Heated bed

I have started my 3d printer from rather unusual point – making the heated bed platform.
What the use of making it as it is possible just to buy it for £20? It is affordable, but I do not like the idea of using 10A power supply, because it adds £20 more for the costs, it requires thick wires. The most important is that it is a very strange idea of making 100W heater with low-voltage power supply for so simple application like keeping the temperature near 100C.

So my idea was to use mains-powered (220-240V AC) heated bed. Brief overview of the existing ideas (http://reprap.org/wiki/Heated_Bed) was not encouraging – large resistors, thick aluminium sheets required to distribute the heat – neither simple nor convenient or reliable. So I decided to make on my own.

The simplest heater is a piece of a resistive wire (nichrome or kanthal). Here we have a dilemma – the wire should be rather thick, because thin wire burns quickly; however, thick wire has low resistance, so a very long piece of wire should be packed into the heated bed area. I have selected a piece of ~5m of 260 micrometer kanthal wire (£3 from the mesh-company via ebay), this means that 220x220 mm bed should be tiled with the wires 5mm apart.

The next component of the heated bed is glass, which is a traditional choice, with no additional heat-spreading materials, because I consider that 5mm wires will generate rather uniform temperature. The last component is insulator that will separate 5 m of the wire from short-circuiting – I have selected high-temperature silicone and applied it on the glass.

This process is a bit tricky, because it requires high accuracy and consistency. I have printed a piece of graduated paper (with Inkscape you can draw it easily) that marks the location of the wires, glued it on the other side of the glass (3mm usual soda-lime glass).

Then I used a ruler (a piece of aluminium profile), put another square piece of glass and moved it along the ruler, while drawing silicone lines with the syringe (the nose of the syringe was cut a bit to allow me to push a consistent 1x1 mm silicone trace.

Quite well done.

Another tricky (and unexpected) step is to put the wire into this mask. It is really tricky - the wire is springy and tries to escape, so it must be glued (temporary) to the glass. I have played with cyanoacrylate super glue, glass glue, epoxy, hot-melt glue – they all did not work well with the glass. However, hot melt glue, handling carefully, made the job. I have glued the wire with small droplets of the glue – it will melt away on heating, but it will not be needed this time.

The last step was to add a bit more silicone, add the second piece of glass, put a weigh and wait until it settles.

Ready! I have tested the bed, its resistance is 260 Ohm (a bit higher than I expected), so I have just plugged it into AC and waited until it heats.
It was a disaster – the temperature was raising (~10C per minute), but the glass started cracking at 60C. At 10oC it finally cracked. Most likely, the heating should be performed slower and, the most important, I was not lucky with the piece of 3mm glass. Fortunately, silicone worked well and my assembly held. Bottom glass, 4mm thick, did not break, so I flipped it and decided to use as a working table.

Afterthought. When (and if) I decide to re-do the heat bed, I will use a more simple way – I will make two kind of hairbrush-like templates with the holes 5mm apart (the desired inter-wire distance). I will pass the wire through the template, glue the wire to the heat-bed glass with the Kapton tape (it really works well at high temperature). And this will be the end of the process – much easier.

Meanwhile, I have attached the bed to an MDF sheet and applied few 3D-printed linear bearing holders with rather unusual plastic reverse sides made from a milk bottle. Quite a good and cheap temporary solution – I will make the reverse holders when the printer works.