Tough as steel, my a...

I had originally intended to rebuild my extruder with the barrel I bought from BitsFromBytes but I ran into a compatibility problem with the nichrome wire I am using. My wire, as well as being un insulated, is a bit lower resistance than the recommended stuff, which I can't buy in the UK. I need about 300 mm rather than 200 mm to get the required resistance.

Going from a 6 mm barrel to an M6 threaded barrel means that I can only wind it with the pitch of the thread (1 mm) and the diameter is reduced, so there isn't enough room to accommodate 300 mm. A friend suggested using a finer thread which seemed like a plan. I have an M6 x 0.75 tap and die so I thought I would use that. Unfortunately finer pitch means shallower, so I would need a 5.25 mm drill bit which I don't have and I thought it was probably a bad idea to use a shallower thread in the PTFE. So in the end I made a new barrel with a thread on each end and an un-threaded section for the heater :-



I used J-B Weld for my last heater but it did not stand up to the heat very well. The bit near the thermistor, which I know was at 240C, and the ends of the heater remained strong, but the rest turned to a light brown dust with a harder darker skin over it. It was still functioning as a heater until I touched it at which point it started to flake away.



On the packet it states "J-B Weld maximum temperature is 600°F" which is 315°C. I know the heating element is going to be a bit hotter than the barrel, particularly when I was extruding with a fan on, because of the temperature drop across the thermal resistance of the J-B Weld. I doubt that it got to 315°C though. I emailed J-B Weld about this but I didn't get any reply other than an auto acknowledgment. Looking at their website I see the following "withstands temperatures up to 500°F" which is only 260°C so no wonder it failed.

The RepRap instructions suggest using Dulux Spraykote BBQ paint as a substitute for J-B Weld under the heading "But They Don't Sell JB Weld Here". Ironically they only seem to sell that in New Zealand so I got a local BBQ paint.



It turned out to be quite nasty stuff. Probably not a good idea using it in doors but it is too cold to do it outside at the moment. It went on easy enough but the RepRap instructions suggest three coats under the wire and four over it.



There are no instructions on the paint about drying times and re-coating. Impatiently I dried it with a heat gun, as it is rated to 450°C, but it blistered. I should probably have used a hair dryer and / or been a bit more patient. I apologise for the rubbish photo but you can see the size of the blisters.



I tried turning them down with the lathe but the paint just flaked off so I was back to square one.

I then thought I would give ThermoSteel a try. Supposedly it is a steel filled water based epoxy paste similar to J-B Weld but rated to 1318°C, although I don't know how you can have water based epoxy. I read somewhere else it was a ceramic paste which makes a bit more sense to me although I am not a chemist.



My plan was to put down a thin layer and then machine it flat with the lathe, wind the heater and cover it with a thicker layer, a technique I used successfully with the J-B Weld. It does say it is machinable.

When mixed up it resembles wall paper paste with iron filings in it. It was impossible to spread thinly, I had to dab it on to get it to stick.



I let is set over night and then as it says it gets stronger when heated (although it does not say to what temperature), I heated it up to gas mark 9 which is 260°C at the rate our oven warms up and then let it cool down at the rate the oven cools down when switched off. When it came out it looked like this :-



It looked a bit fragile so I scraped it with my fingernail and it came off in much the same way as the J-B Weld did!



So not getting very far with making a new heater. I have no idea why my ThermoSteel is a weak crumbly substance instead of something resembling steel. Should I have heated it a lot more? Have I been sold a small pot of wallpaper paste with Iron fillings in for £12.75?

I am not sure what to do now, perhaps try the BBQ paint again, use J-B Weld as I know it at least works for several months or make an induction heater.

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.