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Shrinkage

Shrinkage is a reduction of the volume and the linear dimensions of objects in the course of their formation and also cooling from molding temperature to ambient temperature.

Shrinkage is one of the main indicators characterizing casting properties of materials as it is a reason of formation of shrinkage cavities, porosity, emergence of internal tensions, etc. It is considered at development of technological processes of foundry engineering and of molding equipment. Relative numerical characteristics of change of product sizes at crystallization and cooling are termed as shrinkage coefficients.

Linear shrinkage is a difference between the sizes of the model and the sizes of the casting received by this model (when using disposable casting molds) or a difference between the sizes of the working cavity in the reusable casting mold and the sizes of the casting received in this mold. Linear shrinkage is measured in shares or percentage of the specified sizes of the casting in comparison with the sizes of the model or of the casting mold cavity.

The main reason for linear shrinkage is thermal compression of the casting at its cooling up to the room temperature. Linear shrinkage starts at the temperature above the solidus line when the sufficient amount of the solid phase for formation of the casting framework was already formed. This temperature is termed as the linear shrinkage onset temperature. Further reduction of the sizes at casting cooling is defined by the thermal coefficient of linear expansion that usually makes one third of the thermal coefficient of volume expansion.

In castings the hindered linear (or casting) shrinkage usually takes place. It is shrinkage of the material at which its form does not remain and the casting material shrinks in certain points according to the expansion coefficient. The value of hindered shrinkage is less than the value of free linear shrinkage because of emergence of plastic deformations caused by internal tensions arisen owing to shrinkage.

Volume shrinkage is a reduction of the volume of the material at transition from the liquid state to the solid one and also owing to thermal compression at cooling up to the linear shrinkage onset temperature. Volume shrinkage is observed in castings in the form of shrinkage cavities and/or shrinkage porosity.

Comparison of polyamides by shrinkage