Achieving Your Surface Finish

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.

Sure, it may cost a bit more, but a quality cover can sometimes mean a quality piece of literature, with the potential of rewarding you in the end.

And so it goes with castings. Generally achievable at some cost increase to the buyer, a smooth, uniform surface finish is visually appealing, and it can indicate that the casting supplier has done a good job with the interior of the component. And, like a good book, it can be rewarding in the end, leading to a cost savings in the cleaning, finishing and machining process.

“With green sand, typically without coating or anything, you produce a fairly good cast surface,” said Vic Lafay, vice president of research and tech development, Hill and Griffith, Cold Spring, Ky. “But if the end-user wants to pay a little more, you can improve it.”

Depending on your specifications, finding a casting supplier that knows how to impart a slick surface finish could be the smoothest move you’ve made since bellbottoms were cool.

Know the Sandboxes

For sand casters, sand manipulation can be as important as sand makeup. So, before concerning yourself with your potential supplier’s sand mixture, make sure that the existing materials are being used to their greatest 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 also generate superior surfaces.

Play in the Sand

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

“Since sand makes up 80-90% of the molding material, if you’re going to have a good casting finish, you’ve got to have good sand,” LaFay said. “Most metalcasters 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 by a metalcaster 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. Overused sands won’t pass the tests, as they will have accumulated an excess of organic material that Lafay called “decomposed, various forms of carbon.” 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 supplier’s metalcasting facility, the smoother the surface finish you will see. 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 they also will produce a less permeable mold, trapping gases within that would have seeped out of more porous sands. This can cause porosity and other gas defects in the finished casting.

“You would want to do more venting [to the mold and cores],” said Trinowski. “And depending on the operation, that may be practical, or it may not be.”

The surface area of the sand used also increases with finer grains. This too presents a tradeoff—more resin is required to coat the sand mixture when the surface area is increased. This can equate to a rise in cost that may be passed on to the buyer.
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 a casting with few micro-peaks. LaFay says that the ideal metalcaster will 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.

Rhyme or Resin

Resins and coatings can improve casting surface quality with the use of 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.

According to Trinowski, shell sands produce some of the best surface finishes in the sand casting arena because they 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, chemical binders have a bench-life, and after a given amount of time, they 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. Suppliers may try to push the limits of that bench-life to save on material costs.

Nobake sands operate similarly—they too have a bench-life—but the casting surface results also are superior. “Nobake is going to be more expensive but with absolutely spectacular surface results,” Lafay said.

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 a metalcaster is using is important,” he said. “A lot of it comes down to what’s least expensive and most available in that area of the country.”

And what’s least expensive 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. Of course, this means moving toward materials that are harder to come by.

“I’ve seen metalcasting facilities use refractory coatings in just one spot to improve the surface,” Lafay said. “But that cost would be passed on to the end user.”

Melt Matters

Sand preparation is the foremost concern when trying to limit roughness, but metalcasters 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, they should be wary of extremely high temperatures when trying to smooth out their surfaces. This concern is far more prevalent for iron casters than for the nonferrous facilities.

“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 a 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.

In the end, you receive what you pay for when it comes to surface finish, according to Lafay. “Even when costs increase, most of the time the end user will say, ‘I don’t care,’ and pay a little more. Better castings cost more money.” METAL