Hi,

I'm building a RepRap, well... acually, I bought a complete and assembled hardware kit and electronics from BotMill. It costs some $$ but looks very nice with the parts made on a Stratasys machine. The reason I bought the assembled kit is that I use it at our company, we are planning to buy a Stratasys or 3D systems machine and got really inspired by this project so we decided to make one. And to save precious time we decided to take a short cut.

After assembling the electronics, and programming the boards the RepRap came a live. After that I have been busy with adjusting the X,Y and Z axis.
But... I have some problems with the tempature, I trying to print with HD-PE (maybe a little to ambitious I'm afraid, after some reading on the net it seems that PE is not the easiest material to print with), when I turn on the extruder heater and set it to about 220C and measure the temerature at the nozzel with a mulitmeter (using a thermocouple) it display's a much lower temperature. I have to set the temp. to more then 360C to get the temp. up to 230C at the nozzel, measuring with the multimeter. According to the RepRap host software it does get heated to the given value. To check the mulitmeter I have also used a IR temp. measurement tool, but that shows even a lower value.
I use a 'standard' thermistor, order from RS. And have already made a complete new nozzel, PTFE isolator (with a M8 thread instead of the M7), a PEEK support and a bras heater element with a resistor. Do I need to calibrate the thermistor or some thing?

I have also ordered ABS by BotMill, but it hasen't arrived until now unfortunately, they say that the have some problems with there supplier. I will try to order some PLA localy to start with.

I really like to know what is wrong with the temperature values... so I can move on to finally start print some thing. The HD-PE doesn't come quick enough out of the extruder for now to print...


B.t.w. I'm from The Netherlands, are there more people here from The Netherlands?


Regards,


Guy Brus

Well either you are not using the correct Thermistor which should be either this:

[uk.farnell.com]

or this:

[uk.rs-online.com]

or your thermistor is not making good thermal contact with the tip of your extruder nozzle, or you have mounted it further away from the tip than usual.

Either way you are in luck since you have a thermocouple that you can use to calibrate your thermistor! One technique to do this is as nophead does:

"When I make a new extruder I put a rod type thermocouple down the barrel, before it has any plastic in it. I use a multimeter to measure the temperature both hot and cold and get the corresponding ADC values from my firmware and use those to calculate beta and Ro.

I find the thermistor tracks the thermocouple within 2 or 3 degrees after calibrarion." --nophead

Regards

Giles

Hi Giles,

Thanks for the tip about calibrating the thermistor, I will try this.

The problem can't be the thermistor to far from the nozzle. I placed it as close as possible to the nozzle, and even if I didn't then the value measured by the thermistor should be lower than the real temperature. I my case it is the opposite, real temp. is lower then meassured by the thermistor.


Regards,

Guy

I find I only get an accurate measurement with a thermocouple placed inside the barrel. When you put it on the outside you only get a small contact area and a larger area exposed to the air. To get an accurate reading on the outside you need to embed the thermocouple into the metal, or cover it with an insulator.

An IR thermometer always give a low reading because the emissivity of metal parts is too low and the they are also too small for the aperture of cheap IR thermometers.

This is why I calibrate with a thermocouple inside the barrel as that is the temperature the plastic is heated to.

---

[www.hydraraptor.blogspot.com]

That's true, I measured on the outside of the nozzle. I will clean up the nozzle, and place the thermocouple inside of it. Let's see if that will give some beter results.


Regards,

Guy

Did some measurement inside the nozzle yesterday. Temp. values are more closer to the values displayed in het RepRap host software. But there is still some deviation between them. The higher the temp. the higher the deviation, as expected.

What is the best and most simple way to calibarte this?


Regards,

Guy

I measure the resistance at room temp and ~ 240C. I use a thermocouple inside the heater to get the actually temperatures and from that I can calculate the beta of the thermistor. When I use the new value of beta the two then track each other to within a degree or two.

I also measure the resistor rather than use the value marked on it and I also calibrate the ADC in my micro with a voltmeter.

---

[www.hydraraptor.blogspot.com]

Ok, but the beta value in the RepRap host software isn't used anymore as I have read somewhere, is that right? So do I need te re-generate the look-up table in the AVR software with the Python script, after I have calculated the Beta value?

B.t.w. today I have printed for the first time, see my new topic. Doesn't look disapointing for a first time print in my opinion.


Guy

Yes you need to make a new table and put it in the firmware to change the beta value.

---

[www.hydraraptor.blogspot.com]