FAQ – Frequently Asked Vacuum Pressure Impregnation Questions
Puzzled about porosity? Questioning your castings? Inquiries about impregnation? Voracious for vacuum pressure info? You’ve come to the right place for VPI FAQ. Read through our frequently asked questions, and don’t hesitate to contact Imprex with questions of your own.
What is porosity?
Porosity is an area of sponge-like texture in an otherwise sound metal casting. Micro-porosity are minute interconnected air filled cells, while macro-porosity are bigger defects in the casting observable by sight. Porosity is usually triggered during the casting process by internal shrinkage, gas cavitation, oxide films and inclusions, and combinations of these activities.
How is porosity initiated?
While the metal casting process is refined, natural deficiencies remain. When liquefied and injected into a mold, metal generates gas bubbles that get trapped inside the molded form as the metal hardens. Gas bubbles produce air pockets, folds, and inclusions. Based on their size and arbitrary location in the casting, this porosity can instigate leaks when placed under pressure.
What are the categories of porosity?
- Blind Porosity: From only one surface, with no forming of an uninterrupted pathway for liquid.
- Through Porosity: Extending from one inner face to another, initiating a leakage route.
- Fully Enclosed Porosity: Impregnation is not needed at this level.
What is vacuum impregnation?
Vacuum impregnation, which seals the porosity in metal castings, thwarts the passage of fluids or gases out of or into a fabricated component.
What is the method of vacuum impregnation?
The impregnation sealant is inserted into the cavities within the wall depth of the casting through vacuum methods, with ensuing processing hardening the sealant.
What are the benefits of impregnation?
Impregnation blocks porosity from both sides, avoids leaks even if the membrane ruptures, salvages castings, and increases quality. It is also used to seal sintered mechanisms to prevent corrosion. Impregnation is also utilized before electro-plating to avoid bleedout from acid etches and electrolytes absorbed into porous areas that are sealed in by subsequent plating. Where bleed out and blistering due to porosity are issues on other types of finishes, impregnation before finishing eliminates out-gassing and blistering.
What substances can be impregnated?
Ferrous and non-ferrous metals can be impregnated to remove porosity. Aluminum, bronze, iron, magnesium, plastics, sintered metals, steel, and zinc, plus alloys of these metals, can be impregnated.
Will impregnation repair split castings?
Impregnation will not enhance the casting’s strength, as the cracks will reappear when the casting is under pressure.
Are surface flaws corrected through vacuum impregnation?
This is not possible, as impregnation is inside the part and not a surface remedy.
Should impregnation ensue before or after machining?
Impregnation occurs after a casting is machined, as machining may possibly expose further porosity.
Is the casting tarnished or marred by impregnation?
As impregnation transpires within the casting’s walls, there is no coating or film on the surface that will change any dimensional tolerances.
Is impregnation costly?
The cost of impregnation pales in comparison to remelting, recasting, remachining and overruns. Vacuum impregnation seals the innate issue of porosity, allowing parts once considered as scrap to be functional. Costs associated with impregnation are contingent on different aspects, such as the casting’s magnitude and complexity, the quantity of castings to be impregnated, and the kind of material used to impregnate.
What size porosity can be sealed?
Micro-porosity is generally straightforward to seal. For macro-porosity, sealing is subject to wall depth and the form of porosity. While straight through porosity in thin walls is problematic to seal, sponge like porosity can typically be sealed. One hundred percent solid resin will seal porosity several times larger than other impregnants.
Will impregnated sealant fall out?
When sealing porosity in the wall thickness of a casting, the sealant is secured and will endure for the casting’s life.
Will the impregnant tolerate extreme temperatures?
For most sealants, 400°F is the maximum temperature normally advocated for constant usage, but will endure 500°F temperatures for brief, sporadic intervals. Extreme surface temperatures, up to 1400°F, can be tolerated without resin malfunction when parts are water-jacked or forced air-cooled, as sealant endures in the porous areas of the cool side. The sealant may char next to the hot side, but it does not melt and is safeguarded by the metal’s thermal conductivity.
More FAQ Information
If you’re still looking for answers, you can check out the Imprex glossary of impregnation terms.