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.

J-B Weld Heater

I ordered some Cerastil H-115 high temperature ceramic glue today but it will not arrive for a few weeks so I made another heater with J-B Weld. It is much quicker and easier than using BBQ paint.

I spread a thin layer on the barrel to insulate it and left it for 6 hours to set. Last time I used a thicker layer and turned it down but this time I just spread it very thinly with a spatula while hand rotating the lathe.



Next I wound the nichrome using a nut with a small hole drilled in it to anchor the start. I anchored the end using a piece of copper wire tied to it and pulled over a support round the back of the chuck. I expect this could be done with a drill chuck if you don't have a lathe.



I then added a second, thicker, layer of J-B Weld and left it over night to set.



Finally I slowly heated it up to 200°C in an oven to cure it.



I haven't tested it yet, apart from checking it for opens and shorts but it is pretty much the same method I used first time around: hydraraptor.blogspot.com/2007/07/hotting-up

Back to square one

I moved on from J-B Weld as a means of making a heater because it does not handle the high temperatures I have been using for HDPE. I completely failed with Thermosteel so I decided to have another go with BBQ paint. It seems that I must have the wrong sort of paint because despite helpful advice from Vik Olliver and Forrest Higgs, after a week of trying I can not get it to work.

After many attempts the final method I used was to put down three coats of paint using a paint brush in my lathe. Each layer has to be allowed to dry for many hours and then baked with a heat gun. If I apply heat too soon then it blisters. If I don't apply heat then the next coat simply dissolves the coat underneath. The paint has a lot of very volatile solvent in it.



Once I had three coats, I baked it in the oven on full blast (gas mark 9+) to make it hard enough to take the wire. I anchored one end of the wire with a nut that has a small hole drilled through it and attached a weight to the other end to keep it taught while winding.



To keep the wire in place while I painted over it I tied it to a piece of copper wire wrapped round the back of the chuck. To keep the tension in the right direction I used a piece of PTFE left over from a previous experiment to support it. It was an ideal shape and could stand the heat from the heat gun.



The picture above is after one coat of paint. When the paint was applied it was thick enough to completely cover the wire but when it dries it is very thin. I put five more coats on and baked it in the oven. I was somewhat disappointed when it came out like this :-



The paint resembles soot and has no strength to it at all. I can scrape it away with my finger, like I could with the Thermosteel.

Time to step back from this and think again. It is crazy trying to use high temperature paint as a high temperature adhesive. Some makes of paint may work, but you can't complain when other makes don't. I think it makes a lot more sense to use something designed to do the job such as Cerastil H-115. I will order some on Monday. In the meantime I will go back to J-B Weld because it is easy to use and will last for months if I keep the temperature down.