Die Castings

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Die Castings

 For many years, die castings have presented a cost-effective and highly versatile alternative to other metal shaping methods such as forging and hand tooling. Die casting designs are capable of reaching much closer tolerances and far more complex shapes than forgings or stamped metals while requiring minimal secondary tooling and processing. Die casters can manufacture large quantities of aluminum die castings, zinc die castings and other metal castings with exceptional detail, surface quality and dimensional consistency at relatively low cost. Die castings are manufactured by hot chamber or cold chamber die casting, under carefully controlled pressures. The demand for die castings, particularly automotive castings and other castings used in industrial products, is very high throughout the world.
Many manufacturing industries use die cast parts both in their products and equipment. Since the advent of cost-effective plastic formation processes like vacuum forming and injection molding, the prominence of die cast metal shapes has diminished somewhat. For example, store cash register housings used to be made almost exclusively of die cast metal. Today, almost all cash registers are made of molded plastic. Despite this shift, the number of industrial, commercial and consumer utilities that are products of die casting is too large to be accurately counted. Parts that require the strength of metal still use whole die cast parts, such as gumball machine bases, sink faucets, gas pump handles and a wide range of other metal parts. Computer and electronics industries use high tolerance magnesium die castings as housings and interior EMI enclosures as well as miniature zinc die cast parts for various electronic equipment applications. In the automotive industry, almost every engine block is a die cast product. Die cast aluminum valve covers are also very commonly applied in automobile engines. Cast zinc products are also widely used as door and cabinetry handles.

Every die cast metal part is formed in a die. Dies are also sometimes called molds, and every mold is specially designed for shaping metal in a certain way. For closed die castings, which are the majority of castings, this mold is cut into two separate metal blocks; in order to form a complete mold, the tooled blocks are placed together with cavities aligned. Once the die has been created, it is sprayed with a lubricant that helps control its temperature and assists in part removal once the cast is complete. The die is then closed, and molten metal is poured into the shot sleeve and injected into the die under high pressure by a plunger. Pressure is maintained within the die until the cast has solidified, then the die is opened and ejector pins push out the solidified "shot." This metal piece is considered a shot until the excess material that has solidified around it during casting is removed. This excess material typically consists of sprue, gate, runners and flash that have formed in channels leading to the die mold and possibly in leakage areas between the mold cavities. The cast part is tooled and deburred to remove this excess and is sometimes put through additional secondary processes such as surface finishing, plating and CNC machining.
Die casting is a high volume, low cost means of forming relatively complex metal parts. Because this forming method does not create a uniform molecular structure or grainflow in the same way that forging and extruding processes do, die cast parts do not have high strength and often have microfractures and grainflow inconsistencies that can lead to part failure or breakage under strain, corrosion or heat stress. For this reason, die cast parts are often heat-treated and carefully tested after manufacturing. There are some cases in which a die cast metal part is less appropriate for a task than a forging is. Consistently, though, die castings can be applied and relied on for their strength and durability if installed and maintained correctly. It is also important to carefully pair the correct die casting with its application; choosing the wrong metal for a die casting application can be catastrophic, particularly when it comes to engines and heavy machinery. A combination of careful design, materials selection, installation and maintenance will ensure the continued reliable operation of a die cast part for many years.

Images Provided by A&B Die Casting.

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Die Casting Types

• Alloy castings are metal shapes made by a metal formation machine called a caster. Alloys are combinations of at least two metals and sometimes non-metals like carbon and silicon.
• Aluminum casting are die castings made from aluminum.
• Aluminum die castings are commonly used in the production of hardware and tools because of the great resistance of aluminum castings to corrosion and high temperatures as well as their conductivity. In addition, aluminum castings are used in applications requiring the production of intricate part features and components.
  
• Automotive castings which include engine blocks and cylinder heads as well as brake and suspension components, are increasingly being manufactured out of lightweight material, such as aluminum, to economize fuel consumption by decreasing the overall weight of the vehicle.
  
• Brass castings are used in the architectural and construction industries and in decorative applications, such as doorknobs.
  
• Bronze castings are used in applications requiring high strength, making them ideal for use in the architectural and construction industries.
• Cold chamber die castings are cast metals that are formed in a cold chamber as opposed to a hot chamber.
• Copper die castings are used in applications requiring exceptional strength and resistance to corrosion. Copper castings consist of brass and bronze castings.
  
• Die casters shape metal by putting pressure on molten metal in a die.
• Die casting design is the process of planning and deciding the configuration of a die cast product.
• Gravity die castings are produced by pouring molten aluminum into a metallic tool at a casting temperature of 750º. Gravity die castings have good tolerances and surface finishing.
  
• High pressure die castings have very thin walls. The metal enters the die cavity at high speeds and under great pressure, which increases the chance of porosity.
• Hot chamber die castings are cast metals that are formed in a hot chamber as opposed to a cold chamber.
• Lead die castings offer great corrosion resistance and are used in the production of parts in the plumbing industry. Lead castings and tin castings also provide close production tolerances but are generally not as strong as other die cast metals.
  
• Low pressure die castings are formed when metal from an airtight furnace is transferred into a metallic tool through a rising tube. Low pressure die casting is suitable for relatively small lot sizes and for applications in which heat treatment is needed to improve the mechanical properties.
  
• Magnesium die castings are particularly useful in applications where weight is a factor. Magnesium die castings are very lightweight and cost-effective, especially in comparison to other die cast metals.
  
• Metal castings are metal impressions made with molds.
• Pressure die castings are metal castings that have been formed under carefully controlled pressure conditions; all die castings can be considered pressure die castings.
• Zinc die castings are often used in the production of small parts for the electronics and automotive industries. Zinc castings lend themselves well to secondary operations such as painting and plating.

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Die Casting Terms

Automation - Industry term commonly used to describe the mechanization of various aspects of die casting process.

Biscuit - An excess of molten metal that is leftover in the shot sleeve of a cold chamber die casting machine. Biscuits are considered part of the cast shot and are removed from the die with the casting. 

Blister - Die casting deformation consisting of a gaseous bubble on the surface of the casting and a hole in the casting underneath the bubble. 

Blow holes - Voids or pores that may occur due to entrapped gas or shrinkage during solidification, usually evident in heavy sections (see porosity).

Cavity - The recess or impressions in a die in which the casting is formed.

Cold chamber machine - A type of die casting machine in which the metal injection mechanism is not submerged in molten metal.

Checking - Fine cracks on the surface of a die that produce corresponding raised veins on die casting. Caused by repeated heating of the die surface by injected molten alloys. Also called heat checking.

Die lubricants - Liquid formulations applied to the die to facilitate casting release and prevent soldering.

Dimensional stability - Ability of a component to retain its shape and size over a long period of service.

Draft - The taper given to walls, cores and other parts of the die cavity to permit easy ejection of the casting.

Ejector marks - Marks left on castings by ejector pins.

Ejector pins - A rod that forces the casting out of the die cavity and off of cores.

Ejector plate - A plate that actuates the ejector pins attached to it. 

Fillet - Curved junction of two surfaces. For example: walls that would meet a sharp angle.

Flash - Excess metal on a die cast part extending past the parting line of the die set, which blocks metal from flowing past the die lines and filling the die impressions. 

Gate - Passage for molten metal that connects runner with die cavity. Also, the entire ejected content of a die, including castings, gates, runners sprue (or biscuit) and flash.

Growth - Expansion of a casting as a result of aging or of intergranular corrosion, or both.

Heat checking - (See checking)

Hot chamber machine - Die casting machine in which the pressure chamber is subjected to the molten metal through immersion. Hot chamber machines are used in the die casting of metals with low melting points, such as zinc die casting, copper die casting and magnesium die casting.

Impact strength - Ability to resist shock as measured by a suitable testing machine.

Ingot - Metal or alloy cast in a convenient shape for storage, shipping or remelting.

Injection - The processes of forcing molten metal into a die.

Insert - A piece of material, usually metal, which is placed in a die before each shot. Molten metal is cast around it to produce certain features on and within the die cast part.

Loose piece, knockout - A type of core (which forms undercuts) that is positioned in, but not fastened to, a die. It is so arranged as to be ejected with the casting from which it is removed. It is repeated for the same purpose.

Metal saver - Core utilized to prevent sections of excessive thickness from occurring and to reduce the amount of metal used in a metal casting.

Multiple cavity die - A die having more than one duplicate impression.

Overflow - An opening or cavity on the parting line of the die outside of the main die cavity into which unacceptable metal flows during the shot process. Reasons for metal unacceptability include improperly heated metal and metal that contains impurities.

Parting line - A line along the surface of a die casting reflecting the small space between the die halves; also, the mating surface of the cover and ejector portions of the die.

Plunger - Ram or piston that forces molten metal into a die.

Porosity - Very small voids or pores in cast metal parts resulting from trapped gas or shrinkage during solidification.

Process control - Where parameters of a process are studied and correctly applied in the manufacturing process to produce high quality parts.

Runners - Horizontal mechanisms connected to the sprue that transfer the molten metal from the sprue to the mold.

Shot - The process in which the metal is poured into the die. The shot may also refer to the die cast part as it exists as a cast before secondary operations are performed to remove flash and to fix imperfections in the casting.

Shrinkage, solidification - Dimensional reduction that accompanies the freezing (solidification) of metal, passing from the molten to the solid state.

Shrink mark - A surface depression that sometimes occurs next to a heavy section that cools more slowly than adjacent areas.

Slide - The portion of a die arranged to move parallel to die parting. The inner and end forms a part of the die cavity wall that involves one or more undercuts and sometimes includes a core or cores.

Soldering - Adherence of molten metal to portion of the die.

Sprue - Mechanism with which the molten metal first makes contact during the shot process. The sprue transfers the molten metal to the runners.

Trim die - A die for shearing or shaving flash from a die casting.

Trim - The process in which overflow, flash and other parts are removed from the die cast part or shot.

Unit die - A die interchangeable with others in a common holder.

Undercut - Recess in the side wall or cored hole of a casting disposed so that a slide or special form of core (such as a knockout) is required to permit ejection of the casting from the die.

Vent - An orifice in the casting die that facilitates the release of air from the die cavity into the atmosphere to prevent casting imperfections, such as porosity.

Void - A large pore or hole within the wall of a casting usually caused by entrapped gas. A blow hole.

Waterline - The channel in the casting die into which water flows to precipitate the cooling of the die casting.

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