SAND CASTING TECHNOLOGY

Most of the Japanese sand casting technology we saw has been disclosed outside Japan. In general, Japanese foundries use good foundry practice typical of American foundries. Much of the emphasis in Japanese foundry process development is in the area of automation with an emphasis on the use of pick-and-place robots to replace workers and to obtain more reproducible processes. Robotic core package assembly, robotic mold spraying, and robotic lost foam pattern assembly have been implemented in a number of plants.

Morikawa Lost Foam Process

Morikawa, a sand foundry and major supplier to Honda, developed its variation on the lost foam process and put it into production in October 1985. The WTEC team's hosts at Morikawa noted that pulling a vacuum on the sand as the casting was poured and solidified helped to make the mold more rigid, but the complexity of fitting each mold with a vacuum hook-up and the energy costs involved posed a problem. Their solution was to burn the vapors of the evaporating pattern above the mold, thereby drawing the gases up out of the mold and creating a partial vacuum in the mold.

This process, implemented and licensed to several other foundries in Japan, is available to foundries in other countries. While it clearly is effective in combusting pattern vapors (there is no lingering styrene odor, as in some lost foam operations), the use of a vacuum has not been universally embraced as being absolutely necessary for successful lost foam casting. In addition, because the amount of vacuum which can be pulled in this manner is limited to about one meter, the casting clusters are shallow. This limits the number of castings that can be poured at one time.

Morikawa cooperated with and strongly encouraged Mitsubishi Yuka Badische, a Japanese joint venture of Mitsubishi Chemical of Japan and Badische of Germany, in the development of a lost foam pattern material, Clearpor. This material burns more cleanly, thus decreasing the environmental load from the process and reducing lustrous carbon defects that otherwise may occur in the lost foam process. Morikawa's analysis of the requirements of lost foam has attracted interest in the United States, but no foundry has licensed this technology. Morikawa has also developed automated pattern assembly techniques which reduce manpower in the cluster build operation. Castings produced by their process include bearing caps, automotive and fork lift truck differential gear cases, Diesel engine turbocharger center housings, Diesel truck engine exhaust manifolds, electric motor stators weighing up to 13.6 kg, and large diameter pipe fittings weighing up to 42 kg.

It is reported that 100 foundries in Japan are interested in the lost foam process and that 40 are in production with it today. Principal markets for lost foam castings in Japan are automotive, engine and powertrain, and water distribution and treatment systems.

CADIC Convert Mold Process

This process converts resin-bonded cores to silicate-bonded cores suitable for use in preheated molds, thus introducing the possibility of using sand cores and molds at elevated temperatures, as in the pouring of thin section castings where mold preheat is required to impart the requisite fluidity to the metal. In the convert mold process, conventional resin-bonded molds and cores are immersed in a sodium silicate solution, which is allowed to fully penetrate the sand body. Then the mold is fired at an elevated temperature, burning out the resin and setting the silicate bond. The core or mold can be poured immediately, at elevated temperatures. This allows the pouring of ferrous alloys in thin sections or under conditions where the cooling rate can be slowed to improve casting characteristics. The sand cores can be removed from the castings without problem.

This process has not been fully characterized, so its limitations are not known. A preliminary assessment suggests that the potential for manufacture of thin section iron and steel castings may be limited because of inherent problems in attaining the required dimensional accuracy from resin-bonded sand. The sodium silicate solution will be sold commercially worldwide, so foundries interested in evaluating the process should be able to do so. Both monolithic and shell sand molds can be treated by this process.

Hitachi HMRAC Process

This process involves the application of a vacuum during pouring to produce thin section (2.5 - 3.0 mm thick) ferrous castings. Castings are gravity poured while the vacuum is applied through the bottom of the flask. Then pressure is applied to aid feeding. Only Hitachi currently uses this process in production.

Hitachi engineers also have been developing methods of casting thin wall cast iron. They have developed a method of making castings with walls as thin as 2.5 mm without carbide formation. One of the major problems they encountered was with dimensional control of the castings; these they approached by using shell cores in a permanent mold and adjusting the position of the cores by set screws.

Other Sand Casting Advances

Kubota Porous Core Box

Kubota has developed a porous core box material using powder metallurgy techniques. The material has a controlled porosity, which can be varied from 15% to 30%. This suggests that it would be useful for introducing catalyst gas and purge air into core boxes, thus eliminating the problem of gas and purge vent location. However, there is no data about whether cured resin builds up in the pores, blocking them. This is a new development, and further study is needed.

Kubota Lost Pattern Material

Kubota has also introduced the use of naphthalene as a pattern material for lost pattern processes. Naphthalene can be easily formed by injection molding and should burn out cleanly. However, it is a polycyclic aromatic (a double benzene ring), which in the United States is on the EPA list of industrial substances that must be controlled, and it is doubtful whether U.S. foundries would be interested in using it for this reason. There is also an odor problem.

Toyota Differential Pressure Process

This process, currently applied to permanent mold castings, is also applicable to sand castings. Toyota also pulls a vacuum on its core boxes when making the lacy cores required in cylinder heads. The core room on this production line makes use of automated guided vehicles to transport the cores to the molding line. Robots load and unload the cores so that breakage due to human clumsiness is avoided.

Komatsu Automated Jolt-Squeeze Machine

This machine is of interest because it runs a circulating pattern loop that can cycle four patterns at once.

European Advances in Sand Casting

European foundries appear to be more willing to risk breakthrough technologies in developing new sand casting techniques than Japanese or U.S. foundries. One example is the VAW Alucast plant in Dillingen, Saar. This plant, which produces aluminum automotive engine parts (blocks and heads), is designed to operate in a completely automated mode. The molds are made of core "packages," which are built up of resin-bonded cores, injected, and assembled automatically (in the case of heads) and semiautomatically (in the case of blocks). These packages progress to a pouring station where they are automatically poured. The castings and core packages then move on a cooling line until they have cooled below the solidus temperature.

At this point they move into a heat treat furnace, saving the cost of reheating the castings. Some of the heat for the furnace is provided by the burning resins. As the resins are combusted, the sand falls to a conveyor that cools it and carries it back to the core making stations. The castings are air quenched when they exit the furnace; then a robot picks each one up, manipulates it so that the sand remaining in it is poured out, and places it on a shipping pallet.

Another concept is under development by Disa, which recently merged with Georg Fischer. This concept is aimed at the aluminum casting market and is based on recent work by the University of Birmingham, which showed that controlled filling of the mold from the bottom eliminated the formation of oxide folds in the casting, which are responsible for most casting failures. The method uses the Disa vertical parting line flaskless mold and introduces the metal from below using an electromagnetic pump. Pressure may then be applied to the risers to increase their efficiency. A further enhancement appears to be possible by using magnetite ore instead of silica sand; significant refinement of dendrite arm spacing is reported. The equipment has not yet been offered for sale commercially; if it becomes a commercial success, it could challenge permanent and semipermanent molding of automotive components.

Europe is behind the United States in lost foam technology, although there have been a number of research projects in the area, particularly in France. Pechiney has developed a lost foam technique that carries out the solidification of lost foam castings under a controlled atmosphere and a pressure differential. However, the persistence of folds in the casting, which cannot be detected by nondestructive testing, has prevented its market penetration.

In summary, Japanese foundries have made some significant contributions to foundry process technology in the years since World War II. However, they appear to be lagging behind Europe in the development of innovative casting techniques today. In contrast, Europe appears to continue to lead in the development of advanced sand casting systems.


Published: March 1997; WTEC Hyper-Librarian