Different Types of Pattern Allowances
- Shrinkage Allowance or Contraction Allowance
- Machining Allowance or Finishing Allowance
- Draft Allowance or Taper Allowance
- Rapping Allowance or Shake Allowance
- Distortion Allowance or Camber Allowance
1. Shrinkage Allowance or Contraction Allowance :
Generally during the time of solidification of the molten metal in the mold, molten metal shrinks in the cavity so the casting produced becomes smaller than the cavity and pattern. To compensate the shrinkage of the molten metal, the pattern is made larger than the casting produced. The allowance provided to compensate the shrinkage is called shrinkage allowance or Contraction allowance.
In practice it is found that all common cast metals shrink a significant amount when they are cooled from the molten state. The total contraction in volume is divided into the following parts:
- Liquid contraction, i.e. the contraction during the period in which the temperature
of the liquid metal or alloy falls from the pouring temperature to the liquidus
- Contraction on cooling from the liquidus to the solidus temperature, i.e. solidifying
- Contraction that results there after until the temperature reaches the room
temperature. This is known as solid contraction.
The first two of the above are taken care of by proper gating and risering. Only the last one, i.e. the solid contraction is taken care by the pattern makers by giving a positive shrinkage allowance. This contraction allowance is different for different metals. The contraction allowances for different metals and alloys such as Cast Iron 10 mm/mt.. Brass 16 mm/mt., Aluminium Alloys. 15 mm/mt., Steel 21 mm/mt., Lead 24 mm/mt. In fact, there is a special rule known as the pattern marks contraction rule in which the shrinkage of the casting metals is added. It is similar in shape as that of a common rule but is slightly bigger than the latter depending upon the metal for which it is intended.
2. Machining Allowance
It is a positive allowance given to compensate for the amount of material that is lost in machining or finishing the casting. If this allowance is not given, the casting will become undersize after machining. The amount of this allowance depends on the size of casting, methods of machining and the degree of finish. In general, however, the value varies from 3 mm. to 18 mm.
2. Draft Allowance or Taper Allowance
Taper allowance is also a positive allowance and is given on all the vertical surfaces of pattern so that its withdrawal becomes easier. The normal amount of taper on the external surfaces varies from 10 mm to 20 mm/mt. On interior holes and recesses which are smaller in size, the taper should be around 60 mm/mt. These values are greatly affected by the size of the pattern and the molding method. In machine molding its, value varies from 10 mm to 50 mm/mt.
4. Rapping Allowance or Shake Allowance
Before withdrawing the pattern it is rapped and thereby the size of the mould cavity increases. Actually by rapping, the external sections move outwards increasing the size and internal sections move inwards decreasing the size. This movement may be insignificant in the case of small and medium size castings, but it is significant in the case of large castings. This allowance is kept negative and hence the pattern is made slightly smaller in dimensions 0.5-1.0 mm.
5. Distortion Allowance or Camber Allowance
Distortion allowance is applied to the castings which have the tendency to distort during cooling due to thermal stresses developed. For example a casting in the form of U shape will contract at the closed end on cooling, while the open end will remain fixed in position. Therefore, to avoid the distortion, the legs of U pattern must converge slightly so that the sides will remain parallel after cooling.
Question and Answer
- A Taper provided on the pattern for it’s easy and clean withdrawal from the mould is known as
Answer: Draft Allowance or Taper Allowance
- Shrinkage allowance is added to pattern dimensions to care of
Answer: Solid shrinkage