Thursday, 11 February 2010

Suck it and see

The magnetic steel and polyimide tape bed works very well for manual operation but I am pursuing the vacuum bed idea for fully automated production. I went through a few designs in my head before actually making anything.

The first idea was to drill an array of holes through an aluminium plate and connect them on the back by milling a network of channels. I would close the top of the channels with some Kapton tape. The problem with that idea was there was then nowhere to mount the heating resistors unless it was on top of the Kapton sealing tape, which didn't seem ideal. My solution to that was to mill a channel into the edge of the plate and wind a coil of nichrome all the way round it. That was my plan until I realised there would then be nowhere to attach the vacuum hose.

A solution might be to use a Kapton or silicone stick-on heater and use it to seal the channels in the underside.

What I actually did was to mill a grid of very fine channels into the top surface allowing me to attach the vacuum hose to the side edge and drill a small hole down to meet it, leaving the bottom free for the resistors and thermocouple.

The channels are about 0.5mm wide and 0.5mm deep on a 5mm grid. I milled them with a 0.3mm conical bit that I bought for milling PCBs.

I used a feed rate of 2mm/s and 0.1mm cut depth per pass. My MiniCraft drill runs at about 20,000 rpm. The results were not very good. I have only ever milled plastic before with HydraRaptor. It struggled cutting aluminium such that the shaft of the drill was being displaced in the direction of the bed travel. It raised a burr about as high as the channel is deep. My friendly local milling consultant told me afterwards that aluminium does not like lots of flutes. He recommended a D-shaped cutter with a single cutting edge and a higher spindle speed.

I sanded the surface flat with 240, 600, 800, 1200 and 1800 grade wet-and-dry sandpaper and then polished it with metal polish. I did this to get as good a seal as possible with whatever was placed on top.

I attached some polythene pipe using an M5 copper welding nozzle screwed into a tapped hole in the side of the plate. I use a tapered tap so that the thread would bind to form a seal. I used Fernox LS-X jointing compound to make sure it was airtight. I think it is silicone, so should handle the temperature.

I was hoping to get a perfectly air tight seal and be able to use a static vacuum generated by a syringe. It doesn't seal fully though. I believe normal vacuum tables use rubber o-rings set into a groove to form a seal. I reasoned that would not work in this case because, whereas sheets of stock for milling are stiff enough to remain flat and squash the rubber, thin films would just bend upwards. My idea was that the thin film would be sucked into the channels and be compliant enough to seal it. I think it fails because the edges of the channels are too rough due to my poor milling.

I first attached a small vacuum pump that I made for my jukebox. It is just an aquarium pump with a pipe attached to the air inlet and the case is sealed with rubber glue.

It is not a very strong vacuum, but it is enough to pick up a CD with a suction cup made from the end of a child's rubber dart. I plan to use it for SMT pick and place soon. I measured it at 960 millibars, which is also the extreme low reading on our barometer. I knew that the vacuum it created was less than the atmospheric variation because I started off with an absolute pressure transducer on my jukebox to detect if a disk had been picked up. I had to change the trip point about twice a year because one setting would not work for both extreme high and low weather conditions. In the end I added a second sensor to make it differential.

I placed a piece of 0.075mm polyimide film on top. This is about twice as thick as the tape I use.

The video below shows the effect of the vacuum. It pulls flat and has some resistance to sliding but is not a very strong grip.

I built a Mendel part at 100°C for my first test. The film stayed flat during the build but a few corners lifted. When the part cooled it broke the vacuum and wrinkled the sheet. It was past the point where it had hardened so the base was perfectly flat apart from where the top corners had lifted slightly during the build.

I measured the temperature of the surface and found that it was 10°C lower than that measured underneath by the thermocouple. I raised the set point to 110°C and made another part. This time only one corner lifted (left side of the boss in the middle) .

Here is a speeded up video of the film releasing as the bed is cooled down to 40°C by a fan.

And here is me simulating removing the object by sliding the film. Ignore the ×16 annotation, I am not that slow!

The next thing I tried was a really big part of Mendel. I didn't trust my weedy aquarium pump to hold it down so I used a 1/4 HP 180W 3 cubic feet per minute pump rated to go down to 0.1 millibars. I bought it for £150 over two years ago to make a vacuum bed for milling but never got round to it, so it has been sitting on a shelf, like a lot of other parts and materials I have bought for experiments but not had time to use.

When connected directly to the vacuum gauge with a length of plastic hose it goes down to about 30 mb. I think I would need better quality fittings and pipe to get down to 0.1 mb. When connected to the vac table it gives 40 mb, so although it does leak, it still gets most of the available downforce from atmospheric pressure, i.e. ~15 lbs / square inch.

The part I made ended in disaster because the vacuum broke during the build. I think it was mainly because the object was not quite centred on the table so its outside perimeter was on top of the last vacuum channels. Before it failed some corners had lifted a little, early in the build, so it looked like the ABS does not stick to film as well as it does to tape.

I centred the table and made a slightly smaller piece. Actually this was the same depth as the last piece, so the perimeter falls about half way between the last two channels. Ideally I think you don't want to be that close to the edge.

I raised the temperature to 120°C, so the top of the film was probably ~110°C.

The film stayed vacuumed down during the build but still broke the vacuum and wrinkled during the cool down period. That means that even with close to the maximum vacuum it cannot hold the contraction force. Not a big surprise as I realised a long time ago the warping can generate a lot more than 15lbs / square inch of pull. The only reason I thought this might work is because the plastic does not warp while it is kept hot and indeed the vacuum holds during the build. The problem is that the object does not stick to the film well enough. One corner peeled early on and lifted further as the build progressed. The rest of the base is flat though, so the part is easily good enough to use. The higher temperature and vacuum meant that the grid lines are just visible on the object base if you get the light right.

So just to make sure I can make an object this size on tape without warping I made the larger part again on my magnetic bed. That failed because the bed slipped part way through. I knew that was a likelihood and that I need to add a couple of dowel pins in the corners, but I didn't want to do that while I was experimenting with vacuums. I gambled on it not slipping and lost.

It did build enough to show that it sticks much better though. The corners stayed down and the build lasted long enough to go way past the point where the corners lifted on the vacuum bed. The base was perfect.

I could only think of three possible reasons why the corners would lift on the vacuum bed and not on the magnetic bed.
  1. The surface of film might be different to tape. After all, tapes have the magic property that the glue only sticks to one side and peels from the other without leaving any residue.
  2. The film is thicker, so has more thermal resistance, which might have some influence.
  3. Perhaps the film can lift a little in between the vacuum channels allowing the plastic to peel away and then be sucked flat again.
My best guess was that the third explanation was the most likely. Perhaps closer spaced channels or thicker film would solve it. I had a sample of 0.15mm film so that was the easiest thing to try next. It stuck much better but towards the end of the build I heard a snapping sound and saw that one corner had lifted.

More importantly though I could see that the other corners were deforming the film upwards as I had suspected. It is hard to see here, but the slightly raised blister of film was over a channel, so subject to the full vacuum force.

This shows that even with a heated bed, there is sufficient warping force at the corners to beat atmospheric pressure. The part ended up with one chamfered corner and dimples in the others.

So in general the experiment is a failure as it does not work as well as the magnetic bed. It does allow easy automated removal though. All you need to do is tape down one edge of the film. When the object cools it wrinkles the film and breaks the vacuum. A fence could then push the object off the bed with not too much force, as the film peels easily. When the object is gone the film springs back to being flat and the vacuum can pull it down again ready for the next object.

Although corners lift, the objects are usable for making a Mendel, it is only an aesthetic issue in this case. I think rounding the corners of the parts would fix it. I guess PLA might stay stuck as it warps less, and ABS only fails in extreme cases.

Thanks to Paul for providing the polyimide film samples and lending me the vacuum gauge.


  1. Having not really thought this through it may be a silly idea, but would it make sense to keep the film taught between two rollers slightly below the plane of the heated bed? When the part was finised it could be rolled off, but while being printed the tension between the rollers would serve to keep the film flat (if the film were strong enough to be kept at the necessary tension).

  2. Your parts still look like something to shoot for, Great!

    Why the flat tape? Wouldn't it be better with dozens of holes to use some of the Vacuum with the part? IMHO, a pebble finish might work better with the parts & Vacuum combo.

  3. What do you think of filling channels (think t-channel) or holes with plastic to form a captive raft? Make it from two parts, the top part would have the channels/holes and the bottom part would be solid. The channels/holes would be tapered and go from small at the top to larger at the bottom. The extruder would position itself above the channels/holes and fill them with plastic forming a sort of rivet that would hold to the print to the bed. For automated part removal, there would be a heated bed of nails that would locally remelt the pieces of plastic that the channels/holes were filled with to pop out the part. I could see some issues with this. You would need an absolute positioning system to properly fill the channels/holes. There is some loss of plastic, but not as much if a raft is used.

  4. nophead, ive been watching this and think that a magnetic clamping system will be better overall than trying to do a vaccuum system. for release you can have mechanical louvers that cover the magnets to break the connection or use electromagnets for hold down instead. once i get my mendel finished i was planing on making a magnetic hold down like you have designed and have a pop off lever that the z axis can depress if the build table is all the way to the antimotor side. the plan is also to have a feed tray of plates that can be put on it again from the motor side.

    have to say that your expiriments are both a joy and inspiration to see :)

  5. We are used to you doing great work, but nevertheless it is so helpful to all of us!!

    Some thoughts you may or may not have already had:
    "Before it failed some corners had lifted a little, early in the build, so it looked like the ABS does not stick to film as well as it does to tape."
    This could be because the tape was thinner (right?). Plus, the parts of the polymide that cover the grid will not be heated up by contact from the hot aluminium. It is here the object will exert more force while the polymide is held down the least. And if it is the bed that is mostly leaking (not couplings, fittings etc. elsewhere), this might cool it as well.

    Also, when the bed moves, the difference between the thermocouple reading and the top of the polymide will increase. Even more so with thicker polymide.

    Do you really need a grid, can't you get it to be held down just by making an outline? Or perhaps only channels in 1 direction and meeting in a channel at the sides. I would think that under a vacuum there is nothing heating up the polymide above the channels, no contact from the aluminium, no air (perhaps a slight bit of radiative heat). These local lines lacking heat might be where the warping begins or may create a new sort of warping: through a more indirect temperature flow through the plastic after a few layers. It might be the the ABS above the channels extends a little bit when it heats up while being covered by more ABS.

    @rbisping: If the electromagnets add to the heat of the base, they become pretty efficient... :)

    I'm going a different route (for now) as I have different constraints. My second Darwin has a 230x230x200 build envelope and the bed is static. This means I can remove parts during printing in a round robin fashion. It becomes of a software scheduling problem to optimize production. Parts do warp a little bit, but 6mm acrylic can withstand quite some force. Heating this big a surface would require quite some energy, so I'm experimenting with a local heat stream that heats up the area where the tool is. A radiative heater might also be feasible, but it's harder to control the resulting temperature.

  6. erik, you might go to zoned heating. have 4-8 separate heating zones so your not heating the entire bed. of course you would have to limit your heating to about 60 deg on the plexi but abs sticks well at 60 on plexi. and pla sticks at even lower temps i believe nopheads previous work on that showed about 35 deg if i remember correctly. though it might have been someone else's blog i was reading about that on.

  7. I think you might have a lot better luck using a roller setup similar to what overhead projectors use. That would help holding the film down flat (the vacuum would probably still be useful here), but would also make part removal as easy as spinning one of the rollers to remove the part.

  8. that idea has been proposed but i believe all expiriments to date indicate that warping would be a major issue with that set up. thin membranes of any sort that have been tried invariably induce or allow warping.

  9. @Wes,
    I was planning to do exactly that so the objects can be auto ejected, but you can't keep the sheet flat by horizontal tension. It would require infinite tension and thus infinitely strong film.

    When the film is flat any horizontal pull produces no vertical component at the centre, unless the centre is displaced upwards. Say we wanted no more than 0.25mm lift from a force of say 10N with a bed 100mm wide. A vertical displacement of 0.25mm at a distance 50mm from an edge would stretch the length by 2 * (sqrt(50^2 + 0.25^2) - 50), which is only 0.00125mm. So the mechanical advantage is about 200:1, so you would need a tension of 2000N just to get it nearly flat.

    The tape makes it stick but is also peelable and leaves a perfect glass finish. ABS does not stick to hot aluminium well enough. You can't have a vacuum without the film, it would just leak where the object wasn't.

    It might work but removal is tricky and would leave marks.

    Yes the magnetic system works well but slightly harder to automate the removal. I was thinking of running three rollers along two opposite edges of the steel plate such that they ripple it slightly and the ripple travels the length of the plate like a wave. That should release the object and allow it to be swept off.

    The other problem with the magnetic system is that you have to buy an expensive roll of tape and it is VERY hard to put on without air bubbles and creases.

    I may be wrong, but I don't think the cooling effect of the channels is that significant. They are only 0.5mm wide so the plastic above is only ever 0.25mm away from a hot spot. I think conduction in the Kapton and the plastic itself will spread the heat enough.

    If the vacuum was only round the outline then the middle could easily lift and the outline is object specific?

    The vacuum area of the channels is small, so it only provides a small hold down force until it starts to lift a minute fraction and locally breaks the seal. At that point the vacuum is acting over a bigger area and you get the full 1 ATM force in that area. I think the trick is to have the lines close enough so that the film cannot lift in between channels without breaking the seal. If I was making another I would make them closer.

    The fact that I saw a blister form over a channel tells me the ABS warping pressure at corners is more than 1ATM so cannot be held by vacuum. It would need thicker film to spread the load, or a raft, or Forrest's apron technique, or I guess a 5mm radius at the corners.

    I don't think the movement will cool the bed much because the area covered by the object is insulated by the object. The object itself will be a little cooler but I am running a little hotter than for the mag bed. I can't go much hotter or the vac pipe will melt. I would have to replace it with PTFE tubing or make a coupler.

    See my reply to Wes.

  10. Another very clever design the prints look great again.

    Here are my possibly stupid questions.

    Would having more evacuation points connected instead of one improve/spread the power of the vacuum hold?

    I was thinking four points one in each corner.

    Also making the origin of the the vacuum point deeper into the grid say X4 , Y4 instead of X2,Y0 position in each corner.

    A longer drill would be needed possibly difficult to get the two holes to intersect.

    Thinking this could reduce the chances of losing the seal.

    Excuse my complete ignorance of vacuum system design.

    Only used temprature controlled Vacuum chambers with -2 Torr I've remember it correctly.

  11. BodgeIt,
    Once the film has sealed the flow rate is very low so it makes no difference how many, or where the exits are. The small amount of leakage will find its way through the grid.

    Depending on the flow rate of the pump, it might make a difference pulling the film down to start with. I.e. if the film curls I sometimes have to press it down to start the vacuum. If there were more exits it might have sufficient flow to pull it down itself. Not really an issue though as if the film was between two rollers it would be pulled flat enough with the pump I am using.

    BTW, that is way too big for the job. It needs a reasonably good vacuum, but nowhere near the 3 cfm flowrate. A slower pump would waste far less power, or a reservoir and a pressure sensor to turn the pump of and off.

  12. Thank you.
    I dont have thick enough AL or time to try this yet. I have a small Low voltage vaccum pump from an old food bag sealer.
    It has a 1" diamiter piston assembly on a largeish DC motor.

  13. Unless you want to create an automatic production line I would recommend sticking with the steel plate and magnets. It works better, is easier to make, and doesn't use any power.

  14. Have you thought of using a Teflon sheet, like a baking sheet, instead of Kapton?

  15. No Teflon is the most non-stick / slippery substance I can think of, so not good in an application where I want to stick plastic to it.

  16. I may not understand the process here, but would a vaccuum type clamp work for this kind of bed? That is, if you're familiar with them, this kind of base which has a large suction cup on it, and a lever or a pull which pulls the air out of the suction cup and pulls the clamp down to the surface. Might be a way to avoid the need for a vaccum constantly running.

  17. The problem is I don't have a perfect seal with the film.