Casting Processes for General Information

Metal-Mold Process:

Metal-Mold Processes - Reusable or metal-mold processes are used more extensively for copper alloys in Europe and England than in North America; however, they are gaining recognition here as equipment and technology become increasingly available. Permanent mold casting in North America is identified as gravity die casting or simply die casting in Europe and the U.K. The process called die casting in North America is known as pressure die casting abroad.

Refractory Molds:

Refractory Molds - Of the several refractory-mold-based methods, the Shaw process is probably the best known. Here, the wood or metal pattern halves are dipped into an aggregate slurry containing a methyl silicate binder, forming a shell. After stripping the pattern, the shell is fired at a high temperature to produce a strong refractory mold. Metal is introduced into the mold while it is still hot. This aids feeding but it also produces the relatively slow cooling rates and coarse-grained structures that are typical of the process.

Dimensional accuracy as good as + 0.003 in (_+ 0.08 mm) is attainable in castings smaller than about one inch (25 mm), while tolerances as close as + 0.045 in (+1.1 mm) are claimed in castings larger than 15 in (630 mm) in cross-section. Additional allowances of about 0.010-0.020 in (0.25-0.5 mm) must be included across the parting line. Surface finishes are typically better than 80 gin (2 gm) rms in nonferrous castings.

Waterless Molding:

Waterless Molding - aims to eliminate the sometimes detrimental effects of moisture in the molding sand. Clays are treated to react with oils rather than water to-de them bond to the sand particles. The hot strength of the waterless-bonded sand is somewhat lower than that of conventional green sands. This reduces the force needed to displace the sand as the casting shrinks during solidification, which in mm reduces the potential for hot tearing. On the other hand, sands with low hot strength have a greater tendency to be damaged by hot metal flowing into the mold.'

For large castings, molds may be baked or partially dried to increase their strength. The surfaces of skin-dried molds are treated with organic binders, and then dried by means of torches or heaters. To make dry sand molds, simple organic bonding agents such as molasses are dissolved in the bonding water when making up the green sand mixture. The entire mold is then baked to develop the desired hot strength. Besides hardening the mold, removing water also reduces the chance for blowholes and other moisture-related casting defects. Baking and skin drying are expensive operations and the dry sand methods are rapidly being replaced by a variety of no-bake processes, described below.

There are three general types of low-temperature-curing, chemical binders: Cement has traditionally been used as a bonding agent in the extremely large molds used to cast marine propellers and similar products. Cement bonded molds are extremely strong and durable, but they must be designed carefully since their inability to yield under solidification shrinkage stresses may cause hot tearing in the casting.

Organic binders utilize resins that cure by reaction with acidic catalysts. Furan-, phenolic-, and urethane-base systems are the most popular of the large variety of currently available bonding agents. Of the inorganic binders, the well-known liquid sodium silicate-CO2 process is most widely used for copper alloy castings.

Pressure Die Casting:

Pressure Die Cast - Deeco has partnered with a leading designer and manufacturer of sophisticated forged and die cast-engineering products. These products and services are supplied to a growing number of international customers and markets.

Our approach to design is both unique and innovative. We recognize that in today’s global market it is no longer appropriate to simply adopt a one dimensional, low cost raw material die cast sourcing strategy. Our aim is to provide total solutions to our customer’s component/systems requirements by becoming an integral part of the design team offering advice on product design, assembly and design for manufacture. This allows customers to reduce the total cost of the assembly or system rather than just the raw die cast component cost, thus adding value to their product or service.

This multi dimensional approach provides for innovation and flexibility throughout the design process and assures that our customers remain competitive at all times in today’s dynamic, and rapidly changing global market. We consider factors such as quality, performance and aesthetics to be equal in importance to cost. In this regard we have adopted a market driven philosophy in order to maintain our customers competitive advantage.

Our lean manufacturing philosophy enables us to maintain total control at every stage of the design, development and production processes. As well as offering traditional die cast products in aluminum and zinc, we also offer post production machining capabilities, surface finishing such as polishing, plating and powder coating, facilities for the assembly of related components, application-specific packaging as well as a range of distribution services.

Front-ending the various manufacturing segments of our operation is our technical capability in product and systems design. The latest computer hardware, software, and CAD/CAM technologies support our manufacturing processes. The in-house applications of these processes to metal die casting techniques enables us to become an integral part of the design process with our customers. This total systems philosophy and approach means our customers enjoy short development lead times and world class, technically superior products and services.

Sand Casting:

Sand Casting - Sand casting currently accounts for about 75% of U.S. copper alloy foundry production. The process is relatively inexpensive, acceptably precise and above all, highly versatile. It can be utilized for castings ranging in size from a few ounces to many tons. Further, it can be applied to simple shapes as well as castings of considerable complexity, and it can be used with all of the copper casting alloys.

Sand casting imposes few restrictions on product shape. The only significant exceptions are the draft angles that are always needed on flat surfaces oriented perpendicular to the parting line. Dimensional control and consistency in sand castings ranges from about + 0.030 to + 0.125 in (+ 0.8 to 3.2 mm). Within this range, the more generous tolerances apply across the parting line. Surface finish ranges between approximately 300 and 500 gin (7.7 - 12.9 gm) rms. With proper choice of molding sands and careful foundry practice, surprisingly intricate details can be reproduced. There are a number of variations on the sand casting process.

Plaster Molding:

Plaster Molding - Copper alloys can also be cast in plaster molds to produce precision products of near-net shape. Plaster-molded castings are characterized by surface finishes as smooth as 32 gin rms and dimensional tolerances as close as _+ 0.005 in (+ 0.13 mm), and typically require only minimal finish machining. In some cases, rubber patterns can be used. These have the advantage of permitting re-entrant angles and zero-draft faces in the casting's design.

Gypsum plaster (CaSO4) is normally mixed with refractory or fibrous compounds for strength and specific mechanical properties. The plaster must be made slightly porous to allow the escape of gases as the castings solidify. This can be achieved by autoclaving the plaster molds in steam, a technique known as the Antioch process. This produces very fine cast surfaces suitable for such precision products as tire molds, pump impellers, plaques and artwork. It is relatively costly.

Foaming agents produce similar effects at somewhat lower costs. Labor cost remains relatively high, however. Foamed plaster molds produce very fine surface finishes with good dimensional accuracy, but they are better suited to simple shapes.

Most plaster mold castings are now made using the Copaco process, which utilizes conventional wood or metal patterns and gypsum-fibrous mineral molding compounds. The process is readily adapted to automation; with low unit costs, it is the preferred plaster mold method for long production runs. On the whole, however, plaster molding accounts for a very small fraction of the castings market.

Machine Molding:

Machine Molding - method is 'automated and therefore faster than bench molding, but the casting process is essentially similar. Molding machines sling, ram, jolt or squeeze sand onto patters, which in this case may consist of several parts arranged on a moldboard. Dimensional control, surface finish and product consistency are better than those achievable with bench molding. Favorable costs can be realized from as few as several dozen castings. Machine-molded sand casting is therefore the most versatile process in terms of production volume.

Investment Casting:

Investment Casting - Very fine surface finishes and excellent reproduction of detail are some characteristics of investment casting or lost wax process. The process was practiced by several ancient cultures and has survived virtually without modification for the production of artwork, statuary and fine jewelry. Today, the process's most important commercial application is in the casting of complex, net shape precision industrial products such as impellers and gas turbine blades.

The process first requires the manufacture of an intricate metal die with a cavity in the shape of the finished product (or parts of it, if the product is to be assembled from several castings). Special wax, plastic or a low melting alloy is cast into the die, then removed and carefully finished using heated tools. Clusters of wax patterns are dipped into a refractory/plaster slurry, which is allowed to harden as a shell or as a monolithic mold.

The mold is first heated to melt the wax (or volatilize the plastic), then fired at a high temperature to vitrify the refractory. Metal is introduced into the mold cavity and allowed to cool at a controlled rate.

Investment casting is capable of maintaining very high dimensional accuracy in small castings, although tolerances increase somewhat with casting size. Dimensional consistency ranks about average among the casting methods; however, surface finishes can be as fine as 60 gin (1.5 gm) rms, and the process is unsurpassed in its ability to reproduce intricate detail.

Investment casting is better suited to castings under 100 lbs (45 kg) in weight. Because of its relatively high tooling costs and higher than average total costs, the process is normally reserved for relatively large production runs of precision products, and is not often applied to copper alloys.

Precision Dry Sand:

Precision Dry Sand casting is a process using chemically bonded sands and other mold media to produce unique casting properties. It is considered the best choice for applications requiring intricate coring, large complex parts, high physical requirements, dimensional repeatability and stability. The Precision Dry Sand process also allows for excellent detail, maintains a high degree of dimensional accuracy and promotes metallurgic integrity. Finally it ensures stability of 0.100" wall thickness.

No Bake (Air Set):

No Bake (Air Set) -In this process, silica sand is mixed with a resin that hardens when exposed to the atmosphere. The process requires no water. It can be used for molds as well as cores. It is applicable to products as small as 20 lb (9 kg), although it is mainly used for large castings weighing up to 20,000 lb (9,100 kg). The no-bake process has become very popular in the past 10 years.

Bench Molding:

Bench Molding - operations are performed by hand. Quality and part-to-part consistency depend largely on the skill of the operator. The labor-intensive nature of bench molding usually restricts it to prototypes or short production runs. Patterns are another significant cost factor, especially if their cost cannot be amortized over a large number of castings. Still, bench molding remains the most economical method when only a few castings must be produced.

Permanent Mold:

Permanent Mold - casting utilizes a metallic mold. The mold is constructed such that it can be opened along a conveniently located parting line. Hot metal is poured through a sprue to a system of gates arranged so as to provide even, low-turbulence flow to all parts of the cavity. Baked sand cores can be provided just as they would be with conventional sand castings. Chills are unnecessary since the metal mold provides very good heat transfer. The nature of the process necessitates adequate draft angles along planar surfaces oriented perpendicular to the parting line. Traces of the parting line may be visible in the finished casting and there may be some adherent flashing, but both are easily removed during finishing.

Permanent mold castings are characterized by good part-to-part dimensional consistency and very good surface finishes (about 70 gin, 1.8 [tm). Any traces of metal flow lines on the casting surface are cosmetic rather than functional defects. Permanent mold castings exhibit good soundness. There may be some micro shrinkage, but mechanical properties are favorably influenced by the castings' characteristically fine grain size. The ability to reproduce intricate detail is only moderate, however, and for products in which very high dimensional accuracy is required, plaster mold or investment processes should be considered instead.

Permanent mold casting is more suitable for simple shapes in mid-size castings than it is for very small or very large products. Die costs are relatively high, but the absence of molding costs makes the overall cost of the process quite favorable for medium to large production volumes.

Green Sand Casting:

Green Sand Casting - still the most widely used process--molds are formed in unbaked (green) sand, which is most often silica, SiO2, bonded with water and a small amount of a clay to develop the required strength. The clay minerals (montmorillonite, kaolinite) absorb water and form a natural bonding system that holds the sand particles together. Various sands and clays may be blended to suit particular casting situations.

The mold is made by ramming prepared sand around a pattern, held in a flask. The patterns are withdrawn, leaving the mold cavity into which metal will be poured. Molds are made in two halves, an upper portion, the cope, and a lower portion, the drag. The boundary between cope and drag is known as the parting line.

Cores, made from sand bonded with resins and baked to give sufficient strength, may be supported within the mold cavity to form the internal structure of hollow castings. Chills of various designs may be embedded in the mold cavity wall to control the solidification process.

Risers are reservoirs of molten metal used to ensure that all regions of the casting are adequately fed until solidification is complete. Risers also act as heat sources and thereby help promote directional solidification. Molten metal is introduced into the mold cavity through a sprue and distributed through a system of gates and runners.

Shell Molding:

Shell Molding - Resin-bonded sand systems are also used in the shell molding process, in which prepared sand is contacted with a heated metal pattern to form a thin, rigid shell. As in sand casting, two mating halves of the mold are made to form the mold cavity. Common shell molding binders include phenol formaldehyde resins, furan or phenolic resins and baking oils similar to those used in cores. Non-baking resins (furans, phenolics, urethanes) are also available; these can claim lower energy costs because they do not require heated partems.

The shell molding process is capable of producing quite precise castings and nearly rivals metal-mold and investment casting in its ability to reproduce fine details and maintain dimensional consistency. Surface finish, at about 125 gin (3.2 gm) rms, is considerably better than that from green sand casting.

Shell molding is best suited to small-to-intermediate size castings. Relatively high pattern costs (pattern halves must be made from metal) favor long production runs. On the other hand, the fine surface finishes and good dimensional reproducibility can, in many instances, reduce the need for costly machining. While still practiced extensively, shell molding has declined somewhat in popularity, mostly because of its high energy costs compared with no-bake sand methods; however, shell molded cores are still very widely used.

Low Pressure Mold:

Low Pressure Mold - Low Pressure Permanent Molds produce castings with consistent quality and highly exacting dimensional stability. Unusual shapes can be cast using this method with little or no machining. The Low Pressure Permanent Mold process also allows for design flexibility, lending itself to specialized inserts and coring. It ensures dimensional stability to 0.090" wall thickness and permits high reproducibility. Finally it retains extremely tight tolerance controls