A pattern is a model or the replica of the object to be casted.
Utility of a Pattern
1 Pattern prepares a mould cavity for the purpose of making a casting.
2 Pattern possesses core prints which produces seats in form of extra recess for core placement in the mould.
3 It establishes the parting line and parting surfaces in the mould.
4 Runner, gates and riser may form a part of the pattern.
5 Properly constructed patterns minimize overall cost of the casting.
https://www.metalcastingdesign.com/articles/2014/07/01/10-cost-considerations-your-castings
6 Pattern may help in establishing locating pins on the mould and therefore on the casting with a purpose to check the casting dimensions.
7 Properly made pattern having finished and smooth surface reduce casting defects.
Patterns are generally made in pattern making shop. Proper construction of pattern and its material may reduce overall cost of the castings.
The common materials used for making patterns are wood, metal, plastic, plaster, wax or mercury. The some important pattern materials are discussed as under.
Advantages of wooden patterns
Wood is cheap, light in weight, easily available and can be easily worked. It is easy to join and repair.
But it is susceptible to moisture, tends to warp and wears out quickly due to sand abrasion.
Metallic Patterns
Metallic patterns are not much affected by moisture as wooden pattern. The wear and
tear of this pattern is very less. Metal can be shaped with good precision, surface finish and intricacy. The main disadvantages of metallic patterns are higher cost, higher weight and tendency of rusting.
It is preferred for production of castings in large quantities with same pattern. The metals commonly used for pattern making are cast iron, brass and bronzes and aluminum alloys.
Cast Iron
It is cheaper, stronger, tough, and durable and can produce a smooth surface finish. It
also possesses good resistance to sand abrasion. The drawbacks of cast iron patterns are that
they are hard, heavy, brittle and get rusted easily in presence of moisture.
But it is heavy, brittle and hence it can be easily broken and it may rust.
Brasses and Bronzes
These are heavier and expensive than cast iron and hence are preferred for manufacturing
small castings. They possess good strength, machinability and resistance to corrosion and
wear. They can produce a better surface finish than cast iron. Very thin sections can be easily casted. Brass and bronze pattern is finding application in making match plate pattern
But it is costlier than cast iron and it is heavier than cast iron.
Aluminum Alloys
Aluminum alloy patterns are more popular and best among all the metallic patterns
because of their high lightness, good surface finish, low melting point and good strength.
They also possesses good resistance to corrosion and abrasion by sand and there by enhancing
longer life of pattern. These materials do not withstand against rough handling. These have
poor repair ability and are preferred for making large castings.
They can be damaged by sharp edges. . They are softer than brass and cast iron. Their storing and transportation needs proper care.
White Metal (Alloy of Antimony, Copper and Lead)
Advantages
1. It is best material for lining and stripping plates.
2. It has low melting point around 260°C
3. It can be cast into narrow cavities.
Disadvantages
1. It is too soft. Its storing and transportation needs proper care. It wears away by sand or sharp edges.
Plastic
Plastics are getting more popularity now a days because the patterns made of these materials
are lighter, stronger, moisture and wear resistant, non sticky to molding sand, durable and
they are not affected by the moisture of the molding sand. Moreover they impart very smooth
surface finish on the pattern surface. These materials are somewhat fragile, less resistant to
sudden loading and their section may need metal reinforcement.
The plastics used for this purpose are thermosetting resins. Phenolic resin plastics are commonly used. These are originally in liquid form and get solidified when heated to a specified temperature. To prepare a plastic pattern, a mould in two halves is prepared in plaster of paris with the help of a
wooden pattern known as a master pattern. The phenolic resin is poured into the mould and
the mould is subjected to heat. The resin solidifies giving the plastic pattern. Recently a new
material has stepped into the field of plastic which is known as foam plastic. Foam plastic is
now being produced in several forms and the most common is the expandable polystyrene
plastic category. It is made from benzene and ethyl benzene.
Plaster
This material belongs to gypsum family The main advantages of plaster are that it has high compressive strength and is of high expansion setting type which compensate for the shrinkage allowance of the casting metal. Plaster of paris pattern can be prepared either by directly pouring the slurry of plaster and water in moulds prepared earlier from a master pattern or by sweeping it into desired shape or form by the sweep and strickle method. It is also preferred for production of small size intricate castings and making core boxes.
Wax
Patterns made from wax are excellent for investment casting process. The materials used
are blends of several types of waxes, and other additives which act as polymerizing agents,
stabilizers, etc. The commonly used waxes are paraffin wax, shellac wax, bees-wax, cerasin
wax, and micro-crystalline wax. The properties desired in a good wax pattern include low
ash content up to 0.05 per cent, resistant to the primary coat material used for investment,
high tensile strength and hardness, and substantial weld strength. The general practice of
making wax pattern is to inject liquid or semi-liquid wax into a split die. Solid injection is
also used to avoid shrinkage and for better strength. Waxes use helps in imparting a high
degree of surface finish and dimensional accuracy castings. Wax patterns are prepared by
pouring heated wax into split moulds or a pair of dies. The dies after having been cooled
down are parted off. Now the wax pattern is taken out and used for molding. Such
patterns need not to be drawn out solid from the mould. After the mould is ready, the wax
is poured out by heating the mould and keeping it upside down. Such patterns are
generally used in the process of investment casting where accuracy is linked with intricacy
of the cast object.
SELECTION OF PATTERN MATERIAL
The following factors must be taken into consideration while selecting pattern materials.
1. Number of castings to be produced. Metal pattern are preferred when castings are required large in number.
2. Type of mould material used.
3. Kind of molding process.
4. Method of molding (hand or machine).
5. Degree of dimensional accuracy and surface finish required.
6. Minimum thickness required.
7. Shape, complexity and size of casting.
8. Cost of pattern and chances of repeat orders of the pattern
____________
____________
The types of the pattern and the description of each are given as under.
1. One piece or solid pattern
2. Two piece or split pattern
3. Cope and drag pattern
4. Three-piece or multi- piece pattern
5. Loose piece pattern
6. Match plate pattern
7. Follow board pattern
8. Gated pattern
9. Sweep pattern
10. Skeleton pattern
11. Segmental or part pattern
PATTERN ALLOWANCES
CORE AND CORE BOX
Functions of cores
1. Core is used to produce hollowness in castings in form of internal cavities.
3. It may be deployed to improve mold surface.
4. It may provide external under cut features in casting.
5. It may be used to strengthen the mold.
6. It may be used to form gating system of large size mold
7. It may be inserted to achieve deep recesses in the casting
CORE PRINTS
DESIGN CONSIDERATIONS IN PATTERN AND CORE MAKING
http://www.themetalcasting.com/casting-designing-framework.html
1. All Abrupt changes in section of the pattern should be avoided as far as possible.
2. Parting line should be selected carefully, so as to allow as small portion of the
pattern as far as possible in the cope area.
3. The thickness and section of the pattern should be kept as uniform as possible.
4. Sharp corners and edges should be supported by suitable fillets or otherwise rounded
of. It will facilitate easy withdrawal of pattern, smooth flow of molten metal and
ensure a sound casting. Wherever there is a sharp corner, a fillet should be provided, and the corners may be rounded up for easy withdrawal of patterns as well as easy flow of molten metal
in the mould.
5. Surfaces of the casting which are specifically required to be perfectly sound and
clean should be so designed that they will be molded in the drag because the
possible defects due to loose sand and inclusions will occur in the cope.
6. As far as possible, full cores should be used instead of cemented half cores for
reducing cost and for accuracy. This will reduce cost and ensure greater dimensional accuracy.
7. For mass production, the use of several patterns in a mould with common riser is
to be preferred.
8. The pattern should have very good surface finish as it directly affects the corresponding
finish of the casting. As for as possible, the pattern should have a good surface finish because the surface finish of the casting depends totally on the surface finish of the pattern and the kind of facing of the mold cavity.
9. Shape and size of the casting and that of the core should be carefully considered to
decide the size and location of the core prints.
10. Proper material should always be selected for the pattern after carefully analyzing
the factors responsible for their selection.
11. The use of offset parting, instead of cores as for as possible should be encouraged
to the great extent.
12. For large scale production of small castings, the use of gated or match- plate
patterns should be preferred wherever the existing facilities permit.
14. If gates, runners and risers are required to be attached with the pattern, they should
be properly located and their sudden variation in dimensions should be avoided.
15. Proper allowances should be provided, wherever necessary.
PATTERN LAYOUT
PATTERN CONSTRUCTION
https://www.indiamart.com/proddetail/cast-iron-casting-pattern-with-core-box-15991422588.html
Utility of a Pattern
1 Pattern prepares a mould cavity for the purpose of making a casting.
2 Pattern possesses core prints which produces seats in form of extra recess for core placement in the mould.
3 It establishes the parting line and parting surfaces in the mould.
4 Runner, gates and riser may form a part of the pattern.
5 Properly constructed patterns minimize overall cost of the casting.
https://www.metalcastingdesign.com/articles/2014/07/01/10-cost-considerations-your-castings
6 Pattern may help in establishing locating pins on the mould and therefore on the casting with a purpose to check the casting dimensions.
7 Properly made pattern having finished and smooth surface reduce casting defects.
Patterns are generally made in pattern making shop. Proper construction of pattern and its material may reduce overall cost of the castings.
Materials for Patterns
The common materials used for making patterns are wood, metal, plastic, plaster, wax or mercury. The some important pattern materials are discussed as under.
Advantages of wooden patterns
Wood is cheap, light in weight, easily available and can be easily worked. It is easy to join and repair.
But it is susceptible to moisture, tends to warp and wears out quickly due to sand abrasion.
Metallic Patterns
Metallic patterns are not much affected by moisture as wooden pattern. The wear and
tear of this pattern is very less. Metal can be shaped with good precision, surface finish and intricacy. The main disadvantages of metallic patterns are higher cost, higher weight and tendency of rusting.
It is preferred for production of castings in large quantities with same pattern. The metals commonly used for pattern making are cast iron, brass and bronzes and aluminum alloys.
Cast Iron
It is cheaper, stronger, tough, and durable and can produce a smooth surface finish. It
also possesses good resistance to sand abrasion. The drawbacks of cast iron patterns are that
they are hard, heavy, brittle and get rusted easily in presence of moisture.
But it is heavy, brittle and hence it can be easily broken and it may rust.
Brasses and Bronzes
These are heavier and expensive than cast iron and hence are preferred for manufacturing
small castings. They possess good strength, machinability and resistance to corrosion and
wear. They can produce a better surface finish than cast iron. Very thin sections can be easily casted. Brass and bronze pattern is finding application in making match plate pattern
But it is costlier than cast iron and it is heavier than cast iron.
Aluminum Alloys
Aluminum alloy patterns are more popular and best among all the metallic patterns
because of their high lightness, good surface finish, low melting point and good strength.
They also possesses good resistance to corrosion and abrasion by sand and there by enhancing
longer life of pattern. These materials do not withstand against rough handling. These have
poor repair ability and are preferred for making large castings.
They can be damaged by sharp edges. . They are softer than brass and cast iron. Their storing and transportation needs proper care.
White Metal (Alloy of Antimony, Copper and Lead)
Advantages
1. It is best material for lining and stripping plates.
2. It has low melting point around 260°C
3. It can be cast into narrow cavities.
Disadvantages
1. It is too soft. Its storing and transportation needs proper care. It wears away by sand or sharp edges.
Plastic
Plastics are getting more popularity now a days because the patterns made of these materials
are lighter, stronger, moisture and wear resistant, non sticky to molding sand, durable and
they are not affected by the moisture of the molding sand. Moreover they impart very smooth
surface finish on the pattern surface. These materials are somewhat fragile, less resistant to
sudden loading and their section may need metal reinforcement.
The plastics used for this purpose are thermosetting resins. Phenolic resin plastics are commonly used. These are originally in liquid form and get solidified when heated to a specified temperature. To prepare a plastic pattern, a mould in two halves is prepared in plaster of paris with the help of a
wooden pattern known as a master pattern. The phenolic resin is poured into the mould and
the mould is subjected to heat. The resin solidifies giving the plastic pattern. Recently a new
material has stepped into the field of plastic which is known as foam plastic. Foam plastic is
now being produced in several forms and the most common is the expandable polystyrene
plastic category. It is made from benzene and ethyl benzene.
Plaster
This material belongs to gypsum family The main advantages of plaster are that it has high compressive strength and is of high expansion setting type which compensate for the shrinkage allowance of the casting metal. Plaster of paris pattern can be prepared either by directly pouring the slurry of plaster and water in moulds prepared earlier from a master pattern or by sweeping it into desired shape or form by the sweep and strickle method. It is also preferred for production of small size intricate castings and making core boxes.
Wax
Patterns made from wax are excellent for investment casting process. The materials used
are blends of several types of waxes, and other additives which act as polymerizing agents,
stabilizers, etc. The commonly used waxes are paraffin wax, shellac wax, bees-wax, cerasin
wax, and micro-crystalline wax. The properties desired in a good wax pattern include low
ash content up to 0.05 per cent, resistant to the primary coat material used for investment,
high tensile strength and hardness, and substantial weld strength. The general practice of
making wax pattern is to inject liquid or semi-liquid wax into a split die. Solid injection is
also used to avoid shrinkage and for better strength. Waxes use helps in imparting a high
degree of surface finish and dimensional accuracy castings. Wax patterns are prepared by
pouring heated wax into split moulds or a pair of dies. The dies after having been cooled
down are parted off. Now the wax pattern is taken out and used for molding. Such
patterns need not to be drawn out solid from the mould. After the mould is ready, the wax
is poured out by heating the mould and keeping it upside down. Such patterns are
generally used in the process of investment casting where accuracy is linked with intricacy
of the cast object.
SELECTION OF PATTERN MATERIAL
The following factors must be taken into consideration while selecting pattern materials.
1. Number of castings to be produced. Metal pattern are preferred when castings are required large in number.
2. Type of mould material used.
3. Kind of molding process.
4. Method of molding (hand or machine).
5. Degree of dimensional accuracy and surface finish required.
6. Minimum thickness required.
7. Shape, complexity and size of casting.
8. Cost of pattern and chances of repeat orders of the pattern
TYPES OF PATTERNS
____________
____________
The types of the pattern and the description of each are given as under.
1. One piece or solid pattern
2. Two piece or split pattern
3. Cope and drag pattern
4. Three-piece or multi- piece pattern
5. Loose piece pattern
6. Match plate pattern
7. Follow board pattern
8. Gated pattern
9. Sweep pattern
10. Skeleton pattern
11. Segmental or part pattern
PATTERN ALLOWANCES
Pattern may be made from wood or metal and its color may not be same as that of the casting. The material of the pattern is not necessarily same as that of the casting. Pattern carries an additional allowance to compensate for metal shrinkage. It carries additional allowance for machining. It carries the necessary draft to enable its easy removal from the sand mass. It carries distortions allowance also. Due to distortion allowance, the shape of casting is opposite to pattern. Pattern may carry additional projections, called core prints to produce seats or extra recess in mold for setting or adjustment or location for cores in mold cavity. It may be in pieces (more than one piece) whereas casting is in one piece. Sharp changes are not provided on the patterns. These are provided on the casting with the help of machining. Surface finish may not be same as that of casting.
CORE AND CORE BOX
Cores are compact mass of core sand that when placed in mould cavity at required location with proper alignment does not allow the molten metal to occupy space for solidification in that portion and hence help to produce hollowness in the casting. The core has to withstand the severe action of hot metal which completely surrounds it. Cores are classified according to shape and position in the mold. There are various types of cores are horizontal core), vertical core, balanced core, drop core and hanging core.
Functions of cores
1. Core is used to produce hollowness in castings in form of internal cavities.
3. It may be deployed to improve mold surface.
4. It may provide external under cut features in casting.
5. It may be used to strengthen the mold.
6. It may be used to form gating system of large size mold
7. It may be inserted to achieve deep recesses in the casting
CORE PRINTS
When a hole blind or through is needed in the casting, a core is placed in the mould cavity to produce the same. The core has to be properly located or positioned in the mould cavity on pre-formed recesses or impressions in the sand. To form these recesses or impressions for generating seat for placement of core, extra projections are added on the pattern surface at proper places. These extra projections on the pattern (used for producing recesses in the mould for placement of cores at that location) are known as core prints. Core prints may be of horizontal, vertical, balanced, wing and core types. Horizontal core print produces seats for horizontal core in the mould. Vertical core print produces seats to support a vertical core in the mould. Balanced core print produces a single seat on one side of the mould and the core remains partly in this formed seat and partly in the mould cavity, the two portions balancing each other. The hanging portion of the core may be supported on chaplets. Wing core print is used to form a seat for a wing core. Cover core print forms seat to support a cover core.
DESIGN CONSIDERATIONS IN PATTERN AND CORE MAKING
http://www.themetalcasting.com/casting-designing-framework.html
1. All Abrupt changes in section of the pattern should be avoided as far as possible.
2. Parting line should be selected carefully, so as to allow as small portion of the
pattern as far as possible in the cope area.
3. The thickness and section of the pattern should be kept as uniform as possible.
4. Sharp corners and edges should be supported by suitable fillets or otherwise rounded
of. It will facilitate easy withdrawal of pattern, smooth flow of molten metal and
ensure a sound casting. Wherever there is a sharp corner, a fillet should be provided, and the corners may be rounded up for easy withdrawal of patterns as well as easy flow of molten metal
in the mould.
5. Surfaces of the casting which are specifically required to be perfectly sound and
clean should be so designed that they will be molded in the drag because the
possible defects due to loose sand and inclusions will occur in the cope.
6. As far as possible, full cores should be used instead of cemented half cores for
reducing cost and for accuracy. This will reduce cost and ensure greater dimensional accuracy.
7. For mass production, the use of several patterns in a mould with common riser is
to be preferred.
8. The pattern should have very good surface finish as it directly affects the corresponding
finish of the casting. As for as possible, the pattern should have a good surface finish because the surface finish of the casting depends totally on the surface finish of the pattern and the kind of facing of the mold cavity.
9. Shape and size of the casting and that of the core should be carefully considered to
decide the size and location of the core prints.
10. Proper material should always be selected for the pattern after carefully analyzing
the factors responsible for their selection.
11. The use of offset parting, instead of cores as for as possible should be encouraged
to the great extent.
12. For large scale production of small castings, the use of gated or match- plate
patterns should be preferred wherever the existing facilities permit.
14. If gates, runners and risers are required to be attached with the pattern, they should
be properly located and their sudden variation in dimensions should be avoided.
15. Proper allowances should be provided, wherever necessary.
PATTERN LAYOUT
A layout of the different parts of the pattern has to be made first. The next stage is to shape them. The layout preparation consists of measuring, marking, and setting out the dimensions on a layout board including needed allowances. The first step in laying out is to study the working drawing carefully and select a suitable board of wood that can accommodate at least two views of the same on full size scale. The next step is to decide a working face of the board and plane an adjacent edge smooth and square with the said face. Select a proper contraction scale for measuring and marking dimensions according to the material of the casting. Further the layout is prepared properly and neatly using different measuring and making tools specifying the locations of core prints and machined surfaces. Finally on completion of the layout, check carefully the dimension and other requirements by incorporating all necessary pattern allowances before starting construction of the pattern.
PATTERN CONSTRUCTION
From the layout, try to visualize the shape of the pattern and determine the number of separate pieces to be made and the process to be employed for making them. Then the main part of pattern body is first constructed using pattern making tools. The direction of wood grains is kept along the length of pattern as far as possible to ensure due strength and accuracy. Further cut and shape the other different parts of pattern providing adequate draft on them. The prepared parts are then checked by placing them over the prepared layout. Further the different parts of the pattern are assembled with the main body in proper position by gluing or by means of dowels as the case may be. Next the relative locations of all the assembled parts on the pattern are adjusted carefully. Then, the completed pattern is checked for accuracy. Next all the rough surfaces of pattern are finished and imparted with a thin coating of shellac varnish. The wax or leather fillets are then fitted wherever necessary. Wooden fillets should also be fitted before sanding and finishing. The pattern surface once again prepared for good surface and give final coat of shellac. Finally different parts or surfaces of pattern are colored with specific colors mixed in shellac or by painting as per coloring specifications.
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