Problem with polymer feeding

[Jorge Villegas]

Hello. It´s good to see a community around the "fantastic" world of plastics.

I´ve been dealing with a problem, but i´m all out of ideas. we have various brands of injection molding machines, one of them is a Toshiba with a screw size of 22mm. As soon as the machine got here, it had problems feeding the material (PP). Toshiba sent us a new screw. The problem improved a little, but we still have the problem.

Problem description:

With heavily coloured materials (PP) the screw just won´t feed, it seems as if the material slips around the screw. It also happens with polymer regrind. I´m limited to 20% of regrind, more than that the feeding times are very unstable. I´ve tried grooving the cylinder, and changing the screw with one from another manufacturer. The feeding response improved, but I still have the problem sometimes. The funny thing is, we have another machine (different brand), same screw size, and the feeding reponse is very good. I´ve tried comparing screw designs, but they are the same.

I would like some feedback, if someone has already been through the same!.

I have a video of the problem but i don´t know how to upload it here, if someone has an insight of the problem i could send it by e-mail.

Thank you.

[rickbatey]

I think you need to check the actual psi required to rotate the screw and confirm that your barrel heats are too low. Remember that the set point may not be as close to the actual due to bad thermocouples or bad/weak heaters and a bad controller. Just because machines are similar don't mean that tge settings won't require tweaking for the best performance. NO TWO MACHINES ARE EVER THE SAME!!
Rick.

[Jorge Villegas]

Thank you for your answer. I`ve already checked the status of the barrel heats, they are ok. The machine is electric so it measures torque. Something i`ve noticed is that when charge times go up, the torque needed to move the screw goes down. A sign that the pellets are rotating with the screw and not moving forward.

[brentb]

What kind of colorant and what LDR (loading?) Is pp "barefoot" or does it have lube, anti-stat,or anti-block?

KOM

brent

[rickbatey]

If the torque drops then it's a sign that you're too hot in the rear or too cold from the center forward. That's why the pellets are sticking on the screw. Do you ever see signs that you're over heating the resin? Cold slugs or pellets? Then start raising the temps. Brent brings up valid points concerning additives as well. Are these pellets or strands of resin?
Rick.

[Valentin Leung]

For PP, we had issues with a lubricant. :)
In one of our part, we added lubricating agent and this stuff would make the resin spin with the screw. I think that in the mixing zone, the resin needs to stick to the surface of the barrel, and in order to stick, you need to compress them at the right pressure and the barrel needs to be at a temperature high enough to give the right amount of energy to help the melting process while shearing.

With the lubricating agent, no matter how we set up the process, material will just spins around and around. With virgin material, it was fine, we added it on PP and HDPE, PP would allow us up to 1:100 and HDPE 1:100, beyond that it will spin, use of regrind was problematic, the lubricant migrates to the surface of a part while cooling, so in the mixing zone, the pellets were just moving around without never creating enough shear stress to mix. So you might want to check with your supplier and see if the color they use is any special.

Can you buy the colorant in a masterbatch and mix it down instead of colored pellets?

[Jorge Villegas]

Thank you for all your responses.

I`ve found that if I modify the amount of a lubricant that is used in the color, it actually got better. a lot better. And the thing that solved the problem was using a volumetric dosifier for the Polymer, the color and the regrind.

Regards!