Vacuum hold-down for 3D build surface.

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leadinglights

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the_digital_dentist

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leadinglights
I was going to use PTFE shrink sleeving over 0.71mm nichrome but when I tried it out I found that the resistance was too high - 4 Ohms for each half. I have sent for some constantan, also of 0.71mm, which should have a resistance of just under 2 Ohms per half. This is enameled but I will back that up but coating the inside of the grooves with a tough silicone elastomer (Electrolube FSC) and sealing the wire in with the same. silicone.

Mike

PTFE is thermal as well as electrical insulation, as is the silicone elastomer. Your controller is going to be measuring the plate temperature. It will take time for the applied heat to get through the thermal resistances and heat up the bed. During that time the insulated wire is heating to ever higher temperatures. Teflon decomposes at 350C. How do you guarantee that the wire won't get to 350C?

The wall thickness whether PTFE or silicone is 0.2mm or less, total area of the interface between the insulator and the aluminium is about 7800 mm² and a little over half of that at the wire. Long experience with heater elements tells me that 144 watts through an indifferent thermal insulator will be less than 100°C without even doing the maths - though I may look up the thermal resistances and formulae to allay any suspicions.

Mike

Working with very conservative values for everything I get a temperature drop of 53°C between the wire and the aluminium. I may look around for tough thermally conductive silicone or epoxy but if I don't find anything I will be happy with the FSC silicone.

Mike

I want to fuel up this discussion with an additional proposal:

I always wanted to add a "conveyor belt"-like foil on a heated bed to allow overnight production of multiple parts. Maybe adding a rectangular bed to a delta or the tripteron?
With such a vacuum bed, it would be much more likely to avoid warping.
When a print is done, switch off the pump and run the conveyor until the part reaches the turning point of the belt. It peels off the foil ( I hope ) and drops in a box for next day delivery

Edited 1 time(s). Last edit at 11/24/2016 03:46AM by o_lampe.

That sounds like an excellent idea o_lampe. You used to be able to buy transparency material on rolls for overhead projectors, I guess it's still available. Using the roll in open-ended, one-time-use mode would mean you didn't necessarily have to worry about getting the parts off the foil overnight - it could all just fall into some sort of holding bin. Moving to a closed loop would obviously be more cost efficient if you can reliably get print separation, and a mechanism to avoid printing on the join. It's a lot of complexity for normal level use cases, but as a way of making a print farm more effective the combination of a moveable print surface and vacuum hold down sounds very good indeed. Delta's for the win on this one I think!

Those both sound like excellent ideas o_lampe and JamesK although how well the basic vacuum hold-down idea will work on thin materials I don't yet know. I have been trying to progress this but have been waiting on materials - the heater wire in particular is taking its time to get here from Germany. I backed myself into a corner where only one material and diameter will do and not available in the U.K. in affordable quantities.

I have managed to assure myself that only a small vacuum pump is needed - I used a Druck pressure calibrator to check for leaks and got a good seal for 20 minutes with a vacuum of 750mb - about 3/4 of atmospheric pressure or 3psi pushing down on the sheet.

Mike

The grease sealing would be problem for the conveyor belt. Since the bed wouldn't move, the holding force would be against warping only.

Electrostatic hold down would work for a continuous belt, no moving parts, no grease, no noise, cheap, easy. All that is required is a thin, electrically insulating substrate (PCB'), and conductive pattern of interdigitated fingers (copper on that PC, and a high voltage DC supply like this: [www.goldmine-elec-products.com]. The PCB and power supply are probably a lot cheaper than the vacuum pump, and they're a completely silent. If you don't want to etch the PCB pattern yourself, you can just put metal tape strips on one side of a thin piece of FR4, or even modify a PCB' type bed heater (already has a pattern of lines- just make a few cuts to create two sets of parallel conductors).

When the print finishes, turn off the electrostatic hold-down supply, move the belt to the next position, as the belt bends around the roller the 3D print will pop off and drop into a bucket (assuming it doesn't stick to the plastic so strongly that it tears the belt), turn on the hold-down supply, and start printing the next part.

Edited 2 time(s). Last edit at 11/25/2016 11:07AM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

DD, I hear you, but high voltage on a modified bed heater is probably dangerous. The gap between the traces is probably to small ( especially the cuts must be done twice )
A rule of thumb is: 1mm per 100Volt

OTOH the vacuum pump could be switched by a pressure sensor and would only run occasionally.

Edited 1 time(s). Last edit at 11/25/2016 01:57PM by o_lampe.

I think that the high voltage would not necessarily be dangerous as it could be fed from a very high impedance source so that there is no real shock risk. My worry is more that the mechanical attraction available from electrostatics may be too small to stop warping - this may even be the case with a vacuum but I think a reasonable vacuum of 25% below atmospheric is possibly many times what is practical with electrostatics.

Mike

I made me a frozen pizza the other day and used a roll of oven-paper ( this brown paper, you know? )

Funny how boring things become interesting in a certain light... Would that work as vacuum foil?

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o_lampe
DD, I hear you, but high voltage on a modified bed heater is probably dangerous. The gap between the traces is probably to small ( especially the cuts must be done twice )
A rule of thumb is: 1mm per 100Volt

OTOH the vacuum pump could be switched by a pressure sensor and would only run occasionally.

I'm not suggesting you hook it to a pole-pig power transformer or even a neon sign transformer. Since there is almost zero current required to operate an electrostatic hold-down, I linked a perfectly suitable, cheap, and safe power supply. It's about as dangerous as the electric shock you get when you walk across a carpeted floor and touch a door knob. I don't know who's thumb gave you the 1 mm /100V rule, but it's pretty silly. Here's a 15 kV diode that is 12 mm long. By your rule of thumb it would have to be 150 mm long to be "safe". Separating the conductive strips by 10 mm would probably work fine.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

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o_lampe
I want to fuel up this discussion with an additional proposal:

I always wanted to add a "conveyor belt"-like foil on a heated bed to allow overnight production of multiple parts. Maybe adding a rectangular bed to a delta or the tripteron?
With such a vacuum bed, it would be much more likely to avoid warping.
When a print is done, switch off the pump and run the conveyor until the part reaches the turning point of the belt. It peels off the foil ( I hope ) and drops in a box for next day delivery

I think that there may be a problem with this as the freshly printed part will still be at the heated bed and/or heated chamber temperature. Moving it directly to the "turning point" of the conveyor belt will tend to bend or distort it.
A possible solution is to have an intermediate stage for cool down - so that the first printed part is cooling down while the next one is printing. The main additional complexity of this it the extra length of conveyor belt.

Mike

Oops- missed the link: [www.amazon.com]

and another : [www.deantechnology.com]

After rethinking the whole thing, I don't think electrostatic hold down will work. It can provide enough force to hold the film in place, but not if the print warps. I'm not sure a vacuum would help with that, either.

Edited 1 time(s). Last edit at 11/28/2016 03:55PM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

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the_digital_dentist
.....
After rethinking the whole thing, I don't think electrostatic hold down will work. It can provide enough force to hold the film in place, but not if the print warps. I'm not sure a vacuum would help with that, either.

Having seen ABS prints tear chunks of glass out of a build stage and even PLA lift blue tape due to edge curl, I don't expect that it will be a magic bullet. Having said that, materials with a low intrinsic warp and techniques like heated chambers would go a long way to eliminating warp. Even in the worst case, if the thin film is replaced with something with enough rigidity to resist warp, the vacuum or electrostatic hold down might still have a lot going for them: Thin epoxy glass (FR4) could allow very rapid build surface swap and reheat while allowing the print to be peeled off by bending the FR4 away from the print. Such materials can be quite cheap, 0.4mm FR4 is available from Presspahn workd out at less than £1 for a 200mm by 200mm 0.5mm thick sheet.

As Lykle said in an earlier post, time to suck it and see. I hope to be able to report back on victories (and defeats) in then next few weeks.

Mike

O.K., at long last I have started to "suck it and see". Delays have been largely my own fault but with some input from Murphy's law and sluggish suppliers.

So far I haven't got as far as printing anything but I have checked a number of materials to see if they hold down well - results as follows:-

• FR4 epoxy glass fiber sheet of 0.4mm and 0.8mm thickness hold well to a cold bed with little leakage. Applied to a bed at 110°C they lift all around the edges but if pulled down to the bed for 16 seconds for the 0.8mm material and 5 seconds for the 0.4mm material they then remain flat. Attempting to lift a 25mm diameter section in the middle with about a 1kg pull does not lift it.
• 1mm "Kite" brand Tufnol (SRBF) behaved very similarly to FR4 but at 90°C and 0.6mm. SRBP at 90°C flattened out within 10 seconds.
• OHP transparency film pulled down immediately and was tried from 60°C to 100°C. There was some embossing which prevented a lift test with a suction cup being done so I will prepare some with glued on tabs to try for local loss of adhesion.
• Document pouch film stood proud on the outside and needs a lightweight ring to hold the edges down. This takes considerable embossing from the hexagonal pattern on the vacuum plate. The glue side must be upward as this surface becomes very tacky even at 60°C.

Mike

First tests have been semi successful. I have put a short video on YouTube [www.youtube.com] but this only shows changing between various thin materials which have potential for build surfaces. The first prints that I have tried were ABS on OHP transparancy film but the ABS had poor adhesion to the OHP laminating film on either side. I had hoped to see if the vacuum would hold down the edges of the print from curling but the print to film bond failed without any lifting of the film from the vacuum plate.
The three thicker materials all show dishing of the edges when bought into contact with the hot plate but can be pushed down with light pressure after only a few seconds - then held for a further few seconds to equilibrate after which they remain flat with very little leakage of the vacuum. On a couple of materials the laser spots of an IR thermometer are visible checking how fast the temperature comes up.

Mike

That's interesting. My test with OHP film and abs the adhesion was good enough to pull the sheet off the adhesive holding it to the glass (but the adhesive was poor, so that may not be saying all that much). I guess one of the problems with using OHP is that there is likely to be a lot of brand to brand variation. PETG works very well on my OHP film, but has very little warp so perhaps isn't a challenging enough test for your purpose.

What temperature do you print your ABS at? I print on the hot side and probably put the first layer down at 250C for my test.

Edited 1 time(s). Last edit at 01/20/2017 10:04AM by JamesK.

I think if I had the ability to make what your making I'd be exploring airbearings, put the pump on suck.

Edited 1 time(s). Last edit at 01/22/2017 04:50PM by MechaBits.

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JamesK
That's interesting. My test with OHP film and abs the adhesion was good enough to pull the sheet off the adhesive holding it to the glass (but the adhesive was poor, so that may not be saying all that much). I guess one of the problems with using OHP is that there is likely to be a lot of brand to brand variation. PETG works very well on my OHP film, but has very little warp so perhaps isn't a challenging enough test for your purpose.

What temperature do you print your ABS at? I print on the hot side and probably put the first layer down at 250C for my test.

My tests were at 235°C.
Having used my last two transparency films I was going to buy some more but I find that even within one brand there are 5 types of OHP transparency film - Ink Jet, Mono Laser, Colour Laser, Copier and 100 micron. Since it can be nearly £1 per sheet, what has worked best for you?
edit: The one that didn't work was Dataline transparency film for inkjet
Mike

Edited 1 time(s). Last edit at 01/23/2017 05:42AM by leadinglights.

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MechaBits
I think if I had the ability to make what your making I'd be exploring airbearings, put the pump on suck.

Sorry MechaBits, I don't understand - a little more detail please.

Mike

Hi Mike, yeah Air Bearings...take a look at this site [www.newwayairbearings.com]
they have lots of different types, rails, bushings etc, there are other people doing similar stuff, but this one(disc) in particular sparked my imagination,
some sort of porous material, probably very expensive, but maybe there other ways to produce it, or similar,
I figured with pump on suck it could be used to hold something,
In one of the demo video's he scores the underside and it still performs...
but then I thought what about using a cheap ceramic air stone disc, maybe polished with an oil stone
or milled?
Video's here [www.youtube.com]
I just hope some affordable variants trickle down, so I can build some other tools.

Edited 2 time(s). Last edit at 01/23/2017 05:48AM by MechaBits.

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leadinglights
Since it can be nearly £1 per sheet, what has worked best for you?
edit: The one that didn't work was Dataline transparency film for inkjet
Mike

I tried with Apollo Laser Jet Printer and Copier Transparency Film ( [www.amazon.ca] ). My thinking was that the toner fusing stage of laser printers uses quite high temperatures (approx 200C) so the plastic used would have to have reasonable thermal stability. I've heard of people putting the wrong sort of transparencies through lasers and photocopiers and having the plastic melt on the drum.

I wasn't able to visually figure out which side of the transparency is which, so I assume both sides are the same in this case.

Edited 1 time(s). Last edit at 01/23/2017 07:47AM by JamesK.

That is the sort of thing that would work, but likely to be too expensive.
A couple of ways that I have heard about of making a porous surface are:-

• Use MDF as it is amazingly porous and is used for CNC for this purpose (mentioned earlier by etfrench). The edges and bottom would have to be sealed and its poor conductivity would make it inappropriate with a heated bed but for some things may work well particularly if the build film were sacrificial - it stays on the print.
• FDM 3D prints can be quite porous; it may be possible to cold extrude a ceramic paste which has high thermal conductivity to make a porous plate. An example is powdered glass with a small amount of copper powder, borax flux and water subsequently fired in a glass kiln.

Mike

Hmm, there's an experimental filament with a mixture of two plastics one of which is soluble. After printing you dissolve away the soluble one to leave a highly porous substrate. I wonder if it would be strong enough for this.

This is the stuff I was thinking of, PORO-LAY LAY-FOMM

[www.amazon.com]

Edit: on reflection, that looks too soft to be useful, you'd end up with a rather vague Z datum depending on how much vacuum was applied. However, you can get water soluble filament (eg [filaments.ca] ), so we could design for dual extrusion and print a bed of suitable porosity using whatever was the most suitable second plastic. You're still unlikely to be able to heat such a bed though, so perhaps just drilling a lot of holes through aluminum sheet might be the more direct solution

Edited 2 time(s). Last edit at 01/23/2017 08:10AM by JamesK.

Holes in an aluminium plate may work quite well and a small drill may be usable if fitted instead of the hot end in a 3D printer. You would still need to find a way to get the vacuum to all of the holes - which is why I used the pattern on the front face of the build plate.
I have attached a picture of the bottom of the build plate showing the manifold which connects to the suck holes (??) and the heater under a layer of thermosetting glass fabric.


I know that it is complicated but if it works I will try to make it somewhat simpler
Mike

... this "porous material" is mostly made by sintering small aluminium spheres for the "super-fine" types, or by galvanically filling the spaces in a dense package of small styrofoam spheres and then firing the styrofoam, so the aluminium "3D-grid" will remain.

Have samples of both types from micro-assembly projects (used for vacuum tables too), but not in sizes needed for a printing bed ... and yes, the bulk material is pretty pricey

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Viktor
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leadinglights
Holes in an aluminium plate may work quite well and a small drill may be usable if fitted instead of the hot end in a 3D printer. You would still need to find a way to get the vacuum to all of the holes - which is why I used the pattern on the front face of the build plate.
I have attached a picture of the bottom of the build plate showing the manifold which connects to the suck holes (??) and the heater under a layer of thermosetting glass fabric.

[attachment 89434 20170116_1200001.jpg]
I know that it is complicated but if it works I will try to make it somewhat simpler
Mike

That looks great. Given the conflicting requirements of transmitting heat and vacuum I'm not sure that I see a better way of doing it. The drilled plate (plus a back plate with a cavity for the vacuum) is going to struggle to transmit heat evenly. If you attach the heater to the back plate you'd need to add columns between the back plate and the drilled plate to transmit the heat, which would block some of the holes. Putting the heater on the drilled plate (inside the vacuum cavity) would have it's own complexity of weaving between the holes.

So far very promising. I got a pack of Ryman's OHP laser transparency film and have made several attempts to print. ABS has not been in a cooperative mood today and a number of attempts to print resulted in the ABS separating from the film after only a few layers. Success came with my first attempt to print PLA which went very well and I was able to change from one print to a new one in a very short time. I have filmed this and hope to put it on YouTube soon; I did however forget that the temperatures of all layers after the first are dropped so there is a looooong wait for it to actually start again. Having said that, I thing that a change over time well under 30 seconds is achievable.

The main objective remains getting it to work with edge-curly ABS as nothing is really proved until I know that the vacuum system will hold it against the bed

Mike

As a one-off proof of concept, you could use glue stick on the OHP to get the ABS first layer adhesion you need to test the vacuum. Obviously not what you'd want long term, but it would be interesting

I tried Elmers purple glue but it didn't work with ABS and wasn't needed with PLA. Careful cleaning of the film with acetone helped but not enough.

Mike

Maybe that's the difference between OHP foil and laser printer foil? The printer foil is rough on ( at least ) one side to catch the toner, this helps with ABS too, I guess.