White streaks in my black part

[sogeka]

i am using a TPE material to make black parts. Our parts are about 0.5lbs each on average. For these parts, we use 40% regrind since the black can cover up any color contamination. Recently we have been trying to run some test parts using more regrind and we started to get white streaks, even when none of our materials are white going in. Could this be burning? I started to question the regrind particle size, since this streak only happened when we regrind more. Currently we have a 1/8" or less regrind particle size.

Thanks!

[tim12871]

What other types of material and color do you run in this press? The regrind being 1/8" can still act as a purging compound. Do you have allot of fines in your regrind?

[rickbatey]

One or both resins could have a mineral filler, like talc and be the source. Have you tried slowing screw rpm to improve mixing of the resins? I also have seen certain resins act as a purging agent for other resins or colors.
Rick.

[Suhas]

Good point by Tim and Rick. If the streaks are all over, then it is not burning. Also burning is not seen in streaks. Can you heat some pellets on a hot plate or similar and smear them to see if there is something white in there?
Suhas

[sogeka]

Good point by Tim and Rick. If the streaks are all over, then it is not burning. Also burning is not seen in streaks. Can you heat some pellets on a hot plate or similar and smear them to see if there is something white in there?
Suhas

to answer a few of the questions
- we do have some small regrind fines in the mix
- the other colors are red, pink, green, orange, blue and black
- I can heat the pellets, but I know there is no white in our material. The virgin is clear and we add the colorant which is in the regrind. It all happens in house
- we have not tried to slow down the screw yet, but we can. The materials we melt and mix are virgin TPE and colorant.

Should I try to slow down the screw speed as a next step?

[Chris]

yes, and generally the screw should be back to shot size about 5-7 seconds before the cycle is over.

this does a few things, slower screw help mixing, material does not sit and burn up waiting for the next cycle to start....

[pjhall]

I suggest increasing the back pressure and set the screw speed so that, as Chris said, your charge is completed 5 seconds before the cooling time expires. If the cooling time is very long say 50 seconds or so then add a charge delay. It better for the screw to sit idle before charge than after.

"Color that is in the regrind" - This might be your problem. Is there any virgin concentrate? I suggest adding some virgin black concentrate. UN-0055 from Clariant is a good general purpose black.

Also, you might want to try a reverse temperature profile on the barrel.

[rickbatey]

There are other ingredients in a color than say just carbon black to make black color. Many of the colors have other shades of pigments in them for numerous reasons; just like a color mixed at Lowe's or Home Depot, and white can be added into a red or blue to lighten them several shades.
You shouldn't have to have more than 100 psi hydraulic back pressure to mix this resin, they tend to love shear thinning from injection but excessive back pressure just gives you water to mold parts with. The correct barrel heat settings and profile ( I don't think you really need a reverse one but I've been wrong before) with a good rpm that finishes at that 5 s. before cooling time ends is all that's missing. If you run the screw real fast then let the melt sit for too long before shooting the next parts, may not remain mixed.
Rick.

[tim12871]

Sogeka, using regrind at 40% you defiantly have a difference in melt, the virgin may not be seeing total melt down. I had a similar problem with TPE 50 to 1 concentrate the defect appears as white streaks and looking closer you could see the TPE was not mixing with the concentrate . Increasing back PSI slowing screw down lowering temps was hit and miss. I had to purchase a mixing nozzle and bundle from Synventive formerly Kona . My problem was solved. Hope this helps!

[Suhas]

Good suggestion Tim. Did the required pressure increase with the mixing nozzle?
Thanks,
Suhas

[tim12871]

Good suggestion Tim. Did the required pressure increase with the mixing nozzle?
Thanks,
Suhas

If memory serves me right, I think it was 100 to 150 psi.