High quality UK sand castings using high quality sand

Published - 14th Jun 2024

The importance of sand in producing precision sand castings 

Sand casting accounts for a large proportion of our work at Haworth Castings and we are experts in the process of casting metal components using sand moulds.

In sand casting, sand is packed around a pattern to form a solid mould. Molten metal is poured into the mould and allowed to solidify, after which the sand is removed to reveal the casting.

Choosing the right quality and type of sand is key to achieving a high-quality casting. In this blog we will look at how Haworth Castings selects the best sand for UK sand casting.

The role of sand in sand casting

Sand has long been used in casting moulds due to its practical properties:

• Strength: with the right binders, sand is strong enough to support the molten metal until it solidifies.
• Thermal conductivity: sand conducts and dissipates heat from the molten metal effectively to ensure even cooling and solidification.
• Permeability: the moulded sand allows gases generated during the casting process to escape, preventing defects.

Sand is a cost-effective choice for casting, as it is a relatively cheap and widely available resource. It is also recyclable, reducing the impact on the environment.

The best sand quality for sand casting

Many different grades and sizes of sand are available for UK sand casting. Choosing the right sand involves considering a range of factors, including:

Grain size

The size of the sand grains will affect both the permeability of the sand mould and its surface finish. The wrong choice of sand can lead to rough surfaces requiring additional machining.

Finer grained sand will give a smoother surface to the sand mould, so the cast component will have a better surface finish. However, moulds using finer grains are less permeable, so trapped gases may cause defects in the cast component.

Finer grained sand will also have high flowability, the capacity to fill small cavities in the pattern. High flowability creates a more precise mould and is therefore useful for detailed castings.

Grain shape

Sand is graded according to the roundness of individual grains. Generally, three grain categories are used in UK foundry sand:

• Rounded grain sands provide relatively poor bonding strength, but good flowability and surface finish. 
• Angular grains have greater bonding strength because they interlock, but have poorer flowability and permeability than rounded grain sands. 
• Sub-angular grains are between the two. They possess better strength and lower permeability relative to rounded grains, but lower strength and better permeability than angular grains. 

Thermal stability

The sand must resist the high temperature of the molten metal without expanding excessively, to ensure that the component will be cast to the correct dimensions. Sand that expands or does not retain its shape well can lead to dimensional inaccuracies.

Collapsibility

The ability of the sand mixture to collapse under force. Greater mould collapsibility allows the metal casting to shrink freely as it solidifies, without the risk of hot tearing or cracking.

Moisture content

Moisture affects the strength and permeability of the mould. A mould with too little moisture may break apart, while a mould with too much moisture can cause steam bubbles to be trapped in the casting.

Refractory strength

Moulding sand must be able to withstand extreme heat as molten metal is poured in, without cracking, melting or burning.

Reusability

If the sand can be re-used to produce more sand castings at the same premises, this has both environmental and economic benefits.

As you can see, it is important to choose sand with the right qualities to achieve castings which meet the required specifications. At Haworth Castings, we have years of experience in selecting the right grade and size of sand to produce the optimum quality and finish for our UK sand castings.

Types of sand used for sand casting

Green sand

The type of sand used at Haworth Castings has evolved as our processes have become more efficient. Until the 1980s, we used green sand, a ‘damp’ sand typically including bentonite clay and water. Green sand is still commonly used in the casting industry.

Silica sand

At Haworth Castings, we now use a high-grade thermally reclaimed silica sand mixed with resin binders for UK sand castings. Silica sand has a higher melting point than iron, copper, aluminium and other alloys, so is an ideal choice for sand casting.

Silica sand is classified into different sizes, which affect the amount of binder required and the final surface finish of the casting. Our sand is recycled on site and is approximately AFS 80 under the American Foundry Society classification.

Using resin binders to improve sand strength

The sand used in casting must have sufficient strength to maintain its integrity under the force of the molten metal as it fills the sand mould.

When ‘dry’ silica sand was first introduced, it was purged with carbon dioxide to make the moulds firmer. But carbon dioxide presented health and safety risks due to its combustibility. Today, we use polyurethane cold set binders to bind the sand particles together. These materials are much safer.

The cold-set binders offer many benefits, giving the sand:

• excellent flowability
• high strength
• rapid curing
• a good surface finish, and
• low odour.

Adding a catalyst to the sand mixture

Our foundry team must carefully control the time it takes to produce sand castings. Once the sand mixture has been added to the moulding box, it needs time to harden or set. The outside air temperature can affect the rate at which this process occurs, which can have a negative impact on production times.

To address this problem, we adjust the quantity of the catalyst used in the sand mixture for different temperatures. In cold weather conditions, a higher quantity of the catalyst is needed for the sand to set. Conversely, in hot weather, the sand will set at a much quicker rate naturally, so less catalyst is used.

Recycling our sand

Before we purchased our sand recycling plant in 2013, we bought more than two thousand tonnes of sand every year, and three skips of contaminated sand would go to landfill every week.

Following our £300,000 investment in a sand reclamation plant, we have no sand disposal skips on our premises, waste is minimal, and we recycle 100% of the sand used at Haworth Castings.

As well as offering huge environmental and economic benefits, recycling also allows us to control the properties and ingredients of the sand.

Chemical binders added to our sand enable us to ‘shake out’ the sand from moulds very easily, breaking them down for recycling. The sand is passed through a thermal cleaning area and the heat removes all the residual chemicals.

The result is pristine sand. This reclaimed sand can then be used for further casting work, giving our UK sand castings excellent definition.

Storing sand for sand casting

When not in use, our sand is stored in a silo in the foundry to keep it dry. Any dampness could cause the sand mould to split. More importantly, there is a safety risk to our foundry personnel if hot molten metal mixes with any moisture in the mould during production.

Haworth Castings, experts in sand casting

We consider ourselves experts in sand casting and are always happy to answer your questions – for more information please do get in touch with a member of the team. Contact Haworth Castings on 01794 512685 or email sales@haworthcastings.co.uk.