VariPower Heated Bed - modulated power zones for optimal performance.

Posted On: 2016-01-07 18:28:53 ; Read: 12187 time(s)
Big Bed for BigBox

We’re incredibly proud of our heated beds, which have a bunch of features that make them a step above what else is out there We wanted to show you around the technical details of what makes them awesome.

We’ve got a large 200mm*300mm print surface to heat up, and to be able to print the full range of materials that BigBox is capable of we need a good deal of power to hold as high as 140°C over this surface. We’we've got lots of available power to draw from our 400w, 24v power supply, this gets us good fast heat up times.

We tested quite a few bed designs and types, but they all had the same critical problem: the temperature at the edges was significantly cooler than the measured temperature in the centre. This means that despite having your bed set and reading 110C to print ABS comfortably in the middle of the bed, the temperature towards the edges could be as low as 95C and this means parts printed at the full extents of the bed are going to suffer from warp and detaching from the bed.

annotated board

The overlaid boxes indicate the regions of varying power density, controlled by altering the width of track in that region.

Our solution was to provide more power at the outer regions of the bed in order to compensate for the lower temperatures being measured there. By using slight variations in the width of the PCB heater traces we are able to selectively provide more power to regions that would otherwise be running cooler. This results in a much more even temperature distribution over the surface of the bed.

Not only are the measured temperatures on the bed significantly improved, in practice it makes a really noticeable difference. On our print farm we have the beds filled to the edges with parts, and would often suffer from the parts on the corners and edges warping or coming off the bed. With our new beds using variable power density the problem has gone away and printing these huge plates has become noticeably more reliable and easy.

Even heat distribution

Being able to really utilise the full bed area boosts productivity.

Unlike some beds, we’re positioning the tracks so that they are face up and in contact with the glass build surface. This give the best possible transmission of heat to the glass build surface.

tracks

Look closely, there are tracks hiding on there!

We have connection pads for the thermistor on the underside of the bed to which the legs are soldered to, and traces carry signal back to the connector so there aren’t any stray wires below the bed.

bed soldering

Lovingly hand soldered here at BigBox

To improve heat up times and generally increase performance we’re insulating the underside of the bed. For this we’re using a laser cut layer of a rubber resin bonded cork composite, which is affixed with 3M 467MP adhesive sheet, this type of adhesive is designed for use in demanding temperature conditions and can withstand the extremes of temperature and repeated thermal cycling.

cork insulation on BigBox

 

Only the finest of insulation and adhesives!

The thermistor isn’t just attached to the bottom of the bed: the head of the thermistor is potted into a small hole in the centre of the board, with thermal cement, so it is also in contact with the glass build surface for the most accurate readings.

thermal cement

Thermal adhesive junkies

We’ve left the copper on the rest of the bottom surface in place which provides a certain amount of heat­spreading.

heated mirror

Mirror mirror on the wall, show me the finest heated bed of them all.

Other beds we’ve used have required thick cables to be permanently soldered to large connection pads, this is hard to do, and means you can’t easily disconnect the bed. So we’ve added a connection point that uses a very nice Molex Microfit connector. Each pin on this connector can carry a very impressive 5 amps, it has a locking latch to keep things secure, and is keyed so the connector can’t be accidentally reversed. We’re using 4 pins for bed power, and two pins for the thermistor. We’ve also got a purpose made cable that has the appropriate connectors on each end to connect without fuss to the controller board.

Clunk Click

Clunk Click

On the same tab as the connector we’ve added a position for an optional bed power indicator LED. A lot of people expressed that they didn’t want a flashing LED on their machine, so we’re not including one by default, but if you do want one there’s a position for a standard LED and it’s a simple through­hole solder job. We filleted the corners where the tab meets the rest of the bed for strength, and carried that over to all the external corners too. It’s petty, but sharp corners on other beds are really irksome, they scratch you, and fibreglass from the board substrate gets scratched into your skin.

LEDs can be added

Optional LEDs are catered for. Note the smooth filleting of the corners!

We’ve added as much useful info to the silkscreen on the top of the bed as we could. There’s a 10mm spaced grid, with bolder lines every 100mm, all numbered along the X and Y axes. We’ve called out the home and centre points too, for easy reference. This will allow you to properly align where the print head homes to so you can truly use all the build space on the bed.

Homing

Home is where the arrow is!

We had many more issues getting appropriately flat, strong, and temperature resistant glass than we imagined. We went through 5 different suppliers, we tried plain float glass, tempered glass, heat strengthened glass, and borosillicate glass. Either they weren’t flat enough, weren’t strong enough, or weren’t resistant to heat cycling. In the end we had to go to a specialist optical/scientific glass manufacturer in order to get very resilient borosilicate glass that was flat enough. It wasn’t cheap, but this part of the machine is just too critical to compromise on. The edges all have nice ground chamfers to make things easy on the fingers too.

chamfered glass bed

Chamfered edges on the borosilicate ultraflat glass. Nice!

In the past we’ve used bulldog clips to secure glass to the bed, which is convenient and works well. However the height and size of these clips can interfere with the motion of the print head and get snarled up on the fan duct and nozzle. We sourced these awesome little clips which have a very low height profile on the top of glass, and small footprint. This means the clips reside on the margin of the print bed, never encroach onto the build surface, and cannot interfere with the print head. Once again, this means that you really can use the full extent of the build surface. They look really neat too.

Homing

 

One unexpected issue we did have to overcome was the pins on the opposite side of the bed connector are exposed. This normally wouldn’t be much of a problem, because despite the high power at those pins the voltage is too low to be a hazard. However it’s all too easy when using print removal tool like a paint scraper with a metal blade to slide over and off bed and short these pins. Shorting a 400w power supply is a somewhat dramatic event, with much emission of sparks and magic smoke. The shorting was powerful enough to do a little unintentional electrical discharge machining.

darn.

See the burn holes in the print spudger? 'Don't try this at home.'

To overcome this we simply added a small printed cover to shield the pins from shorting. A new high temperature material was used for this as it will obviously be subjected to higher than average temperatures.

A printed part that slots into place from the side means that sparks will no longer fly.

A printed part that slots into place from the side means that sparks will no longer fly.

In summary, we've achieved a huge boost in heating performance, not just in power and heat up times but consistency and accuracy of heat all over the bed. We've made the whole system really nice to integrate into a printer and we think that even those who don't want a BigBox would be interested in using one. So we're offering heated bed kits as a product in its own right. The kit we're selling comes assembled with the insulator, thermistor and connectors. The borosilicate glass and bed clips are included. We're throwing in the cable too, which should be suitable for most electronics setups out there.

VariPower Heated Bed - modulated power zones for optimal performance.

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Comments
papabur | 2016-03-10 16:12:40

Nice work guys, looks good!
Nick | 2016-01-30 05:42:15

Would running this on 12VDC be possible?
Attero | 2016-01-26 02:21:15

This looks like a really nice heated bed solution. Any chance you'll be offering it in different (or custom) sizes?
Ashok M Karnad | 2016-01-09 14:26:08

I went through the details of your BigBox-Heated-Bed. I wish I work on this in near future. As retired 70+ electronics & automation engineer, I hope to get hands on experience on 3D printers, as I am sure, the technology will soon be within the reach of common man & that too at a faster pace than that of internet.
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