Cold shuts and misruns are both common defects in metal casting.
In this blog, we will be looking at the main causes, how they are identified and the measures we put in place to prevent them.
How do cold shuts and misruns differ?
Cold shuts occur when two relatively cold streams of molten metal from different gates meet and do not fuse together properly during the casting process. This problem is visible to the naked eye – giving the appearance of a crack separating the two sections. Cold shuts can either extend through part of the casting or the entire workpiece.
A misrun occurs when the molten metal freezes before it reaches all parts of the mould cavity – which leaves a completely unfilled part of the mould.
Both problems can lead to weak spots in castings, so it is important that we prevent these issues from occurring during production.
What causes these issues?
Cold shuts and misruns can be caused by a lack of fluidity in the metal. Fluidity is a very important consideration in metal casting. It can affect the maximum thickness (and thinness) of the part, the intricacy of the casting and the filling of the mould. Fluidity is influenced by a range of factors, most notably the pouring temperature. The temperature must be sufficiently high to avoid casting defects but not excessive – otherwise it could have a negative effect on the sand or mould.
Our foundry personnel use fractional calculations to determine the coherency point – this is the point at which the material cannot flow. They must carefully manage all the varying, and sometimes conflicting, factors to ensure the integrity of the casting.
The running system also plays a crucial role in preventing casting defects such as misruns and cold shuts. A good gating system ensures that the metal is kept ‘alive’ (in a liquid state) for sufficient time to fill the extremities of the mould cavity – thus preventing misruns. Sometimes a simple adjustment, such as adding an additional riser to the running system, is all that is needed to prevent this kind of defect.
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