Sand Casting: Finishing and Treatment

Published - 20th Dec 2023

Our expert guide to sand casting post-processing

Haworth Castings has over 60 years’ experience as a leading metal casting company specialising in the production of high quality sand castings for customers in the UK and abroad.

Using 80 ASF sand and CAD cut tooling we guarantee exceptional dimensional accuracy, with fine cast definition and excellent surface aesthetics, suitable for use in sectors from defence and aviation to energy, automotive and safety-critical industries.

Surface finish

The physical integrity of the pattern and cores we use in our sand casting, and the choice of sand we use in the moulding boxes, all contribute to the good surface finish achieved on sand castings once they are removed from the mould.

After casting, we are often asked to use additional finishing techniques on our sand castings, designed to give the component the desired surface texture and enhance its aesthetic qualities according to the end use.

Finishing also acts as a preparation for other post-processing treatments such as heat treatment, anodising, alochroming, painting and moly spraying, which are applied to the casting in order to increase its strength, and increase resistance to wear and corrosion.

In this article we will look at the finishing and treatments we offer for sand castings.

1. Fettling & linishing

Castings require different levels of finishing depending on the type of metal used and the end application. The first stages in the casting finishing process are:

Fettling

The removal of any excess material such as runners and risers which have been attached to facilitate the sand casting process. At Haworth Castings a vertical band saw is generally used for this purpose, although a hand-held chop saw can be used for very large castings. The blade must be able to cut through the full range of aluminium alloys used, including those with a high silicon content such as LM6. Silicon is notoriously difficult to cut through, so the choice of blade is important.

Linishing

Grinding or sanding of the component to improve the surface flatness using manual or automated tools, which again must be sufficiently strong to smooth the cast metal.

2. Shot blasting and peening

Shot blasting and peening are methods used to clean and strengthen castings. Both shot blasting and shot peening involve firing small steel balls at a casting at high speed, but the two techniques differ in purpose. Shot blasting uses abrasives to clean or smooth the surface to prepare it for processing; shot peening uses the plasticity of metal to prolong the life of the part.

Shot blasting

Manufactured metal parts are rarely ready for use straight out of the mould. Shot blasting cleans metal parts for further processes such as painting or powder coating, ensuring that these treatments will adhere properly to the part. Shot blasting can clean off contaminants like small particles of sand, or deburr the surface to make it smooth.

Shot blasting also brings out the natural colour of the metal and can be used for purely aesthetic reasons.

Shot blasting involves shooting a high-pressure stream of abrasive material against the surface of a metal part. The shape, size and density of the shots will determine the final results.

Shot peening

The material properties of metal can be strengthened by applying stress to its surface, increasing component fatigue life and reducing cracking in components that will be operating in high-stress environments. Working the surface of metal in this way to increase its strength is called peening.

Stainless steel ball bearings are propelled at high speed at the surface of the casting. The impact of each shot produces a small indentation, which compresses the sub-surface layer and strengthens the metal. Cracks cannot form in this compressive environment.

3. Heat treatment

Heat treatment is used to further improve the mechanical and physical properties of a casting – primarily to increase the tensile strength and reduce the ductility (stretch) of the metal.

Castings are made up of tiny crystals called ‘grains’. Heat treatment allows us to manipulate the grains to alter the properties of the metal.

The choice of heat treatment processes depends on the time and temperature needed for isothermal transformation. At Haworth Castings, we use various heat treatment conditions to produce the different degrees of hardness required for our castings.

• Water quenching for fast cooling of heated parts: TF (fully heat treated) or TB7 (solution treated)
• Polymer quenching: an alternative quenchant offering greater flexibility.
• Stress-relieving: heating a part to a temperature at which internal stresses can relieve themselves: TE (age hardening)

Once the heat treatment has been completed, the castings can be put through crack detection analysis to check for any defects.

4. Anodising

Anodising is used to increase corrosion resistance. It is an electrolytic process that thickens the naturally-occurring oxide layer which forms on the surface of aluminium when it is exposed to air. Anodising creates a hard, protective layer. The transparent anodic surface is integral to the metal, rather than just a coating.

How the process works

The cast part is first cleaned and pre-treated using either chemical or mechanical processes. It is then immersed in an acidic solution.
An electric current is passed between the part (the anode), the electrolyte (the acid) and a cathode. Using electrolysis, a thick oxide layer is formed. The part is removed once the required thickness has been reached and it is rinsed to remove any acid.

The component is then sealed to remove any porosity and create a smooth finish.

5. Alochroming

Alochroming involves coating the casting in a chemical solution to form a protective film. Whilst both anodising and alochroming use chemical oxidisation, alochroming uses a chemical treatment rather than an electrical current to produce a protective chemical film or surface layer.

This process is also known as chromating or alodining, and can be carried out by chemical immersion (in a bath), spraying or brushing.
An alochrome protective film is generally much thinner than other finishes and is not particularly durable. It is therefore useful as a decorative finish, but not for components experiencing significant abrasion.

The key benefits of alochroming

This technique is often used as a pre-treatment as it provides excellent adhesion for paints and other coatings. It also has excellent electrical conductivity and therefore alochromed parts are widely used in electrical and electronic components.

6. Painting

Painting gives components exceptional wear and corrosion resistance, and an aesthetically pleasing surface finish. The two main techniques are summarised below.

Wet painting

Liquid paint is sprayed onto the surfaces of the casting as a coloured pigment suspended in a solvent. Wet painting is particularly useful for parts that cannot be heated for powder coating. It can also produce a thinner finish and an attractive aesthetic look.

Powder coating

Powder coatings offer higher levels of abrasion and wear resistance than wet paint finishes due to the thermal bonding involved. Finely ground particles of dry powdered paint pigment are sprayed onto the surface of the component using an electrostatic spray gun. The part is then heated in a curing oven to melt the powder and form a hard, durable skin.

7. Moly Spraying

Moly spraying provides added protection for extreme temperatures. ‘Moly’ is the shortened name for molybdenum, a chemical element which, when added to various alloys, increases the strength and wear resistance of the resultant materials.

Molybdenum was used extensively in armour plating and tanks during the First World War and is now widely used in industrial applications.

The benefits of this process include:

• Reduced friction and wear (longer life parts)
• Lower abrasion on any mating parts
• Ease of penetration and bonding to metal surfaces
• Excellent corrosion resistance and chemical resistance

At Haworth Castings, we use moly spraying for castings that need to withstand adverse weather conditions and temperatures.

Providing solutions to complex casting challenges

Haworth Castings is proud to offer sand casting as a complete manufacturing solution, including full machining, finishing and treatments to ensure your metal castings meet or exceed your material and aesthetic specifications. We have extensive experience in metal casting and finishing, and the expertise to resolve the most demanding casting challenges.

Our dedicated support team is here to help you through all stages of the sand casting process, including advising on the best manufacturing process for your part, and the most effective and efficient finishing and treatments.

If you are looking for high quality manufacturing, particularly for performance-critical applications, find out if we can help by contacting Haworth Castings on 01794 512685 or email sales@haworthcastings.co.uk.