Thin machine cover made from light and shape stable material

[W205]

Hello,
I would like to explore what options do I have in replacing steel (welded plate + ribs) cover with something much lighter, machinable, stiff and shape stable. Price does not play an important role.
Current welded piece is apprx: 30 x 20 inches, with 0.03" thick base plate, ribs are 0,15 x 0.15"

As mentioned: at approx. 0.8 kg this part is somehow heavy. It is stiff enough not to bend too much when manually handled. I would like to know, if there any options in plastic/carbon range of material, that would allow us to reproduce part with similar mechanical characteristics and yet benefit in weight reduction.

Thanks for all answers in advance.

 

[MikeHalloran]

You're looking at half a million dollars for an injection mold, just because of the size.

It's not clear from your picture whether there's an overall continuous membrane to the part, or if it has three large windows in it. No effect on mold cost either way.

You might be able to produce a usable part with additive manufacturing, but you may be disappointed with porosity/ incomplete fusion issues. ... or not.

Be clear on this: no plastic is anywhere near as stiff as steel.

Mike Halloran
Pembroke Pines, FL, USA

 

[tygerdawg]

Mike is absolutely correct on this. The elastic modulus & tensile strength of plastics vs. steel are in different hemispheres. A 1:1 substitute doesn't exist. Carbon fiber construction may get you there, but you won't like the cost.

If you redesign the part, you might (MIGHT) be able to consider thermoformed + trimmed plastic. This design path presents a less costly solution than injection molding tools, but only if the total production quantity is suitable. But you do not mention the critical piece of information: bending & flexural allowances. Any plastic part would require a lot of stiffening ribs & reinforcements.

Unless, of course, it is simply something dumb like a decorative fascia.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[RPstress]

Plastic filled with short carbon fibers will not be stiffer than steel and at best will typically be about as stiff as aluminum if 40% fibers are used (50% of the highest modulus short carbon fibers available (hard to get) might give a modulus of about 80 (maybe 100) GPa). Continuous high modulus carbon fibers could be made stiffer than steel but would need a complete redesign compared with a metal. Carbon can be machined quite easily though it's harder to machine than steel (carbide tools and a lot more tool wear). Normally design of a carbon part minimises machining necessary. Beryllium (as featured in MMPDS; from memory E about 400 GPa) is stiffer and less dense than steel though far less tough and not as strong (see MMPDS; ^50 ksi on a good day) and must be handled with great caution as it is toxic: beryllicosis can result if it is machined in a way that produces particles that can be breathed in. A particulate reinforcement of steel (say 25% SiC particles) will make it a bit stiffer with a reduction in toughness and elongation. Stiffer-than-steel limits options severely. You could also try Googling "tungsten alloys"; a bit dense for me but seem pretty stiff.

 

[W205]

Thank you everybody for your fast answers.
Yes, those are large three windows. And to be even more complicated: size and number of those windows might change: different versions of part will be needed.

Unfortunately this is not a decorative piece and flatness tolerances should be about the same, as achieved with steel welded part (the benefit of still is also possibility of local "manual" bending to correct shape).
Redesign of part, so that details would better suite different material and manufacturing process is not a problem. However: basic shape is defined and can not be changed (size, three windows, total thickness).

I will try to get some more information about additive manufacturing.

 

[3DDave]

Why does no one recall the use of diagonals on structures like this as a stiffening method?

I do enjoy the problem statements that, at the end, include the requirement that it be identical in every way except performance, and cost is no object (until it is.)

 

[W205]

3DDave: Thank you for your comment.
Shape is defined by other parts of machine and process in which part is involved. Main shape features are defined.

I of course believe, that steel is optimal choice for this, however I am pushed "from above" to find other - lighter - solutions. On basis of MikeHalloran's comment I have started to search for a workshop that could print it out of some Titanium alloy.

 

[3DDave]

Are the middle stiffeners defined by other parts of the machine?

Titanium is half as stiff as steel and half the density so unless the section/depth can be increased there will be no improvement for flexure under its own weight. It's main advantage is that the strength is somewhat higher than half and the properties don't change as much with temperature. 0.028 Ti sheet, 24 inch by 36 inch is about $200. 0.035 sheet is about $30 more.

 

[RPstress]

If you want it lighter and stiffer than steel your options are pretty much limited to small benefits from aluminum and titanium, slightly bigger ones from an advanced aluminum-lithium alloy or larger ones from beryllium or HM or UHM continuous carbon epoxy (there are other resins but epoxy is usually the the best bet). A redesign in really stiff carbon would be sensible; this will need a big change in design practices. An Al-lith alloy might be good enough (the MMPDS is the best reference for such things - I think it's up to versiin -9 now; good Al-lith needs the most recent). How much lighter and stiffer do you need it? A good redesign in steel might give you what you need (see 3DDave's comment).

 

[RPstress]

It's worth mentioning that all metals' young's moduluses divided by their densities are the same for all useful metals (and many useful composites), except beryllium (high stifness for its density), Al-lith (a tiny bit high stiffness for its density) and copper alloys (copper, brass, bronze, etc.), all low stiffness for their (rather high) densities. High stiffness for low density is close to breaking a law of nature. Graphs of density vs. Young's modulus have one big spike (Be), a few much smaller ones (Al-lith alloys) and a bunch of big ones (different types of carbon composite), plus a few horribly low ones (all the Cu alloys). The horribleness of Cu becomes an asset when absorbing impact energy with low rebound. Useful in some test fixtures.

 

[W205]

Everybody,
Thanks for all your contribution.
Regarding stiffness: I am happy with current part stiffness. Steel performs well, it does bend during manual operation, but it is all in elastic range. Material currently used is AISI 420.
My task is, to make part with same stiffness and some 30% lighter.
Regarding beryllium that was mentioned: I have never worked with it. Can someone name one of its alloys that could be safely handled? Finish product is constantly in contact with human hands!

 

[dgallup]

Geometry is your friend. Increase the moment of the cross section and the part will get stiffer. Then you can thin out the material.

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[RPstress]

You could try AlBeMet, see

http://www.materion.com

. AlBeMet's a beryllium/aluminum alloy that's about as stiff as steel but quite a bit less dense. There are a few alloy variants. Materion are a useful reference for some fairly exotic alloys and claim to have advice for handling them. They also supply pure beryllium. I'd need to chevk my notes but I have a feeling AlBeMet is a metal matrix particulate composite that's hard (but not impossible) to machine.