Why the Pattern allowances in casting affected:
Sometime the patterns are not exactly the same size as the desired casting shape. To compensate for patterning process, removal of pattern, dimensional and structural changes during the casting, allowances are made in pattern. For difference types of Pattern allowances in casting as follows. The Most common of shrinkage allowance in casting possible for molding.
Various pattern allowances in casting process
- Shrinkage allowance
- Draft allowance
- Finishing or machining allowances
- Shake allowance
- Distortion allowance
They are the Pattern allowances in casting process used for making mold.
Shrinkage in casting or shrinkage allowance in casting process:
The liquid melon metals are poured into cavity and the mold together after cooling and solidify. In this case the phase change in metal shorten it`s volume. It is known as shrinkage. Now, the patterns require incorporate suitable allowance for shrinkage. So the patterns made slightly more than require shape finish.The exact value of pattern depends on the alloy being cast as follows.
Typical pattern make’s shrinkage of various metals:
|SI.No||Metal alloy||Solidification shrinkage (Volume %)||Cooling shrinkage (Volume %)||Solidus Temperature (⁰C)|
|Fe (pure)||3.5||6.0 to 7.5||
|Steel||2.5 to 3.0||4.2 to 6.0||
|Ductile cast iron||-4.5 to +2.7||3.0||
|Gray cast iron (2% c)||1.05||1.0||
It also can be classify into solid shrinkage and liquid shrinkage. The volume is reduced during the metal solidification process is called solid shrinkage.
The volume is reduced during the metal cooling process is called liquid shrinkage.
The leading edge are may be break off or get damaged during the pattern is removed from mold. So that is avoid, the taper is to allow on the pattern and easily remove from the edge of mold. In this taper having some of angle is called draft angle. The draft angle depends upon the type of molding, height, and complexity of pattern.
Finishing or machining allowance:
The produce the part by using sand casting method. The surface finish of the part will be poor. So this avoid, the casting of product is machined together in surface like turning or grinding to improve the surface finish during the machining process, some sufficient amount of metal removed from the mold part. To compensate this, the machining or finishing allowances are given in the casting. This allowance is added together in basic size of the pattern.
The amount of allowance added to pattern based on,
- Type of machining process
- Material used
- Requirement of accuracy
- Molding method
It ranges from lower than 2 mm to 15 mm based on size and material of the part. For too large machining allowance to increase cost of molten metal and more removal of metal from mold part by machining.
Shake or rapping allowance:
The pattern are removed from the mold cavity, sometime the pattern are rapped in easily remove from cavity. In this case the final mold cavity is enlarged. So to compensate for enlargement, the pattern dimension to be reduced. It is called shake or rapping allowance.
Distortion or camber allowance:
When the mold is cooling process, the stress is developed in solid metal because of the uneven metal thickness or more cooling to allow in the casting. These stresses may be inducing distortion or bending in the casting. This can be avoided by initially distorting the pattern in opposite direction.
The casting part thickness is uneven. Hence, the solidification process is not uniformly and thermal stress provided on the section. So those castings are distorted. To avoid that, the chamber provided in the opposite direction.