What is the right casting process for your project?

Depending on what you are trying to achieve, different casting processes offer different directions of travel.

Casting can produce complex, functional shapes quickly, easily and reliably. While casting is one of the oldest production methods (dating back thousands of years), modern technology has transformed casting into a precise, data-driven process involving multiple technologies. As a result, complex castings are now created which support the innovation required in the approach to 21^st century engineering challenges.

There are many casting processes, including investment casting, sand casting, high pressure and low pressure die casting. It’s often incorrectly assumed that each casting method can produce the same outcome, but different casting methods achieve different results.

In this article, we’ll take a look at the differences between the main casting processes, as well as considerations for choosing the right one for your cast part.

All casting processes are the same, aren’t they?

It’s a common misconception, but to put it simply, no they're not. Every casting method involves pouring molten metal into a mould. But there are important, distinct differences in process steps, inputs and outcomes for each casting process. As a result, different processes will suit different production volumes, budgets, materials and type of part required.

Which casting process is best for my project?

There’s no clear, right or wrong answer to this. Each casting process will suit different project requirements, and the best choice of casting to produce your component will be determined by a range of constraints. These include:

• Product timescales
• Purpose of the part (for example, prototyping or high volume production)
• Design functionality
• Shape complexity and topology
• Volume
• Material selection
• Stage of production and/ or innovation. For example, prototyping projects will better suit sand casting, whereas high production runs are most suitable for high pressure or investment casting

Casting processes: an overview

Sand Casting

Sand casting is one of the oldest production methods. It involves pouring molten metal into a sand mould cavity, which then cools and solidified. The mould cavity must keep its shape until the metal has cooled and solidified. After the metal has cooled, the mould is broken apart to remove the casting.

The flexibility offered by sand casting allows for the development of complex shapes by altering different components in the process; including alloys, shape topology and process design to control solidification rates in order to achieve specific material properties. Sand casting is a relatively low cost and quick casting process, making it particularly suitable for prototyping and low volume production runs.

Sand casting can also be considered an eco-friendly process, especially when compared to other production methods. The sand and metal used in the process is recyclable, contributing to a circular economy and carbon-neutral aims. The ability to create complex, net shapes is also important for lightweighting and part count reduction.

However, the relatively low production volumes associated with sand casting mean that it isn’t particularly suitable for high production runs. Porosity caused by the sand mould can be associated with reduced part strength, and the surface finish due to the sand often needs additional machining to achieve casting integrity. Good process design within sand casting helps to mitigate some of these potential concerns.

Low pressure die casting

Low pressure die casting uses pressure, rather than gravity, to introduce the molten metal into the chill mould from below. Gas pressure holds the metal in the die until it solidifies, removing the need for runner feeder systems. This method produces a casting with excellent dimensional accuracy, reduces porosity and achieves exceptional and consistent density and strength in the final casting. As a process, it’s well-suited to high volume production rates.

Low pressure die casting has slower production times, and requires investment in machinery, making it less suited to prototyping and more suitable for high volume production. There’s also a minimum wall thickness requirement of approximately 3mm which can limit shape geometry and design function that can be achieved.

High pressure die casting

Often shortened to Pressure Die Casting, High Pressure die casting is a repetitive process that can create high volume production parts by injecting molten metal into a metal die under pressure. High pressure die casting has relatively short production cycles, but it’s more suited to higher volumes.

There’s a high start-up cost, but this is absorbed by the high volume production potential. Pressure die casting requires complex machinery, and can achieve wall thickness in parts as low as 1-2.5mm. Machine size will limit the casting size achieved, and core placement is inherently complex to allow disassembly.

Investment Casting

Investment casting creates a pattern of the part with wax, which is then dipped into ceramic slurry. The mould is then heated and dried, creating a ceramic shell mould for casting. Investment casting can created precise, complex geometries which thinner walls and high surface finish requirements (for example in in industries such as aerospace and automotive).

It’s a relatively expensive casting method compared to other processes and is best suited to low production volumes, typically less than 10,000 pieces. Investment casting is not suitable for designs requiring cores.

Choosing the best casting process for your project

As even our brief overview demonstrates, choosing the most suitable casting method for your next project will be determined by factors such as project stage, design function, need for innovation and iteration (for example in prototyping), volume and timescales. Your casting supplier will work with you to fully understand your requirements in order to advise on the best process for your project.

Want to know more about sand casting? We've written an overview eBook, The Basics of The Sand Casting Process to help. It's also full of our GW insights and tips to help you achieve the best quality casting possible.