Browsing by Author "Muga, Zacharia Chimoto"

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    Investigation of properties of selected local sands for foundry use
    (University of Dar es Salaam, 1994) Muga, Zacharia Chimoto
    A study has been made on the unbonded and bonded sand process variables of sand material obtained locally from selected areas: Mbande, Pugu, Boko and Kunduchi. The unbonded sand process variables studied include composition, sintering paint, size grading and grain shapes. The bonded sand process variables studied include: green sand properties using clay (bentonite) bonded sands; while thermal properties were investigated using sodium silicate-CO2 bonded sands. The results from the unbonded sand tests showed that two of the sands investigated, that is, Pugu and Mbande, are high silica sands with 96% silica and about 4% of impurities. Both the investigated sands showed high sintering point above 1200°C indicating high purity and refractoriness. The other two sands from Boko and Kunduchi, showed to have low silica sands with 87% and 88% silica hence the percentage of impurities, and low sintering point or refractoriness (below 1200°C). The study on thermal properties of the bonded sands revealed that all the sands are thermal stable. Linear thermal expansions of 5.2, 50.0, 4.8 and 4.6% in decreasing order were shown by Pugu, Mbande, Kunduchi and Boko sands respectively. High silica sands showed high linear expansions than low silica sands. Mechanical analysis of the sands revealed that all the sands are sub angular in shape with Mbande and Pugu sands being graded as fine sands with A.F.S. grain fineness of 64.25 and 56.01 respectively. Boko and Kunduchi are graded as coarse sands with A.F.S. grade 39.6 and 30.5 respectively. As for synthetic sand (10% clay, 3% water), Boko sand showed to have higher permeability followed by Kunduchi, Pugu and Mbande sands. However, higher compression strength was achieved by Kunduchi sand followed by Boko, Pugu and Mbande sands. From the above results the investi gated sands were assessed and graded according to their suitability in the production of both non-ferrous and ferrous alloys and type of binder to be used in moulding. The sands were graded as follows: (1) Mbande and Pugu sands: for production of high alloy castings (grey and ductile irons) in both green sand and chemical bonded sands. (2) Boko and Kunduchi sands: for small ferrous castings and non-ferrous alloys castings and green sand only. Further work is however required to investigate other sand pits around Tanzania which are believed to have sands with satisfactory moulding properties. These areas include: those around the lakes Victoria, Nyasa and Tanganyika; and Central Tanzania areas.