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Deep Cryogenic Treatment of Castings, Tools, and Components
Improving Die Casting Productivity by Increasing Die Life
By Dr. H. E. Trucks, PE
Cryogenics International, Scottsdale, Arizona
In today's highly competitive world, finding ways and means to reduce the cost of a product is never ending. In the various production methods of manufacturing a part, the cost can be reduced by improving the life of tools used by the various processes used for production.
Cost reduction for die castings can be achieved by good casting design, reduction of costly specifications that are not needed for the product, and selection of the proper alloys that are functional for the end use of the product. Die castings offer closer dimensional tolerances than any production casting process producing comparable results. However, as in any other casting design, tight tolerances should be specified only for dimensions which so require.
Cost Analysis Recommended
The design engineer should make a cost analysis to determine if it is more economical to leave machining stock on the critical areas in order to improve die life and reduce die cost. If the analysis shows that there will be a cost reduction by machining the critical areas, then this is the approach to take. However, the designer also must take into consideration that all aluminum castings (including die castings) have built-in stresses due to shrinkage of the molten metal after it cools in the die. Some of these stresses in some areas can be reduced by good casting design. The proper casting alloy also is important in stress reduction. However, if there is no surface machining, these stresses have little or no effect on the function of the casting.
Die castings that have to be machined for closer tolerances and flatness will most likely distort on these surfaces, which in turn adversely affects these critical areas. Distortion starts when the outer layer is removed by machining. The outer layer locks in the stresses, and-when removed-leaves the machined surface distorted.
Stress Relief Through Cryogenics
What can be done to relieve these stresses before machining? Through advances in cryogenics, it has been discovered, researched, and proven that when these castings are treated in a cryogenic chamber at temperatures of 88°K (-300°F) at a controlled time/temperature cycle, all internal stresses are relieved, thanks to the alignment of the molecular structure that had been distorted during the casting process. When these cryogenically treated castings are machined, there is no distortion on the machined surface; any distortion which may occur is caused by improper machining, dull cutting tools, too slow speeds/and feeds for the aluminum alloy, or insufficient material removed by the first cut. The cryogenic treatment is not only for aluminum and its alloys but for all other die casting metals with the exception of the zinc alloys. The zinc alloys are no longer ductile at temperatures lower than -30°F. This cryogenic process also can be used to good advantage for die casting dies and the cutting tools used to produce the dies. In recent years, improvements have been made in the production of hot work tool steels and high speed tool steels that are used to cut the die cavity. Since producing hot work tool steels is very expensive, it is vital that the tool steel be such that a long and uniform tool life can be expected.
What's Behind the Benefits of Cryogenics?
Cryogenics is an exciting and important frontier that already has led to major discoveries...and holds much future promise. In the case of steel and its alloys, cryogenic treatment removes the built-in kinetic energy of atoms, which is the energy of motion. There is a normal attraction between atoms that makes them want to get together. But their energy of motion keeps them apart unless that energy is removed by low temperature cooling. After heat treatment of the dies, the final treatment at temperatures of 88°K (-300°F) transforms soft austenite into hard martensite. This transformation improves die surface wear and heat resistance; more parts can be produced before costly rework is required. This reduces the scrap rate of the casting, thus improving the production rate before rework of the tooling is required.
Cryogenics International, Scottsdale, Arizona (480/991-0299) designs and manufactures electronically controlled cryogenic chambers (which are available for the treatment of aluminum products) and the required tooling to produce them.
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