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Waiting for a heatbed to get up to 85˚C for a relatively small part got me wondering why beds aren't hardware/G-code configurable for what area is heated? I'm sure it would be an increase in parts costs and electronics, but it seems that being able to just heat an area a little larger than the part(s) being built would save in time and energy use.

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    $\begingroup$ That is a very good question. It would require switchable sections of the PCB track, but how would those sections be configured? Differing radii of circular PCB tracks? or if in square blocks how would they be arranged, and how many (in halves, quarters, eighths etc.)? I guess the permutations for a square one are a bit more complex and numerous to be practicable. Doing it for a round heatbed seems simpler though. Also, how big would the market be, and would the added complexity in electronics (i.e. cost) and code be worth it. It seems a good idea though... Design a prototypePCB in EagleCAD? $\endgroup$ – Greenonline Jul 8 '18 at 21:35
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    $\begingroup$ @Greenonline RepRapFirmware already has an optional parameter (H for M140) to select heatbed, this could possibly be used for that. For Marlin it will require some extra coding as H is not yet a supported parameter. Nice project! $\endgroup$ – 0scar Jul 9 '18 at 9:31
  • $\begingroup$ You must learn patience, grasshopper. No, seriously: when you're printing anything that takes longer than 30 minutes to complete, the heating time is insignificant. $\endgroup$ – Carl Witthoft Jul 9 '18 at 15:21
  • $\begingroup$ Well, not just all about patience ... do we really need a full 200W (~) element heating up non-used space for hours (days)? $\endgroup$ – Jer Jul 10 '18 at 0:03
  • $\begingroup$ @Jer well, Using the whole heating element the heat bed reach the printing temperature in 3 or 5 minutes, using the half heating element the time increases between 8 to 12 minutes. I don't want to wait to get roots on my feet. $\endgroup$ – Fernando Baltazar Jul 11 '18 at 5:21
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I've wondered that myself a while ago and fact is that such beds or silicone heating pads do exist. Usually these are quite large (and expensive) and usually referred to as "dual zone heat beds/pads".

As far as energy consumption; less area to heat is faster heat up times (depending on the control) and less energy consumed. For small prints this may be beneficial. The price of such beds are very high, so to break even you would have to print a lot. An alternative to buying would be to etch your own bed.

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    $\begingroup$ Thanks. As I've been thinking over people's answers I can see you'd want to segment the heated areas in a number of ways: ideally a bit of a heat break, some kind of switching to turn on/off zones, they'd need their own thermistor, and finally something to control/interpret all of this back to the main board. Not something you'll find any time soon on a sub-$200 printer. $\endgroup$ – Jer Jul 20 '18 at 19:56
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The heated bed is a reasonably good thermal conductor, so the difference in energy between heating all or 10% of the bed (assuming a 3x3 grid split 1 and 8) isn't going to be that significant.

In terms of heating speed, 200W across the whole bed will heat it faster than 40W applied to the centre square, and will also be less likely to cause warming or heat cycling effects (unless PSU regulation is a problem).

If the bed is much larger, or has thermally separated zones, then there might be some justification in the increased control complexity.

As an example, although glass has a thermal conductivity around 1% of a metal, it is still 30x better conducting than air. Conductivity towards the unheated areas of even a solid glass bed will roughly match the surface loss - so best case you would reduce losses to somewhere 20%-50% of what heating the whole bed might cost (assuming the same 3x3 grid).

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  • $\begingroup$ I had the same concerns as you about conduction, but not come to putting it in words! The only thing I can think of to dispute this is thermal silicone heating pads (on an insulated body) in conjunction with a glass bed, glass is a poor heat conductor. Nice addition! $\endgroup$ – 0scar Jul 11 '18 at 12:22
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As the etched version is very impressive (thanks 0scar for that), there are other possibilities to build zones on the bed:

  1. Using resistance wire;
  2. Using a etched bed with zones.

The challenge with zones is mainly down to:

  1. When and how to switch on/off particular zone;
  2. Temperature control needs to be added to every zone to avoid over/under heat in the particular zone.

This last point brings even more challenges as that requires a PWM channel and a temp sensor (per zone), so standard RAMPS need some extensions in the wiring. One could overcome that using custom G-Code to set on/off zones and a double temp sensor for the main zone to follow the temperature changes.

In detail: a dedicated Arduino with PWM outputs that will read the temperature from a secondary temperature sensor in the main zone and follows it.

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    $\begingroup$ The commercial dual zone beds come with a controller to do that, indeed a simple Arduino should be able to do that. You could even alter the Marlin bed heating code to include an extra parameter that determines which bed to heat up. It seems that reprap firmware already has such functionality (parameter H). Recent versions of RepRapFirmware also provide an optional 'H' parameter to set the hot bed heater number. If no heated bed is present, a negative value may be specified to disable it. $\endgroup$ – 0scar Jul 9 '18 at 9:24
  • $\begingroup$ thanks for that @0scar, I have a 500mm X 500mm bed, so I am thinking to slice it in 100mm zones, that gives 5 zones to be controlled, and RAMPS will not handle that without extending it as that requires at last 8 extra pins and some extra computing power which we know is limited. $\endgroup$ – profesor79 Jul 9 '18 at 10:11
  • $\begingroup$ 100 mm squared makes 25 zones ;) What about control using i2c? $\endgroup$ – 0scar Jul 9 '18 at 10:19
  • $\begingroup$ it extends by 100mm so we have 5 elements $\endgroup$ – profesor79 Jul 9 '18 at 10:20
  • $\begingroup$ A rotary switch can help 2P6T, also are called selectors. $\endgroup$ – Fernando Baltazar Jul 11 '18 at 5:29
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it is probably too late for the party but i thought about grid heated bed recently. about controlling which zone to turn on and off, i think we can treat the grid as Led matrix, the matrix led signal can be used to drive mosfets to power the heat pads.

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