Skip to content
Wiki
Main site

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:

  • Time at temperature. Long sessions, and zones set hotter than the material actually needs
  • Long runs without a purge or a material change — nothing sweeps the dead zones out

Low flow does not accelerate the build-up. It is what makes the build-up visible, which is why the splashes turn up on slow prints and on the purge line rather than mid-job at speed.

Fix

Section titled “Fix”

Work up in cost. The nozzle is not a consumable, and most of the time it does not need to come off.

  1. Purge at temperature with clean material. This is the cheap first move and it clears residue that has not hardened — see Material change and purge
  2. If the splashes survive a full purge, change the nozzle — see Nozzle set
  3. If they survive that too, the residue is above the nozzle, in the barrel → Barrel clog purging procedure

Prevention

Section titled “Prevention”

There is no service interval for the nozzle. It is not a consumable you replace on a schedule — you change it when it stops doing its job, and black splashes at low flow are one of the ways it tells you.

What actually reduces the build-up:

  • Purge between materials and colours, and give it more than feels necessary — see Material change and purge
  • After a long run with coloured material, purge with natural before leaving the machine standing
  • Keep the zone temperatures no higher than the material needs. Time at temperature is what carbonises residue, so the cheapest prevention is not running hot “to be safe”

Notes

Section titled “Notes”
  • Easy to misdiagnose as a masterbatch problem. The clue is when it shows: at low flow, after a long run, on a machine that has not been purged through
Section titled “Related notes”