Pattern (casting)

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Wooden pattern for a cast-iron gear with curved spokes Roue en bois.jpg
Wooden pattern for a cast-iron gear with curved spokes
The top and bottom halves of a sand casting mould showing the cavity prepared by patterns. Cores to accommodate holes can be seen in the bottom half of the mould, which is called the drag. The top half of the mould is called the cope. SandMoldCopeDragCores.jpg
The top and bottom halves of a sand casting mould showing the cavity prepared by patterns. Cores to accommodate holes can be seen in the bottom half of the mould, which is called the drag. The top half of the mould is called the cope.

In casting, a pattern is a replica of the object to be cast, used to form the sand mould cavity into which molten metal is poured during the casting process. Once the pattern has been used to form the sand mould cavity, the pattern is then removed, molten metal is then poured into the sand mould cavity to produce the casting. The pattern is non consumable and can be reused to produce further sand moulds almost indefinitely.

Contents

Due to the fact that almost all metals contract or shrink as their temperature falls, casting patterns must be made larger in size than the actual casting they will produce. Aluminium casting contraction is ~1.3% for example, so patternwork for a cast aluminium part would be made 1.3% bigger than the cast part itself. [1] [2]

Patterns used in sand casting may be made of wood, metal, plastics or other materials. Patterns are made to exacting standards of construction, so that they can last for a reasonable length of time, according to the quality grade of the pattern being built, and so that they will repeatably provide a dimensionally acceptable casting. [3]

Patternmaking

The making of patterns, called patternmaking (sometimes styled pattern-making or pattern making), is a skilled trade that is related to the trades of tool and die making and moldmaking, but also often incorporates elements of fine woodworking. Patternmakers (sometimes styled pattern-makers or pattern makers) learn their skills through apprenticeships and trade schools over many years of experience. Although an engineer may help to design the pattern, it is usually a patternmaker who executes the design. [4]

Materials used

Typically, materials used for pattern making are wood, metal or plastics. Wax and Plaster of Paris are also used, but only for specialized applications. Sugar pine wood is the most commonly used material for patterns, primarily because it is soft, light, and easy to work. Honduras mahogany was used for more production parts because it is harder and would last longer than pine. Once properly cured, it is about as stable as any wood available, not subject to warping or curling. Once the pattern is built, the foundry does not want it changing shape. True Honduras mahogany is harder to find now because of the decimation of the rain forests, so now there is a variety of woods marketed as mahogany. Fiberglass and plastic patterns have gained popularity in recent years because they are water proof and very durable. Metal patterns are long lasting and do not succumb to moisture, but they are heavier, more expensive and difficult to repair once damaged. [5]

Wax patterns are used in an alternative casting process called investment casting. A combination of paraffin wax, bees wax and carnauba wax is used for this purpose. In this case the wax "pattern" is melted out from the mould cavity which is normally a rigid plaster like material rather than sand, so the wax "pattern" can only be used once. [5]

Plaster of Paris is usually used in making master dies and molds, as it gains hardness quickly, with a lot of flexibility when in the setting stage. [5]

Design

Sprues, gates, risers, cores, and chills

The patternmaker or foundry engineer decides where the sprues , gating systems, and risers are placed with respect to the pattern. Where a hole is desired in a casting, a core may be used which defines a volume or location in a casting where metal will not flow into. Sometimes chills may be placed on a pattern surface prior to molding, which are then formed into the sand mould. Chills are heat sinks which enable localized rapid cooling. The rapid cooling may be desired to refine the grain structure or determine the freezing sequence of the molten metal which is poured into the mould. Because they are at a much cooler temperature, and often a different metal from what is being poured, they do not attach to the casting when the casting cools. The chills can then be reclaimed and reused.

The design of the feeding and gating system is usually referred to as methoding or methods design. It can be carried out manually, or interactively using general-purpose CAD software, or semi-automatically using special-purpose software (such as AutoCAST)

Types of Patterns

Patterns are made of wood, metal, ceramic, or hard plastics and vary in complexity.

A single piece pattern, or loose pattern, is the simplest. It is a replica of the desired casting—usually in a slightly larger size to offset the contraction of the intended metal. Gated patterns connect a number of loose patterns together with a series of runners that will be detached after shake-out. Segmented or multi-piece patterns create a casting in several pieces to be joined in post-processing.

Match plate patterns are patterns with the top and bottom parts of the pattern, also known as the cope and drag portions, mounted on opposite sides of a board. This adaptation allows patterns to be quickly moulded out of the molding material. A similar technique called a cope and drag pattern is often used for large castings or large production runs: in this variation, the two sides of the pattern are mounted on separate pattern plates that can be hooked up to horizontal or vertical machines and moulded with the molding material. When the parting lines between the cope and drag are irregular, a follow board can be used to support irregularly shaped, loose patterns.

Sweep patterns are used for symmetric molds, which are contoured shapes rotated around a center axis or pole through the molding material. A sweep pattern is a form of skeleton pattern: any geometrical pattern that creates a mold by being moved through the molding material.

Skeleton pattern comes into play when the entire setup made of wood or metal is costlier. It is made usually as a part with some gaps left unfilled and those unfilled parts are filled or covered by loam sand or clays. Strickle board or Strike-off board is used to scrape the excess clay if applied to the gaps. E.g. Turbine Casing, Soil and Water pipe bends, valve bodies and boxes.

Allowances

To compensate for any dimensional changes which will happen during the (solid) cooling process, allowances are usually made in the pattern. [6]

Liquid Shrinkage

Almost all metals shrink volumetrically during solidification, this is known as liquid shrinkage. Another way of saying that is almost all metals undergo a volume increase upon melting, or liquidus temperature. Typical "volume shrinkage" is in the range between 3.5% to 10.0% depending on the alloy. Some graphitic cast irons, when cast in heavier sections, under well controlled conditions, can exhibit a slight positive yield. Type Metal is also known, and used, for its ability to hold a true and sharp cast, and retain correct dimensions after cooling. Normally when making engineering cast parts the "method" is designed along with the pattern - being the riser size, number of risers, and location of risers. Additionally downsprue(s), runner bar(s), and ingate(s) are also designed in "the method". The "method" thus ensures the molten metal is delivered, the mould filled correctly, and the risers filled to "feed" the "shrinking volume" of liquid to the casting during solidification. This "method" is done by a "methods engineer", who may be a patternmaker (with additional training), a founding engineer, or metallurgist who is familiar with concept of volume increase / volume loss associated with melting and casting / solidification. Example: Assume steel at 7.85 density (solid) and 6% shrinkage, or better said, a 6% volume increase when molten. A mould has been made to cast a 100 kg block, based on the solid density of steel. The liquid density of steel is only 94% that of its solid density value - about 7.38 when liquid. Thus when the 100 kg block (solid calculation) is filled with liquid it contains a mass of only 94 kg. The 6 kg, has to be supplied from a "riser" or "feeder" during solidification - thus the solid object now has a mass of 100 kg. The method is a system to deal with the volume loss during solidification. This (technically) is not an allowance.

This extra size that is given on the pattern for metal contraction is called "the contraction allowance". These values are typically between 0.6% and 2.5%. This is accounted for using a contraction rule, which is an oversized rule. Contraction rules are generally available for the common industrially cast alloys. Alternately, the Patternmaker will simply add a nominated percentage to all dimensions. An example of this allowance - if a bush were required to be 1500mm O/D, 1000mm I/D and 300mm high using a 2% contraction rule: The Patternmaker would make the pattern 1530mm O/D, (as it will contract in), 1020 I/D (as material tend to contract towards the centre or Centre of gravity ) - important to note that allowance is added even to the inside diameter as material tend to contact towards the centre. [7] The core used is either made up of collapsible sand or it is given enough hollow space at the centre of the core to allow metal to expand. Finally, the height dimension would be 306mm.

The contraction amount can also be varied slightly by the sand system being used for the mould and any cores, for example clay-bonded sand, chemical bonded sands, or other bonding materials used within the sand. Exact values can vary between different foundries due to the sand systems being used. Each foundry, by gauging its own patterns and castings, can refine its own contraction allowances.

Shrinkage and Contraction can again be classified into liquid shrinkage and solid contraction. Liquid shrinkage is the reduction in volume during the process of solidification (liquid to solid), the liquid shrinkage is accounted for by risers. Solid contraction is the reduction in dimensions during the cooling of the (solid) cast metal. Contraction allowance takes into account only the solid contraction.

Draft allowance

When the pattern is to be removed from the sand mold, there is a possibility that any leading edges may break off, or get damaged in the process. To avoid this, a taper is provided on the pattern, so as to facilitate easy removal of the pattern from the mold, and hence reduce damage to edges. The taper angle provided is called the Draft angle. The value of the draft angle depends upon the complexity of the pattern, the type of molding (hand molding or machine molding), height of the surface, etc. Draft provided on the casting is usually 1 to 3 degrees on external surfaces (5 to 8 internal surfaces). [7]

Finishing or Machining allowance

The surface finish obtained in sand castings is generally poor (dimensionally inaccurate), and hence in many cases, the cast product is subjected to machining processes like turning or grinding in order to improve the surface finish. During machining processes, some metal is removed from the piece. To compensate for this, a machining allowance (additional material some times referred to as green) should be given in the casting. [7] the amount of finish allowance depends on the material of the casting, size of casting, volume of production, method of molding, etc.

Shake allowance

Usually during removal of the pattern from the mold cavity, the pattern is rapped all around the faces, in order to facilitate easy removal. In this process, the final cavity is enlarged. To compensate for this, the pattern dimensions need to be reduced. There are no standard values for this allowance, as it is heavily dependent on the personnel. This allowance is a negative allowance, and a common way of going around this allowance is to increase the draft allowance. Shaking of the pattern causes an enlargement of the mould cavity and results in a bigger casting. [7]

Distortion allowance

During cooling of the mould, stresses developed in the solid metal may induce distortions in the cast. This is more evident when the mould is thinner in width as compared to its length. This can be eliminated by initially distorting the pattern in the opposite direction. [6]

Demand

Patterns continue to be needed for sand casting of metals. For the production of gray iron, ductile iron and steel castings, sand casting remains the most widely used process. For aluminum castings, sand casting represents about 12% of the total tonnage by weight (surpassed only by die casting at 57%, and semi-permanent and permanent mold at 19%; based on 2006 shipments). The exact process and pattern equipment is always determined by the order quantities and the casting design. Sand casting can produce as little as one part, or as many as a million copies.

Although additive manufacturing modalities such as SLS or SLM have potential to replace casting for some production situations, casting is still far from being completely displaced. Wherever it provides suitable material properties at competitive unit cost, it will remain in demand.

Related Research Articles

<span class="mw-page-title-main">Metal casting</span> Pouring liquid metal into a mold

In metalworking and jewelry making, casting is a process in which a liquid metal is delivered into a mold that contains a negative impression of the intended shape. The metal is poured into the mold through a hollow channel called a sprue. The metal and mold are then cooled, and the metal part is extracted. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods.

<span class="mw-page-title-main">Injection moulding</span> Manufacturing process for producing parts by injecting molten material into a mould, or mold

Injection moulding is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals, glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed, and injected into a mould cavity, where it cools and hardens to the configuration of the cavity. After a product is designed, usually by an industrial designer or an engineer, moulds are made by a mould-maker from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is widely used for manufacturing a variety of parts, from the smallest components to entire body panels of cars. Advances in 3D printing technology, using photopolymers that do not melt during the injection moulding of some lower-temperature thermoplastics, can be used for some simple injection moulds.

<span class="mw-page-title-main">Ingot</span> Piece of relatively pure metal

An ingot is a piece of relatively pure material, usually metal, that is cast into a shape suitable for further processing. In steelmaking, it is the first step among semi-finished casting products. Ingots usually require a second procedure of shaping, such as cold/hot working, cutting, or milling to produce a useful final product. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods. Precious metal ingots can be used as currency, or as a currency reserve, as with gold bars.

<span class="mw-page-title-main">Die casting</span> Metal casting process

Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly to an injection mold during the process. Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, pewter, and tin-based alloys. Depending on the type of metal being cast, a hot- or cold-chamber machine is used.

<span class="mw-page-title-main">Sand casting</span> Metal casting process using sand as the mold material

Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand—known as casting sand—as the mold material. The term "sand casting" can also refer to an object produced via the sand casting process. Sand castings are produced in specialized factories called foundries. In 2003, over 60% of all metal castings were produced via sand casting.

<span class="mw-page-title-main">Riser (casting)</span>

A riser, also known as a feeder, is a reservoir built into a metal casting mold to prevent cavities due to shrinkage. Most metals are less dense as a liquid than as a solid so castings shrink upon cooling, which can leave a void at the last point to solidify. Risers prevent this by providing molten metal to the casting as it solidifies, so that the cavity forms in the riser and not the casting. Risers are not effective on materials that have a large freezing range, because directional solidification is not possible. They are also not needed for casting processes that utilized pressure to fill the mold cavity.

<span class="mw-page-title-main">Lost-foam casting</span> Type of evaporative-pattern casting process

Lost-foam casting (LFC) is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of polymer foams to simplify the investment casting process by removing the need to melt the wax out of the mold.

Ceramic forming techniques are ways of forming ceramics, which are used to make everything from tableware such as teapots to engineering ceramics such as computer parts. Pottery techniques include the potter's wheel, slip casting and many others.

Spin casting, also known as centrifugal rubber mold casting (CRMC), is a method of utilizing inertia to produce castings from a rubber mold. Typically, a disc-shaped mold is spun along its central axis at a set speed. The casting material, usually molten metal or liquid thermoset plastic, is then poured in through an opening at the top-center of the mold. The filled mold then continues to spin as the metal solidifies.

<span class="mw-page-title-main">Metal injection molding</span> Metalworking process in which finely-powdered metal is mixed with binder material

Metal injection molding (MIM) is a metalworking process in which finely-powdered metal is mixed with binder material to create a "feedstock" that is then shaped and solidified using injection molding. Metal injection molding combines the most useful characteristics of powder metallurgy and plastic injection molding to facilitate the production of small, complex-shaped metal components with outstanding mechanical properties. The molding process allows high volume, complex parts to be shaped in a single step. After molding, the part undergoes conditioning operations to remove the binder (debinding) and densify the powders. Finished products are small components used in many industries and applications.

<span class="mw-page-title-main">Foundry</span> Factory that produces metal castings

A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals processed are aluminum and cast iron. However, other metals, such as bronze, brass, steel, magnesium, and zinc, are also used to produce castings in foundries. In this process, parts of desired shapes and sizes can be formed.

<span class="mw-page-title-main">Continuous casting</span> Process for solidifying molten metal

Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a "semifinished" billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary molds to form ingots. Since then, "continuous casting" has evolved to achieve improved yield, quality, productivity and cost efficiency. It allows lower-cost production of metal sections with better quality, due to the inherently lower costs of continuous, standardised production of a product, as well as providing increased control over the process through automation. This process is used most frequently to cast steel. Aluminium and copper are also continuously cast.

<span class="mw-page-title-main">Investment casting</span> Industrial process based on lost-wax casting

Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term "lost-wax casting" can also refer to modern investment casting processes.

A chill is an object used to promote solidification in a specific portion of a metal casting mold. Normally the metal in the mould cools at a certain rate relative to thickness of the casting. When the geometry of the molding cavity prevents directional solidification from occurring naturally, a chill can be strategically placed to help promote it. There are two types of chills: internal and external chills.

Permanent mold casting is a metal casting process that employs reusable molds, usually made from metal. The most common process uses gravity to fill the mold, however gas pressure or a vacuum are also used. A variation on the typical gravity casting process, called slush casting, produces hollow castings. Common casting metals are aluminium, magnesium, and copper alloys. Other materials include tin, zinc, and lead alloys and iron and steel are also cast in graphite molds.

Ceramic mold casting, also known ambiguously as ceramic molding, is a group of metal casting processes that use ceramics as the mold material. It is a combination of plaster mold casting and investment casting. There are two types of ceramic mold casting: the Shaw process and the Unicast process.

<span class="mw-page-title-main">Full-mold casting</span>

Full-mold casting is an evaporative-pattern casting process which is a combination of sand casting and lost-foam casting. It uses an expanded polystyrene foam pattern which is then surrounded by sand, much like sand casting. The metal is then poured directly into the mold, which vaporizes the foam upon contact.

<span class="mw-page-title-main">Casting</span> Manufacturing process in which a liquid is poured into a mold to solidify

Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various time setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods. Heavy equipment like machine tool beds, ships' propellers, etc. can be cast easily in the required size, rather than fabricating by joining several small pieces. Casting is a 7,000-year-old process. The oldest surviving casting is a copper frog from 3200 BC.

Shell molding, also known as shell-mold casting, is an expendable mold casting process that uses resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labour requirements. It is used for small to medium parts that require high precision. Shell molding was developed as a manufacturing process during the mid-20th century in Germany. It was invented by German engineer Johannes Croning. Shell mold casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable mold. However, in shell mold casting, the mold is a thin-walled shell created from applying a sand-resin mixture around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell molds. A reusable pattern allows for higher production rates, while the disposable molds enable complex geometries to be cast. Shell mold casting requires the use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal.

A casting defect is an undesired irregularity in a metal casting process. Some defects can be tolerated while others can be repaired, otherwise they must be eliminated. They are broken down into five main categories: gas porosity, shrinkage defects, mould material defects, pouring metal defects, and metallurgical defects.

References

  1. Bawa, H S (2004). Manufacturing Processes – I. Tata McGraw-Hill. pp. 1–12. ISBN   978-0-07-058372-6.
  2. Ammen, C. W. (1999). Metalcasting. McGraw-Hill Professional. pp. 159–176. ISBN   978-0-07-134246-9.
  3. "Types of patterns used in Casting Process - Mechanical Engineering".
  4. Shelly, Joseph Atkinson (1920). Patternmaking: A treatise on the construction and application of patterns, including the use of woodworking tools, the art of joinery, wood turning, and various methods of building patterns and core-boxes of different types. New York: Industrial Press. pp. 2-5 et seq. The common types of patterns are:

    1) Single piece pattern
    2) Split piece pattern
    3) Loose piece pattern
    4) Gated pattern
    5) Match pattern
    6) Sweep pattern
    7) Cope and drag pattern
    8) Skeleton pattern
    9) Shell pattern
    10) Follow board pattern
    11) segmental pattern

  5. 1 2 3 Radhakrishna, K (2011). Manufacturing Process - 1. Bangalore: Sapna Book House. p. 20. ISBN   978-81-280-0207-6.
  6. 1 2 Praveen, Kestoor (2011). Manufacturing process - 1. Bangalore: Star – tech education. p. 16.
  7. 1 2 3 4 Rao, P.N. (2003). Manufacturing Technology. New Delhi: Tata McGraw-Hill. p. 68. ISBN   0-07-463180-2.