The Objective

Extruding PETG filament through Filabot EX6 extruder.

PETG extrusion for 3D printer filament.

There is a growing number of hobbyists who are interested in plastic extrusion for either creating their own 3D printer filament or for injection molding of small parts. However, screw extrusion is not an intuitive process, largely due to the complex rheological properties of plastics and their tendency to degrade when handled incorrectly. The goal of this guide is not to provide a comprehensive overview of polymer (i.e., plastic) processing but instead to hit the most important considerations when extruding plastic on a small scale (< 5 kg/hr) with commodity plastics. This is a companion guide to my two videos on 1) Filament Extrusion and 2) Plastic Recycling (coming soon).

For more detailed analysis of plasticizing screw extrusion Chris Rauwendaal's textbook on Polymer Extrusion is an excellent resource.


Plastics can be broadly classified into two major categories: thermosets and thermoplastics. Liquid thermosets are chemically reactive resins that are mixed together or with a catalyst to form a solid component after curing. A two-part epoxy is perhaps the most well-known liquid thermoset. Once a thermoset has been cured, it cannot be returned to its precursors (liquid, powder, etc.) through thermal processing. In fact, instead of having a melting temperature, thermosets have a decomposition temperature because they burn once a certain temperature is reached.

Thermoplastics, on the other hand, can be melted, cooled, and re-melted multiple times without undergoing a significant chemical change. When melted, the molten plastic can be poured or injected into a mold, extruded through a die/nozzle, or thermoformed around a desired geometry. When cooled, the plastic regains its impressive strength-to-weight ratio. The ability to reform thermoplastics without loss of their desirable properties allows these materials to be recycled. If a plastic is sold as a pellet, then it is almost always a thermoplastic.

Plastic pellets for extrusion, including PLA, ASA, PETG, HIPS, ABS, and TPU
Commodity thermoplastics in pellet form that can be readily extruded.

Thermoplastics are incredibly versatile materials that can be rapidly shaped into end-use parts when heated above their melting point. When cooled, the plastic regains its impressive strength-to-weight ratio. However, large volumes of plastics are difficult to melt due to their thermal conductivity that is, on average, 1000x less than metals. Further exacerbating the problem is the tendency of plastics to degrade when held above their melting point for long durations. Therefore, conductive heating (i.e., placing the plastic in an oven) is unsuitable for the thermal processing of polymers.

Screw Extrusion

The screw extruder was invented to overcome the challenges of efficiently heating significant quantities of plastics. This design relies on an Archimedean screw to melt, mix, and force plastic out of a die. Pelletized material is fed into a barrel and screw assembly, where the screw's rotation initiates the melting process through inter-pellet friction. When plastics melt, they become viscous fluids that resist the conveying motion of the screw. This resistance further heats the material, which is known as viscous or shear heating. The most important takeaway of this process is that the heating comes from within the plastic itself. As a result, the relatively low thermal conductivity of the plastic is less likely to impede the melting process.

Extruder screw spinning - animated GIF

Plasticizing Screw Design

There are many exotic screw designs, but this section will focus on a simple screw with a single compression region. While I used the adjective “simple,” these screws are still far more specialized than a metal drill bit. Low cost extruders that use augers, drill bits, or other home improvement screws should be avoided. What makes an extruder screw unique are the following properties:

Anatomy of a screw for a plasticizing extruder. Highlighting different levels of screw compression, materials (chrome vs nitrated) and sections of the screw (feed, compression, and metering).
Extruder screws with varying levels of compression that are compatible with Filabot's EX6 Extruder.

Size Matters

Larger screws have more space between the flights to convey and melt the plastic pellets but will require more power to turn. In an attempt to make plastic extrusion more accessible to hobbyists, low-cost extruders with short screws and low powered motors have been offered (e.g., Filastruder). While cheap, these designs have a low output and cannot sufficiently mix colorant and other additives with the bulk material, yielding an inconsistent product. Extrusion speeds can be as slow as 100 grams per hour! For time and quality-sensitive applications, larger extruders should be used. The Filabot EX6 extruder has a screw that is twice as long as the Filabot EX2, but 4x the output.

Line up of hobbyist filament extruders, including the Filabot EX6, Filabot EX2, and Filastruder
Hobbyist filament screw extruders.

Extruding Filament

Filament extrusion instruments, including screw extruder, air cooling, filament diameter measuring instrument, and spooler.

EX6 Extrusion Parameters and Tips

Download PDF Here

Recycling Plastics into Filament

Recycling plastics into filament start with shredding the waste plastic, then granulating it, then extruding, then pelletizing and finally extruding once more.


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