Abstract
Skin dried mould, that is composed mainly of silica sand, bentonite and water, is commonly used for medium and large steel castings of about 0.5ton to 10ton weight.
Skin dried mould is prepared by removing the surface moisture to 50mm depth heating the surface at about 200°C for 2 to 4 hours.
In this paper, the relations between properties of bentonites and dry strength of sand mixtures were sought in order to determine the influence of bentonite, the main bonding material, on the strength of sand layer of skin dried mould surface.
The results may be summarized as follows.
(1) When skin dried moulds of steel castings are being assembled, such characteristics as high strength of mould coupled with preservation of mould edge as well as good adhesion of sand particles against rubbing or shaving are mostly desired in the shop.
(2) When the bentonite of higher liquid limit value is used, higher dry strength mould can be obtained.
(3) When the bentonite used is one with higher liquid limit value, better adhesion can be secured of the sand particles.
(4) Therefore, it is in accordance with the practical demand to employ bentonite of higher liquid limit value in assembling the satisfying skin dried mould for steel castings.
(5) Sand contained 3.2% or 4.2% moisture, and when moisture content was in this range, it may be concluded that the test results were not influenced by aggravation of sand mould due to crack formation on drying.
