As the metalcasting industry’s second favorite method for producing cast components (green sand molding is the first), nobake molding has proven its worth as an efficient means to produce medium and low volumes of complex castings in both ferrous and nonferrous metals.
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 sec to as long as the foundry 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;
- compatability with most pattern materials, including wood, plastic, metal, fiberglass and styrofoam, 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 1-5000 parts/yr.
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.
When to choose nobake molding for production?
With the multitude of casting process available for casting designers and purchases to choose from to produce their components, when should nobake molding be selected? Although it is impossible to provide absolute guidelines to follow for casting process selection, the following questions should help aid your decision.
How many cast components will you require per year?
Nobake molding typically is an option for production runs from 1-5000 castings/yr . 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 prefers cast components with higher complexities in low to medium volume runs.
When should you choose nobake molding vs. green sand molding?
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 of the casting design. Since unfinished nobake molded castings (without machining) typically cost 20-30% higher 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 this comparison. 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 innovative tooling options to increase casting complexity and add design features to the components.
What about pattern cost because you only require a few parts?
Pattern materials for nobake molding include wood, plastic, fiberglass, metal and styrofoam. This allows the tooling cost to be minimized as much as if not more than any other production casting process. In addition, with the styrofoam option for the Full Mold process (see “Nobake Meets Lost Foam in Full Mold Process” sidebar), hard tooling doesn’t even have to be created for small production runs.
How do you design cast components for nobake molding?
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 foundry with your ideas to determine how best to accomplish a specific casting challenge.