[walter]

For those just joining us, here's how it works (thanks for digging it up Jsage!)


Polymer Castings Take on Metals :: The structures of equipment such as lathes, milling machines, coordinate-measuring machines, and pump bases are typically cast-iron or steel weldments. The iron-casting process produces a rough casting, which is then stress relieved, precision machined, stress relieved again, and finally painted, a process that may take four months or more.


But polymer-composite (PC) castings are cast to tolerance, demolded, and ready to use, all within days, and cost about 30% less than steel or cast-iron equivalents. They can be cast in color to eliminate painting as well.



The mineral aggregates are precisely graded and range in size from a fine powder to particles 0.375 in diameter. Precise grading minimizes air voids and resin use and promotes stronger castings. In general, castings incorporate coarser aggregates, though fine aggregates better fill thin sections. Mechanical properties of PC are about the same, regardless of aggregate size.

Aggregates
High-hardness mineral aggregates including quartz, basalt, and granite are typically used, though recycled glass is another option. Granite, for example, leaves a jagged edge when it is broken up so it better grips the resin. But the jagged edges can also hinder flow into mold features. In contrast, high-strength, high-purity (99.5% SiO2) quartz aggregate has a more round shape that improves flow and compaction. Vibratory compaction during the molding process tightly packs the aggregate together, which boosts part strength. The quartz has a Mohs hardness of 8 (diamond = 10) and makes up about 92% of a part by weight.

Resins
Resin systems include epoxy, polyester, vinyl ester, methacrylate and furan, the most common of which is epoxy. Epoxy has excellent chemical resistance and long-term stability, as well as good mechanical properties. The addition of wetting agents improves resin/aggregate adhesion, while antifoaming agents reduce trapped air. Volumetric shrinkage of epoxy is minimal and additives can nearly eliminate it.

Epoxy makes sense when precision and high strength are key design considerations. A downside of epoxy is that it takes several hours to cure. Polyester, in contrast, has a high shrink rate and is therefore inappropriate for high-precision parts. It's also mechanically weaker than epoxy. But it can cure in just 10 min, making it ideal for small, high-volume parts.

The PC casting process blends resin, hardener and aggregate in a batch or continuous mixer. A batch mixer is preferred because components can be accurately weighed prior to mixing. The mixture pours into a mold and cures in just a few minutes or perhaps several hours, depending on the resin system and formulation. Curing typically takes place at room temperature, though some resin systems are heat treated for added strength and stability. Features such as tapped holes are cast in place. This a big advantage over machining that depends upon the machine locating the holes properly each time. Casting eliminates the need to inspect hole locations, once established.

Unlike metallic castings, wall thickness can vary without inducing internal stresses. Resins have less mechanical and impact strength than steel or cast iron, and tend to be stronger in compression than in tension. Highly stressed sections are simply made thicker. Thin sections in tension are internally reinforced with steel or fiberglass.

Molds
A big advantage of polymer casting is the ability to use molds of various materials. Wood molds work well for extremely large parts or when short lead times are important. Parts cast from these molds can be held to relatively close tolerances. Common methods to form precision surfaces on bulk castings include secondary machining, grouting, or casting in premachined steel components.

Wood mold for a 15-ton polymer miller base



Surface finish is another consideration. Obviously, a highly polished mold surface will produce parts with the same finish. Parts can be made with a mat finish or cross-hatching by applying the features to the mold itself.

PC Cuts Vibration
Improved wear, lower costs, and ease of manufacture are why many users choose polymer castings over metal counterparts. But the primary reason is improved vibration damping. Tests show polymer composites dampen vibration 10 better then cast iron and 45 better than steel.

Some grinding-machine makers claim polymer-composite bases help grinding wheels last 30% longer and produce smoother ground surfaces. Makers of lathes and milling machines using polymer bases report similar results with cutting tools. In other applications, better vibration dampening lets high-speed printing machines and scanners deliver higher resolutions at faster operating speeds. Makers of computer chips use polymer bases for high-speed inspection machines and for chip bonders that perform 400 welds/min.

MECHANICAL PROPERTIES OF POLYMER COMPOSITE

Tensile strength 4,000 psi
Compressive strength 18,000 psi
Density 0.084 lb/in.3
Modulus of elasticity 4.5 10 6 psi
Thermal expansion 7.0 in./in./°F
Thermal conductivity 1.6 Wm/°K
Temperature limits –50 to 250°F
Dielectric constant 4 @ 1 kHz, 25°C
Chemical resistance Excellent

________________________________________________
This article was written by Terry Capuano, P.E., President, Accures Casting LLC and edited by Machine Design Senior Editor Lawrence Kren.

[lgalla]

Mike two componet urethanes have isocyanate which is the bad stuff.Need body suit air hood spray booth.
Gerry you reminded me of a difficult to release mold we had.We sprayed the mold with black latex.Why not take the mold with release agents on to a bodyshop to spray polyurethane which is uv stable?
Larry

[jsage]

Larry,

Works for me. I was struggling with how you use a mold release while creating a surface which won't delaminate with the vibration. Walter, thanks for summary. Finding and reading that stuff took me a few hours.

I think that alumina component might be very helpful. I realize it sounds like hype, however, the better mixed the stronger the PC will be. Of course provided it is not ridiculously costly. I was trying to figure out what the special additives anocast might be referring to.

On the treatise if you skip down to the bottom it gets to vibration loss for different materials. It does drag on forever.

Cheers.

Mike

[jsage]

Geof,

That's it. The non female mold paint is Awlgrip. I almost bought some once but the warnings gave a bit of pause.

So I guess my question to you folks. I how massive is everyone looking on going. I feel there can be some compromises. In the treatise it talks about hysteresis which to me meant the composition created vibratory loss and perhaps prevented the return of vibrations creating higher amplitude vibrations. So the conclusion I came away with was to create a design that balances the need to reduce vibration while not building a goliath.

For my needs I am looking for grinding, milling, routing, and potentially the equivalent to an ink jet printer for creating consistent application of concrete acid stains. A tall order.

I'd like a basin type structure which could potentially involve another material for support/elevation. I'll need to sit down and really figure out what I need, how much it would weight etc.

[Geof]

....In the treatise it talks about hysteresis which to me meant the composition created vibratory loss and perhaps prevented the return of vibrations creating higher amplitude vibrations....

You have it sort of reversed here. Hysteresis means that something, when distorted by a force which is applied and removed at the same rate, does not return back to the original shape as fast as the force is removed. The stress/strain curve does not follow the same path during deflection and relaxation. An example of a sort is right at your finger tips; push your finger against something and then pull it away quickly. The flesh gets depressed but when you pull it away it does not rebound instantly but takes a noticeable time to do so.

Steel springs, most metals in general, do not display hysteresis, they return instantly as the deflecting force is removed; this is why they ring. Rubbers and many plastics do display hysteresis and do not return from a deflection quickly so they do not ring. For vibration damping hysteresis is good because you get vibratory loss.

Down at the molecular level what is happening is that the energy that is put in as vibration is turned into heat due to intermolecular friction.

[jsage]

Geof,

Yes that sounds right. Last thing I read, my brain was taking short cuts. It went into a lot detail about selection of materials for expected working temps and gave some tables on loss at different temps. Also changes from visco inelastic to elastic as temperature changed. Suprised on it's discussion of glass. Perhaps playing your music loud in you car on a cold day would be an example.

[lgalla]

Hi Mike
I cannot find the alumina componet you mentioned.Please elaborate.
Thankyou
Larry

[jsage]

Larry,

This is the one. If you look at the first Author that is how I navigated to this company pdf. Roughly, this alumino silicate in very small sizes is supposed to improve mixing while not degrading the mechanical characteristics of the PC. The information refers to it as a counterintuitive fact that adding a solid improves mixing, perhaps in similar manner to other applications of graphite powder. I'm also inferring since they are looking at non nucleated structures then in some fashion it promotes motion without being bound in the polymer composite during mixing. Of course, I don't have the background to make full sense of this research or conclusion.

BREAKTHROUGH POLYMER FINISHING TECHNOLOGY

http://www.vitroco.com/discovery/polymer.pdf


Mike

[lgalla]

Mike that PDF refers to injection molded plastics and PP PolyProplene.If you want to improve flowability inE/G the Zeeospheres I suggested a while ago will enhance flow as they are microscopic perfect spheres 20to200 micron and are at par with the hardness of E/G.I appears to be a fine powder but is millions of tiny microspic ballbearings.I posted previously on Zeeospheres but cannot find it in the millions of E/G posts.3M is the manufacturer Zeospheres grade 850 is the least costly.Check it out.
Larry

[jsage]

; )

I never found it either. Not that I would have remembered. I'll take a gander when I get a chance. Reasonable is good. Especially since we are not specialists or have specialized tools but we would rather have good return on our investment.

[walter]

You can buy 3M Zeeospheres here:

http://www.thecarycompany.com/products/3M/G-800.html

G-800 and G-850 grade is recommended for polymer concrete.

[BobWarfield]

Wow, 46 pages and counting?

I left this thread at about page 25, but as far as I can see, nobody has poured any epoxy? Not even a little surface plate or something as an experiment?

Who will be the first to try it and post the pix? Or did I just miss them a long ways back?

Best,

BW

[walter]

WilliamD already started some testing:

http://www.cnczone.com/forums/showthread.php?t=33434

My first "pour" will be in about 2 weeks...

Shopmaninc epoxy is on it's way, and I found local suppliers of Zeeospheres and ground silica sand - all I need is a chunk of steel (reinforcement).

Edit: Black carbon thing will also be needed (for added sportiness)
Well, it may take longer than 2 weeks..

[Zumba]

Hey Walter, did you end up buying the 635 Thin Epoxy? I will probably pick up the 1/2 gallon and 32 oz hardener set for 34 bux.

How much are the Zeeospheres running you? what are you using for large aggregate?

[martinw]

Edit: Black carbon thing will also be needed (for added sportiness)
Well, it may take longer than 2 weeks..

Dear Walter,

Aesthetics tend to. Much higher price as well.

Best wishes

Martin

[walter]

Agreed Martin. Good looks cost money.

Zumba,
I will skip the "large" aggregate this time (to speed up the whole process). The article on top of this page (EDIT: the article in post 541) says the "sand only" performance is about the same. I just need to see some results.

Yes, I went with slow 635 kit from shopmaninc.

Don't know the pricing on quartz sand/zeeospheres. Suppliers are 90 min away- I'll just go for it.

The sand is 40-250 microns, 50 lbs bags.

Zeeospheres- that is a novelty to me, I just have to have it...
Same with Monarch 800 black carbon pigment. Thanks to Larry for all the info, you guys rock!

[Armstrong]

I have not tried it, but I have heard about it many times.

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Armstrong Industrial Hand Tools Catalogs - Get The New Armstrong Industrial Hand Tools Catalogs

[lgalla]

Walter the carbon black is extremely black and fluffy.Opening a bag carefully would create a large black cloud.I am sure you do not want a 25lb bag it would last a lifetime.Try to get a sample.An alternative source for carbon black or lampblack is art supply retailers.Glad you posted on Zeeospheres.That showed what I had been trying to explain about advantages of micro fillers properties.
Any of the products I have recomended I have used over the years and obtained good results for my purpose.Good luck with the 635.
Larry

[Zumba]

What would be a suitable substitute for zeeospheres? Preferably something cheap and easy to find?

[BobWarfield]

FYI, I did a design sketch for an epoxy granite table for a gantry-style milling machine:

Pour Frame with Sub-Frame:



Threaded Inserts:



Inserts installed in pour frame:



Poured Epoxy-Granite Table:



More details about how all of this works and could be built is available on my web site:

http://www.thewarfields.com/cnccookbook/CCEGGantry.html

Epoxy granite is quite an intriguing material. If professional quality results can be attained in a home shop, e.g. smooth finishes and 0.001" accuracy, it seems quite tractable to build very impressive machines in a relatively modest shop.

I look forward to seeing more "pour experiments" before diving in myself.

Best,

BW

PS Anyone here have a good recommendation for reasonably priced FEA software? This sort of thing cries out for it.