Hill and Griffith
Nobake: The Basics (Including Mold Coatings)
This article focuses on the nobake process and when it would be appropriate for use.
An MCDP Staff Report Click here to see this story as it appears in the January/February 2018 edition of Metal Casting Design & Purchasing
Dozens of chemically bonded sand molding methods exist, but they can be divided into three main categories: coldbox, heat-activated and nobake. The basic principle is that a binder and catalyst are mixed with the sand to help form the mold into a “brick-like” product when cured. The differences in the process focus on the sand resin binder and catalyst used and how the mold or core is cured.
Coldbox—with this method, sand is coated with one of several binders, such as liquid sodium silicate or phenolic urethane, and catalyzed by a gas (such as carbon dioxide or sulfur dioxide) passing through the sand. This causes the resin binder to harden (cure) and lock the sand grains in place to maintain a solid mold wall.
Heat-Activated/Shell (also called hotbox and warmbox)—heat is used as the curing means in shell molding. Plastic resin-coated sand is compacted around a pattern and allowed to rest until a “shell” forms. The mold then is heated to temperatures higher than 500F (260C) to cure the mold.
Nobake (also called airset, dry sand and precision sand)—like coldbox, several binders are optional. However, a liquid catalyst is used. The sand is processed in a continuous mixer and then formed around the pattern until it is fully cured.
Mold coatings are applied to more than 75% of noble molds. The coatings help prevent defects and improve surface finish.
In the nobake process, sand is mixed with a chemical binder/catalyst system and then molded around the cope and drag halves of the tooling. After a specified period of time (from as little as 10 seconds to as long as the metalcasting facility requires depending upon mold size), the sand mixture hardens (resembling a brick in strength) to form the mold halves, and the tooling is drawn. Then, a refractory coating may be applied to both mold halves before they are brought together to form one complete mold for pouring. (Nobake molded cores also can be produced using a similar method and assembled into the mold to form more complex shapes.)
Nobake molding, like green sand molding, is known for its versatility. Virtually all metals can be cast via nobake molding with component weights ranging from less than a pound to several hundred thousand pounds. For casting designers, nobake molding offers:
Good dimensional tolerances (±0.005-0.015) because the rigidity of the mold withstands the pressures exerted by the molten metal during casting.
Compatible with most pattern materials, including wood, plastic, metal, fiberglass and polystyrene, allowing for inexpensive tooling options for casting runs as low as one. In addition, nobake molding imparts minimal tooling wear.
Design flexibility for intricate casting shapes. The rigidity and tensile strength of nobake molds allows for thin sections of 0.1-in. to be routinely produced. In addition, mold strength allows for minimal draft and radii requirements in casting design.
Reduced opportunity for gas-related defects as the nitrogen content of most binder systems used for nobake molding minimize susceptibility to gas porosity.
Fine surface finishes that can be upgraded further with the mold and core coatings to support special finishing on the cast components such as paint or dressing. In addition, nobake casters can alter their molding media make-up from basic silica sand to higher-end media such as chromite or zircon sand for applications requiring X-ray quality and extreme pressure tightness.
Ability to work well with unique metalcasting quality enhancement tools such as metal filters, ceramic runner systems and exothermic risers to improve casting properties.
Low to medium volume production capability with runs from 1-5,000 parts per year.
The key, as with any casting process, is to ensure the casting design is optimized to take advantage of the benefits afforded by nobake molding.
With the multitude of casting processes available to casting designers and purchases to produce their components, when should nobake molding be selected?
Nobake molding typically is an option for production runs from 1-5,000 castings/year. Due to the curing time required for the chemicals to harden the mold, as well as the methods to distribute the molding media on the pattern, the high productions achievable with green sand, permanent mold or diecasting aren’t possible with nobake. Nobake molding is most often best used to cast components with higher complexities in low to medium volume runs.
Anything that can be cast in a green sand mold can be cast in a nobake mold, but the reverse isn’t true.
Besides the number of castings that need to be produced, the decision between green sand and nobake comes down to the complexity and size of the casting design.
Nobake molds can take on castings of all shapes and weights. Depending on the nobake process, cast components typically range from 1 to 50,000 lbs. (0.45 to 22,680 kg). Often, the nobake process is a good fit for multi-ton castings, such as diesel engine blocks for heavy industrial applications or parts for wind turbines.
Large, multi-ton castings that are too large for a flask are produced via pit molding, in which the pattern is set in a pit and nobake sand is compacted around the sides. Large castings are cooled slowly, often over the course of few days, to prevent excessive internal stress.
The nature of the chemically bonded sand in nobake casting allows for rigid mold walls that can sustain the pressure of a large amount of metal (molds for extremely large castings still require internal rodding and other support). Sand in nobake molds has less of a chance of breaking or moving than green sand. This is because the cured sand tends to be more structurally stable than green sand.
Since unfinished nobake molded castings (without machining) typically cost more than green sand, designers and purchasers sourcing to nobake molding must offset this price difference by taking advantage of what the process offers. Significant reductions in machining costs can be achieved through the process’ tight tolerances and minimal dimensional variability and by designing in complex shapes and geometries, thin walls, and reduced draft, radii and machine stock.
Tooling cost also plays a factor in choosing between green sand and nobake molding. Green sand molds require compaction force during the molding process, which means that the tooling must be able to withstand this force. Nobake tooling doesn’t have to withstand a strong compaction force (often only light vibrations), allowing wood and plastic to be viable tooling materials. In addition, the lack of compaction force in molding also allows nobake molders to use loose pattern pieces and other tooling options to increase casting complexity and add design features to the components.
Pattern materials for nobake molding include wood, plastic, fiberglass, metal and polystyrene. This allows the tooling cost to be minimized as much as, if not more, than any other production casting process.
Designing castings for traditional nobake molding follows many of the same principles used in all other casting processes. Draft is required so patterns can be drawn, sharp corners and angles should be minimized, and uniform section thicknesses (especially in the same plane) should be employed as much as possible. However, the process does allow for more daring designs. Consult a nobake facility with your ideas to determine how best to accomplish a specific casting challenge.
Nobake molds can be made via 3-D sand printing, which removes the necessity of tooling.
This article is an excerpt from Designing & Purchasing Metal Castings.
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