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quantity of parts required per year or batch, as this will affect machine size and manufacturing costs and be used to estab- lish the die design. Casting quality criteria, finish, accuracy and other fac- tors influence die design. During the quotation stage, the die designer can calculate the feed system and gate size needed to achieve the finish, cycle time and machine size required, as well as predict the flow path of the molten material in the cavity. Te size of ejector pins and position of fixed and sliding


cores are critical to the performance of the die. Tese factors are influenced by the casting profile, section thicknesses, aesthetic features and limitations. Positioning ejector pins on areas of the casting thick enough to support the ejection force when the casting alloy is hot and ductile will reduce surface bumps. Placing ejectors on bosses within the casting can limit this problem, as can the use of vertically gated ejec- tor bosses outside the casting edge. However, these mea- sures add weight and cost to the casting and/or shot. Sleeve ejectors can be used to apply force directly around a problem core or casting feature where zero draft cores are employed. Te designer should introduce die features that allow


adjustments of die faces to remove flash, such as wear pads and overflow wells, which change fill patterns and remove trapped air/fumes in the cavity. Vacuum technology can be used if casting performance requires minimal levels of poros- ity. Te use of robotics will have design implications and may lower the casting-to-shot weight yield ratio. Te surface finish of any zinc alloy die casting relies on


die surface temperature, which is controlled with water or oil pipes designed into the die. Te size and position of the pipes are determined at the die design stage and will be related to the weight/volume, number of cavities, cycle time and finish required. Tis is also influenced by the cavity fill time and gate velocity, which can be correlated to the diecasting machine performance and requires optimizing gate and runner size for efficient operation and yield. Die face temperature requires consistency and control.


To achieve the defect-free matte finish required for elec- troplating with minimal polishing, the die temperature must be controlled at an optimum 356F (180C). Higher temperatures may be required if the casting has longer flow distances or where the casting has thin sections less than 0.05 in. (1.25 mm). Achieving the necessary surface finish in these cases requires good die and metal injec- tion temperature control, as well as a cavity fill time of less than 20 milliseconds. These parameters should yield an end of fill metal temperature greater than 761 (405C) in the cavity and prevent the formation of oxide surface defects and cold shuts within the casting. Te elimination of this type of defect also requires a


smooth cavity fill pattern designed to reduce oxide generated by turbulence in the die cavity. To maintain these conditions, the diecaster must create consistent process conditions, cycle times and interruption-free running.


Die Lubrication In zinc alloy die casting, die spray is a lubricant, not a cool-


ant. It is normally water-based soluble oil (for health, safety 28 | MODERN CASTING September 2012


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