Dozens of chemically bonded sand molding methods exist, but they can be divided into three main categories: coldbox, heat-activated and nobake.
Excerpt from the Metal Casting Design & Purchasing January/February 2018 issue.
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.
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.)
- 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.
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