Thursday, 16 June 2011

Hot and fast...

This is mainly relevant for anyone else making/using a shapercube (which is what I have). It is fun, and I have been experimenting and finally have some useful details...

I am working on PLA at the moment, as that is lower temperature than ABS. The ABS needs a new hot bed which I am working on and will post in due course.

Hot bed: What I have is a Prusa PCB Heatbed MkI connected to an ATX 12V power supply. It is around 1.7 ohm and so uses around 7A. Obviously, contrary to what they say on the IRC channel, just having a beefier PSU at 12V will not change that. I have a Homebase Heat Shield Mat under it as insulation (the sort of thing plumbers use to protect stuff from a blow torch). It can comfortably handle 60C which is fine for sticking down PLA but not good enough for ABS. I attached a thermistor to it to T1 on the RAMP board and make the s/w regulate the temperature for me.

Hot end: This is the standard shapercube 0.5mm diameter nozzle hot end. It is a short hot spot at the end, and a brass tube with a PEEK insulator. Works well.

You must ensure you calibrate the steps/mm and measure the filament. I have blue PLA that is nominally 3mm but actually 2.45mm! Packing density of 1 is fine on PLA. Get that right and you avoid a whole heap of issues. I also suggest using Skeingforge to make the GCODE and SCAD for the 3D models. if you get it wrong either you print stuff with loads of gaps or you get bobbles on the print and as you go up the layers it forces the hot end in to what you printed badly, causing displacement of the print and/or the hot end jamming up and not working. Get it right and a fill layer is a flat surface with no bobble and no gaps.

I have been tinkering with stepper control settings. These are controlled by links on the RAMPS board. The shapercube site says to set all for 1/8 step. For Z axis there is no point and there may be torque benefits in using less. I tried full step (no links) and that shakes the whole thing and makes a noise, so half step (just first link) is fine for Z axis. You then set the arduino code to 320 steps/mm for Z instead of 1280.

On X and Y there is no need for 1/8, why the hell not use the 1/16 which the controllers will do. Just put all three links and change the steps/mm from 40 to 80 in the arduino code.

The extruder (Wade design) is fine - but needs the springs set as loose as they go really. Basically, if all is well the filament will feed through with very low pressure. If you tighten too much you just strip the filament when something goes wrong.

Now, the trick I have been working on for two days since I got a replacement hot end is temperature (top tip, do not try and clear the nozzle on the hot end using a 0.6mm pin in a 0.5mm nozzle - you bugger them up badly).

I fitted a fan to blow across the print as well. I have an STL for a nice fan mount if anyone wants.

PLA will extrude right down to around 170C it seems. And I was told on the Internet [citation needed] that lower temps were better... I initially tried 190C and worked down to lower and lower. The problem was part way through printing the hot end would sort of jam up, and the extruder could not feed anything. You first get silly fine strands of PLA and then nothing and print is ruined.

The solution seems to be hot and fast. I am now running reliably with 215C on the hot end, and 60mm/s feed rate. This way of working has the benefit that things print quickly as well :-)

So, that is my tips for the day for shapercube users... Have fun. Catch me on irc on #reprap if you want to know more.

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