Polylactic acid (PLA) is a bio-degradable polymer that can be produced from lactic acid, which can be fermented from crops such as maize. This makes it an ideal candidate for use in certain energy rich, cash poor areas of the world.
It is harder then PTFE and melts at a lower temperature (around 180°C to 220°C), and a glass transition temperature between 60-65 °C, so is potentially a very useful material. It does exhibit higher friction than PTFE however which can make it difficult to extrude and more susceptible to extruder jams.
Usage
PLA is the ideal material for a Mendel RepRap. It is dimensionally stable, so there is no need for a heated bed. It is relatively inexpensive, and is not hard to source in filament form.
You can get slightly higher quality surface finish with ABS over PLA, but on the whole PLA works better in the machine, requiring lower temperatures and giving stronger, more hard-wearing products. It does have a slightly higher coefficient of friction in the drive and transport than ABS, but this is more than compensated for by its lower viscosity when molten. This means lower pressure in the melt chamber and hence a lower driving force.
Heater Settings
Depending on which material you use, you should adjust your heat accordingly.
4032D requires higher temperatures and may need to be set as high as 230°C
4042D should extrude at 190°C
4043D 160-220°C around 180 is a good start (see http://ultimachine.com/pla)
Extrusion Width
There has been some evidence that pigment may affect extrusion width. If you are switching plastics a lot, it is a good idea to measure the extrusion before going through the toolpath process.
Build Surface
PLA bonds very very firmly to Acrylic, and it is not recommended to print directly on an Acrylic build surface.
It does stick well and is removed easily from various forms of Tape.
Heated Bed
It can also be printed on Polyimide (Kapton) that is pre-heated, but will be hard to remove until both the part and the surface are cooled.
It can also be printed directly onto heated glass
Polycarbonate
PLA prints nicely directly to a very lightly oiled sheet of polycarbonate/Lexan. Some tips for printing on Polycarbonate:
- If you have trouble getting it to stick, try raising your extrusion temp a few degrees. At 185C I have no trouble, but at 180C it is almost impossible to get a good first layer. On large prints, bumping the print temp by 5 degrees or so can improve first layer bonding.
- another method that helps PLA stick to the printing surface that can be used with or without heatbed, is wiping the surface with a bit of lemon juice. the sugars (and?) the acid in lemon juice help PLA stick very well to print surface even with big prints.
- Use a VERY small amount of oil. Too much and it will not stick, too little and it will stick too well. I wipe a small amount of cooking oil on a a paper towel and wipe that on the surface. it should feel just slightly oily. It takes practice to get it right, but it is not hard.
- Start your print a bit higher than you would expect. I do about .45-.6mm for .35mm layer height, and the prints just pop off when done.
- Keep a chisel on hand just in case. If you surface isn’t oily enough or you start too low, it will bond pretty tightly. You will need a chisel to remove it and scrape off the remnants. With practice you will rarely need it, but keep it on hand just in case.
- No Heated build platform is necessary, and I have not tested it with one.
- If you are in the US, Home Depot sells an 8x10x0.093″ piece of Polycarbonate for under $4. It attaches perfectly to the existing bed of your Prusa with double sided tape. It will need to be replaced as it gets scratched up, but it will be usable for many prints, and is far less hassle than blue tape.
- Home Depot also sells acrylic and clear styrene sheets for less money. They may work fine, but I have not tested. I know acrylic is not recommended without oil, but I am not sure if anyone has ever tested it with oil.
- This is tested with PLA only, I have not tried it with ABS.
Moisture Issue
PLA can absorb moisture from the air. When it is heated this moisture can turn to steam bubbles which with certain hot end (extruder head) designs can interfere with printing. The symptom is that when the extruder motor stops the PLA kept coming out. When the stepper starts again there is a significant delay. Occasionally the tip may blow a bubble with a tiny puff of what looked like steam.
Small amounts of PLA filament (Natureworks PLA4043D has been tried) can have some moisture removed by putting it on a piece of aluminum foil in an oven heated to 170F for an hour. The filament in the oven is floppy, but sticks to itself only slightly. Flexing the coils after cooling unsticks them from each other. Heating a whole spool this way has not been tried, and may result in the spool becoming unusable, so caution is advised.
Interestingly, a weight change can be seen after baking. One coil went from 120.5 grams to 120.0 grams (almost 1/2%).
Color Influence
It was observed that the color of the PLA (maybe the 4042D from ultimachine) seems to alter some of its properties (from multicolour prints)
- Black is lovely and glossy about 75% opaque
- Yellow is clean and precise when extruding at 196°C, it layer bonds very well and the printed parts feel very similar to ABS, being strong but with a little give
- Black and Yellow don’t snap when you bend it, unlikely for blue/green
- The blue is particularly odd giving micro-bubbles inside the extruded filament if I run it at 196°C, but these are minimized at 187°C.
- The blue/green is more brittle in it’s filament form, but produces a very hard part when extruded.
- Green filament fall between Yellow and Blue, looks really nice when printed, it seems to give the best definition of the printed object.
- The Red is more ‘sticky’ and is a little prone to very fine strings, maybe a change in temperature will resolve this