What is Pattern in Casting?
In casting, Pattern is defined as the replica of the original shape desired to be cast.
A pattern can be defined as a model of desired casting which when molded in sand forms an impression called mold.
Few Important things about Casting Patterns
- When the mould is filled with the molten metal, it forms a solid casting when it solidifies.
- If the pattern is made accurate, then the casted metal piece will also be accurate.
- There are various materials used for making patterns; like wood, plastic, wax and metal (including cast iron, brass, aluminum, alloy steel, etc).
- In simple words, first of all the pattern is made, and from this pattern, the mould cavity is made. From this mould cavity the solid casting is manufactured by pouring the molten metal into the mould cavity.
- Even if you want to make a single casting, then also you need to make a pattern first. And by using a single pattern, you can manufacture many castings.
Having said that, let us discuss some important functions or uses of patterns in casting.
Functions of Pattern In Casting
The main function (or use) of pattern is to make a mould cavity.
But besides the main function of preparing the mold cavity, there are many other functions that a pattern should perform.
These functions of casting pattern are mentioned below.
#1) Molding the gating system
It is also very important to have a good gating system so that the molten metal can easily pass into the mould cavity, through feeding reservoirs (gate and risers).
The gating system can also get the benefit of machine moulding.
#2) Establishing the parting line
Generally if we talk about the flat pattern plate, then the parting surface is a simple plane.
Sometimes, the curve parting surfaces are also required in some castings. These parting surfaces can be established by using a pattern.
#3) Making core print
Some of the casting requires cores. In some cases, the provisions are made on the pattern for the core prints.
Core prints are simply the portion of the pattern and cavity which anchors the core at proper position in the mold cavity.
The core print is added to the pattern, but it does not appear on the casted workpiece because the core print is blocked off by the core itself.
#4) Establishing locating points
In casting shop or foundry shop, various markings or points are made on the casting for checking the dimension of casting. Some machining operations also require marking on the casted piece, which makes the machining operations easy.
#5) Minimizing casting defects
The pattern required for casting operation should be clean and it should have smooth surface finish.
The patterns with rough surface, having cuts, loose particles generally leads to imperfections and defects in casting.
#6) Providing for ram-up cores
Sometimes a part of the mold cavity is made with cores which are positioned by the pattern before the molding is rammed.
The ram-up core is then held by the sand which has been packed around it.
#7) Providing economic in moulding
The construction of the pattern should be such that it fulfills all the requirements at the minimum cost.
The appropriate material should be selected for the construction of the pattern.
For some castings, the wood pattern is preferred while for other castings, wax pattern is preferred. So selection of pattern materials also plays a vital role in reducing the cost of pattern manufacturing.