Sunday, February 28, 2010

Rep strap Mini/Micro Mendel Heated Bed using Fire Glass

Building Mini or Micro Mendel for the show I don't know where I can get Aluminum sheet easily so I had an odd thought on Saturday when passing a Glass supplier so popped in to see if I could get some heat resistant glass 140mm x 138mm to use as a heated bed.

I was told I had to buy fire glass it can take up to 600C. I got lucky they had an off cut so I had two cut for £11 note this is good price as its usually £100 Square Meter. 

It has a slight yellow tint to it but as its not for looking through who cares.

Here are pictures of my attempts to get the element right using 50ohm per meter nichrome.


Note as I re-make them I start using much less Kapton tape  


The last attempt was the best achieving 100C in 6 minutes

Then 110C  in 7 minutes. I'm running from power supply no temperature control yet..

So resistance of final solution is about 20 ohms based on a 30.5 voltage and current of 1.5A total wattage is about 46 W. 

The original attempts were mounted on Plastic chopping board using bulldog type clips as the wattage went up the Plastic started to smoke so the final version has 3mm MDF bulldog-ed to the Fire Glass. I removed the two silver levers on each of the bull dog type clips.

I just need to figure out how to mount a Thermistor then I can add PID or Bang bang control of temprature using an extruder controller.

The nichrome has a 30mm spacing the other attempts were 10mm, 15mm, 20mm, 25mm to fit a 100mm x 100mm central print area.


Casainho said...

Looks like you are measuring the temperature in the middle of two pieces of heated wire.

Since glass is "half" temperature insulator, would be important that you verify that you get the same temperature values over almost all glass area.

Casainho said...

And if you are going to make heater control, could you please consider doing and testing the following circuit?

Or maybe you can find one more simple and report it :-)

Triffid Hunter said...

for more even heating and ability to run from 12v, consider two or more runs of nichrome in parallel, in different areas of your glass plate.

Insulating the underside would probably help too

Unknown said...

@Casainho I've not noticed any hot spots with my infrared thermometer that I used to look at the how even the temprature was.
Was kind of hopping that the high temprature glass conducted heat more evenly tho..

I think its called ceramic glass or LAS glass-ceramic as a whole has a thermal expansion coefficient that is very close to zero. LAS stands for lithium, aluminum, and silicon.

Thank you sorry I alredy have 4 of the RepRap Extruder 2.1 PCBs built its similar idea to your CCT I think.

I'm not sure how to hold a glass thermistor to the glass plate maybe supperglue / Araldite rather than a glass on glass friction connection.

@Triffidhunter every part of the glass is 15mm from the heating element. Once at temprature I think the effect is reasonably even. All of the other nichrome runs did get the glass up to temprature but they took a little too long. I thought
it was better to have a fast heat up time 6 Minutes is fast. Much faster than my aluminum bed.
I had thought of parallel runs though. If I get problems I could have 6-8 straight runs at 10mm apart.

I'm using 24-30 Volt supply its better for stepper motor driving.

I have MDF as my insulator as plastic kinda burns with fast heating the MDF seems to be a better insulator than the plastic was. What material were you thinking of as an insulator?

Unknown said...

Thank you both for your sugestions as they are usefull for any one else who wants to do this.

Getting a lower resistance Nichrom wire would much better!!
particularly for running on 12V,

I have less than 14 days to get this working for the show. So Im stuck with what I have/could get easily.

Heres the wikipedia link for ceramic glass

I dont know if sheet Aluminum is more expensive to get than the fire glass. It was just easier for me to get fire glass as wood burning stoves are a bit of a fashion / eco thing here.