Liquid color issue

[Suhas]

Good points above.

For now, if you are getting good mixing with higher back pressures and screw speeds, then set the back pressures and the screw speeds to these higher values and then adjust your pack and hold pressures (may need to go lower to compensate for the increased back pressure). If this does not help, try the pack and hold times (are you separating pack and hold or is it just one time?). Next adjust melt temps.

the best is to do a good DOE at the higher back pressures and temps.

Let us know so that we can all learn.

Thanks,
Suhas

[nikom]

Thanks for the input, I'll be doing a DoE soon. We have a smaller screw for the machine that is a barrier screw designed for acetal, but the supplier was not able to install it while they were here due to them missing (forgetting?) some parts.

For an update, we tried a different color that is a standard concentrate in pellet form. This eliminated most of the color 'swirl' issues we were seeing. However we are still seeing some discoloration across the knit line in a few of the cavities. It is not always the same cavities but it does seem to more frequent in certain cavities. Our next step will be investigating the hot runner, and older system from Mold Masters.

[nikom]

Another update, smaller barrier screw is installed. Mixing in the liquid color is very homogeneous but the knit line issue remains. Waiting to hear back from Mold Masters and hopefully get a technician out to visit from them.

[iamplwirtz]

Thanks for the update Nikom. Curious to hear of the resolution from Mold Masters.

[nikom]

A small update, we ran another tool that is a cold runner with the same shot size but different parts. No color issues were observed with this mold, but some processing changes were made that will hopefully translate back to the tool that was having issues.

After talking to Reiloy we are using a pretty severe reverse heat profile (410 front zone, 460 rear zone).
We installed a mixing nozzle body with four twist elements from PPE.
We have started mixing the liquid color by essentially rolling it around the shop before use.

Some additional steps we tried that did not help:
We tried using the smaller screw that was designed for mixing acetal and saw worse performance, possibly due to a short residence time of about 35 seconds.
We varied backpressue from 300 to 5000 psi and screw speed from 30 to 300 rpm.
Attempted various ways of feeding the liquid color into the material path and premixing the color with the pellets.
Mold Masters has not provided us with any technical support but has sent us all drawings of the hot runner system.