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.

Hot bed

Making a heated bed to combat warping has been on my "to do" list for a long time. In fact I ordered the materials more than a year ago. My plan was to use an aluminium plate with many small power resistors screwed on the back.

The plate is 8" square to match my table and 6mm thick. A friend with a CNC machine shop kindly machined it for me. It saved me a lot of hard work with a hacksaw and file and looks a lot better as well.



I estimated that it would need about 50W to raise the temperature to 100°C, so I aimed for 100W to give a reasonable margin for control. I used 9 10W 12Ω resistors wired in parallel. Driven from 12V this would take 9A giving a power of 108W.

The holes in the resistors are only big enough for M2 screws. I drilled blind holes and tapped them with a plug tap, actually a broken tapered tap that I ground to a flat end.



Tapping small holes in aluminium is tricky, that was how the tap came to be broken in the first place. The correct size hole for M2 is 1.6mm but I drilled it 1.7mm to make it easier to tap. In fact aluminium is so ductile that the peaks of the thread are still the correct diameter. I.e. the 1.7mm drill would not fit the hole after tapping and the thread was a good tight fit on the bolts. I used paraffin for lubrication.

Soldering the resistors was fun.



I used stout wire to handle 9A and high temperature solder because I fancied using it as a hot plate for soldering. My 50W iron did not have enough power to melt the solder when the resistors were mounted with two thick copper wires leading from them. To get round that I placed it on a silicone matt and powered it up to raise the temperature to 100°C and then soldered it while hot and live, not something I would recommend. As the iron bit is grounded I had to solder all the 0V connections first and then swap the polarity.

The original plan was to power it from a 12V PC power supply and switch it with a big MOSFET. Initial tests with a bench power supply showed it took about 15 minutes to warm up to 80°C. When calculating the power I had forgotten take into account the specific heat capacity of the thick sheet of aluminium. I didn't want to add 15 minutes to the build time, so I decided to double the power. I have abused these resistors before and got away with it. I changed the wiring slightly to make a series parallel combination with a total resistance of 12Ω and fed it from 48V AC giving 192W.

I used a big 350W transformer and controlled the mains to it with a solid state relay. Since the temperature is controlled there is no real point in using a regulated DC supply. It is much more efficient to use AC and avoid the losses associated with rectification and smoothing. It also allows me to use the same control hardware and firmware that I used for the SMT oven.

I made some PEEK insulating stand-offs to mount it on my XY table with a gap of about 6mm below the resistors: -



I wrapped the feed points around two of these to make the transition to a lower temperature with PTFE sleeving before using normal flex to handle the movement of the table.



I also added some foam board to insulate the top of my X-Y table.



This just fills some of the air gap under the plate to prevent air circulating and convecting heat downwards.

I made some PTFE washers to go under the nuts that hold it down by slicing up a failed extruder insulator: -



These deformed considerably when I heated the table to 230°C, highlighting why PTFE insulators fail when used in an extruder.

Here is the final result mounted on the machine: -



I added Kapton tape around the edge as I thought it would stop hot air escaping from underneath, but it didn't seem to make a lot of difference.

Here is the open loop response at full power: -



Although it can reach the required temperature, it is much too slow for SMT soldering. It needs to be able to rise at about 1°C / second for that. So I will stick to using the oven for soldering for now. I was hoping to be able to paste boards, place components and then solder with the board still on the table, but it obviously needs a lot more power.

Here is the response using bang-bang control from the host at one second intervals.



Some analysis: the initial rise rate is about 20°C in 75 seconds. The specific heat capacity of aluminium is 0.9 J /gK and the total weight of the bed plus resistors is 700g. So with 192W the time taken to rise 20°C should be 0.9 × 700 × 20 / 192 = 66 seconds, reasonable agreement as we ignored any heat loss.

The initial fall rate is 5°C in 85 seconds while at a temperature of ~80°C above ambient. So the rate of heat loss is 0.9 * 700 * 5 / 85 = 37W. Looking at the steady state the power is on for about 1 in 6, which would be 32W, so again reasonable agreement.

The plate is ~ 200mm square so its area is 0.04m² so it looks like we need about 1kw / m² to reach the sort of temperatures needed for HDPE and probably twice that to have reasonable warm-up time and control. Mendel's build area is also 200mm square, so would require a similar power.

You might have noticed the thermocouple is covered with a piece of ceramic cloth in the photo above. This is what happens if it is just stuck down with Kapton tape:-



You can see that as the temperature rises you get increasing thermal noise. Even with the ceramic cover in place you can see similar noise on the open loop test when the temperature was much higher. I think the reason for this is the convective air currents causing chaotic air turbulence. If you think about it you have hot air rising but, away from the edges, the only way cold air can replace it is by falling through the rising air.

A better place to put the thermocouple would be under the bed to avoid the convection currents, but I wanted to try controlling the surface temperature when it was covered by a bed material. Here is what happens with the thermocouple on top of a 3mm thick sheet of smoked acrylic: -



The set point is 95°C in this case. Clearly a case where bang-bang does not work too well, with 5°C overshoot and 3°C undershoot.

The acrylic loses about 15°C between the bottom and the top surface. That makes it curl upwards, so it would need a frame around the edge to hold it down. Fortunately I have one made from HDPE laminated with aluminium so it should stand the heat. It also adds a significant time lag.

Another problem is that acrylic has a glass transition at about 114°C. When the control was of the top surface temperature, the bottom surface exceeded that during the overshoots and went soft.

So I will need to implement PID for top surface control, but I had a suspicion that a transformer was not going to like PWM into its primary much. Anyhow I put the thermistor back onto the plate and moved the bang-bang control from the PC to the firmware in preparation for building something. Bang-bang was an apt name for what happened next. When the temperature crossed the set point it started dithering the mains on and off. The transformer sounded like it wanted to jump off the desk and then blew its 3A anti-surge mains fuse.

The solid state relay turns the power on at the zero point crossing of the mains, and off when the current is zero. Current builds up slowly through an inductor so what could possibly be wrong? I had noticed big transformers thump when you connect them to the mains, but I had always assumed it was because the secondary usually has a big smoothing capacitor to charge up. However, this was a purely resistive load, and even with no load attached the transformer thumps on start-up, so some reading up on transformer theory was required!

It turns out that transformers take a big surge current and turning on at the zero crossing point is actually the worst point to turn them on. The reason is that when a transformer is running, being an inductor, the current lags behind the voltage by 90°. So normally when the voltage is crossing zero, the current is at its maximum reverse polarity and over the next half cycle of voltage it goes though zero and then to its maximum positive value: -

If the current starts at zero then over the first half cycle it will rise to twice its normal value: -

That would not be too bad except for the fact that transformers usually run with their core close to magnetic saturation for efficiency reasons. That means the core saturates during start-up. The inductance disappears and then the only thing limiting the current is the DC resistance of the primary, about 3.3Ω in my case, so the current can be enormous. Counter intuitively, the best time to turn a transformer on is when the mains is at its peak voltage.

So I learned something I didn't know about transformers. The fix was simple, I added a solid state relay to the secondary circuit and plugged the transformer into the mains. Bang-bang control then was able to pulse it very quickly due to the dithering caused by noise, which ends up with some proportional control, that reduces the temperature swings to a fraction of a degree.



So I can add PID control firmware and it can be shared with my oven control but I have an extra solid state relay and a big transformer. A better solution would be to pick the resistor values to give the correct wattage when wired in series across the mains. Of course mains on a moving table is not the safest design. I would use a heavy three core flex with an earth lead to the plate and a second independent earth strap for safety.

It turns out the first use for my heated bed was not to combat warping, but actually something more essential, details coming soon ...