Die casting - Nguyen Ngoc Ha

ducts 1.1. Advantages and Limitations of Die Casting • Advantages: – Economical for large production quantities – Good dimensional accuracy and surface finish – Thin sections are possible – Rapid cooling provides small grain size and good strength to casting • Disadvantages: – Generally limited to metals with low metal points – Part geometry must allow removal from die cavity 1.2. Common Characteristcs • Tolerances: Al and Mg  .002/in. Zinc  .0015/in. Brass  .001/in. Add  .001 to  .015 across parting line depending on size • Surface Finish: 32-63RMS • Minimum Draft Requirements: Al & Mg: 1° to 3° Zinc: 1/2° to 2° Brass: 2° to 5° • Normal Minimum Section Thickness: Al & Mg: .03 Small Parts: .06 Medium Parts Zinc: .03 Small Parts: .045 Medium Parts Brass: .025 Small Parts: .040 Medium Parts • Ordering Quantities: Usually 2,500 and up. • Normal Lead Time: Samples: 12-20 weeks Production: ASAP after approval. 1.2. Common Characteristcs • Wide range of shapes • Lower melting point alloys • High mold costs – large quantity product • High production rates with short cycle times • Extra dies required for trimming flash and runners 1.2. Common Characteristcs • Product weight <0.01 to 50 kg • Good surface finish, low porosity, capable of high complexity parts • Excellent dimensional accuracy +/- 0.001 tolerance • Takes several weeks before first product output • Produces 2-200 parts/mold hour • Requires approximately 10,000 parts for use 1.3. Method Application • High-melting-point alloys of aluminum, magnesium, and copper are commonly cast by this method, although other metals (including ferrous metals) can also be cast in this manner. Molten-metal temperatures start at about 600 ˚C for aluminum and magnesium alloys and increase considerably for copper- base and iron-base alloys. 2. DIE-CASTING ALLOYS ASTM Number Al Cu Mg Zn AG40A(XXIII) 4.1 0.1 max 0.04 Remainder AC41A(XXV) 4.1 1.0 0.04 Remainder A relatively wide range of nonferrous alloys can be die-cast. The principal base metals used are ZINC, ALUMUNIUM, MAGNESIUM, COPPER, LEAD and TIN. They are classified in two groups: 1. Low-temperature alloys (casting temp. below 5500C), 2. High-temperature alloys Zinc-Base Alloys : Over 75% of die castings are produced from zinc-base alloys. Melting point is around 4000C. So, they are cast by Hot-chamber die casting 2. DIE-CASTING ALLOYS • Aluminum improves mechanical properties. • Copper improves tensile strength and ductility. • Magnesium makes casting stable in microstructure. • Zinc alloys are widely used in: automotive industry, washing machines, refrigerators, business machines, etc. 2. DIE-CASTING ALLOYS ASTM Number Cu Si Mg Al Uses S12A&B -- 12 -- Remainder Large Intricate castings S5C -- 5 -- Rem. Gen. Purpose G8A 3 -- 8 Rem. High strength, res. corro. SG100A& B -- 9.5 0.5 Rem. Gen. Purpose, Good Property, excellent casting charac. SC84B 3.5 9 -- Rem. Good machinability and castability Aluminum-Base Alloys : They are used due to their lightness in mass and resistance to corrosion. Compared to zinc alloys they are more difficult to cast (melting point around 5500C). Since molten aluminum will attack steel if kept in continuous contact with it, the cold-chamber process generally is used. Principal elements used as alloys with aluminum are SILICON, COPPER, and MAGNESIUM. Silicon increases hardness and corrosion resisting properties, copper increases mechanical properties, magnesium increases lightness and resistance to impact. They are generally used in aerospace industry and production of pistons. 2. DIE-CASTING ALLOYS ASTM Number Cu Si Sn Pb Zn Uses Z30A 57 min -- 1.5 1.5 30 min Yellow brass, good machinability Z5331A 65 1 -- -- Rem. Gen. Purpose casting, Corr. res., castability Z5144A 81 4 -- -- Rem. High strength, hardness, wear resistance-but most difficult to mold Copper-Base Alloys : Die casting of brass and bronze have presented a greater problem due to their high casting temperature. Temperatures are around 870 to 10400C, which need heat resisting die material. Copper alloys are cold chamber die-cast. Copper-base alloys have extensive use in miscellaneous hardware, electric-machinery parts, small gears, marine, automotive and aircraft fittings, chemical apparatus, and numerous other small parts. 2. DIE-CASTING ALLOYS ASTM Number Al Zn Mn Si Cu Ni Mg B94 9 0.5 0.13 0.5 0.3 0.03 Rem. Magnesium-Base Alloys : It is alloyed principally with ALUMINUM, but may contain small amounts of SILICON, MANGANESE, ZINC, COPPER, and NICKEL. They have the lowest density. Their casting temperatures are around 670-7000C, so, cold chamber die casting is suitable. Properties and Applications of Die-Casting Alloys 3. MOLDS FOR DIE CASTING 3.1. Introduction • Usually made of tool steel, mold steel, or maraging steel • Tungsten and molybdenum (good refractory qualities) used to die cast steel and cast iron • Ejector pins required to remove part from die when it opens • Lubricants must be sprayed into cavities to prevent sticking 3.1. Introduction • Die Materials: - Hot-worked tools steels - Mold steels - Maraging Steels - Refractory Metals Properties: 1. High hot strength 2. High temperature wear resistance 3.2. Mold Materials • For Al, Zn, Mg Alloys: - 40Cr5MoV1Si (SKD 61) - 40Cr5MoVSi (SKD 6) • For Cu Alloys: - 30Cr3Mo3V - 30Cr2W8V (SKD4, SKD5) - 40Cr8W2 • For steel: Mo, Mo-W Alloys 3.3. Types of Cavities in Die-Casting Die Various types of cavities in a die-casting die. Source: Courtesy of American Die Casting Institute. 3.3. Types of Cavities in Die-Casting Die • Die-casting dies, Fig., may be made single- cavity dies, multiple-cavity dies (with several identical cavities), combination-cavity dies (with several different cavities), or unit dies (simple small dies that can be combined in two or more units in a master holding die). 3.3. Types of Cavities in Die-Casting Die • Typically, the ratio of die weight to part weight is 1000 to 1. Thus the die for a casting weighing 2 kg will weigh about 2000 kg. Dies are usually made of hot-work die steel or mold steels. Die-wear increases with the temperature of the molten metal. Heat checking of dies (surface cracking from repeated heating and cooling of the die) can be a problem. When die materials are selected and maintained properly, dies may last more than half a million shots before any significant die wear takes place. Example of a Die Casting Mold Example of a Die Casting Mold 4. DIE CASTING MACHINES • Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity • Two main types: 1. Hot-chamber machine 2. Cold-chamber machine 4.1. Hot-Chamber Die Casting Metal is melted in a container, and a piston injects liquid metal under high pressure into the die • High production rates - 500 parts per hour not uncommon • Applications limited to low melting-point metals that do not chemically attack plunger and other mechanical components • Casting metals: zinc, tin, lead, and magnesium 4.1. Hot-Chamber Die Casting Cycle in Hot Chamber Casting Cycle in hot-chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber Cycle in Hot Chamber Casting Cycle in hot-chamber casting: (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification 800-ton hot-chamber die-casting machine 4.2. Cold-Chamber Die Casting Machine • Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity • High production but not usually as fast as hot-chamber machines because of pouring step • Casting metals: aluminum, brass, and magnesium alloys • Advantages of hot-chamber process favor its use on low melting-point alloys (zinc, tin, lead) 4.2. Cold-Chamber Die Casting Machine Schematic illustration of the cold-chamber die-casting process. These machines are large compared to the size of the casting, because high forces are required to keep the two halves of the dies closed under pressure Cycle in Cold Chamber Casting Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber Cycle in Cold Chamber Casting Cycle in cold-chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification 4.3. Process Capabilities and Machine Selection • Machines are rated according to the clamping force needed to keep the dies closed. • Capacities range: 25 to 3000 tons. • Other factors involved in the selection of die- casting machines are die size, piston stroke, shot pressure and cost. • Ratio of die weight to part weight is 1000 to 1 • Dies are usually made of hot-work die steels or mold steels. 4.3. Process Capabilities and Machine Selection • Die design includes draft to allow removal of the casting. • Die casting has the capability for rapid production of strong, high-quality parts with complex shapes. • It also produces good dimensional accuracy and surface details, (net-shape forming). • Because of the high pressures involved, walls as thin as 0.38 mm are produced. Ejector marks remain. 4.3. Process Capabilities and Machine Selection • Because the molten metal chills rapidly at the die walls, the casting has a fine-grained, hard skin with higher strength. • Strength-to-weight ratio of die-cast parts increases with decreasing wall thickness. • Components such as pins, shafts, and threaded fasteners can be die cast integrally Called insert molding. • For good interfacial strength, inserts may be knurled, grooved, or splined. 4.3. Process Capabilities and Machine Selection • In selecting insert materials, the possibility of galvanic corrosion should be taken into account. • If galvanic corrosion is a potential problem, the insert can be insulated, plated, or surface- treated. • Equipment costs, particularly the cost of dies, are somewhat high, but labor costs are generally low. • Die casting is economical for large production runs. • Lubricants (parting agents) often are applied as thin coatings on die surfaces. The usually are water-based lubricants with graphite. 4.3. Process Capabilities and Machine Selection – Insert Molding Grooved bolts cast into a part, after the molten metal solidifies, they become a part of the product 5. DESIGN CONSIDERATIONS IN DIE CASTING 5.1. Construction of casting parts 5.1. Construction of casting parts 5.1. Construction of casting parts 5.2. Gating System 5.2.1. Direct Gating System 5.2.2. Inner Gating System 5.2.3. Outer Gating System 5.3. Gating System Design a) Poor; b) Better; c) The Best 5.4. Vent 5.5. Washing Canals 6. LOW PRESSURE CASTING • The basic process is shown in Fig. - In basic permanent and slush casting processes, metal in cavity is poured under gravity. However, in low-pressure casting, the metal is forced into cavity under low pressure (0.1 MPa) of air. Advantages - Clean molten metal from the center of ladle (cup) is introduced into the cavity. - Reduced- gas porosity, oxidation defects, improvement in mechanical properties Products 7. VACUUM PERMANENT-MOLD CASTING • This is a variation of low-pressure permanent casting • Instead of rising molten into the cavity through air pressure, vacuum in cavity is created which caused the molten metal to rise in the cavity from metal pool.