I’ve started working on a simple design for a printable modular raygun. The image to the left shows a 1/5th scale prototype print.
The problem is warping. This is a wide and shallow print. Each half of the raygun is 75mm wide, and only 12mm thick.
PLA warps less than ABS, so up until now I’ve not needed to worry about warping. I’ve noticed a little on objects with flat bases, like the Sim City 2000 factory but it has not been too noticable. This raygun print changes that.
I designed the raygun as simply as I could imagine. The barrel is a cylinder, with a cone at each end. One cone streamlines the back end, and the other is inset as a ‘reflector’. The grip is a rectangle set at an angle to the barrel.
Then I added some twiddly details. There’s a hollow tube down the centre of the barrel for darts or a light. I put a football shape on the bottom of the handgrip. The finger cutouts are made by subtracting toroids from the grip. All the sharp edges are smoothed with constant radius curves.
Since undercuts and overhangs are hard to print on a ‘fused filament deposition’ printer like my Mendel Prusa, I’ve had to cut the model into two pieces with flat bottoms, to print it.
Just like most plastic toy guns, I cut it down the middle. This is easy to do in OpenSCAD. You can see the parts in the image on the left.
The disadvantage to this method is that the prints have to stay exactly flat on the build surface. Otherwise the two halves won’t fit together perfectly.
Elastic bands hold the flat centre portion of the barrels together, revealing the warping you see in the third photo. This occurs because PLA shrinks slightly as it cools and sets.
Imagine the printer extrudes a long thin noodle of plastic, and assume it’s exactly 100mm long. As it cools down, it shrinks a little, becoming shorter – say 98mm.
The printer then lays another noodle on top of it, exactly 100mm long. There is a 1mm overlap at each end. The two pieces bind together due to the heat in the top strand. Then the top strand cools and wants to shrink to 98mm, trying to make the bottom strand shrink with it. But the bottom strand has done all the shrinking it can. So it bows up a little instead.
This happens with each new layer, and the result is the ends lift up off the print bed.
Really good adhesion of print to printbed helps a little, but you have to be able to get the print off when its finished. Only printing small objects helps, but that’s no fun. I’m going to have to stop the lowest layers from cooling down so soon, so there is less shrink stress forcing the edges up and away from the bed.
That is why the heated build platform was invented. It warms the plastic from below, without being so hot as to make it melt. Then, when the print is finished and the heated bed turned off, the whole print shrinks slightly as it cools, and pops neatly off the bed without having warped.
That’s the theory. There is less consensus about how to mechanically achieve this holy grail. That is why I haven’t built my heated bed yet. Maybe tomorrow. Which is already today, because it is now 1:30 am.