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.

Extruder broke already

Well my best attempt at making a reliable extruder again resulted in one that only lasted a few weeks! The brass worm pulley that was pushed onto a splined shaft worked loose while extruding PMMA.



PMMA is quite hard work to extrude, but probably no worse than HDPE. On reflection splines into brass are not going to hold the massive force that occurs at 2mm radius. A better idea would be to have a boss on the side of the pulley and use a set screw onto a flat on the shaft. I would also add smaller diameter bosses at each side to meet the centre rim of the bearings. That would automatically position the pulley dead centre.

But to do that I would have to make a new pulley cutting jig and redesign the motor bracket to be a bit wider. I would need a working extruder to make the new bracket of course, so I decided to bodge the existing design.

I drilled out the centre of the pulley to 6mm and then reamed it to 6.4mm. I then turned a steel hub from a piece of hex pillar. I made it about a tenth of a millimetre oversized, added a chamfer to the hole in the pulley and forced it in with a vice, creating a very tight fit.



I didn't trust that to hold on its own so I left a hex flange on the other side and soldered it to the brass: -



Certainly not my best soldering, but bodging is bodging. The hub is twice as wide as the wheel and steel is harder than brass, so it should have a much better grip on the splines. I don't know if it will last or not. The constant back and forward motion of the anti-ooze fix means that if anything is weak it gets worked loose.

With the repaired extruder I made a third lamp shade clip leaving 1mm of the acrylic rod left above the pulley, how lucky is that?



Then I pushed my luck too far. When I bought the 3mm PMMA rod I also got a 2mm rod to compare results. Stiffness of a rod is a fourth power on diameter I think, so 2mm filament is five times more flexible than 3mm.



This would certainly be feasible to use in coils as it has a similar minimum bend radius to 3mm PLA, we just need to find somebody to supply it in that form at a reasonable price. 2mm rods are even more expensive than 3mm rods, £1.24 on eBay as opposed to £1.49, but are only 44% of the volume!

I decided to give it a try in my newly repaired extruder by printing a whistle. I had to scale it down because with 0.4mm filament it would use more than 1m of 2mm filament, so I printed the same g-code using 0.3mm filament and scaled the dimensions accordingly.

It managed to print a couple of layers and then the extruder jammed. I think the problem is that with a 3.6mm bore and 2mm filament there is too much of a gap, so molten plastic can flow upwards and freeze in the cold part of the tube above the taper. I think it would work fine with an extruder designed for 2mm filament. The drive mechanism just about works because although it does not have as much grip, it only needs 44% of the force that 3mm filament needs. The barrel and heater block would need a smaller bore though and could be made smaller. Similarly the smaller motor I used before would have plenty of torque, in fact a high torque NEMA14 should work.

So there are a lot of advantages to using 2mm feedstock like commercial machines do, BUT stiffness falls as a forth power, but force required only falls as a square law, so I expect soft plastics like HPDE, PP and PCL may buckle when being fed. Certainly the gap between the pinch wheel and the barrel entrance would need to be very small.

I fixed the jam by putting a drill down the hot barrel and hitting it with a hammer. That fixed it and I hand fed some ABS before reassembling the extruder. After assembly it would not work at all. The thermistor had shorted out to the metal work!

Nothing much to see from the outside, just a weird furry slimy deposit on the back of the AL tube and a green stain on the thermistor lead that was shorted.



I cannot get to the thermistor or heater without removing the PTFE cover, but that can't be removed without unscrewing the barrel, another slight design flaw. If I had tapped the stainless steel pipe all the way up I could just unscrew it from the AL tube that surrounds it, but it is really hard work tapping stainless steel.

I unscrewed the barrel while the extruder was hot to reveal this mess: -



The plastic that leaked when I first built the extruder has been stewing for weeks and has boiled down to something resembling bitumen. I expect the more volatile products condensed on the cold AL tube above it forming the Vaseline like deposit.

I couldn't tell why the thermistor was shorted because it came away with the PTFE cover. The Cerastil that I glued it in with seems to have decomposed in the chemical soup around it. My last few attempts at sticking thermistors with Cerastil have not been very successful. I am not sure if I mixed it to the wrong consistency, or if it is now too old to cure properly. It doesn't look any different, but instead of rock hard cement I seem to get something crumbly.

I cleaned it all up and stuck the thermistor back in with RTV silicone. I am sure it is not as conductive as Cerastil, but over such a short distance (between the thermistor and the wall of the hole it is in) I am hoping it will not have much effect.

I made the hole for it a bit deeper and opened out the top so it was big enough to accommodate the PTFE sleeving as well. That should keep it from touching the metal. It is surprisingly difficult to glue something into a small hole with a viscous glue. It is hard to get the glue to go down the hole without leaving an air pocket. A better idea might be to drill out a small screw, all the way through, fill it with glue from both sides. Then when it has set simply screw it into a tapped hole in the heater block.

I am waiting 24 hours for the silicone to cure now, so back to work tomorrow and less blog posts.

New Year New Plastic

Just over a year ago a friend asked me to make replacement for a broken clip that was part of a light fitting. It was not too difficult to model and I made a copy in ABS with 0.3mm filament, 0.24mm layers.



It did the job mechanically, but with one obvious aesthetic problem: -



The original clips were made from transparent polycarbonate and all I had at the time was green ABS. I didn't use PLA because I worried the lamp could easily get hot enough for the clip to go soft and drop the shade. The only transparent thermoplastic that I could get hold of in filament form was PMMA (AKA acrylic / Perspex, etc), which is available in 1m rods. It is too stiff and brittle to use in my previous extruder, so I promised to have a go when I moved to a pinch wheel design.

The first attempt was a complete failure. It melts at 130 - 140 or 165°C depending where you read. It has a relatively high glass transition, 100-114°C, again depending where you read. I found I could extrude it with a fair amount of force at 180°C. It is very viscous with plenty of die swell. I couldn't get it to stick to anything, including itself, at that temperature though. It isn't sticky like PLA, so it wouldn't stick to masking tape. The obvious second choice was a sheet of acrylic as all thermoplastics will stick to themselves.

The general rule of thumb to make plastic weld to itself is that the average of the temperature of the hot part and the cold part has to be higher than the melting point. So to get it to stick to the base, which is at room temperature it would have to be extruded at twice the melting point minus the ambient temperature. The only plastic that seems to break this rule is PLA which melts at 160°C but will bond to itself at 180°C. I think it is something to do with it having a low glass transition and / or that it is sticky like a glue when it is molten.

I upped the temperature to 240°C but it started to hiss and smoke and still did not bond to the base. Lots of places quote the boiling point of PMMA to be 200°C! I dropped the temperature back to 220°C and it is much happier, but still does not stick .

So the only way to make it bond with itself is to raise the ambient temperature. Cue the heated bed. I set the temperature of my aluminium plate to 100°C, the hottest it can safely be below the glass transition. I taped a small scrap of 3mm acrylic sheet to the middle of the bed with Kapton tape. From my experiments before I estimate the surface temperature would be about 85°C. That gives an interface temperature of about 150°C and that seems to be enough to get it to bond to itself.



Here is a short video of 0.3mm PMMA filament being extruded at 16mm/s: -



Here is the finished object: -



It was not too difficult to release from the bed with a penknife once the bed has cooled that is, I keep forgetting that it is hot! The bed takes ages to cool unless I blow it with a fan.

I am very pleased with the final result. I only had 1 meter of 3mm filament to get this right and I managed to find a suitable bed material, temprature settings and make three clips. The build quality is excellent even if I say it myself.



So another useful material in the RepRap arsenal. Apart from HDPE I think it has the highest working temperature. It is very stiff and brittle though. I had a couple of jams due to it snapping where it enters the extruder barrel. The alignment is not quite right because being so hard it does not press into the worm pulley as far as other plastics. The extruder could do with an adjustment there perhaps, or a bigger entrance to the pipe.

It is a bit more transparent than PLA. It smells a bit more when it is extruded, but it is not an unpleasant smell, I would describe it as sweet and aromatic. The major downside is that it is only available in rod form, so the biggest object you can make in one go is 7 cm³ and at £1.49 per meter on eBay, it is comparatively very expensive.