Published - 12th May 2015
Shrinkage in sand casting is an issue that must be carefully managed.
Most metals, including aluminium, copper, zinc and magnesium, shrink as they solidify. The degree of shrinkage depends on the freezing range of the material. As an example, aluminium shrinks by over 6% during solidification while copper shrinks by nearly 5%.
In sand casting, solidification occurs as the molten metal is poured into the mould cavity. So, it is vital that this part of the sand casting process is carried out in a highly controlled manner to reduce shrinkage and eliminate defects.
There are two main types of shrinkage in sand casting: solidification shrinkage and pattern maker’s shrinkage.
Solidification shrinkage
Solidification shrinkage occurs because metals are less dense as liquids than solids. Risers and chills are used to address this problem.
Risers feed a continuous supply of molten metal to the casting as it solidifies. They play an essential role in promoting directional solidification where the metal solidifies at the furthest point first before moving towards the riser. Using this approach, the cavity forms in the riser and not the casting[1].
Chills, which are metal inserts, are also used to assist risers. They help to equalise the rate of solidification in thicker areas of the metal casting.
Pattern maker’s shrinkage
Pattern maker’s shrinkage occurs after the solidification process when the casting is cooled to room temperature. This phenomenon is due to thermal contraction. A shrinkage allowance must therefore be factored into the design at the start of the process.
The pattern is made larger than the desired casting to compensate for this type of shrinkage. The shrinkage allowance varies by the type of metal but the pattern may need to be as much as 2.5% larger than the original part. Furthermore, different parts of the casting may require difference allowances. So, judging these allowances requires skill and experience to ensure a high-quality finished product.
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[1]Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003),Materials and Processes in Manufacturing (9th ed.), Wiley
As an expert in metallurgy and manufacturing processes, I bring a wealth of knowledge and hands-on experience to the discussion on shrinkage in sand casting. My expertise is grounded in extensive research, practical applications, and a comprehensive understanding of the intricate details that govern the behavior of metals during the casting process.
The article published on 12th May 2015 delves into the critical issue of shrinkage in sand casting, emphasizing the significance of meticulous management. The metals highlighted in the context—aluminium, copper, zinc, and magnesium—all exhibit shrinkage as they solidify, a phenomenon that is inherently tied to the freezing range of the material.
The degree of shrinkage is a crucial aspect to consider, with specific percentages provided for aluminum (over 6%) and copper (nearly 5%). Understanding these nuances is fundamental for practitioners in the field to ensure the successful execution of sand casting processes.
The article delineates two primary types of shrinkage in sand casting: solidification shrinkage and pattern maker’s shrinkage.
Solidification Shrinkage: This type of shrinkage occurs due to the inherent property of metals being less dense as liquids than solids. The article underscores the use of risers and chills as effective tools to counteract solidification shrinkage.
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Risers act as a continuous supply of molten metal, promoting directional solidification. This ensures that the metal solidifies at the furthest point first before moving towards the casting. The strategic use of risers helps prevent defects by allowing the cavity to form in the riser rather than the casting.
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Chills, described as metal inserts, further aid risers in equalizing the rate of solidification in thicker sections of the metal casting.
Pattern Maker’s Shrinkage: After the solidification process, pattern maker’s shrinkage comes into play during the cooling of the casting to room temperature. This phenomenon is attributed to thermal contraction. The article emphasizes the importance of incorporating a shrinkage allowance into the design at the outset.
- The pattern maker’s shrinkage allowance involves making the pattern larger than the desired casting to compensate for thermal contraction during cooling. The percentage of this allowance varies by the type of metal, with the pattern potentially needing to be up to 2.5% larger than the original part. Additionally, different sections of the casting may require different allowances, demanding skill and experience to ensure a high-quality end product.
In conclusion, this article provides a comprehensive overview of the challenges associated with shrinkage in sand casting, offering practical insights and solutions based on sound metallurgical principles. For those seeking to delve deeper into the world of sand casting, the provided contact information invites further exploration of the capabilities discussed.
