High pressure required to shoot with PC

[Joel JS]

Hello,

Here is one that is stumping me....

We have a mold that uses a PC/PBT blend, (and it says High flow, but I am suspicious of that.) This mold has one hot drop/hot sprue, about 6 inches long which is direct gated into part. Wall thickness is .068" All temps are are 550F wich is on the high end for mat'l specs. If I just purge through the open mold I need about 17,000 plastic psi, and I have nothing left for the part (machine is 17,500 max) (Purging just the machine is fine, low psi, maybe 4k plastic psi.)

We tried to remove torpedo from hot drop and that did not work..... and would quickly freeze off. (This helps keep the unheated part of the drop (last 1") from freezing and we found it's necessary!)

We opened gate up from .068 x .0100 to .068 x.0120 and it did not help much at all.....

Afraid to open up gate further as it might not break cleanly, and may not provide more benefit.

Purging through this hot drop should be easier.... but heats are checking out... not sure if this material needs a bigger orifice or we have a bad drop/design ... any help is appreciated.

thx - Joel

[rickbatey]

You need to prove distance between torpedo tip and gate insert. I've seen them grow when hot to the point that the actual gate area becomes restricted due to thermal expansion. Also you can't mold the part if your using that much of the available press pressure.
Rick.

[Joel JS]

You need to prove distance between torpedo tip and gate insert. I've seen them grow when hot to the point that the actual gate area becomes restricted due to thermal expansion. Also you can't mold the part if your using that much of the available press pressure.
Rick.

OK......thx Rick!

[Joel JS]

Still no luck although we opened up gate orifice further.... we also tried shooting a high flow ABS and it had almost the same pressure restriction to purge through the open mold, so we can rule out the material. The torpedo looks ok (it has ample room between flights and gate opening) We are looking at the drop again.... cleaning it out and examining....this sucker is long!...... it's 14" long, and has an inner ID of 6mm..... wondering if the ID is too small for that length causing our pressure, any thoughts on that?

Thanks! Joel

[Joel JS]

Also, the heater is wrapped on the outside of the drop, and it looks suspect as well. It has three windings on each end of the drop, and then is wound between the two ends in a fashion that there is about 1.5 inches between the winding, possibly allowing it to get cool in the middle. (I only have a 4" temp probe.)

thx Joel

[rickbatey]

Joel,
Did you check the distance between the tip of the drop and the gate orifice in the mold? I think there's no room for the melt to get part the tip and into the gate insert.
I do think 6mm is too small for 14"L drop. Maybe 8 mm is better pressure and still keeps residence time down in drop.
Rick.

[Joel JS]

OK, thx Rick. Right now we are talking to DME..... they said it looks like someone custom made a hot sprue using DME components, and stretched a 7" heater to 13"...... they further said that this set up would never be recommended for an engineering grade resin. So we are working with them for a solution.

Thanks again, Joel

[MTUHusky]

Hey Joel,

A couple questions.

1. When you say 550 degrees, do you mean that is what the screen reads? or is that what you measured with a small diameter thermal couple/temperature probe? I ask this because I have seen a 50 degree difference between what the screen is telling me and what my temperature probe tells me for an air shot. I would definitely check that first, and I would have you and someone else do an air shot temp and average the results for consistency. I would do this test right from the nozzle and right from the hot tips to make sure your material is coming out of the barrel at the right temperature and out of the tips at the right temperature.

2. Did you try taking the caps off of the drops and purging through the open manifold? This would tell you quickly if your restriction is the tips or the manifold itself. You might have a dead spot, degraded material or even worse, metal in your manifold.



-Husky-