MK2A Heated Bed Upgrade

[orgizmo 0]

We have recently built our UM and now we're looking to upgrade to a heated bed for doing larger parts. I have a few questions and would appreciate any help anyone can provide =]

We have a MK2A PCB (http://www.geeetech.com/wiki/index.php/PCB_Heatbed/Aluminium_Bed_Mount_Plate/Borosilicate_glass) as the plate is designed for the reprap prusa the aluminum plate will not quite reach between the bolts that normally hold the acrylic print bed on the UM. I've seen people using plywood/MDF as a base to mount the heated bed to the UM. Has anyone had experience with using these materials for this purpose? Any feedback on the pros/cons of using plywood or MDF would be appreciated.

All the build logs I've been finding have used either the aluminum plate (x)or the glass plate, I've yet to find one that uses both. Should these boards be mounted:

Glass

---------- <BullDog Clips

PCB

----------- <Bolts/ (Spacers?)

Aluminum

----------- <Bolts/Spacers

MDF/Plywood Base

Would I be better off have spacers between the aluminum, the PCB and the Plywood layers? Or do I only need spacers between the aluminum and the wood?

From what I've read either nylon or PTFE washers would be sufficeient to withstand the heat involved. Any thoughts?

I have a PSU harvested from one of my parts computers that can supply 350W, 17A @ 12V, my PCB has a measured resistance of 1.8Ohms which means I should be getting a cold start draw of 6.7A... is this going to be sufficient for doing ABS/PLA or should I upgrade to a more powerful 24V supply?

With the 4.7kOhm resistor going to the UM electronics will a 1/4W resistor do the job or should I get a higher wattage resistor?

I'm not in any particular rush to get the heated bed up and running (slow and steady winning the race and all) so if there's anything I've overlooked in all of this please let me know. I love our UM and would perfer not to see it burn/explode

 

[gr5 2,181]

will a 1/4W resistor do the job

 

yes. That's plenty. I got mine off some old junker electronics. yellow,violet,red. Double check with ohmeter after removing.

 

[gr5 2,181]

Order some cheap kapton tape from ebay. You can use this to connect many of these pieces together and if you use the aluminum only instead of glass also then kapton can be your topmost layer. Aluminum should be mic6 (it matters! look up mic6 aluminum on wikipedia) which is not particularly expensive or anything but very very flat and won't warp.

Personally I just put the heater taped under the aluminum, reversed the UM bolts so the screw head is down instead of up, got rid of the springs, and support the aluminum bed from below. The heater is small enough to fit between the screws for my paticular heater. If you do this then you can get screws with large heads (or print something to go over the screw head!) for easy turning by finger when leveling.

 

[edr360 0]

I used the original acrylic plate and added a few washers between the acrylic and the heating element to keep the acrylic cool. Did not remove the springs either. I print on glass covered with kapton tape.

I suppose, the setup without springs and with plywood base may be better in some ways, maybe I'll try it later.

 

[amano 0]

Glass

---------- <BullDog Clips

PCB

----------- <Bolts/ (Spacers?)

Aluminum

----------- <Bolts/Spacers

MDF/Plywood Base

 

I built my heated bed nearly like this. For the base I used a milled 5mm aluminium plate manufactured after a drawing of mine and with a middle hole for the thermistor. Directly on it comes the PCB (MK2b in my case) - no spacers! On top of the PCB and again with no spacers, followed a 4mm oven glass (resilient up to 800 degrees) also manufactured for me. I covered the glass with a big single PET tape sheet.

I would say that no perfectly milled aluminium plate can beat the smoothness of the glass. BUT the heat conductance of the glass is much more worse than with a aluminium plate. But for me and with the PET it works very well also with ABS. But a good heat chamber is necessary for good ABS prints.

Many shops that sells the PCB says that the PCB is good till 130 degrees because the PCB becomes bumpy at higher temperatures. Thats why I decided to built a sandwich with no spacers between aluminium, PCB and the glass. Because of that there is absolutly no space for the PCB to bend. Air (gaps between the sandwich layers) ist one of the biggest insulators for the heat.

In addition there is no need for a MDF plate in my opinion. I talked to a workmate (cabinet maker) and asked him for the heat resistance for MDF - I didn't get a precise answer. Thats why I decided to go with aluminium (also the alu plate is much thinner and stiffer as MDF).

Under my sandwich there is about 1cm space (air insulator) before the standard UM build-plate follows. I added a self-adhesive heat shield sheet (resilient up to 1200 degrees) on it for safety.

The whole sandwich are held together by bulldog clips - very strong no slipping. I used simple plastic washer from hardware store. Probably PA or something like that - no problems.

350W 17A at 12V are enough. I'm still using a similar combination and my MK2b draws about 13A at 12V.

If there are any questions I can upload a photo tomorrow if you want.

 

[orgizmo 0]

Thank you all so much for the quick and helpful replies, very interested to see a picture amano =]

What are you using for a heatshield between the acrylic UM plate and the heat sandwich? With the thicker sandwich what does your maximum z travel become?

Thanks again =]

 

[edr360 0]

I use 2 hex nuts and 1 washer for each angle. Basically, the heated plate is screwed down to the acrylic sheet and the hex nuts and washers create a gap between the heater and the acrylic. As heat conductivity of air is very low, the acrylic gets only a little warm. Its maximum safe operation temperature is about 80 C, so there is no problem with that.

Max Z travel is 8-9mm less than the original one.

 

[amano 0]

Oh.. I've poorly expressed myself - caused by my mean english - sorry. I replaced the entire acrylic plate. With "build-plate" I mean the Z-axis birch wood stage.

I bought the heatshield on ebay. Normally they are for reflecting the heat on motorcycles and cars. For instance like this: http://www.ebay.de/itm/Helix-Racing-Products-Heat-Shield-with-Adhesive-18-x-18-401-1302-/330951515175

On Z height I lost around 5mm if that. Pictures tells more then words :