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and environmental reasons) and is sprayed in mist form onto the die faces in sufficient volume to maintain the die temperature while evaporating the water content of the spray. Under ideal conditions, evaporation


leaves the die face covered in a micro- scopic coating of lubricating oil. Control of the volume of spray and use of the correct type of lubricant are critical to maintaining good casting surface finish. Surface defects can arise from oil deposits burning onto the die cavity face and forming fine coking, which shows on the casting surface. Tis is usually the result of exceeding the die temperature guidelines (more than 392 F [200C]), using an oil rich die spray, improper selection of lubricant or excessive application of lubricant. Te applica- tion of die lubricant to a die that is too cold (less than 320F [160C]) will result in cold flow defects, excessive porosity from fumes and steam, and interrupted cycling, which exacerbates the problem.


Post Casting Defects In order for a zinc alloy die casting to


accept high quality finishes, several stages must be considered after the part is com- plete. Most of these stages involve post casting operations that identify, create or reject defective parts after finishing. Secondary operations include clip-


ping, de-gating, grinding and polish- ing, vibro- or mass media finishing, machining and plating/painting, among others. Rejects can arise from each of these operations, but from a finishing point of view, some forms of reject can be ignored, such as machining scrap due to dimensional inaccuracy. Part movement after casting gener-


ally is responsible for the same number of rejects or surface defective parts as the casting process. Often, the level of polishing or mass media finishing is insufficient to remove the edge cuts, nicks and impingements introduced by multiple handling points after the casting leaves the die. After the casting cycle is complete, the part is ejected and, in some cases, dropped onto a conveyor or slide tray and into a steel or plastic storage pan. During this fall, minor impacts with steel edges and other castings can occur, causing impingement or nicks on susceptible faces or edges. Te die


September 2012 MODERN CASTING | 29


designer should consider this factor when positioning the casting in the die. Ejector and face pins and core slide edges on both die sides need to be accommodated when the shot falls forward or off the ejector pins. Te same discipline should apply during the clipping stage. Te use of robotic pick-and-place units reduces the risk of impingement damage in many


ONLINE RESOURCE


Visit www.moderncasting.com to read another article on avoiding casting defects.


instances, assuming the same precautions are put in place to prevent it. Brock Metal is an independent supplier of zinc alloy ingot; for more information visit www.brockmetal.com.


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