to adjust the timing or polish the mechanism.
Rick.

[M&M]

Looks like a mechanical release issue when the tool opens. Did some parts like that, just about the same resin and just about the same issue.

Got the sample done with a 50 degree delta onthe mold heat and put spring loaded "A" side ejectors on the P/L, toenails I guess.

Use air poppets on the a side next time. Cyrstal PS will stick like glue to the steel, it does not "slide" across anything.

Fix it don't fight it