Die casting
Die casting is the process of forcing molten metal under high pressure into mold cavities (which are machined into dies). Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter and tin based alloys,[1] although ferrous metal die castings are possible.[2] The die casting method is especially suited for applications where a large quantity of small to medium sized parts are needed, ensuring precise surface quality and dimensional consistency.[2]
This level of versatility has placed die castings among the highest volume products made in the metalworking industry.[1]
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History
Die casting equipment was invented in 1838 for the purpose of producing movable type for the printing industry. The first die casting-related patent was granted in 1849 for a small hand operated machine for the purpose of mechanized printing type production. In 1885, Otto Mergenthaler invented the linotype machine, an automated type casting device that became the prominent type of equipment in the publishing industry. Other applications grew rapidly, with die casting facilitating the growth of consumer goods and appliances by making affordable the production of intricate parts in high volumes.[3]
Equipment
There are two basic types of die casting machines: hot-chamber machines (a.k.a. gooseneck machines) and cold-chamber machines.[4] These are rated by how much clamping force they can apply. Typical ratings are between 400 and 4,000 short tons.[1]
Hot-chamber machines rely upon a pool of molten metal to feed the die. At the beginning of the cycle the piston of the machine is retracted, which allows the molten metal to fill the "gooseneck". The pneumatic or hydraulic powered piston then forces this metal out of the gooseneck into the die. The advantages of this system include fast cycle times (approximately 15 cycles a minute) and the convenience of melting the metal in the casting machine. The disadvantages of this system are that high-melting point metals cannot be utilized and aluminium cannot be used because it picks up some of the iron while in the molten pool. Due to this, hot-chamber machines are primarily used with zinc, tin, and lead based alloys.[4]
 injection molding machine. |
 open tooling and injection nozzle. |
 Complete working cell. |
Cold-chamber machines are used when the casting alloy cannot be used in hot-chamber machines; these include aluminium, zinc alloys with a large composition of aluminium, magnesium and copper. The process for these machines start with melting the metal in a separate furnace.[5] Then a precise amount of molten metal is transported to the cold-chamber machine where it is fed into an unheated shot chamber (or injection cylinder). This shot is then driven into the die by a hydraulic or mechanical piston. This biggest disadvantage of this system is the slower cycle time due to the need to transfer the molten metal from the furnace to the cold-chamber machine.[6]
The dies used in die casting are usually made out of hardened tool steels because cast iron cannot withstand the high pressures involved. Due to this the dies are very expensive, resulting in high start-up costs. Dies may contain only one mold cavity or multiple cavities of the same or different parts. There must be at least two dies to allow for separation and ejection of the finished workpiece, however its not uncommon for there to be more sections that open and close in different directions. Dies also often contain water-cooling passages, retractable cores, ejector pins, and vents along the parting lines. These vents are usually wide and thin (approximately 0.13 mm or 0.005 in) so that when the molten metal starts filling them the metal quickly solidifies and minimizes scrap. No risers are used because the high pressure ensures a continuous feed of metal from the gate. Recently, there's been a trend to incorporate larger gates in the die and to use lower injection pressures to fill the mold, and then increase the pressure after its filled. This system helps reduce porosity and inclusions.[7]
In addition to the dies there may be cores involved to cast features such as undercuts. Sand cores cannot be used because they disintegrate from the high pressures involved with die casting, therefore metal cores are used. If a retractable core is used then provisions must be made for it to be removed either in a straight line or circular arc. Moreover, these cores must have very little clearance between the die and the core to prevent the molten metal from escaping. Loose cores may also be used to cast more intricate features (such as threaded holes). These loose cores are inserted into the die by hand before each cycle and then ejected with the part at the end of the cycle. The core then must be removed by hand. Loose cores are more expensive due to the extra labor and time involved.[8]
Dies
Two dies are used in die casting; one is called the "cover die half" and the other the "ejector die half". Where the they meet is called the parting line. The cover die contains the sprue (for hot-chamber machines) or shot hole (for cold-chamber machines), which allows the molten metal to flow into the dies; this feature matches up with the injector nozzle on the hot-chamber machines or the shot chamber in the cold-chamber machines. The ejector die contains the ejector pins and usually the runner, which is the path from the sprue or shot hole to the mold cavity. The cover die is secured to the stationary, or front, platen of the casting machine, while the ejector die is attached to the movable platen.[9]
A die's life is most prominently limited by wear or erosion, which is strongly dependent on the temperature of the molten metal.[10] Dies for zinc are often made of hot working tool steels (e.g. SAE grade H13) and only hardened to 29-34 HRC.[11] Cores are either made of hot working tool steel (H13) or heat treatable stainless steel (e.g. SAE 440B), so that the wearing parts can be selectively nitrided for hardness, leaving the exposed part soft to resist heat checking.[11]
| Zinc | Aluminum | Magnesium | Brass (leaded yellow) | |
|---|---|---|---|---|
| Maximum die life [number of cycles] | 1,000,000 | 100,000 | 100,000 | 10,000 |
| Die temperature [C° (F°)] | 218 (425) | 288 (550) | 260 (500) | 500 (950) |
| Casting temperature [C° (F°)] | 400 (760) | 660 (1220) | 760 (1400) | 1090 (2000) |
Other failure modes for dies are:
- Heat checking: surface cracks occur on the die due to a large temperature change on every cycle[10]
- Thermal fatigue: surface cracks occur on the die due to a large number of cycles[10]
Process
There are four base steps in any die casting process: die preparation, filling, ejection, and shakeout. The dies are prepared by spraying the mold cavity with lubricant. The lubricant both helps control the temperature of the die and it also assists in the removal of the casting. The dies are then closed and molten metal is injected into the dies under high pressure; between 10 and 175 megapascals (1,500 and 25,400 psi). Once the mold cavity is filled, the pressure is maintained until the casting solidifies. The dies are then opened and the shot (shots are different from castings because there can be multiple cavities in a die, yielding multiple castings per shot) is ejected by the ejector pins. Finally, the shakeout involves separating the the scrap, which includes the gate, runners, sprues and flash, from the shot. This is often done using a special trim die in a power press or hydraulic press. Other methods of shaking out include sawing and grinding. A less labor-intensive method is to tumble shots if gates are thin and easily broken; separation of gates from finished parts must follow. This scrap is recycled by remelting it.[4] The yield is approximately 67%.[13]
The high-pressure injection leads to a quick fill of the die, which is required so the entire cavity fills before any part of the casting solidifies. In this way, discontinuities are avoided, even if the shape requires difficult-to-fill thin sections. This creates the problem of air entrapment, because when the mold is filled quickly there is little time for the air to escape. This problem is minimized by including vents along the parting lines, however, even in a highly refined process there will still be some porosity in the center of the casting.[14]
Most die casters perform other secondary operations to produce features not readily castable, such as tapping a hole, polishing, plating, buffing, or painting.
Lubricants
Water-based lubricants, called emulsions, are the most commonly used type of lubricant, because of health, environmental, and safety reasons. Unlike solvent-based lubricants, if water is properly treated to remove all minerals from it, it will not leave any by-product in the dies. If the water is not properly treated, then the minerals can cause surface defects and discontinuities. There are four types of water-based lubricants: oil in water, water in oil, semi-synthetic, and synthetic. Oil in water is the best, because when the lubricant is applied the water cools the die surface by evaporating while depositing the oil, which helps release the shot. A common mixture for this type of lubricants is thirty parts water to one part oil, however in extreme cases a ratio of 100:1 is used.[15][16][17]
Oils that are used include heavy residual oil (HRO), animal fats, vegetable fats, and synthetic fats. HROs are gelatinous at room temperature, but at the high temperatures found in die casting, they form a thin film. Pigments are added to control the emulsions viscosity and thermal properties; these include graphite, aluminum, and mica. Other chemical additives are used to inhibit rusting and oxidation. Emulsifiers are added to water-based lubricants, so that oil based additives can be mixed into the water; these include soap, alcohol esters, and ethylene oxides.[18]
Historically, solvent-based lubricants, such as diesel fuel and kerosene, were commonly used. These were good at releasing the part from the dies, but a small explosion occurred during each shot, which led to a build-up of carbon on the mold cavity walls. However, they were easier to apply evenly than water-based lubricants.[15]
Pore-free casting process
When no porosity is required for a casting then the pore-free casting process is used. It is identical to the standard process except oxygen is injected into the die before each shot to purge any air from the mold cavity. This causes small dispersed oxides to form when the molten metal fills the dies, which virtually eliminates gas porosity. An added advantage to this is greater strength. Unlike standard die castings, these castings can be heat treated and welded. This process can be performed on aluminium, zinc, and lead alloys.[6]
Heated-manifold direct-injection die casting
Heated-manifold direct-injection die casting, also known as direct-injection die casting or runnerless die casting, is a zinc die casting process where molten zinc is forced through a heated manifold and then through heated mini-nozzles, which lead into the molding cavity. This process has the advantages of lower cost per part, through the reduction of scrap (by the elimination of sprues, gates and runners) and energy conservation, and better surface quality through slower cooling cycles.[6]
Advantages and disadvantages
Advantages:[8]
- Excellent dimensional accuracy (dependent on casting material, but typically 0.1 mm for the first 2.5 cm (0.005 in. for the first inch) and 0.02 mm for each additional centimeter (0.002 in. for each additional inch).
- Smooth cast surfaces (1–2.5 micrometres or 0.04–0.10 thou rms).
- Thinner walls can be cast as compared to sand and permanent mold casting (approximately 0.75 mm or 0.030 in).
- Inserts can be cast-in (such as threaded inserts, heating elements, and high strength bearing surfaces).
- Reduces or eliminates secondary machining operations.
- Rapid production rates.
- Casting tensile strength as high as 415 MPa (60 ksi).
- Castings are made as large as an 8 feet (2.4 m) across and 30 pounds (14 kg) in weight. In magnesium
Disadvantages:[8]
- Casting weight must be between 30 grams (1 oz) and 10 kg (20 lb).
- High initial cost.
- Limited to high-fluidity metals.
- A large production volume is needed to make this an economical alternative to other processes.[19]
In the standard die casting process the final casting will have a small amount of porosity. This prevents any heat treating or welding, because the heat flows into the porosity.[20]
Die casting materials
The main die casting alloys are: zinc, aluminium, magnesium, copper, lead, and tin. Specific dies casting alloys include: ZAMAK, zinc aluminium, AA 380, AA 384, AA 386, AA 390, and AZ91D magnesium.[21] The following is a summary of the advantages of each alloy:[1]
- Zinc: the easiest alloy to cast; high ductility; high impact strength; easily plated; economical for small parts; promotes long die life.
- Aluminium: lightweight; high dimensional stability for complex shapes and thin walls; good corrosion resistance; good mechanical properties; high thermal and electrical conductivity; retains strength at high temperatures.
- Magnesium: the easiest alloy to machine; excellent strength-to-weight ratio; lightest alloy commonly die cast.
- Copper: high hardness; high corrosion resistance; highest mechanical properties of alloys die cast; excellent wear resistance; excellent dimensional stability; strength approaching that of steel parts.
- Lead and Tin: high density; extremely close dimensional accuracy; used for special forms of corrosion resistance. Such alloys are not used in foodservice applications for public health reasons.
Maximum weight limits for aluminium, brass, magnesium, and zinc castings are approximately 70 pounds (32 kg), 10 lb (5 kg), 44 lb (20 kg), and 75 lb (34 kg), respectively.[22]
The material used defines the minimum section thickness and minimum draft required for a casting as outlined in the table below. The thickest section should be less than 13 mm (0.5 in), but can be greater.[8]
| Metal | Minimum section | Minimum draft |
|---|---|---|
| Aluminium alloys | 0.89 mm (0.035 in) | 1:100 (0.6°) |
| Brass and bronze | 1.27 mm (0.050 in) | 1:80 (0.7°) |
| Magnesium alloys | 1.27 mm (0.050 in) | 1:100 (0.6°) |
| Zinc alloys | 0.63 mm (0.025 in) | 1:200 (0.3°) |
See also
- Fusible core injection molding
References
- ↑ 1.0 1.1 1.2 1.3 FAQ About Die Casting, http://www.diecasting.org/faq/, retrieved 04-12-2008.
- ↑ 2.0 2.1 Degarmo, p. 328.
- ↑ About die casting, The North American Die Casting Association, archived from the original on 2010-10-15, http://www.webcitation.org/5tVCFPCyL, retrieved 2010-10-15.
- ↑ 4.0 4.1 4.2 Degarmo, pp. 329-330.
- ↑ Parashar, Nagendra (2004). Elements of Manufacturing Processes. City: Prentice-Hall of India Pvt.Ltd. p. 234. ISBN 9788120319585. http://books.google.com/books?id=CU5-1iWZl6AC&pg=PA234.
- ↑ 6.0 6.1 6.2 Degarmo, p. 330.
- ↑ Degarmo, p. 329-331.
- ↑ 8.0 8.1 8.2 8.3 Degarmo, p. 331.
- ↑ Davis, p. 251.
- ↑ 10.0 10.1 10.2 Degarmo, p. 329.
- ↑ 11.0 11.1 Stefanescu, D.M. (1988), ASM Handbook: Casting, 15, ASM International, p. 789, ISBN 0871700212.
- ↑ Schrader, George F.; Elshennawy, Ahmad K.; Doyle, Lawrence E. (2000), Manufacturing processes and materials (4th ed.), SME, p. 186, ISBN 9780872635173, http://books.google.com/?id=Nz2wXvmkAF0C&pg=PT207.
- ↑ Brevick, Jerald; Mount-Campbell, Clark; Mobley, Carroll (2004-03-15), Energy Consumption of Die Casting Operations, Ohio State University, (US Department of Energy Grant/Contract No. DE-FC07-00ID13843, OSURF Project No. 739022), http://www.osti.gov/bridge/servlets/purl/822409-gPzbT4/native/822409.pdf, retrieved 2010-10-15.
- ↑ Degarmo, p. 330-331.
- ↑ 15.0 15.1 Andresen 2005, p. 356.
- ↑ Andresen 2005, p. 357.
- ↑ Andresen 2005, p. 358.
- ↑ Andresen 2005, p. 355.
- ↑ Die casting is an economical alternative for as little as 2,000 parts if it eliminates extensive secondary machining and surface finishing.
- ↑ Liu, Wen-Hai (2009-10-08), The Progress and Trends of Die Casting Process and Application, archived from the original on 2010-10-19, http://www.webcitation.org/5tbnPal1W, retrieved 2010-10-19.
- ↑ Die Casting, efunda Inc, http://www.efunda.com/processes/metal_processing/die_casting.cfm, retrieved 04-12-2008.
- ↑ Alloy Properties, The North American Die Casting Association, http://www.diecasting.org/faq/alloy_prop.htm, retrieved 04-12-2008.
Bibliography
- Davis, J. (1995), Tool Materials, Materials Park: ASM International, ISBN 9780871705457.
- Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4.
- Andresen, Bill (2005), Die Casting Engineering, New York: Marcel Dekker, ISBN 9780824759353, http://books.google.com/books?id=QDPLgfCSoqoC.
External links
- Die Casting Information and FAQ's
- Die Casting Pictorial Library
- Tolerances for die casting (according to DIN 1688 GTA 13, German Standards)
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