It took me more time then I thought, but finally, I built my improved heated bed.
The initial test shows, the bed's temperature can go up to 164.3C, which is pretty hot:-)
(on the picture it only shows 148.7C, but it is because I already disconnected the transformers from
mains, so it already cooled for about 3-5mins.)
I thought a lot about heated bed insulating, as my previous one
was made of MDF sheet, which smelled a lot and burned:
I bought stone wool covered on one side with aluminium foil (it has a fancy "thermal mirror" name;-), which makes the fixation more easy, and also helps to keep the fibers inside.
This version of the aluminium sheets also contains some more bugfixes,
like rotated power resistor's holes (aesthetically better),
was misplaced one hole of the 9 resistors,
I also made sure about the magnets placement (it turned out pretty important).
The 5 red resistors are 15Ohm values, all connected parallel, giving a total resistance of 15/5 = 3Ohm.
The 4 blue resistors (at the two edges) are 10 Ohm values, connected parallel, giving a 10/4 = 2.5Ohm total resistance.
I also discovered the last time, that the total resistance varied with the heated bed temperature. At room temperature it was about 3Ohm, while when hot, it was 6.2Ohm!
It could be because of bad joints. So this time I additionally soldered everything together too.
Five 15 Ohm values and four 10Ohm ones.
I forgot to mention this is also a walkthrough tutorial, if you are crazy enough to buy a heated bed kit from me.
(but I hope it is equally useful for your tinkering fellas too!)
Step 1: Cover the bottom of the bed with Polyimide (Kapton) tape
Cover it, also the top of the bed should be covered around the edges (lets say 2-3 cm).
Step 2: Cut out the power resistors
You should also cut out all the magnetic holes too.
Step 3: Metal bar
This steps needs a bit more explanation I think.
I was always worried about the lifetime of my magnets, so I consulted with a magnetic expert, and incorporated in the design what I gathered during the short conversation.
First, the neodymium magnets are usually stable until 80C. But the temperature which it can withstand vary on many factors, like the size of the magnets, its shape, the exact alloy and manufacturing (it can be manufactured for up to 140C). And also it depends a lot from the magnetic fields and how the magnetic forces travels around the magnets.
A magnet operating in a "closed circuit" is more stable at higher temperature, and also it has more magnetic force directing upwards.
For better explaining I draw two explanatory diagrams (sorry for my limited drawing skills).
So I decided this L-shaped metal is width enough and I cut 4 bars:
- 3 pieces 190x10x4
- 1 piece 120x10x4
So I decided to put the magnets on the table just like a chess board, one is reversed compared to the other.
So one magnet up, one magnet down. Putting all magnets upwards, would be worse then doing nothing at all.
So I put together all the 14 magnets, and drawed the direction with a marker on all the magnets:
Step 4: Placing the magnets
For "gluing" in the magnets into the holes, I decided to wrap around each magnets with Kapton tape.
So I can hardly push in the magnets. Here is a photo of two magnets wrapped around (and also the short metal bar):
Step 5: Wiring
I also covered all the metal bars with Kapton tape, for insulating (and also for additionally fixing them).
Step 6: Thermistor
Step 7: Insulating
I covered the table with stone wool, also note the bolts, which fix the insulating to the table.
On the top I put the metal sheet on it, and cut around with the blade, so it has a nice finish:
Maybe on the photo it is not obvious, but in reality it is incredibly strong. I cant slide the metal
sheet on it...
Note: it reached 164.3C, just I disconnected the table, and it cooled down a little while I was grabing a camera.
I hope you enjoyed the article.