Polymers that have spring properties similar to spring steel

[adil_mohammad]

TL;DR Summary

Looking for a plastic material for prototyping a design. Needs to be durable and cheap for mass production.

Hello everyone,

First of all a very happy new year to everyone! And a big thank you to all the people who contribute to this forum, I have learned so much from here.

I am prototyping a design for a part that will be used in a consumer product. I am in the early stages of researching materials. The design can be built with spring steel however I was wondering if there are any plastics out there that have similar properties. Most importantly I am interested in flexural strength at different weights. If I were to apply weight ranging from 125-250 lbs, I need it to bend but then return to original position, like a flat spring almost. The closest thing I have come across is fiber-reinforced plastic; such as glass filled nylon or some type of carbon filament in epoxy. Cost and ease of manufacturing is also an important factor so it can't be too high tech a material. I am pretty stuck right now so any guidance and direction would be greatly appreciated. I have included a drawing to help visualize it, the weight would be applied to the curve that sits on the x-axis and as a result the y curve would move laterally, that movement is very necessary, that's what makes the whole thing work, the figures are in inches. Thank you.

 

[jrmichler]

Fiber reinforced composite materials are entirely suitable for making one or several parts by hand. It does not matter if the fibers are glass, carbon, Kevlar, or even paper. A good source of supplies and books on the subject is Aircraft Spruce and Specialty (https://www.aircraftspruce.com/). Click on Composite Materials, then on Books/Videos. A good book to start with is Moldless Composite Sandwich Aircraft Construction.

Your part could be made by cutting a piece of foam plastic, such as the stuff sold for home insulation, to the shape, laying on several layers of resin soaked cloth, then cutting away the foam. Start with 5 or 10 layers of fabric, then adjust the number of layers to get the strength and stiffness you want.

Hint: Use epoxy resin, not the polyester resin sold in hardware stores as "fiberglass resin". Epoxy is stronger and polyester resin dissolves Styrofoam.

 

[ChemAir]

Summary:: Looking for a plastic material for prototyping a design. Needs to be durable and cheap for mass production.

The closest thing I have come across is fiber-reinforced plastic; such as glass filled nylon or some type of carbon filament in epoxy.

These would be my first suggestions as well. The only common non-composite that that comes to mind is polycarbonate (Lexan tradename), but it can be bent to the point of permanent deformation.

I expect the composite design to require less machinery to manufacture that part than if it was polycarbonate.

 

[berkeman]

Welcome to the PF.

I have included a drawing to help visualize it,

I am interested in flexural strength at different weights. If I were to apply weight ranging from 125-250 lbs

What are the units on your drawing? (apologies if I missed a note somewhere)

 

[adil_mohammad]

Fiber reinforced composite materials are entirely suitable for making one or several parts by hand. It does not matter if the fibers are glass, carbon, Kevlar, or even paper. A good source of supplies and books on the subject is Aircraft Spruce and Specialty (https://www.aircraftspruce.com/). Click on Composite Materials, then on Books/Videos. A good book to start with is Moldless Composite Sandwich Aircraft Construction.

Your part could be made by cutting a piece of foam plastic, such as the stuff sold for home insulation, to the shape, laying on several layers of resin soaked cloth, then cutting away the foam. Start with 5 or 10 layers of fabric, then adjust the number of layers to get the strength and stiffness you want.

Hint: Use epoxy resin, not the polyester resin sold in hardware stores as "fiberglass resin". Epoxy is stronger and polyester resin dissolves Styrofoam.

Thank you for pointing out that great resource, I will look into it over the weekend, hopefully my local library will have it available or something similar.

I was wondering if this technique you've mentioned is viable for mass production?

How well will this material hold up to deformation, I imagine the resin will start cracking and breaking over time?

 

[adil_mohammad]

These would be my first suggestions as well. The only common non-composite that that comes to mind is polycarbonate (Lexan tradename), but it can be bent to the point of permanent deformation.

I expect the composite design to require less machinery to manufacture that part than if it was polycarbonate.

Sorry if this comes off as a dumb question but I am still very new to all this. I have never manufactured anything before, what kind of machinery would be needed to produce this? Also, would this even be worthwhile to build with plastic or am I better off to just go with spring steel? The reasons I was considering plastic was lower cost, ease of manufacturing (mass scale), and shipping and handling purposes. But I am also worried about the durability of the plastic part, I can not have any deformation or else it defeats the purpose of the design.

 

[adil_mohammad]

Welcome to the PF. What are the units on your drawing? (apologies if I missed a note somewhere)

Thank you for the welcome, I'm glad to be here.

The units are in inches.

 

[jrmichler]

How well will this material hold up to deformation, I imagine the resin will start cracking and breaking over time?

They make airplanes from it. One famous example is the Rutan Voyager, the first airplane to fly around the world nonstop without air to air refueling. Photo below from NASA:

Fiber reinforced composites can be made in quantities of one, or in mass production. The manufacturing techniques are different for mass production than for small quantities.

There are plastics that can be bought in sheet form, sawed to size, then heat bent into shape. Polycarbonate, as mentioned above, is a good choice to start with. You would need to run a test to find if it is strong enough.

 

[anorlunda]

Sorry if this comes off as a dumb question but I am still very new to all this. I have never manufactured anything before, what kind of machinery would be needed to produce this? Also, would this even be worthwhile to build with plastic or am I better off to just go with spring steel? The reasons I was considering plastic was lower cost, ease of manufacturing (mass scale), and shipping and handling purposes. But I am also worried about the durability of the plastic part, I can not have any deformation or else it defeats the purpose of the design.

I don't think you are ready for this project yet. You can't make judgments on cost for ease of manufacturing without more theory and more experience.

Zero deformation is a direct contradiction to wanting spring properties. The difference between springs and rigid bodies only becomes evident when we deform them. And no material is 100% deformation free, so if that is critical, you have a problem.

It might work better if you tell us what you are trying to accomplish with this device.

 

[256bits]

Sorry if this comes off as a dumb question but I am still very new to all this. I have never manufactured anything before, what kind of machinery would be needed to produce this? Also, would this even be worthwhile to build with plastic or am I better off to just go with spring steel? The reasons I was considering plastic was lower cost, ease of manufacturing (mass scale), and shipping and handling purposes. But I am also worried about the durability of the plastic part, I can not have any deformation or else it defeats the purpose of the design.

You mean 'plastic deformation' which is when the body is stressed beyond its elastic limit, like when you bend a paper clip.
How thick, or bulky, should your part be - that can limit your choice of material.

Wood can be considered a plastic. Laminated wood products can be made curved much the same way with gluing veneers together and letting dry in a form of your curve. That's one way to test your curve and how it functions, without the messiness of the resin, or dealing with spring metal fabrication.

 

[adil_mohammad]

They make airplanes from it. One famous example is the Rutan Voyager, the first airplane to fly around the world nonstop without air to air refueling. Photo below from NASA:
View attachment 255123
Fiber reinforced composites can be made in quantities of one, or in mass production. The manufacturing techniques are different for mass production than for small quantities.

There are plastics that can be bought in sheet form, sawed to size, then heat bent into shape. Polycarbonate, as mentioned above, is a good choice to start with. You would need to run a test to find if it is strong enough.

If it works for planes I'm sure this will work for my application. However, I like the idea of working with polycarbonate sheets. Will a standard heat gun be enough to heat and form it to shape? Does the heating process change the properties or strength of the polycarbonate?

 

[adil_mohammad]

I don't think you are ready for this project yet. You can't make judgments on cost for ease of manufacturing without more theory and more experience.

Zero deformation is a direct contradiction to wanting spring properties. The difference between springs and rigid bodies only becomes evident when we deform them. And no material is 100% deformation free, so if that is critical, you have a problem.

It might work better if you tell us what you are trying to accomplish with this device.

I agree with you, nothing is ever 100%. I guess I should have been more careful with my wording. I am looking for something that will flex under tension but return to it's original state relatively deformation free. The reason I bring up cost and ease of manufacturing is because I am at the stage of selecting the material for prototyping and all these factors are important for final production. There's no point in prototyping with a material that is going to be cost prohibitive or require specialty manufacturing, if it can be done simpler and cheaper.

That's why I am here asking experts because there is only so much that can be learned from books the knowledge and experience of a real person is worth 100 times more. I realize their is no way to know for sure until I actually start prototyping, but I am just looking for general direction because there are so many different options out there and I want to tap into the collective knowledge of everyone here to help me move in the right direction.

I hope that makes sense. I'm not looking for a hand out, just a nudge in the right direction. I will still have to do all the research and work on my own, which I am ok with.

 

[berkeman]

apply weight ranging from 125-250 lbs

The units are in inches.

You are going to apply a transient load of 250 lbs to a thin spring member with dimensions 8"x13"x3"? I do not envy that spring at all... (ouch)

 

[256bits]

I am at the stage of selecting the material for prototyping

Then you have done stress analysis already?
What you have is a beam, albeit a curved one at that.

 

[adil_mohammad]

You mean 'plastic deformation' which is when the body is stressed beyond its elastic limit, like when you bend a paper clip.
How thick, or bulky, should your part be - that can limit your choice of material.

Wood can be considered a plastic. Laminated wood products can be made curved much the same way with gluing veneers together and letting dry in a form of your curve. That's one way to test your curve and how it functions, without the messiness of the resin, or dealing with spring metal fabrication.

Yes, I guess that's the term I am looking for plastic deformation, I need it to bounce back into its original shape, unlike a paper clip. The part can be as thick as it needs to be, it is not a limiting factor at all, it can be as thick as 2 inches if need be.

I like the idea of wood, it never came into consideration but I understand what you're saying. I would imagine certain woods would work better then others for this? I am leaning towards bamboo perhaps? Would a simple wood glue be enough? I was thinking of laying thin veneer sheets in a criss cross pattern to provide a bit of rigidity to the shape, however, I wonder how well it would hold up to constant stress.

 

[berkeman]

I like the idea of wood

If you're considering using wood for this, we need to page @phinds

 

[adil_mohammad]

You are going to apply a transient load of 250 lbs to a thin spring member with dimensions 8"x13"x3"? I do not envy that spring at all... (ouch)

Lol I have the same concern. However, the good thing about this design is that I have some leeway when it comes to the dimensions; especially the thickness. In all honesty this is really just a sketch of the final shape, the dimensions are relatively close to what they need to be at; however, there will be no way of knowing for sure until I can build it and physically test it my self.

I was thinking of making multiple "sizes" so to speak. Where I can adjust the thickness for the weight range ie. 125-150 lbs, 150-175 lbs etc. Do you think that would be a better apporach rather then forcing everything into a 1 size fits all?

 

[phinds]

If you're considering using wood for this, we need to page @phinds

I've been puzzled by this whole thread, given that the subject line say properties similar to spring steel. I don't know much about material properties but I question whether any of the suggestions made so far meet that. Certainly I don't think anything made with wood would do that.

 

[anorlunda]

https://www.ottobock.com/en/paralympics/technology/running-prosthesis-system-tf/

These things claim to be made of carbon.

 

[adil_mohammad]

Then you have done stress analysis already?
What you have is a beam, albeit a curved one at that.

I agree, all I have is a design in principle, and there is no way of knowing how well it will or won't work until I actually build it and test it for myself. I am not really sure how I can do stress analysis without having something to apply the stress to? Since there are 100's if not 1000's of different polymers out there I have hit a bit of a dead end. I don't have an academic background so I am having difficulty understand the technical data that describes all the different properties of the materials. I would just like to know which plastics could possibly work well for this application, if at all?

 

[adil_mohammad]

I've been puzzled by this whole thread, given that the subject line say properties similar to spring steel. I don't know much about material properties but I question whether any of the suggestions made so far meet that. Certainly I don't think anything made with wood would do that.

Hmm you make a valid point, I guess what I mean is plastics that have the "spring" property of spring steel. If i recall correctly, I think bamboo strips are very pliable and spring-like, depending on the application and assembly process. I know skateboards are made from wood material, although on the surface they seem very solid and rigid, they have quite a bit of flex to them. Do you think similar processes and techniques can be used for this design?

 

[adil_mohammad]

View attachment 255189
https://www.ottobock.com/en/paralympics/technology/running-prosthesis-system-tf/

These things claim to be made of carbon.

Yes sir! Now this is what I am talking about! I am going to dig into this some more; I can't imagine this material is cheap? If I am not mistaken this is carbon fiber?

 

[Laura]

The material property that determines a material's spring effect is the elastic modulus. The flexural modulus is based on both the elastic modulus and the geometry of the part. The elastic modulus of steel is 200 GPa, for polymers it is much much less than that. So without either reinforcing the polymer with fibers or significantly changing the geometry, I'd estimate that this is not possible. You may want to explore "filled polymers" - these are basically composites, but the glass fibers are so small that the part can still easily be injection molded. Even with those, a geometry change might be necessary.

As for permanent deformation, the likelihood of preventing this is controlled by the yield strength of the material. Spring steel has a very high yield strength. Fiber reinforced composites (not filled polymers) are probably the only "plastic" option here if you want to compare to steel.

 

[adil_mohammad]

The material property that determines a material's spring effect is the elastic modulus. The flexural modulus is based on both the elastic modulus and the geometry of the part. The elastic modulus of steel is 200 GPa, for polymers it is much much less than that. So without either reinforcing the polymer with fibers or significantly changing the geometry, I'd estimate that this is not possible. You may want to explore "filled polymers" - these are basically composites, but the glass fibers are so small that the part can still easily be injection molded. Even with those, a geometry change might be necessary.

As for permanent deformation, the likelihood of preventing this is controlled by the yield strength of the material. Spring steel has a very high yield strength. Fiber reinforced composites (not filled polymers) are probably the only "plastic" option here if you want to compare to steel.

Thank you for the insight and suggestion, I will definitely look into filled polymers. I am very much intrigued by the idea of injection molding. As of right now I am going to start with polycarbonate sheets as mentioned by @jrmichler. I think it will be a good starting place since the material is readily available and fairly straight forward to work with. I am starting to think the only way this will really work, material wise, is fibre reinforced composites. As pointed out by @anorlunda, carbon seems to be the go to as it is already being used for similar purposes. I need to do some more research on the kind of fibres I can use and learn some more about the production techniques involved, à la diy. However, I am curious about what you mean by geometry? Does that refer to the overall shape of the design or something else?

 

[anorlunda]

I need to do some more research on the kind of fibres I can use and learn some more about the production techniques involved, à la diy.

If you search "carbon fiber DIY" on Youtube, you'll see lots on materials and production. Here is one about DIY carbon skis which sounds close to what you want.

 

[Stormer]

I've been puzzled by this whole thread, given that the subject line say properties similar to spring steel. I don't know much about material properties but I question whether any of the suggestions made so far meet that. Certainly I don't think anything made with wood would do that.

Bows for hunting has been made out of wood for thousands of years. And if they did not spring back they will not function...

Modern compound bow limbs are made from glass fiber / epoxy composites and a lot of leg prosthetics like the ones pictured above is made from carbon fiber / epoxy composites.

But for ease of manufacture and low cost high volume production i wold think that a fiber reinforced thermoplastic will be a better option. But i don't know if it has the same properties as the fiber reinforced epoxy materials mentioned above.

 

[phinds]

Bows for hunting has been made out of wood for thousands of years. And if they did not spring back they will not function...

Any of them made from spring steel?

 

[Stormer]

Any of them made from spring steel?

Yes bows can be made with leaf springs. But it will be really heavy.

 

[anorlunda]

The reasons I was considering plastic was lower cost, ease of manufacturing (mass scale), and shipping and handling purposes.

I don't see why you think plastic has lower costs. Especially laminated plastics or carbon fiber laminates, when you factor in manufacturing cost.

It could be true that spring steel is the less expensive choice. In any case, you need to look at the alternatives in more detail before making a fully informed decision.

 

[phinds]

Yes bows can be made with leaf springs. But it will be really heavy.

Yeah, I should have been specific. I was talking about long bows, not cross bows, which I did realize had been made w/ steel.