A colour change tells you how well your screw cleans itself. If the changeover takes hours, the screw holds material in places you cannot see.
Dead spots sit where the flow stalls. A flight land against a barrel step, a shallow groove in a kneading disc, a worn flight tip that no longer wipes the wall.
Each spot holds a pocket of the old colour, and that pocket releases slowly into the new product.
Look at the screw tips and the end of the shaft. Look at the gap behind the last element. Look at the seals and the die adapter.
Most colour complaints trace back to these areas, not to the screw body.
A line that stops with the screw full of hot melt leaves colour in every gap. A line that purges before the stop starts clean.
Run a transition compound through the barrel while the screw still turns. Stop with the barrel full of that compound, not with the coloured product.
Go from dark to light through a natural or a white compound. Go from light to dark through a mid tone. Two steps clean faster than one jump.
Raise the temperature slightly during the transition. The higher temperature lowers viscosity and helps the new material lift the old film.
A screw with a long metering section holds more material, and more material means more waste. If colour changes happen often, a shorter compression zone helps.
Ask about the dead volume when you order a new element. A small change in geometry can shorten the changeover by minutes, and those minutes add up over a year.
Weigh the purge that comes out during a change. Write the number in the log. When you change a routine, compare the new number with the old one.
Most plants cut purge waste by a third once they measure it, because the measurement shows which step actually costs the material.
A chipped flight or a gouged land holds colour that no purge reaches. Inspect the screw after a difficult change and replace the damaged element.
We supply elements with finishes that release melt cleanly, and we can quote geometries that reduce dead volume for lines with frequent colour changes. Tell us the number of changes per week.
The hopper, the feeder and the die adapter hold colour as well. A clean screw with a dirty hopper gives you the same complaint next morning.
Wipe the feeder screw and the hopper walls between colours. Use a vacuum, not compressed air, because air spreads pigment into every corner of the shop.
A liquid or a pellet concentrate doses into the feed and clears faster than a pre-coloured compound. The changeover shortens and the inventory of coloured resin drops.
Check the dosing point. A concentrate added too late mixes poorly and shows streaks even after a long purge.
The die adaptor and the screen pack trap old colour in corners. Clean both at every change, and keep a spare screen set ready.
Write the changeover steps on a card and hang it at the line. A routine that depends on one operator's memory breaks down on the night shift.
