Foundry Production

Centrifugal Casting

Casting made in molds which are rotating so as to produce a centrifugal force in the molten metal.

Centrifugal casting are produced by forcing molten metal to solidify in rotating moulds. The speed of the rotation and metal pouring rate vary with the alloy and size and shape being cast.

Centrifugal casting has greater reliability than static castings. They are relatively free from gas and shrinkage porosity. Many times, surface treatments such as case carburizing, flame hardening and nitriding have to be used when a wear resistant surface must be combined with a hard tough exterior surface. Conventional static casting defects like internal shrinkage, gas porosity and nonmetallic inclusions are less likely to occur in centrifugal casting.

Die Casting

A rapid, water cooled permanent mold casting process, still quite limited to nonferrous metals. This process involves melting a metal (i.e. Aluminum), injecting it into a die and cooling it to form a part.

The die casting process has a number of advantages over other processes used to form metal products.

    1. Parts are made to near net shape with good repeatability.
    2. Die castings can be made to form complex geometry.
    3. The die casting process is a rapid production process where high volume parts can be made economically.

There are three types: the plunger type operated hydraulically, mechanically or by compressed air with or without a gooseneck; the direct-air injection which forces metal from a gooseneck into the die, and the cold-chamber machine. All force the metal into the die with a pressure greater than that of gravity flow.

After casting, the parts made can be taken through a number of finishing steps to create an end product that meets a wide variety of customer needs.

Die Casting (Brit. Pressure Die Casting)

Investment Casting

Investment Casting is the process of casting metal into an investment mold. Also known as “precision casting” or “the lost wax process”, investment casting is a process that uses a wax or thermoplastic pattern. The pattern is invested (or coated) in refractory slurry, and after the mold is dry - the (wax) pattern is melted or burned out of the mold cavity.

The production technique of investment casting is both one of the oldest and most advanced of the metallurgical arts. The root of this production technique dates back to at least the fourth millennium B.C. The artists and sculptors of ancient Egypt and Mesopotamia used the rudiments of the investment casting process to create intricately detailed jewellery, pectorals and idols. Remarkably, civilizations as diverse as China’s Han Dynasty, the Benin Kingdom in Africa and the Aztecs of pre-Columbian Mexico employed similar techniques.

The investment casting technique was largely ignored by modern industry until the dawn of the twentieth century, when it was “rediscovered” by the dental profession for producing crowns and inlays. During World War II, with urgent military demands overtaxing the machine tool industry, the art of investment casting provided a shortcut for producing near-net-shape precision parts and allowed the use of specialized alloys which could not readily be formed by alternative methods. The investment casting process proved practical for many military components, and during the postwar period it expanded into many commercial and industrial applications where complex metal parts were needed. In the decades that followed, many innovations such as the shell process, the steam autoclave, conveying, automation, and robotics have modernized and transformed the process.

In today’s world, investment castings touch all of our lives. When we fly on an airplane, drive an automobile, play golf, use a utility tool, power tool or hand tool, we are using investment castings. Once thought of as suitable only for low volume, high cost applications, investment casting has evolved into a technology capable of producing quantities of millions of parts per year, at costs rivalling those of less flexible and less desirable processes.

The investment casting process begins with the production of wax replicas of the desired castings. These replicas, called patterns, are injection moulded in metal dies. A pattern must be manufactured for each casting to be produced. A number of patterns (depending on size and complexity) are attached to a central wax stick, or sprue, to form a casting cluster, or assembly. After some initial pre-dips, which thoroughly clean the wax, the assemblies are immersed, or “invested,” into a liquid ceramic slurry, and then into a bed of extremely fine sand to form a shell. The first critical layers are often applied by hand. Between each layer the ceramic is allowed to dry. The later, heavier layers are often applied by automated equipment or special shell building robots. Enough layers must be applied to build a shell strong enough to withstand subsequent operations. After the shell is completely dry, the wax is melted out in a high pressure steam autoclave, leaving a hollow void within the mould, which exactly matches the shape of the assembly. Prior to casting, the shells are fired in an oven where intense heat burns out any remaining wax residue and prepares the mould for the molten metal. In the conventional gravity pouring method, metal is poured into the shell through a funnel-shaped pour cup and flows by gravity down the sprue channel, through the gates and into the part cavities. As the metal cools, the parts, gates, sprue and pouring cup become one solid casting. After the casting has cooled, the ceramic shell is broken off and the parts are cut from the sprue using a high speed friction saw. After minor finishing operations, the castings, which are identical in configuration to the wax patterns which shaped them, are ready for shipment to the customer.

Jobbing Foundry

A foundry or casting facility engaged in the manufacture of numerous types of castings. Such a foundry is equipped to economically produce a single casting or in small quantities from a pattern.

No-Bake Molding

Nobake molding is an efficient means to produce medium and low volumes of complex castings in both ferrous and nonferrous metals. In the nobake process, sand is mixed with a chemical binder/catalyst system and then molded around a pattern. After a specified period of time, the sand mixture hardens (resembling a brick in strength) to form the mold halves - at which time the pattern is drawn from the mold. Then, a refractory coating may be applied to both mold halves before they are brought together to form one complete mold for pouring. Nobake molded cores also can be produced using a similar method and assembled into the mold to form more complex shapes.

Nobake molding is known for its versatility. Virtually all metals can be cast via nobake molding, with component weights ranging from less than a pound to several hundred thousand pounds. For casting designers, nobake molding offers:

     1. good dimensional tolerances (±0.005-0.015) because the rigidity of the mold withstands the pressures exerted by the molten metal during casting
    2. compatability with most pattern materials, including wood, plastic, metal, fiberglass and styrofoam, allowing for inexpensive tooling options for casting runs as low as one. In addition, nobake molding imparts minimal tooling wear
    3. design flexibility for intricate casting shapes. The rigidity and tensile strength of nobake molds allows for thin sections of 0.1-in. to be routinely produced. In addition, mold strength allows for minimal draft and radii requirements in casting design.
    4. reduced opportunity for gas-related defects as the nitrogen content of most binder systems used for nobake molding minimize susceptibility to gas porosity
    5. fine surface finishes that can be upgraded further with the mold and core coatings to support special finishing on the cast components such as paint or dressing. In addition, nobake casters can alter their molding media make-up from basic silica sand to higher-end media such as chromite or zircon sand for applications requiring X-ray quality and extreme pressure tightness
    6. ability to work well with unique metalcasting quality enhancement tools such as metal filters, ceramic runner systems and exothermic risers to improve casting properties.
    7. low to medium volume production capability with runs from 1-5000 parts/yr.

Nobake molding typically is an option for production runs from 1-5000 castings/yr . Due to the curing time required for the chemicals to harden the mold as well as the methods to distribute the molding media on the pattern, the high productions achievable with green sand, permanent mold or diecasting aren’t possible with nobake. Nobake molding prefers cast components with higher complexities in low to medium volume runs.

Since unfinished nobake molded castings (without machining) typically cost 20-30% higher than green sand, designers and purchasers sourcing nobake molded castings must offset this price difference by taking advantage of what the process offers. Significant reductions in machining costs can be achieved through the process’ tight tolerances and minimal dimensional variability and by designing in complex shapes and geometries, thin walls, and reduced draft, radii and machine stock.

Designing castings for traditional nobake molding follows many of the same principles used in all other casting processes. Draft is required so patterns can be drawn, sharp corners and angles should be minimized and uniform section thicknesses (especially in the same plane) should be employed as much as possible. However, the process does allow for more daring designs. Consult a nobake foundry with your ideas to determine how best to accomplish a specific casting challenge.