Industrial manufacture of a parabolic mirrored surface

Skills materials and processes
1

I’m not sure where to start researching into this for my A-level product design coursework, but this board has been so friendly and helpful to me in the past, so I’ll start here.

I am designing a high-volume production collapsible solar cooker, for use in the third world. I have all almost all the product designed to a tea, except the actual reflective surfaces.

The “petals” which fold out to create the parabola (shape which focuses all incoming rays of sunshine into one point above it, where the cooking pan rests) are injection-mouled plastic (material to be decided - something highly impact resistant).

The mirrored surfaces must be parabolic, and as reflective as possible, and not-too-fragile (considering the conditions of the great outdoors). Also, if possible, the mirrored surface must require no adhesive/welding/permant fixing to attatch to the main assembly. So far all of the components I have incorporated into the product snap into place.

Options I have thought of:


Simply spray-paint the plastic parabolic surface with a metallic paint

PROs: Cheap, and one less component in the assembly.

CONs: How environmentally friendly are metal-sprays? The spray (i imagine) will chip-off pretty easily, leaving the user with the problem of buying more spray (which may not be readily available.), or, more likely, discarding the product altogether). Also, I have not seen any applied-finishes that are up to the reflective-quality of a real mirror (which is what I need).


An actual mirror

PROs: Highest reflectivity possible.

CONs: Extremely fragile, surface finish easily marred through use. How are mirrors made in high volumes? Can they be actually made parabolic?


A plastic component

PROs: light, easily and cheaply manufacturable/replacable, wears well. Moulded-in fixings can easily be made to ‘snap’ to the main assembly.

CONs: Can a plastic material that is reflective enough actually be found?

IDEA: Is it possible to copy a mirror’s design, but instead of a glass sheet covering a metal-film, an acrylic sheet is used instead? THe problems I can think of with this solution, is that the acrylic itself will reflect light, and that it won’t be perfectly transparent.


I’ve given you a quick brainstorm of the ideas in my head at the moment. Please could you give me some feedback on these ideas, and give me a clue as to which one is the most viable? Also, any new ideas that I have overlooked would be most appreciated!

Also, it would be great if anyone could point me in the direction of any and all information on the manufacture of reflective surfaces! :slight_smile: thank you all[/b]

2

If you’re looking for ruggedness, I would consider chrome plated metal (like an old automotive bumper).

The parabolic form could be injection molded (if you go with plastic), or stamped (if you go with the plated metal).

My concerns for plastics would be that would be fairly easy to scratch or break, eventhough it would be lighter. Molding a suitable surface for chroming required the tool surface itself to be chromed (added tooling costs). You can’t really injection mold a reflective mirror surface (i.e., metalic). You have to chrome plate or paint it afterwards.

Yes, the mirror has to be good, but not accurate. The less perfect you can tolerate, the lower your costs.

A cheap solar oven could be made with sheets of aluminum foil.

The frame where the cooking pan rests could be a wire form. Again, in remote locations, you can’t fix broken plastic, but you can staighten out a bent wire, or even solder or weld metal pieces back together. Think of the worst conditions this product could experience and then add a safety factor to that. Don’t worry about aesthetics quite yet - just make sure it works first.

Best of luck,

Warren

3

Thank you very much! You’ve been a great help.

4

I wonder if injection moulding LIQUID METAL (yeah that new stuff you heard of) with a chromed mould would make a mirror-like surface.
Just a thought, no investigation went into it but if you look into it, please post.

5

http://www.liquidmetal.com/applications/dsp.jewelry.asp

from the liquid metal manufacturers website

claims a “luxurious finish”

Injection moulding large sheet-like components may result in a very weak component, as the material cannot be injected throughout the mould evenly?

6

Just get some plastic mfg to make chrome over an injection. They do it all the time. Not quite astronomical, but good enough for proto.

:)ensen.

7

I’ve got to echo Warren on this one. Remember that you’re not designing a cooker for the Aesthetics-above-all-else-no-maintanence-throw-it-away-if-it-breaks-the-slightest-bit developed world.

With that in mind, there are a few things to consider:

  1. Can it be made locally in the region you are going for? With such a simple product as a solar cooker, and with pre-industrialized countries having such high levels of manufacturing labor, it would be pretty ridiculous not to let the local labor market take care of the manufacture.

Because of this, definitely lose the idea of using Liquid Metal for anything. For this project it would be total design overkill. Injection molding plastic is also probably out, since making large molds like you would need is pretty much out of the question without sophistocated manufacturing.

  1. Like Warren said, can it be repaired? In countries without a fashion-driven disposeable goods economy, repairs and upkeep of products isn’t seen as a bad thing. Bicycles are repaired. Copper pots are re-tinned. Wicker furniture, rather than being thrown out, is re-woven for repair.

So make your material and manufacturing choices with this in mind. For example, using giant mirrors is a bad idea, because once they break, you can’t fix them, but using a bunch of tiny mirrors glued to a woven basket is much more easily repairable, and in that way, more durable than a plastic or glass surface.

If you were obsessed with durability, the suggestion of a chromed steel mirror is a good one, but you might even consider straight polished steel with a coating of oil or wax, or tinned copper, which could be re-finished if it got dammaged much more easily than chrome. Again, the small amount of upkeep that these devices will require is much less than a cooking fire which needs constant feeding and vigilance.


It’s always tempting to use the newest, coolest thing in our products. In this case, however, the newest, coolest thing about this cooker is that it won’t use wood, or require constant stoking. Don’t worry so much about making a golfclub-style pitch (lots of mega-engineered materials designed to appeal to a person reading an add from their executive’s desk); If your design realy fits with the culture you’re introducing it into, then you’re much better off.

8

Couldn’t you die cast the porabola shape and then polish or chrome plate it. I think a nice polish though would be more environmentally friendly, vs. Chrome. Chromium is some pretty bad stuff.

Maybe you can even add a condensing lens at the focal point. That way it can redirect the heat and create a more efficient heating source. Of course this is only a theory. But it’s something I saw while researching DLP chips.

9

Oops… I thought you were still in prototype… didn’t realize you wanted to make a working pre-production model.

In that case, based on the third world parameter… either steel sheet or aluminum foil wrapped around bent or carved wood slats.

Start with a parabolic form in wood. In it’s simplest version, a circle jig will suffice as paraboloids and spheres have roughly the same math for the amount of “dish” you require. This form can be cut, carved or otherwise fabricated by the local factory.

The form is used to compare against the petals. Each petal would be a 20mm or 30mm strip long enough to form a section of dish. There would be hole at one end for fastening together with a bolt or pin. With enough strips the petals would fan out to form the reflective dish or collapse for travel.

Each strip is polished to a mirror finish by hand or by foot operated buffing wheel. Maintenance in the field by the user is through continual polishing. The entire thing is carried in a Zip-Loc bag for waterproofness.

When unfolded for use, a cheap telescoping metal antenna is attached to some exposed threads of the petal connector bolt. When extended, the antenna is the exact focal length of the reflector, so the end must touch the surface of the cooking pot. Once the focus is set, the antenna is retracted to prevent it from becoming cooked.

These are relatively cheap materials that are readily available in third world countries. More importantly, they could be fabricated and assembled by hand.

:)ensen.

10

wow thank you all you’ve given me a lot to consider here! You’ve all definately helped me above and beyond what I expected! cheers

11

Just re-read your original post, you mention ‘petals’
There already is a ‘new product’ that uses petals for solar cooking (or energy gathering) I saw info about it a while ago, maybe even on core, but don’t remember much. Except the petals looked beige on the picture and most likeley our of abs or some other acetal.

Also there was a nice ‘baloon like’ structure of 2 halves the bottom mirrored (like those ‘chromed’ baloons) and the top half was transparent. First I saw it was on the Dave Chalk computer show, seen it elsewhere. You already probably know about both of them, but posting these just in case.

on the subject of heating things up, check out this month’s materials section on core77, an alternative way of heat thing up (to a 100°F is I recall).

you also mention that you already designed other components. what are they made of? what process?

12

wow i didn’t know about other products like that! if anyone has any links that would be very handy!

Design has been on the backburner for a while, whilst i was doing my maths and physics A-level exams! (did well in all except pure maths…bleurgh who needs it?:roll: )

I’m back on track now (hopefully) … im in the process of making a presentation board of my product, as it is developed so far, I will scan and post an image on here, the annotations explain the product, its manufacture, etc. quite well… will give me some feedback on my presentation/annotation too :stuck_out_tongue:

13

Those expandable colanders may be adapted for this use. The same mfg may be able to re-shape each petal to the correct curve.

:)ensen.

14

As far as similar “petaled” solar cookers/engines go, take a look here:

お酒の肴にはチーズは相性が良いってホント？ (look at the 2003 links)

EI is one of Idealab’s think farms, and they are cranking along on the solar energy thing. They have abandoned the petal+stirling engine design for a more efficient solar Photovoltaic system (you can see it in the later timeline images)

Definitely keep in mind, though, that this is a company designing for california corporations to have backup power during brownouts. They want to keep costs low, but it’s a totally different field than developing countries.

15

Wow those designs look very good! Though a little more hi-tech than my solution.

I was hoping my design would be marketed like the wind-up radio was (by trevor Bayliss?), in that it was primarily designed for developing countries, but was still sold in the west.

EDIT: here’s a digital photo of the concept board of my product (the way it will be sold in the west, sans substitute materials/components that are made from local materials).




sorry, my scanners broken, and the perspective screws up collages in photoshop! The pictures so large so you can read the annotations.

16

yes, the petal thing is what I was talking about.

now let me look for the other one.

oh, and there’s a water ‘purifier’ distiller that functions like the ‘baloon’ but a bit different. if that’s what your ultimate goal is than perhaps look at that.

now…

solar cooking documents in the solar cooking archive <these are a few, but don’t see the one I saw there…

this is a worthwhile read http://www.fsec.ucf.edu/Solar/PROJECTS/SolarCooker/reflectivematerialsreport.pdf


after a while of searching i gave up thinking what I saw was a first version of the CookSack from Soltac