Pattern, Pattern Materials, Types of patterns & Pattern allowance (Casting)

Pattern, Pattern Materials, Types of patterns & Pattern allowance (Casting)

Patterns:

        A pattern is an element used for making cavities in the mould, into which molten metal is poured to produce a casting. A full-sized model of the part, slightly enlarged to account for shrinkage and machining allowances in the casting. Made from Wood, Plastic, Metal. They must be tough enough to he used repeatedly. Coated with a parting agent to facilitate there removal.

Pattern Materials:

• The selection of pattern material depends on the following factors:

• Service requirements like quality and degree of accuracy.

• Type of production.

• Type of moulding.

• Possibility of design changes.

• Number of castings to be produced.

The requirements of good patterns are:

• Secure the desired shape and size of the casing.

• Easily worked, shaped and joined.

• Light in weight.

• Resistant to wear and abrasion, corrosion and chemical action.

• Easily available at low cost.

• Able to take good surface finish.

• Dimensionally stable in all situation.

The common materials used in pattern makings are:

• Wood

• Metal

• Plastic

• Waxes

• Rubbers

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Types of patterns:

• Solid pattern.

• Split pattern.

• Loose piece pattern.

• Gated pattern.

• Match plate pattern.

• Follow board pattern

• Sweep pattern .

• Shell pattern.

Solid Patterns:

        Pattern made exactly like the desired casting. It is the easiest pattern to fabricate. Pattern made exactly like the desired casting It is the easiest pattern to fabricate. Determining the location of the parting line between the two halves of the mould can be problem. Limited to very low production quantities.

Split Pattern:

        Split pattern consist of two pieces. These patterns are split along the parting line. Parting line may be flat or irregular surface. These patterns are appropriate for complex part geometries. Used in moderate production quantities.

Match Plate Pattern:

        Two pieces of the split pattern are attached to opposite sides of a wood or metal plate. The gates and runners are also mounted on the match plate. It is used in large number of castings production.

Cope and Drag Pattern:

        A cope and drag patterns mounted on separate match plates. Each half of the mould made separately. Two separately made mould halves are assemble together to form the proper mould cavity. Its is used in higher rate of production.

Gated Pattern:

        A gated pattern is simply one or more loose patterns having attached gates and runners. It is ribbed construction of wood which forms an outline of the pattern to be made. This enables a considerable saving in moulding time and a uniform feeding of molten metal. They are used for mass production of small components.

Loose Piece Pattern:

        Some times, it is very difficult to remove the pattern from the sand if it is made in a single piece. So the pattern is made by two pieces. The main pattern and the obstructing parts as a loose pieces. After moulding the main pattern is removed first and then the loose pieces are removed.

Skeleton Pattern:

        A pattern consisting of wooden frame and strips is made called skeleton pattern. It is ribbed construction of wood which forms an outline of the pattern to be made. It is filled with moulding sand and rammed properly. The surplus sand is removed by means of a strickle.

Sweep Pattern:

        Sweep is a section or proper contour that is rotated about one edge to shape mould cavities. These type of patterns used to produce parts having shapes of rotational symmetry. The moulds are made manually, either in a pit or on the foundry floor.

Segmental Pattern:

        Also known as “part patterns”. Its consist of pivot spindle and a small segment of pattern. Moulding sand is rammed between the outside and inside of the pattern not at the ends of the pattern. After ramming one section, the pattern is lifted up and forward to the next section for ramming and so on. Generally applied to circular work such as rings, Wheel rims, Big size gears, etc...

Shell Pattern:

        This pattern is mainly used for large pipe fittings. Pattern made of metal mounted on a plate and parted along the parting line. These type of pattern is hollow in construction. The outside shape is used as a pattern to make moulds. The inside shape is used as a core box for making cores.

Follow board pattern:

        Patterns that have thin section tend to become distorted or collapse under the stress of ramming. Constructing a block which will fit inside the thin is called “ Follow Board". Its serve as a support during the ramming operation. Also used to form an irregular parting line in the mould when the outlines of the pattern are not regular.

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Pattern allowance:

        Pattern always made larger than final casting. Excess dimension in the pattern is called Pattern Allowances. Allowances considered while designing a pattern for shrinkage, draft, machining, distortion. Types of allowance such as,

• Shrinkage allowance

• Draft allowance

• Machining allowance

• Distortion allowance

• Shake allowance

Shrinkage Allowance:

        As metal solidifies and cools, it shrinks and contracts in size. Pattern is made larger than the finished casting by means of a Shrinkage Allowance to compensate the contact in size. Normally shrinkage allowance is give as mm/m.

Machining Allowance:

        Finish and accuracy achieved in sand casting are generally poor. Casting is functionally required to be of good surface finish on particular surfaces or dimensionally accurate those indicated on the finished working drawings. The extra amount of metal provided on the surfaces to be machined is called “Machining Allowances”. The allowances added to the pattern depends upon the following factors,

• Material of the casting.

• Size and shape of the casting.

• Method of moulding.

Draft Allowance:

        Given to pattern to easy withdraw from mould. When a pattern is drawn from a mould, there is always some possibility of injuring the edges of the mould. The vertical surfaces of a pattern will damage the mould edges. Slight taper on the vertical surfaces of a pattern is known as the “Draft”.The vertical surfaces of a pattern will damage the mould edges by,

• Length of the vertical surface.

• Method of mould.

• Design and economic restrictions imposed on the casting.

Distortion Allowance:

        Applicable to castings tend to warp or distort during the cooling period. The shape of the pattern is thus bent in the opposite direction to overcome this distortion is called “Distortion Allowance”. For example, a U-shaped casting will be distorted during cooling with the legs diverging, instead of parallel.

Shake or Rapping allowances:

        Due to rapping of the pattern in the mould, the size of mould cavity increases slightly. This increase is insignificant for small and medium size castings, but it must be considered for large casting. A shake or rapping allowance shall be given to pattern by making it smaller to compensate for rapping.

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