Why are cores used in the sand casting process?

What is a core?

A core is used in sand casting to produce internal cavities and achieve complex design angles. A core is an (often complex) insert placed into a mould. The main purpose of the core is to direct where the molten metal will fill within the cavity.

How do cores create complex shapes in sand casting?

Cores are a vital part of the sand casting process, allowing design complexity by creating cavities and angles that would otherwise be impossible to be created from a pattern. Cores can also be necessary for the final function of the part.

A great example of this is the water jacket inside an engine - cylinders could be machined in theory, but it' s impossible to machine something inside that you can't even see with a CT scanner.

How do you make and place cores accurately in the sand casting process?

Making and placing sand cores accurately is crucial to the quality of the final casting. Cores must have high levels of permeability, be able to resist temperatures from the molten metal and have good hardness properties. Sand cores are designed to be broken down, or ‘ shaken out’ at the end of the casting process with the rest of the sand mould.

Good mould design is important for sand casting accuracy

Good core print location design is important and core media must be suitable to resist temperature and forces during the casting process. Good mould design takes into account how the mould is assembled too, resisting any movement of cores which can create a scrap part.

What are the different ways to make a core?

Hand rammed This involves manually ramming the cores using sand
Blown Created by shooting a sand / binder mixture into a tool cavity under pressure and cured with a gas.
3D printing 3D sand printers can create tool-less cores rapidly with complex geometries. Multiple features can be combined into printed assemblies reducing sand piece count.

Can cores reduce a part's material usage?

Well-placed and well-designed cores can help to reduce the weight of a final product, reducing the total amount of metal required. Cores can be used to aid lightweighting by taking redundant material from component designs.

HOW IMPORTANT IS THE TYPE OF SAND USED IN A CORE?

Sand strength is important. If this is below the pressure created by the liquid metal filling the mould, this could cause the core to split. Different sands have different properties including thermal expansion, density and heat capacity affecting dimensional outcomes and solidification rate. Core strength, permeability and outgassing need to be considered.

WHAT IS THE BEST CORE PRODUCTION METHOD FOR MY PROJECT?

This depends on your part requirements and is influenced by factors such as volume, your lead time requirements and your budget. For example, 3D printing of cores is rapid, but expensive. Ask your supplier about potential hybrid core solutions too.

When should you consider using 3D sand printing for cores?

3D sand printing isn’t only a rapid prototyping tool. It also offers the ability to print cores for sand casting. This can provide economic and design benefits to sand casting, including:

Lead time benefit 3D printing removes the need to create tooling for sand moulds, speeding up the process.
Volume For lower volumes (particularly in prototyping), 3D printing can reduce investment cost, as well as the total project price.
Geometry freedom As you don ’t need to be concerned with sand extraction from the tooling, or reassembly afterwards, 3D printing allows huge freedom in the geometrical complexity of parts that can be created. There is no need to consider draft angles and split lines.
Fast iteration If a design has many iterations, 3D printed sand allows these to be created very easily and quickly. For example, water jackets in racing cylinder blocks.

How fast can 3D printing produce cores?

Instead of weeks with traditional blown cores, 3D sand printing can create a core in a matter of hours. This can speed up lead times significantly in the casting process. However, 3D printing is not suited for higher volume production.

Why are cores used in sand casting?