Introduction
Remaining shrinkage hole is a common defect in castings, caused by a number of possible factors related to the manufacturing process. This paper attempts to thoroughly review the causes and prevention of remaining shrinkage hole defects in investment castings, and to discuss various methods of defect evaluation and treatment.
Definition and Categorization
Remaining shrinkage holes are cavities that are left in a solid casting due to the inability of the metal to completely fill all portions of the mold during casting. Generally, the size, shape and position of the remaining shrinkage holes depend on the design of the mould and the type of casting process used. Cast Iron, aluminum and other alloy materials are prone to shrinkage holes and they can arise from various sources in the investment casting process, including mould design, sand cores, and casting techniques.
Causes
Mould Design
The mould design is a major cause of shrinkage hole defects in investment castings. Inadequate flow channels and complex casting geometries, such as thin walls and narrow corners, can prevent adequate metal flow into all regions of the mould, leading to shrinkage hole defects. Additionally, improper gating and pouring systems can impede the effective flow of metal into the mould, resulting in residual shrinkage holes.
Sand Cores
Investment castings involve the use of sand cores to create internal cavities and details. If the sand cores are not properly packed, air pockets can form within the core, resulting in the formation of shrinkage holes after casting. Additionally, sand cores with inconsistent permeability can lead to inadequate metal infiltration into the core and the development of shrinkage holes.
Casting Techniques
Casting techniques can also influence the formation of shrinkage holes. Poor mold packing and improper pouring can impede the effective flow of metal into the mold, resulting in residual shrinkage holes. Additionally, rapid chilling of the casting due to inadequate cooling can lead to shrinkage holes.
Evaluation and Treatment
Evaluation
In order to properly evaluate the presence and extent of remaining shrinkage hole defects, casting samples may be examined with visual and dimensional inspection methods. If a qualitative evaluation is necessary, a magnifying glass or microscope may be used to examine the casting in greater detail. Additionally, radiographic evaluation may be necessary to gain information on internal shrinkage hole defects.
Treatment
The best method for treating remaining shrinkage hole defects is to modify the design and methods of the casting process. After evaluating the causes of the defect, corrective measures may be taken to reduce or eliminate the occurrence of shrinkage hole defects. For example, changes may be made to the mold design and the sand core packing process to reduce the formation of shrinkage holes. Additionally, process changes such as improved gating, runner design, and/or quicker solidification times may be necessary to prevent shrinkage holes. In cases where these corrective measures are unfeasible, additional machining processes and/or welding operations may be necessary to remove the remaining shrinkage holes.
Conclusion
Remaining shrinkage hole defects are a common problem in the investment casting process, and can significantly impact the quality of a casting. The causes of shrinkage hole defects are varied and complex, but they can generally be attributed to factors such as mould design, sand core packing, and casting techniques. To identify and address the root cause of the defect, it is important to evaluate the casting sample with visual, dimensional and/or radiographic inspection methods. If the underlying cause of the defect is identified, corrective measures can be taken to reduce or eliminate further shrinkage hole defects. In cases where these corrective measures are not possible, additional machining and/or welding operations may be required to remove the remaining shrinkage holes.
