PMMA on PI

Previously I described how I made some lamp shade clips for some friends from PMMA (Acrylic). Unfortunately the clips got lost in the post so I had to run off some more. I used a sheet of acrylic as the bed the first time, but since then I have developed my magnetic bed with PI (Kapton), so it was an interesting experiment to see if PMMA would stick to PI.

The originals were made with the bed at 100°C, but the insulating effect of 4mm of PMMA gave a surface temperature of only~85°C. That was sufficient to form a partial weld strong enough to resist warping. My first test was with the bed at 100°C, but that came loose after a few layers. With the other plastics I have found I need a higher temperature on PI because it does not form a weld. It sticks by some magic that I don't understand, possibly Van de Waals forces.

I tried again with the bed at 130°C and this time it stuck well but was easily peel-able by bending the bed. So another plastic that works on hot polyimide.



I have also been doing multi-part builds one layer at a time, something I hadn't dared to do until recently because my previous extruders were not reliable enough to risk a bed full of objects.

If you get the height just right on the polyimide bed the first layer comes out almost perfect. You can actually make one layer thick objects, i.e. 0.3mm in this case.



These came out very well.



I arrange multiple items by reading the STLs into AOI and orienting and positioning them. I then union them together in pairs and then union all the unions until I have a single object. I then convert that to a triangle mesh and export it as a GTS. If I don't convert first I only get one object in the GTS rather than the composite. I then slice as one object.

These springs show a very rare bug in Skeinforge or my code where it does a move with the extruder on. That is the only time I get any string.



Here is an assortment of smaller parts :-



It is best to group the taller parts together so when it gets to the higher layers the head does not have far to travel.



There were so many in this build that it overlapped the hot part of the table, so a few corners lifted a bit. I think ideally the table should be a bit bigger than the movement to keep some warm air around the objects. Perhaps a wall around the edge would keep some heat in.

I did have the crazy idea of building a wall around the objects as they are made so that there is a small gap of trapped hot air around them. I.e. the machine builds a disposable cocoon around the object to act as a heated build chamber. It would be a bit wasteful of plastic of course but it could be made from a cheap paste if we had a second head.