hotend postmortem

August 17, 2011

Following the last post I’ve sliced open the old ptfe thermal barrier in order to get an idea as to why it leaked/jammed. In the first image, below, it can be seen that the pla made quite a bit of room for itself increasing the ptfe inner diameter! If you look closely in the lower section close to the transition from threaded to smooth section there is a little blob of orange pla.

I have a cheap’n’cheerful usb microscope which is useful for this type of situation…

Above is a slightly closer view of the orange blob.

Above is a closer image of the pla blob. Sorry about the lack of a scale, if it helps the threaded section in zoomed-out images is M6.

Above is a microscope image of the od, in particular the threaded section. The dark region just above the (admittedly dark!) threads is possibly of interest….

Above is a closer view of the dark region in question. There does appear to be some orange pla exiting this hole. It is in the same position as the blob on the id… so this is likely the leak.  I suspect you then get into a vicious cycle of jamming-leaking-jamming which eventually leads to a plug which is too much for the extruder to overcome.

On a brighter note here are some images of the makergear hotend:

The hotend has whats called a groovemount, this is visible in the first image. This slides onto a plywood plate which can be purchased from makergear. In my case, having a laser and some ply lying around, I cut one out myself (sorry for the lack of photo!). Its basically a rectangular piece with a U-shaped slot/cutout for the groovemount to slide onto. The plate’s thickness was about 0.5 mm too much so I shaved that off with a sharp chisel. The plate mounted on the Prusa x-carriage without fuss, over the existing ply plate. This raises the extruder a shade but the hotend is shorter than the old one so no z-axis height is lost.


New hotend

August 12, 2011

Since the last post, a while back, the hotend gave up. Initially it started leaking…. gently oozing pla out somewhere between the teflon and the brass. I took it apart and used sink/drain unblocker fluid to clean the pla out (most came out with gentle heating first). I added an extra turn on the teflon barrier threads with a die so I could better tighten the teflon into the brass nozzle in an effort to seal any gaps that may have formed and put it all back together. Unfortunately it leaked again quite soon afterwards so I repeated the process a second time.

Pretty soon it was leaking again. This had the effect of putting down ever so slightly less plastic than skeinforge anticipated and so the top layers were a shade sparse. In any case I rightly or wrongly I continued printing… but not for very long…. a couple of days at most. Then the hotend just packed it in altogether.

I opened it up again and the teflon was clogged so I drilled it out (forgetting I could just heat it in hot water!). Anyway with a 2mm channel drilled in the teflon I could still see some pla in the walls of the teflon (at the time I was printing with some nice orange from Vik!). I reckon the pla had forced itself into the teflon and was actually leaking from there and not at the brass interface though I should really split the teflon down the middle to verify that.

So I’d been hearing good things about Makergear’s hotend for quite some time and now being hotendless it seemed like a good time to get one. So I ordered a 0.35 mm nozzle superpack for 1.75mm filament, Makergear shipped the same evening and the kit arrived here (Ireland) a few days later. The first pressing task was to eat the free chocolate sample! Then down to the business of assembling the kit. This turned out to be pretty straightforward and all the instructions are on their website.

I’ve been printing with this for a few weeks now and so far its been printing really well. I’m getting 60 mm/s out of it easily and I plan to push it a bit further and see how far it’ll go (the old hotend struggled beyond 30 mm/s).

Something else thats been happening in the past few weeks is Action 68’s fork of skeinforge called SFACT. Check out reprapfordummies.net. Action 68 has deleted out unused plugins and the default values in SFACT are pretty good out of the box. In my opinion its a nice step forward and I’ve been getting nicer prints as a result. Its integrated into Kliment’s pronterface host which is pretty cool. Hard to keep up with all the development…. 😉


Hotend: temperature fluctuating

May 10, 2011

I’ve spent the past few days trying to optimise the Prusa Mendel printer with respect to speed and temperature, having already  calibrated steps/mm etc. The printer however, was having none of it and very kindly gave me wildly varying prints where the top surface rarely bridged properly. I noticed that the temp was varying quite a bit so I decided to change out the thermistor which had probaby come away from the heater block. I replaced it with a different model and decided to tape it to the block with kapton tape. However the fluctuations were worse (+/- 15 degrees C around setpoint!). The new thermistor, too, was struggling to stay in contact with the heater.

I had reinforced the previous thermistor/fire cement setup with a flat brass piece so i bent this to try to pin the new thermistor in place in the (now widened) hole. This helped but didn’t solve it completely. So then i stripped some light wire and wrapped it around the brass and the heater block to hold the brass tight against the thermistor and the heater block. So the temperature is now nice and stable! However I need to set it to 300C to get the flow i used to get at 210C….. must be a temptable problem… but at least its extruding repeatibly again!

Printing from SD using sprinter firmware works well – its nice not to have blobs everytime you do something that forces the netbook to work hard 😉


hotend assembly

February 16, 2011

I’ve commenced work on the hotend. I had to drill out the 5mm hole in the Al heater block to take the >6mm  5.8 ohm resistor. In addition I had to drill a hole for the 0.8 mm thermistor. Not having a 0.8 mm drill bit, I turned to my trusty laser which did the job nicely.

laser drilling of Al heater block (material ejection and plasma emission visible):

laser drilled hole + thermistor alongside:

After that I followed Arcol’s excellent assembly instructions – hope he doesn’t mind 🙂

Resistor in heater block:

thermistor + fire cement:

I’ve left the fire cement to set overnight. There’s probably too much of it but I was trying to encaspsulate as much of the crimped section as possible for strength. I’ll test it tomorrow. One worry is that the resistor protrudes from the heater block. I’m afraid of hotspots where the temperature rise outside the block may exceed the limit and fry the resistor… but we’ll see tomorrow.