My printer has a very large table, but since it is driven by rods and not belts the speed is very slow. As a result I made a small heat bed aprox 120 x 120mm for ABS. This gives me roughly 100 x 100 to print on. I also have onother larger bed 280 x 260 for PLA.
Most parts are small and do not require a large bed and it is easy to heat up a smaller bed to 110deg than it is to heat up a large bed. My small bed heats up in about 10 minutes to 110deg and works fantastic (I need to time it to give exact time). The large bed goes to about 70deg but struggles to go beyond that.
All my beds are made from 3mm aluminium with a 4mm glass on top. As far as I know, the glass I use is just normal glass. It breaks or shatter just like normal window glass and is easy to cut/snap using a glass cutter.
PLEASE REMEMBER: Use Safety glasses AND gloves when handling glass. When cutting it there are always VERY sharp edges and splinters can fly into your face. And if you live in a hot country , as I do, make sure you wear closed shoes. If you drop the glass on your feet/toes you will know it if you don’t wear closed shoes.
You don’t need glass on any of your beds, but it is an easy way to get the surface pretty straight. Just stick Kapton tape on to the alu plate is ok for ABS, provided the plate is straight and flat. For PLA (I have not printed with PLA yet) you can print on blue tape or directly on glass. You need to check this out on the reprap site.
I have also read that some reprappers use a mixture of water and sugar as a coating on glass when printing PLA and a glue/soup made from ABS dissolved in acetone for ABS printing. Easy to make, cheap and easy to clean. I have not used this myself but will give it a try one day. Again you need to check this out on the reprap site as well.
I don’t like to run too many amps through my Ramps board, so I connected my bed up to a relay which is controlled by the Ramps. This way the 8 to 10 amps needed to heat up the bed goes through the relay and not the Ramps. No it does not tick/click a lot as the bed is good in holding it’s temp. The glass has got a lot of mass.
I glued the resistors using JB Weld to the underside. Soldered it all together using normal solder. I have not added any insulation to the under side yet, as the bed reaches temp of 110deg fairly quickly. The glass plate is stuck on with silicon of type Permatec 26B. This is high temp silicon used in cars. If you would like to be able to remove the glass plate DO NOT USE SILICON. It sticks permanently or you can always brake it I guess.
The thermistor I stuck on top of the glass with same silicon. It measures very accurate according to my infrared thermometer but I found that measuring a large part as it prints gives me a better indication of how hot the plastic gets. I then adjust the temperature accordingly. You can’t measure directly on the glass as it will not read off the shiny surface but on the silicon.(which may well be the same temp as on glass surface). I have found that lets say I set the temp to 90degrees, there is so much heat built up in the mass of the table, that it actually continues to rise after it has been turned off. Could be as much as 15-20degrees. This way if you set it to 110 you may find that the part is actully a bit gelly like after a while, as it is 120-130 degrees hot. Not ideal. I run mine at around 85 and the part measures around 105, Placing the thermistor under the bed or glass will most likely measure the temperature the surface will eventually end up at. That is probably the way I should have mounted the thermistor.
Here are some photos of it all:
I used some scrap 3mm aluminium and glued the resistors underneath.
This is the silicon I use to glue the glass onto the aluminium plate with.
The resistors used for the small bed are 5W 0.22 ohm x 24.
I wire them up in 2 groups of 12.
One group of 2 rows of 6 resistors in series gives me 12 x 0.22 = 2.64 ohms
Two groups of 2.64 ohms each, wired in parallell gives me 2.64 / 2 = 1.32 ohms
That gives me about 12v / 1.32 ohms = 9.09 amps (4.5 amps for each group)
Due to this high amperage I have installed a relay that is controlled by the ramps board and then the relay switches the bed in and out. This to avoid all those amps going through my board.
If my calculations are correct that should mean that each resistor has a voltage drop of 0.22 x 4.5 =0.99V. That again gives me 0.99 x 4.5 = 4.45watt per resistor. And that is within the range of the 5w rating for the resistor. Should be safe. (4.45w x 24 = 106.92w total)
That is a total of approx 12v * 9.09 amps = 110 watt
This small table reaches operating temperature of 110C in under 10 minutes (I have to time it and put the right time here) But it is quite quick.
If you glue the resistors this way using JB weld, you will not be able to get them off again in one piece. Hammer and kiesel is the only option. So make sure you know how many to glue down. I learned the hard way. To reach 110c you need at least 100w power to heat the bed (this kind of bed).
Remember to use appropriate thick cable when powering the bed. Cable must be rated higher than the current the bed will draw. Any old extension leads should do the job. And don’t forget the plug as well. It must also be able to handle the proper current or it will burn. Getting cable from any reputable electronics store, staff should know and be able to give advice.