How are filaments made Know Your Filament Part 3
Anyone with an interest in 3D printing will be well aware of the mountain of jargon, science, engineering and often down-right confusion that surrounds 3D printing filaments. If you've ever wondered what crystallinity has to do with melting temperature or what on earth a 'block-copolymer' is then fear not. In this 4 part series of short articles, we're going to lay-out exactly what goes into a filament and why it's important for 3D printing, as well as debunking some common misconceptions about filaments.
In part 1 we covered what polymers and copolymers are and their importance in 3D printing.
In part 2 we looked at crystallinity and why it's so important to your filament.
In part 3 we'll briefly discuss what goes into a filament and how it's made.
Making Filaments - The Ingredients
By this point we're all pretty familiar with what a filament is, yet exactly how a filament is made may come as a surprise!
The journey to filament starts with a plastic resin, such as PLA or ABS, which is produced in the form of clear or white pellets. These resins do not contain pigments or any additives, they are the plastic in its raw form. Most resins are purchased directly from large suppliers. Some of the largest resin suppliers in the world are Dow Chemicals, Lyondell Basell, Exxon Mobil, and Ineos. Some resin producers specialise in specific plastic types, for example, NatureWorks specialises in PLA production. By the end of 2016, global annual resin production reached 335 million tonnes.
There are two broad families of plastic resins, the thermosets and the thermoplastics. The key difference between the two families is that thermoplastics can be heated and cooled multiple times without loss of integrity. On the other hand, Thermosets can only be melted once, after they have cooled they cannot be melted again, they just burn! Obviously, this makes thermosets entirely inappropriate for 3D printing. That's why the plastics we know and love in 3D printing are all thermoplastics.
Additives are added to the raw plastic resin to give it specific, desirable properties. Additives may be added when the resin is first synthesized, or during the filament production process by the filament manufacturer. Through the use of additives, a plastic resin can be optimised to meet certain needs.
There are far too many additive for us to name, instead we've made this handy table for you to see the different types of additive and what they do!
Additive Class Function Plasticizer Plastics are generally not inherently elastic. The use of plasticizer additives increases flexibility elasticity. Impact Modifiers Strengthens the plastic's impact resistance, this is particularly important for very brittle plastics. Fillers Are used to give the plastic more mass, making it heavier and stronger. Flame Retardants Do what it says on the tin - reduces the impact of flames on the plastic. Antioxidants A large family of additives that are used to reduce the wear and tear caused by chemical or sunlight exposure, which plastics are quite susceptible to. Colourants Add colour, obviously. It's not always so simple though - the colourant added must meet the same specifications as the resin, e.g. is it food safe? Antimicrobial Help the plastic to resist microbial growth. Thickeners Increase the viscosity of the plastic in its liquid phase. Effects the mixing of other additives as well as extrusion. Anti-stats Prevent the build up of static electricity and the resulting electrostatic discharge.
Making Filaments - The Process
The plastic resins arrive at the manufacturer in the form of clear or white pellets a few millimetres across. The first step in the process is to mix the resin pellets with an additive, like a colourant or an impact modifier, in a blender.
The second step is extremely important in producing high quality filament. The resin/additive mixture must be dried for about 2 hours anywhere between 60°C to 80°C. Skipping this step results in filament that is prone to popping and jamming.
The third step utilises one of the most important machines in manufacturing today - the single screw extruder (SSS). An SSS is basically a machine that heats and extrudes the product into a long filament. Here's where things get really interesting because this does not happen as you'd expect it to!
The common perception is that the resin is extruded through different sized nozzles to produce filament of difference diameters (1.75mm, 2.85mm etc.) but this is absolutely incorrect! In reality, the resin/additive mix is moved through several heated chambers to melt and mix it. It is then not pushed out of a nozzle (like a 3D printer), instead it is pulled out of a die of a fixed size. The speed at which it is pulled out of the die is what determines the filament diameter! Most manufacturers will immediately run the filament through a laser diameter gauge to check in real time that the tolerance is within a reasonable range.
The fourth step in the production process is the complex cooling process that the filaments must go through! This process is what determines the roundness of the filament, not the extrusion as you might expect! If the filament is cooled incorrectly you can end up with an oval shaped filament.
The filament is first run through a warm tank, often filled with warm water, which is what gently cools the filament and gives it its round shape. Cooling at the incorrect temperature here is what leads to misshapen filament. Next the filament runs through a cold water tank, down to room temperature.
The final step is wrapping the filament onto a spool, or wrapping for MasterSpool refills. Filaments have to be airtight and usually vacuum sealed as most of the plastics used are hygroscopic, meaning that they absorb water from the air. You can read about our blog post on how to store your filaments to protect them from humidity.
So there you have it, after 3 short parts you now know how filaments are made, what they are on a molecular scale and how this affects their properties. The more we know about the tools we use, the better we will be able to generate truly innovative solutions! To break the rules, we must first understand them implicitly.
Go forth and print, filament experts!
Some useful links and resources for further reading.