Not every line has a side feeder. You can still run filler at a useful loading through the main feed, if you accept the limits.
Filler added at the main feed meets cold granules and a partly filled channel. The screw has to convey a mix of solid filler and solid granules, and that mix conveys poorly.
Expect lower output. Expect more torque. Expect the feed zone to become the bottleneck.
Use a large pitch and a deep flight in the feed zone. The channel needs volume for both the polymer and the filler.
Keep the first kneading block well downstream. A block close to the feed opening chokes the mix before it melts.
A starve feeder works better than a flood feeder here. Meter the filler and let the polymer granules carry it.
Watch for segregation in the hopper. Filler and granules separate when the hopper vibrates, and the mix that reaches the screw changes minute by minute.
Filler carries air. Air has to leave somewhere, and with no side feeder the vent does the work.
Check the vent filter often. A blocked filter raises pressure and pushes powder back into the feed.
Most lines reach a practical ceiling on filler loading through the main feed. Above that ceiling the torque runs out, or the vent blocks, or the strand turns rough.
Find your ceiling on a trial, not on a datasheet. Raise the loading in steps and note where the symptoms start.
A downstream feed port costs less than a new line, and it solves most of the problem. Some buyers add a simple vertical feeder on a spare barrel section.
Another option is a masterbatch. A highly filled masterbatch feeds through the main hopper more easily than raw filler does, and the screw sees a gentler mix.
Filler through the main feed wears the first elements harder than a side feeder does. Check that zone more often, and keep one spare stack.
Tell us the loading you want and whether you can add a downstream port. We will suggest a feed zone design, and we will be honest about the ceiling your line can reach.
The zone after the feed carries the mix into the melt. Use a short conveying section, then a mild kneading block. A hard block too early stalls the mix and raises torque before the polymer melts.
Keep one element of margin. A screw that runs at its torque limit on a clean polymer has nothing left for a filled grade.
Mineral filler carries moisture. Moisture turns into steam in the barrel, and steam pushes back against the feed.
Check the filler moisture before a trial. A simple oven test tells you more than the supplier certificate does.
Raise the loading in five steps and note the torque at each one. Stop at the step where the torque climbs faster than the output.
Write the ceiling in the line file. The next engineer then knows where the limit sits without repeating the trial.
A rough strand on a filled compound often means unmelted filler, not a die problem. Look at the screw before you touch the die.
