The 5 Types of Pattern Allowances in Casting You Must Know

Pattern allowances in casting are extremely important—if this aspect is overlooked during the casting process, the parts we produce may end up smaller than the required dimensions, leading to significant economic losses. Therefore, selecting appropriate allowances in casting patterns can greatly improve efficiency and avoid waste. So, how should pattern allowances be applied in the casting process? And what forms do pattern allowances take in casting?

The allowances typically considered in pattern design and casting are as follows:

• Shrinkage Allowance for Casting

• Machining Allowance for Casting

• Draft Allowance in Casting

• Rapping or Shake Allowance

• Camber Allowance (or Distortion Allowance) in Casting

1. Shrinkage Allowance for Casting

Shrinkage allowance refers to the reduction in dimensions of a casting during cooling or solidification. This is an inherent property of all materials. The shrinkage rate is affected by the casting material, shape, and temperature, and the magnitude of shrinkage varies with different materials. As a result, the final casting will be smaller than the mold.

Shrinkage is divided into liquid shrinkage and solid shrinkage:

• Liquid Shrinkage: The reduction in volume when molten metal changes from a liquid to a solid state at the solidus temperature.Solution: A riser is installed in the mold.

• Solid Shrinkage: The reduction in volume of solid metal due to temperature decrease.Solution: The shrinkage allowance for the casting is specified in the pattern allowances.

In general, all materials undergo shrinkage unless subject to special constraints. The shrinkage rate is fixed under specific conditions such as material type, temperature, and thickness. Thus, experienced manufacturers can provide pre-shrinkage data before production.

Below is the shrinkage allowance for other common casting metals:

2. Machining Allowance for Casting

It is well-known that casting is one of the most cost-effective and direct metal forming methods. However, the allowances commonly used in sand casting processes have certain limitations in terms of dimensional accuracy and surface finish. Machining allowance is designed to address the issue of casting surface finish—freshly produced castings have relatively rough surfaces and require manual grinding or polishing.

Therefore, we must ensure a certain thickness for surface machining, which is the machining allowance in casting. Depending on the part size and material, a machining allowance of 2–15 mm can be reserved. If a higher surface polishing finish is required, the machining allowance should be increased accordingly.

3. Draft Allowance in Casting

When a pattern is removed from the mold, the surfaces parallel to the removal direction may suffer slight damage, and a tapered surface may form on the vertical surfaces of the pattern. To compensate for these changes, these parallel surfaces of the pattern are slightly tapered (typically at an angle of 1–2 degrees). This design allows the pattern to be easily removed from the mold without damaging the casting. The draft allowance refers to the modification of the pattern surface to protect the pattern from damage. The draft depth on the vertical surfaces of the pattern is usually 5–30 mm per meter; for precision casting, the draft depth is approximately 3–6 mm per meter.

The draft allowance required for a pattern depends on the following factors:

• Shape of the casting

• Mold material

• Casting depth

• Molding method

4. Rapping or Shake Allowance

During the casting process, minor dimensional changes that occur to the mold are referred to as "rapping allowance" or "shake allowance." After completing the casting setup, the pattern is removed from the mold, and slight shaking may occur. This process can cause the mold cavity to expand slightly. Therefore, the pattern is usually made smaller than the actual dimensions of the desired casting to compensate for this expansion.

5. Camber Allowance (or Distortion Allowance) in Casting

Thin sections of the casting may cause metal distortion during cooling. Additionally, uneven heating during the casting process can also lead to gradual distortion or bending during cooling. To avoid such bending or distortion during casting, the pattern can be adjusted to have a curvature in the opposite direction—this way, when the casting bends due to uneven metal thickness, it will straighten out to the desired shape. Camber allowance is a modification to the pattern design to compensate for bending during the casting process.

The above are the common types of allowances in casting patterns. We must take various preventive measures before casting to ensure that the casting meets the requirements from the very beginning.