×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Metal VS Plastic

Metal VS Plastic

Metal VS Plastic

(OP)
Hey guys,

I am working on an invention which involved a frame with a build in arm. The arm slides out of the frame and then is clamped forn at various lengths. I'm trying to figure out the most cost effective material to use for the frame and arm. They will be running against each other and will be clamped in place. It needs to be durable and cannot bend, if it does it must go back to shape, it must also be light.

I have the prototype being built right now our of aluminum but its too soft and will corrode. When it goes into mass production I was thinking about possibly using some sort of plastic, but the whole frame and arm must remain perfectly straight. I also need to keep costs down.

Does anyone have any ideas on what I should use?


Thanks!

RE: Metal VS Plastic

You'll probably need to nail down your requirements a bit more quantitatively and explain your device a little bit.

For instance "cannot bend" is impossible, for given load pretty much any material will deflect some amount (I can't think of an exception off the top of my head). However, through correct design and material choice the deflection can be kept to acceptable limits. " if it does it must go back to shape" means that at maximum load you mustn't exceed the yield strength of your chosen material (assuming fatigue etc. isn't a significant issue).

What do you mean by too soft? Do you mean it dents and dings too easily? Or do you actually mean it bends too far in which case it is too flexible/not stiff enough not too soft. Or do you mean it actually fails either by permanent deformation or actual breakage in which case it is not strong enough.

In bending situation the Youngs or elastic modulus is only one parameter that determines how much something deflects. The cross section (second moment of area) is potentially more significant - think I beam verses flat strip. http://en.wikipedia.org/wiki/Bending

Back to aluminum, if you improve the cross section you may be able to give it adequate stiffness in your application. While different grades of aluminum vary quite a lot in strength, the youngs modulus doesn't vary much from memory. However, different grades can be more or less corrosion resistant. Additionally there are various finishes that can be applied to aluminum to improve it's corrosion resistance. Finally while aluminum is fundamentally very reactive (so prone to corrosion) the corrosion (aluminum oxide) adheres to the surface creating it's own protective layer (unlike typical red rust on iron which flakes off). So for some grades of aluminum alloy no finish may be required, as long as you are not scraping off the oxide layer during operation.

What environment is this item being used in as this may affect choice/definition of corrosion resistant?

What volumes of production as this will impact potential manufacturing methods.

From memory I want to say Youngs modulus of most simple plastics is somewhat lower than aluminum. Some fiber reinforced plastic composites may be options though.

As described, I'm tempted to suggest http://en.wikipedia.org/wiki/Unobtainium.

What kind of engineering education do you have, no offense but much of this should be pretty elementary structures to any kind of mechanical, structural, aeronautical, civil or similar engineering degree.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Metal VS Plastic

While I was reading the OP, I had several of the same thoughts that KENAT described. I also wondered about scale. Is this arm 3 inches unsupported or 30 m unsupported? Is the force on the extended arm a few grams or a couple of tons? Does it cycle daily or on a nano-second scale? How are you planning to transfer the forces to ground?

All that stuff matters. If this is a device to count grains of rice then you may be able to afford carbon fibers or titanium. If you are loading a ship then those materials get kind of pricey. I'm an inventor as well (3 patents and a dozen or so inventions that I've given to industry because they didn't have enough market potential to pay for the cost of getting a patent) and I applaud anyone able and willing start down that road, but if your design is lacking as much as your description of it, I don't hold out much hope for a successful result.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

RE: Metal VS Plastic

When it comes to structures, metals are usually a better choice. High strength structures made of composites or plastics can be quite expensive.

As the old joke goes: "Lightweight, strong, corrosion resistant, low cost. Pick any two."

RE: Metal VS Plastic

The OP hasn't been back since the day he made his first post. I Think we're talking to ourselves.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

RE: Metal VS Plastic

As long as we don't start answering ourselves...

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies to make the best use of these Forums?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources