Additives & Composites (Vol. 2)

Additives and composites bring even more diversity on a market already full of polymers. Yet these remain simple concepts accessible to anyone.

It is highly recommended to first read the Additives & Composites Vol. 1 before you move to this article.

Have you heard of conductive filament ? Copper-filled plastic ? How can PLA be sometimes transparent and sometimes of a deep dark blue ? All of these filaments contain additives, that’s what makes them special. Let’s dive into two related concepts: additives and composites.

How to process a polymer + additive ? You can find an introduction to extrusion here. In a nutshell, extrusion is the process that transforms pellets or powders into filament or other “continuous items”. The goal here is to manufacture a filament of plastic, while loading it with certain fillers, for all kinds of applications. How do we guarantee a good dispersion of the additive inside the polymer matrix, while preserving the high quality of the filament ?

Twin-screw VS single-screw

There are two main ways to "compound" composite filaments. Each way corresponds to an extrusion method.

  • Twin-screw extrusion. Double the fun, right? Those extruders are simply equipped with, well, two screws. The screws rotate side by side and generate a turbulent flow: great for mixing, but it is impossible to control the diameter of a filament. Twin-screw extrusion will give you compounded pellets, not filament.
  • Single-screw extrusion. Because only one screw is involved, the mixing capacity is lesser, BUT this is the only viable method to produce good filament.

So which one do I need ?

It depends. The best way is to use both. First, you must compound the plastic and fillers into pellets, via twin-screw extrusion. You can then feed those pre-compounded pellets in your single-screw extruder to produce good quality filament. With this method, you can achieve the highest percentages of fillers, which will be homogeneously dispersed. The fastest way is to go straight for single-screw extrusion, feeding in the filler and the plastic like you would pour different ingredients at the same time in a blender. This method will sometimes suffice and result in great composite filament. It is possible to integrate additional mixing mechanisms in the design of the machine, such as a mixing section in the screw.

Different types of additives There are many types of additives: metallic powders, ceramic powders, ground coffee, glass fiber, wood... But knowing what additive you want to use does not suffice to predict exactly what filament you will obtain. The filament-making process will depend on:

  • filler type: powders are easier to process than fibers
  • filler particle size: the smaller, the better
  • filler percentage: the smaller, the easier
  • polymer grade: viscous polymer work best for extrsion
  • polymer form: pre-compounded pellets work best, then powder/powder mixes, then the other types of additives/plastics
  • settings used for extrusion: as always, the process must be well-tuned
  • whether the material was pre-compounded or not: if yes, there are almost no limit

Case study: Colorant masterbatches

It is surprisingly easy to customize the visual aspect of a 3D printing filament using colorants. A masterbatch is made of the same polymer as the one it is to be mixed with, but is also carries a high concentration of filler - in this case, colored pigment. Typically, including only 2%w of pigmented masterbatch, along with 98% virgin plastic, will give a homogeneously, strongly colored filament. One might even find it enjoyable to search for the perfect tone by mixing different masterbatches in different proportions, or to create rainbow filament. The usage of masterbatches may not be the most appropriate method for mass-production of colored spools, because it is not the most reliable process when it comes to replicating precisely the exact same color tone in dozens of spools. It is however the preferred method to experiment with colorants and create smaller batches of custom filament. You can find a case study about the making of colored PLA filament here.

Conclusion If you want to produce a filament filled with a special additive, here are the takeaways:

  • it is recommended to pre-compound the filler inside the plastic, during a preliminary twin-screw extrusion. It will allow you to achieve the highest percentages of filler, and work with the most difficult ones, such as fibers.
  • it is sometimes possible to skip this pre-compounding step by jumping straight into single-screw extrusion. Especially when using masterbatches. In that case you will produce filament will mixing the matter at the same time.

There is a lot of room to experiment with the polymer grades, the additives, the combinations of both, the parameters,... But with a bit of experimentation, sooner than you think, you might obtain some PEEK filament containing 250% (weight) of steel!

A word about recycling additives When talking about composites in general, the main concern is that they are manufactured specifically with a certain infill and a set shape. It is then impossible to separate the reinforcement from the matrix. As far as 3D printing is concerned however, because the additives have to be small in size and well dispersed in the plastic, they can also be recycled with it. The process is rather straightforward:

  • Shred the printed parts, typically 3D-printing waste
  • Obtain a regrind which is still compounded, instead of obtaining pellets or powders
  • When the particles become more or less as small as pellets (a few millimeters), they can simply be extruded again, almost like virgin plastic, while still containing the fillers.
Certain fillers such as carbon nanotubes being rather expensive, recycling them becomes very advantageous.