Dimensions in PLA vs PETG
Dear All,
I'm a total beginner: I've been playing with my Prusa 4S for a week.
So far I've been using PLA (because that's what everyone seems to start with) and I've found that external diameters seem to be reliable. I mean, if I define a cylinder in Fusion to have a diameter of 5 mm, then the printed cylinder is pretty much 5 mm in diameter.
But for holes, PLA seems to always produce a significantly smaller hole than that specified. For example, I need a structure to guide a 1.5 mm needle. If I design a 1.6 mm hole in Fusion, my needle will not even begin to fit. It's only when I tell Fusion that I want a 3 mm bore that my 1.5 mm needle will go in.
It's not been an insurmountable problem. The parts I've been making are small and I can get what I need iteratively. If I need a 7.5 mm hole, I know that I'm going to have to specify something closer to 7.7 mm so I try that, and then adjust until my printed model actually does what I want in the real World.
Tonight I had my first foray into printing with PET-G. I ended up with a nice-looking model. But: I printed off my needle guide (the thing that's supposed to take a 1.5 mm dia needle) using the model that works for PLA (3 mm bore) and, according to Mr Mitutoyo, the bore produced is exactly 3.0 mm. This is true to the Fusion model, but is way too sloppy for a 1.5 mm diameter needle.
What I don't understand is why. All my reading suggests that PLA gives *better* dimensional accuracy than PET-G. My (very limited) experience suggests the complete opposite. It looks to me that if I want a long hole with a diameter of 1.6 mm in PET-G, then that's what I should design in Fusion... so why do I have to fudge things in PLA when it's supposed to give more accurate prints?
I'm probably missing something fundamental, being a total novice. What is it that I'm missing, please?
RE: Dimensions in PLA vs PETG
Each mass has a different thermal and mechanical expansion. This affects the design of the parts relative to each other, where the parts are supposed to fit together. Over time, you get used to it and introduce a clearance parameter into the Fusion and it will recalculate your parts so that they fit together. This parameter depends on the material from which the part will eventually be made. For starters, this includes a few parts that will be thrown away before they fit together to your satisfaction.
RE: Dimensions in PLA vs PETG
Thank you.
RE: Dimensions in PLA vs PETG
Plastic shrinks as it cools. In a nutshell this is what your experiencing. The higher the print temperature usually the more it shrinks. Aka ASA will shrink more than PLA.
If you're part has dimensionally critical features, then you'll need to compensate for the shrinkage in your design.
RE: Dimensions in PLA vs PETG
I understand. For the project that I'm working on, dimensions are critical, so I'll have to rework my components to compensate.
Thanks for taking the time to reply.
RE: Dimensions in PLA vs PETG
There are shrinkage errors with PETG being more than PLA, but there are also “quantizing” errors. That is errors due to the fact that you have stepper motors with discrete step limitations and discrete layer height. When printing circles on vertical surfaces, the bottom and top will be flattened. One technique to help is to add a “V” to the top and bottom so the hole is slightly tear drop shaped and then the flat areas don’t exist and won’t block a circular shaft being inserted. These are all limitations of the manufacturing technology one is using. You don’t necessarily want to adjust your printer so it is exact for one filament, because it might be much worse for another. I think the best is to investigate the shrinkage for the filament you are using and then adjust sizes accordingly But you also need your design to take into account this quantizing error that is due to discrete steps/layers that are used to create models.
RE: Dimensions in PLA vs PETG
Also novice here, what bit me with holes are seems. You can get relatively well sized hole, but another part won't fit, since there is the seem line.
Slicer will put it at a sharp edge, so one option to have little recess in the hole. Another is file 😂
I got feeling that with PETG the seem lines are more pronounced. I don't have direct comparison, though.
RE: Dimensions in PLA vs PETG
I often forget to look where the seams are located.. or the slicer has inherited a setting that doesn't work. Make sure you look at where the seams are placed.. "Aligned" setting works well.. but you can also put the seams right where you want with "Seam Painter".. then they will always be where you want..
Also novice here, what bit me with holes are seems. You can get relatively well sized hole, but another part won't fit, since there is the seem line.
Slicer will put it at a sharp edge, so one option to have little recess in the hole. Another is file 😂
I got feeling that with PETG the seem lines are more pronounced. I don't have direct comparison, though.
RE: Dimensions in PLA vs PETG
yeah, I know... A bit annoyingly, the seams are not visualized by default in PrusaSlicer. I might remember, if they were on by default.