PLA is Poly-Lactic Acid filament, and it's usually the go-to choice for people looking to print larger models or for longer periods of time. The important properties of the material dictate the reasons for using it; it doesn't warp at all and can print fine in cooler environments.
Most of the things I have seen say that 90% of the time, PLA is simply a better material to be 3D printing. That may be true, but now that I have some experience with our printers & have helped a few customers with theirs, a couple of issues surrounding the use of PLA filament have surfaced. It seems that although you can load it into your tool head and print with it, sometimes things happen not entirely as we would expect.
Filament grinding & extruder issues from the get go are things we've seen at Core - we have our Lulzbot 3D Printers going all the time. Usually, we get a fix quickly for these problems, so we've decided to make a straightforward walkthrough that will show you how to do swap your filaments out and prime your print head successfully.
A quick filament swapping process
There's a primary filament swapping process you should follow with your printer, just like in Our First Print video. You should be:
- Heating the filament in the tool head up, at the very least to above its glass transition temperature
- Safely remove the filament from the tool head once at this temperature
- Insert your new filament into the tool head
- Set the new extrusion temperature
- Extrude any remaining filament before printing with the new type.
Essentially, if you follow this process, you will be able to print any filament you choose with no filament issues.
That said, there can be some interplay between plastic types when you are regularly swapping out print material. Take ABS and PLA for example; we know that at higher temperatures of 230-240 degrees, PLA can begin to crystallize. That can be a real issue if we are swapping our ABS to PLA constantly. Our question is that when swapping out our filament from PLA to ABS or vice versa, how do we make sure we aren't cooking our filament above its recommended temperature?
Hot-End Design and Operation
Well, that all comes down to the operation of our hot-end, and how it works. And to understand that we need some understanding of plastics. The filament is essentially the ink for our 3D printer; it’s made up of synthetic polymers (these are collections of atoms that chain together on an atomic level). Additional materials (additives) can combine with the base polymers that will enhance/change their properties. These additives are a big part of how we get different colors and strengths for our filaments.
At room temperatures, our filament is a hard plastic that won't be printable, and as you know, we heat it up so that we can melt it and print it. We tend to see 3D printing as this practice where our filament is going into our hot end as cold and hard plastic and coming out as the soft and runny filament; as if it goes from rock solid to runny in one simple motion.
That is not entirely the case, there's a point in the heating up process, where the properties of our plastic begin to change dramatically. We call that point the glass transition temperature (listed in the datasheet of your filament). At the Glass Transition Temperature (Gtt), your plastic loses some of its rigidity (or we say the viscosity of the plastic increases) and continues to grow for every degree we go above this point.
The Hot End Heat Gradient
So our hot end is made to reach the 'extrusion' temperature but stay above the Gtt, which seems logical enough as it's the perfect state to 3D print in. The hot end design is such that in the heater cartridge near the extruder, the temperature of the plastic is high enough to lay our filament down simply. Just above that heater, we have a part called the heat break, and that is where our plastic reaches this glass transition temperature, becoming soft and malleable.
The cool thing I learned when researching extruder design was the plastic forms a seal between the hot plastic and cold plastic in the extruder. This seal forms in heat break above the heater cartridge, and as the plastic seal melts it slides down into the heater cartridge for extrusion more filament melts to replenish the seal! So at any point below our heat break, we expect the plastic to be hotter than the glass transition temperature. Whereas inside and above the heat break we expect our plastic to remain cooler and harder than the Gtt.
Jamming and Grinding
The common problems we tend to see with PLA printing is grinding, where the filament is eaten away by our feed gear and extrudes 'cobwebs.' It's that or we just get no extrusion at all without any sign of grinding (Jamming).
Let's run through that process now with our Lulzbot Mini. We'll be swapping some ABS to PLA and printing with it. I have some ABS in the tool head. I've got the Mini heating up to around 210 degrees, which is a great purging temperature for most of the conventional plastics we use in 3D printing.
To remove the ABS, we will undo the tensioner, and pull on the filament to get it out of the hot end. Then we will unload our filament spool of ABS and load up the PLA spool. Now we will take the PLA filament and load it into the extruder.
While the printer is still up around 210 degrees, we are going to extrude (or purge) the remaining ABS and new PLA, which you will see coming out of the extruder.
Once we have a consistent extrusion, we will be all set to print.
Sometimes, especially with PLA, we can have heat creep up the filament, past the heat break and cause our filament to set above the heat break. We've noticed that this seems to happen toward the end of prints. The next time we go to print, our filament seal is hard-set and won't get up to the right temperature to extrude. The plastic isn't moving, but our feed gear keeps turning. The gear will eventually chew out our filament, and the small wisps of filament that do come out of the hot end will look stringy cobwebbed. To fix this, we want to make sure:
- The cooling fan is on during our PLA prints
- We aren't printing inside an enclosure that insulates the heat of the printer.
These two things will ensure our heatsinks do their job, and we can happily PLA print over and over with no issues.
Alternatively, if we have some ABS or other filament left over in the heater cartridge, they could be jamming our extruder. Usually jamming happens if the filament isn't removed cleanly and can be fixed (90% of the time) with a long (100mm) purge at 210 degrees.
That pretty much brings us to the conclusion of our PLA printing guide. Thanks for watching, good luck with your prints!
