Saturday 7 January 2012

Chapter 24 - In which I design and build a new hot end

Happy new year! I'm still behind with my blog postings (no surprises there...), but am trying to catch up. One of the biggest and most significant event in the last few months has been the design and build of my new extruder hot end.

In my last post, I described how my PTFE barriers had all failed. With that in mind, I set about designing something that would be incapable of failing as the PTFE had done. I wanted to steer away from PEEK due to the cost, and the fact that the operating temperature range is only slightly above that of PTFE - I know that with my luck I'd start to push nozzles out of it.

Inspired by one of Nophead's designs, I chose to make the new thermal barrier out of stainless steel. Although stainless is considered quite a poor thermal conductor (amongst metals) it was still going to get pretty hot when the nozzle is at 200C. I designed the barrier to be made from 16mm bar stock so it will fit in the body of wades extruder, with a set of cooling fins along the body. The high temperature of the hot end, the fact that the barrier is only 40mm long, and the relatively large mass of the barrier means that I will have to actively cool it to prevent the feedstock from swelling and jamming. A small fan will blow across the fins to remove the heat.

To prevent jams at the transition between the barrier and the nozzle, I will be using a PTFE liner. I had considered using sleeving, but eventually settled on some penumatics pipe (ebay again - 5mm outside diameter, 3mm internal diameter). This would run down the inside of the barrel and nozzle, right to the tip. This did create a problem though - in order to get the liner down into the nozzle (a MIG tip), I would have to reduce it's external diameter. The MIG tip's thread size is M6, which has a 5mm core diameter - if I drilled the MIG tip out to 5mm, there would be no thread left! I settled on a 4mm internal diameter for the tip, and a stepped liner design - 4mm in the nozzle and 5mm in the barrel.

In a last minute change-of-heart, I dropped the copper sleeves and car battery clamp of my earlier design. I would have to use machine tools to make the new barrier, so why not treat myself to a new heater block too? It is aluminium, 16x16x10mm, has a grub screw to hold the nozzle in place and has the thermistor positioned near the nozzle for a more accurate temperature reading.

Here is a render of the new design:



And here is a cutaway:


I borrowed the machine shop at work for the lathe- and drill-work. I want to thank Martin and Wesley for giving up their lunchtimes to help me!

No inverted drill here - a proper tool!
We used a long M3 bolt down the inside of the PTFE pipe to keep it straight whist machining the reduced section:

5mm pipe on an early drawing for the barrier
Lots of machining later, we have a completed barrier assembly:



The liner fits very snugly into the MIG tip, which screws nicely into the barrier. The liner sits flush with the top end of the barrier.

About the MIG tip itself - it has been drilled internally to 4mm as mentioned above, but has also had the outer bottom face machined back. We also reduced the nozzle hole size by tapping it with a hammer. While it's great that the hole is smaller, it also presents a problem - just how big is it? I guess I'll find out when I try to extrude.

I forgot to take a photo of the new heater block being made, but here it is wired up with the resistor in place:

I hope blue marker pen fumes aren't toxic...

So, does it work? You'll have to wait till next time to find out...!