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Heated bed v1.6 – Thermal insulating and more

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:

Also it left an unremovable stain:

My primary concern was, the bed was not hot enough. So I decided to improve on the insulating, and I bought stone wool, which can withstand this temperature easily:

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.

I also modified the aluminium sheet again. I added four additional threaded blind hole, to easily fix the stone wool:

(It is the bottom of the finished heated bed, but please keep reading;-)

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).

I also decided to put 9 resistors at the bottom of the bed. But I also wanted to arrange them
into two group. My girlfriend came to the rescue, and proposed this resistors arrangement:

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.

Here are the resistors placed on the vanilla aluminium sheet:

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

Once you covered the bed, you should place the power resistors on it, and mark them around:

And also cut it with a retractable blade. Here is it half done:

Also one finished:

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).

Open circuit:

Closed circuit:

Directing magnetic forces is possible via irons (and other magnetic metals).
Determining the width of the metal bar, I have done with trial and error.
Here is the little experiment setup:

The bottle cap is (obviously) attached to the magnet. Putting a width enough metal bar between
the cap and the magnet, catch all the magnetic force:

So I decided this L-shaped metal is width enough and I cut 4 bars:
- 3 pieces 190x10x4
- 1 piece 120x10x4

Here is a photo of one of the long ones:

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:

That way I can make sure I put it right on the table. I also wrote neath all the magnet holes its direction (up or down):

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):

For double-checking if they are attached to the aluminium sheet, I placed the sheet in the air, and watched if one of the metal bar (with magnets) are falling down:

Step 5: Wiring

I also covered all the metal bars with Kapton tape, for insulating (and also for additionally fixing them).

I wired the middle Z shape:

Here is a perspective photo:

I bit more advanced stage (also with the "schematic"):

Here is finished:

Once I finished all the wires, I soldered each joints (remember the explanation from the beginning of the article):

I covered everything with Kapton tape. Everything.

Step 6: Thermistor

I drilled a little the table, and put the 200k thermistor in it. I also soldered a piece of wire on each thermistor legs, and covered them with Kapton tape:

Also the magnets are trying to grab the thermistor, so I placed near the terminal:

Step 7: Insulating

I covered the table with stone wool, also note the bolts, which fix the insulating to the table.

Bottom side:

On the top I put the metal sheet on it, and cut around with the blade, so it has a nice finish:

Also here is a little demonstration how strong are the table magnetically:

Maybe on the photo it is not obvious, but in reality it is incredibly strong. I cant slide the metal
sheet on it...

Also one more time the temperature proof:

Note: it reached 164.3C, just I disconnected the table, and it cooled down a little while I was grabing a camera.

Also here is an explanation what are the 6 wires coming out of the bed:

I hope you enjoyed the article.

Comments (5) Trackbacks (1)
  1. This improvements sound really good.
    I will have to solder my resistors too.

  2. Are you still selling heated bed kits?

  3. kuhlivisj: yepp. If you are interested, shoot me an email.

  4. I’d recommend using any magnet except neodymium. Samarium-cobalt and Alnico magnets are both very strong and good for high-temperature use. You’d have no problems with either of those at temperatures like 160 C.

  5. I agree with Jacob, there are plenty of higher temperature resistant magnets around. Sure, they aren’t quite as strong as Nd but they will be stronger after a year or so of heating! Nd magnets also fail due to oxidation and exposure to water and water vapour, which is why they are coated in silver. I strongly suspect that the thermal stress will lead to that coating failing, too.

    The heated bed arrangements you have done are great. :-)

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