A high temperature extruder?

The standard RepRap extruder can't quite handle the temperatures for HDPE for very long. I have found a high temperature replacement for J-B Weld. The main weak point remaining is the PTFE thermal barrier. PTFE is an excellent thermal insulator but it is not very strong mechanically. It also expands by about 0.5mm at 225°C. Worse than that it seems to slowly creep the more I use it, which makes a mockery of my z axis calibration. Since I got it working again I have re-calibrated it four times and each time it has grown: 0.3mm, 0.2mm, 0.15mm and 0.3mm. I.e. it is now 0.95mm longer than when I built it and a further 0.5mm when it is hot.

I have come to realise that stainless steel is quite a poor conductor of heat compared to other metals:-

Stainless Steel	Brass	Aluminium	Copper
17 W/mK	109	250	400

I bought some stainless steel pipes on eBay that have an outside diameter of 6.4mm and an inside diameter of about 3.5mm. I cut a 50mm length, tapped it and screwed in into a medium sized heatsink. I tapped the other end and screwed in my experimental high temperature heater. I applied heatsink compound to both threads.



I put a thermocouple in the heater and adjusted the power to get 240°C inside the brass part of the barrel. That only required 7.3W. I put another thermocouple at the top of the stainless steel barrel and that only reached 50°C.



Although this is just a lash up, it looks really promising. I can get the temperature even lower by using a CPU heatsink or a small fan. I will make a nozzle out of aluminium or copper with a built in heater and thermistor.

Not only will this stand temperatures up to the limit of the thermistor, which is 300C, but it is also much more rigid and does not change in length significantly with temperature. It should also reduce the amount of molten plastic because of the thermal gradient down the SS barrel. That should give less extruder overrun.

A riveting read

A recent modification to the RepRap extruder is the addition of two 3mm pins through the clamp and the PTFE insulator to prevent the PTFE slipping out. My PTFE tube is only 12mm diameter compared to the current design which is 16mm so 3mm pins are a bit too big for it. Instead I used some shafts from pop head rivets which are about 2.3 mm.



I drilled the holes 2.2mm to make them a snug fit.



Here are the pins installed through the clamp after being cut to length and rounded of with a grinder:-

Bottoming

As well as repairing my extruder I aimed to bring it closer to the latest RepRap design. The machined heater barrel has been replaced by a drilled out brass bolt. Partly out of laziness, and partly out of a lack of confidence in my skill with a lathe, I bought a ready made barrel from BitsFromBytes.



I was surprised to find it came with a modification that I had blogged when I made my first nozzle. That was to turn down the end that fits into the PTFE holder to get round the fact that I didn't have a bottoming tap to make the thread go right to the end of the hole in the PTFE. Ironically, I had bought a set of taps which included a bottoming one in the meantime. Well at least I thought I had :-



The tap on the left is the one from my cheap set of taps which only has one for each thread. The boxed set are the new ones that I bought with a taper, second and bottom tap. The single tap seems to correspond to the second tap, which makes sense for a compromise. The thing I don't understand is why the bottom tap still has a point and some taper. I was expecting it to be straight with a flat end, so I made it thus with a grinder :-



To try it out, I made a test thread in a scrap of PTFE and cross sectioned it :-



As you can see the flat end of the heater barrel butts up nicely to the end of the thread in the PTFE. I think it is important not to have a void here as it will fill with molten plastic which will freeze when the heater is switched off. It might then be difficult to melt it again as it is insulated from the heater.

The end of the barrel that is turned down is then a pretty good fit for the acorn nut, although a simpler solution is probably to bottom the thread in the nut and the PTFE and then go back to a plain heater barrel.

Nuts

Since my first attempt at making the RepRap extruder the design has moved on to use a brass acorn nut as the nozzle. It has the advantages of making the extruder easier to fabricate, allows the aperture size to be changed by swapping nuts and allows blockages to be cleared. I have to say that I never experienced a blockage with my single piece nozzle, but I can see how it could easily happen if a bit of dirt bigger than the aperture gets into the barrel.

Unfortunately brass acorn nuts, otherwise known as dome nuts and cap nuts, are expensive and hard to get hold of. I got a couple of un-drilled ones from BitsFromBytes.



My plan was to start with the smallest hole I could drill and expand upwards to see what effect it had and then drill the other to the size I found to be the best. Stupidly, I overestimated how thick the dome was and put a centre drill right through the first one. So now I have one with a 0.3mm hole and the other is about 1.1mm.

This is the 0.3mm bit I used :-



If you use a drill or a drill press it is easy to snap drill bits this small but it is actually very easy to drill 0.3mm holes with a lathe. I spin the nut in the chuck and hold the drill in my fingers. I drill from the inside of the dome. The drill finds its own centre and then I apply light pressure. I expect the same could be done by spinning the nut in a drill chuck.

The RepRap design for the heater barrel is just a flat ended threaded brass tube made from an M6 bolt. This is easy to make but not the ideal shape. Brass acorn nuts seem to be machined from a solid piece of brass. The internal thread is made by drilling and tapping. Because it is a blind hole that means that the thread does not go all the way to the end. If you screw a flat ended barrel into it then it stops short of the end, leaving a void that will fill with molten plastic as can be seen here. Molten thermoplastics compress under pressure, so ideally the amount of molten plastic in the extruder should be kept as small as possible to make the start stop response as fast as possible.

I decided to sacrifice my over drilled nut to find out the inside profile by cross sectioning it :-



Not surprisingly, the inside profile matches a 5mm drill as that is the correct size for tapping an M5x1 thread.



This is how far a flat ended barrel can enter :-



This is my attempt to match the profile :-



And this is the improved fit :-



The chamfer at the end is not quite right. My DeWalt bits have a 110° angle but the standard appears to be 118°.

I decided to take a look at steel acorn nuts :-



These are a completely different animal. Rather than being machined out of one piece they consist of a nut with a dome pressed into it.



They have some advantages and disadvantages : -

• They are cheaper and more commonly available.
• They are smaller so less thermal mass.
• The dome is much thinner, about 0.4mm rather than 1mm, so it is easier to get a short hole.
• Steel has a much lower thermal conductivity than brass so plastic may cool down in the nozzle.
• Steel has a different thermal expansion rate than brass. Fortunately it is less so it should get tighter as the extruder warms up.
• The steel dome might spring out under the pressure of extrusion.
• The flat ended heater barrel goes in further but leaves voids at the side.

These are just the nuts I have managed to buy. I have no idea how much they vary from one supplier to another.

The heat insulator in the picture above is an experimental one turned from a bar of soapstone.

Extruder postmortem

So my extruder died at the beginning of the month. I have been busy with other things so I have only just got round to thinking about rebuilding it. Here is a list of things that failed :-

I got my extruder working in mid August. At the end of September it ejected its PTFE barrel from the clamp, breaking the heater wires. This is a common problem and stems from the fact that PTFE has the lowest coefficient of friction of any known solid material. I tightened it further and it didn't slip out again. However, when I came to dismantling it I noticed that the end that was in the clamp has been compressed by about 0.3mm. The 3.2mm drill that I made the hole with is now quite a tight fit so the hole may have shrunk slightly. I will drill it out to 3.3mm because some of my 3mm HDPE filament is slightly more than 3.2mm where it is a bit oval.

One curious thing is that it now looks to have a thread in the entry hole.



I am struggling to explain this. The only way I can think it may have happened is as follows :-

The filament has a thread cut into one side by the drive screw. Before I added my feed spool the filament used to rotate as it went through the extruder and had a thread cut all the way round. Perhaps it transfered its thread to the PTFE, which is quite soft.

The final failure mode was the nozzle jumping a couple of threads and leaking molten HDPE. It rammed the nozzle through the object being made and damaged the bed underneath.



The bottom end of the PTFE barrel has swollen by about 0.3mm. PTFE is known to creep, i.e. when subjected to a prolonged force it very slowly flows. It has no memory so it does not spring back when the pressure is removed. I think this is why the top of the barrel shrank and the bottom expanded. There is a lot of pressure in the barrel and it is close to PTFE's maximum operating temperature. PTFE melts at 327°C but it starts to degrade at 260°C. I have been running at 240°C which is a bit too close for comfort.

My barrel, at 12mm, is smaller than the current recommendation which is 16mm. This may have contributed to the failure.

The recommended solution is to fit a pipe clip, but I didn't have one small enough, so I pressed a short section of 15mm OD pipe over the end, which is a tight fit. It had the desired effect. When I first made the barrel the thread was a snug fit. When I dismantled it I noticed it had become quite sloppy. With the ring of metal in place it is now a tight fit. I will use some plumber's PTFE tape to seal it as well.



The other thing that was on the point of failing was the flexible drive shaft. Strands started breaking and the more that broke the more it flexed, so it was a kind of avalanche effect. I estimate that the shaft had rotated about 100,000 times so the flexing backwards and forwards must have caused metal fatigue. I probably have the most mileage on this part of anybody so far so it could be a sign of a design flaw. There are a couple of problems with my implementation which certainly won't have helped.

The first is that I used some 2.5mm cable I had to hand rather than the 3mm recommended. It doesn't seem like a big difference but I think the rotary strength is at least a cube law which would make my cable roughly half as strong. It was left over from a garage door installation but I don't think I have any worries there as 100,000 flexes is about 75 years use!

The other contributory factor is that the bearing lands on my drive shaft are a little bit eccentric. This stems from the fact that my watchmaker's lathe is not really big enough for this work. Fundamentally the hole through the headstock is not big enough to take the 5mm threaded rod.

Each time the shaft rotates it opens and closes the pump halves a little. This makes a big torque variation over a revolution because of the strong springs holding it closed. That caused the cable to wind up and unwind a little each revolution. It actually modulated the filament width and gave the objects a basket weave appearance. Here is a good example :-



I bought some new parts from www.bitsfrombytes.com, which wasn't an option when I first made the extruder.



That will allow me to give the flexible drive a fair test. If it proves to be unreliable then I will switch to direct drive. I found out from the core team that it is not required for any of the polymers currently used, only things like Field's metal.

Another thing that was starting to fail was the J-B Weld holding the heater wire to the nozzle. It is supposed to be rated to 315°C but it had started to crumble with my extruder running at 240C. The other problem I had with my J-B Weld is that it does not cure in the specified time at room temperature. I have to bake it to make it strong. I emailed J-B Weld but got no response apart from an automated reply.

David commented on my last post suggesting Thermosteel ,which is good for 1300ºC, so I will try that next. A couple of the core team use BBQ paint which handles 600°C so I will try that as well.

If it's not one thing it's another!

The third post in a row about my extruder breaking, not a good start to the year!

Now that I am making solid test shapes rather than hollow ones the extruder is working a lot harder and all it seems to do is break down. The drive cable started to disintegrate this afternoon, I could hear the strands breaking :-



It was still limping on however when this happened :-



The brass nozzle started to come out of the PTFE heat barrier. This was ironic because it was only yesterday that I said I had not had this problem in a forum discussion. Others have had it happen and the collective wisdom is to use a pipe clip round the end of the PTFE to secure it.

And the JB Weld which insulates the heater wire has turned to dust :-




So some rebuilding to do!

Running repairs

No sooner than I had fixed my heater, the extruder motor failed!

I bodged the heater connection by putting some more solder on it. It's not a permanent solution because the solder is molten while the heater is on so it slowly oxidizes away. The last time bodged it that way it lasted six months though. It really needs a crimped connection.

The GM3 motor failed by running slowly, getting very hot and drawing lots of current. It eventually caused the protected MOSFET that is driving it to shut down. Opening it up soon revealed how it had failed :-



It has two pairs of copper brushes. Three of them have holes worn right through and the fourth has broken off. Its stub was touching the wrong side of the commutator, causing a short.

More expensive motors have carbon blocks on the end of arms which can wear down a lot further before they fail. Bigger motors have spring loaded carbon rods. The gearbox shows no sign of wear so it is let down by the cheap motor.

This motor is not really up to the job of driving the extruder. It is being severely abused by running it from 12V PWM when it is only rated at 6V. I anticipated it would not last long and ordered a spare when I bought it. I fitted that and HydraRaptor is up and running again. Curiously the second motor seems a lot quieter than the first.

At some point I think I will upgrade to a stepper motor. They are more expensive but, as long as you don't load the bearings, they last virtually forever. In the long run they probably work out cheaper and I can also dispense with the shaft encoder and the interference suppressor.

Wear and tear

My extruder's heater went open circuit so I removed the heat shield to have a look at it. I have actually run it for many hours now and have extruded quite a lot of HDPE. I have about 200g of extruded test objects and scrap which represents about 13 hours operation. I only recently started saving my scrap so I must have extruded a lot more. The 2.5Kg reel of HDPE is noticeably smaller.

The heater has also run for a lot longer than the extruder has been extruding. I got fed up of waiting for it to warm up at the start of each run so my host software leaves it on. I keep meaning to put a timeout in the firmware to turn it off when there hasn't been any Ethernet messages for a while as I have left it on for long periods a few times.

The extruder is starting to show some signs of aging. The plastic shield which keeps the fan draft away from the nozzle looked like this when I made it :-



But now it looks like this :-



The nozzle itself now looks like this :-



The JBWeld that surrounds the heater wire has gone very dark and has several cracks in it. One of the heater connections broke off in a previous accident so I dug it out and joined a piece of copper wire by squeezing it tight and soldering it. There is now no sign of the solder which is why it has gone open circuit.

The black stuff which looks like bitumen must be slow cooked HDPE. I am surprised that long term heating to 240°C causes it to decompose. I don't know if the white surface on the shield is just due to its surface melting a bit or whether something boiled off the nozzle and condensed onto it or reacted with it.

Even the high temp insulation over the thermistor wires is starting to look a bit sad!

I also noticed that the steel wire that forms the flexible drive coupling is starting to break up. A couple of strands have snapped and there is a pile of black dust on top of the pump shell.



The heater connection should be easy to fix. I have a few planned improvements to make to the extruder but I will wait till parts wear out before replacing them with better ones to get the most use of it.

Extruder dimensions

I have been asked for dimensioned drawings of the extruder. I made these by manually inspecting the 3D models in ArtOfIllusion. It is not the easiest application for extracting dimensions so I made 2D drawings in Visio which does have good dimensioning tools. I then made Python scrips to do the milling. My dimensions may differ in places but I did make the extruder from these drawings and it does work. I tightened up some of the hole clearances because my milling machine holds much tighter tolerance than FDM.

This is the motor shaft coupler. I adjusted the slot to suit my GM3 motor. I think there are now two versions of this part. The official design is tapered but this is not necessary with the offset motor mount so I simplified it to a cylinder.



Here is the finished article milled with a 2.22 mm bit. The step on the outside and in the shaft slot are there because my milling tool's shaft is wider than the bit, so to go deeper than 9mm I need to have some clearance. The material is some sort of metal loaded resin.



Here is the clamp drawing I used. It has now been superseded by a larger design. Note that I adjusted the hole for the PTFE to suit my 12mm rod. I think the official design was 10mm but is now 16mm. I also widened the slot to allow the 2.2mm milling tool to get in and added some extra mounting holes to suit my machine.


I milled it from 9mm Delrin.



Here is the pump drawing :-



The poly channel on the official version slopes outwards at the entry but that is only needed for the version without the offset motor.

And here is the milled version :-



The material I used is not as slippery as CAPA so, to reduce friction in the channel, I smoothed it with emery paper, polished it with metal polish and sprayed it with PTFE dry film spray.

I split the motor mount into three pieces for milling from a sheet of 5mm perspex. I fixed the pieces together with M2.5 screws, tapped into the perspex.





If anybody wants the Visio source file it is here:- forums.reprap.org

Stretching a point

In his article: x-idler-bracket-continued Vik Olliver alluded to the fact that you can extrude filament with a smaller diameter than the hole in the nozzle. I did some experiments to see how fine I could go. In fact the final filament diameter is simply determined by the feed rate of the extruder and the travel rate of the nozzle, or in my case the bed. The filament stretches to the length that matches the rate of travel while it is still liquid. You can then calculate the mean diameter from the volume of material extruded. The nozzle height has to be a bit less than that mean diameter and then the width becomes a bit wider.

Here are three 20 x 20 x 20 open cubes with different wall thicknesses :-



The first was 1mm diameter filament extruded at 4mm per second with a height of 0.8mm giving a wall thickness of about 1.2mm.

The second was the same feed rate but with the extruder traveling over the bed at 16mm per second to give 0.5mm filament, the same as the nozzle hole diameter. The height was set to 0.4mm giving a wall thickness of about 0.6mm. As you can see it warps more but I expect it would behave if it was building a solid object. The bottom layer which was stuck to the table has better corner definition.

The third attempt was 0.35mm filament extruded at 16mm per second with a hight of 0.28mm and a width of about 0.5mm. As you can see holes started appearing but I think that was just because the sides buckled so badly. Interestingly the holes can be bridged by filament above that needs no support. Again, I think this would be OK making solid objects, or at least objects with thicker walls.

This is really good news as it means I can get down to the sort of resolution commercial machines get (0.25mm) without having to have a very small nozzle aperture, which would limit the flow rate. It remains to be seen what effect stretching has on the polymer but as it is still liquid at that point I think it wont increase the contraction much, if at all. It does mean I need very fast head movement to keep up the deposition rate, about 64mm per second. I think my machine will do that if I reconfigure the steppers for speed rather than torque, a simple one wire change.

Scaling new heights

In my article laying-it-on-the-line I showed how I arrived at this test shape, a 20mm open ended cube :-



I decided to try different sized shapes to see how the process scales. It turns out that it doesn't and 20mm cubed is the magic size that is easiest to make!

The first thing I tried was taller :-



As you can see at 50mm tall it is starting to sag and 100mm is hopeless. The problem is that as the height increases, the plastic already laid down contracts as it cools and leaves the nozzle high and dry. I fixed that be reducing the Z increment from 1mm per layer to 0.95mm. That allowed me to make a good 20 x 20 x 95mm square tube :-



I presume with this increment I can keep going up, but who knows, I thought that at 20mm!

Next I tried low and wide. This was an attempt to make 120 x 120 x 20mm :-



I stopped it when the first two layers failed to weld. This was because with an object this large, by the time it has traced the perimeter to start the next layer, the first layer has cooled down to room temperature. In my post sticking point I predicted that to make an instantaneous weld between molten plastic and plastic at room temperature requires the molten plastic to be at temperature

T = 2 x Tm - Tr, for HDPE and 20°C this means about 240 - 250°C.

I set my extruder to 240°C and made this mess :-



I don't like running the extruder that hot because, although HDPE is not supposed to burn until 350°C, it smells like burning plastic and the end of the nozzle is glazed black. Also, the toothbrush that wipes the nozzle is showing signs of melting.

The object came unstuck from the foam board because of the extreme corner curling due to shrinkage. This is a fundamental problem with HDPE and room temperature FDM. HDPE shrinks about 2% when it cools from its melting point to room temperature. Commercial FDM machines use ABS, which has a lower shrinkage, and they keep the work piece in an oven close to the melting point. That means the hot plastic does not need to be so much hotter than the melting point, and most of the shrinkage occurs after the object is complete. The problem here is that the first layer cools and shrinks before the next layer lands of top. The next layer is bigger when it welds on top but then it shrinks, contracting the bottom layer more. Each subsequent layer increases the tension on the layers below. The bigger the object is, the worse the effect is because the mismatch between the size of the hot layer and the cold layer below it is bigger in absolute terms.

Following in Forrest's footsteps I tried laying down a raft of HDPE first to anchor the object to the foam base. The raft is 120 x 120mm but the object is now only 100 x 100 x 20mm.



As you can see that gives a big improvement but it wasn't strong enough to hold the corners down fully. A bigger raft, and perhaps a second layer might help but as it was an hour to build the raft and half an hour to build the object I didn't bother trying again. The blob, by the way, is where the firmware crashed on the last layer!

Here are some 40 x 40 x 20mm tests made with rafts, the second one had a bigger raft:-



Here the corner curl with a raft is comparable to the 20 x 20 x 20mm test without a raft showing how this effect gets dramatically worse as width increases.

Next I tried tall thin objects :-



Both were made on rafts and, the first is 15 x 15x x 75 mm, the second is 10 x 10 x 100mm. The photo is not very good but they both flare towards the bottom. The one on the left has an untidy surface as each layer is not well aligned with the layer below it and the one of the right has a completely wavy surface like basket work.

The reason for this is that because the perimeter is shorter, the layer below is still molten when the next layer is extruded on top of it. It moves around giving an untidy surface and also does not resist the contraction of the layer above. The bottom few layers are the correct size because they are welded to the solid raft but the layers above are too small as they have contracted inwards. A 5 x 5mm test shows this effect even more :-



The only way around this is either to wait for the layer below to cool, speed up its cooling with a fan, or extrude very slowly. I decided to experiment with a fan. It was immediately obvious that if you have a fan blowing near the nozzle you have to insulate it otherwise it doesn't reach its target temperature.

The RepRap design uses fiberglass wool but I wanted to be able to see the state of my heater so I decided to make a transparent cover. I started with a plastic test tube, donated by my wife, which used to contain bath salts.



I cut the end off this and drilled a hole to clear the nozzle. I converted a large plastic nut into a mounting flange by stripping out the thread on my lathe so that it was a push fit.



Here it is mounted on the extruder :-



The first fan I tried was a small North bridge cooling fan. It was so light that I could mount it on a stiff wire attached by ring tail crimps and bolts. :-



Unfortunately it wasn't very powerful so the next fan I tried was a PC case fan complete with speed control and blue flashing LEDs.



This worked a lot better but it is difficult to get it as close as I would like it. Here is the tallest thing I have made so far, it is 10 x 10 x 150mm. At this point I changed to 4mm per second travel and a feed rate to give a 1mm filament. I found that you can get finer filament just by stretching it as it leaves the nozzle so the work I did with flow rate and filament size is not really relevant. I had to reduce the layer height to 0.8mm.



This worked well on the windward side, with a nice tight corner, but not so well on the leeward side. The corner away from the fan is more rounded and the layers are less tidy. A cross section shows that the two sides cooled by the fan are straighter and longer.

The 5 x 5 x 20mm test is much improved but its surface area is so small that the fan fails to keep it cool enough. I think the only option with something as small as this is to slow down, and possibly drop the temperature.



Again the leeward side is not as good. The filament short cuts the corner because the layer below is not strong enough to hold it in place. I think to make the fan effective a cowling and duct is needed to get a strong flow of air directed downwards around all sides of the nozzle.



I have noticed that there is always an excess of material at the corners. This is because the head makes a perfect right angle but the filament has a minimum bend radius and takes a short cut. I am not sure how to compensate for this. I can't really pause the extruder because its response is too slow, so I either have to speed the head up as it takes a corner, or perhaps make it move in an arc that matches the bend radius rather than a right angle.

And finally here is an improved version of the magic 20mm cube :-



This is with the benefit of a raft, finer filament and fan cooling. The corners are a bit sharper and the corner curling a bit less. The reasons why this turned out to be the optimum shape are :-

1. It is small enough that the filament does not cool too much when you go round it.
2. It is large enough to make it long enough to traverse so that it does not stay too hot.
3. It is short enough for the fan to be able to cool the back wall from the inside as well as the front.
4. It is small enough for corner curling to not be too extreme.

From these experiments I now think I have a good understanding of how the parameters: temperature, flow rate, traversal rate, z increment and fan use affect the result. I have only looked at thin walled boxes, I expect solid objects to add more thermal and contraction issues.

The only reason I am using HDPE is that it seems to be the only thermoplastic filament I can buy off the shelf in the UK without getting it specially made.

With a bit of trial and error I expect I could make the machine produce a wide range of shapes and some useful objects but therein lies the problem. It is not supposed to be trial and error. The dream is to be able to input an arbitrary 3D model, of any size within the build volume, and have the item appear a few hours later. At the moment I can't see how that can happen with room temperature extrusion of HDPE. Its melting point is too high and its contraction too great. Managing the temperature of the object being built is very tricky as the features of the object vary from large to small.