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critical area. However, for nodules on margins or on internal surfaces, removal of these irregularities might alter the fit of the casting. As previously noted, the best method to avoid air bubbles is to use the vacuum investing technique.
If a manual method is used, various precautions can be observed to eliminate air form the investment mix before the investing.
As
previously outlined, the use of a mechanical mixer with vibration both before and after mixing should be practiced routinely. A wetting agent may be helpful in preventing the collection of air bubbles on the surface of the pattern, but it is by no means a certain remedy. As previously discussed, it is important that the wetting agent be applied in a thin layer. It is best to air dry the wetting agent, because any excess liquid dilutes the investment, possibly producing surface irregularities on the casting.
Water Films. Wax is repellent to water, and if the investment becomes separated from the wax pattern in some manner, a water film may form irregularly over the surface. Occasionally, this type of surface irregularity appears as minute ridges or veins on the surface.
If the pattern is moved slightly, jarred, or vibrated after investing, or if the painting procedure does not result in an intimate contact of the