The shell process: taking a new look.

Environment and casting quality benefits are creating new interest in the use of this well-known sand core and mold process.

The shell process, developed in Germany during World War II by Johannes Croning, is still referred to as the Croning process in many parts of the world.

The Germans used the process experimentally during the war to produce mortar and artillery shells and other projectiles. Though the Germans tried to keep the process secret, it was discovered by a U.S. government agency and made public in 1947.

The initial process blended raw sand with a mixture of powdered phenolic resin and hexamethylenetetramine (hexa). The dusty resin blend could be used for making molds or fabricating cores only where it could be introduced into the corebox by gravity.

Many suppliers and users continued to develop the process, and in the mid-1950s, a liquid phenolic resin was created by dissolving the resin in an alcohol solvent. The liquid resin assured that each sand grain was coated with resin before the solvent evaporated. An added release agent allowed the dry, free-flowing, resin-coated sand to be blown to produce cores of many shapes and solid sizes without separating the resin from the sand.

Further work in the 1960s led to the development of flake resins that incorporate release agents, are easier to handle and contain no flammable solvents. Today, flake resins account for more than 85% of shell resins.

The shell process gets its name from its thin-walled, hollow cores and thin, lightweight molds that follow the contours TABULAR DATA OMITTED of the coreboxes or patterns. Shell cores are used in many U.S. foundries and about 280 foundries also use the process to produce molds.

The shell process' chemistry is relatively simple: phenolic novolak resin and hexa are combined and coated on sand. When the resin-coated sand is exposed to a heated (450-600F) corebox or pattern, the resin melts and bonds the sand to form a cured shell core or mold.

Environmental Regards

For years there has been considerable discussion of the environmental concerns associated with phenolic-based foundry resin binders. Shell process resins and resins from many other chemical binder processes are phenol-based. Controlled tests (Table 1) show that waste sand from shell cores and molds often contain less leachable phenols than waste sand from most other common core and mold processes.

In past years, decisions on the selection of a chemical binder core or mold process were made without much thought to environmental impact even if the binder were phenol-free. In today's ecology-minded world, decisions based on scant information or unfounded assumptions are no longer adequate.

Phenols can be formed as a result of high-temperature decomposition or the rearrangement of organic binder molecules. Table 1 lists leachable phenol levels from waste sands from various common chemical binder systems.

Shell process waste sands have the lowest leachable phenol values because of their low sand-to-metal ratios (0.3:1 to 1:1) and the fact that hollow shell cores result in a more complete binder burn-out during pouring. The high density of shell cores and molds aids heat transfer, a factor that assists burnout.

The data in Table 1 was generated from typical waste sands, but it should not be used to predict exact leachable phenol levels. Any given foundry practice can affect the level of leachable phenol. Other methods for testing leachate levels, such as the toxicity characteristic leaching procedure (TCLP), may result in slightly different leachate levels but shell process waste sands will be relatively low.

Formaldehyde

The final regulation on occupational exposure to formaldehyde was issued last year by the Occupational Safety and Health Administration (OSHA) and contains several new requirements. It lowers the permissible exposure level (PEL) for formaldehyde from 1 ppm to 0.75 ppm.

In addition, the regulation mandates annual training for all employees exposed to formaldehyde between 0.1-0.5 ppm for an eight-hour, time-weighted average. This undoubtedly will affect foundries using binder systems containing and emitting formaldehyde.

Shell resins and resin-coated shell sand essentially contain and emit no formaldehyde during core- and mold-making, unlike some commonly used chemical systems. Small amounts of phenol and ammonia are emitted, but at emission levels generally well below currently established threshold limit values (TLV).

Resin-coated shell sand contains no organic solvents, making it low in content and emission of volatile organic compounds (VOCs).

Waste Minimization

The shell process minimizes waste in three areas:

* Resin-coated shell sand has an indefinite bench life and remains usable for many years when properly stored. This eliminates the normal need to dispose of mixed sand that has exceeded its useful bench life and cannot be used in core or mold production. Disposal of sand/resin mixtures from any chemical process where the mixture has not been reacted or cured by exposure to metal pouring temperatures may increase leachate levels because none of the resin has been cured or burned away.

* Cured shell cores and molds have an indefinite shelf life and are unaffected by storage in high temperature/humidity conditions. This eliminates disposal of cores and molds that have lost strength in storage. Disposal of cores or molds not exposed to metal pouring temperatures may increase leachate levels.

* Most shell cores made today are hollow, use less sand to be reclaimed or disposed compared to solid cores. Shell molds typically are lighter in weight and, therefore, have lower sand-to-metal ratios than the same mold produced by another process. Shell process sand also can be thermally reclaimed and reused, and is less affected by contaminants in new or reclaimed sands, an advantage where sand disposal and new sand purchase costs are high. In addition, the chemistry of the shell process is relatively unaffected by acidic or basic sand components.

Casting Quality

In addition to its environmental characteristics, the shell process is used by many foundries to produce precision castings. Japanese foundries use it more extensively than North American foundries.

Several Japanese automakers report using the process for 90-95% of their core work. Many Japanese-owned metalcasting operations in North America mandate that suppliers use the shell process.

There are three paramount reasons why many casting purchasers and users specify the shell process:

* Superior casting finish--Resin-coated shell sand is dry and free-flowing, unlike resin/sand combinations in other processes. This allows cores to be blown to a greater density, producing castings with cored surfaces having excellent finish often without the use of a refractory coating. Shell molds produced by gravity flow of resin-coated sand onto the heated pattern also produce castings with excellent finish. The free-flow characteristics of shell process sand often provide the required casting finish by using a coarser, more permeable sand than would be required using an alternate process.

* Superior dimensional tolerances--Shell process cores have higher hot-strength than any other commonly used coremaking processes (Table 2) and produce castings with precise dimensional tolerances.

* Reduced gas defects--Hollow shell cores can provide ample venting of gases even when fine sands are used. When hollow cores are combined with the increased permeability of coarser sands, castings can be manufactured without the gas defects sometimes present when using solid cores. The process limits the volume of sand used due to hollow cores and low sand-to-metal ratios, reduces dimensional variations and produces an excellent casting finish.

These advantages have assured the continued broad use of the shell process worldwide and have encouraged expanding applications in North America.

Bibliography:

Formaldehyde Institute, Washington, D.C.

Commercial Sand Coaters Assn., Aurora, IL

C. K. Johnson, "Phenols in Foundry Waste Sand," modern casting, Jan. 1981

S. Raja Iyer and W. Ward, "Bonding Properties of Core Process Binders on Reclaimed Spent Sands Containing Bentonite," Acme Resin Corp., Forest Park, IL, March 1992.

Metal Casting Industry Directory, Penton Publications, Cleveland, OH, 1992 Ed.