How-To & Guides22 November 2024

Purging the Barrel & Screw: A Step-by-Step Guide

A step-by-step guide to properly purging your extruder barrel between material changes — preventing cross-contamination, reducing waste, and maintaining filament quality across different polymer types.

Why Purging Matters

When switching between materials — particularly between different polymer families — residual material left in the barrel and on the screw will contaminate your next extrusion run. Even small amounts of contamination can affect the mechanical properties, appearance, and processing behaviour of the new material. For research-grade work, thorough purging is not optional.

The Three-Stage Purging Process

Effective purging involves three stages: a mechanical purge to displace the bulk of the previous material, a chemical or thermal purge to clear residues from the screw flights and barrel wall, and a confirmation pass with the new material before beginning production.

Stage 1: Mechanical Purge

With the barrel at the processing temperature of the current material, feed a commercial purging compound or compatible transitional polymer at moderate screw speed. Run until the colour and appearance of the extrudate matches the purging compound without traces of the previous material — typically 3–5 barrel volumes. Increase screw speed briefly at intervals to generate high shear.

Stage 2: Temperature Transition

If the new material processes at a significantly different temperature, adjust the barrel setpoint while continuing to run purging compound. Do not allow the barrel to sit static during a temperature transition — keep the screw turning slowly to prevent material from sitting in a dead zone and degrading.

When transitioning from a high-temperature material (e.g. PEEK at 380°C) to a lower-temperature material (e.g. PLA at 200°C), purge at the high temperature first with a high-temperature purging compound or HDPE, then reduce temperature while continuing to purge.

Stage 3: Confirmation Pass

Once the purging compound runs clean and the barrel is at target temperature, introduce 100–200g of the new material and extrude to waste. Inspect the extrudate carefully for colour contamination, black specks (degradation products), and surface defects. Only begin production once the confirmation pass is clean.

Selecting a Purging Compound

For general-purpose purging between commodity polymers (PLA, ABS, PETG, nylon), a high-MFI HDPE or commercial purging concentrate is appropriate. For high-performance polymer transitions (PEEK, PEI, PEKK), specialist purging compounds rated for the required temperature are essential — HDPE will degrade at these temperatures.