Carbon Black and other finely divided stuff
Here is a quote from K. Gamski in the 1975 proceedings about the purpose of the regulator charge:
<blockquote>For a chemically stable composite, chemically resistant charges must be used to regulate the reactivity of the binder and its viscosity and to reduce the hinderancy effect of inert charges on the evolution of shrinkage, regulation charges must be used. This type of charge is characterized by a high specific surface area of 3,000 to 5,000 cm^2/g (300-500 m^2/kg). In order to neutralize the hindering effect of inert charge on the evolution of binder shrinkage, a binder regulator charge ratio of at least 1:1.5 to 1:2 is recommended.</blockquote>
The carbon black that we have actually has a much higher specific area than the stuff they specified. closer to 10,000 cm^2/g. The purpose of the carbon black is to prevent strains in the material caused by shrinkage and to provide some dispersion hardening as I and DAK3333 mentioned before. From Walter's results, it looks like the russian paper cited must have meant by volume. Either that or it's measured in terms of specific gravity of Peter the Great's urine or something. . .
Gamski's data along with Gupta's and Walter's data show that by weight 9% epoxy is the minimum useful amount (less ruins strength) and that strength seems to go up until 12.5% and starts to go down above 14%. These were from specific mixes with fewer fines than the one I wrote up. Like I said before, the equation Gamski says should be used to predict the amount of epoxy combined with Gupta's optimal 33 micron thick epoxy layer result gets 35% epoxy by weight with the mix I put in the formula. This right amount of epoxy goes down with the larger rocks added by walter so I think he's got a reasonable mix.
As for easier to obtain mixes, I still contend that Dupont Starblast Ultra Staurolite sand is near optimal for a no large rocks mix. Besides, it's heavy and one of the papers I read says that strength can be accurately predicted by a formula where weight per cubic foot is the only variable.
Qualitatively, the homeshop folks will likely do a lot better with the larger aggregates as they are much less subject to the effects of poor deairing ruining the mix. Smaller aggregate mixtures appear to be stronger according to Gamski however.
I think Walter ought to get some fairly decent results out of one of today's batches.
I'll write up some more formulas as I get a chance. The formula in the Huntsman literature may be more appropriate for instance. I also owe calls to dow corning, agsco, and byk today.
In summary, Rock on Walter. Thanks for the commentary on the carbon black, Larry.
Why Are We So Concerned With Static Deflection?
All the static deflection number tells us is that the beam will bend by a thousandth or two under no load. Why is that a big deal? We should be looking at the dynamic deflection under actual (varying) loads.
If we use the beam as a support for a rail (or for a metal support for a rail), we can just machine out the static deflection. We would still be left with the dynamic deflection, though.
So let's talk about the dynamic load and the deflection under that load. To do that, we need to know the mass of moving components and the forces (cutting and other) that act upon our beams.
Ken
Flexural Modulus of Accures Material and Modulus Argument
In keeping with my promise to Andrew to reread all of his postings, I am. My first comment is that I corresponded with the president of Accures castings today and confirmed that the correct elastic modulus for their material is 4.5*10^6 psi. The 4.5*10^5psi number on their website is in fact a typo that they are going to fix!
As for my rough calculations on this thread , I have used a flexural modulus of 2.0*10^6 psi. If anybody thinks that this flexural modulus value is higher than we can achieve at home please post. This is towards the bottom of the range for either compressive or tensile modulus in the nist report originally posted by brunog: http://www.fire.nist.gov/bfrlpubs/bu...PDF/b99032.pdf
--Cameron