These guides are intended to give more information about what you can do with your MakerBot 3D Printer.
PLA, or Polylactic Acid, is a biodegradable plastic with a lot of features that make it great for 3D printing -- it doesn't give off fumes like ABS does, or warp nearly as much. It's also really shiny.
PLA is harder than ABS, but more brittle, but that doesn't mean that it's easily breakable -- it's actually super strong -- just that when it does give, it's more likely to snap than bend. And it also stays flexible for a short while as it cools, which can be handy. And it doesn't warp and crack on larger objects the way ABS can. Basically, each is better for different kinds of things, and getting comfortable with PLA will give you choice and flexibility.
Before you start printing, there are a couple of things you should do: first, if your extruder is compatible, make sure you're using a MK8 nozzle. If you're printing with The Replicator you've already got one, and if you have a MK7 Stepstruder it's an easy upgrade. Then forget what you might have read about printing with PLA in the past -- there's a lot of information about PLA out there, but much of it won’t apply when you're printing with an MK8 nozzle.
MK7 v. MK8 The main difference between the MK8 Stepstruder and the MK7 is the ratio of melted to unmelted plastic inside the heated nozzle. You don't want PLA to stay melted for too long, because it will start to degrade, but the new design should eliminate that problem. About the same amount of plastic fits into each nozzle, but in the MK8 nozzle, more of that plastic remains solid. That means that any given bit of PLA will spend less time sitting in the heated nozzle in it's liquid state, and won’t have much time to degrade.
When you first load your PLA filament, you'll want to make sure all the ABS filament has been purged before you start printing. Running the extruder for 300 seconds should flush out all remnants of ABS. Purging old filament will also get rid of any degraded PLA, so if your bot is already loaded with PLA filament and you're worried that the plastic inside the extruder has been heated for too long, run the extruder for a little while to get fresh plastic into the extruder -- fifteen seconds should do it.
The Anchor Have you noticed the little blob of plastic that your extruder puts down before it starts printing? That's the anchor, and one of its functions is to get rid of the plastic that's been sitting in the nozzle as the extruder and build plate heat up. If your PLA has been sitting in the nozzle for too long, you may notice that the anchor has a slightly brownish tone.
Before you start printing, you should also make sure you're using an appropriate build surface. Blue painter’s tape makes a great surface for printing PLA -- the PLA will adhere well whether or not you're printing on a heated plate, and your printed objects will be pretty easy to remove. You should be able to find blue tape at any hardware store. The Acrylic plate on the Replicator 2 is another great printing surface, although sometimes items stick so well to the acrylic that it's difficult to remove them. For those occasions, we recommend a thin metal metal spatula. Users have also reported that a heated glass plate is great with PLA.
You can print with PLA on Kapton tape, but it's not a good idea. In order to get it to stick to the Kapton, you have to heat your platform, sometimes just as much as you would with ABS. And PLA retains heat, so the hot platform can degrade the plastic and have negative effects on your print, like causing the lower layers to sag or discolor.
The default profile on The Replicator is calibrated to work well with both ABS and PLA, but it's a good idea to learn how to change your Heated Build Platform temperature. PLA will stick well to blue painter's tape even on a cool build platform, so for most prints you can just delete the line of code that heats the platform altogether.
PLA can warp a little in cold spaces, or on large objects. There are a couple of ways of dealing with those conditions: if you have a heated build plate, you can set the platform temperature to 40° or 50° C, which will relax the lower layers of your print without overheating them. The easiest way to deal with this, though, is to have an active cooling fan cooling down the PLA immediately after it exits the nozzle.
Small Details PLA melts at a lower temperature than ABS, but we find it prints really well at higher temperatures -- 220° C for slow speeds and 230° C with acceleration. There's a theory that when you print PLA with high temperatures, heat creeps up the filament and melts it just enough that the drive gear can't pull it into the extruder effectively. That should not be an issue with stock MakerBot extruders since the Stepstruder MK7. PLA failures are more likely to be due to a worn or miscalibrated Delrin plunger. That's the case for ABS too, but PLA, beng harder and more brittle, may be more prone to these kinds of failures.
Printing with PLA can be a little bit tricky, just because it's more sensitive to high temperatures than ABS. Here are some tips to keep your PLA printing process smooth.
Most problems people encounter when printing with PLA arise from the PLA degrading after being heated for too long. If your extruder motor starts skipping instead of drawing filament into the extruder, that's probably the issue. Remove the filament, cut a new edge and reload it. Then purge the old plastic before you start printing again. You also might want to try lowering your extruder temperature a little. Try to avoid letting heated PLA sit in your extruder for too long.
On the other hand, you might have more extrusion problems if you've set your extruder temperature too low. That's because PLA uses a lot of heat to change from its solid to its liquid state, and it's actually pulling that heat out of the nozzle and cooling the nozzle down. If the nozzle isn't hot enough to keep melting PLA, the filament inside it can harden. This shouldn't be an issue unless you're printing at very low temperatures or at very high speeds.
Slow speeds can also cause extrusion problems, and if your print keeps failing at its narrowest point, that might be the issue. The Cool plugin in Skeinforge makes sure you spend a certain amount of time on each layer before moving on to the next one, and on layers that cover a very small area, the feed rate can slow down so much that your PLA will degrade in the nozzle. Go into Skeinforge, select Cool, and change the Minimum Layer Time from 5 seconds to something shorter -- 2 seconds should be enough.
If your print fails at a point where extrusion stops and starts a lot, or on small details, the Restart Extra Distance setting might be at fault. Some versions of Skeinforge have this set to a value that makes your bot start a tiny bit late each time it begins to extrude. Changing it from -0.2 to 0 will let your extruder run through the plastic a tiny bit faster, which might be all you need to turn a failure into a success. Find the setting in Skeinforge -- it's the last setting in the Dimension plugin. This could also be the culprit if details just aren't printing right -- if sections of your print have a really tiny horizontal diameter, that tiny length you’re subtracting could become a problem.
If you're having trouble unloading filament, it's possible that the end of the filament has swelled slightly and gotten stuck on the other side of the drive gear. Use ReplicatorG's control panel to run the extruder motor forwards for a few seconds in order to extrude the swelled section. Then try unloading the filament again.