SAND MOULD CASTING
Purpose of the work: to study the sand mould -foundry practice; to assess the metal utilization coefficient and quality of castings. Theory. Foundry is one of the main preparatory bases of modern industry. Intricate castings (blanks or finished products) are the usual foundry products manufactured by pouring molten metal into moulds. Foundry accounts for almost 66.7% of total blank production in machinery-building, 49% – in tractor and agricultural machine-building, 40% – in chemical and oil machine-building, 33.7% – in automobile-producing, 32.6% – in instrument-producing, 32.5% – in civil and road engineering machine-building industries (for 1987). Such extent may be attributed to possibility of castings 0.5...500 mm and over in width, from a few grams to hundreds of tons in mass, of intricate shape and wide alloy range to be manufactured. Casting frequently occurs the only (unique) process to obtain a proper shape. The world overall foundry production nowadays exceeds 80 mln tons annually. Approximately 70% of them are manufactured in sand moulds, because of simplicity and universality of the process. For expendable mould manufacturing, besides molding materials, the following set is needed (Fig. 3.1): pattern of a future casting; core boxes in case if there are openings or cavity in the casting; gating system patterns; pattern board (hand molding) or pattern plate (machine molding), etc.
Fig. 3.1. Casting and set of patterns and molding accessories: l – casting; 2 – pattern; 3 – molding board; 4 – core box with core; 5 – gating system pattern
Pattern's dimensions are in excess of casting's ones by shrinkage value. Split structure of a pattern and tapers promote its easy withdrawal from amould. Wooden patterns are usually employed in hand (manual) molding. Aluminium or steel half-patterns and gating system patterns are disposed on a metal pattern plate in machine molding. Core boxes of wood or metal are intended to manufacture cores used for cavity (openings) formation. Metal frames (flasks) promote sand containment in molding, transportation and pouring. Molding materials include molding and core sands. Molding sand consists of used sand and fresh materials: sand (fireproof base), fireproof clay (binder) and water (service ingredients). Three kinds of molding sands are distinguished: facing, backing (hand molding) and unified (machine molding) sands. So far as cores (except core prints) are surrounded while pouring by molten metal, core sand should exhibit higher strength, gas permeability and compliance in comparison with molding one. It contains, besides clay bond, or instead of it, other binders (pitch, oil), sawdust and peat (for proper compliance during crystallization). Hand molding in two mould sections over split pattern has found the wid est use. Pattern drag (bottom half) is disposed on a pattern board with its joint face abut on the board (Fig. 3.2) and a drag flask is mounted next. The flask is filled at first with facing and then with backing sand and rammed. The flask has to be turned over. The upper (cope) half-pattern is joined to the drag one and, next, the upper flask has to be set up. After setting up of the gating system pattern, filling the flask with sand, the last must be rammed in much the same way. Then the upper half-mould is removed and the patterns are withdrawn.
Fig. 3.2. Hand two-part molding: bottom half-mould (a) and top half-mould (b) molding; c – assembled mould: 1 – molding board; 2 – bottom flask; 3 – bottom half-pattern; 4 – top half-pattern; 5 – top flask; 6 – gas relief sprue pattern; 7 – core; 8 – gating system
Hand molding is employed in individual and batch-production. Nevertheless, low productivity and high manual labour consumption are the demerits peculiar to this process. The main drawbacks of castings produced in hand-manufactured moulds are following: rough surface and inaccuracy of dimensions.
|
