Surface Finishes for Casting Processes
There’s good news, and there’s bad news. The bad news is no published standard exists for the surface finishes available from metalcasting facilities. The good news is recently compiled data has been published that shows the average finishes produced by plants across the country, as well as the best case scenario when all corrective measures are put to use.
According to the data, gathered by Product Development and Analysis LLC (PDA), Naperville, Ill., plaster casting produces the best surface finishes on average, with average root mean squared (RMS) values between 40 and 125. Other ceramic based processes, including investment casting, are similarly effective at delivering the high gloss shine. When the metalcaster at these facilities pulls out all the stops, he or she can deliver RMS values as low as 25.
Also at the low end of the spectrum, particularly when additional measures are taken, are the metal molding processes. Diecasting and squeeze casting both can reach 20 RMS in extreme cases.
Sand processes typically produce the roughest surfaces, but chemically bonded sands, including shell, nobake and coldbox, can rival the ceramic processes at their best. Shell sand produces the most favorable range of average RMS values at 75-150, followed closely by vacuum casting (150-200 RMS).
The full results of PDA’s findings are shown in Fig. 1. Exact values are listed in Table 1.
Who Can Do What?
Even if you know what kind of surface finish you require, how do you find the right metalcaster to deliver it?
According to Jiten Shah, PDA president, the values published in Fig. 1 and Table 1 are drawn from a random sample of metalcasting facilities in the U.S. and abroad. The company collected published capabilities from the participating metalcasting facilities and used them to create an average range. The extreme values are based on the lowest RMS values the company discovered published in the literature.
You can use the published values to zero in on the appropriate process, but the next challenge is figuring out who falls where on the provided spectrum.
“The processes can be stretched,” Shah said. “But you have to partner with the metalcasting facility or shop around to get what you want.”
You’ll also likely have to pay for the surface finish improvements available in certain processes.
“Nobake is going to be more expensive but with absolutely spectacular surface,” said Vic LaFay, vice president of research and technology development for Hill and Griffith. “I’ve also seen [metalcasting facilities] put coating in one spot to make a particular surface spectacular. But the cost would be passed on to the end user.”
According to LaFay, shifting processes for a fine surface finish can also come at non-monetary prices.
“If you go to lost foam, you have to realize that the surface finish looks like the Styrofoam pattern,” LaFay said.
As an alternative to changing processes, you can work with your metalcaster to see what finishing processes might be available to improve the surface finish of your component. The cast component producer sometimes can use a different shot blast material, such as glass beads, to smooth a rough surface. Machining certain surfaces might also present an economically viable alternative.
Why the Variation?
Shah said that a metalcasting facility can achieve the lower end of the surface finish range—and even those extreme best case surface finishes—by controlling several factors: weight/section thickness, alloy and mold material treatment.
The first two are straightforward. As weight and section thickness increase, the quality of the surface finish decreases. And alloys with higher melting points produce lower RMS values than alloys with lower melting points. In other words, iron castings will not be as smooth as aluminum castings, and steel castings typically will be rougher still. According to the data gathered by PDA, aluminum can be cast to lower surface finishes than can iron particularly in the permanent mold process. Copper falls somewhere in between the two materials. The same is true for nonferrous alloys vs. ferrous alloys in the shell molding process.
Mold material treatment is a more complicated matter. For all metalcasters, the quality of the tooling is paramount if the extreme best surface finish is to be obtained. For permanent mold casters, coatings and routine mold die maintenance both play a role. For sand casters, a variety of variables come into play. The sand itself creates the first level of variation. The finer the sand used, the better the surface finish. However, fine sand limits permeability, which can create casting defects.
“The type of sand you’re using is important,” LaFay said. “A lot of [metalcasters] go with the cheapest and available in the area of the country. If the sand they have to use is on the coarse side, coatings come into play.”
Coatings fill the voids between sand grains and can also act as a refractory, limiting the defects and loss of integrity that can occur with higher melting point alloys.
In addition to sand type and additives, compaction method also influences sand casting surface finish—a blown or slung mold will have greater compaction and thus better surface finish than a hand-packed mold. Similarly, chemically bonded sands depend on the material used to hold them together to produce a better surface finish.
“With nobake and coldbox, [metalcasters] can go to a higher hot strength resin, which can provide surface finish improvement,” said Doug Trinowski, vice president of technology for HA International, Westmont, Ill. “Shell sand has the highest hot strength of any chemical sand. It is typically used at a higher resin level than other nobake or coldbox sands. And, typically the coated sand is a little bit finer, so you have the combination of high hot strength and a fine grain, producing a very excellent surface.”
Whatever your expectations of any process, don’t allow yourself to be constricted by norms. Averages are a good place to start, but you don’t know what a metalcaster is capable of until you ask.
“The extreme scenario is not the best practice you normally find, but where people have their own little magic niche,” Shah said. “You shouldn’t expect it, but it is doable.”