polymer equivalent to a bi-metallic strip? 2

[opticsman]

Hi all, Is there a commercially available all polymer/plastic equivalent to a bi-metallic strip? I am looking for something that could be integrated in an injected moulded product. It needs to be reasonable cheap.
Many thanks in advance for any advice or leads.

 

[patprimmer]

I have not seen it, but it should be easy. Mould a strip in GF whatever polymer suits with direction of flow along the strip.

Overmould with same polymer but no glass fibre.

It will certainly bend toward the glass filled side when heated.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Compositepro]

The principle of the bimetallic strip requires two things. A difference in coefficient of thermal expansion between two materials and modulus or stifness of the materials to generate useful forces. So as Pat suggested, glass fiber (or carbon fiber, or metal wire or strip) in plastic will create warpage with temperature change. However, if the plastic is not stiff enough it may creap or deform in other ways.

 

[opticsman]

Hi all , great help, thanks for that. Is there a polymer or combination strip that will bend with moisture. I am thinking of the fortune fish material ( cellophane I think), but something more robust and repeatable?

 

[patprimmer]

In principle you could use a material that swells in water, like a nylon, and a material that does not swell, but you would need to check what will overmold to what.

It would have a very slow response rate as (depending on a number of factors) it takes hours or even days to absorb enough to swell noticeably. This probably makes it unworkable in the real world.

A glass filled nylon on one side and an unfilled on the other might work. Nylon 6 is probably best as it absorbs most water.

Re water or temperature, re material choice, a lot depends on other factors

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[opticsman]

hello Pat .Again very Useful Thanks . What is your background /role ? is it permissible to have your contact details? Opticsman

 

[patprimmer]

My original training was in textiles technology and I ran a colour and wet processing lab, but I have worked since 1977 in Engineering Plastics in technical marketing and product management roles.

I am a committee member of PIMA (Plastics Injection Moulders Association).

http://www.pima.asn.au/home.asp

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Demon3]

Pat's approach is a good one. I would not choose glass fiber though as the filler. Each layer is basically a spring and the one spring has to expand more than the other one, thus bending the second spring. If you put glass fiber in then one spring is much stiffer than the other so that the unfilled polymer will not be able to bend it. Therefore I would use a filler that does not increase modulus much. Examples are CaCO3 (chalk) and glass spheres. Or, if you do use glass fiber then make that layer only 1/4 the thickness of the unfilled nylon layer.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[patprimmer]

Demon3

You are correct. Although the glass fibre severely reduces expansion due to moisture absorption, it also increases modulus to an unworkable extent.

Co-extruded films with nylon/tie layer/PE curl a lot on moisture absorption, but are probably to thin for your purposes. It may be possible to make them very thick by the so called film casting process where melt is extruded onto a set of rollers then pressed to size by the rollers as they freeze.

The costs of samples by this process will be high.

If it is thick enough to have the required rigidity, it will have a VERY slow response rate.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[unclesyd]

Here is just one article from a search on Google using "bi-polymer switches"

http://www.iop.org/EJ/abstract/0964-1726/2/1/001

There is a very large pdf in the above search that leads to this.

http://ntrs.nasa.gov/search.jsp?N=0&Ntk=all&Ntx=mode matchall&Ntt=polymer+switches

I don't know if any of the above info will be of use but may give you some leads. At leas you know people have worked on same.