Rebore

My Mendel has been very reliable and consistent running virtually 24/7, but about a week ago, after putting on a new reel of plastic things started to go wrong. The initial symptoms were that small parts built fine, in fact I printed a mini Mendel or Huxley that came out well: -

It took just two full Mendel beds, plus a few parts on HydraRaptor. I did the gears on Hydra for accuracy and the Bowden clamps at 100% fill because they look weak to me for the job they are intended to do. The plastic weighs 335g (including a Wade's extruder), slightly more than 1/3 of a Mendel by weight but the print time is about 1/2, because small parts need finer filament. I printed most of these at 0.5mm whereas I do a lot of Mendel at 0.6mm.

But getting back to the problem, the quality of large parts had started to fall off a bit. They were coming out with blobs on the outside formed by the nozzle oozing as it moves from one object to another. These were not well bonded, so they could be simply scraped off with a fingernail, but something I had tuned out ages ago. Another change was that it was not doing 45 degree overhangs well, so it left filament hanging down in the tops of tear shaped holes. Again, not a big problem as they just get drilled out anyway.

I started to suspect the temperature was too high so I pushed the thermistor well into the heater block. Then the filament started jamming after the first layer (which I do very slowly). After a few attempts the extruder drive gear broke where the captive nut for the grub screw is. This seemed more like the temperature was too low, so I suspected the thermistor was no longer reliable. I decided to rebuild the heater assembly as my last one was put together in a hurry from parts left over from an experiment. It had been in the wars as well, being entombed in ABS and hacked out again, not to mention running almost continuously for about 2500 hours. Originally the thermistor was glued in with RTV silicone, but that was long gone and it relied on the wires holding it in place.

Since my original heater hack using a vitreous enamel resistor I had moved on to a smaller resistor on Hydra and found that worked better. The surface area of the block is a lot less and that is where most of the heat is lost from, so the amount of power required goes down. It also warms up faster of course, both due to less heat being lost and also less thermal mass. The resistor I have settled on is a Vishay / Sfernice RWM04106R80JR15E1

The thermistor is drilled as close as I dare to the thread for the nozzle and then counter-bored so that the entrance is wide enough for the PTFE sleeving. The wires have PTFE insulation to withstand the temperature and the resistor is soldered with 300°C HMP solder. I think I could also get away with ordinary unleaded solder as well because of the length of the resistor leads, but I didn't want to chance it.

After a tip from Giles I used Rothenberger high temperature glass rope adhesive to glue the resistor and the thermistor. It sets in only half an hour, which is a big advantage over other things I have tried. I also used it to stick ceramic tape on the outside of the block to insulate it.

When I first heated it up the adhesive bubbled causing a downward slope in the temperature graph. I thought at first the thermistor had been dislodged by the blistering, but I think it was just temporarily cooled by the out-gassing. I should have heated it much more slowly the first time I think.

The new heater works much better than the old one. The warm up time to 255°C is about 280 seconds, whereas the old one took about 400 seconds (the bed takes about 350 seconds to get to 140°C). It also runs at about 70% to maintain 240°C while extruding, whereas the old one needed about 90%. The bang-bang control cycles much faster and only deviates by one degree. That is because of the close proximity of the thermistor to the heater. Because it is mounted between the heater and the barrel I can be sure the swing at the barrel is even less. I calibrate against a thermocouple inside the barrel, so any temperature difference across the block is calibrated out. It should be negligible though because the thermistor is also very close to the barrel and aluminium is a very good conductor. The extra power needed to heat the ABS when extruding 0.6mm filament at 32mm/s is about 10%, i.e. ~2W.

The new improved heater didn't solve any of my problems though. While reassembling the extruder I tried pushing filament through by hand. It was much harder than I remembered it was when I first built the extruder. At this point I was beginning to suspect the plastic was different in some way although it looked identical and was part of the same purchase.

I noted that the filament was coming out very curly. That was something I had noticed happening on both my machines when I do a test extrusion, but I had ignored it. I measured the diameter though and found whereas it normally swells to 0.7mm this was coming out oval and about 0.5mm by 0.6mm. It all fell into place then. I have read that the difference between straight hair and curly hair is whether it is round or oval. The only way the filament could be oval is if the nozzle aperture is no longer round. I put a 0.5mm drill bit through it and it started to extrude round, straight, 0.7mm filament again. The hole must have been partially occluded by the burnt plastic that tends to glaze the end of the nozzle. That caused the plastic to come out thinner and faster. It was fine when making objects with 0.5mm filament because it was still being stretched but when building with 0.6mm filament it was being compressed, so would hang loose if given the chance. The smaller hole increased the barrel pressure, which is why it oozed. The plastic would be compressed more, so require more backing up to release the pressure and stop the flow. Also the extra pressure was too much for the pinch wheel when extruding at the top flow rate I use, which is 0.6mm at 32mm/s. I think the M8 hobbed bolt is below the ideal diameter for softer plastic like ABS.

I also re-bored HydraRaptor (with a 0.4mm drill) and that stopped the filament being curly as well. It seems nozzles need occasionally re-boring. I had assumed that the hot flow of high pressure plastic would have kept the hole clean, but not so.

So a simple fault had my machine out of action for days because I didn't recognise what the symptoms meant collectively.

No compromise extruder

I have settled on using vitreous enamel resistors embedded in an aluminium block for the heater. I think they are the easiest heater to make and likely to be the most durable. They also work fine with simple bang-bang control, whereas it would appear that the Nichrome and Kapton version requires PID.

One of the aims of my new design is to reduce the amount of molten plastic to minimise ooze. Also less molten plastic means less viscous drag. I also wanted to reduce the thermal mass (to reduce the warm up time) and completely cover the hot part with insulation to allow a fan to blow on the work-piece without cooling the nozzle.

To achieve these aims I switched to a smaller resistor (same resistance but less wattage) and mounted it horizontally rather than vertically. There is some risk that the resistor may fail but I think as long as it has good thermal contact with the aluminium block, so that its outside temperature is less than 240C, then I have a good chance it will last.

The smaller resistor also means a much smaller surface area so less heat is lost. T0 keep the molten filament path as short as possible I combined the heater and the nozzle and made it from one piece of aluminium. That also gives very good thermal coupling between the nozzle tip, the melt chamber, the heater and the thermistor.




I turned it out of a block of aluminium using my manual lathe and a four jaw chuck, but I think I could also mill it out of 12mm bar using HydraRaptor.

A feature that I have used on my previous extruders is to cover as much of the nozzle as possible with PTFE. That stops the filament sticking so that it can be wiped off reliably with a brush. It also insulates the nozzle.

My previous nozzle cap implementations have been turned from PTFE rod. The downside of that is that the working face, that has been cut and faced on the lathe, is not as smooth and slippery as the original stock.



To cover the face of this version I used a 3mm sheet of PTFE so it has the original shiny surface.



Normally PTFE is too slippery to glue so my original plan was to screw it on with some tiny countersunk screws. However, the sheet I bought was etched on the back to allow it to be glued, so I stuck it on with RTV silicone adhesive sold for gluing hinges onto glass oven doors.



To insulate the rest of the heater I milled a cover out of a slice of 25mm PTFE rod.



I normally stick items to be milled onto the back of a floor laminate off-cut using stencil mount spray. I didn't think that was going to work with a PTFE cylindrical slice that is only a little bigger than the finished item. Instead I milled a hole in a piece of 6mm acrylic sheet that was already stuck down with stencil mount. The hole was slightly smaller than the PTFE so I faced it and chamfered it on the lathe and then hammered it in.



I roughed the shape with a 1/8" end mill and then sharpened the internal corners and cut the slots for the resistor leads with a 1mm end mill. I tried to mill the whole thing with a 1mm bit but it snapped due to a build up of burr in the deep pocket. On reflection it was silly to expect to be able to mill deep pockets with a 1mm bit and of course it is much faster to rough it with a bigger bit.



I used my normal technique of taking 0.1mm depth cuts at 16mm. That allows me to mill plastic with no coolant, but I expect I could have made much deeper cuts in PTFE. It mills very nicely, probably because it is soft and has a high melting point and low friction.

I haven't done any milling for a long time so for anybody new to my blog here is my the milling set-up: -



It is simply a Minicraft drill with some very sturdy mounts. The spindle controller I made originally would need its micro replaced as the one I used has a bug in its I²C interface. Instead I just connected it to the spare high current output on my new extruder controller.

The remaining part of the extruder is the stainless steel insulator.



I made the transition zone shorter than the last one I made because I wanted all of the inside of the transition to be tapered. The aluminium sleeve carries away the heat from the cold end of the transition to an aluminium plate that forms the base of the extruder. That in turn carries the heat to the z-axis via an aluminium bracket. I used heatsink compound on the joints.

Here is a view of the bottom half of the extruder: -



And here is a cross section showing the internal details: -



So that was the plan, what could go wrong? Well everything really! The first problem was that the resistor shorted out to the aluminium block. The smaller resistor only has a thin layer of enamel over its wire. Normally I wrap aluminium foil round it to make it a tight fit. I didn't drill the hole big enough so it was a tight fit with only one layer and pushing it in abraded the enamel. The solution would be a bigger hole and more layers of foil, but I just glued it with Cerastil as a quick fix. Of course it only failed after I had fully assembled it and run some heat cycles so I had to strip it down again to fix it. Not easy once the wiring has been added.

The next problem is that it leaks. I think it is because I dropped the extruder when I was building it and bent the thin edge at the end of the stainless steel barrel. That forms the seal with the heater block, so even though I straightened it I think the seal is compromised. I keep tightening it and thinking it is fixed but after hours of operation plastic starts to appear at the bottom of the PTFE cover.

The other problem is that mostly it extrudes very well, I now do the outline at 16mm/s and the infill at 32mm/s, but sometimes the force needed to push the filament gets higher and causes the motor to skip steps, or the bracket to bend so far that the worm gear skips a tooth.

I have made several objects taking between one and two hours and it worked fine. Other times, mainly when I was making small test objects with Erik, it will completely jam. Actually it seems to jam when it is leaking badly, which implies the pressure of the molten plastic is much higher as well as the force to push the filament. The only explanation I can think of is there is an intermittent blockage of the nozzle exit. More investigation required.

Constipated Extruder

My "New Year" extruder, which is the one on HydraRaptor that I use to build things, stopped working while building the first layer of an object. That is the lowest temperature layer, so the plastic is at its most viscous.



I couldn't get it to work again, so I removed the drive and tried pushing the filament by hand. I couldn't shift it. I measured the temperature of the molten plastic with a thermocouple and it was correct, so I deduced that the nozzle must be blocked. I removed the nozzle and when I pushed the filament this came out: -



It is dark and glassy looking. No idea what caused it, but it seemed to have blocked the nozzle. I cleared it out with a drill and reassembled it. I took the opportunity to measure its performance with my "lead kebab" test jig.

Even though this extruder has a 0.3mm nozzle and no taper in the PEEK insulator, it works better than the tapered PEEK extruder with a 0.5mm nozzle.



The most notable difference is that this one has a much bigger heater chamber, so perhaps a smaller heater bore melts the plastic quicker.

I got this interesting graph of flow against force, averaging over five runs of 20mm : -


I think the steep part of the curve is where the flow through the nozzle dominates the force required and the first part is where the plug friction dominates. The point where I operate it is right on the knee of the curve. I suspect adding a taper would straighten it out, but I don't want to strip down my only working extruder to prove that.

So I don't know what caused the blockage, but it is the second time I have had an extruder block, so it goes to show that a detachable nozzle is advisable.

Simply better

I find it very satisfying when making something simpler also makes it better. I tested the simplified heater / nozzle design using the same stainless steel insulator and heatsink arrangement, so I could get a direct comparison of the results.



The heater warms up a lot faster than the one made with two AL clad resistors. It also extrudes faster and the times are more consistent. ABS pushed with 2.32Kg went from 3.7 mm³ to 4.6 mm³, an increase of 24%. HDPE pushed with 4.6Kg went from 3.8 mm³ to 9.3 mm³!

The nozzle is 0.6mm rather than 0.5mm, which reduces its contribution to the pressure by a factor of 2, but all my other tests have shown that what happens at the other end of the heater dominates the force requirement. As I improve things though, the nozzle hole becomes more significant.

Here are the drawings :-



Although it looks complex it isn't difficult to make with a drill press, drill vice, and some taps and dies.

I glued the thermistor in with Cerastil, but I expect it could just be wrapped in tin foil and jammed in like the ceramic resistor, taking care to insulate the wires of course. I use PTFE sleeving.

I didn't need to seal the threads with PTFE tape. I just screwed them up tight and there was no sign of any leakage.

The next thing to try is putting a taper in my PEEK version to see if that can be made to perform as well as this one.

Of course I haven't built anything yet with any of these designs, so caveat emptor.

Top tip

I got the tip to use welding tips for an extruder nozzle from Andy. They come in packs of five from Halfords for £4 on-line and £5 in the shops.



They are made from copper and have a 0.6mm hole down the middle. The thread is M5.

I drilled out the one on the right to 3mm, almost to the end, to reduce the pressure needed to extrude. They drill easily if the drill is lubricated with a little paraffin. It's a shame they don't work as is, but all the same it is much quicker and easier than turning, drilling and tapping the standard design.

They also simplify my evolving extruder design because the heater block no longer needs a spout. I can simply drill and tap the bottom of the melt chamber M5 and screw these in. I can also change the M8 penny washer for an M6 one. That allows me to reduce the outside diameter of the PEEK collar to 8mm so it can be made from the same stock as the thermal transition. The area of the collar will be less so it will conduct less heat.

The 0.6mm orifice can be made smaller if necessary by filling it with high temperature solder and then drilling it with a fine drill. Solder is very easy to drill so less chance of breaking a fine bit. Also Vik Olliver suggested you can make a small hole by soldering in some fine Nichrome wire and then pulling it out again to leave the hole (solder does not stick to Nichrome).

I haven't tried one yet but I can't see any reason why they wont work well.

HydraRaptor's New Year's Resolution

My new extruder has a 0.3mm nozzle compared to 0.5mm that I have used before. The actual filament diameter is controlled by the flow rate versus the head feed rate, so a single nozzle can give a range of filament diameters.

The maximum diameter is governed by the hole size and the die swell. The head movement has to be about the same as the rate that the filament leaves the nozzle, or faster, otherwise the filament squirms about and makes a zigzag instead of a straight line. Fortunately the faster the flow rate, the more die swell there is, which works in our favour when trying to extrude the maximum diameter filament. With the 0.5mm nozzle I could extrude up to about 1mm with ABS and I used that to good effect when making the first layer of the raft. With a 0.3mm hole die swell is more but even so I can only get 0.8mm filament. That makes the first raft layer thinner, so it is less tolerant to the bed being uneven.

I normally extrude at a rate that produces filament the same diameter as the nozzle but it can be stretched further making it smaller than the nozzle. The limiting factor is when the filament starts to snap. I did make some 0.3mm filament with the 0.5mm nozzle but I don't think I got the full benefit of the extra resolution because the filament was less constrained as the nozzle changed direction.



These two gears are both made from the same gcode with 0.3mm filament giving a layer height of 0.24mm. The one on the left was made with a 0.5mm nozzle and the one on the right with the new extruder with 0.3mm nozzle. The latter is slightly better defined. The benefit is more apparent on the underside.



The bottom of the one on the left feels perfectly smooth due to being made on a raft with a very fine surface. It is actually smoother than a sample I have from a commercial machine.

I was disappointed that it did not improve the clockwise slant of the teeth. This must be due to the same effect that makes holes come out too small. The filament likes to cut corners, so when the head moves on a curved path the filament takes a smaller radius path. I noticed that the teeth are straight at the base but slanted at the top, so the effect is somewhat cumulative.

I made another one with the outlines anti-clockwise on every second layer. Here is a video of it being made: -

HydraRaptor RepRapping a gear from Nop Head on Vimeo.

The teeth came out straighter but the edges are slightly more ridged because each layer alternates a little.



The surface is not quite as good as the previous one. I put that down to variations in the feed stock diameter. You need exactly the right amount of plastic to get a good surface.

I also need to up the resolution of my z-axis. 0.05mm is significant with 0.24 mm layers, so I will have to add microstepping like my other axes.

So in summary 0.3mm nozzle gives noticeably better results and can still make 0.5mm filament due to die swell. It is harder to get the raft heights and temperatures correct. To get the same build rate with 0.3mm filament I would have to extrude at 44mm/s, but HydraRaptor is currently limited to 32mm/s. I could probably tune it up to 44 but the vibration gets a bit ridiculous as the moving mass of the table is 9Kg.

The pros and cons of nozzles

HydraRaptor seems to be running reliably again, touch wood. I did have one scare when it started making noises like a machine gun when I had left it running unattended. It turned out that the shaft encoder code wheel on the extruder motor had fallen off. That caused the firmware to think it was far behind and so it applied maximum power in an attempt to catch up, which caused the GM3 gearmotor's torque liming clutch to slip. I added it to the list of sanity checks to put in my extruder firmware :-

• If the shaft position gets more than, say, half a turn behind then give up.
• If the thermistor resistance is too high then the thermistor is open circuit so turn the heater off.
• If the thermistor resistance is too low then the thermistor is short circuit.
• If the heater has been on for more than 5 seconds and the temperature has not risen then the heater is open circuit.
• If the heater has been off for 5 seconds and the temperature has not dropped then panic, the transistor is short circuit.

All these checks are necessary for safe unattended operation in my opinion.

The solution to the code wheel problem was to extend the shaft of the GM3 with a piece of brass rod :-



I have managed to perform quite a lot of tests with HDPE and it is clear that the new acorn nut nozzle behaves quite differently to the previous one piece design.

The original nozzle looked like this and had a 0.5mm hole that was about 0.6mm deep: -



The new nozzle is made from an acorn nut turned to a point. I also has a 0.5mm hole, but it is tapered at about 45° so the the part of the hole that is 0.5mm diameter is very thin :-



The differences this seems to make are: -

1. The die swell, i.e. the amount the filament expands from the hole diameter, is a little less.
   
2. The amount of filament that extrudes after the motor is switched off has increased quite a lot. The excess is wiped from the nozzle, but by the time the head has moved from the brush back to the workpiece, a few more mm have leaked out making for a messy line start. I think this is because the shorter exit hole makes it easier for the plastic to escape.
3. If I move the head quickly with the extruder off, then the filament snaps. It quite often leaves a blob that sticks to the workpiece. With the longer hole it stretched to a long thin string rather than snapping.
4. I used to be able to lay down 0.5mm filament at 16mm/s by stretching, but now I can only do this reliably at 8mm/s as the filament has a tendency to snap. I think it is too easy to pull it from the new shaped hole.
   

When stretching the filament it has a greater tendency to cut corners. I think this is mainly due to not running the fan, but may also be because the nozzle is too pointy. A wider nose will help to push the corners down.

I can't run the fan because the heat loss from the bigger nozzle causes the heater to work harder, raising the temperature of the barrel above the point where the PTFE distorts. I need to insulate the nozzle so I will try making a new one similar to this one with a PTFE cover over it.

Here is about where I am at with extrusion quality: -



This is a rectangular block about the size of the extruder pump (60 x 20 x 15mm), with a 50% fill. I forgot to put a top surface on it but that is perfectly possible. It was extruded at 220°C (measured at the nozzle) with filament stretched to 0.75mm at 7mm/s. The layer height is 0.6mm and the pitch is 0.9mm. Some warping still evident but it has come a long way from my first attempts.

Nutty Nozzle

My old extruder nozzle was made from a solid brass rod with a 0.5mm hole in the end.



It was drilled 3.2mm from the other end to accept the filament. The problem was gauging how far to drill from the back. Ideally the hole in the end should be as short as possible to provide less resistance to the filament flow. Drilling too far would write it off so I erred on the side of caution.

I suspected it was a bit on the long side as I got more die swell than I was expecting. Since it is now scrap I sectioned it to find out exactly how long the hole was. It turns out it was about 0.6mm.

The latest RepRap design uses an acorn nut which gives much better access to the back of the hole. I used the smallest drill I have, which is 0.3mm, to start with, I might open it up later.



The dome of a brass acorn nut is quite thick so I opened the hole out from the back using a conical milling bit that I bought for PCB track isolation milling.



The point is actually about 0.3mm so I was able to countersink the hole from the back until the point came through. That means the rim of the hole is very thin indeed and there is a double taper leading to it. The first is created by the drill that made the thread hole in the acorn nut and then a shallower taper made by my mill bit. It will be interesting to see what flow rate and die swell I achieve with this close to ideal shaped aperture.



I turned down the front of the nozzle to a point to give some clearance to the work piece as suggested by Vik Olliver somewhere I can't find now.



The extra hole on the face next to the thermistor is to allow me to introduce a thermocouple during calibration.

Boring

Well turning, boring and tapping to be precise. I made the extruder nozzle and PTFE heat barrier this evening :-



I am making a mixture of an older RepRap design and the latest, mainly because I bought the parts before the design was changed. I made a small modification to the nozzle design. You are supposed to use a bottoming tap to thread the inside of the PTFE tube. As I don't have one of those, I drilled the hole 5mm deeper to accommodate the tapered end of the tap. I also turned the last 5mm of the nozzle down instead of threading it. I should have made the PTFE tube 5mm longer but I forgot so I have slightly less heat barrier.

I am not totally happy with the nozzle because I bent it while I was threading it with a die. I don't think it will affect the operation it just looks a bit scrappy. Here they are screwed together and inserted into the clamp :-



I need to add the heating element next.