Sunday, 31 January 2010

Quick release bed

I am in the process of making a heated vacuum table to hopefully allow automatic ejection of finished objects. In a conversation with Laszlo he mentioned he was planning to use a heated steel bed and use magnets placed around the object to hold down a sheet of Kapton. I turned the idea upside down. Why not stick Kapton tape to a sheet of steel and clamp it to a heated aluminium bed using magnets underneath?

I found a thin sheet of bright springy steel that was part of an electric toaster. My best guess is that it is one of the grades of stainless steel that is magnetic. It is only 0.3mm thick so it is relatively flexible, but it always springs flat. It came from a Kenwood toaster that gave good service until our cleaner suggested to my wife that she should turn it upside down to get rid of some persistent crumbs. The next time it was used it burst into flames because a crumb got wedged between the element and the steel plate and burnt through the nichrome.

I made a tiny heated table from an off-cut of 6mm aluminium. It is only 105mm x 73mm, which is smaller than a MakerBot CupCake bed but I think it is just big enough to make all the Mendel parts.

I have run out of AL clad resistors so I made my own from vitreous enamel ones embedded in aluminium blocks with tin foil. I used two 6.8Ω resistors in series driven from ~ 26V AC. That gives about 50W and a similar warm up time to my larger bed driven with 200W.

I milled flat bottomed holes to within about 1mm of the surface and embedded five neodymium magnets which are held in with Kapton tape.

I used M3 threaded nylon stand-offs as insulated table legs and mounted it onto my XY-table using a sheet of 4mm aluminium / plastic laminate called Dibond. It is very nice material to work with.

The steel plate covered in Kapton tape then sticks to the top of the table. I heated it to 100°C and tried making some ABS objects.

This worked well and the objects were easy to remove by bending the plate and peeling them.

The magnets are strong enough to hold down even big objects. The only problem I had was that the nozzle snagged on the first layer of this object and managed to slide the steel plate, causing the first layer to be offset.

Contrary to popular belief, FFF does require significant force and benefits from a stiff extruder mounting.

A couple of pins in the corners to act as dowels would solve the sliding problem.

Here is a video showing how easy it is to remove the objects: -

It is still a manual process though, so I will pursue the vacuum table idea to attempt to make a bed that can eject the object itself.


  1. cool i like that idea, good clean and simple

    and easy to do, kudos

  2. Nicely done - it's pretty simple to make and looks convenient to use.

    One question, though: what grade of magnets are you using? If the bed heats up above the Curie temperature of the magnets they will die...

  3. Another great innovation and write up!!.

    Knowing how Weller use a magnet for soldering iron temprature control.
    I don't think 100C/212F will have much effect as its a lot less than 600F/315C to 900F/482C the temprature range of the Weller TC bits.
    Bit temprature is shown in 100's of degrees F by a digit(6-9) stamped in the bottom of the bit.

    I'm sure someone suggested a conveyor belt for the Mendel print bed in the RepRap forum.

    Using electromagnets with a Kapton-ply-plate replacement system. (Maybe a delta arm from the RepRap Forum).
    Its not much of a stretch to imagine some kind of automated production machine of RepRap parts.

    Spelling corrected version.

  4. Damn Chris you beat me again. Im in the middle of manufacturing a bed like this(18 of it, if everything goes well). Im currently waiting for the magnet to arrive(8x1mm, 10x1mm, etc), and also I have an arrangement with a cnc shop at monday.

    Im a bit behind of blogging, I try to catch up.


  5. > I milled flat bottomed holes to within about
    > 1mm of the surface and embedded five neodymium
    > magnets which are held in with Kapton tape.

    I dont get, if you are fixing the magnets using Kapton tape, why the trouble of milling holes for the magnets?


  6. @theirreverent,
    I read the Currie temperature of neodymium magnets is low for a magnet but it is ~300C so no problem in this application.

    The magnetic force distance equation is an inverse square law so the magnets need to be very close to the top side of the aluminium. An easier way to make it would be to drill all the way through and glue them flush with something like JB Weld. I needed to keep the top surface intact as I plan to mill channels into it to apply a vacuum.

    Yes a Kapton belt held down with vacuum is what I will try next. The belt does not need to be continuous, just a sheet that rolls onto a roller will guarantee the object falls off.

    I was hoping this system would completely release the object so it could be knocked off with a swinging arm but a small section stays stuck requiring a vertical pull to release it. Easy for a human but tricky for a robot as the object shape is arbitrary.

    Electro magnets would waste a lot of power unless the heat was used to heat the bed as well. The control would get tricky as you can't afford to turn the magnets off during the build.

  7. Came accross this a couple of years ago when trying to convince a technician that Stainless Steel didn't have to be non magnetic. I had just orderd a 600mm square 20mm thick Stainless Steel sheet with holes at 50mm pitch. It was magnetic and there was panic as it was a back plate to be used for a test rig which would be used for environmental simulation of a component being driven through a ford every 5 minutes or so.

    To make non magetic Stainless Steel the material has nickel added which modifies the structure and improves low temperature performance making it less brittle.

    Probably not a requirement for a toaster :)

    If I recall correctly, its cheaper as well of course.

    If the magnets are as specified they will be fine and easily handle higher tempertures than you intend to use currently. ;)

  8. Trudged in the snow to Shed @ my house.
    To recover/recycle some AL sheet.
    240mm x 130mm x 3.3mm thick~Ex Vero Rack End plate?

    Found 2 AL clad resistors 2.2 ohm
    4.4 ohms at 20V 90W.

    Nothing for insulator maybe some recycled Nylon screws 190°C-350°C?

    Have 20mm Magnets too with maybe metal shield of a CD drive if I have one without holes in.

    Don't really want to cut the AL sheet down if I can avoid it what do you think?

  9. This is very interesting! You could also remove the objects with a scraper. The lower side of it should be of a soft material that will prevents damage to the Kapton sheet. You could wedge it under or have something that mechanically wedges itself under. On the other hand, this is mechanically much more complex than a conveyor belt would be. But wouldn't the material under the Kapton sheet need to be porous to hold the kapton down? That may get in the way with the nice surface quality that you have right now. On the other hand, it's only one face that will become so smooth, the value of automation is much larger than perfecting polish.
    If you happen to have a small bit of steel left, maybe hairspray would increase adhesion at lower temperatures, allowing you to achieve lower power consumption and still counter warping (it may very will not work at all, though!).

  10. @BodgeIt,
    90W should be plenty for that size so no need to cut it. Nylon screws should work but tricky to get it level. PM me with your address if you want some nylon standoffs.

    Yes I expect the vacuum table to leave some imprint on the part, but fine for mass production as most things don't need the shiny finish.

    I think the temperature needs to be above the glass transition to completely eliminate warping, so not a lot of energy to be saved unless slightly warped is acceptable. It is for most Mendel parts I suppose but the energy used is tiny compared to the price of plastic.

  11. Thank you.. Only flat thin magnetic material I could find was a Matsumi 3.5" disc drive cover so I had to cut it to fit :-( 99mm X 130mm

    So I have just tested my Evolution Fury Chop Saw with 3.3mm Aluminum. Have swarf in my hair though.

    Will try Nylon bolts with 2mm or 3mm threaded hole in end of bolt to screw plate on.
    Nylon bolts should all be same length.

    Need to get things working quickly.

    Didn't think I stood a chance in Hell of a Stand in March, as application was over 2 months after the dead line.

  12. This seems like a better idea than the film/vacuum/convayor...

    Ideally any automated system need to let the printed object cool before removing it, and this would allow you to simply swap out build surfaces, meaning you only have the minute or 2 for that thin steel to warm up rather than letting the whole platform cool and then re-heating.
    (Which is going to drive me nuts one day... only having the one platform is adding 20 mins to everything I build right now.)

  13. Also, I see one big advantage to using many strips of kapton... It looks like the uneven surface makes it much easier to remove a cool part!
    Although the finish of the underside is nowhere near as good, and I suspect linked to the minor warping I'm getting on very large parts.

  14. Hello all

    What if you make a convayor out this steel sheet and after the build is completed the convayor transports the part to its edge and there a wedge waits a mm under and lower the convayor face the part will follow some mm the surface and than laying partly on the wedge and the convayor push the part off the surface. I hope you get the picture. then the convayor turns back to its build position only slithly cooled and ready in a few seconds for the next part.

    Cu Atztek

  15. @The Ruttmeister,
    I think this is ideal for ABS and PLA but I need the vacuum for HPDE as it doesn't stick to Kapton. My plan is use a sheet of polythene held down by vacuum for that.

    I cool the plate before removing it but I suppose you could remove it with oven gloves and stick it to a cold magnetic surface. The bottom of the object is jelly like so you risk distorting it.

    I didn't notice any less grip when using thins strips of tape. I suspect that if you get less grip and still some warping that the surface is not hot enough.

    To speed up the warmup you simply need more power. It doesn't waste any energy, in fact it saves a little because less heat is lost while warming. A big fan can cool it pretty quick.

    Yes the plate could slide over the edge and be bent downwards slightly to allow a wedge to get under the object. The object would need to have cooled though otherwise it would deform when it hit the wedge.

  16. Good point nopehead i had not known how the plastic acts at this tempreture. so the conveyor needs a cooldown station. When the build is complet the conveyor moves the part out the reach of the reprap and cools down, so when the next part is finished the first is solid and the conveyor move it to the wedge and the second part to the cool position.

    Sorry for this long post.

    Cu Atztek

  17. Off topic:
    I am seeing this video of yours:

    Nice to know that you like 8bit music :-)

  18. Yes a bit different from my normal choice of music. I think Zach got me into them by using them on one of his videos.

  19. Nophead,
    do you know of anyone looking into recycling any of the plastics used in 3D printing? Here in the US, we have recyclable plastics labeled with their type. I suspect grinding up the plastic and heating them in some kind of press would extrude a filament, but I don't know of anyone looking into this as source of plastic for printers.
    Sorry to contact you by your blog. I can't find your e-mail address (not a good stalker, I'm afraid). :)

  20. Hello Lane,
    It is on the list of things to do but most people are concentrating on improving build quality and reliability first.

    There are two approaches: grind it into granules and use a granule extruder or melt it into a 3mm filament and use the existing extruder. Adrian Bowyer did some experiments with a granule extruder but they were not totally successful //

    Forrest Higgs has done some experiments melting granules to make filament. Again proof of principal but not a practical working device yet.

    I plan to have a go sometime but so many other experiments to do first. I have shredded HDPE milk bottles with a paper shredder. I would heat it in a cylinder, use a vacuum to remove air bubbles and then push it through a nozzle with a piston to make 3mm filament. Then pass it through a water bath to freeze it.

  21. Chris,

    You never fail to amaze me. What a lovely design!

    Have you tried printing HDPE on it, yet?

  22. HDPE does not stick to Kapton but I have milled some channels in the AL bed and will try holding down a polythene sheet with a vacuum for HDPE in the next day or so.

  23. HDPE prints really well on polypropylene sheet. :-)

  24. Tumbled upon this:

    Spacers water dissolving; interesting for support material


  25. Interesting indeed. I e-mailed them but they cannot supply it in 3mm filament form. I'll follow up on it some more. I'd like to know what it is.

  26. Do you know if it is actually a thermoplastic? It would have to withstand soldering temperatures so I am guessing it isn't. It could be something like ceramic plus a water soluble binder.

  27. Would it be possible to use electromagnets to do the release - i.e. you use permanent magnets to hold it in place and when you want to release it activate the electromagnets to counter the force of the permanent magnets.

    This would then allow easily removing the steel plate while hot to place elsewhere to cool down w/o a chance of deforming the jellylike objects.

    Then you stick the second kapton covered steel plate on the preheated bed and within 1 minute you can start printing the next batch.

    Would this work?

  28. I think it would be possible but it would need 5 fairly powerful solenoids to do it.

    The standard way to make a magnetic clamp that can be released it to put a cylindrical magnet between two pole pieces. That gives more force than my arrangement because it uses both poles. To release it you rotate the magnet 90 degrees so that each pole of the magnet contributes equal and opposite fields to each of the two pole pieces.

  29. Sorry to interupt people, but you guys are amazing, I could only dream of it, but want to play. Where can I get the components to build a Mendel or even a Hydraraptor if possible. I am interested in organically shaped arched structures made of PP.

    Peter Wynd

  30. Hello Peter,

    You can get most of the parts in eBay if you search for RepRap Mendel: extruded parts, motors, bearings and electronics. I have a set of the plastic parts on there at the moment. You can also get electronics from The rest of the parts like nuts and bolts come from local hardware shops or online.

  31. The multi seal material is made from:

    You can read it in their MSDN. It melts between 150-180C and is partial solutable in water.