Smooth Moves: Sand’s impact on Surface Finish

Different types of sand molds and cores used by your supplier will impact the surface finish of your castings.

An MCDP Staff Report

(Click here to see the story as it appears in the January/February issue of Metal Casting Design & Purchasing.)

For many casting buyers, a smooth, uniform surface finish is more than a visually appealing bonus. It can be a sign that the casting supplier has produced a high quality component. Perhaps more importantly for the casting buyer, a proper surface finish can lead to eventual cost savings in the cleaning, finishing and machining of the part.

Castings produced in sand molds can have relatively smooth surfaces, but if need be, the baseline surface finish can be improved through various means, though such gains will come with additional costs. Discussions with casting suppliers can help buyers understand potential solutions and associated costs.

Sand Selection & Other Variables

The surface finish of your cast components depends greatly on the type and quality of sand used in the casting process. The sand’s manipulation and makeup are vital considerations. Your potential supplier invests time, money and effort in ensuring these materials are being used to their greatest potential.

The method of compaction in sand molding 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. Blowing 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.

Sand makes up a large majority of the casting mold, and it has an equally significant impact on the casting’s eventual surface finish.

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.

Smaller grains sit more closely to one another 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.

The surface area of the sand used also increases with finer grains. This too presents a trade-off—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.

Resins and coatings can improve casting surface quality with the use of almost any type of sand. Resins, such as phenolic urethane, can smooth out casting surfaces and provide some refractory properties. Generally, resin-coated sands are used for applications that require exceptional surface finish.

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.

Lower cost sand also includes more impurities, including 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 source.

Considerations During Casting

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, president, Product Development & Analysis LLC (PDA), Naperville, Ill., the values published in Figure 1 and Table 1 are drawn from a random sample of metalcasting facilities in the U.S. and abroad. PDA collected published capabilities from the participating metalcasting facilities and used them to create an average range. The extreme values are based on the lowest root mean square (RMS) in micro-inches 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. You’ll also likely have to pay for the surface finish improvements available in certain processes. The nobake process is more expensive than green sand, but the resulting surface finish is generally improved.

As an alternative to changing processes, you can work with your metalcaster to see which finishing processes might be available to improve the final surface of your component. The cast component producer may be able to 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.

Shah said 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 Shah, aluminum can be cast to lower surface finishes than can iron. Copper falls somewhere in between those two materials. The same is true for nonferrous alloys versus ferrous alloys in the shell molding process.

Sand preparation is the foremost concern when trying to limit roughness, but metalcasters must address the molten metal’s entry into a mold. Aside from achieving a metallurgically sound melt prior to pouring, casting suppliers should be wary of extremely high temperatures when trying to smooth out their surfaces. Radiant heat from the molten metal can lead to degradation of the mold surface, which can produce conditions that lead to poor surface finish.

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, which can lead to flaws in the casting’s surface. 

ighter, stronger aluminum components is the ultimate goal in a recently started research project that joins industry, research and academic institutions to develop innovative manufacturing solutions and transfer them into real-world production.  
In August, Lightweight Innovations for Tomorrow (LIFT) announced a new project intended to advance technologies for diecasting and heat treating aluminum parts, primarily for aerospace, defense and automotive applications.
“If we can reduce just a few ounces of metal from automobile engine mounting cradles or the housings that hold transmissions, we can deliver an impact that is multiplied by the millions,” said Larry Brown, executive director, LIFT. “In aerospace, an added benefit might lower manufacturing costs as well as increase fuel savings from the lighter weight designs.”
LIFT is operated by the American Lightweight Materials Manufacturing Innovation Institute and is one of the founding institutes in the National Network for Manufacturing Innovation, which is a federal initiative to create regional hubs to accelerate the development and adoption of cutting edge manufacturing technologies. It was formed in 2014, and the vacuum diecasting project is one of the first two started. The other project focuses on thin-walled gray iron parts.
Lead partners for the project are Boeing and The Ohio State University. The focus is to develop key process technologies (super vacuum diecasting and a shortened heat treatment) and computer engineering tools for 300 series aluminum diecasting alloys to improve mechanical properties and reduce the minimum wall thickness (up to 40%) and weight (up to 20%). According to Alan Luo, professor of materials science and engineering and integrated systems engineering, The Ohio State University, the project will reduce the variability in quality and improve the mechanical properties of high pressure die castings. The project also will explore new design methods of lightweight castings using local mechanical properties predicted by the new computer engineering tools, as opposed to the current casting design using minimum properties of cast alloys.
“If you can take a common part, such as an access panel you see on the wing of an airplane and use high integrity die castings, it could reduce weight and manufacturing costs,” said Russ Cochran, associate technical fellow, Boeing. “We hope to demonstrate that advances in high vacuum diecasting will produce parts that meet all the rigorous performance specifications we require, while realizing weight and efficiency goals.”
In current high-speed aluminum diecasting, microscopic air bubbles can form inside the part as the molten metal races through the mold. These tiny bubbles are not an issue for most diecast parts in typical applications, and engineers allow for them by using more metal and making parts thicker to meet strength and other performance requirements. These aluminum parts can achieve tensile strengths up to 47 ksi and minimum wall thicknesses of 0.04 in.
For this project, however, researchers are looking at methods to cast thinner walls with increased strength for structural applications, and for that, the bubbles can be detrimental. By applying a vacuum to the mold, diecasters remove air from the environment. Air is the culprit for porosity.
“We know in the laboratory that if we pull all the air out of the mold just before the molten metal flows in, we can eliminate the bubbles,” Luo said. “Without bubbles, we can design thinner parts that are just as strong and durable, but with less metal and lighter weight. There are other benefits, as well, because the new process allows us to heat treat parts after they are cast, which will improve their performance in service.”
The group also will be working on a shortened solution heat treatment to improve mechanical properties cost efficiently. A simple T5 heat treatment (where castings are cooled from an elevated temperature and artificially aged) has shown in preliminary work to increase yield strength by 40% for E380-type alloys. Now researchers want to see if a shortened T6 heat treatment (where castings are solution heat treated and artificially aged) can be developed to achieve even better properties in 300 series aluminum.
An important part of the two-year project will be enhancing the ability of computer models to predict the performance of aluminum diecast parts by combining information about the microstructure of the metal with a host of design and production parameters. The process, called integrated computational materials engineering (ICME), has great potential for reducing the time it takes to design and qualify new components for vehicles and will address some of the key challenges in implementing thin-wall diecasting technologies: die design, process control, casting design and process simulation.
Currently, castings are designed using the minimum properties of alloys as a baseline for the entire part. The ICME approach will allow designers to pinpoint higher or lower minimum properties to localized regions.
The aim is to connect the thermodynamic prediction of alloy composition and heat treatments to process modeling, which will enable designers to locate specific properties in specific areas of a part to meet service loading conditions. When load paths are clearly defined, the research group also plans on establishing topology optimization techniques to enhance design optimization.  
In the first three months of the project, the group has selected the baseline alloy (A380) and identified an experimental high strength aluminum alloy for structural casting development for aerospace and automotive applications. A concept design on a thin-wall casting die also has been started. The die is based on a thin-walled zinc die casting, which is being redesigned for aluminum. Researchers have begun mechanical property evaluation and ICME model validation in thin-wall casting development. Key real-world applications, such as an aerospace wing fuel door, to demonstrate the benefits of vacuum diecasting and ICME have been identified.  
“What we are doing here is bridging the gap between great research in laboratories and great manufacturing skills in private industry,” Brown said. “Once you bring these innovations into production, the results just multiply.”
Lightweight aluminum diecast components have a significant market to fill, particularly in the transportation industries, including aerospace, automotive, military and marine. Industry partners from these markets include Eaton Corp., Comau and Nemak. On the research side, Worcester Polytechnic Institute, Southwest Research Institute, the University of Michigan and Massachusetts Institute for Technology have joined the effort, while the American Foundry Society and North American Die Casting Association are assisting with modeling, technology oversight and dissemination of knowledge on how to manage the new thin-wall aluminum diecasting process in a production environment.
LIFT is operated by the American Lightweight Materials Manufacturing Institute and was selected through a competitive process led by the U.S. Department of Defense. It is one of the founding institutes in the National Network for Manufacturing Innovation, a federal initiative to create regional hubs to accelerate the development and adoption of cutting-edge manufacturing technologies.
After the two-year project is concluded, the group plans to deliver design guidelines and property specifications for thin-walled aluminum diecasting and ICME models for thin-wall casting design with location-specific properties. As these technologies and guidelines are incorporated into production diecasting operations, the options for aluminum components in structural applications will expand.   ■