Master Pattern

Master Pattern :
         Master patterns are used for preparing the mouldsfor metal castings which are later used as patterns for further moulding work, called metal patterns. The masterpatterns are accurately finished wooden patterns, whichcarry double shrinkage allowance and the required machiningallowance. For example, an alluminium pattern is to bemade which is to be used further for making moulds forbrass castings. The alluminium pattern should, obviously,be larger than the desired brass casting by an amount equalto shrinkage that will take place during solidification ofthis casting. For making this alluminium pattern a woodenpattern is to be used which should be larger than thealluminium pattern by an amount equal to the alluminiumshrinkage, added with proper machining allowance forfinishing the alluminium casting. Mathematically, it can be represented thus :
Let Sb represent the size of the desired casting in brass.
And  Let Sa represent the size of aluminum pattern.
And  Let Cb represent the contraction allowance for brass.
Then  Sa=Sb+Cb
Again, let S represent the size of the master pattern.
And let Ca represent the contraction allowance for aluminum.
Also let Am represent the machining allowance required
to finish the aluminum casting to the required size of
pattern and to give smooth surface finish.
Then S = Sa+Ca+Am = Sb+Cb+Ca+Am
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.

Core Prints

Core prints :

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.

Pattern Materials

Pattern materials :

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 Advantages :
(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.
(v) It can be preserved for a fairly long time by applying proper preservatives like shellac varnish.

On the other hand, it has certain disadvantages 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.

2) Metals :

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 Disadvantages :
(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.

3) Plaster :

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.

4) Plastics :

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.

5) Wax :

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.


Pattern :

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.


1.Showel: It consists of iron pan with a wooden handle. It can be used for mixing and conditioning the sand.


2. Trowels: These are used for finishing flat surfaces and comers inside a mould. Common shapes of trowels are shown as under. They are made of iron with a wooden handle.



3. Lifter: A lifter is a finishing tool used for repairing the mould and finishing the mould sand. Lifter is also used for removing loose sand from mould.



4. Hand riddle: It is used for ridding of sand to remove foreign material from it. It consists of a wooden frame fitted with a screen of standard wire mesh at the bottom.


5.Strike off bar: It is a flat bar, made of wood or iron to strike off the excess sand from the top of a box after ramming.
     Its one edge made beveled and the surface perfectly smooth and plane.



6. Vent wire: It is a thin steel rod or wire carrying a pointed edge at one end and a wooden handle or a bent loop at the other. After ramming and striking off the excess sand it is used to make small holes, called vents, in the sand mould to allow the exit of gases and steam during casting.



7. Rammers: Rammers are used for striking the sand mass in the moulding box to pack it closely around one pattern. Common types of rammers are shown as under.


8. Swab: It is a hemp fiber brush used for moistening the edges of sand mould, which are in contact with the pattern surface, before withdrawing the pattern. It is also used for coating the liquid blacking on the mould faces in dry sand moulds.
9. Sprue pin: It is a tapered rod of wood or iron, which is embedded in the sand and later withdrawn to produce a hole, called runner, through which the molten metal is poured into the mould.
10. Sprue cutter: It is also used for the same purpose as a sprue pin, but there is a marked difference between their use in that the cutter is used to produce the hole after ramming the mould. It is in the form of a tapered hollow tube, which is inserted in the sand to produce the hole.