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Metal Casting Technologies : December 2006
Back to the Practical quality control in the melt shop any work steps are required for the production of castings and these include factors which cause defects; but it is rather difficult to ascertain such causes. In order to identify the causes of defects, a certain amount of technological experience is required; but it is also necessary to standardize the technology and work contents for each plant and to establish the relationships between any recurring defects and the preset standards, i.e., the differences between the output and the required standards should be checked in order to pinpoint such causes. Standardization must be made for raw materials, works, preparations, etc. For example, a raw material acceptance standard must be established, and only materials which pass this standard should be used. Normally, it is desirable to adhere to the following standards: (a) Raw material acceptance and storage standards. (b) Melting work standards, including melt control. (c) Casting sand mixing work standards, including the mixing standard. (d) Molding and pouring work standards. (e) Cleaning work standards. (f) Product inspection standards, including the product standards. With regards to such specifications, for instance in the case of cupola melting, the size of the foundry coke to be used should be specified as 1/10 to 1/8 of the inner diameter of the cupola, and the ash content should be specified as 15% or less. With regards to the cupola charge, the mixing-in of material with a high chromium (Cr) content should be avoided. Such minimum standards are established for control of the process and, at the same time, they should be used as teaching material for the training of personnel.* The recommended minimum number of quality control facilities for the proper control of the various casting processes for the production of the various types of metals is given in the tabulation on the next page: *JETRO, Manufacturing Technology Guide: No. 13, Casting. (Tokyo, Japan: Japan External Trade Organization, 1984) pp. 9-10. REQUIRED ATTRIBUTES OF A GOOD FOUNDRY A. Has the ability to design castings and gating systems B. Uses dimensionally stable patterns C. Has the ability to make good molds and cores 1. Practices good sand control 2. Uses a good moldmaking process and practice D. Has the ability to produce good molten metal 1. Has the ability to select scrap properly 2. Uses a good melting process and practice 3. Practices good melt control E. Has the ability to produce good surface finish 1. Uses proper grades of sand for molds and cores 2. Uses surface treating facilities and practice F. Has enough skilled personnel for key operations QC CHECKPOINTS FOR PROPER FOUNDRY OPERATION A. CASTING DESIGN 1. Section design (knowledge) 2. Gating design (knowledge) 3. Design check (trial castings) B. PATTERNS 1. Material selection (knowledge) 2. Patternmaking process (equipment, skill) 3. Dimensional check (instruments, skill) C. MOLDS AND CORES 1. Sand control (sand testing laboratory) 2. Molding/coremaking processes (equipment, skill) 3. Dimensional check (instruments, skill) D. MOLTEN METAL 1. Charge selection (knowledge) 2. Melting process (equipment, skill) 3. Composition check (chem. testing lab, skill) E. CASTING PHYSICAL CONDITION 1. Pellet/abrasive selection (knowledge) 2. Cleaning process (equipment, skill) 3. Surface finish check (instruments, skill) 4. Soundness check (NDT instruments, skill) 5. Metal properties check (metallographic lab, physical lab, mechanical lab, knowledge, skill) SPECIAL QUALITY CONTROL TECHNIQUES Cause-and-Effect Diagrams Cause-and-effect diagrams (C&E Diagram, Ishikawa Diagram, or "Fish-bone" Diagram) have evolved from one-sided cause analysis, or a single-sided diagram, developed and widely acclaimed in Japan. Its creator was Professor Kaoru Ishikawa who devised this form of analysis in 1953 while doing quality-control consulting. He used it to communicate with managers who felt the complexities of quality problems were overwhelming when explained orally. Today, nearly all Japanese quality control texts contain a section devoted to the Ishikawa diagram. m Prof. J. Hermes D. Bautista, Technical Consultant, PMAI (Note: This paper serves two sections within this edition - Back to the Floor as well as being an excellent technical article to keep on file) METAL Casting Technologies December 2006 48