shrinkage
Shrinkage is the reduction in volume of a material or workpiece without removing material or applying pressure. Shrinkage takes place through drying , cooling or chemical or physical remodeling mechanisms in the material. Shrinkage due to drying is often reversible, so the material can also swell .
Depending on the material structure, shrinkage can also be anisotropic , i.e. different in size in different directions (e.g. wood during drying). If the shrinkage is inhomogeneous , i.e. unevenly at different points (e.g. when cooling or drying from the outside in), shrinkage cracks can occur if the tensile stresses that result in the material exceed its tensile strength. Otherwise, temporary stresses or, in the case of ductility, latent stresses can arise in the material ( internal stress ). A parameter for the shrinkage is the shrinkage .
Shrinkage when pouring
After casting , workpieces shrink by a certain percentage of their volume as they cool due to the change in volume during crystallization and thermal expansion, and shrinkage takes place. This value differs depending on the material and is already taken into account in the production of the original mold (model construction) (see values in the following table). The shrinkage occurs after the solidus temperature of metal alloys is reached up to room temperature. It is not only the material, but also its geometry (shrinkage restraint) that determines the exact value of the shrinkage in practice. Steel castings, for example, shrink in a range of 1 to 3% under practical casting conditions.
The longitudinal shrinkage amounts to:
| Cast material | Shrinkage in% |
| Cast iron with flake graphite | 1 % |
| Cast iron with spheroidal graphite | 0.5% - 1.2% |
| White malleable cast iron | 1.6% |
| Black malleable cast iron | 0.5% |
| aluminum | 1.25% |
| copper | 1.9% |
| Brass , bronze | 1.5% |
| Cast steel | 2% |
| plastic | strongly dependent on the type of plastic and processing conditions - therefore no absolute values can be given. However, the shrinkage of plastics is usually much higher than that of metals. At least in the case of plastics, shrinkage is caused by increasing crystallization, which causes a local increase in density. The lack of volume with the same shape is then referred to as shrinkage. |
| Ceramics | In ceramics, too, the shrinkage of a workpiece is called shrinkage. A distinction is made between the shrinkage during drying, which is caused by the evaporation of water, and the shrinkage during fire. The shrinkage can reach values of over 10%. |
The molds for the workpieces must be built larger by the amount of shrinkage depending on the material being cast, so that in the end a precisely fitting workpiece is obtained. In mold making , measuring tools are used for this which already take the shrinkage factor into account. For example, a meter rule in the mold construction of a steel foundry is 102 cm long and is divided into 100 cm particles (1000 mm particles).
Definition:
Shrinkage = (shape - dimension of cold workpiece) / shape * 100%
Shrinkage in casting resins
Low shrinkage in casting resins is a quality criterion, as otherwise built-in components can come under compressive stress and gaps can arise with other parts to be wetted if the adhesion is insufficient. In the case of castings used in electrical engineering / electronics, this can lead to moisture penetration and reduced dielectric strength. If magnetic cores come under compressive stress, their magnetic properties can change. They are therefore partially padded, that is, provided with an elastic intermediate layer before potting.
Low shrinkage can also be achieved with fillers.
Others
Primary shaping without shrinkage is possible, for example, with amalgam , which is one of the reasons why dental fillings made with it are more durable than plastic.
Only a few materials show no shrinkage when solidifying, but even an increase in volume. In addition to water, this includes, for example, swelling cement : Concrete normally shrinks , but can be made shrink-free or swelling with swelling additives.
See also
- Shrinkage (material science)
- Swelling (increase in volume due to fluid absorption)
- Delay (mechanics)