Prototyping process produces sand molds, cores for production castings. (Technology in Progress).

Time to market is a critical measurement for today's OEM. It determines how efficient manufacturers are in developing, implementing and producing ideas generated.

As suppliers to OEMs, foundries are a vital link in the time-to-market chain. The faster foundries are able to deliver prototypes and first-run cast components to an OEM for testing and initial production runs, the sooner foundries will be perceived as valued suppliers and castings as quick solutions.

An obstacle facing sand foundries in the production of prototype cast metal components is finding a prototyping process to produce sand molds and/or cores. For efficient and optimal testing of form, fit and function by the casting end-users, sand molds and cores must be used to produce prototype sand castings to ensure that the prototypes have the same material and mechanical properties as those that will be manufactured in full-scale production.

Joint Development

In 1999, International Truck & Engine Corp., Melrose Park, Illinois, was searching for a method to speed up the development process for its engines. Specifically, the firm wanted to improve the development process for its cast cylinder heads and blocks, manifolds and housings--the foundation components for the diesel engines it produces.

The manufacturer teamed (via a joint development agreement) with Clinkenbeard & Assoc., Rockford, Illinois, in search of a sand casting prototyping solution. Combining the international engine group's foundry and sand casting experience (it has foundries in Waukesha, Wisconsin, and Indianapolis) with Clinkenbeard's rapid prototyping knowledge, the Clinkenbeard Process was born (and a patent has been awarded) as a method to machine sand mold copes and drags and cores from blocks of cured sand. These molds and cores are ready for pouring to cast production-equivalent sand castings.

Sand Mold & Core Prototyping

As with all castings, this prototyping process begins with a CAD file of the component as delivered by the casting design engineer. This file then must be reconfigured, following steps similar to those used to machine patterns. For cores, the internal passages must be separated from the external geometry, the core prints must be designed and, using computer aided manufacturing software, the tool paths must be brought together. Once the mold and/or core design is complete, it is inputted into the CNG machining system configured for the sand block. The system runs automatically.

The cured sand blocks for machining have been produced via the nobake and coldbox processes with binder levels as low as 0.8-1% and as high as 3%, depending upon the application. The key to this stage of the process is efficient sand mixing to ensure that all the grains are coated thoroughly with binder. Therefore, when the reaction (through mixing or gassing) occurs, the entire sand block is cured through. Although some soft pockets of undercured sand in core and mold production can be ignored in production foundries because they will not affect the casting, a soft pocket uncovered during sand block machining will result in a scrapped core or mold.

Machining a block of cured sand is a more delicate process than machining metal. Sand machining is a dry process without lubricant. When machining the sand blocks, a fluid diamond cutter passes more times across the block of sand to machine the shape of the mold and/or core. The sand block will chip, crack and/or fragment with any imperfection (inconsistency in curing).

The largest cores and/or mold halves produced measure up to 30 x 60 in. This allows large cores and/or molds to be produced in one piece as well as the production of multiple cores and/or molds from one sand block. Once machined, the cores and/or molds are ready for casting.

The downside to the core and mold production system is the surface finish of the final prototype castings produced. The passes made by the diamond cutter on the cured sand block result in a stair-stepping surface on the resultant cores and/or mold halves, which is then replicated on the castings. This downside can be controlled by the cutter direction, cusp height and/or the use of hand finishing on the affected area of the mold/core. Another factor that must be considered is the size of the machining envelope.

Currently, the process has been used to produce mold halves and cores for iron, aluminum and magnesium prototype casting. Once the appropriate binder system has been developed, steel casting production will be available.

In Application

Current production times for cores and/or molds are based on the complexity of the casting design. After three years of production experience, cores and molds for components such as manifolds, aircraft pumps and truck cylinder heads have required machining times from 1-24 hr.

This prototype process is most efficient when machined sand cores (often the more complex portion of the mold) are used in conjunction with molds made from rapid tooling. Prototypes produced thus far with this new process have shown a 50% reduction in lead time from design to casting compared to the use of current rapid tooling methods alone, and an 80% reduction compared to traditional production methods.

The key to this prototyping process is its ability to produce cast prototypes in a similar method as those that will be cast in production. As a result, mechanical and material property testing will be accurate. For sand casting foundries, this prototyping method can provide a way to bridge the lead-time gap and provide cast components before production-ready tooling is available.

For More Information

Proceedings from the 2001 Society of Manufacturing Engineers (SME) Rapid Prototyping & Manufacturing Conference, SME, Dearborn, Michigan (2001).

"Evaluating RP Methods: NASA's Side-by-Side Comparison," K Cooper, G. Williams and P. Salvail, Engineered Casting Solutions, Fall 2001, AFS, Des Plaines, IL.

The Worldwide Guide to Rapid Prototyping at http://home.att.net/~castleisland/

About the Author

Ronald Gustafson is president of Clinkenbeard & Assoc., Rockford, Illinois.