Tuesday 6 April 2010

Dibond bed

I had been making Mendel parts with my Mendel, using PLA on blue masking tape, as it didn't have a heated bed . When I made a frame vertex on its own it came out completely flat. Larger parts like the z-base brackets warped a little at the corners, but were still acceptable. However, when I made a bed full of parts the warping was much worse. Frame vertexes warped a little and z-base brackets curled up several millimetres and jammed the y-axis, ruining a bed full of parts. I think the reason they warp more is that it takes so long for each layer that the parts are completely cold when the next layer is deposited. The odd thing is that Adrian Bowyer manages to print trays full of parts on blue masking tape without a heated bed. I have added it to the growing list of things that work better in Bath than they do here: AOI and PTFE being another two.

I had some aluminium plate on order but I wanted to knock something up quickly. I figured PLA on blue tape would only need 40-50°C to stop it warping. My bed is made from Dibond, which is 3mm thick and has the following characteristics:
  • Thickness of aluminium layers 0.3mm.
  • Core polyethylene, type LDPE.
  • Surface: lacquering - modified polyester lacquer system.
  • Temperature resistance from -50 ° C to +80 ° C.
  • Aluminium grade premium A1Mg aluminium alloy.
The great thing about it is that it appears to come pretty flat and is strong, light and easy to machine. I wondered if the aluminium layer was thick enough to spread the heat. I didn't think heat would flow though the LDPE very well so I mounted 10 47Ω 50W resistors around the top edge. I have found that for some reason 47Ω are cheaper than the 12Ω ones I used on HydraRaptor's bed. I wired them in pairs in series and then all the pairs in parallel giving 18.8Ω. I connected them to my 48V AC transformer with a small solid state relay. The total power is about 120W. Not as much as I use on my aluminium beds, but plenty of power to get to 50°C quickly. In fact, it warms up faster that my extruder does.

An initial test showed that the middle was about 10°C cooler than the edge. Not a big surprise considering how thin the aluminium is and how far the heat has to travel. When I measured the other side the difference was only about 5°C, so I decided to mount it upside down with the resistors on the bottom and the thermocouple on the top.

It works very well, and the objects stay flat. The first multi-part build I did though failed after the first few layers.

The extruder jammed because the top of the thermal insulator got hot enough to allow the PLA filament to go soft before the entrance. The extruder was finding PLA very hard to push anyway and the maximum speed I could get was about 24mm/s of 0.5mm filament. This is because the thermal transition zone is too long. The extra heat rising from the bed must have pushed it over the edge, literally!

The insulator is a combination of PTFE for slipperiness and PEEK for strength, but I think PEEK conducts too much heat. It doesn't help that my heater is not insulated yet and the Mendel carriage traps any rising heat.

I am quite happy with with Wade's drive mechanism but decided it was time to try another hot end design, coming soon ...

I think that for PLA, Dibond and blue tape / Kapton is a good solution. It won't handle the temperatures for ABS on Kapton though, but it might be good for ABS on PMMA or PC.


  1. NopHead, nice to read about this kind of design for heated bed :-)

    PLA go soft before enter in PTFE remember my last problem. Are you sure that your extruder motor wasn't helping PLA getting soft?

    (I am still assembling my Mendel and I need an Heated Bed on it :-) )

  2. The motor does not get hot enough to soften PLA (I use PLA for the motor bracket on HydraRaptor) and it is a long way from the PTFE. I think it is heat conducted by the PEEK outer jacket and convected from the heater and the bed.

  3. slacker!
    dont tell me your stranded!
    you owe us 4 posts for the past 2 weeks!

  4. If anything, this volcanic crisis should be making us double our efforts on the Reprap project.. Our imports are crippled and we need to be able to make stuff ourselves.

  5. hi
    you know dibond has a flame resistant version!
    It is class B en 1350 thats 900Kw for 1200seconds, so no problem stable state 300 degrees C
    dibond FR aluminium plus mineral wool sandwich
    # Flame retardant’ fire classification class B, s1, d0 according to EN 13501-1, the new European standard norm.
    # Core with mineral core, without halogenated fire retardants making the material non-toxic
    # All well-known DIBOND® processing possibilities possible (except for hot air welding)
    # Excellent performance, including with bending and mill cutting technology

    do tell us if it works for abs as well
    kind regards

  6. Tan,
    No I didn't know about Dibond FR, thanks for the tip. I will have to see if I can buy some to try it.