Using only 5% of barrel size

[meltflipper]

hi every one, I hope that every one is doing ok
today I run in to a very comment problem, I resave a new mold this week and the only machine that we have available to run this mold have a pretty large barrel, process development is pretty much don but I am running in to a lot of problems as expected.
what can I do to control my process? and what should I do with my cushion? should I run with a large cushion to use more barrel or just live a small cushion?

[rickbatey]

I think smaller is better but I think most of your problems stem from the poor resolution using such an abbreviated screw stroke. You may have to run it higher with such a large screw. Will the residence time hurt the resin or cosmetics? Can you reach and maintain the dimensions?
Rick.

[nemanja]

Hi,

Did you tried to use bigger decompression volume to increase movment of screw? Depends on mold desin and material it can help. It worked for me in same situation.

Regards,
Nemanja.

[meltflipper]

hi, rickbatey and thank you for your help
dimension are all over and I think that residence time is not an issue we did a 30-30 melt test several times and it coms up ok 420 degree's material is PP the other issue that I have is that the part has e living hinge close to the end of fill it looks like the flow is shutting of at the hinge and when I transfer from velocity to pressure we are over packing everything before the hinge. Going back to my question y will follow your recommendation to run with a higher cushion at this moment my shot size is 1.2in and my cushion is 0.46

thank you very much

[meltflipper]

hi, nemanja and thank you for your help
I am currently using 0.25in of decompression and I been tall that we shouldn't be using decompression because is a valve gated mold
is this correct?

[nemanja]

In my case it was cold runner mold, shot size 0.5in, cussion 3in and decomression 3in (values are more less correct, because im useing metric units). with this setup i was able to get more less stable process. Im not sure this will be posible with valve gated mold.
Something that also cross my mind is mold widouth injection speed, almost only with holding preassure. That also give me some resaults.

BR,
Nemanja

[JayDub]

the other issue that I have is that the part has e living hinge close to the end of fill it looks like the flow is shutting of at the hinge and when I transfer from velocity to pressure we are over packing everything before the hinge.


If at all possible I would suggest pushing your VPT to the far side of the living hinge. The endurance of the hinge relies on the polymer chains being aligned perpendicular to the axis of the hinge, which requires a high flow rate. If you are filling the hinge on pack pressure, the polymer chains may be balled up instead of stretched out and the part may fail early in service.

Good luck with this problem – I once justified buying a new (smaller) screw and barrel for this issue. My argument was that with such a short distance of screw travel, a good portion of your shot is taken up with just shutting off the check ring, during which time the plastic is going who knows where – not conducive to process control or consistency.

[brentb]

Are you running polypro?

KOM

brent

[meltflipper]

Yes polypro 3620WZ

[meltflipper]

Hi, and thank you all for your help and support this is grate

yes my VPT is set to end after the living hinge so I am using first stage speed to get as much molecular orientation as possible, however you just mention something that I really was overlooking, that more then half of my shot size is been used to move the check ring forward this is going to be a grate argument for my meeting on Monday

Agent thank you all and have a grate weekend

[rickbatey]

Decompression is OK with a valve gate. It will help get the check ring to shut more consistently but using a faster initial fill rate will help as well. Remember to flex the hinge after molding to help align the molecules in the living hinge to ensure it lasts. I also believe the dimensions are due to the inconsistency in the check ring closure and resolution of the press trying to make such a short injection stroke.
Rick.