Tuesday, 23 August 2011

High power hot-end for fast printing - dual resistor & easier thermistor

This is the first of a few new hot-end designs I have been testing, I mounted it to my new ‘Bling’ machine for some speed and temperature testing, it’s performing well, here is how I made it - 

I decided to try a new shape for the heater block, normally you want a low profile block with a short hot-zone, but I specifically wanted to try a triangular shaped block for a number of reasons / goals - 
  • I wanted to mount 2 x Resistor heating elements in the block so a wide base made this easy.
  • I wanted to try a longer heating zone to see how fast I could extrude.
  • Triangle gives smaller mass at the top, hopefully to reduce oozing due to the longer heating-zone.
  • I wanted to be able to easily wrap the block in PTFE sheet, a triangle gives a easy to wrap shape and is insulated at the top with two thickness of PTFE.
  • For the future I want to be able to extrude fast @ 300 degrees C with less than a 4 degrees C temperature drop while printing solid layers.

Next I REALLY wanted to switch away from using the tiny dipped thermistors the size of pixi-dust with ultra thin connecting wires, they are such a pain to use and don’t really have many plus points apart from the low thermal mass/quick response. I now only use a ‘normal’ Axial style. I can fully recommend doing this, it’s 1000 x easier to build and much less fragile, I also think I get much better temperature reading stability, response time is fine.

Thermistor Type:NTC; Resistance:100kohm; Thermistor Tolerance:± 10%; Beta Value (K):3974K; Operating Temperature Range:-60°C to +300°C;

You can buy them from Farnell -  only £0.25p

I have used 2 x 10ohm Vitreous enamel resistors in parallel; this gives approximately a 5ohm load where you would normally use a single 6.8ohm resistor.
The resistors are either side of the nozzle, giving nice constant heating.
The axial thermistor is slightly above the resistors and is just touching the side of the nozzle barrel, this placement gives me a quite constant 3 degrees C lower reading than the inside of the nozzle throughout the temperature range, which is fine.
Wrap the thermistor body and legs in Kapton to insulate.

Use Aluminum foil or tape around the Resistor body, it should be a good tight fit in the block. 

All in place and secured with Fire cement.

The part make-up looks like this. (Ignore the PTFE washers; I didn’t use these in the end).

I also wrapped the heating block in a strip of 0.5mm PTFE sheet.

Still using PEEK in this design, I expect to remove this part in the next version.

All wired up using high temperature heat-shrink and silicon coated wires.
And assembled.
I added a small heatsink, but I doubt this is really required.

The PEEK block is only luke-warm to the touch even after 4+ hours of printing – Note I am using a small 40mm directional fan aimed at the PTFE.
I will do some more tests to see if I can switch off this fan and still keep a cool cold-end.

My Fan-duct for Greg's Linear bearing X Carriage is on Thingiverse here.

It’s printed just over 2Kg of material so far at ~160mm/Sec and it’s performing well; I’m really happy with it. All of the multicoloured Faberdashery printing in my previous blog update was done with this setup along with some ABS parts and build sheets.

I have not yet found an upper speed limit as I need to do more testing, but it manages to print PLA at 180mm/sec with only a 2 degrees C drop in temperature when doing whole build-plates of full infill layers.

I will be testing ABS more extensively with it over the next week and seeing how fast I can get it working with different materials.

My main observation now is that you do need to run this style of hot-end at a minimum of 80mm/sec, and I need a 1.5mm extruder reverse to make sure oozing is not a problem. At speeds lower than 60mm/sec it’s better to use a smaller heating zone/block.

Next time I will still use a triangular block but make it slightly smaller overall and a little shorter.

Thanks for looking, I would be really interested in any comments or opinions on high-speed printing and hot-ends designed for speed and also high (290+ temperature) - you'll see why in a future post....