Plate to support the Heated PCB?

The kit I just acquired seems to assume that you'll supprt the heated PCB just on the corners and then rest the aluminum print plate on top of the PCB. I have a MK2a and the small holes at the 4 corners seem fragile to be the sole support for the print surface, but that's what seems to be shown on this page about the heated PCB's: [reprap.org] . It shows standoffs holding up the PCB with a piece of glass on top.

I've seen others which have MDF or aluminum plates for supports to the PCB's and this seems like a good idea to me so I was planning on adding something like this, but wasn't exactly sure of mounting. The kit came with a plate that's the same size as the PCB which was intended for the Y-base plate (which limits the Y travel far too much because it's too wide). I was thinking to use this as the support for the PCB and then use a smaller plate for the Y-base to increase the Y range. I'll probably convert to only 3 supprt points between the base and the support plate to make leveling the bed easier but I'll have to work out all the details. (also need a hole in the middle for the thermistor.)

Does this sound reasonable?
I've heard of people putting heat reflecting insulation between the PCB and the support plate. Is this recommended? What type?

Thanks for your help as a newbie.
-Bret

Any advice on this? What do all of you run to support your print surface?
Thanks,
Bret

I agree that those 4 corner holes on the PCB are fragile to say the least. What I did, was bond, with contact adhesive, the PCB directly to 9mm MDF with a glass plate on top. This unit sits on the print bottom plate.

Thanks Waitaki. I like the MDF idea. I think I'll cut a piece of MDF (1/4") and attach the PCB to the MDF with the corner holes and then put glass on top similar to your setup. I think I'll suspend the MDFabove the Y base plate with the standard bolt/spring setup.

Thanks,
Bret

What is the kit? If it's Prusa2, then I'm guessing that you mounted the y smooth rods below the threaded rods across the front and back. They're supposed to go over the threaded rods so that the build plate can clear the threaded bars.

Also, you have to put some air or insulation and air between the mdf plate and the pcb heated bed. A cheap insulation is a sheet of corrugated cardboard. For a spacer, you can simply use M3 screws and nuts.

I used washers under the four posts holding the PCB to spread out the weight a little. Even with a couple head crashes I have not had any issues with the four corners of the PCB holding the weight.

Yes, some sort of heat barrier between PCB and top plate will help to stop the MDF from possible warping.

Edited 2 time(s). Last edit at 03/03/2013 04:32AM by waitaki.

Having an insulation layer does make sense. Cardboard seems really combustable. Is it really OK? I saw one recommendation for wool and that sounds good. Maybe I'll use some other fire resistant cloth and washer/nuts for spacers. It's interesting the several instruction pages show putting the MDF right next to the PCB (like [reprap.org] ).

brnrd, the kit is indeed a Prusa2 came with the frame mostly put together and it did have the Y bars below the frame and the print bed plate there which was limiting the Y-travel. I'd seen some instructions which have the Y bars above the frame as you indicate and that would improve the Y-travel, but the print bed ends up being higher which limit's the Z-range.

Then I discovered that on the main Prusa2 build page it shows the Y-bars below the frame but the key is that the Y-base plate they show is only 140mm deep. I measured it out and this provides enough travel that you can cover the whole print bed so it should be the same as if I put the bars on top. The trick is that I need to then suspend the main print bed (MDF board) at points above this smaller base plate. This is exactly what they do on the reprappro model ( [reprap.org] ). I think this ends up providing more Z-range as well as the maximum Y-range. The only possible issue is that the suspension points for the print bed are closer together rather than all the way at the corners, but I don't think that should be a problem (and seems to work OK for the reprappro).

Did I miss something in my figuring?
Thanks,
Bret

It doesn't matter whether you install another plate below the plate where the pcb heatbed mounts into. In the end it still has to go over the threaded rods across the front and back. Also, installing springs on the heat bed only leads to vibrations that shows up as waves in the x-y direction along the walls of your printed parts in my experience.

Read the blog from Josef Prusa dated 11/11/2001 here on the design of Prusa2 (Iteration 2). I'm guessing that there might be some confusion between the original Prusa and iteration 2 instructions.

Here's a good way to increase your z build height: [www.robosprout.com]

Also, it seems that wool is considered self combustible and highly flammable. Note that the bed is not going to be heated to no higher than 130C so cardboard is perfectly fine.

Edited 2 time(s). Last edit at 03/03/2013 11:24AM by brnrd.

Here is the stack I have been running for several weeks on my mendelmax running 110-115 without any issues.
Lower plate Lexan bolted to Y carriage.
PCB bolted with risers to Lexan just enough space for 1 thickness cardboard wrapped in aluminum foil.
Glass plate clipped to PCB

No springs anywhere tried that for a short period and found out how bad it was.
Have not had to level my bed since assembly, still nice and flat nd level.
Lower Lexan just gets slightly warm , been running non-stop all weekend.
Have not had any issues with only using the 4 corners to support the PCB.

I have heard others using an aluminum plate on top the PCB to help spread the heat and extend the build surface slightly.


Edited 1 time(s). Last edit at 03/03/2013 11:32AM by Spk64.

Thanks for the links brnrd. I like the looks of the plate that holds directly to the linear bearings. I may have to make such a modification on mine.

If you don't have springs, do you just use a nut to hold the bottom of the PCB and then lock them at the right hight to do the leveling? So it's positioned just the same but doesn't have the springy-ness? Maybe the springs just needed to be stronger or more compressed to avoid the bouncing on the other models as it seems like a very tight spring would provide the same stability as if they were locked in place.

About the wool, it was the strongest recommendation on the main heated bed page ( [reprap.org] ): "Commonly used insulator materials are cardboard, wool, and cotton cloth on top of medium density fiberboard MDF. Wool may be a good option for insulation because its ignition temperature is 600C". If what you say is true, we should probably fix this page so it isn't the recommendation.

Spk64, thanks for the picture. That really helps me understand the setup. I like that it reduces the number of pieces to make up the bed as opposed to going underneath the frame with the Y-rods and needing another plate in the system. Is that just aluminum foil wrapped around the cardboard? What is your experience with the springs?

Thanks for the tips.
Bret

I'm not sure if the sites that claim that wool is self flammable and highly combustible is reliable. According to wikipedia, sheep wool is naturally flame retardant unless mixed with some polyester which can make it flammable.

How about fibre glass matting?

I was thinking of some kind of fibre glass insulation, but it's typically not thin enough. But basic fiberglass mat is a good idea.

I was also considering finding some insulation that they use under the carpets in cars. It should be pretty thin and I assume flame resistant. Any other ides?

Hardware stores should have self-adhesive insulation for wrapping around hot pipes, but it works nicely for printer heat beds too. Armaflex HT is one type, rated to 150 C, if you can find it. More common types are rated to 105 or so, which should still be OK for ABS temps, but barely.