A pattern may be defined as a replica or facsimile model of the desired casting which, when packed or embedded in a suitable moulding material, produces a cavity called mould. This cavity, when filled with molten metal, produces the desired casting after solidification of the poured metal. Since it is a direct duplication, the
pattern very closely conforms to the shape and size of the desired casting, except for a few variations due to the
necessary allowances. The ways in which a pattern differs from an actual component are :
1. It carries an additional allowance to compensate for metal shrinkage.
2. It carries additional allowances over those portions, which are to be machined or finished otherwise.>
Let Sb represent the size of the desired casting in brass.
And Let Cb represent the contraction allowance for brass.
to finish the aluminum casting to the required size of
pattern and to give smooth surface finish.
Size of master pattern = Size of the final casting to be Made + shrinkage allowance for the material of final casting + shrinkage allowance of the metal of which the pattern is to be made + Finishing allowance for the metal pattern.
When a casting is required to have a hole, through or blind, a core is used in the mould to produce the same.
This core has to be properly seated in the mould on formed impressions in the sand. To form these impressions, extra projections are added on the pattern surface at proper places. These projections are known as core prints.>
Sand Grains :>
The shape and size of the sand grains has a remarkable effect on the physical properties of the foundry sand. The sand grains may have smooth, conchoidal or rough surfaces. Out of these the first type i.e., smooth, is preferred for moulding for the reason that such a surface renders higher permeability, sinter point and plasticity to the sand mass, but the percentage of binder required is also equally high.>
Similarly the sand grains may have different shapes. The commonly formed shapes are rounded, sub-angular, angular and compound. The rounded grains do not bind together two well when rammed and, hence, render the sand mould highly permeable but the strength of the mould is also reduced.>
Sub-angular grains give a relatively stronger bond than above but the permeability is reduced. Angular or
sharp grains produce a much stronger bond and a low permeability when rammed. Thus they enable a mould of greater strength. Sand grains which are cemented together such that they do not separate when screened are called compound. They may consist of one, two or a combination of all the above three shapes. They are not much preferred.>
Like the shape the size of sand grains also effects the mould structure and its characteristics. Large, regular and uniform grains increase permeability. Smaller grains increase smoothness on mould surfaces.>
Pattern and its types>
Pattern-its types and its allowances>
A pattern may be defined as a model of desired casting which when moulded in sand forms an impression called mould. The mould when filled with the molten metal forms casting after solidification of the poured metal. The quality and accuracy of casting depends upon the pattern making. The pattern may be made of wood, metal(cast iron, brass, aluminium and alloy steel.), plaster, plastics and wax.>
A pattern is always made larger than the required size of the casting considering the various allowances. These are the allowances which are usually provided in a pattern.>
1: shrinkage or contraction allowance:>
The various metals used for casting contract after solidification in the mould. Since the contraction is different for different materials, therefore it will also differ with the form or Type of metal>
2: Draft allowance>
It is a taper which is given to all the vertical walls of the pattern for easy and clean withdraw of the pattern from the sand without damaging the mould cavity. It may be expressed in millimeters on a side or in degrees. The amount of taper varies with the type of patterns. The wooden patterns require more taper than metal patterns because of the greater frictional resistance of the wooden surfaces.>
3: Finish or machining allowance>
The allowance is provided on the pattern if the casting is to be machined. This allowance is given in addition to shrinkage allowance. The amount of this allowance varies from 1.6 to 12.5 mm which depends upon the type of the casting metal, size and the shape of the casting. The ferrous metals require more machining allowance than non ferrous metals.>
4: Distortion or camber allowance>
This allowance is provided on patterns used for casting of such design in which the contraction is not uniform throughout.>
5: Rapping or shaking allowance>
This allowance is provided in the pattern to compensate for the rapping of mould because the pattern is to be rapped before removing it from the mould.>
Types of Patterns:>
The common types of patterns are as follows:>
1. solid or single piece patterns
2. split or two/multiple piece patterns
3. match plate pattern
4. cope and drag pattern
5. loose piece pattern
6. gated patterns
7. sweep pattern
8. skeleton pattern
9. shell pattern
10. segmental pattern
11. follow board pattern
12. lagged up pattern
13. left and right hand pattern>
The common materials of which the patterns are made are the following :>
1) Wood :>
It is the most common material used for pattern making because of the following >
(i) It is cheap and available in abundance.>
(ii) It can be easily shaped into different forms and intricate designs.(iii) Its manipulation is easy because of lightness in weight.>
(iv) Good surface finish can be easily obtained by only planning and sanding.>
On the other hand, it has certain >
also as follows:>
(i) It wears out quickly due to its low resistance to sand abrasion. As such, a wooden pattern cannot stand a long constant use.>
(ii) It is very susceptible to moisture, which may lead to its warping or splitting. This needs its careful storing in a dry place and the application of preservatives.>
(iii) Its life, owing to the above reasons, is short as compared to other pattern materials. This confines its use to such cases only when a small number of castings are required.>
Metals are used with advantage, as pattern material, only when the number of castings to be made is very high and a closer dimensional accuracy is desired. They have a much longer life than wooden patterns and eliminate the inherent disadvantages of wood to a great extent.
But they also carry the following >
(i) They are costlier than wood and, therefore, cannot be used with advantage, where a smaller number of
castings is to be made.>
(ii) For giving different shapes and fine surface finish they need machining. This again adds to their cost.>
(iii) Most of them are very heavy and in case of large castings the weight of the pattern always poses a problem in its manipulation.>
(iv) A large number of them have a tendency to get rusted.>
Plaster of Paris or gypsum cement is advantageously used as a pattern material since it can be easily casted into intricate shapes and can be easily worked also. Its expansion can be easily controlled and it carries a very high compression strength. Its specific use is in making small patterns and core boxes involving
intricate shapes and closer dimensional control. A marked feature of this cement is that contrary to the action of metals, it expands on being solidified. Thus, if a cement of proper coefficient of expansion is selected, the effect of shrinkage of casting can be automatically neutralized.>
Plastics are gradually gaining favor as pattern materials due to their following specific characteristics :
1. Lightness in weight.
2. High strength.
3. High resistance to wear.
4. High resistance to corrosion due to moisture.
5. Fine surface finish.
6. Low solid shrinkage.
7. Very reasonable cost.>
The plastics used as pattern materials are thermosetting resins. Phenolic resin plastic and foam plastic suit best for this purpose. For making the pattern, first the moulds are made, usually from plaster of Paris. The resin is then poured into these moulds and the two heated. At a specific temperature, the resin solidifies to give the plastic pattern.>
Wax patterns are exclusively used in investment casting. For this a die or metal mould is made in two
halves into which the heated wax is poured. The die is kept cool by circulating water around it. As the wax
sets on cooling, the die parts are separated and the wax pattern taken out.>
Quality and detail wax patterns hold clean, crips detail. Excellent in the burnout cycle. Models are workable and exhibit bounceability.
We offer various patterns including:>
- Men’s/ Ladies’ Rings (with/ without stone settings)
- Wedding Bands/ Wedding Sets
- Engagement Sets
- Charms, Novelties, Pendants
- Earrings and Pendants
- Astrological Signs