What post processing options are available for sand casting?
How to give your part its final mechanical and material properties.
Post processing for sand castings to improve the final finish
There are a number of different post processing options available to further modify the properties of the casting. Physical processing options can be used to change the shape/surface finish, chemical processes can include painting or surface treatments. Heat and pressure treatments can be used to change casting mechanical properties.
What are the most common post processing options?
Some of the most common post processing options in the sand casting process include:
- HIP'ing
- Heat treatment
- Impregnation
- Surface treatments
- CNC machining
post processing with hip'ing
HIP'ing can make a good casting great, but it can't make a bad casting good. It involves heating the casting, under pressure in the presence of an inert gas (usually Argon). This helps remove porosity from the casting, improves mechanical properties, and offers a lower cost route to high performance castings.
What are the benefits of HIP'ING for aluminium castings?
HIP'ing has a number of benefits, including potentially removing micro porosities, reducing impact of wear and corrosion and
improving the mechanical properties of part. HIP'ing also increases the reliability and 'part
life' of a component, helping to make parts longer lasting by improving fatigue resistance.
heat treatment
Heat treatment is used on aluminium castings to modify the mechanical properties, including balancing tensile strength and ductility. Understanding how different temperature and exposure times influence the mechanical properties of the casting is key to getting the desired outcome.
How does heat treatment improve material properties of a part?
Heat treatment can change the structural properties of the metal by redistributing material precipitated from grain boundaries to homogenise the microstructure. This balances tensile strength and ductility. Heat
treatments can also change and improve other material properties such as corrosion or electrical resistance.
Impregnation
Where micro porosity and micro voids are a possibility (or have been identified via inspection), impregnation protects against the chance of the casting leaking air or fluids. It's a thermosetting plastic that seals against potential leak paths.
How does impregnation improve casting quality?
Impregnation helps maintain function of the part with sealing, reducing both internal corrosion and moisture. This ensures parts
and components are pressure and leak tight, helping to remove potential leak paths.
Surface treatments
Treatments such as painting, electrolysis and chemical coating can all be used to give a casting enhanced surface properties. This will usually happen after the casting has been machined (if this is required). The use of these techniques will be driven by the alloy composition of the casting, application and desired surface properties.
Should you use powder coating on your part?
Painting, or powder coating is one of the most common surface treatments. This requirement will be determined by the specification, and can be used for aesthetic reasons or to enhance corrosion resistance.
Read more: What are the most common casting methods?
Read more: The GW Machining Buyer's Guide free download.
Read more: Controlling mechanical properties of sand casting parts with alloy selection.
Want to know more about the sand casting process?
Download our free ebook, 'The Basics of the Sand Casting Process'.
