In theory* a vacuum bed could supply a force of around 1kg per centimeter. I have it on good authority that this is not enough enough to stop the belt from lifting with a warping object. There is also another problem, it seems from anecdotal evidence that kapton tape starts to lose its adhesive properties after a number of uses. This would mean the constant replacement of the belt material and wouldnt allow for true continues printing. I wonder if the mylar tape used by Charles Pax has the same problem?
This system would involve a large spool of kapton or mylar tape being mechanically unspooled at one end, drawn across the bed and then respooled at the other. If a vacuum system was used this could possibly solve any lifting problems and reel to reel conveyor belt also wouldnt suffer from degradation as its only used a few times. It would also remove the problem of how to reliably join kapton and mylar tape to make belts.
For obvious reasons this is not the most economical or environmental approach. As far as I can tell it hasn’t been tried with a reprap type printer.
The segmented conveyor belt
This is a system that uses a conveyor belt much like the tracks on a tank. As each track is a solid segment that can be stiff enough to resist the warping of the plastic it could do away with the need for a vacuum pump.
However it is likely this would be a very complicated and possibly expensive build build that could be prone to mechanical break down. Some commercial products have been suggested such as the one shown below.
The ‘Lazy Susan‘ rotating print bed
This idea is simple enough, have two or more printer bed surfaces that rotate into and out of the printing area. There are a lot of problems with this idea, not least that you still then require a way to physically remove the part from the bed autonomously and that the Mendel would have to be redesigned to accommodate the path of rotation.
As far as I can tell no one has tried this approach.
The changeable plate based print bed
In this approach a number of printer bed ‘plates’ are used. One approach could be have a stack of plates at the rear of the Mendel and one by one they are fed into the printer. One a print is complete the plate is then moved off the bed from the front of the printer and into some sort of collection area.
Here is a video of a Hass pallet pool machine, it gives you the general idea. It uses multiple ‘pallet’ to store objects to be worked on. It then collects the pallet, undertakes a task and returns the pallet with the completed object. Thanks Dale for pointing this example out.
This has many advantages in that there is no need for a vacuum solution or any requirement for a flexible conveyor material. Unfortunately it would also be difficult to construct and not a true ‘continuous’ printing system. As far as I can tell no one has tried this approach either.
So as you can see there is no shortage of different approaches and ideas for how to achieve continuous, reliable and unattended printing. I personally feel that with enough tinkering any of these different approaches could be made to work. However they all seem to fall down in one or more ways:
- They reduce the total percentage of parts that can be self replicated by a Mendel.
- They add considerably to the build cost of a reprap.
- They add considerable complexity to DIY.
- They require the replacements of perishables. Eg: caypton tap.
I have a few idea of my own about how I may the tackle the problem, but that’s for another day.
* Just how much force can be supplied by atmospheric pressure at sea level using a realistically priced vacuum pump?
Lets assume you go all out and buy a $300 USD vacuum pump that supplies a 15 micron vacuum level. Lets also assuming your bed isnt a perfect seal and you can only reach 100micron of vacume. This converts to 13 pascals of pressure being maintained under the bed, pushing upwards.
Its worth noting here just what a pascal is. A pascal is a measure of force per unit area of 1N per meter squared. Force is equal to mass by acceleration (F=ma). So an object weighing 1kg thats accelerated down by gravity at 9.8ms^-2 exerts a force of 9.8N.
Now assuming your ambient air temperature is 20 degrees C and your at sea level (STP) , the ambient air pressure is 100,000 pascals (100kPa). This is equal to just under 10,000kg spread over one square meter. To put this in perspective, thats equal to the same force that 10,000 1kg bags of flour would exert on a 1m by 1m table. So why are we not all compressed into the floor by the 10 tons of atmosphere stacked on our heads? Because we are mostly made of water which is incompressible and our body’s are at equilibrium with the pressure around us.
Anyway, so thats just under 10,000 kg per meter squared (Kg.m-2) of atmospheric pressure pushing down on the printer bed with only 13Kg.m^-2 pushing up from below (near vacuum). This gives a product of 9,987 kg.m^-2 pushing down, or just under 1kg per square cm.
Thanks nophead for pointing out a mistake I made.