Plasticicsing time variation

[processtech]

Hello, Why would we be seeing a variation in plasticising time of a tpe, approx. 1-1.2 sec variation.Thanks

[Pilot]

Hi,

could you send to us more info? temp. profile, backpressure, material name, machine type, age??

[rickbatey]

There are several possible causes, and varying remedies. I agree we need more info, as well as press brand and age along with actual extruder motor pressure at startup and say 2-3 hours later.
Have you purged the barrel to see what the actual melt temp is and how it looks? Also what resin? Many have release or UV agents in them that can affect the process. Are you coloring the resin or pre-cololored? Is this related to only one color?
Rick.

[Suhas]

Other that the above valuable suggestions, see if you can increase your back pressure up a little and check to see if the consistency gets better.
Thanks,
Suhas

[rickbatey]

One other possible cause, the hopper opening may be too small or some other part above the feed throat causing the screw to be slightly starved for resin.
Rick.

[processtech]

Hello,
Thanks for the replies.
Temp profile °C, Nozzle 181 Zones, 181,176,176,165
Screw rpm 200, back pressure 31Bar,
Material gls 273-054
Increased hopper throat temp improved the variation, but also there is sporadic short shots, could this be a check ring issue.
Mike

[Pilot]

Hi, according me the screw rpm is too big ... 50-80 must be OK, but it depends on your homogenity, but also for TPE lower rpm can bring a better homogenity. Is your sprue really cold when plastication started?

[processtech]

It has ran perfectly at those conditions in the past,is it pointing towards a machine issue seen as we get shorts on and off,its an 80 tonne Krauss maffei,relatively new,knocked off hold pressure and calculated the shot to shot variation over ten shots,about 1.3%,rules out the checkring,but new before I done the test it would as the melt cushion is consistent

[tim12871]

Can you physically see if the screw is starving? If you increased the hopper throat temp and this improved the inconsistency, you might consider pre drying the material.

[rickbatey]

My experience and those that have worked on KM for years; NRV is not shutting consistently. Though it might be leakage at the bottom of the barrel. Try adding like 10mm to all screw strokes and see what happens.
Rick.

[processtech]

Hello,
Didn't get back to this thread in a while, Rick what is the NRV,
A different question why would we see sporadic increases in injection time, approx a half of a second increase, nothing has changed,same material batch, same moulding parameters, i cant figure what the issue is, the plasticising is relatively consistent, little or no variation.
Thanks in advance for any advice opinions you may have

[rickbatey]

Non Return Valve or the check ring, seat and screw tip. This being a KM is what leads me to believe all of your issues are from the inconsistent closing of the NRV. I work on KM's everyday now and a former KM guy told me that is an issue for them. We use a QSO tip (quick shut off) from Glycon I think it is but there are supposed to be newer and better ones today.
I feel this is the issue as you now have injection time variation. All stem from inconsistent NRV cycling. Replace it and see what happens. Being the resin this is, I'd have the screw and barrel checked and measured as well.
Rick.

[tim12871]

How much post decomp are you using and is the screw decompressing back to the same position every cycle? I work with a Husky with 4 injection units and had the issue with inconsistent post decompression. This would cause a variation in fill time. End fix was adding an in line flow controll.

[pjhall]

I think Rick is on the right path suggesting the NRV. Post decompression can also help the NRV seat. I had a plant manager many years ago that didn't want to spring for a new screw tip / check ring assembly and insisted that I install a "brody ring" Bring. I had a hard time explaining to this guy that although it worked this is a "band aid"!!! FYI...That molding shot went out of business!!!

[ArikB]

A tact temp study should have been done during process development. This will tell you the exact temp to have your rear zone set for the most efficient throughput by your equipment. If this has already been done just grab a pyrometer and check actuals to the documented value. If this hasnt been done you definitely should do it now. The people above have mentioned some other things to check but finding out your screw tact temp is the first place I would start.

Remember, the plastic needs to stick to the barrel and slip on the screw for proper recovery.