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# Aluminum Sheet Between Heater and Glass

Posted by Shachar85
 Re: Aluminum Sheet Between Heater and Glass June 17, 2016 05:22AM Registered: 5 years ago Posts: 125
Controlling it with enough thermal mass (thick plate) shouldn't create overshoot issues with the more powerful heater, since it'll all average out. The plate will still distribute the heat quickly enough, it'll just be more mass to heat up.
 Re: Aluminum Sheet Between Heater and Glass June 17, 2016 06:24AM Registered: 6 years ago Posts: 14,359
I hope you don't mind me interrupting.

Quote
deckingman
Do you have heat insulation under your bed? Any idea what 8mm or 10 would do to the warm up time? If it's say 25% thicker would the warm up time be 25% longer or is that too simple? Price wise, 6 mm would cost me £31.57, 8 mm £36.03, 10 mm £37.96 so there is hardly any difference and on that basis I'd be tempted to go for 10 mm. Just a bit concerned about warm up time though.

It is worth insulating under the bed to avoid wasting energy and to prevent things under the bed getting too hot, but it won't cut the heating time by very much. The warm up time is dominated by the heating power and the thermal capacity of the bed plate.

That insulation sounds expensive to me - what material is it?

Quote
deckingman
Also, I haven't bought the heater yet but I've seen a 400 x 400mm 1.5kW which seems insanely high unless it was a miss print. Assuming it isn't a miss print, should I be tempted? As control is going to be "bang bang" via a SSR, would a heater that powerful lead to overshoot probs? I'm guessing not if the warm up time is in the order of 5 minutes but correct me if I'm wrong.

DD says he used 0.46W/cm^2 on his large machine, and I use 0.41W/cm^2 on my 330mm diameter delta. The heater you have seen is 0.9375W/cm^2 which is more than double. This would lead to high temperatures which would ruin your bed heater and could pose a fire risk of the SSR fails short circuit or the electronics leaves the heater fully on. Also you are right, overshoot may be a problem. I recommend you use a less powerful heater. These people [www.aliexpress.com] will make a silicone heater to your own size, voltage and power specifications at an economical price. I recommend you get a bed heater a little smaller than the bed, so as to leave a margin of 15mm or so around the outside for fixings, bed plate clips etc.

You can use low-frequency PWM with a DC/AC SSR. The Duet uses a PWM frequency of 10Hz for the bed and chamber heater outputs for this reason.

Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
 Re: Aluminum Sheet Between Heater and Glass June 17, 2016 08:11AM Registered: 4 years ago Posts: 579
Quote
dc42
I hope you don't mind me interrupting.

Never for a moment - your advice is always most welcome.

Quote
dc42
It is worth insulating under the bed to avoid wasting energy and to prevent things under the bed getting too hot, but it won't cut the heating time by very much. The warm up time is dominated by the heating power and the thermal capacity of the bed plate.

That insulation sounds expensive to me - what material is it?

No, you misunderstood. Those prices were for the aluminium plate. For insulation I've bought a sheet of this to try. [www.amazon.co.uk]. For £8.99 you get 600mm x 1250mm x 6mm thick which is far too large but I can't find it in smaller sizes. Might use 2 layers. I looked up the spec and the heat transfer is 1/2 that of cork, it's fire proof, and it's pretty rigid too so I reckon I can use it without anything like an mdf base.

Thanks for the other advice. I'll stick with the 800W and get it say 360mm x 360mm which will leave 20mm all round but will still be slightly bigger than the actual print area.
 Re: Aluminum Sheet Between Heater and Glass June 17, 2016 08:21AM Registered: 7 years ago Posts: 5,197
1.5 kW is definitely overkill for a bed that size. I would use PID control for the temperature. Some folks have demonstrated Z artifacts that occur with variations in bed temperature, and PID control will minimize that sort of thing. An SSR will happily switch power on and off at 10 Hz, but it is always best to use one with a built in zero crossing detector.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
 Re: Aluminum Sheet Between Heater and Glass June 17, 2016 09:36AM Registered: 4 years ago Posts: 579
Thanks DD. I'll bin the "kettle element" idea then.
Control will be whatever the Duet offers. I didn't think it was PID for the bed but DC42 tells me it's 10Hz PWM so maybe it is.
 Re: Aluminum Sheet Between Heater and Glass June 17, 2016 12:01PM Registered: 8 years ago Posts: 548
Quote
dc42
could pose a fire risk of the SSR fails short circuit or the electronics leaves the heater fully on.

That's why you install a thermal fuse to cut power if the bed heater gets too hot. I use one that trips at 152C and I haven't had to replace it yet.
 Re: Aluminum Sheet Between Heater and Glass June 17, 2016 05:50PM Registered: 6 years ago Posts: 14,359
The Duet bed heater control defaults to bang-bang, but you can switch it to PID. When I get round to implementing predictive temperature control, I'll probably make it PID by default.

Edited 1 time(s). Last edit at 06/17/2016 05:52PM by dc42.

Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
 Re: Aluminum Sheet Between Heater and Glass June 17, 2016 11:28PM Registered: 3 years ago Posts: 175
Lets see
400x400 or 40cmx40cm size

6mm = 960cm cubed
8mm = 1280cm cubed
10mm = 1600cm cubed

using 0.5W (maths is easier)

6mm = 480W
8mm = 640W
10mm = 800W

My bead has around 2450cm cubed of 8mm Cast and with the 1200W heatpad it will get to 60c in less than 3 minutes from say 15c ambient but it also well insulated underneath so all heat goes to the bed. i've used Ceramic fibre paper in 3mm thickness as the initial insulator.

Still using the default bang bang control of the SSR and have no overshoot issues or temp stability issues, it will stay inside a degree for the entire print.
 Re: Aluminum Sheet Between Heater and Glass June 18, 2016 03:59AM Registered: 4 years ago Posts: 579
Thanks guys. Always nice to see theoretical calcs backed up with empirical data. I think I'll go with 10 mm thick aluminium as it's only a couple of quid more than 8 mm. Apart from layer changes, it'll be static so a bit of extra mass won't be a bad thing. I'll use 2 layers of that insulation board so that'll give me 12 mm of insulation underneath. Just thinking about that, 12 mm insulation plus say 4 mm for the heat pad, plus 10 mm for the aluminium 4 mm glass all supported on a frame made from 20 x 20 aluminium extrusion means the bed assembly will be 50mm thick. My doesn't it soon add up?
 Re: Aluminum Sheet Between Heater and Glass June 19, 2016 04:20AM Registered: 4 years ago Posts: 579
Just a quick update on this. Ordered 10mm plate from Aluminium Warehouse for £37.96. Then there was postage @ £12.49 making it over 50 quid. Then when I got to the check out, there was VAT on top making it over 60 quid which made me cough and splutter a bit.
 Re: Aluminum Sheet Between Heater and Glass June 21, 2016 06:55PM Registered: 4 years ago Posts: 1,873
It could be worse - I order most of my metal from across the border in the USA. Not only is the shipping expensive, but you get hit for import duties, taxes and exorbitant brokerage fees (particularly by UPS). Fortunately it's becoming more common for the suppliers to use shipping services with the duties and brokerage paid up front, so there are less nasty surprises these days. Even so, it's not unusual for the metal to be only around half of the total cost.
 Re: Aluminum Sheet Between Heater and Glass June 25, 2016 05:27PM Registered: 4 years ago Posts: 579
So my expensive lump of aluminium turned up the other day, the heater is still on the slow boat from China. The heater comes with a thermistor but I have a spare PT100 so I got to thinking about how best to fit this. Did you know that you can "mill" aluminium quite successfully with a woodworking router and HSS bit.

Set the slowest speed and take it easy. The groove is 3mm wide (the size of the cutter) and 2mm deep. I did it in 2 passes of 1mm depth.
I'll fix the PT with some Kapton Tape or some such when I come to fit the heater.

Drilled the mounting holes as well, and tapped an extra hole to take a 5mm bolt for an earthing terminal

Haven't got a drill press so I had to do these "free style" with a cordless drill.

It was a bit scary attacking a 60 quid lump of aluminium this way but I got lucky.
 Re: Aluminum Sheet Between Heater and Glass June 25, 2016 05:42PM Registered: 7 years ago Posts: 1,275
WOW, braver than I am as I can't free hand drill, nor saw, worth squat. Those holes would not have been salvageable if I had attempted to free hand drill them for sure.

_______
I await Skynet and my last vision will be of a RepRap self replicating the robots that is destroying the human race.
 Re: Aluminum Sheet Between Heater and Glass June 25, 2016 07:56PM Registered: 4 years ago Posts: 1,873
I picked up a drill press on sale a couple of years ago - now I find it quite unsettling if I have to use a hand drill. The only problem is that once you get a cheap drill press, it doesn't take long before you want a better one...
 Re: Aluminum Sheet Between Heater and Glass June 26, 2016 04:39AM Registered: 4 years ago Posts: 579
I used to have a cheap drill press many years ago but the belt snapped and I couldn't find a replacement anywhere (that was in the days before Ebay and Amazon) so I ended up throwing it out and living without it. A glance at my user name will give you a clue to what I do these days, so I use cordless power tools all day long and have managed to do some quite crazy things which definitely should not be tried at home kids.

Edited 1 time(s). Last edit at 06/26/2016 04:40AM by deckingman.
 Re: Aluminum Sheet Between Heater and Glass June 27, 2016 01:14PM Registered: 4 years ago Posts: 579
Just a quick update on this. I've given the plate a coat of high temperature stove paint as I intend to use one of DC42's probes which prefers not to have a highly reflective surface under it. I baked it in the oven at 220 degC for half an hour to cure the paint ("she who shall be obeyed" was out at the time). It took forever for the plate to cool so I'm definitely having a removable glass print surface on top. On the quest of the holy grail of print surfaces, I spoke to a man who knows nothing about 3d printers but everything about glass. I explained what it is for and his suggestion is 4mm toughened float glass (good for 200 degC apparently), sandblasted, dubbed edges and BriteGuard coated which he says " reacts chemically with glass and forms a high strength functional layer that protects against glass corrosion". Personally I hadn't heard of glass corrosion and I got lost in all his other technical talk as well. Anyway, the glass is ordered (3sheets). I'll report back if it's any good. If not, I can always turn it over and using some other coating/medium on the other (plain) side.
 Re: Aluminum Sheet Between Heater and Glass July 06, 2017 02:30PM Registered: 5 years ago Posts: 63
Quote
the_digital_dentist
I don't get it. Everyone knows you need a flat surface to print reliably, yet so many keep buying thin, flexible aluminum and then put glass on top of it. Why do people keep buying the thin, flexible aluminum? It makes no sense. Why not just get a flat aluminum plate such as cast tooling plate in the first place and be done with it? Unlike galss, aluminum is thermally conductive so the temperature will be even across the surface and you won't have print adhesion problems. Surely the cost of a thin flexible aluminum plate, plus glass to fit, and the goop you're going to have to slather on to get prints to stick can't be much less than the cost of a piece of cast tooling plate. We're talking about a few dollars/euros here, not hundreds. Are the few dollars/euros you save worth all the screwing around to find a magic formula to try to get your prints to stick?

Since you're developing the printer yourself, I advise you to try out a 3 point leveling system. When you level the print bed you're adjusting it so that it lies in the XY plane defined by the printer's guide rails. It takes only three points to define a plane. The typical 4 corner "leveling" system bends the bed plate and the undercarriage. You can't level a bent plate, neither can autoleveling. With enough screwing around you may get the center of the bed flat/level enough to print on but you aren't likely to be able to use the entire bed. The further you get from the center the worse it will get. Auto leveling assumes that the bed is a plane so its not necessarily going to help.

Here's a 250 mm diameter ABS print on my 317 x 305 x 6.35 mm cast tooling plate bed supported by a three point leveling system:

I have never seen anything of similar size done on a thin piece of aluminum with a glass plate and a 4 point leveling system.

What's the point of building a machine with a 200 x 400 mm bed if you can't print on the entire surface? The thin aluminum/glass/4 corner leveling is done in cheapo (and some not so cheap) printers because it is a couple dollars/euros cheaper than doing it the right way, or the "designers" don't know any better. The whole point of developing your own printer is to do something better than you can get elsewhere. If you're just going to copy their mistakes, just get one of those cheapo kits and save yourself a lot of trouble sourcing the parts.

On a new printer I am about to start building I'm going to use the 6mm tooling plate but I was still considering using 3mm glass on top of that. Not so much for switching the printing surface out quickly but to be able to remove the print in the first place. Presently I use the old school glass over a the circuit board heaters, the glass has kapton tape on it for a printing surface. I usually need to put the glass in the freezer to pop the print off, and since the new build will have a heated chamber I'm assuming the print will stick even better. So how do you get a print that large off the bed without having to hammer it? Is the little extra movement of the aluminum vs. glass during cool down enough to release the print easily?

Thanks!
 Re: Aluminum Sheet Between Heater and Glass July 06, 2017 03:23PM Registered: 7 years ago Posts: 5,197
That print came up with a little prying, as they all do.

The kapton I used was 5 mil think and I used to get months of use out of it before it had to be replaced. Then I took that printer to the makerspace and people there managed to destroy the kapton weekly. That's when I decided to give PEI a test. I got a 60 mil piece of PEI (12x thicker than the kapton I was using) via amazon for about \$30 and put it on there a year ago and no one has managed to damage it yet. I've been putting 30 mil PEI on other printers since then.

I have been using one of these tools for years and it works very well. I've watched people at the makerspace using it and think I figured out why they were damaging the kapton so quickly. They would attack the print with the scraper and if it didn't instantly come up, keep pushing harder and harder and pounding at it until they gouged the kapton. Common sense isn't very common, so now I explicitly tell them that if the print doesn't lift off when you first try, move to a different point on the print and try again. There's always a weak spot - all you have to do is find it.

I don't like glass because it is a thermal insulator and breaks easily (and I don't need to do mass production on my printers). I would think that if you just have to take the plate and print off the bed you could clamp a piece of PEI down on it, but maybe it would be too flexible. Cooling prints in the freezer to get them off the glass might work, but what will happen to the prints? If you cool ABS too quickly it will start to split between the layers.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
 Re: Aluminum Sheet Between Heater and Glass July 07, 2017 05:35AM Registered: 6 years ago Posts: 14,359
I too find that I often need to put prints in the fridge or freezer to release them. So I print on either uncoated 4mm float glass, or thin PEI on top of float glass. In both cases, clamped to an aluminium bed plate. I have never broken a glass plate, although once when testing a new filament, the filament stuck so well to the glass that when it eventually came off, it took a small shard of glass with it.

Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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