Casting treatments and finishes

Published - 23rd Feb 2022

Improving physical properties and appearance for specific applications

Investacast supplies high-quality castings in a wide range of ferrous and nonferrous alloys to demanding sectors including the Aircraft Seating, Semiconductor and Electronics industries.

Investment casting is a precision manufacturing process capable of meeting demanding specifications and finishes. This blog looks at how cast products can be processed to improve their aesthetic appearance and physical properties.

Preliminary surface treatments for castings

The surface finish achieved by the investment casting process can be enhanced to a greater or lesser degree depending on the end use.

We offer three main blasting processes to remove any surface contamination, improve surface finish and prepare the part for other operations such as painting or coating. Abrasive blast media include:

• Aluminium Oxide – effective for removing any residual scale from the surface of the casting, to produce a matt, uniform finish. Used on both ferrous and non-ferrous alloys.
• Glass Beading – a gentler blasting process for aluminium, using graded glass beads to achieve an aesthetically-pleasing, slightly textured finish.
• Stainless steel shot blasting – used for the harder grades of steel to create a uniform, slightly textured finish.

The impact of the blasting material leaves small concave indentations that can improve the reflectivity of the surface for greater aesthetic appeal. By eliminating surface dirt and increasing the surface area of the part, blasting will also improve the adhesion properties of the surface for painting and other coatings.

Additional treatments to improve corrosion resistance

Metals can be vulnerable to corrosion when exposed to environmental factors such as air and water, so for some applications it is important to increase corrosion resistance during manufacturing. At Investacast, we recommend that customers take advantage of surface finishing treatments for such parts, in the form of pickling and passivation, or electro-polishing.

The chromium in stainless steel provides natural resistance to corrosion due to a passive oxide film that forms on any exposed surface. Where this resistance is compromised or needs to be enhanced, passivation can be promoted through:

• Electro-polishing – an electrochemical treatment for stainless steel castings which improves corrosion resistance by exposing a fresh surface. This is also known as electrochemical polishing, anodic polishing or electrolytic polishing.
• Pickling – the chemical removal of a thin layer of scale and contaminants from the surface of stainless steel to expose a fresh layer. Passivation can also be promoted with an oxidising acid treatment. Investacast uses a trusted professional service to pickle and passivate our castings, using a high-quality acid bath treatment.

Surface finishes for castings

We offer a wide range of surface finishes, giving customers the ability to further colour, protect or enhance the capability of their part. These include:

• Powder Coat Paint – this baked-on powder spray will assist corrosion resistance (provided the existing surface provides a complete barrier) and provide colour. However, powder-coating is relatively thick at 80-100 Microns, so customers need to consider this in the fit and functionality of their product.
• E-Coat Paint – Electro-coating is a wet paint finishing process that uses electrical currents to attract the paint product to the metal surface. Despite being only a few microns thick, the E-coat provides extra durability against knocks and abrasion, while maintaining the detail which can sometimes be lost through thick powder coat paint. It is widely used in the automotive sector and is increasingly popular in other spheres, partly due to the low cost. E- coat finishes achieve extremely high levels of corrosion resistance on carbon steels that would normally corrode quite quickly.
• Anodising – many industrial and commercial applications demand anodised parts to enhance resistance to corrosion and wear. Anodising is an electrolytic process that creates a hard protective layer which thickens the naturally occurring oxide layer on the surface of aluminium. The transparent anodic surface is integral to the metal rather than just a coating.

However, it can be challenging to anodise some grades of aluminium since the silicon added to improve fluidity acts as an insulator. Good Design for Manufacturing will take account of the need for specific finishing to ensure that the right material and process are selected from the start.