Tuesday, February 06, 2007


Forensics on the 2 amp low thermal intertia extruder barrel

I had a few extra minutes yesterday so I decided to amp up the 2 amp low thermal inertia extruder barrel on the Mk 2.1 Fused Deposition Modelling (FDM) extruder and try to run some of the 3 mm acrylonitrile butadiene styrene (ABS) filament through it before I broke it down for refurbishing. I needed to refurbish it because the insulated nichrome 80 heating coil had become partially delaminated from the barrel. I knew that eventually the lose nichrome 80 would overheat and fail.

Sure enough the heating coil failed while I was extruding ABS.

This morning I stripped the barrel and discovered that the heater coil was not partially delaminated from the barrel but completely delaminated. The insulation had completely disappeared from the nichome in places. While I didn't not look closely for the break I suspect that the coil shorted and overheated at a contact point causing the break.

This time around I am going to follow Adrian's suggestion and use about 7 inches of nichrome 80 instead of two lengths of 14 in parallel like I did last time. I am also going to space the wire out a bit on the lower end of the barrel instead of wrapping it tightly. This will be more like what I did with the first test barrel that I build.

I am also going to give it 8 coats of BBQ paint before laying down the wire and another 8 coats to tack it down this time. As well, I plan to secure the two ends of the coil to the barrel with copper filament before I paint it.

Isn't the problem that the wire needs to expand and contract so it will always try to break away from the paint?
Yup! The wire is coated with a silicone and glass fibre sheath, though, which hopefully takes up a bit of the expansion/contraction cycle. The point is to get that sheath tacked firmly down on the surface that is to be heated. If you let it wave around in the air it gets super hot (> 1200F) compared to what it gets to when it's tacked down (~400F) and the sheath perishes.
I wonder if you could heat the barrel more directly without any thermal contact by using an induction technique similar to induction cooker hobs. Bit tricky if your limiting yourself to a 12V DC supply but not impossible. I might look into it myself.
If we could get away from having to use nichrome wire and all of the hassle that goes with insulating it that would be a REAL blessing.
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