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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 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.

First, a steel mold capable of producing tens of thousands of castings in rapid succession must be made in at least two sections to permit removal of castings. These sections are mounted securely in a machine and are arranged so that one is stationary (fixed die half) while the other is moveable (injector die half). To begin the casting cycle, the two die halves are clamped tightly together by the die casting machine. Molten metal is injected into the die cavity where it solidifies quickly. The die halves are drawn apart and the casting is ejected. Die casting dies can be simple or complex, having moveable slides, cores, or other sections depending on the complexity of the casting.

The complete cycle of the die casting process is by far the fastest known for producing precise non-ferrous metal parts. This is in marked contrast to sand casting which requires a new sand mold for each casting. While the permanent mold process uses iron or steel molds instead of sand, it is considerably slower, and not as precise as die casting.

Regardless of the type of machine used, it is essential that die halves, cores and/or other moveable sections be securely locked in place during the casting cycle. Generally, the clamping force of the machine is governed by (a) the projected surface area of the casting (measured at the die parting line) and (b) the pressure used to inject metal into the die. Most machines use toggle type mechanisms actuated by hydraulic cylinders (sometimes air pressure) to achieve locking. Others use direct acting hydraulic pressure. Safety interlock systems are used to prevent the die from opening during the casting cycles.

Die casting machines, large or small, vary fundamentally only in the method used to inject molten metal into the die. These are classified and described as either hot or cold chamber die casting machines.

Hot chamber machines (Fig.1) are used primarily for zinc, and low melting point alloys which do not readily attack and erode metal pots, cylinders and plungers. Advanced technology and development of new, higher temperature materials has extended the use of this equipment for magnesium alloys.

In the hot chamber machine, the injection mechanism is immersed in molten metal in a furnace attached to the machine. As the plunger is raised, a port opens allowing molten metal to fill the cylinder. As the plunger moves downward sealing the port, it forces molten metal through the gooseneck and nozzle into the die. After the metal has solidified, the plunger is withdrawn, the die opens, and the resulting casting is ejected.

Hot chamber machines are rapid in operation. Cycle times vary from less than one second for small components weighing less than one ounce, to thirty seconds for a casting of several pounds. Dies are filled quickly (normally between five and forty milliseconds) and metal is injected at high pressures (1,500 to over 4,500 psi). Nevertheless, modern technology gives close control over these values, thus producing castings with fine detail, close tolerances and high strength.

Figure 1: Hot Chamber Machine. Diagram illustrates the plunger mechanism which is submerged in molten metal. Modern machines are hydraulically operated and equipped with automatic cycling controls and safety devices.

Cold chamber machines (Fig. 2) differ from hot chamber machines primarily in one respect; the injection plunger and cylinder are not submerged in molten metal. The molten metal is poured into a “cold chamber” through a port or pouring slot by a hand or automatic ladle. A hydraulically operated plunger, advancing forward, seals the port forcing metal into the locked die at high pressures. Injection pressures range from 3,000 to over 10,000 psi for both aluminum and magnesium alloys, and from 6,000 to over 15,000 psi for copper-based alloys.

In a cold chamber machine, more molten metal is poured into the chamber than is needed to fill the die cavity. This helps sustain sufficient pressure to pack the cavity solidly with casting alloy. Excess metal is ejected along with the casting and is part of the complete shot.

Operation of a “cold chamber” machine is a little slower than a “hot chamber” machine because of the ladling operation. A cold chamber machine is used for high melting point casting alloys because plunger and cylinder assemblies are less subject to attack since they are not submerged in molten metal.

Figure 2: Cold Chamber Machine. Diagram illustrates die, cold chamber and horizontal ram or plunger (in charging position).

Die casting dies (Fig. 3) are made of alloy tool steels in at least two sections called fixed die half and ejector die half. The fixed die half is mounted on the side toward the molten metal injection system. The ejector die half, to which the die casting adheres, and from which it is ejected when the die is opened, is mounted on the moveable platen of the machine.

The fixed die half of the die is designed to contain the sprue hole through which molten metal enters the die. The ejector half usually contains the runners (passage ways) and gates (inlets) which route molten metal to the cavity (or cavities) of the die. The ejector half is also connected to an ejector box which houses the mechanism for ejecting the casting from the die. Ejection occurs when pins connected to the ejector plate move forward to force the casting from the cavity. This usually occurs as part of the opening stroke of the machine. Placement of ejector pins must be carefully arranged so force placed upon the casting during ejection will not cause deformation. Return pins attached to the ejector plate return this plate to its casting position as the die closes.

Fixed and moveable cores are often used in dies. If fixed, the core axis must be parallel to the direction of the die opening. If moveable, they are often attached to core slides. Should the side of a die casting design require a depression, the die can be made with one or more slides to obtain the desired result without affecting ejection of the casting from the die cavity. All moveable slides and cores must be carefully fitted, and have the ability to be securely locked into position during the casting cycle. Otherwise, molten metal could be forced into their slideways causing a disruption of operations. Although slides and cores add to the complexity and cost of die construction, they make it possible to produce die castings in a wide variety of configurations, and usually more economically than any other metalworking process.

Figure 3

Sprue holes are tapered with the small end located at the breaking point when the die is opened. A sprue pin, located in the ejector half, makes the sprue hollow and deflects metal entering the die into the runner system.

Runners are channels located at the parting line to route liquid metal from the sprue hole to the gate.

Gates are passages through which metal enters the die cavity. They have an important function in directing metal flow so that the cavity is correctly filled. Air is expelled through vents as molten metal enters the die cavity.

Guide pins assure proper alignment of die halves and correct register of cavities.

Side walls and cores are designed to have a slight taper or draft. The largest diameter or cross section of a cavity must be located at the parting line (unless slides are used) so the casting can be removed from the die. As molten metal in a die cavity solidifies, it shrinks away from walls onto core pins and other projections. The design of the die must permit withdrawal of core pins, and ejection without applying too much pressure which could cause deformation to the die casting.

Cores, fixed or moveable, as well as “loose pieces,” must be positioned to facilitate removal either mechanically or by hand. Loose pieces used to form undercuts must be positioned by the operator. This requires extra labor, and usually slows the casting cycle.

Inserts can be cast integrally to provide special characteristics.

Most dies are cooled by water circulating through channels drilled for that purpose

Dies are classified as: single cavity, multiple cavity, combination and unit dies (Figures 4-A to 4-D).

A single cavity die requires no explanation. Multiple cavity dies have several cavities which are all identical. If a die has cavities of different shapes, it’s called a combination or family die. A combination die is used to produce several parts for an assembly. For simple parts, unit dies might be used to effect tooling and production economies. Several parts for an assembly, or for different customers, might be cast at the same time with unit dies. One or more unit dies are assembled in a common holder and connected by runners to a common opening or sprue hole. This permits simultaneous filling of all cavities.

Figure 4

“Automation and Mechanization” are terms often used interchangeably by the die casting industry. Automation of a manufacturing process is more sophisticated and complicated than the mechanization of various operations.

A die caster progresses towards automation by mechanizing various steps of the die casting process. For example:

1. Die lubrication can be accomplished by installing fixed or reciprocating spray systems.
2. An automatic ladling device can replace the hand ladle.
3. Castings can be removed from the die by extractors or robots, or by “drop through” to a conveyor below the machine.
4. Die casting machine operation can be integrated to cast, quench, trim and eject castings and return scrap to the furnace by conveyor.

By closing the loop, the die casting process can be automated, utilizing feed-back controls and microprocessors which automatically self compensate for process variables. As die casting plants move toward automation, noticeable changes occur. The operation tends to become safer. Production and quality improves considerably. Automation upgrades the labor force by requiring the development of new skills for machine set-up, programming, electronics, mechanics and maintenance.

Quality in die casting is maintained through the use of process controls and feedback between the process control computer and the die casting machine.

Process controllers may utilize microprocessors to access transducers mounted on the die casting machine, to obtain velocity, position, hydraulic pressure and tie-bar strain data, etc. The microprocessor then adjusts the die casting machine operation through special valves, thus assuring consistent castings shot after shot. The process controller also collects machine performance data for statistical analysis in quality control.

Step 1 — Alarm monitoring — A computerized remote terminal mounted near the die casting machine continuously monitors the following: velocity, position, pressures at the accumulator, die lock-up cylinder, head and rod side of the injection cylinder, temperatures of hydraulic oil, metal (up to ten locations in the die), and strains on four tie-bars. Each of these variables is high/low limit checked every shot. An alarm is sounded or flashed if a casting variable goes out-of-limits.

Step 2 — Control — Automatic control valves and tie-bar adjusting motors are installed on the die casting machine and connected to the computerized remote terminal unit. The computer adjusts controls to maintain satisfactory die locking force, slow shot velocity, optimum fill time and proper intensifier timing. The computer may also be interfaced to a robot and/or a programmable controller.

Step 3 — Data acquisition — After installing computerized remote terminal units on each machine, a data acquisition system may be formed by interconnecting these units and communicating all information back to a master terminal computer in the office. The master computer prints out a management information report which summarizes the operation of the entire die casting shop, including which machines are running, when they stop, shots made (good and bad), which machines are out-of-limits and what to do. At the master station, the manager can analyze the process by studying shot profiles on a video monitor and statistical reports from a high speed printer.

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.

There are many sources for information on die casting design. These include text books, technical papers, literature, magazines, seminars and courses conducted by engineering societies, trade associations and industry. Often, the die caster selected to produce a component part is an excellent source for information.

To gain maximum advantage of the die casting process, it is always a good idea to draw upon the wide ranging experience of a custom die caster. New designs should be reviewed during the early stage of development. Significant savings may be realized during this interchange of ideas.

The data appearing (Table 5) on approximate dimensional and weight limits for die casting of different alloys may vary under special conditions. When in doubt, ask your die caster. He is thoroughly familiar with his machinery and equipment and can make suggestions (during the design stage) which may affect tooling and production changes, resulting in lower costs.

Table 5:

APPROXIMATE DIMENSIONAL^* AND WEIGHT LIMITS FOR DIE CASTING IN DIFFERENT ALLOYS

Data apply to average conditions. For exceptional conditions, larger castings, closer dimensional limits and thinner sections may be feasible.

Advice on designing die castings is usually based upon desirable practices or situations to avoid. However, like most rules, there are exceptions. These affect either costs, appearance and/or quality of final products. Listed below are guides which should be considered when designing for die casting:

1. Specify thin sections which can easily be die cast and still provide adequate strength and stiffness. Use ribs wherever possible to attain maximum strength, minimum weight.
2. Keep sections as uniform as possible. Where sections must be varied, make transitions gradual to avoid stress concentration.
3. Keep shapes simple and avoid nonessential projections.
4. A slight crown is more desirable than a large flat surface, especially on plated or highly finished parts.
5. Specify coring for holes or recesses where savings in metal and overall costs outweigh tooling costs.
6. Design cores for easy withdrawal to avoid complicated die construction and operation.
7. Avoid small cores. They can be easily bent or broken necessitating frequent replacement. Drilling or piercing small holes in die castings is often cheaper than the cost of maintaining small cores.
8. Avoid use of undercuts which increase die or operating costs unless savings in metal or other advantages fully warrant these extra costs.
9. Provide sufficient draft on side walls and cores to permit easy removal of the die casting from the die without distortion.
10. Provide fillets at all inside corners and avoid sharp outside corners. Deviation from this practice may be warranted by special considerations.
11. Die casting design must provide for location of ejector pins. Take into consideration the effect of resultant ejector marks on appearance and function. The location of ejector pins is largely determined by the location and magnitude of metal shrinkage on die parts as metal cools in the die.
12. Specify die cast threads over cut threads when a net savings will result.
13. Die castings which affect the appearance of a finished product may be designed for aesthetics, and to harmonize with mating parts.
14. Inserts should be designed to be held firmly in place with proper anchorage provided to retain them in the die casting.
15. Design parts to minimize flash removal costs.
16. Never specify dimensional tolerances closer than essential. This increases costs.
17. Design die castings to minimize machining.
18. Where machining is specified, allow sufficient metal for required cuts.
19. Consider contact areas for surfaces which are to be polished or buffed. Avoid deep recesses and sharp edges.

Dies can be produced for simple and complex parts. Parts having external undercuts or projections on side walls often require slides which increase costs. In many cases, however, resultant savings of metal or other advantages such as uniform wall sections, offset the extra cost or affect a net economy in overall costs. This is especially true when large quantities are involved.

Plastics injection moldings
Compared with plastic injection moldings, die castings are stronger, stiffer, more stable dimensionally, more heat resistant, and are far superior to plastics on a properties/cost basis. They help prevent radio frequency and electromagnetic emissions. For chrome plating, die castings are much superior to plastic. Die castings have a high degree of permanence under load when compared to plastics, and are completely resistant to ultra-violet rays, weathering, and stress-cracking in the presence of various reagents. Manufacturing cycles for producing die castings are much faster than for plastic injection moldings. Plastics, however, may be cheaper on a unit volume basis, may have color inherent properties which tend to eliminate finishing, are temperature sensitive, and are good electrical insulators.

Sand castings
Compared with sand castings, die castings require much less machining; can be made with thinner walls; can have all or nearly all holes cored to size; can be held within much closer dimensional limits; are produced more rapidly in dies which make thousands of die castings without replacement; do not require new cores for each casting; are easily provided with inserts die cast in place; have smoother surfaces and involve much less labor cost per casting. Sand castings, on the other hand, can be made from ferrous metals and from many non-ferrous alloys not suitable for die casting. Shapes not producible by die casting are available in sand castings; maximum size can be greater; tooling cost is often less and small quantities can be produced more economically.

Permanent mold castings
Compared with permanent mold castings, die castings can be made to closer dimensional limits and with thinner sections; holes can be cored; die castings are produced at higher rates with less manual labor; have smoother surfaces and usually cost less per die casting. Permanent mold casting involves somewhat lower tooling costs, and can be made with sand cores, yielding shapes not available in die casting.

Forgings
Compared with forgings, die castings can be made more complex in shape and have shapes not forgeable; can have thinner sections; can be held to closer dimensions and have coring not feasible in forgings. Forgings, however, are denser and stronger than die castings; have properties of wrought alloys; can be produced in ferrous and other metals, and in sizes not suitable for die castings.

Stampings
Compared with stampings, one die casting can often replace several parts. Die castings frequently require fewer assembly operations; can be held within closer dimensional limits; can have almost any desired variation in section thickness; involve less waste in scrap; are producible in more complex shapes and can be made in shapes not producible in stamped forms. Stampings, on the other hand, have properties of wrought metals; can be made in steel and in alloys not suitable for die casting, and in their simpler forms, are produced more rapidly, and may weigh less than die castings.

Screw machine products
Compared with screw machine products, die castings are often produced more rapidly; involve much less waste in scrap; can be made in shapes difficult or impossible to produce from bar or tubular stock; and may require fewer operations. On the other hand, screw machine products can be made from steel and alloys which cannot be die cast. They have the properties of wrought metals, and they require less tooling expense.

Die casting produces stronger parts with closer tolerances that have greater stability and durability. Die cast parts have greater resistance to temperature extremes and superior electrical properties.

Die casting produces parts with thinner walls, closer dimensional limits and smoother surfaces. Production is faster and labor costs per casting are lower. Finishing costs are also less.

Die casting offers the same advantages versus permanent molding as it does compared with sand casting.

Die casting produces more complex shapes with closer tolerances, thinner walls and lower finishing costs. Cast coring holes are not available with forging.

Die casting produces complex shapes with variations possible in section thickness. One casting may replace several stampings, resulting in reduced assembly time.

Die casting produces shapes that are difficult or impossible from bar or tubular stock, while maintaining tolerances without tooling adjustments. Die casting requires fewer operations and reduces waste and scrap.