Flashing:
Almost all moulds are made in more than one piece, that fit together with bolts, dog clips or elastic bands. The fitting together of the mould always leaves a parting line sometimes referred to as the seam line. This is where the casting material runs into the gap between the parts of the mould leaving a visible, protruding edge all aroid the object that has been cast, regardless of the material used.
Sometimes the casting material seeps further into the space between the parts of the mould leaving “ flashing “ which looks like thin fins of material which has to be removed by the mould operator or technician using a knife or power tools. This process is called “ fettling “ a term coined by 18th century ceramicists working in large factories like Wedgwood, where “ fettling” was a specific job for one highly skilled person.
Stepping :
Often the parting line has a step in it where the two parts of the mould are not completely aligned. This is extremely common and almost all moulds create some stepping. But sometimes it can be improved by the operator ensuring they have put the mould back together properly. Some casting materials and techniques create more stepping than others and it is down to the operator to choose the most appropriate materials for their process. To repair stepping, it is required to add more casting material to the lowest side and to remove material from the highest side. This takes time and experience.
Air bubbles:
There are two types of air bubbles. The first type are air bubbles in the mould rubber.
They often happen on leading edges and are created by the manual application of thick rubber working air into it. Almost all moulds have these air bubbles but it is the mould maker’s objective to reduce them to a minimum.
When casting material is painted into or poured into a mould with air bubbles, it creates “spuds”.
As the casting material runs into the negative space created by the air bubble, it fills it up and makes a spherical protrusion on the side of the casting. Normally, this is quickly removed manually or with a power tool. But sometimes the spuds are larger and more complex, they require more skill and training to remove. These are also quite common and should be expected on any cast.
Air bubbles in the cast:
When the casting material is poured or painted into the mould, it can create its own air bubbles. Some castings require a “vacuum chamber” to suck out the air bubbles. When the mould is opened to reveal the cast, small holes can be seen in certain areas, these are air bubbles in the cast. And require filling by the casting technician. This process is normally easy to do and should be expected.
No - runs :
Some very small or complex moulds trap air in the thinnest, detailed parts. This prevents the casting material from reaching fully into the mould detail.
This can be prevented by the mould operator painting in a layer of the material first.
Sometimes the Mould Maker can be asked to create vents in the mould to stop air being trapped. And you should ensure the design of your original object is suitable for casting to reduce the likelihood of this happening. Removal of the stick like protrusions created by vents, is a common job to be completed by the “ fettler” or casting technician.
Before undertaking a custom mould making commission you will be asked if you are competent or have a competent person to complete the type of technical work above. If you are not confident that you are able to complete this type of work, it is recommended that you ask us to do the casting work instead, to achieve best results and avoid disappointment.
No responsibility can be taken by MouldMaking UK for the quality of castings completed by the customer or their agents, from custom moulds completed by us.
