Why is my casting slip so slow and difficult to drain from the mould?

When the cast thickness has built up to the desired amount, it must be possible for the excess slip to be emptied from the mould without needing to resort to shaking the mould. When this problem occurs it can be caused by a problem with the slip or the mould. If the mould has a narrow filling hole, the cast thickness may restrict this to a point where the slip cannot pass through. To rectify this, the mould opening needs to be enlarged. However in cases where the mould opening is a normal size, then the properties of the slip should be suspected. The slip is too thick and must be thinned down by either a water or deflocculent addition. This problem is quite common with ‘closed back’ flatware moulds which have two small ‘filler’ and ‘riser’ holes. In these types of mould it is often impractical to increase the size of the holes because of the shape of the finished piece, so the slip should be thinned with water for this mould.