Tuesday, July 26, 2011

Extruder Hot-end v2

The previous extruder hot-end served me well as a 'proof of concept'. It showed that a stainless steel down tube, attached to a fan-cooled heatsink, could ensure the filament remained solid and ridged until it entered the hot-end melting chamber, and my single resistor hot-end had the heating capacity required to melt the PLA moving at a steady, if not slow, flow rate. It also allowed me to exercise my cold-end feed mechanism. It worked well initially in a clunky sort of way, but due to it's hand-built nature is soon suffered from repeated stalls. I put this down to the imprecise nature of my hand drilled holes in the SS tube and aluminium nozzle. I felt the introduction of an inner PTFE tube liner would help matters, so over the past few weeks I designed and build a new extruder. Here's some detail:

The stainless down tube is made from a stainless socket head bold. It's head was drilled and tapped with M8 thread, it's shaft drilled out with 6mm hole. The nozzle is an M8 threaded brass rod, tapered cone tip with .4mm hole. The nozzle is drilled out with a 3mm hole to within about 2mm from the tip. The back end of the nozzle has a cone lead-in to the 3mm hole, visible in the next picture.
(A big thanks goes out to a friend for drilling out these components.)
PTFE tube is from http://www.adtech.co.uk/TubeSize2.html   I put the PTFE in the freezer for 10min and tapered the tip with a pencil sharpener until it matched the cone in the back end of the brass nozzle. The heater block is aluminium and unchanged from the previous extruder. I like the heat conductivity capability of the aluminium so am sticking with it for now. Many other designs use brass heater blocks.
I assembled the unit by first threading the brass into the stainless steel tube. There's about 8mm of thread in the SS. The brass was wrapped with plumbers PTFE tape and tightened home into the SS tube. I filed two flats on the brass nozzle to get a good grip. It's a good hand-tight. I had to widen the hole in the heatsink and clamp block to 8mm to receive my new SS tube. I inserted the ss tube into the heatsink as far as it would go and tightened the clamping block up (clamp nuts visible in picture above).

The last assembly step was to thread the heater block onto the brass nozzle. I fitted a Teflon sheet between the heater block and the ss shaft to act as a heat shield. This is off-the-shelf baking tray protector (rated to 260deg C). It also provided a nice air duct for the little fan which I moved to the face of the heatsink, keeping the cold air away from the heater block. The fan is held on with a dab of hot-melt. In testing, the heatsink remained cold to touch at all times once the fan was running.

Detail note: Initial testing of this new design showed that some method was needed to ensure the PTFE tube was held firmly against the cone seat at the top of the brass nozzle. Initially the PTFE tube moved upwards within the SS tube and plastic leaked between the two, jamming the hot-end. I have addressed this issue by chilling the PTFE, inserting it and trimming it flush with the top of the SS tube, and then placing a small washer to cap the PTFE flush with the SS tube. The wooden block was then slid on over the SS tube to hold the washer firmly in place.
Here you see the wooden block that sits on top of the SS tube, (washer not in position). This wooden block pushes down of the washer that caps off the SS tube and is bolted in place with two bolts through the aluminium heatsink bulkhead.
I've had limited printing success with this new extruder, as you can see from the scrappy objects in the photo above. I do need to step back and get the extruder calibrated properly. On the other hand my x and y axes is square and general dimensional accuracy is good. The 40mm square is 40mm! I also need to learn a whole lot more about GCode. The above tests were printed by just downloading the STL files, converting to GCode in RepSnapper and hitting Print!

I'm reading a lot about people using Skeinforge instead of RepSnapper. Should I get into Skeinforge at this point? I don't know... opinions welcome.

Thanks for viewing!
No.6

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