Casting of Metals سباكة المعادن Dr. Talaat A. El-Benawy الفصل الدراسي الاول 1433/ محاضرة رقم

Forces acting on the mould As the mould fills it becomes exposed to high metallostatic pressures which tend to displace or distort the mould sections and cores. These forces can be accurately predicted and contained by foundry measures. The first need is for a dense, rigid mould, since the pressure tends to expand the mould cavity, especially in greensand practice. Rigidity of the mould parts can be increased by using box bars or cover plates to reinforce the sand mass; these measures become increasingly necessary with moulds of large area.

Assuming a rigid mould, the next concern is with the force tending to separate the mould parts. The upward force acting on a flat mould surface is equal to 9.81  h A [N], where  = density of the metal [kg/m3] h = head of metal, m A = superficial area, m2 A completely flat mould surface gives rise to the maximum lifting force: calculations for other shapes can thus be safely based upon their projected areas. The force is resisted by using box clamps and arrangements of plates and tie-bars to hold the mould parts together and by weighting the top part. In the latter case the minimum weight required is r h A [kg] including the weight of the cope itself.

Cores too are exposed to an upthrust, equivalent in this case to the weight of metal displaced; the net upward force is thus considerable except in the casting of light alloys. The force is countered by high mechanical strength and rigidity in core construction, enhanced by reinforcing grids and irons. Cores must be firmly supported against movement in the mould. A core relying upon a single coreprint in the mould bottom, for example, tends to float out of position and must be anchored with wires or sprigs. Cores of large dimensions may require several points of support: if these cannot all be provided in the form of coreprints, studs or chaplets may be needed to prevent movement on casting.

Example 1: It is required to cast the cast iron part shown in the figure. If the flask dimensions is 300 × 300 × 200 mm 3, the density of steel is 7.8 g/cm3 and the density of sand is 1.67 g/cm3, calculate the required weight that should be used to prevent the buoyancy force from lifting the upper flask.

Example 2: It is required to cast the cast iron tube shown in the figure. If the flask dimensions is 800 × 600 × 300 mm 3, the density of steel is 7.8 g/cm3 and the density of sand is 1.67 g/cm3, calculate: 1.the required weight that should be used to prevent the buoyancy force from lifting the upper flask and 2.the force acting on the core