Wealth of molding methods meet every casting need.

Wealth of Molding Methods Meet Every Casting Need

Last month, the Metalcasting Process covered mold making with sand, the most commonly used molding material. However, foundries use many other materials to make molds, such as ceramic shells, metallic permanent molds and plaster. As this installment shows, each of these molding methods has casting applications for which they are best suited.

Investment Casting 

The investment casting process, also referred to as the lost-wax process, is the process of completely investing a three-dimensional pattern in all of its dimensions to produce a one-piece mold into which molten metal will be poured.

Although the making of castings by the lost-wax pattern method has been used by artists for thousands of years, it was early in the 20th century that the dental profession began the succesful application of the technique to making of metal fillings and crowns.

This led to the demand for larger cast-to-shape surgical prosthetic devices in chromium-cobalt and other advanced alloys that could not be forged or machined. During World War II, the process was adapted to the manufacture of engineered castings of many types of metals which were not amenable to fabrication by other than casting methods.

Investment casting, as encountered in production facilities today, uses expendable patterns usually made of wax, though plastic and even frozen mercury patterns are used as well. Patterns are produced by injecting wax into a master die and removed when solid. A number of patterns are then attached to suitable gates and risers, forming an assembly referred to as a "tree."

The assembly is precoated by being dipped into a refractory coating material. The coated wax assembly is invested in a liquid molding material, or slurry, by a series of dipping operations. A carefully formulated slurry, consisting of a selected binder plus refractory, usually silica or alumina, forms a thin ceramic shell around the pattern.

Then, refractory grain is sifted onto the coated patterns. The shell assembly is allowed to dry and the coating and drying process is repeated until a ceramic shell is built up to the desired thickness. The molds are allowed to dry, then the molds are fired to burn out all the wax pattern material, develop mold strength and to pre-heat the mold prior to pouring.

The principal advantages of this process over conventional sandcasting are smoother surfaces, closer as-cast tolerances and minimum of machining required.

Some disadvantages of the process are that the process does not lend itself well to mechanization. This, plus drying times needed between each dipping cycle, make investment casting a lower volume process. Robotic applications for shell dipping and automated conveyor systems for shell drying and handling are improving productivity in dipping and drying areas, however.

Investment castings are produced in a wide range of alloys, including steel, copper, aluminum and magnesium alloys, as well as ductile iron and specialty alloys for corrosion and high temperature applications. With each different alloy cast, attention must be paid to the varying insulative and heat transfer characteristics of the ceramic mold material. This is essential in preplanning the gating and risering design for the particular casting to be produced.

Ceramic Process

Investment casting today also incorporates ceramic molding processes which employ a reusable master pattern, rather than expendable patterns.

Ceramics mold materials are made from a clay base and contain various oxides and ingredients other than sand. The raw clays are calcined or fired at high temperatures and are then blended, mixed with water and formed into mold components. Completed molds are then fired again. Ceramic molding may be accomplished through three techniques: "true" ceramic molding, ethyl silicate slurry molding and the Shaw process, a variation of the ethyl silicate process.

In true ceramic molding, the refractory grain may be bonded with calcium or ammonium phosphates. The preferred method for producing the mold is the dry pressing method, in which the molds are made by pressing the ceramic mixture in dies under pressure. After pressing, molds are stripped from the dies and fired.

With the ethyl silicate variation, a mixture of refractory filler, hydrolyzed ethyl silicate and a liquid catalyst are blended together to a slurry consistency. The slurry is poured over a pattern and allowed to gel. After gelation, the mold is stripped and torched with a high pressure gas torch. The mold may then be cooled, assembled and fired, or it may be assembled after firing.

In the Shaw process, split molds are made by pouring a refractory slurry over a prepared pattern. The slurry is composed of a mixture of coarse and fine sillimanite, hydrolyzed ethyl silicate and a liquid catalyst. These are mixed together in less than one minute of mixing time and then poured over the pattern.

After pouring, the slurry is allowed to gel. During the gel stage, the mold is moderately flexible, which facilitates the stripping operation. After two to three minutes, the mold can be stripped away from the pattern and the mold is flamed with high temperature gas flame. The mold burns until all volatile matter is consumed.

Flaming also causes the mold surface to form a system of "craze cracks." Crazing is progressive throughout the mold, producing a mold of high permeability and partial immunity to thermal shock. After mold closing, the mold is fired at red heat until all vestiges of moisture are driven off. This further strengthens the bond and produces an inert, gas-free, erosion-resistant, collapsible mold which can be poured either hot or at room temperature.

Plaster Process

Plaster casting is highly adaptable to the production of nonferrous castings, particularly aluminum, zinc and magnesium, as well as silver and gold. But because the sulfur of the gypsum, which is a major component of most molding plasters, reacts chemically with ferrous metals poured at high temperatures, ferrous casting produced in plaster have poor cast surfaces.

Intricately designed impellers, components of electronics gear and molds for producing tires are examples of parts that are produced in plaster molds. The three generally recognized plaster mold processes are: conventional plaster molded castings; the Antioch process; and the foamed plaster process.

In all these processes, the principal mold ingredient is calcium sulfate. This is usually compounded with various types of fibrous and refractory aggregates and water, which makes the plaster mixture a pourable slurry.

The plaster slurry is poured over a pattern and is allowed to set. The set mold is dehydrated completely to liberate the free and chemically combined water, since the presence of even minute amounts of water in the mold would result in casting defects, due to the low permeability of this type of mold.

Dehydration is accomplished by oven-drying the mold. The high temperatures employed in drying (400-1400F or 200-760C) causes mold shrinkages ranging from 1-2% depending on the temperature. In addition, burnout reduces the strength of the mold by about 50%.

The Antioch plaster molding process has been succesful in producing castings with complex shapes requiring fine detail and thin sections. The major advantage of the process is that it develops a high degree of permeability in the mold. Also, Antioch process molds do not shrink.

A specially-formulated plaster slurry mixture forms the mold, which is subjected to a drying cycle involving steam-autoclaving, air curing and oven drying. When undried molds are partially dehydrated in the autoclave, then allowed to rehydrate in air, gypsum crystals slowly recrystallize into granules about the size of sand grains and the mold acquires a porous structure of high permeability.

The foamed plaster process or permeable metalcasting plaster, has been designed to produce mold permeabilities comparable to those obtained in the Antioch process, but without the autoclaving cycle.

In this process, a foaming agent is added to the plaster slurry and air is beaten into the mix with a rapidly rotating rubber disc. After baking, a properly made mold of permeable metalcasting plaster contains as much as 50% air holes, imparting a high permeability to the mold.

Centrifugal Casting 

Centrifugal force has been used in a variety of way in the production of ferrous and nonferrous castings. By spinning a mold while the molten metal is being poured, centrifugal force acts to distribute the metal properly in the mold, which may be either sand or a permanent metal mold. The centrifugal casting process makes possible the production of large rolls, gas and water pipes, automotive cylinder liners and other castings requiring a cylindrical cavity without cores.

There are three types of centrifugal casting: . true centrifugal casting, in which the casting is spun about its own axis, without risers and cores; . semi-centrifugal casting, in which the object, such as a wheel with spokes, is spun about its own axis, but risers and cores are needed; . centrifuged casting, where the mold impressions are grouped around a central downgate, as in static casting and centrifugal force is used mainly as a mold filling device.

Permanent Mold Processes

The foundry term "permanent mold" is used to describe a mold that can be used repeatedly. Molds are usually made of cast iron, though steel and graphite also are used as mold materials. Permanent mold castings can be produced from all of the metals, including iron and copper alloys, but most production is in lighter metals such as zinc-base, magnesium and aluminum.

When the molten metal is poured into a permanent mold, it is cooled much more rapidly than in a sand mold, producing a sound, dense casting with fine grain structure and superior mechanical properties. Owing to the stability of the metal mold, dimensional tolerances of 0.010 in. can be held, making possible the casting of surfaces to dimensions that would otherwise require machining.

A consistent high quality of finish on the casting surface is inherent to permanent mold processes, so castings require less finishing. For these reasons, parts can be cast very close to finished dimensions. These advantages add up to a near net shape capability for most permanent mold processes. Many parts are cast to dimension with finished holes requiring little or no machining.

The advantages of permanent mold casting of aluminum can be realized when ferrous metals are cast in metal molds, but there has been little documentation of these results to date. The major difference between the permanent mold casting of aluminum and iron lies in their differing solidification characteristics.

Aluminum solidifies as a mass, whereas iron solidifies as water does, with a skin strong enough to retain its shape. This will allow an iron casting to be ejected from a mold quickly, but this advantage has not been capitalized upon until recently.

Previously, annealing was necessary to remove strains inherent to rapid solidification and to remove carbide formations in permanent mold castings. But, according to one manufacturer of ferrous permanent mold systems, annealing destroyed much of the as-cast properties and is not necessary when a recently developed system of controlled rapid ejection is employed.

Rapid controlled ejection of iron castings from a permanent mold is like a continuous bar caster, where any carbides are annealed by the casting's inner heat. Recent developments enable the ejection time to be computer calculated for each mold. For example, a 40 lb flywheel can be ejected in 17 seconds; a small connecting rod, in about three seconds.

In the case of ductile iron, the more rapid solidification rate obtained in a metal mold is said to increase nodule counts to over 1000 nodules per sq. mm for one in. sections. Sections up to four in. may have over 400 nodules per sq. mm. These high counts are said to provide increased strength, elongation and fatigue properties. The casting of carbide-free parts ensures better machinability without annealing.

Permanent molding does have some disadvantages, however. The cost of tooling--the engineering and manufacture of the methods--can be quite high. The initial cost of casting and/or machining the molds must be amortized, therefore, a basic requirement for using the permanent mold process is that the number of castings to be made must be fairly large.

Gravity Permanent Mold Process--The molten metal is poured manually into a permanent mold under gravity conditions, with no external pressure. Static pouring involves manually introducing the metal to the top of the mold through downsprues. Tilt pouring, on the other hand, is a method of pouring metal into a basin while the mold is in a horizontal position. The metal flows into the mold cavity as the mold is gradually tilted to a vertical position.

The mold halves for gravity casting are often comparatively simple. A typical two-part die is split along a vertical joint line passing through the die cavity, the gating, feeding and venting system. Internal cavities can be produced by using separate metal cores, which can sometimes be directly retracted from the casting. However, cores for permanent molds also can be expendable sand, plaster, or shell cores.

Permanent molds are also usually equipped with arrangements of pin locators, clamping devices and casting ejection systems. Die halves may also be hinged. Modern permanent mold practice has taken advantage of mechanization technology with the availability of automated opening, closing, locking and ejection systems.

Low Pressure Permanent Mold--This method uses a minimal amount of pressure (usually 5-15 psi) to fill a permanent mold. The process utilizes a split die similar to other permanent mold processes. The molten metal is contained in a crucible which is housed in a sealed pressure vessel inside of which is a heat shield supporting electrical elements.

A tube passes vertically down through the top of the vessel, with its lower and immersed in the molten metal and its top flange sealed against the furnace lid. A permanent mold is mounted on the working table of the machine, located and sealed onto the open end of the tube. When air pressure is admitted to the furnace, it displaces the molten metal, causing it to travel up the tube and into the cavity.

Diecasting--Essentially a high pressure permanent mold process, diecasting machines inject the molten metal into a permanent mold under pressures of 5000 psi or more. The process is typically is used to produce large volumes of zinc, aluminum and magnesium castings of intricate shape.

The diecasting process is capable of producing castings at the rate of one every few seconds. The rate of production depends largely on the complexity of the casting's design, its sectional thickness and the properties of the alloy. Great care must be taken with the design and gating to avoid the high-pressure porosity to which this process is prone.

Squeeze Casting

Introduced in the U.S. in the mid-1970s, the squeeze process, sometimes called liquid metal forging, uses a high pressure solidification and rapid cooling process in a permanent mold. The process is said to combine the advantages of forgings and castings.

Modified and filtered metal is poured into a preheated die cavity which is located on the bed of a hydraulic press. The press closes the die and pressurizes the liquid metal. The pressure, which exceeds 10,000 psi, is maintained until solidification is complete. The casting is then ejected from the cavity.

Solidification under pressure eliminates internal porosity and the rapid cooling improves metallurgical structure. Properties such as strength and fatigue life are said to exceed those of conventional permanent mold castings, approaching those obtained by forgings.

The process is compatible with a wide range of casting alloys. Squeeze casting also has been found to successfully bond ceramic reinforcing fibers into a molten aluminum matrix, to produce a high strength metal matrix composite casting.

Replicast

The Replicast Ceramic Shell (CS) process is one of the newest molding process, developed in this decade by the Steel Castings Research and Trade Association (SCRATA) to improve steel casting quality.

The ceramic shell process, similar to that used in investment casting, is the principal molding material. However, the process is unique in that it utilizes patterns of expanded polystyrene, similar to those used for the evaporative pattern process, instead of wax. The pattern is coated with a ceramic slurry. The resulting shell is dried and then fired to remove the foam pattern.

The next innovation of the Replicast CS process is the method of preparing molds for pouring. A vacuum is drawn on the shell molds when they are placed in the flask. The area around the shells are filled with loose sand that is vibrated for compaction and then vacuum is applied to the sand as well. The combination of the strength of the ceramic shell and the added support of the vacuum develops a mold system of very high wall strength.

Surface finishes obtained are comparable to those produced by investment casting. Further, casting integrity and as-cast quality levels are very high. Shrinkage, inclusions, porosity and other surface and sub-surface defects are drastically reduced.

It can be concluded from the long list of molding methods described over the two preceding articles that the metal-casting industry has come a long way in the development of stronger, more accurate molds.

Yet, due to foundries' needs for both economical methods of producing quality castings and for precision molding methods for premium castings, all the molding methods discussed, from green sand through Replicast, play important roles in the foundry industry.

PHOTO : In the investment casting process, individual wax patterns are attached to gates and risers to prepare the patterns for dipping into a ceramic slurry, which forms a ceramic shell mold around the patterns.

PHOTO : The next step in investment casting it to form a ceramic shell mold. The foundry shown uses a robot to dip a large cluster of wax pattern trees into the ceramic slurry tank.

PHOTO : The surface of a plaster mold is carefully finished by hand.

PHOTO : A centrifugal mold is spun as it is filled with molten metal.

PHOTO : Within minutes after pouring, a permanent mold is opened and casting removed. Note retractable metal core at right.

PHOTO : The Replicast process requires the ceramic shell mold to be surrounded by loose sand in the flask. Vacuum is applied to sand and mold prior to pouring.