Friday, December 10, 2010

Rebuilding the Original Heater

I've rebored the PTFE to remove the notches I previously made on the inside of the barrel. It now has a diameter of 5.5mm and is completely smooth again. I've remounted it on the extruder base and attached the nozzle.

I've also rebuilt the original nichrome wire heater wrapped around the nozzle, just in case the new heater block I've made is causing the problem with molten PLA backing up into the PTFE barrel.

The heater part works fine, unfortunately the nozzle is still filling with molten PLA. Since it never used to do this before the original catastrophic failure of the PTFE barrel that this is the cause. I'll need to make a new barrel.

The problem is I don't have a lathe, so do I spend $95 on the techshop lathe class so I can use their lathes and then pay $125 for the one month membership, or pay ~$500 for a mini-lathe for my workshop. 

I'd like a lathe, but I doubt I'd use it again after this. Perhaps Santa will bring me one?

Sunday, December 5, 2010

Extruder Problems Continue

After rebuilding the extruder and running some test prints the extruder is still having problems. After about 10-minutes of printing the filament stalled again. The PTFE of the nozzle had backed up with molten PLA again.

I think I'll try two more things tomorrow. Firstly, I'll rebuild the extruder with the original nichrome wire heater - perhaps the block heater is more suited to the new style extruder where the brass doesn't get inserted as far into the PTFE. I think that perhaps because the brass nozzle extends into the PTFE 15mm or so, it is transferring heat further up the insulator than with the new style extruder.

Secondly, I'll try boring out the PTFE insulator. There is a slight notch on the inside where the drill slipped when I was drilling out molten PLA previously. This notch appears to be catching the molten PLA, which could be causing the feed problems. This will increase the internal bore diameter slightly which hopefully won't cause any problems.

If none of this works, I'll be rdering some PTFE and joining TechShop and using their lathes.

Saturday, December 4, 2010

Building the Block Heater

As part of rebuilding the extruder I thought it would be a good idea to build the block style heater, rather than than the original heater. I made a couple of modifications to the build, we'll see how they work out.

The basic components of a block heater.
I followed the mechanical drawings and plans on the RepRap Site. It did require me to buy a drill press, metric drills and a metric tap and die set, but I do enjoy having new power tools!

I used a 3/8" ultra-machinable brass bar from McMaster. Cutting and machining it was fairly straightforward. The new drills and drill press did their job well.

I haven't tapped anything since I was at school, but the technique came back quite quickly. A quick test fit showed that everything went together easily.

I used 1mm PTFE tubing I found at Frys to insulate the leads for the thermistor.

To increase the thermal conduction between the thermistor and the resistor and the block I used Biostar Nano Diamond thermal grease. It is good up to 240c and is an electrical insulator. I filled the thermistor hole with the grease before pushing it home. The resistor was coated with the grease before sliding it in to its hole. 

With all the parts inserted.
While I was shopping for the fire cement I found a Rutland caulk good to 600F at Ace. It doesn't require heat curing and is flexible. I'll give it a try and see how it works out. It was a little messy to apply but should be better than fire cement.

After wiring it up and waiting for it to cure first tests appeared to be promising. It heated up quickly to the specified temperatures.

First tests with filament did not go so well. The filament did not feed very well and the output from the nozzle curled up! - Not sure why?

Then catastrophe struck again, the nozzle appeared to seize up. It appeared that molten PLA had backed up the PTFE and solidified.

It was another late night dismantling the nozzle and cleaning it out and drilling out the solidified PLA out of the PTFE. After it was all clean I checked the nozzle opening in case there was something stuck in there causing the curling.
Complete and back on the extruder. Note the new sprung loaded mech.

Using another thermistor as a temperature probe inside the nozzle it I checked the temperature inside the nozzle. There appears to be a significant difference between the block temperature and the nozzle temperature. The block has to register between 250 - 260c for the internal temperature of the nozzle to register between 205 - 220c. This may have been the cause of the feed problems. At a block temperature of 220c the internal nozzle temperature was only at 185c.

Cranking the temperature up to 260c and trying some manual filament feed tests shows this temperature setting was much better. Tomorrow I'll try rebuilding it completely and testing it with the motor feed.