To say that all vacuum impregnation processes are equal would be to say that every die casting process is the same. Nothing could be further from the truth. The vacuum impregnation process will have a direct impact on the sealing quality.
We posted a blog in May about the common frequently asked questions about vacuum impregnation. With the complexity of the subject, these questions and answers do not fully address all porosity and vacuum impregnation questions. The following are additional answers to commonly asked questions about porosity and vacuum impregnation.
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.
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.
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.
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.
As long as the casting itself.
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.
We are sometimes told by customers that they are unaware of what vacuum impregnation is. Ultimately, they have a casting with porosity that is leaking whatever is passing through the part (i.e. air, gas, water, etc.), and they need a solution to their problem. Here are some of the more common questions that we receive in regards to vacuum impregnation and porosity. Our intent is that you can use this information as a guide when you need to correct castings with porosity.
Would you like this information in a printable form? Of course! Click here to download it as a pdf.
What is porosity?
Porosity is an area of sponge-like texture in an otherwise sound metal casting. There are two types of porosity:
Porosity is typically caused during the casting process by internal shrinkage, gas cavitation, oxide films and inclusions and the many combinations thereof.
What is vacuum impregnation?
Vacuum impregnation (also known as “impregnation” and “impreg”) seals the porosity in metal castings, thus making leaky castings pressure tight.
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.
What are the types of porosity?
There are three types of porosity:
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.
These are not the only questions we receive in regards to vacuum impregnation and porosity.
To discuss your specific questions, or situation, contact us at 330-562-1440 or send us an email.
Next Steps
