Machinery's Handbook, 31st Edition
1512 CENTRIFUGAL CASTING The G-factor is the ratio of centrifugal force divided by gravitation force:
mv R mg 2 ·
v Rg 2
F F
(12)
GF
= = c
=
g
where GF = gravitation factor. Solving further, we get:
gGF D
GF D
30 2
(13)
N
=
=
423 .
π
where D = inside diameter of the mold. If the G-factor is too low in centrifugal casting, the liquid metal will not remain forced against the mold wall during the upper half of the circular path but will drop inside the cavity. Too high a speed results in excessive stresses and hot tears in the outside surface of the casting. On an empirical basis, GF = 50 to 100 for a metal mold, and GF = 25 to 50 for a sand cast mold. Semicentrifugal Casting.—During semicentrifugal casting (Fig. 15), the mold is rotated around its axis of symmetry. The molds used can be permanent or expendable and may contain cores. The detailed shape is given by the shape cavity of the rotating mold. The mold is designed with risers in the center to supply the feed metal. Centrifugal force is utilized for refilling of melt metal, increase of the filling power in order to cast parts with thin walls, and slag separation. In general, the rotational speed is lower than speeds used in true centrifugal casting—usually it is set so that a G-factor of around 15 is obtained. As this process produces parts in which the central zone (near the axis of rotation) usually has inclusion defects, it is suitable only for parts where these can be machined away, such as cogwheels.
Fig. 15. Semicentrifugal Casting Centrifuge Casting.—The third type of centrifugal casting, centrifuge casting (see Fig. 16) involves molds arranged about the central sprue at the same distance from the center of the axis of rotation. These molds contain all necessary geometry for the cast part, as well as the gating system. During the pouring phase, molten metal is introduced into the central sprue and then travels via runners to the entrance of each mold. When an object is rotated, force is produced that acts directly away from the center of the axis of rotation. As the centrifuge spins, centrifugal force pushes the metal outward from the center, through the runners into the molds. Once the correct amount of molten metal for the casting is poured and distributed into the molds, the apparatuses continue to rotate while solidification occurs. After the castings have completely solidified, the apparatuses will stop rotating, and the parts can be removed. Use of centrifugal force helps controls specific properties of the cast parts. The method is extensively used for casting smaller parts, including casting gold crowns for teeth in the dental industry.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online