Casting defects and how to prevent them – part 2

Published - 27th Mar 2019

In this second blog, we conclude our study of the most common casting defects and the steps we take at the foundry to avoid them.

Dimensional inconsistencies

Geometrical inconsistencies in the casting can be caused by a number of issues. These include:

• a mould shift (a mismatch between the top and bottom parts of the mould)
• wear or manufacturing issues with the pattern
• core fitting issues
• a miscalculation of the metal shrinkage

These issues can be resolved through good pattern and mould design.

Pressure from the molten metal lifting the top off the mould can also lead to dimensional anomalies. This can be addressed by applying metal weights to the mould during the casting process.

Discontinuities

This category covers a wide range of defects, such as cracks, which can occur when the metal does not cool at a uniform rate.

A cold shut is a common cause of discontinuities. This manifests itself as a partial or whole separation in one of the casting walls. Cold shuts can occur in thin, difficult-to-fill sections or when two streams of molten metal converge during the metal pour and the surface in this area does not fuse together correctly.

This is caused by a lack of fluidity in the molten metal or by cross-sections within the mould that are too narrow. The fluidity can be increased by changing the chemical composition of the metal or by increasing the pouring temperature[1].

Discontinuities may also occur when steel or iron inserts are used to improve the wearability of the casting and this metal does not fuse properly with the aluminium. There are several ways to prevent this problem, such as heating up the insert before the metal pour as well as using special coatings or shot blasting to improve adhesion.

An incomplete casting

Misruns are a common casting defect and they occur when the molten metal is unable to fill the extremities of the mould – which leaves unfilled cavities.

This particular problem can occur if:

• the pouring temperature is too low
• the material is poured too slowly
• there is insufficient fluidity in the molten metal
• there is insufficient material to fill the mould

All of these issues can be resolved through good process design.

At Haworth Castings, our foundry engineers have many years’ experience in managing and preventing all types of casting defects. Please emails us on sales@haworthcastings.co.uk or call +44 (0) 1794 512685 to find out more about our casting capabilities.

[1] Rao, 1999