In a recent processing forum there was a common question regarding color swirls and colorant mixing. There were many common responses such as adding back pressure, speeding up screw speed, slowing down screw speed, changing screws, adding mixing elements, etc.
Let’s get back to basics for a moment…
RE Colorants: The aspect particular to colorants is that they have a low molecular weight carrier to allow it to melt and disperse the colorant early in the screw. If the carrier is compatible with PP, the suggestions below should help you.
RE Melt Temp: If the melt temperature is too low, the carrier will melt too late in the barrel and will not have time to melt, especially in a GP screw. This is why it is critical to know what resultant melt temperature provides the correct color with you colorant and base resin. Too low a temp will provide poor mixing, too high a temp will degrade the colorant resulting in off color or off gloss. If you need more heat, add heat through barrel temperature. If you need more mixing, then you add back pressure, but only after you have verified the melt temperature is correct. Screw RPM can either add heat by breaking chains or lower temp by pumping un-melted pellets to the front of the screw… neither is good.
Re Documentation: The above is why to have good process documentation so you know what specific process parameters give you good parts including melt temp (measured using a thin k-type probe), recovery time, & back pressure (converted to/from plastic pressure if you are going from machine to machine).