Sometimes to understand how something works it’s important to understand what it doesn’t do. Many companies call us and ask about vacuum impregnation. Although this process has been used since the 1940's, many metal casters, machinists and engineers are not familiar with the process, how it works and how it can best be used to meet their needs and requirements.
What is Vacuum Impregnation?
Before we discuss what vacuum impregnation does not do, it is important to clearly define what is vacuum impregnation. Vacuum impregnation seals casting porosity that forms during the casting or molding process. Specifically, it seals the internal, interconnecting path of porosity, which breaches the part’s wall. Vacuum impregnation seals the interconnecting porosity and allows manufacturers to use parts that would otherwise be scrapped. It is the preferred method to seal porous castings in order to prevent fluids or gases from leaking under pressure.
When thinking about vacuum impregnation it’s important to understand the three most common misconceptions:
1. Vacuum Impregnation is Not a Coating.
Although parts are completely submerged and covered with sealant during the process, the sealant is completely emulsified and washed from all the surfaces prior to the final polymerization. Surfactants incorporated in the actual sealant promote the complete washing of the material from all surfaces. Only the sealant that has been drawn into the casting porosity by the force of the vacuum and pressure remains in the part.
2. There are No Dimensional Changes to the Casting after Impregnation.
Vacuum impregnation allows engineers complete freedom to design and make parts to the actual net shapes. An engineer does not need to incorporate dimensional allowance for vacuum impregnation. In the final step of the vacuum impregnation process, the trapped sealant in the porosity polymerized with heat or absence of air.
3. Vacuum Impregnation Does Not Act Like a Cork or Plug.
As mentioned before, the impregnation sealants are drawn into the open casting porosity and deep into the walls by the pressure used in the process. At the surface the sealants are washed away, because there is not enough material for the sealant to adhere. If there is visible or open porosity, the low viscosity sealants will most likely be washed out of any pits or holes.
A part with surface porosity or blemishes before the process will exhibit the same surface porosity and blemishes after the process. However, the porosity below the part’s surface will be sealed.
Vacuum impregnation seals the internal, interconnecting path of porosity, which breaches the part’s wall. The vacuum impregnation process allows manufacturers to use parts that would otherwise be scrapped. In summary, when considering vacuum impregnation it’s important to know what it is NOT going to do:
- Vacuum impregnation is NOT a coating.
- There are no dimensional changes to the casting after impregnation.
- Vacuum impregnation does not act like a cork or a plug