A side feeder is one of the most underestimated components on a twin screw extruder line. It looks like a small auxiliary unit bolted to the side of the barrel, yet it decides whether your filler, glass fibre or talc reaches the screw at a stable rate — and therefore whether your product stays within specification hour after hour. This guide from Joiner Machinery explains what a side feeder does, how to run one properly, how to select the right model for your line, and where side feeders are used in real production.

What a Side Feeder Actually Does

In a twin screw extruder, the main hopper feeds the polymer into the first barrel zone. Fillers, reinforcing fibres and mineral powders, however, cannot simply be dropped into the same hopper: they would bridge, fluidise or wrap around the screws before the polymer melts. A side feeder solves this by injecting the additive downstream, directly into the already molten polymer, at a controlled rate and at a controlled point.

• Downstream addition — filler meets the melt after the polymer is fully fluxed, so dispersion is uniform and the screws are not overloaded.
• Volumetric or gravimetric dosing — the feeder screw speed sets the throughput, which is why stable speed control matters more than maximum speed.
• Vent protection — material is pushed into the screw channel instead of blowing back through the vent port, keeping the vacuum system clean.
• Higher filler loading — with side feeding, 40–80% CaCO₃, talc or glass fibre loadings become practical without starving the main feed.

How to Use a Side Feeder Correctly

Most side feeder problems are installation and operating problems, not hardware problems. Follow this sequence at start-up and during every grade change.

1. Mount at the correct barrel position. The feeder should sit two to three barrel zones downstream of the main feed, after the polymer has melted but before the mixing section. Too early and the additive is not wetted; too late and there is no length left to disperse it.
2. Align the feed screw with the barrel opening. The screw tip must sit flush with, and slightly inside, the barrel bore. A recessed tip creates a dead pocket where powder compacts and eventually blocks the port.
3. Start the main extruder first, then the side feeder. Bring the line to steady melt condition and stable main feed before dosing begins; stop in the reverse order. Running the side feeder into a cold or empty barrel is the fastest way to shear a coupling.
4. Tune screw speed, not just setpoint. Increase side feeder rpm in small increments and watch the motor load and the melt pressure at the die. A rising load with a falling output usually means the port is flooding.
5. Keep the hopper under slight negative pressure. A small vent on the feeder hopper prevents air from being carried into the melt and avoids voids and surging.
6. Clean the port at every shutdown. Purge with a low-viscosity cleaning compound, remove the screw, and clear the barrel opening before the material cools.

Side Feeder Selection Guide

Selection starts with the material, not with the machine size. Answer four questions before choosing a model: what is the bulk density, is the additive free-flowing or cohesive, what dosing accuracy does the end product require, and how much room is there on the barrel?

• Bulk density below 0.3 g/cm³ (fumed silica, carbon black, wood flour) — choose a large-diameter, deep-flighted feeder screw and a tall hopper with an agitator; volumetric feeding struggles here, gravimetric is strongly preferred.
• Free-flowing pellets or granulate — a standard single-screw side feeder is sufficient and the most economical choice.
• Glass fibre and mineral fillers — a co-rotating twin screw side feeder gives self-wiping conveying and resists wrapping; wear protection on screw and barrel is essential.
• High accuracy requirements (±0.5%) — specify a gravimetric loss-in-weight version with a closed-loop control link to the main extruder drive.
• Retrofits — always confirm the barrel opening dimensions and the centre height of the existing extruder before ordering; adaptor flanges are available but change the screw tip position.

CWJ-35 reference configuration: screw diameter 35 mm, screw length 495 mm, L/D 10:1, screw speed up to 300 rpm, suitable for KY35 class extruders, with alternative motor and gearbox ratios available on request. Use this as a starting point and confirm the final drive sizing against your dosing rate and bulk density with Joiner's engineering team.

Typical Applications

• Filled PP and PE compounds — 20–50% talc or CaCO₃ for automotive interior parts, pipe and sheet.
• Glass fibre reinforced PA, PBT and PP — fibre added downstream to protect fibre length and preserve mechanical properties.
• Masterbatch production — high pigment and carbon black loadings, where accurate low-rate dosing sets colour consistency.
• Wood plastic composite (WPC) — wood flour at 40–60% loading, fed with a crammer-assisted side feeder.
• Recycling and refilling lines — calcium carbonate or glass bead addition to adjust density and cost.

Maintenance That Prevents Downtime

• Inspect the screw and barrel liner every 1,500–2,000 operating hours; filler service life is far shorter than polymer service life.
• Check the coupling and torque limiter after any blockage — a correctly set limiter slips before the shaft breaks.
• Replace hopper seals and vent filters when dosing accuracy drifts; air leaks mimic feeding faults.

Joiner Machinery has supplied twin screw extruder side feeders and replacement parts for extruder lines since 2003, in screw diameters from 35 mm upwards, with wear-resistant coatings and full dimensional interchangeability for major extruder brands. Send us your barrel drawing, dosing rate and material data sheet, and our engineers will confirm the correct model, motor sizing and mounting position for your line.