Getting extruder to extrude

I have built a heater block extruder tip. I have purchased 3E3 3watt metal oxide film resistors. I also purchased a number of 10k thermistors. Everything looks and runs...except I'm not melting any plastic. Are these inadequate resistors ? I do know that I will most likely need to change the temperature file to work with the 10k vs 100k thermistors (as soon as I can get that to work). The temperature according to what its reading anyway is 190-200 C...and it very hot. but no plastic. any ideas ??

HI

What plastic are you using, if it's ABS you will need about 230-250C but PLA should melt at 180-190C.
Just check to make sure the nozzle is not blocked or the feed stock is not to tight in the barrel hole.

Auzze

I heated it further and finally got a dribble. I wonder if perhaps connecting 2 resistors in series might work better ? Also, it seems the heating isn't consistant, it heats 5-10 degrees then stops and I have to turn the heat on and off to get it to continue. Is this normal ?

thanks

I'm guessing you might be using RepSnapper. If this is the case, o to the gcode START tab, and change the line that reads

M104 S73.0 ;Set Temperature

to

M104 S200.0 ;Set Temperature

This might cure the turning off and on problem.

To make the change stick, click the Input File tab, and press the SAVE SETTINGS button.

I think it's better to use the 100k for the extruder tip and the 10k for the bed. You may have to change the reference resistor to use the 10k on the extruder. This is what I remember from looking at the tables in the temperature.h firmware files.

Also, do you mean 3R3 or 3.3 ohm, not 3,000 kohm for your resistor for your heater element?

Thanks for the responses. The software I'm running is just the reprap software, not sure what redsnapper is. the resistors I'm using I believe are 3.3ohm I think the 3e3 is just another way of representing that. the colors look like:
orange, orange, gold, gold which i read to be 3 ,3, and .1 multiplier =3.3 +-5%

What is the max temperature rating of the resistor? If it is a 3 Watt resistor, I would expect it isn't very high. With a 12V supply you will get over Watts on each resistor if they are in parallel. They will get hot much faster than they can heat up the thermistor and they might burn out.

The way we use these resistors, the power limit doesn't matter, but the maximum temperature is critical.

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Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
[www.codeerrors.com]

Hey again everyone. Well I tried to hook up two of the 3.3 resistors in parallel and nothing happened. I also ordered and tried some 5w 4.7 ohm resistors and nothing. the little 3.3 3 watt resistors heat up and with some difficulty, eventually melt the plastic...but not enough to do anything. And it seems that there isn't enough power to heat the 5watt resistors. Any ideas ??

The Mendel-parts.com head uses a 4R7 7W resistor and I happily melt ABS at 230 degrees C.

How is the resistor mounted? I have mine through the hole in the block and held in with fire cement.

Geoff

Ok, one more time. Thanks for the advice Geoff. In answer to how it is mounted, I have just placed the resistor in the aluminum block. It is a rather tight fit. I then placed a hi temp silicon sealer at both ends. this is just a temp measure to test my proof of concept. I also have not as yet insulated the unit. Today, I attached a watt meter to the reprap with the following results. At rest 30 watts. When I set it up with the 3watt resistor ~ 50-60 watts. when I connect to the larger 5 watt resistor it remains at 30 watts. I'm curious as to why this might be ? I would expect with the larger resistor that all the energy would pass thru and not heat it...but it appears as thou it does nothing. Again, I am using makerbot electronics. the power goes from the mother board thru an rj45 connector to the extruder board. I have wondered if perhaps the rj45 cannot carry the load.

I am using a Makerbot G3 extruder controller and the power runs from my motherboard to the controller via the RJ454 connector and a CAT5 cable. My heater works fine so the cable can carry the required power... Uses 3 of the cat 5 wires per pole if you examine the schematic.

The power rating or the resistor only tells you how much power it can take before it gets too hot ahd breaks. When in the heater block, it can take more power since the block acts as a heat sink and draws heat from the resistor from getting too hot. So, until the resistor fails, equal resistance should result in the same power given that the voltage stays constant. You won't get more power in the form of heat by increasing the power rating of your resister. To get more heat, you have to decrease the resistance.

P = V^2/R

The commonly used 6.8 ohm resistor gives 21.2 W of heating power.

You are right that the higher wattage resistor should not heat up as much. But only if in air. If they're both inside the heater block, you might not see much difference.