Mohawk College - Introduction to Metal Casting ...

MATL MTB72 Module 1 - Course Notes...

Whutz-Up with Sand?

Since Molten Metal is hot, and the utility of the mold is the space that it does not consume, the mold material has to withstand the heat of the metal without melting, losing it's shape, size etc... Since Sand Molding is done with Sand, the sand must not melt and it's physical dimensioning should remain intact.

Since sand is generally a ceramic based material it typically has a strong inter atomic bonding structure that contributes to it's ability to withstand high temperatures before breaking down.

Since High Temperature is a subjective term, lets generalize that most sands have a melting point above 3000F.

The popularity of sand as a molding material is due to it's abundance and relatively inexpensive cost.

"Daddy, where does sand come from?"

I'm glad you asked... Sand has many sources and compositions, but all sands have their common origin in the fact that they are granular material resulting from the disintegration or crushing of rock.

Of the various bodies that influence and advise the metal casting industry the AFS (American Foundry Society) will provide the standards, terms and ranges of measurement used in this course.

The AFS states that sand is "Mineral Material" regardless of chemical composition, and casting sand falls into a broad range of grain sizes that span 2mm to 0.05mm. 1/12" to 1/500" or #10 to #200 Mesh screening.

The four basic sand compositions discussed in this course are Silica, Olivine, Ziron and Chromite. These types of sand have various properties that effect their application to the broad spectrum of foundry work.

Of the numerous properties the most important are Shape, Melting point, Thermal Expansion, Weight (measured by Bulk Density and Specific Gravity) and pH. The Chemical composition of these sands provides some insight into why the other properties are attributed to the various types.

The following charts are based on AFS data listed in AFS pub "Technology of Metal Casting" ISBN #0-87433-257-5.

Sand Chemistry & Property Table

This chart is important, why the fuck do you think I re-typed it (it's in the text, it's in the lecture, it's a slide, it's even referred to in the audio files and I think references are made to it in the video clips)... I believe that like the infinite constant Pi all answers to this module are contained within the chart above... 

Sand Type Verbiage

Silica

Silica is the most common sand type and can be found where ever water has had the time to erode rock to a granular form over time. Lakes, rivers, river banks etc.

In North America Pure Silica Sand is mined primarily in the Illinois and Missouri States from the St. Peter deposit, all other silica deposits tend to have varying degrees of organic and mineral contaminants that must be removed prior to classification as Casting Sand. These organic and mineral contaminants effect castings in numerous ways including but not limited to introduction of carbon into the cast, unpredictable thermal expansion and binding properties to name a few.

Key Info;

High thermal Expansion 0.018"/inch, Melting point 3110F/1710C, a varied shape and acidic pH.

Olivine

Olivine Sand is an ortho-silicate of Magnesium (Mg) and Iron (Fe) and is found in it's natural state within Forsterite Mg₂SiO₄ and Fayalite Fe₂SiO₄(neither terms are overly relevant to this course) Except that Cast quality Olivine is only derived form 90% Forsterite...

The Forsterite is crushed to reduce the mineral to a granular form, logically increasing the cost due to production of the material.

Olivine sand is typically used in the Non-Ferrous foundry sector, but is used occasionally in the Ferrous Sector to aid in the production of Manganese (Mn) Steel (Austentic) due to it's ability to overcome adverse chemical reactions that would result if Silica sand were used.

Key Info;

Low thermal Expansion 0.0083"/inch, Melting point 3400F/1875C, an angular shape and basic pH.

Chromite FeCr₂O₄

This is an African Sand, as it comes from Africa, and has a price tag to reflect its point of origin.

There is so much I could say about Chromite FeCr₂O₄, but won't as it's not germane to this discussion, but I will note that it is used in Steel foundries, and as a facing or core material where it's superior thermal characteristics are called upon...

Key Info;

Low thermal Expansion 0.004"/inch, Melting point 3800F/2093C, an angular shape and basic/neutral pH.

Zircon

This is the last of the four sand types and has the fanciest name... Zirconium Silicate or ZrSio₄ to his friends is found primarily in Australia, Florida and California. With the most stable thermal properties of the 4 types it is used as a mold or mold facing material where very high temperatures are encountered and refractoriness becomes a consideration.

Oh and before I forget, that High refractoriness can aid in "Directional Solidification" if your a keener and can figure that stuff out when you make your mold.

And another minor point that might be worth knowing Zircon has trace elements of Uranium and Thorium , no big deal... Just so you know... Dispose of as the law tells you, though it doesn't say specifically that the sand itself is hazardous... 

Key Info;

Lowest thermal Expansion 0.003"/inch, Melting point 4600F/2538C, an elliptical or rounded shape and slightly acidic pH, an AFS gfn of 65 to 140 with 100 as the most common.

So why all the hype on Thermal Expansion??? 

Although Silica Sand is the most common sand, it has some characteristics that influence it's behavior when used within a mold.

The silica is considered to be in an Alpha Quartz State when below  approx 2000F, above that temp it changes to a Beta Quartz state that is typified by significant expansion. The results of this expansion can be mold wall movement, Buckles, scabs, Rat's tails etc...

Similarly Silica sand having a relatively lower melting point compared to the other sands, has issues with metals that approach and exceed 2240F in the form of "Wetting" the point where metal starts to cover the surface of the sand grain faces and Fluxes with Iron... (I don't really get the fluxing part, but all will come clear in time...)

AFS Grain Fineness Number (gfn) & Grain Distribution

Words like "Fine, Medium & Coarse" are somewhat vague and subject to interpretation in the abscence of some form of relativistic model... Thank God the American Foundry Society stepped in and implemented a numeric scale that does away with all that inaccuracy...

The AFS gfn covers average grain size and distribution, as this is an average it may be somewhat deceptive.

he AFS scale typically covers 25 to 170 (according to the Text book) But I personally have never heard of anything less than 80 or 90 on the coarse side and have regularly heard of fine grains that top the 200 - 220 range...

And that's the point!!! Don't confuse individual grain Mesh #'s with AFS gfn, as the gfn is an AVERAGE!!! 

Why such a range??? Too coarse a gfn is reflected in the surface of the casting, too fine a gfn and the mold can't pass the gases from the molten metal during the pour and results in gas related defects.

And again on a personal note, I know from painful experience if you can't pass gas, there will be unpleasant results...

Sand Handling

So if your sand is a distribution of Mesh sizes that is averaged to an AFS gfn, then it would stand to reason that you would want that distribution to stay, well, distributed...

Consider a tumbler of sand rotating on an inclined axis, the fines grains move to the bottom, and the coarse grains move to the top. Or just a conveyor that is dumping the sand into a cone shaped pile, the fine grains typically will accumulate in the center of the cone rising vertically as the cone builds surrounded by coarser material rolling off to the sides and surrounding the base...

The above illustrates examples of "Segregation" and will impact casting quality if the AFS gfn distribution is not retained. 

Sand Reclamation

Man does this Module never end... 

So you have some sand, you selected it based on the type that suits your casting specifications, you ensured that you ordered the right AFS gfn for the metal or alloy to be cast, You've employed 100's of 1,000's of dollars to ensure that the AFS gfn distribution is retained, You cast you first casting and now what???

You open your check book and hope that you can afford even more machinery to handle the task of Sand Reclamation!!!

On the most basic level the spent molds have to be broken up to make the sands flow for molding in the next casting. Along with breaking-up the spent mold, "Sand Fines" (sub-AFS gfn sized material that results from mechanical handling of the sand), spent binder material and any metallic refuse need to be removed.

And for high production foundries the sand has to be cooled prior to re-use if the production cycle is that rapid.

As I said, get that check book out as it ain't gonna be cheap...

Mechanical Reclamation Processes

The following are examples of various methods of mechanically reclaiming spent mold sands.

Jaw Crusher

Pneumatic Scrubbers

Vibratory Reduction 

Shot Blast Reduction

Mechanical Sand Scrubber

Heating Units to burn out residual binders - this is employed when dealing with chemically bonded sands that use a resin that has cured and can not be re-activated... Excessive amounts of cured resins that accumulate in the sand impact the LOI or Loss on Ignition rating of the sand.

Sand Coolers 

All of the above should not aggravate the accumulation of "Fines" by excessively rough handling of sands. Sands that are classed as angular impart specific properties to the mold and casting, but those attributes are altered if the sand is literally broken by force during the reclamation process and result in a sand that is now round.

The removal of metal(s) from the sand is also important as it will directly impact the refractoriness of the sand and casting quality and defects. Especially in a Jobber Foundry where various alloys are cast using the same sand if it suits the purpose.

Sand & Molding Technology - Module 1, Mod#2, #3, #4, #5, #6-7, #8, #9, #10

Back to Casting

Disclaimer (an unfortunate necessity)
All Rights Reserved theworkshop.ca © May 04, 2008