Mendel heated bed

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Revision as of 11:53, 16 May 2010 by Adrianbowyer (talk | contribs) (Thermal Insulation)
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Mendel heated bed

Release status: Experimental

Wired-up.jpg
Description
Mendel Dibond heated bed
License
GPL
Author
Contributors
Based-on
Categories
CAD Models
Heated-bed-design-files.zip
External Link


Introduction

Finished-labelled.jpg

If you build fused-filament-fabrication parts (the technology that RepRap uses) at room temperature, they have a tendency to curl as they cool from the bottom up. This is caused by higher layers shrinking as they cool, and bending the already-set lower layers upwards.

Different plastics suffer from this problem to different extents. ABS is quite bad. Polylactic acid (PLA) is quite good. Indeed, if you make a single PLA part in your RepRap, curling probably won't be a problem at all - the part will stay warm enough from one layer to the next to eliminate the problem almost completely.

But if you build whole trays of parts (as in the picture) then, even using PLA, you can get some curling. The cure for this is to build on a bed that is heated.

This heated bed is primarily for making polylactic acid (PLA) parts. It probably won't run hot enough for ABS. But that guess is subject to correction by experiment.

The CAD and other design files for this heated bed can be downloaded from here.

This device was from an idea by Nophead.

Cutting out

The main part of the heated bed is made from Dibond. This is a sandwich of LDPE between two sheets of aluminium. It is very light, very stiff and very flat - all required properties for a RepRap build bed. The bed is made from a sheet of Dibond with heating wires and insulation attached underneath.

The plastic in the sandwich is not the world's best thermal conductor, but the bed is thin enough that it seems to work well.

I bought my Dibond from these people who kindly cut it into rectangles of 232x260 mm for me. It is pretty widely available as it is used for making signs and exhibition stands and such like. It's also pretty cheap.

Heated-bed-plan.png

This shows the pattern to be cut from 3 mm thick Dibond (the .dxf file for this is in the CAD download). The shape is exactly the same as the standard Mendel build bed, but with the addition of the tab at the back for mounting electrical connections. The dimensions of the tab are not shown because you may have to change them a bit to accommodate the electrical connectors you use. My tab was 30 mm deep.

Two of the three holes in the tab are for mounting a three-way chocolate-block screw electrical connector - get a chunky one; it has to handle a significant current. The third is for a small right-angled bracket to mount a connector for a thermistor that will be used to measure the bed's temperature. Virtually any small two-way connector will do for that. Make sure that the connections (which will be on the underside when the bed is in use) do not foul the two reprapped parts and the M4 screws through them that hold the right-hand Y-axis bar at the back of your Mendel.

You could try cutting the Dibond in a laser cutter, but the aluminium may well give trouble if you do. I found that a bandsaw worked best. If you saw out the shape, gently clean the edges with a file afterwards to remove any lips and burrs.

Wiring up

The first electrical question that arises is obviously: how much power is needed to keep the bed at the right temperature? I run my bed at 55oC (though see below), and that seems to need a little under 100 W at a duty cycle of about 70%.

If you are going to run the bed at 12 volts, then this implies a bed resistance of 122/100, or about 1.5 ohms. This further implies a total current of 8A.

Heated-bed-schematic.png

Here is the circuit diagram for the bed, together with its control circuit. As you can see, I set it up so the bed was effectively two resistors in parallel, so each of those needed to be 3 ohms. But they in turn are made up of more wires in parallel. The exact pattern you use will depend on the resistivity of the heating wire you use.

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The heater wire I used was nichrome. It doesn't need to be insulated. Measure the resistance of a length (say 400 mm) of your nichrome, then work out a regular pattern of it covering the bed that will give the total resistance that you want. Make sure that all current paths are equal, so you get equal heating.

Here I have laid out the pattern on graph paper, which is quite a convenient way of setting it up. There are three copper wires, with two ladders of nichrome between them. I used Blu-tack to hold everything in place while I was making it. The connections are made using crimp connectors or bootlace ferrules - you can't solder nichrome.

Wired-up.jpg

When you have the pattern right, place it on the bed and tack it down with short lengths of Kapton tape. Then use more Kapton to hold the entire pattern down completely. Dibond has an insulated covering on it - it won't short out the wires. This means that you can tape them down straight onto the surface.

Finally wire up the connectors using fat wires for the heater. These you can solder, because you can solder onto a ferrule. You can also see the thermistor in place here, also held down with Kapton. Put it somewhere near the middle as far away from the wires as you can.

The thermistor I used was an Epcos B57550G103J. This has a resistance of 10K at 25oC, and a beta value of 3480.

Thermal Insulation

Clearly you want all the heat to go upwards where it will be useful, so we need to insulate the underside of the bed. I used builder's crack-filler foam.

Start by printing four of the parts in the file foam-block.stl from the download in your RepRap. These are to bolt to the bed to prevent the foam getting in the way of the springs that support the bed. Print the parts using very coarse infill - they don't need to be strong. When they are done, wrap a rectangle of paper round each one and tape it tight. This is to stop the foam sticking. Bolt them hand-tight to the wiring side of the bed.

Ready-to-foam.jpg

Place the bed on newspaper. Spray it with a small amount of water using something like a pot-plant mister to help the foam to set and stick. Then spray the foam on the back. Try to avoid clogging the electrical connectors.

Do not be tempted (as I was - see below) to try to even out the foam with a trowel. You will just mess it up. Try to get an even coating with no gaps, and don't bother how thick it is or where it goes over the edges.

Foam-cutting.jpg

When the foam is fully set, cut it to size. I used a bread knife, which seemed to work well. Trim it so that it is about 10mm thick, and cut round the edges of the bed to neaten it up.

Installation

Ready-to-install.jpg

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Testing