HISTORY

HISTORY OF DIE-CASTING

The earliest examples of die casting by pressure injection - as opposed to casting by gravity pressure - occurred in the mid-1800s. A patent was awarded to Sturges in 1849 for the first manually operated machine for casting printing type. 

The process was limited to printer's type for the next 20 years, but development of other shapes began to increase toward the end of the century. By 1892, commercial applications included parts for phonographs and cash registers, and mass production of many types of parts began in the early 1900s.

The first die casting alloys were various compositions of tin and lead, but their use declined with the introduction of zinc and aluminum alloys in 1914. Magnesium and copper alloys quickly followed, and by the 1930s, many of the modern alloys still in use today became available.

 The History die casting process has evolved from the original low-pressure injection method to techniques including high-pressure casting - at forces exceeding 4500 pounds per square inch - squeeze casting and semi-solid die casting. These modern processes are capable of producing high integrity, near net-shape castings with excellent surface finishes.

The Future refinements continue in both the alloys used in die casting and the process itself, expanding die casting applications into almost every known market. Once limited to simple lead type, today's die casters can produce castings in a variety of sizes, shapes and wall thicknesses that are strong, durable and dimensionally precise.

SHRINKAGE

SHRINKAGE

Castings shrink when they cool. Like nearly all materials, metals are less dense as a liquid than a solid. During solidification (freezing), the metal density dramatically increases. This results in a volume decrease for the metal in a mold.

 Solidification shrinkage is the term used for this contraction. Cooling from the freezing temperature to room temperature also involves a contraction. The easiest way to explain this contraction is that is the reverse of thermal expansion. Compensation for this natural phenomenon must be considered in two ways.


Solidification shrinkage 
The shrinkage caused by solidification can leave cavities in a casting, weakening it. Risers provide additional material to the casting as it solidifies. The riser (sometimes called a "feeder") is designed to solidify later than the part of the casting to which it is attached. Thus the liquid metal in the riser will flow into the solidifying casting and feed it until the casting is completely solid.

 In the riser itself there will be a cavity showing where the metal was fed. Risers add cost because some of their material must be removed, by cutting away from the casting which will be shipped to the customer. They are often necessary to produce parts which are free of internal shrinkage voids. One method that assists in keeping the metal molten in the riser longer is the utilisation of an exothermic sleeve.

Sometimes, to promote directional solidification, chills must be used in the mold. A chill is any material which will conduct heat away from the casting more rapidly that the material used for molding. Thus if silica sand is used for molding, a chill may be made of copper, iron, aluminum, graphite, zircon sand, chromite or any other material with the ability to remove heat faster locally from the casting.

 All castings solidify with progressive solidification but in some designs a chill is used to control the rate and sequence of solidification of the casting.


Patternmaker's shrink (thermalcontraction) 
Shrinkage after solidification can be dealt with by using an oversized patterndesigned for the relevant alloy. Pattern makers use special "contraction rulers" (also called "shrink rules") to make the patterns used by the foundry to make castings to the design size required. These rulers are 1 - 6% oversize, depending on the material to be cast.

 These rulers are mainly referred to by their actual changes to the size. For example a 1/100 ruler would add 1 mm to 100 mm if measured by a "standard ruler" (hence being called a 1/100 contraction ruler). Using such a ruler during pattern making will ensure an oversize pattern. Thus, the mold is larger also, and when the molten metal solidifies it will shrink and the casting will be the size required by the design, if measured by a standard ruler.

 A pattern made to match an existing part would be made as follows: First, the existing part would be measured using a standard ruler, then when constructing the pattern, the pattern maker would use a contraction ruler, ensuring that the casting would contract to the correct size.

COOLING RATE

 COOLING RATE

The Cooling rate at which a casting cools affects its microstructure, quality, and properties is termed as cooling rate. It is largely controlled by the molding media used for making the mold. When the molten metal is poured into the mold, the cooling down begins. This happens because the heat within the molten metal flows into the relatively cooler parts of the mold.

 Molding materials transfer heat from the casting into the mold at different rates. For example, some molds made of plaster may transfer heat very slowly, while a mold made entirely of aluminium would transfer the heat very fast. This cooling down ends with solidificationwhen the liquid metal turns to solid metal. Intermediate cooling rates from melt result in a dendritic microstructure.

At its basic level a foundry may pour a casting without regard to controlling how the casting cools down and the metal freezes within the mold. However, if proper planning is not done the result can be gas porosities and shrink porosities within the casting. 

To improve the quality of a Die-casting and engineer how it is made, the foundry engineer studies the geometry of the part and plans how the heat removal should be controlled. Where heat should be removed quickly, the engineer will plan the mold to include special heat sinks to the mold, called chills.

 Fins may also be designed on a casting to extract heat, which are later removed in the cleaning (also called fettling) process. Both methods may be used at local spots in a mold where the heat will be extracted quickly. Where heat should be removed slowly, a riser or some padding may be added to a casting. A riser is an additional larger cast piece which will cool more slowly than the place where is it attached to the casting.

ADVANTAGE DIE CASTING

ADVANTAGE DIE CASTING

ADVANTAGE DIE CASTING

Die casting is an efficient, economical process offering a broader range of shapes and components than any other manufacturing technique. Parts have long service life and may be designed to complement the visual appeal of the surrounding part. Designers can gain a number of advantages and benefits by specifying die cast parts.

High-speed production - Die casting provides complex shapes within closer tolerances than many other mass production processes. Little or no machining is required and thousands of identical castings can be produced before additional tooling is required.

Dimensional accuracy and stability - Die casting produces parts that are durable and dimensionally stable, while maintaining close tolerances. They are also heat resistant.

Strength and weight - Die cast parts are stronger than plastic injection moldings having the same dimensions. Thin wall castings are stronger and lighter than those possible with other casting methods. Plus, because die castings do not consist of separate parts welded or fastened together, the strength is that of the alloy rather than the joining process.

Multiple finishing techniques - Die cast parts can be produced with smooth or textured surfaces, and they are easily plated or finished with a minimum of surface preparation. Simplified Assembly - Die castings provide integral fastening elements, such as bosses and studs. Holes can be cored and made to tap drill sizes, or external threads can be cast.

USEGE APPLICATION

USEGE APPLICATION

Die cast parts are found in many places around the home. The polished, plated zinc die casting in this kitchen faucet illustrates one of the many finishes possible with die casting.

The connector housings are examples of the durable, highly accurate components that can be produced with today's modern die casting.

CHOOSING THE PROPER ALOYES

 CHOOSING THE PROPER ALOYES

Each of the metal alloys for die casting offer particular advantages for the finished part.

Zinc :

The easiest alloy to cast, it offers high ductility, high impact strength and is easily plated. Zinc is economical for small parts, has a low melting point and promotes long die life.

Aluminum :

This Die-casting alloy is lightweight, while possessing high dimensional stability for complex shapes and thin walls. Aluminum has good corrosion resistance and mechanical properties, high thermal and electrical conductivity, as well as strength at high temperatures.

Magnesium :

The easiest alloy to machine, magnesium has an excellent strength-to-weight ratio and is the lightest alloy commonly die cast.

Copper :

This alloy possesses high hardness, high corrosion resistance and the highest mechanical properties of alloys cast. It offers excellent wear resistance and dimensional stability, with strength approaching that of steel parts.

Lead and Tin :

These alloys offer high density and are capable of producing parts with extremely close dimensions. They are also used for special forms of corrosion resistance.

CHOOSING CASTING PROCESS

 CHOOSING CASTING PROCESS

Consider the castingprocess at 645 B.C, the first traces of the Sand Molding was found. Now consider the state-of-the-art Electromagnetic casting process. Truly, the Casting process has traversed a long path and impacted human civilization for nearly five millennia.

 With technological advances, metal casting is playing a greater role in our everyday lives and is more essential than it has ever been.

Tip :

• For any Metal Casting Process, selection of right alloy, size, shape, thickness, tolerance, texture, and weight, is very vital.
• Special requirements such as, magnetism, corrosion, stress distribution also influence the choice of the Metal Casting Process.
• Views of the Tooling Designer; Foundry / Machine House needs, customer's exact product requirements, and secondary operations like painting, must be taken care of before selecting the appropriate Metal Casting Process.
• Tool cost.
• Economics of machining versus process costs.
• Adequate protection / packaging, shipping constraints, regulations of the final components, weights and shelf life of protective coatings also play their part in the Metal Casting process

TYPES OF DIE

TYPES OF DIE

There are four types of dies:

• Single cavity to produce one component
• Multiple cavities to produce a number of identical parts
• Unit die to produce different parts at one time
• Combination die to produce several different parts for an assembly

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 a high startup cost. 

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.

These tooling are made of alloy tool steels in at least two sections, the fixed die half, or cover half, and the ejector die half, to permit removal of castings. Modern dies also may have moveable slides, cores or other sections to produce holes, threads and other desired shapes in the casting. Sprue holes in the fixed die half allow molten metal to enter the die and fill the cavity. 

The ejector half usually contains the runners (passageways) and gates (inlets) that route molten metal to the cavity. Dies also include locking pins to secure the two halves, ejector pins to help remove the cast part, and openings for coolant and lubricant.

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.

A die's life is most prominently limited by wear or erosion, which is strongly dependent on the temperature of the molten metal. Aluminum alloy die usually have a life of 100,000 cycles, if the die is properly maintained. 

Molds for die casting zinc last approximately 10 times longer than aluminium die casting mold due to the lower temperature of the zinc. Dies for zinc are often made of H13 and only hardened to 29-34 RHC. Cores are either made of H13 or 440B, so that the wearing parts can be selectively nitrided for hardness, leaving the exposed part soft to resist heat checking. Molds for die casting brass are the shortest-lived of all.

EQUIPEMENT –MACHINE

There are two basic types of die casting machines:
Hot chamber machines are used primarily for zinc, copper, magnesium, leadand other low melting point alloys that do not readily attack and erode metal pots, cylinders and plungers. The injection mechanism of a hot chamber machine is immersed in the molten metal bath of a metal holding furnace.

The furnace is attached to the machine by a metal feed system called a gooseneck. As the injection cylinder plunger rises, a port in the injection cylinder opens, allowing molten metal to fill the cylinder. As the plunger moves downward it seals the port and forces molten metal through the gooseneck and nozzle into the die cavity. After the metal has solidified in the die cavity, the plunger is withdrawn, the die opens and the casting is ejected.

These machines are then rated by how much clamping force they can apply. Typical sizes range from 400 to 4,000 short tons. 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 fill the "gooseneck". The gas or oil 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 aluminum 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.

Cold chamber machines 

 when the casting alloy cannot be used in hot-chamber machines; these alloys include aluminum, magnesium, copper, and zinc alloys with a large composition of aluminum. This machine works by melting the material, first, in a separate furnace. 

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.

Cold chamber machines are used for alloys such as aluminum and other alloyswith high melting points. 

The molten metal is poured into a "cold chamber," or cylindrical sleeve, manually by a hand ladle or by an automatic ladle. A hydraulically operated plunger seals the cold chamber port and forces metal into the locked die at high pressures.

TYPES OF CASTING FORMATION

TYPES OF CASTING FORMATION 

Hot Forming
Examples are Centrifugal casting, Extrusion, Forging, Full mold casting, Investment casting, Permanent or Gravity Die casting, Plaster mold casting, Sand Casting, Shell Mold casting. The method to be used depends upon the nature of the products to be cast.


Cold Forming
Examples are Squeeze casting, Pressure die casting, Gravity die casting, Burnishing, Coining, Cold forging, Hubbing, Impact Extrusion, Peening, Sizing, Thread rolling.

TYPES OF CASTING PROCESS

TYPES OF CASTING PROCESS

Pore-free casting
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. 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. These castings can still be heat treated and welded. This process can be performed on aluminum, zinc, and lead alloys.


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 slow cooling cycles.

TYPES OF CASTING METHOD

Centrifugal
Centrifugal casting is both gravity- and pressure-independent since it creates its own force feed using a temporary sand mold held in a spinning chamber at up to 900 N (90 gee). Lead time varies with the application. Semi- and true-centrifugal processing permit 30-50 pieces/hr-mold to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3-4.5 kg.

Small art pieces such as jewelry are often cast by this method using the lost wax process, as the forces enable the rather viscous liquid metals to flow through very small passages and into fine details such as leaves and petals. This effect is similar to the benefits from vacuum casting, also applied to jewelry casting.


Continuous
Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. Molten metal is poured into an open-ended, water-cooled copper mold, which allows a 'skin' of solid metal to form over the still-liquid centre.

 The strand, as it is now called, is withdrawn from the mold and passed into a chamber of rollers and water sprays; the rollers support the thin skin of the strand while the sprays remove heat from the strand, gradually solidifying the strand from the outside in. After solidification, predetermined lengths of the strand are cut off by either mechanical shears or traveling oxyacetylene torches and transferred to further forming processes, or to a stockpile. 

Cast sizes can range from strip (a few millimeters thick by about five meters wide) to billets (90 to 160 mm square) to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process before being cut.

Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increases the quality of the final product. Metals such as steel, copper and aluminium are continuously cast, with steel being the metal with the greatest tonnages cast using this method.

DIE CASTING PROCESS

DIE CASTING PROCESS

die casting process

There are four major steps in the die casting process. First, the mold is sprayed with lubricant and closed. The lubricant both helps control the temperature of the die and it also assists in the removal of the casting. Molten metal is then shot into the die under high pressure; between 10-175 MPa (1,500-25,000 psi). Once the die is filled the pressure is maintained until the casting has solidified.

 Finally, the die is 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 scrap, which includes the gate, runners, spruces and flash, must be separated from the casting(s). 

This is often done using a special trim die in a power press or hydraulic press. An older method is separating by hand or by sawing, which case grinding may be necessary to smooth the scrap marks. 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.

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. Most die casters perform other secondary operations to produce features not readily ca stable, such as tapping a hole, polishing, plating, buffing, or painting

DIE-CASTING

DIE-CASTING

Top die-casting

Die casting is a versatile process for producing engineered metal parts by forcing molten metal under high pressure into reusable steel molds. These molds, called dies, can be designed to produce complex shapes with a high degree of accuracy and repeat ability. Parts can be sharply defined, with smooth or textured surfaces, and are suitable for a wide variety of attractive and serviceable finishes.

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 nonferrous metals, specifically zinc, copper, and aluminum based alloys, but ferrous metal die castings are also possible. The die casting method is especially suited for applications where many small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency.

Die-casting process by which molten metal is forced by a plunger or compressed air into a metallic die and the pressure maintained until the metal has solidified. Die castings are accurate, are sharply outlined, have a good surface finish, and can be made in complicated designs. 

Zinc, aluminum, and magnesium alloys are the principal metals used. The high cost of the die usually limits the process to large-scale, high-speed production. Typical products are carburetor bodies and zippers. Type-casting machines are specialized die-casting machines.

Die castings are among the highest volume, mass-produced items manufactured by the metalworking industry, and they can be found in thousands of consumer, commercial and industrial products. 

Die cast parts are important components of products ranging from automobiles to toys. Parts can be as simple as a sink faucet or as complex as a connector housing.