Choosing Between Nobake and Green Sand Metal Casting Systems
Navigate the differences in capabilities and cost factors between nobake and green sand metal casting molding processes to ease your sourcing decision.
Opting for a sand casting process over permanent mold, diecasting, or investment casting is step one in sourcing a casting. But sand casting encompasses several methods, each of which carries its own advantages and disadvantages. The two most common processes used are green sand and nobake molding, and sourcing between the two is not always cut and dry.
Forty-two percent of North American metalcasters employ the green sand molding process. Its popularity stems from its affordability, ability to be used for most metals and flexibility for low to high volume production.
Green sand molding does not hold the tightest tolerances or achieve the finest surface finish of all the casting processes; however, in general, it meets most of the qualifications desired to produce a high quality casting with a good finish.
Nobake sand casting, which is utilized by 40% of North American metalcasters, also is known for its versatility. Virtually all metals can be cast via nobake molding, a wide range of sizes can be produced, and the rigid mold walls provide good dimensional tolerances and control.
This article was originally published in MODERN CASTING
An MCDP Staff Report (Click here to see the story as it appears in the Sept./Oct. issue of Metal Casting Design & Purchasing.)
Although it is one of the most popular molding systems, nobake sand casting production has not been automated to the same extent for high volume production. Up to 500 molds per hour can be made on green sand lines, compared to 20-30 molds per hour in nobake.
Automated green sand molding lines use metal pattern equipment to withstand the high pressure, high density molding method of the high volume machines. This normally results in a well compacted mold that yields better surface finishes, casting dimensions and tolerances than other green sand molding methods.
Typical nobake casting production ranges from low to medium volumes. Nobake sand molds are produced using a chemical binder and catalyst to cure and harden them. Due to the curing time required for the chemicals to harden the mold, as well as the methods to distribute the sand on the pattern, the high production rates achievable with green sand are not possible.
The highest level of green sand automation is found in vertically parted green sand casting, where the parting line between the two mold haves runs vertically rather than horizontally. With a production rate up to 500 molds per hour, these lines produce molds without flasks, thus eliminating the cost to produce and maintain the framing around each mold. Vertically parted molding machines are usually integrated into a highly automated casting line, which is characterized by minimal manual involvement in the areas of molten metal handling, pouring and cleaning operations.
Size and shape-wise, the green sand casting process has almost no limit. Typical green sand castings range from several ounces to 500 lbs., but green sand castings as large as 7,000 lbs. have been made. Nobake’s size range is similar to green sand, but it is often the process of choice for very large castings weighing more than 100,000 lbs., due to its mold strength.
When the binder and catalyst are mixed with sand in nobake molding, a strong, brick-like mold is formed. The rigid walls of a nobake mold can withstand the pressure of a large amount of metal and have less chance of breaking than green sand.
The choice between green sand molding and nobake sand casting is fairly straightforward if the casting job will be high volume (green sand) or large in size (nobake). But for those parts that fall in between, the choice is not so simple.
For many applications, green sand casting is the most cost-effective of all metal forming operations, and the process can be used to produce a wide range of casting sizes in almost all casting alloys. While the process has been automated in many facilities for high volume work, it is economical for short runs and prototype work, as well. Production rates often are increased by producing multiple parts in a single mold. Sometimes, completely different parts are poured in the same mold.
While the green sand process is one of the most economical metal manufacturing methods, when an application requires the process to reach beyond its normal capabilities, it can become more expensive. For instance, if a part requires tighter tolerances or a smoother surface finish, nobake sand casting can deliver better results. The rigidity of a cured sand mold allows for tighter dimensional tolerances than green sand. The hardness of the mold also translates to a smoother mold surface and casting finish.
Unfinished nobake molded castings (without machining) typically cost more than green sand, but the price difference is offset when machining costs are reduced due to nobake’s tighter tolerances. Reduction in scrapped castings and rework also affects the bottom line for castings that overexert the green sand casting process’ capabilities.
Green sand castings can be produced with walls as thin as 0.09375 in. (0.238 cm) with no maximum limit. When thin walls are required, a metalcasting facility will take special precautions for shrinkage allowance, mold preparation, venting, pouring technique and other factors, all of which will add cost.
Shrinkage allowance is a patternmaker’s calculation that accounts for the contraction in metal volume as the liquid metal cools to solidification. Venting is the method of providing a small opening or passage in a mold to facilitate the escape of gases when molten metal is poured into it.
Tolerances as tight as +/-0.03 in. (0.0762 cm) can be cast in green sand, but the tighter the tolerances, the higher the rejection rate. A tolerance of +/-0.0625 in. (0.158 cm) is practical for small castings, but increased tolerances are required for large castings.
As a general rule, nobake thin wall section limits are +/-0.1 in. (2.54 mm), although it varies by metal. Common tolerances for nobake range from +/-0.03-0.47 in. (0.9-12 mm) because cured sand used in nobake molding is more structurally stable than green sand.
If surface finish is a factor, nobake sand casting produces smoother surface finish than green sand. Nobake’s finish can go as low as 150 root mean square (RMS) in typical operations compared to 250 RMS with horizontally parted green sand. Surface finish varies within each sand casting process, depending on the fineness of the sand being used, bonding mixture, mold compaction and sand control methods. Metalcasting facilities also may incorporate mold coating techniques to further improve surface finish. However, this will add cost to the part.
Tooling costs also factor into choosing between green sand and nobake casting. Green sand tooling must be able to withstand the compaction force during the molding process. Because nobake tooling doesn’t have to withstand this force (only light vibration), the pattern equipment in nobake sand systems vary widely among wood, plastic and metal tools. In addition, the lack of compaction force in molding allows nobake molders to use loose pattern pieces and other innovative tooling options to increase casting complexity and add design features to the component.
The versatility in pattern material choice means more opportunities in cost savings, as patterns and coreboxes can be tailored to production and metal. For example, if only a few castings are required, a good grade of pattern lumber can be sufficient. Larger casting quantities and higher accuracy requirements will dictate an aluminum pattern.
Nobake and green sand casting methods can incorporate cores made in another sand process. For instance, a nobake core can be placed in a green sand mold to provide tight tolerances and smoother surface finish for interior passageways and holes. Cores add cost, but they can provide a balance between less expensive molding and complex geometrical requirements. Core design and coremaking details discussed with a metal casting supplier will reveal the cost benefits of pairing cores with either green sand or nobake molds based on a specific part’s requirements.
Several of the benefits of both nobake and green sand molding processes overlap, but a few general questions should help in ultimately deciding:
Is it a high volume part?
Is it larger than 20,000 lbs.?
Is it a relatively simple part with no cores?
Does it have a surface finish requirement?
Does the part have multiple interior passageways, holes, mounting pads or geometries?
Will the part benefit from added features that would reduce machining?
Is it a short-run or one-off part for which tooling costs should be kept to a minimum?
How thin are the part’s walls?
What are the tolerance requirements?
Anything that can be cast in a green sand mold can be cast in a nobake mold, but the reverse isn’t true. If a designer finds the metal casting supplier must add several costly steps to the green sand process through coring, coating, machining, pattern adjustments and trial runs to make a successful casting, the nobake process most likely will be the better choice.