Turn a rough rider into smooth operator: when you're using a bonded sand casting process, improving your surface finish can be a difficult task. But it also can be invaluable.

You may not be able to judge a book by its cover, but you can tell a thing or two about its quality. If there are misspellings on the outside, you'll probably find shoddy writing on the inside. If there are visible imperfections on the binding, there may be a page or two missing from the full text.

Plus, no matter how open minded the buyer, bad covers can make books very tough to sell.

And so it goes with castings. Not only is seeing a smooth, uniform surface finish visually appealing to buyers, it's the first chance they have to determine that you've done your job right.

Depending on your customer's specifications, improving your castings' surface finish could be the smoothest move you've made since bellbottoms were cool.

Take Care of Your Sandbox 

For green sand metalcasters, sand manipulation can be as important as sand makeup. So before getting your hands dirty in the sand mixture itself, you'll want to make sure that you're using all of your sand's potential.

"Anything that can improve compaction can improve surface finish," said Doug Trinowski, vice president of technology at HA International, Westmont, Ill.

It's common sense, says Trinowski, and a concept that influences surface finish at all levels. Sand grains that are packed tightly produce a more uniform surface over which molten metal will eventually flow, mimicking the appearance of the sand itself. Hand ramming makes for the worst compaction, and Trinowski recommends going to more consistent processes to improve surface finish. Blowing, or slinging, for example, produces a smoother surface than hand ramming, while the jolt-squeeze method provides an even smoother, more compact mold than blowing. Automatic molding machines achieve better compaction still.

Play in the Sand

Once you know your sand is being put to good use, it's time to look at your base material, particularly with the use of green sand.

"Since sand makes up 80-90% of your molding material, if you're going to have a good casting finish, you've got to have good sand," said Vic LaFay, vice president of research and tech development, Hill and Griffith, Cold Spring, Ky. "Most foundries don't add enough new sand. They rerun too much, being pennywise and pound foolish."

Five tests--for green compression strength, permeability, moisture content, compactability and temperature--should be run hourly to determine when it is time to mix in some new sand and avoid a degradation of surface quality. All five should fall within acceptable ranges to achieve quality surface finishes. Over-used sands won't pass the tests, as they will have accumulated an excess of organic material--"decomposed, various forms of carbon," said LaFay. That material will make sands fit together poorly, and the surface of molds and cores will show an increase in juts and crags.

Screening also is key to maintaining a fine, flowable sand during reuse. Metalcasting facilities should use a nest of several screens with pores of differing sizes. According to LaFay, a four screen nest is ideal. The 140 screen, he said, is critical in eliminating sands that can lead to a rough surface finish. The 140 produces that perfect mix of fine sand grains and course sand grains that allows them all to sit together in the tightest manner.

In all of the bonded sand processes, the finer the sand grains in use at your metalcasting facility, the smoother the surface finish you will attain. Larger grains produce larger peaks and valleys as molten metal flows into the spaces between them. Those peaks and valleys, generally measured in microns, produce a roughness value--root mean squared (RMS)--where small numbers mean a slick finish.

Going to finer sand does require that metalcasters take some precautions. Smaller grains sit closer together to produce that smooth surface finish, but that also means that the mold produced will be less permeable, trapping gases within that would have seeped out of more porous sands.

"You would want to do more venting," said Trinowski. "And depending on the operation, that may be practical, or it may not be."

The surface area of the sand also increases with finer grains. This too presents a tradeoff--more resin is required to coat your sand mixture when the surface area is increased. This can equate to a rise in cost and the volatility of the molding material. That volatility is compounded by the fact that fine grain sand molds are less gas permeable.

For green sand molding, the 10-20% of the mixture not composed of silica or lake sand will be made up of the clay bonding mechanism. This should be of the highest quality if you want to take the micro-peaks off of your castings. LaFay says that the ideal situation is to use a mixture of the two available bentonite clays--southern and western.

"That's your mineral glue part, and the more effective the gluing characteristic of the mineral, the better the integrity you're going to get," LaFay said. "Together, [western and southern clays] work better than on their own." However, he admits, there is no magic percentage that dictates the mix.

Resin Up Your Mold

Resins and coatings can improve casting surface quality on almost any type of sand. According to Trinowski, resins such as phenolic urethane smooth out casting surfaces and provide some refractory properties.

"If you look at resin-coated sands, typically those are always associated with good surface finish applications," said Trinowski.

Trinowski offers the example of shell sand, which produces some of the best surface finishes in the sand casting arena. This is because shell sands tend to combine the highest hot strength and resin level with the finest grain of any chemical sand.

Hot strength--the ability of a material to maintain its integrity at elevated temperatures--can be improved in nobake and coldbox sands with some resins, which helps to impart a cleaner surface finish, as it makes the molds and cores more resistant to erosion by the molten metal. However, users of chemical binders must be sure to utilize the resins before they have exceeded their bench-life. After a given amount of time, chemical binders begin the process of polymerization and become less flowable. The additive at that time makes molds less compact and dense, and they exhibit poor strength properties--all of which adds up to a surface finish on par with two-day stubble.

Green sand metalcasters also can use an additive to improve their surface finish, but the need to do so really only arises in certain locales, according to Bill Marchisin, Foseco, Cleveland.

"The type of sand you're using is important," he said. "A lot of it comes down to what's cheapest and most available in that area of the country."

And what's cheapest and most available is often impure, containing foreign elements that compromise the flowability of the sand. To combat these contaminants, which can change the way region-specific sand compacts, metalcasters can use a coating that fills in the gaps that result on the mold and core faces. Such a coating will cause particles to be transferred to the surface via a zircon conductor.

"In any case, you're imparting a refractory filler to fill in the surface gaps," said Marchisin. "It's strictly done to improve surface finish."

Melt Matters

Sand preparation is the foremost concern when trying to limit roughness, but you can't just slop any old batch of molten metal into a mold and expect the surface to come out flawless. Aside from achieving a metallurgically sound melt prior to pouring, metalcasters should be wary of superheating when trying to smooth out their surfaces.

"The higher the temperature, the more likely the degradation of the mold surface due to the radiant heat," said Trinowski. "It doesn't automatically mean poor surface finish, but it can make those conditions arise that make it more likely." This is where your mold's hot strength can be important.

Mold faces also can be compromised when liquid metal physically crashes into the sand. Increased turbulence in the melt can cause imperfections in the sand mold that will be transferred to the casting upon solidification.

Seasoned metalcasters know, however, that they can't gear everything they do toward making their product as smooth as a baby's bottom. There are other characteristics--often dictated by their customers--that they have to consider when preparing a mold. It's a balancing act, according to LaFay.

"Every foundry has its own mix," he said. "It's like cake mix. You have yellow cake, chocolate cake, white cake. And people will balance it based on their individual needs."

For More Information

"Effect of Zircon Sand (Added to Silica Sand) on Surface Finish of Al- 11% Si V-Process Castings," P. Kumar and R.C. Creese, 1999 AFS Transactions (99-130).

"Optimizing Sand Control at New Haven Foundry," M.B. Krysiak and V. Raman, MODERN CASTING, August 2001, p. 47.

Keys to a Slick Surface Finish

There are only a few bumps in the road on your way to achieving a smoother surface finish. Follow this checklist to navigate them.

Green Sand

[check] Use a highly consistent, high-pressure packing mechanism.

[check] Add new sand when recycled product begins to lose flowability.

[check] The finer the grains, the smoother the surface but use caution.

[check] Use a combination of western and southern bentonites.

[check] Use a coating when regional sands are of poor quality.

[check] Be wary of superheating and turbulence when pouring.

Chemically Bonded Sands

[check] The finer the grains, the smoother the surface but resins can become volatile.

[check] Use resins with high hot strength.

[check] Use resins within their prescribed benchlife.

[check] Be wary of superheating and turbulence when pouring.

Shea Gibbs, Assistant Editor