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Black splashes and nozzle degradation

Occasional black spots, streaks, or liquid-looking “splashes” in the extrusion, especially visible after running the same material for many kilograms without a nozzle change or purge.

Symptoms

Section titled “Symptoms”
  • Dark / black spots or streaks in otherwise clean extrusion
  • More visible when printing at low volumetric flow (slow or small prints)
  • Splashes appear on the purge line before a print
  • The material looks normal at high flow but shows contamination at low flow

Cause

Section titled “Cause”

Carbonized (burnt) material accumulates in the dead zones of the nozzle bore over time — especially in the corners near the tip. At normal printing pressure this material stays trapped. When the flow rate drops (travel moves, slow prints), the pressure drops and the carbonized material dribbles out.

This is accelerated by:

  • Long runs without color/material changes (no flushing action)
  • Printing at high temperatures for extended periods
  • Masterbatch use — colorant residue builds up faster
  • Running the machine with low or zero masterbatch after previously running with high masterbatch
  • Running at low flow rate after a long session without Liquid Masterbatch — the pressure drop exposes carbonized residue that accumulated faster without the flushing action of colorant. Pattern confirmed: black splashes appear specifically at low volumetric flow after sessions run without masterbatch.

Fix

Section titled “Fix”
  1. Change the nozzle — remove and replace with a clean nozzle; see Nozzle set
  2. Run a full purge at temperature with clean material before resuming
  3. If splashes persist after nozzle change → residue is in the barrel above the nozzle → Barrel clog purging procedure

Prevention

Section titled “Prevention”
  • Change or purge the nozzle every 5–10 kg of material throughput (adjust based on material and color)
  • After any long run with colored material, run a purge with clean/natural material before storing the machine
  • Keep extrusion temperature at the minimum necessary — higher temps accelerate carbonization

Notes

Section titled “Notes”
  • Confirmed by a customer after ~5–8 kg without masterbatch: black splashes appeared at low flow → resolved by nozzle change
  • This issue is easy to misdiagnose as a masterbatch problem — the key clue is that it appears without masterbatch or after a long run
Section titled “Related notes”