Difference between revisions of "Prusa Mendel Print Surface"

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Latest revision as of 10:33, 16 February 2014

Swapping Materials

Changing from glass to acrylic or polycarbonate beds is easy. Just remember the following:

  1. For glass, use the laser cut foam washers, and don't over tighten.
  2. Ordinary glass needs the laser cut plastic as heat spreader and thermal resistor. Not recommended for other materials.
  3. For plastic (acrylic/polycarbonate), don't set the bed too warm, or the printed part will be non-removable.

The full print bed stackup is:

  1. Bottom. MDF backing attached to LM8UU bearings and timing belt.
  2. Printed, plastic, leveling mat. See "Leveling" section below for details.
  3. Laser cut foam mat. Serves as thermal insulator, provides a notch for wiring, and slightly improves flatness.
  4. PCB heater.
  5. Optional. Laser cut red plastic, clear plastic, or silicone. Mostly serves as heat spreader and thermal resistor, sometimes necessary for ordinary glass.
  6. User-selected print surface. Glass, plastic, the choices are elaborated in the "Surface Materials" section.
  7. Foam washers. These are laser cut from the same material as the foam mat, and should not be reused once crushed. Extras are stored in a glass jar.
  8. M3 screws and washers. These are bolted directly through mounting holes in all other layers, including glass.

Surface Materials

By default, we are using an ordinary glass print surface, which has been cut, drilled, and bolted onto the printer. Other surfaces are available, and we plan to upgrade to pyroceramic glass as the default print surface.

Glass

Glass is an ideal print surface material. It binds strongly to the plastic while warm, and releases the part once cool. Additionally, glass is somewhat rigid, flattening out the print surface.

Obviously, is all good for first layer alignment, warp prevention, and easy part removal at the end of a job.

Please be careful to use the laser cut foam washers for installing glass, and apply as little pressure as possible. Tighten all the screws a quarter rotation at a time, just until the glass stops moving downward.

Ideal Glass Surface

Pyroceramic Glass, 9" * 9", Four Mounting Holes. Recommend against the similar bed design here. The wiring cutout and larger 9.5" * 9.5" size may contribute unnecessarily to thermal stress.

Can be ordered from One Day Glass.

Ordinary Glass

Ordinary glass can be purchased from a variety of stores, cut to size, the drilled with a dremel and the cylindrical diamond abrasion tool. After a little practice, new glass beds can be manufactured with just a few dollars and 20 minutes.

Tenleytown Ace Hardware is especially helpful, offering cutting service with the glass.

Unfortunately, ordinary glass does tolerate thermal stress well. Although we are in theory using a PCB heater to evenly warm the bed, in practice the thermal gradient from center to outside edge can be high as 30C. Catastrophic failure becomes likely given mechanical stress, spot heating/cooling from some 3D printed parts, or microfractures on a poorly cut edge. Further, ordinary glass may be less rigid, prone to representing complex warped shapes underneath it.

Ordinary glass surfaces should be double pane and no larger than 8.75" * 8.75" . Optionally add the thin laser cut red plastic, clear plastic, or silicone sheet between ordinary glass and the PCB heater. This will mitigate rapid or uneven heating.

Plastics

Plastics tend to offer extremely tough adhesion, without the quick release behavior offered by glass. While this may be helpful at times, care is needed to prevent parts from permanently binding to the print surface, even if paper or kapton tape layers are used.

When printing on these surfaces, please set the bed temperature as low as possible. Recommend no more than 45C for PLA.

Additionally, plastic surfaces are inherently less rigid than glass, particularly if thinner than ~5mm.

Heavy Acrylic

An 8.25mm acrylic sheet has been salvaged from a disassembled LCD monitor, and laser cut to fit. At this thickness, rigidity seems to be exceptional. Probably the best choice for laying down a paper tape or kapton printing surface.

Polycarbonate

A 5.75mm polycarbonate sheet has been cut and drilled to fit our printer.

Leveling

Suggest others do not attempt this without prior experience. Anyone contemplating this exercise in frustration needs a lot of patience and a fire extinguisher. Additionally, the end result is difficult to undo if incorrect.

Built-in alignment between extruder positioning system and print surface is badly warped on our aging Prusa Mendel, taking as many as six polynomial orders to accurately characterize. Consequently, the only reliable process for realigning this complex system is rather complicated, involving a 3D printed and heat-polished leveling mat. It is roughly equivalent to using a CNC mill to finish off its own table.

Step 1

Use a CAD program to generate a large rectangular object covering most of the print surface at least three layers high. This is our 'leveling mat'.

Step 2

Disassemble the print bed down to an intermediate surface (preferably the hard MDF backing), cover with paper tape. This is the print surface for our 'leveling mat'.

Step 3

Carefully move the Z-axis endstop to match extruder depth with the new print surface.

Step 4

Abort! If you have started printing the leveling mat, you probably didn't set the Z-axis endstop position well enough. When this happens, do not wait for the printer to put down more than one layer of the leveling mat - it may become infeasible to remove.

Use PLA material, and retry printing the leveling mat until the first layer is clearly adhered to the intermediate surface.

Step 5

There will be some rough areas on the printed mat. Heat polishing solves that. Bolt the PCB heater on top of this leveling mat, bolt laser cut foam on top of that, and the heavy acrylic print surface on top of that. Ensure the whole assembly is evenly clamped down under significant pressure.

Heat to ~90C for 5 minutes, and have a fire extinguisher handy. Generally, when the underside of the MDF hits 40-50C, heat polishing will be complete.

Step 6

Unbolt the heat polishing assembly, and check the leveling mat. Should be silky smooth, with no bumps larger than 0.05mm. If not, repeat Step 5 until it is.

Step 7

Reassemble the print bed as it was before, carefully. If lucky, it should be possible to keep extruder nozzle aligned to print surface within <0.15mm at all positions.

If unlucky, not only will the print surface remain warped, but the old leveling mat will need to be removed to repeat the process. Since the leveling mat can adhere very strongly, this can be excruciatingly difficult.