I have been surprised that the heated bed proved to be much more interesting then it would initially seemed. First I had to decide the method of heating – PCB or nichrome wire. At work we have a RepRapPro Mendel which uses a PCB heater. It’s fine but takes ages (up to 7 minutes) to reach temperature. My eMaker Huxley uses nichrome and take moments in comparison (2-3 minutes). Now obviously I have to account for the differing bed size, Mendel at 200mm square and the Huxley at 140mm. That’s a 35% difference in size which isn’t reflected in the times so there is more going on. My unscientific opinion is that I have much less nichrome wire length where the PCB has a long track to give its resistance. Bear in mind I was always taught that heating on PCBs was a bad thing! Due to this the nichrome is able to heat to a higher temperature per mm of wire than the PCB thus able to heat much more rapidly. So that is my long winded way of saying I went for nichrome for the DSF.
By no means is nichrome without its problems. It generally comes with no insulation, not many plastics can withstand the 1000’s of Celsius nicrhome can reach, so shorting on the bed is a consideration. I believe you can get it in a glassfiber but I have yet to find that in the UK. Connecting to it is another issue. You really shouldn’t solder to it, at least not with normal tin/lead solder (it would probably be hot enough to melt it).
So each problem in turn. Insulation has been provided by kapton tape. It can withstand the temperatures the wire should reach and still insulate it from the aluminium bed. To help guide the wire I designed the heat shield with a channel that allows the wire to run in a fixed pattern around the bed. This ensures that there should be reasonable heat coverage over the bed, you are never more than 25mm from it. I then covered the bed in a double layer of kapton tape where the wire will run. That completed the wire was then routed and taped down with kapton again. The connection between the 15A supply cable and the nichrome wire is taken care of with a couple of ferrules. These allow for a solid connection without solder. The supply cable was then routed through the heat shield and a cable tied used to provide strain relief.
The bed thermistor was attached and routed as well and the whole assembly wired up to the Megatronics board. I am always cautious when turning on a greater for the first time. Initially I set the temperature to get to 35C. I monitored the rise in both Repetier-Server and using an infrared thermometer. This is where I discovered something disconcerting.
The bed rose in temperature and settled at the set temperature in Repetier-Server. The infrared thermometer told a different story. It showed the temperature rocket to about 44C before settling down to 40C, 5C more than was set and what Repetier-Server was reporting. The same is true for most temperatures, the thermometer shows the temperature shoot way above the target, sometimes by 10-12C and then settle at about 5-6C above what the electronics see. I have asked for help on the RepRap forums so will see if anyone else has an idea. My current thought is that it is the temperature tables in Marlin, if so then I will need to figure out how I change it.
A safety feature on most beds is having them mounted on springs so that if there is a problem, say with a print curling, then the bed can move out of the way of the hotend. That leads to a question, how much ‘spring’ do I need? What does the spring need to support: the heat shield, aluminium bed, glass and what ever is being printed. Without the print my bed weighs in at a good 780g, adding a theoretical print of about 200g that means I need to support around 1Kg. I found a site, Lee Spring, which supplies all manor of springs. The factors to take into consideration are: the weight (1Kg), size of bolt holding the base (M5), distance to travel. On Lee Spring the ‘springiness’ of a spring is measured in newtons per millimetre. Thus if you wanted a spring of 1N/mm to move by 2mm you would have to apply 2N of force. 1N is equivalent of about 102g. Something else to consider is that my design has 2 springs on one side and one on the other. The weight is split half way per side so the double spring side has about 500g split between the two springs, whereas the single spring has to take the whole 500 itself. I chose the LCM080DG 02 M as it fires the size I needed and has a compression of 1.5N/mm. As the springs will be compressed about 3mm anyway there is enough resistance to hold the rest of the bed up.