Heated Print bed? Not this print thank you.

There I was rocking along. I was smoking hot. Part after part was coming out of the system.

Skeinforge was generating the code while my scripts sucked up the latest
gcode and added it to the queue. The music was pumping and the wine was flowing.

Then disaster. Putting 240C ABS on the printing bed in the same general place time after time had raised the temperature of the print bed. Who would have known? I had a hot spot. My part had now fused to the acrylic. And when I say fused I mean taking a chisel and hammer and trying to tap the two apart resulted in the Acrylic failing rather than the part coming off. So back to the drawing board for the print bed.

Anyone solve the hotspot issue yet?

Stephen

You could use PLA

PLA is $USD 80 from makerbot + postage.

But thanks for the tip.

Why can't you use a random (or changing) starting position for each build.

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

mccoyn () said
Why can't you use a random (or changing) starting position for each build.

I say
Thanks. I did think about that and it would work with smaller objects but what about larger objects that take up the whole print bed?

Also I want the reprap to start putting objects out 24x7 so cooling the print bed is an issue.

I have solved the issue. I removed the 4.5mm Acrylic and switch to a 2mm Acrylic with a aliminum sheet underlay. The aliminum is a good heat conductor and sucks the heat away from the hot spot. Straight after a print the hot spot only has a 2C temperature difference and quickly disappears.

I now seem to have a mirror as a print bed which is kinda cool and useful for calibrating the height of the print head at the start of the print run.

regards

Stephen

If you use a thicker piece of acrylic and sand it lightly between prints you won't be having much trouble. I use 1/4 inch acrylic and have zip trouble. I'm upping that to 3/8 inch so that the HDPE that I use to print polypropylene on and the polypropylene I print HDPE on are all the same thickness. :-)

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

By 240 C do you mean 240 degrees Celsius? ABS's melting point is ~105 degrees C so this seems dangerously high....

Also why are you heating up Acrylic to >0.5*it's melting temperature(~130 degrees C)? Acrylic might permanently deform under load at this temperature.

Printing on something that's an insulator isn't really a good idea...

@Forest Higgs
I was using 4.5mm and since I have been using it for six months it was well and truly "sanded" 4.5mm is about 0.18 of an inch. But I agree it was great. very little "bend" in the bed ment a good print. until I went for mass production and that's when I got my heat spot. The hole in the acrylic is truely impressive.

@Gene Hacker
Yep 240C degrees. It probably goes higher than that as my temp sensor is at the end of the extruder. I have had readings of 255C until things quieten down and hit equilibrium. Yep I am sure ABS melts at 105 but to get the extruder to work you need the extruded plastic to bond to the layer below. That requires lots of heat. The default for skeinforge of ABS is 230C incidentally. What temp do you print at?

Gene hacker said
Printing on something that's an insulator isn't really a good idea...

I say
Agreed. But ABS loves to stick to acrylic. (sometimes too much) hense the 2mm acrylic for the stickness and the aluminium underlay to conduct the heat away.

Has anybody tried tried polycarbonate (lexan) yet? It appears to be what Stratasys use in the later machines and will stand a higher temperature before it softens.

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[www.hydraraptor.blogspot.com]

stephen george Wrote:
> My part had now
> fused to the acrylic. And when I say fused I mean
> taking a chisel and hammer and trying to tap the
> two apart resulted in the Acrylic failing rather
> than the part coming off. So back to the drawing
> board for the print bed.
>
> Anyone solve the hotspot issue yet?

Most heated beds I have seen use a metal surface.
It would have been trivial to use cooling-spray to
contract the bed under the object, thus making the object
pop of due to the metal being a fat superior heat-conductor
to ABS and contracting much faster.

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