Extruder Testing, Part II

After I was able to get a working extruder earlier this week, I had done some really rough testing using some poor techniques.  I still didn't have the chip that I needed to let the thermocouple be read directly by the microcontroller, so I was having to switch the power supply on and off manually to keep the heater barrel from going above 230˚C.  I only had 5V and 12V available to me from the power supply which was limiting things greatly.  5V gave too low a temp and 12V would overheat the system.  I since did a little investigating and figured out a way to run the heater barrel using pwm (pulse width modulation).  I set up potentiometer that could adjust the length of the space time (off time) while keeping the mark time (on time) constant.  This meant that I could go from a 100% duty cycle (always on) to around a 20% duty cycle using my method.  I also hooked up my stepper motor driver that is based of 4 high amperage HEXFETs.  They allow me to run my motors at 3 Amps easy.  I set up a circuit that watches an input button and each time it is pressed it turns the motor one step.  So after I had the whole thing working together, I could use the pwm to keep the heater at a constant temperature and then use the push button to slowly extrude the ABS.  Here is a shot of the setup.

The potentiometer that I had was very sensitive and I could actually adjust the barrel temperature by 1-2˚C which I was pretty happy with.  So I slowly got it up to 230˚C and once it had leveled off I started using the push button to slowly extrude the plastic.  

A few observations from the testing.

1.  The rate that the ABS came out of the nozzle did not seem to be as constant as I would have liked.  Each time I clicked the button and the motor moved one step I expected the same amount of ABS to come out, but that was not always the case.  I am thinking maybe a little higher temperature would be needed to make sure everything is good and melted before trying to push it out the barrel.  

2.  At times I would notice the motor slipping on the plastic.  The nema 23 motor I am using has 60 oz*in holding torque so I am thinking that should be enough for this task.  Maybe I just need to tighten the gap between the roller bearing to get a better grip on the filament.  

Good news is that the thermocouple chip that I ordered should be at my apartment when I get home today so with a little bit of luck I should be able to hook up the extruder motor, motor driver, and temperature sensor to the computer and test out the host software.  I have a feeling it wont be that easy though....