Pattern Allowances
The liquid metal on solidification undergoes various type of shrinkages. this shrinkage may be because of metal characteristics and also may be because of pattern, gating, and risering design. The shrinkage affects the dimensional accuracy needed in the final casting. Hence, in order to get a correct casting, various allowance called pattern allowances need to put on the pattern before making the mold and subsequent pouring in the mould cavity.
Similarly the pattern allowances are also need to be provided on the core patterns. The allowances usually considered on patterns and core boxes are as follows:
1. Shrinkage or contraction allowance
2. Draft or taper allowance
3. Machining or finish allowance
4. Distortion or camber allowance
5. Rapping allowance
1. Shrinkage or Contraction Allowance
The metal when solidifies shrinks volumetrically. The metal shrinkage is of two types:
i. Liquid shrinkage:
After pouring the liquid metal in a mould, metal starts solidifying. When metal solidifies from liquid state to solidus temperature, there is reduction in volume of the liquid metal. This reduction is known as liquid shrinkage. The reduction in volume is compensated by the riser, which stores liquid metal and the metal in riser remains liquid later than that of metal in cavity.
ii. Solid shrinkage:
Once the casting is solidified in the mould cavity, the contraction in volume, when the temperature of the casting moves from solidus to room temperature, of the casting occurs. This contraction in volume is taken care by providing the shrinkage allowances in the pattern. The rate of contraction with temperature is dependent on the material. For example steel contracts to a higher degree compared to aluminum. To compensate the solid shrinkage, a shrink rule must be used in laying out the measurements for the pattern. The various rate of contraction of various materials are given in Table 6.2.1.
2.Draft or Taper Allowance
All the vertical surfaces of the pattern is provided with a taper allowance so that the pattern can be removed safely from the molding sand without tearing away the sides of the sand. This taper allowance is also known as draft allowance. Fig. 6.3.1 (a) shows a pattern having n taper allowance. In this case, till the pattern is completely lifted out, its vertical sides will remain in contact with the vertical walls of mold, thus tending to break it. Fig. 6.3.1 (b) is represents a pattern with taper allowance. In this case the mould wall does not break when the pattern is lifted out of the mould. As a guideline, when the taper allowance is provided on the inner surfaces of the pattern, the allowance is generally higher than that of allowance which is provided on the outer surface. Also, the amount of taper allowance depends upon the length of the vertical side of the pattern to be removed from the molding sand. Table 6.3.1 provides a general guide lines for the draft allowance.
3. Machining or Finish Allowance
The allowance provided on the pattern to take care of dimensional accuracy and surface finish of the end product is known as machining or finish allowance. Sand castings provide, in general, poor surface finish. In order to achieve the desired surface finish, the casting need to be machined. The allowance to take care of extra metal removed to achieve the surface finish is called the machining allowance. The machining or finish allowance depends upon the type of molding process, method of molding, shape and size of the casting, orientation of the different surfaces of the pattern in the cavity, degree of finish desired, and metal of the end product. The machining allowances recommended for different metal is given.
