How-To & Guides7 March 2025

A Complete Guide to PET Extrusion on Desktop Equipment

PET is one of the most widely recycled polymers in the world and one of the most rewarding to process on desktop extrusion equipment. This guide covers drying, processing temperatures, screw speeds, and the specific challenges that trip up first-time PET extruders.

Why PET Is Worth the Effort

PET occupies an unusual position in the desktop extrusion material library. It is abundant, inexpensive (particularly in recycled form), produces filament with excellent print characteristics, and the resulting printed parts have good strength, moderate temperature resistance, and low moisture absorption. PETG — a glycol-modified variant — has become one of the most popular 3D printing filaments precisely because of these properties.

Extruding PET from raw pellets or regrind allows researchers and manufacturers to control exactly what goes into their filament — particularly important when working with recycled streams or when adding functional additives to a PET matrix.

Drying: The Non-Negotiable First Step

PET is hygroscopic — it absorbs moisture from the atmosphere readily, and the consequences of processing undried PET are severe. At melt temperatures, water hydrolyses the ester bonds in the polymer backbone, causing molecular weight reduction that permanently degrades mechanical properties and produces a brittle, low-viscosity melt impossible to draw into consistent filament.

PET must be dried to below 50 ppm moisture before processing. This typically requires 4–6 hours at 160–170°C in a desiccant dryer with circulating air. A conventional oven will not achieve the required dryness. Once dried, PET absorbs moisture rapidly at room temperature — transfer directly from dryer to extruder hopper without delay.

Processing Temperature

Virgin PET typically processes in the range 270–290°C barrel temperature. Recycled PET, which has undergone thermal history during its original processing and use, may require slightly lower temperatures to compensate for degradation-induced viscosity reduction. Observing the melt viscosity through motor torque during initial processing provides the best guidance for temperature setting.

PETG processes at lower temperatures — typically 230–250°C — due to the disruption of crystallisation by the glycol co-monomer. Its lower melt strength makes it somewhat easier to draw into filament, which is part of why it has become the preferred desktop variant.

Common Problems and Solutions

Bubbles in filament: Almost always moisture. Return to dryer for extended cycle.
Brittle filament: Degradation from excessive temperature or moisture. Check drying and reduce barrel temperature.
Diameter variation: Screw speed instability. Consider closed-loop servo control.
Yellowing: Thermal degradation. Reduce barrel temperature or increase screw speed to reduce residence time.
Stringing at die: Melt too hot or too slow. Reduce temperature slightly or increase output rate.