How Does Vacuum and Pressure Affect Casting Materials
When you are hand mixing casting materials, air bubbles are inevitably going to be stirred in. Viscosity, temperature, and surface tension of the material will determine how well the air will or will not self release from the liquid. Other than a few of our low viscosity systems, the mixed material will need to be placed in a vacuum chamber to remove those air bubbles prior to pouring in the mold; also called de-airing or de-gassing.
The vacuum expands the trapped air causing the bubbles to grow, rise to the surface of the material and release. After a period of time these bubbles decrease in quantity meaning that air is effectively being removed from the material. This air would otherwise cause voids and potential mechanical issues with your cast part.
Cold materials will have higher viscosities and may be more difficult to degass. Raising temperatures lowers the viscosity which will aid in pulling air out of the material but may also reduce working time if taken to extremes. Pressure casting can be an alternative or secondary process to vacuum 'degassing' casting material. Pulling air out of the material ensures the material isn't latent with bubbles as it goes into the mold, but air can be re-introduced into the material as it is poured in from splashing or mold features that trap air (e.g. sharp corners).
In some applications, vacuum and pressure must be used in conjunction to produce air free parts. Typical pressure values, when material is degassed, are in the 40-50psi range. Higher pressures may be required if material is not degassed. Pressure does not remove air; it compresses air bubbles down to the point that they are not visible anymore. If pressure is released too soon (before material has had time to set) the bubble could potentially reappear.
The vacuum expands the trapped air causing the bubbles to grow, rise to the surface of the material and release. After a period of time these bubbles decrease in quantity meaning that air is effectively being removed from the material. This air would otherwise cause voids and potential mechanical issues with your cast part.
Cold materials will have higher viscosities and may be more difficult to degass. Raising temperatures lowers the viscosity which will aid in pulling air out of the material but may also reduce working time if taken to extremes. Pressure casting can be an alternative or secondary process to vacuum 'degassing' casting material. Pulling air out of the material ensures the material isn't latent with bubbles as it goes into the mold, but air can be re-introduced into the material as it is poured in from splashing or mold features that trap air (e.g. sharp corners).
In some applications, vacuum and pressure must be used in conjunction to produce air free parts. Typical pressure values, when material is degassed, are in the 40-50psi range. Higher pressures may be required if material is not degassed. Pressure does not remove air; it compresses air bubbles down to the point that they are not visible anymore. If pressure is released too soon (before material has had time to set) the bubble could potentially reappear.