KISS Hot End
For those unfamiliar with the acronym KISS it stands for "Keep It Simple, Stupid". This version is an attempt to design a hot end that performs well and is still quick and easy to fabricate.
Key aspects of the hot end are:
- Minimal part count
- Can be fabricated without the need for a lathe or milling machine.
- If you do use a lathe then you don't need to use advanced techniques like 4-jaw chucks and turning external threads.
- Eliminate joints that can leak plastic (still has one joint that can leak if not tightened)
- Easy to clean out blockages.
- Easy to dis-assemble to swap for a different sized orifice.
The current design still has some issues. Please see known issues for details.
The design uses an off the shelf brass M8x30mm hex head bolt to form the extruder barrel and nozzle. This brass bolt is threaded through a simple aluminium block that carries a heater cartridge and a thermistor. A PEEK insulator prevents too much heat from being transferred to the extruder body.
The M8x30 brass bolt is modified by drilling down the center towards the head at 3.2mm or 2mm depending on the filament size you intend to use.
Drill to within about 0.5mm of the bolt head. This can be done in a drill press. You could probably even do it with a hand held drill if you were careful.
Next drill the hole out to the same size as the OD of the Teflon/PFA tube and to a depth of 5mm. This should be about 5.2mm for 5mm OD PFA tubing.
Remove the first 2mm of thread from the end of the bolt. A bench grinder, angle grinder or file can be used for this. The head of the bolt is then shaped to a cone shape, leaving a flat of about 1mm diameter. This can be done by gripping the bolt in a drill and spinning it while removing material with an angle grinder, bench grinder or a belt sander. You could also use a lathe if you have access to one. Finish it off with a bit of wet and dry sand paper to give it a nice appearance.
The last step is to drill the orifice. Drill a 0.35mm to 0.5mm hole as you prefer in the center of the flat area on the tip of the bolt. This can be done in a drill press with care. Carbide PCB drills are easier to use than HSS jobber drills. They have a 1/8" diameter body. The jobber drills are usually too small for a standard drill chuck to grip. Use the highest speed your drill press is capable of and take lots of small pecks at the hole until it breaks through.
The aluminium block is easier to make. You need a block about 20 x 20 x 12 mm or 3/4" x 3/4" x 1/2". There are a few ways to get this. Buy 20 x 12 mm or 3/4" x 1/2" solid bar and cut a short length from that. 20 mm or 3/4" square bar could also be used. Or you could get some 12 mm plate and cut out a 20 mm square.
Clean it up and square it with a file then mark out the three holes as per the drawing. Positioning is not critical.
Tap an M8 thread into the largest hole. Take care to get the thread started square to the hole. It is best to use a taper tap for this.
If you have a 6mm reamer available then drill the 6 mm hole under size and use the reamer to finish it, otherwise just drill it 5.5 mm and then drill again to 6mm.
The last part is the plastic insulator. Drill and tap as per the drawing. For the M8 thread you should start the thread using a taper tap to ensure it is square and then use a plug tap to finish the thread to the required depth.
Use a hacksaw and a file to cut the groove mount slots near the top.
The heater cartridge used for the prototype is 6mm diameter and 20mm long and provides 25W at 12V. You can also buy 6 x 23 mm 40 W cartridges through e-bay. You will need to make some changes to dimensions if you use the longer heater cartridge. It is important that the brass nozzle bottoms out in the tapped hole of the PEEK insulator and that the PFA tube is the right length for the cavity.
If you want to use a wire wound resistor for the heater you can. Just adjust the 6mm hole size to suit your resistor.
When mounting the heater cartridge the prototype did not require any heat sink paste or glue. With a close fitting hole and a heater cartridge there is plenty of heat transfer. If you are using a wire wound resistor then some fire cement or exhaust gasket cement would be a good idea. Also worth considering is Loctite 5920 CopperMaxx. This is a hi-temp silicone sealant that withstands 316°C and contains copper particles.
Teflon PTFE or PFA tubing
The short length of Teflon PTFE or PFA tubing is important it has a low coefficient of friction and ensures the filament can't swell up and jamb if it softens before it enters the barrel.
The prototype used PFA tubing. PFA has much the same properties as PTFE but it is transparent and is easier to extrude into tubing. Both are types of Teflon.
The heater block will benefit from being insulated, particularly if you have cooling fans directed at it. The prototype was wrapped with 2 layers of red silicone foam rubber sheet about 1 mm thick, held in place with a strip of Kapton tape.
The glass fibre sheet that has been used on other designs of hot end would also be good. With a 25W heater the hot end maintained 200°C with about 60% on time. The heat up time was only a few minutes. If a fan was blowing on the bare hot end then the heat up time was too long.
- Screw the brass bolt into the aluminium block
- Do it up nice and tight
- Insert the PFA tube into the PEEK insulator
- Insert the heater cartridge into the aluminium block
- Screw the insulator onto the end of the brass bolt
- insert the thermistor bead into the aluminium block
- Wrap a strip of Kapton tape around the PEEK insulator and the wires to hold the thermistor and heater in place.
Drawings of all components:File:KISS Hot End.PDF
Bowden Hot End
If you want a bowden type hot end then make a tapped hole at the top end of the PEEK insulator. M5, 1/8"BSPT or NPT thread depending on your choice of pneumatic fitting.
For 3mm filament the bowden tube should be 3mm ID/5mm OD. IMI Norgren manufacture fittings for 5mm tube that work. They require a 1/8 BSPT thread. Norgren P/N101250518.
For 1.75mm filament the bowden tube should be 2mm ID/4mm OD. Norgren P/N 101250418 is 4mm to 1/8BSPT push fit.
If you use this with a PLA extruder body then you need a cooling fan blowing across the PEEK insulator. Without it the heat transferred through the PEEK will get the extruder body hot enough to get a bit soft. A small amount of airflow is required to prevent this. The prototype had a small 40 mm video card fan running at 30% and ran without issues.
Machining grooves into the PEEK insulator would help prevent this. The grooves do this by reducing the conduction path and by increasing the surface area. These grooves could be made using a hacksaw with care but a lathe would make this task easier.
When reassembling the hot end it pays to heat it up and tighten it again. Some leakage will occur between the PEEK insulator and the brass nozzle. A few wraps of PTFE thread tape might reduce this.