Tuesday, April 9, 2019

Top and Bottom

I'm still working on ironing out the kinks in using my new 3D printer. If you remember from last time one of the big issues was the underneath of the parts where the quality was terrible. It wasn't entirely clear why it was so bad so I've done some more experimenting.

What you can see in the photo is the underside of the Sand Hutton wagon chassis printed in three different ways. On the left is the original print I did which had the chassis parallel to all three axis of the printer and raised off the print bed on supports. The middle print was tilted slightly (only around one axis) but still printed with supports. The final print on the right was printed directly on the build plate with no supports at all.

As you can see all three prints have issues but the best one is the one printed directly on the build plate. This is true even if we discount the marks left by the supports. It seems that the resin essentially "sticks" to the cross beams and doesn't drain away as the build plate moves up (I'm guessing surface tension keeps it in place somehow). I was expecting this not to happen when the model was tilted but you can see that it still happens. The right hand print does show that I possibly don't need as many supports as I initially expected though, as the rest of the print is fine. I will need to raise it off the base plate though, so for real prints I'll still need some support structure, but I might have to be a little cleverer as to how and where they are attached.

I have had some better luck playing with the top surfaces of the prints though.

If you remember from last time, the other problem was not being able to see the gap between planks on horizontal surfaces.

The floor should be made of four planks, and the gaps between them should run left/right across the photo, but while there are very faint lines, they are not defined enough and would disappear under the lightest of layer of paint. I had an idea on how I might be able to "fix" this without actually changing the model.

Firstly I produced a much smaller test file; essentially printing just the floor planks and details straight onto the bed of the printer. This gave the following print

As you can see this is just the floor, and the plank gaps (which should now run up/down the photo) still don't appear particularly well. After applying my "fix" and printing the model again I got this

As you can see the planks are a lot more defined, the question is how did I do that?

I've been messing around with some open source software that allows me to view and edit the individual layers once a model has been sliced. What I spotted was that there is no overall setting for layer height within the file, rather each layer has a Z height position. A quick test showed that I could set this value to anything I wanted and the build plate would move to that position before exposing the layer. I videoed the test which shows the first few layers being exposed as normal and then the build plate going first to 50mm then back down to 20mm and then up to 60mm all without any issue:

It's a rather boring video and sorry you can't read the screen very well, my camera didn't seem to like focusing on the UV light -- the layers just tell you the layer number and the Z height.

So having figured out this was possible I set about trying to solve the issue of planks on a horizontal surface by doing essentially a double exposure of a single layer. The gaps between the planks on my test are a single layer deep (0.05mm) and two pixels wide. My fix was therefore to take the original layer (on the left) duplicate it, and then delete alternative planks on both layers to give two new layers (centre and right)

On my first go I gave them both the same layer height. While this seemed to work, gaps were more visible, it seemed that one set of planks was ever so slightly higher than the other. My assumption is that by using the exact same layer height I'd got a small amount of extra resin that cured on the second layer. So I set the second layer to be 0.01mm less than the previous layer (i.e. the smallest increment I think the printer can manage). The result was that the planks seem level with each other but the gaps are more visible than when printed as a single layer.

Each layer in the file also store the exposure time and off time so I was hopeful I could do other interesting things by editing the files (like longer cure times for certain layers) but it seems as if these settings are ignored and those in the file header used instead; at least that seems to be the case with exposure time.

Messing around with the files like this is certainly not straight forward, and you'd need to be careful setting the layer heights to ensure you don't try forcing a print through the bottom of the vat, or lift it up too high causing the second half to stick to the bottom of the vat etc.