What is PATTERNS in Metal Casting ?

It is a replica of the part to be cast and is used to prepare the mould cavity. These are made of either wood or metal. A mould is assembly of two or more metal blocks or bonded refractory particles (sand) consisting of primary cavity. The mould cavity holds the liquid material and essential act as negative of the desired product. The mould also contain secondary cavity for pouring and channeling the liquid material.

Types of Patterns

1.  Loose pattern: It is made in one piece, usually from wood, and is used for castings numbering up to 100.

2.  Gated pattern: This is simply one or more than one loose pattern with attached gates and runners and provides channel through which the molten metal can flow from the pouring sprue to the mould cavity. This pattern is frequently set on follow board conforming to the parting surface of the mould. The follow board helps in an easy removal of the pattern after the mould has been prepared.

3. Match plate pattern: This pattern is made in two halves mounted on both sides of a match plate( of wood or metal )conforming to the contour of the parting surface. The match plate is accurately placed between the cope and the drag flasks by means of locating pins. For small castings, several patterns can be mounted on the same match plate.

4. Cope and drag pattern: The cope and drag halves of a split pattern are separately mounted on two match plates. Thus the cope and drag flasks are made separately and brought together ( with accurate relative location) to produce the complete mould.

5. Sweep pattern: Normally made of wood, it is used for generated surface of revolution in large castings.

6. Skeleton pattern: This consists of a simple wooden frame outlining the shape of the casting. It is used to guide the moulder for hand-shaping the mould and for large casting having simple geometrical shapes. 

Pattern Design

While designing a pattern, the parting line should be chosen so as to have a smallest portion of the pattern in the cope. As the moulding sand has greater strength in compression than in tension, the heavier section of the pattern should be included in the drag. The possible defects due to loose sand in the mould are more frequent in the cope half. For this reason, most critical surface should also be included in the drag.

Pattern Materials

The usual pattern materials are wood, metal and plastics. The most commonly used pattern material is wood. Main reason for using wood is easy availability and low weight and easily shaped. Disadvantage is absorption of moisture.

Choice of pattern material depends essentially on the size of casting, the number of casting to be made from the pattern and dimensional accuracy required. For very large castings, wood may be only practical pattern material. Because of their durability and smooth surface finish, metal pattern are used for large scale casting production and for closer dimensional tolerance.

Though many materials such as cast iron, brass etc. can be used as pattern material. White metal, aluminium are most commonly used. These are light, can be easily worked and corrosion resistant. Since white metal pattern can be made use for making additional pattern without worrying about the double shrinkage allowances, most metal patterns cast in sand moulds from a master wood pattern provide with double shrinkage allowances, plastic also used as pattern material because of their low weight, easily formability, smooth surface finish and durability. They do not absorb moisture. The making of plastic pattern can be done in clay mould or mould made of Plaster of Paris. The most generally used plastics are cold setting epoxy resin with suitable filters.

Pattern Allowances

For metallurgical and mechanical reasons, following allowances are made on the pattern if casting is to be dimensionally correct.

1. Shrinkage Allowance: During casting process, the molten metals solidify inside the mould cavity and solidified casting is smaller than the cavity due to shrinkage. Therefore a pattern is made larger than the size required by an amount equal to the shrinkage that occurs.

Name of Metals	Shrinkage Allowance mm/100 mm
Cast iron Brass Aluminium Zinc Lead Steel Copper Malleable iron	1.0 1.6 1.6 2.4 5.4 2.1 1.9 1.0

2. Draft allowance: To remove the pattern from the mould without damaging the mould cavity, a taper is given to all vertical faces of pattern which is called draft. Draft may be as much as  2 mm per 100 mm of vertical surface.

  

   3. Machining allowance: Certain castings are machined after fettling. The casting should have the additional material that is removed by machining where high dimensional accuracy is needed or a smooth machined surface is needed, either of which cannot be achieved is casting. Therefore the casting is made large as such places that require machining by providing a machining allowance that varies form 1.5 mm to 6 mm for ordinary casting and as much as 12.5 mm for large casting, depending on the material, dimension and surface finish required.

4. Shake and Distortion allowances: These allowances may be provided for large casting and intricate shapes of castings where distortion by thermal stresses may occur.

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