The Mechanical post welcomes you back! Today we’ll talk about pattern, what materials are used for the making, its advantages, limitations and types.
What is pattern in casting ?
Patterns are used to make moulds for the casting into which the molten metal would be poured. Patterns are the copy of the product which is intended to be casted. However, it is not an exact replica of the casting desired. There are certain essential difference.
It is slightly larger than the desired casting, due to various allowances that is shrinkage allowance, machining allowance etc. and it may have several projections or bosses called cold prints. It may also have extensions to produce runners and gates during the moulding process.
Desired characteristics of pattern
- Secure the desired shape and size of the casting.
- Cheap and readily repairable.
- Simple in design for ease of manufacture.
- Light in mass and convenient to handle.
- Have high strength and long life in order to make as many moulds as required.
- Retain its dimensions and rigidity during the definite service life.
- Its surface should be smooth and wear resistant.
- Able to withstand rough handling.
- For short run production wood is the suitable material.
- For large scale and mass production metal being more durable than wood though is costlier but however can survive in the long run.
- For batch production plastics for example epoxy, resins and also from gypsum and cement are preferred.
Advantages of using wood
- Light in weight.
- Comparatively inexpensive.
- Good workability.
- Lends itself to gluing and joining.
- Holds well varnishes and paints.
- Can be repaired easily.
Disadvantages or limitations
- Inherently non uniform in structure.
- Possess poor ware and abrasion resistance.
- Cannot withstand rough handling.
- Absorbs and gives of moisture, so that it varies in volume, wraps and thus changes its mechanical properties. These drawbacks, however, can be remedied by drying and seasoning it and then giving coats of water proof varnishes and paints.
Types of wood commonly used in pattern making
Maple birch and cherry
Advantages of using metal
- More durable and accurate in size than wooden patterns.
- Have a smooth surface.
- Do not the deform in storage.
- Are resistant to wear, abrasion, corrosion and swelling.
- Can withstand rough handling.
- Expensive as compared to wood.
- Not easily repairable.
- Heavier than wood patterns.
- Ferrous metals are prone to rusting.
Common metals used for pattern making are
- Facilitates the production process.
- Makes it more economical in cost and labour.
- Plastic patterns are highly resistant to corrosion, lighter and stronger than wood patterns.
- Moulding sand sticks less to plastics than to wood .
- No moisture absorption.
- Smooth surface of patterns.
- Strong and dimensionally stable.
Finishing of patterns
The finish of the casting depends on the finish of the pattern. If the pattern is to be preserved for a long period and if a colour scheme is to be used for good quality enamel paint should be selected to spray or brush paint it.
C.I., Malleable iron = 10mm/m.
Brass, Cu, Al = 15mm/m.
Steel = 20mm/m.
Zinc, Lead = 25mm/m.
The amount of finish allowance depends on the material of the casting, its size, volume of production, method of moulding, configuration of the casting, the position the wall surface occupies in the mould and during pouring. Machining allowance is larger for hand moulding as compared to machine moulding. The largest allowances are taken for the surface is located in the cope half of the mould, since they are liable to contamination due to slag.
Types of pattern
- One piece or solid pattern.
- Stock split or parted pattern.
- Loose piece pattern.
- Gated patterns.
- Match plate pattern.
- Cope and drag pattern.
- Sweep patterns.
- Skeleton patterns.
- Segmental pattern.
- Follow board pattern.
Colour scheme for patterns
- Red = Surface to be machined
- Black = Surface as cast
- Yellow = core prints and seats
- Yellow/red stripes = Loose piece
- Yellow/black stripes = Stop off
– refered from Textbook of production technology by Dr. PC Sharma.
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