Hi,

For a school design project, I want to make use of thermochromic paint.

I need to heat a specific part of a metal surface to anywhere between 50-60 degrees C. This needs to be a constant temperature. The surface itself will be 300mm x 300mm. The thickness of the surface is flexible but needs to be able to support itself without bending or crumpling.

The desired heated area would be around 100mm x 20mm but this can change, as it probably depends on what I use to heat it.

What would be the best way to heat it? I've been looking at small heating elements, but I really have no idea about any of this. How would I control the heating to reach the desired temperature? What would be the power source? How would I connect it to the power source?

Thanks and apologies if too vague, the project is pretty vague in my head too!

Rob

 

If you heat a small section of the plate to 60C then the heat isn't going to stay within that boundary but will be conducted into the rest of the plate. Is that a problem for this project?

 

Hi Rob,

For the heating you could look at something like a "silicon heat pad" which could be fixed / glued to your metal part:
https://www.aliexpress.com/wholesal...SB_20190301052907&SearchText=silicon+heat+pad

It will help give a reasonably spread out heat source.

For controlling it you are best with something like a "PID controller":
https://www.aliexpress.com/wholesal...d=SB_20190301053010&SearchText=PID+controller

 

As @pmd34 says, the silicone heat pad is a good possibility. Particularly because they are used as bed heaters for 3D printers and so there are adhesives and breakout boards with MOSFETs designed to operate them. You could easily buy a lot of off them shelf stuff and put together a thermostatically controlled heater,

The only problem is that they are likely to be square, and about 200mm on a side or so.

 

If your focus is the thermochromic paint, I would use the simplest heating method possible and a thermocouple (TC) to monitor the temperature. Of course, if you have a substantial budget there are a lot of possibilities. If not, I would check out a Mr. Coffee-type heated base , any warming plate from a used products store, or even an electric range top. For the TC, your cheapest bet is probably a consumer kitchen supply store, DrugMart or Walmart. I would try to find something ready made and not get into the electronics design aspect, unless you are already competent in that subject. Then focus on the chemistry aspect.

On the other hand, this could turn into an electronics project with microcontroller, feedback loop, etc. and just buy an assortment of thermachromic paints.

Only you can answer whether you want a chemistry project or an electronics one.

 

My first thought was to use an "old fashioned" incandescent light bulb and dimmer circuit to control the heat output, 12v car bulbs are cheap and readily available

 

Hallo,

May the plate to be heated be drilled?
Then you could mount some aluminium power resistors on the plate.
They come in various sizes and shapes:
http://www.arcolresistors.com/resistors/hs10-aluminium-housed-resiso/
http://www.arcolresistors.com/resistors/hs25-aluminium-housed-resistor/
http://www.arcolresistors.com/resistors/hs50-aluminium-housed-resistor/
http://www.arcolresistors.com/resistors/arf150-low-profile-wirewound-metal-clad-resistor/

Driving a current through those resistors will heat the plate.

Bertus

 

... shop around for an infra-red heat source. The effective surface temperature could be dialed in by adjusting the distance from the heat source to the surface. Also, it should be possible to form an insulating template from something so that only the exposed, designated surface area is heated. Count on one or two experimental trials to get some idea of heat build-up, temperature rise, uniformity, and other factors.

 

If you heat a small section of the plate to 60C then the heat isn't going to stay within that boundary but will be conducted into the rest of the plate. Is that a problem for this project?

Yes, seems like I overlooked that. I could change the material of the plate, to something less conductive, like acrylic? Would that keep the heat more localised? I suppose I'll have to test.

 

Use a heat lamp.

 

Yes, seems like I overlooked that. I could change the material of the plate, to something less conductive, like acrylic? Would that keep the heat more localised? I suppose I'll have to test.

You will have to use multiple materials if you need tight localization. If some bleeding is OK, you can use a poor conductor with a good conductor the size and shape of the hot spot bonded to the top surface.

 

Thank you to everyone's suggestions! Didn't realise I'd get such quick responses. I'll explore all suggestions.

Thanks again.

Rob

 

Ooo.. @bertus has a very good point, I have actually used some of these:
https://www.vishay.com/docs/50006/rch.pdf

In the past for heating, they can easily be securely bolted to a piece of metal and given a little thermal grease work great.

 

Yes, seems like I overlooked that. I could change the material of the plate, to something less conductive, like acrylic? Would that keep the heat more localised? I suppose I'll have to test.

Yes, if the top surface is something less heat conductive then you can attach a piece of metal underneath and heat that. It will certainly keep the heat more localised. Keep the top layer thin and it will pass the heat from below better and there would be less sideways spread.

 

Hello,

Precise localized heat is not easy to do because pretty much everything conducts heat readily except the best insulators, and actually even they do but to a much lesser extent. The conduction takes longer in less conductive materials, but it gets there eventually unless some cooling is combined with the heating, such that the cooling cools the parts of the plate that do not require heating.

You could also use a thermoelectric cooler device (aka Peltier) but again the heat will conduct to other places. Perhaps multiple thermo devices stacked side by side, with some cooling and some heating, depending on how you need to distribute the heat.

A better idea is probably to sectionalize the plate into regions that have to be heated and those that do not. For example, say you need to heat a 1x1 inch area in a 5x5 plate. You could mill out the 1x1 area to maybe 1.05 x 1.05 and use an insulator that is 0.025 inches thick to isolate the 1x1 area to be heated from the rest of the plate. That would be best unless of course you need to move the area to be heated to some other location on the fly.