Materials & Processes in Manufacturing ME 151 Chapter 15 Multiple Use Mold Casting Processes 1 Introduction Expendable Molds - melting point materials and castings General shortcomings of the expendable-mold processes: Dimensional Variations Property Variations Relatively Low Production Rates Multiple-Use-Molds - melting point materials and castings General shortcomings of the multiple-use-mold processes: Limited to the lower melting point nonferrous metals and alloys Part size is limited Cost of tooling Factors that affect mold life: Alloy being cast Mold temperature Mold material Mold configuration Pouring temperature 2 Multiple Use Mold Casting Processes Permanent Mold or Gravity Die Casting Slush Casting Corthias Casting Low-Pressure Permanent Mold Casting Vacuum Permanent Mold Casting 3
4 Permanent Mold or Gravity Die Casting Molten metal poured under the action of gravity alone Reusable Mold Good Surface Finishes Good Dimensional Accuracy Directional Solidification Fast Cooling Rates Casting Examples: Lower Melting Point Alloys Relatively Short Mold Life Restricted Complexity of Mold Risers Still Required Aluminum Pistons, Cooking Utensils, Electric Irons, Gear Blanks Table 15-1 Page 327 (Process Overview) Slush Casting Molten metal remains in the mold until a shell of desired thickness forms Reusable Mold Good Surface Finishes Good Surface Detail Fast Cooling Rates Casting Examples: Lower Melting Point Alloys Variable Wall Thickness Ornamental Objects, Candlesticks, Lamp Bases, Statuary 5 Corthias Casting Uses a plunger to create a positive pressure and force molten metal into outer portions of the mold cavity Reusable Mold Good Surface Finishes Good Surface Detail Thinner Sections Lower Melting Point Alloys Casting Examples: Thin Walled Ornamental Objects 6
7 Low Pressure Permanent Mold Casting Molten metal pushed into the mold using low pressure (5-15 PSI) Wide Variety of Metals Metal not Exposed to Atmosphere Mold Filled in Controlled Manner No Risers Needed Directional Solidification High Yield Rates Better Mechanical Properties Slow Cycle Times Figure 15-2 Page 327 Vacuum Permanent Mold Casting Molten metal pulled into the mold using a vacuum Same as the Low-Pressure Process Thin Wall Castings Excellent Surface Finishes Superior Metal Cleanliness Low Dissolved Gas Content Better Mechanical Properties Slow Cycle Times Figure 15-3 Page 328 8 Die Casting Molten metal forced into mold under high pressure and held under pressure during solidification Fine Sections & Excellent Detail High Production Rates Trapped Air (Porosity) Excellent Surface Finishes Good Dimensional Accuracy Long Mold Life Special Casting Alloys with Excellent Properties Minimal Machining Table 15-2 Page 331 (Process Overview) 9
10 Types of Die Casting Dies Figure 15-4 Page 329 2 Types of Die Casting Machines (Gooseneck) Machines Lower melting point metals (Zinc,Tin, Lead Alloys) Melting furnace part of the machine Fast cycle times (up to 15/min) 1 oz to 90 lb castings Figure 15-5 Page 329 11 2 Types of Die Casting Machines Machines Higher melting point metals (Brass, Aluminum, Magnesium) Melting furnace separate from machine Longer cycle times Figure 15-6 Page 330 12
13 Other Multiple Use Mold Casting Processes Casting (Liquid-Metal Forging) Centrifugal Casting (Figure 15-8) Semicentrifugal Casting (Figure 15-11) Centrifuging (Figure 15-12) Continuous Casting (Figure 6-5) Electromagnetic Casting Melting Requirements for Casting The source that provides the molten metal for the casting operation should meet the following criteria: Provide adequate amount of material or meet required melting rate Provide material at the necessary temperature for pouring Maintain the desired chemistry and quality required Minimize contamination Hold the material without deterioration of quality Economical to operate No pollution 14 Melting Procedure Selection Factors that determine which melting procedure to use: Temperature needed to melt and superheat the metal Alloy being melted Desired melting rate or quantity of metal Desired quality of the metal Availability and cost of fuel Variety of metals or alloys to be melted Batch or continuous melting Required level of emission control Capital and operating costs 15
16 Melting Methods Virtually all foundries use one of these types of furnace Indirect Fuel-Fired Furnaces Air Furnaces or Direct Fuel Fired (Figure 15-14) Furnaces (Figures 15-15 & 15-16) Induction Furnaces (Figures 15-17 & 15-18) Cleaning, Finishing, & Heat Treatment Most castings require some level of secondary processing such as: Removing cores Removing gates and risers (trim dies, saws, torches) Removing fins and flash (tumbling) Cleaning (tumbling, shot blast) Repairing defects (welding) Heat treatments (annealing, stress relieving) Inspection (liquid penetrant, x-ray, etc) 17 Chapter 15 Multiple Use Mold Casting Processes Difference between expendable-mold & multiple-use-mold casting General understanding of die casting & the two basic types of machines Basic concept of the different multiple-use-mold casting processes General understanding of the different types of melting methods, what they are used for, their advantages & disadvantages General understanding of cleaning, finishing, & heat treatment of castings Review Questions: 18