Irregular Screw Recovery Times on LCP

[BSBMX]

I'm having a difficult time controlling our plasticizing/screw recovery time on an LCP process that we're trying to minimize cycle time. Here are some specs:


Custom reel-to-reel insert 8 cavity mold with cold sprue/runner
Nissei TH-E70 vertical press
Vectra E130i LCP
Custom new Westland screw/barrel 22mm combo, designed for 30% GF LCP
Inject stroke of ~21mm
Fill time of .11s, hold time of .19s, cooling time of 2.0s
Plasticizing time range of 1.5-2.8s, causing our cycle time to vary from 6.2s to 7.5s


Our material is properly dried. The barrel temps are 680F (nozzle)--665F--640F--640F--640F--625F--210F (feed). If I could get the recovery plasticizing time consistent, I could drop our cooling time even further and get closer to a flat 6.0s cycle time, or better.

Lowering the back pressure from 1000psi to 650psi did help some, but a variance of 1.0s+ in recovery time is still occurring. Right now we're running at a metering speed of 250rpm, but changing this from 175 up to 310 doesn't seem to make much of a difference for controlling recovery time.


Any ideas to get a more consistent recovery time? Any help is appreciated. Thanks!

[rickbatey]

I've never ran this resin, but I see two issues that may be causing some of the issue. 1-What percentage of the max shot size doe that 21 mm metering stroke represent? If less that 30%, your heats will need some serious massaging to get the results you want. 2- I think the jump from 625°F to 640°F is too large. In addition I would expect more heat near the center of the barrel to ensure your pumping resin in the correct state forward for the subsequent shots. Especially with that much glass. It could be the temps are too low with the percentage of glass. The polymer starts to transition but the glass is still a solid. Low temps can cause glass fiber length breakage and separation in the barrel as the screw works the melt.
Again, I've no experience with LCP. Where's the Brent(Sifu) when you need him?
Rick.

[Suhas]

Rick has some good points above. Yes, like Rick I will wait for one of our Masters - Mr. Brent's comments :)
There is a study that is done: Plot the recovery time versus the rear one or two zone temperatures. You should see a 'U-shape' curve. Choose the one that will give you the lowest time and see what happens. While doing this test, you may also want to take about 20 shots at each temp and document the screw recovery times. Then you can plot consistency (or variation) versus temperature.
The other thing you can do a full DOE by varying melt temp and screw speed (3 levels) and recording screw recovery times and consistency at the responses. This will be the best and time well spent.
Hope this helps.
Regards,
Suhas

[brentb]

We run lots of E 130, as well as A 130 and medical grade Vectra. Good going with the LCP screw/NRV

We get good results (horizontal) with a fairly slow (70 to 100 RPM) screw rotate, and a back pressure of 1.000 in plastic pressure. Keep feed throat pretty warm and use a small orifice nozzle,

AND

KEEP ON MOLDING!

brent

[Suhas]

Brent, I am assuming that that is 1000 plastic psi and not 1.000 plastic psi, right? I know lately you have been having problems with your decimals :) :) - As in the other post of the melt cushion :)

[brentb]

Brent, I am assuming that that is 1000 plastic psi and not 1.000 plastic psi, right? I know lately you have been having problems with your decimals :) :) - As in the other post of the melt cushion :)


Many of my jobs (all in electric are set with a BP of 1,000 psi. If you are running a hydraulic press DONT set 1,000 psi (or Bar).
It may seem funny to increase BP for erratic feeding, but LCP is a different animal.

KOM
brent