What is Casting Porosity?
Casting Porosity is a phenomenon that occurs in materials, especially castings, as they change state from liquid to solid during the manufacturing process. Casting porosity has the form of surface and core imperfections which either effects the surface finish or as a leak path for gases and liquids.
Impregnation is a cost effective, permanent solution to the problems that may be encountered as a result of such porosity in castings.
Holes or defects may be created that if permeated by gas or fluid, result in leaks or surface defects that incur significant and costly quality problems, even leading to the failure of the component in service.
Vacuum impregnation is the permanent solution to casting porosity problems, filling any voids with a stable yet flexible material that is resistant to attack from heat, oils, chemicals or vibration. The process is sub-surface and can be performed on raw materials or the finished machined part, causing no dimensional change or contamination to the component.
Impregnation as a means of treating casting porosity has been employed since the 1950s, although traditional sealants such as sodium silicate or polyesters have since been replaced by more effective and environmentally friendly methacrylate based products.
It is Ultraseal International's commitment to continuous improvement over the years that have convinced companies of the benefits of impregnation as a genuine quality enhancement rather than a rectification process.
Casting Porosity can be found in castings made from many materials but is particularly prevalent in aluminium or magnesium die castings, where three main types may be identified.
- FLOW POROSITY, resulting in surface or internal defects caused by poor pressure or metal flow conditions in the process.
- GAS POROSITY, usually internal, caused by trapped gases of various kinds in the die. Gas porosity comes from three main sources in die-casting, namely trapped air, steam and burned lubricant.
- SHRINK POROSITY, the most common and most difficult to control, caused by the change in volume as the metal changes state from liquid to solid.
Casting porosity, impacts upon everyone in the manufacturing supply chain since it is encountered to varying degrees in all castings. It appears as a collection of voids in the structure and takes three basic forms:
Consequently, it is vitally important to get the ‘marriage' between the equipment and chemical sealant right. Ultraseal International has invested substantially in research and development to ensure that its latest equipment is designed to accommodate and effectively work with the latest products like MX2 and Rexeal recyclable sealant which makes major contributions towards the environmental issues.
Ultraseal would always recommend that the most reliable method for evaluating the performance of an impregnation sealant is through the testing of impregnated test rings, as established by the international standards and recognised test procedures. By doing so, results that reflect "in use" product performance are generated.
The blue line on the diagram indicates the size and density of the voids (porosity) in a test ring before impregnation.
The red line indicates the size and number of voids (porosity) in the test ring after impregnation with Rexeal 100. This demonstrates that all microporosity is sealed and the gross porosity of the sample was improved by 99%.
Micro Sections showing magnified porosity in test rings