The Objective
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.
Thermoplastics
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.
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.
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:
- High Length/Diameter Ratio (L/D Ratio). Perhaps their most significant parameter, extruder screws are characterized by their long length in respect to their diameter. A high L/D ratio facilitates better mixing and improved thermal processing of the plastic. Common L/D ratios for screw extrusion typically range from 20:1 to 30:1.
- Compression. Nearly all extruder screws feature a tapered design where the depth of the thread or flight decreases from one end to the other. This compression plays important roles in melting the plastic and generating the pressure required to force the plastic through the desired geometry or die. The screw's compression is determined by the plastic being extruded.
- High lubricity. Extruder screws are often chrome plated to provide a slick surface that promotes material flow down its length. Screws with high lubricity are also less prone to wear and have a longer life.
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.
Extruding Filament
EX6 Extrusion Parameters and Tips
Download PDF HereRecycling Plastics into Filament
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