Sand castings have many defects that can occur due to t […]
Sand castings have many defects that can occur due to the mold falling. The mold usually fails because of one of two reasons: the wrong material is used or it is improperly rammed.
The first type is mold erosion, which is the wearing away of the mold as liquid metal fills the mold. This type of defect usually only occurs in sand castings because most other casting processes have more robust molds. The castings produced have rough spots and excess material. The molding sand becomes incorporated into the casting metal and decreases the ductility, fatigue strength, and fracture toughness of the casting. This can be caused by sand with too little strength or a pouring velocity that is too fast. The pouring velocity can be reduced by redesigning the gating system to use larger runners or multiple gates. A related source of defects are drops, in which part of the molding sand from the cope drops into the casting while it is still a liquid. This also occurs when the mold is not properly rammed.
The second type of defect is metal penetration, which is when the liquid metal penetrates into the molding sand. This causes a rough surface finish. This is caused by sand particles that are too coarse or pouring temperature that are too high. If the pouring temperature is too high or sand of low melting point is used, then the sand can fuse to the casting. When this happens the surface of the casting produced has a brittle, glassy appearance.
The following is seven basic categories of defect in sand castings.