Advantages of Die Casting

ADVANTAGES OF 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.

DIE CASTING DESIGN

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.

Type of Alloy (base metal)

ZINC

ALUMINUM

MAGNESIUM

COPPER

Maximum weight of casting, lb.

75

70

44

10

Minimum wall thickness, large castings, in.

.035**

.080

.100

.090

Minimum wall thickness, small castings, in.

.015**

.040

.040

.055

Minimum variation per inch of diameter or length from drawing dimensions over one inch ***

.001

.0015

.0015

.009

Cast threads, max. no. per in. external

32

24

24

10

Cored holes, min. dia., in.****

.050

.080

.080

.250

GUIDES FOR DESIGN

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.

COMPARISONS WITH OTHER PRODUCTS

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.