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Vacuum Impregnation FAQ

 

 

What is porosity?

Porosity is an area of sponge-like texture in an otherwise sound metal casting. There are two types of porosity:

•  Micro-porosity: Very small (almost invisible) interconnected air filled cells.

•  Macro-porosity: Larger flaws in the casting visible to the naked eye.

 

Porosity is typically caused during the casting process by internal shrinkage, gas cavitation, oxide films and inclusions and the many combinations thereof. 

What causes porosity?

The metal casting process is very sophisticated, but it still has inherent imperfections.  When liquedied and injected into a mold, metal creates gas bubbles that get trapped inside the molded form as the metal solidified.  Gas bubbles create air pockets, folds, and inclusions.  Depending on their size and their random placement within the casting, this porosity can cause metal parts to leak when placed under pressure.

What are the types of porosity?

There are three types of porosity:

•  Blind Porosity: From one surface only and therefore not forming a continuous passage for liquid

•  Through Porosity: Stretching from one inner face to another thereby causing a leakage path

•  Fully Enclosed Porosity: This cannot be reached by impregnation.

Porosity_Types.png

 

What is vacuum impregnation?

Vacuum impregnation (also known as "impregnation" and "impreg") seals the porosity in metal castings, thus making leaky castings pressure tight.  Vacuum impregnation prevents the migration of fluids or gases out of or into a manufactured component:

•  From contained areas to the atmosphere as in hydraulic pumps and transmissions

•  From adjacent independent passages as in oil and water circuits in an engine and graphite plates used in fuel cells

•  From atmospheric conditions to the internals of a component as in insert or over-molded plastic connectors, wire, cable and connector assemblies

What is the process of vacuum impregnation?

In a nutshell, the impregnation sealant is introduced into the voids within the wall thickness of the casting through vacuum and/or pressure methods. Subsequent processing solidifies the sealant. 

How does impregnation improve product quality?

When casting porosity is reviewed during inspection, "good" castings are as porous as the "bad" castings because the porosity is blind and not completely inter-connected. Subsequent mechanical or thermal shock or stress often breaks the thin membrane in the "good castings" which keeps the blind porosity from being continuous, thus causing a "leaker". Impregnation fills porosity from both sides preventing leaks even if the membrane does break. Therefore, impregnation salvages castings and improves quality, while inspection only sorts out leakers.

What materials can be impregnated?

All ferrous and non-ferrous metals, whether sand cast, gravity die, pressure die castings or forgings can be impregnated to eliminate porosity. Iron, bronze, aluminum, zinc, magnesium, steel, sintered metals and plastics, as well as alloys of these metals can be impregnated.

Can cracked castings be fixed through impregnation?

No. Impregnation will not increase the strength of a casting. The cracks will reopen when the casting is under pressure.

Can vacuum impregnation cure surface flaws?

No. This is because impregnation is within the part and not a surface treatment.  

Should impregnation occur before or after machining?

Impregnation occurs after a casting is machined.  This is because machining may potentially uncover additional porosity.

Will impregnation discolor or damage the casting?

No, because impregnation occurs within the walls of the casting.  There is no film or coating on the part surface that will change any dimensional tolerances.

How much does impregnation cost?

This question should read "How much can you save?". It is important to remember that impregnation costs are a small fraction of remelting, recasting, remachining and overruns. Vacuum impregnation seals the inherent problem of porosity, thus allowing parts once deemed as scrap to be useable.  The cost of impregnation depends upon several factors including (but not limited to) the size and complexity of the casting, the amount of castings to be impregnated, and the type of material used to impregnate.  

What size porosity can be sealed?

Micro-porosity causing "weepers" is usually easy to seal. For larger micro-porosity and macro-porosity sealing depends on wall thickness and the type of porosity present. Straight through porosity in thin walls is difficult to seal. Sponge like porosity of any type can usually be sealed. 100% solid resin will seal porosity many times larger than other impregnants. 

Will impregnated sealant vibrate loose or fall out?

No. When sealing porosity within the wall thickness of a casting, the sealant is locked in and will remain so for the life of the casting.

What temperatures will the impregnant withstand?

For most sealants, 400°F is the highest temperature generally recommended for continuous usage, but will withstand 500°F temperatures for short intermittent periods.  Higher surface temperatures, up to 1400°F, can be withstood without resin failure when parts are water-jacked or forced air-cooled.  This is because sealant remains strong and solid in the porous areas of the cool side.  The sealant may char next to the hot side, it never melts and is protected by the thermal conductivity of the metal.  Examples of this are automotive cylinder heads and blocks.

How long will the impregnant last?

As long as the casting itself.

What else will impregnation do?

In addition to sealing for pressure tightness, impregnation is used to seal sintered components and other parts to avoid corrosion. It is also used prior to electro-plating to prevent bleedout from acid etches and electrolytes absorbed into porous areas which are sealed in by subsequent plating. Where bleedout and blistering due to porosity is a problem on other types of finishes, such as lacquers and baked enamels, impregnation before finishing eliminates out-gassing and blistering.