CHAPTER 4

METAL MOLD CASTING

Diran Apelian

INTRODUCTION

Metal mold casting refers to all casting technologies in which the mold cavity is made of a metallic material. This is in contrast to other casting technologies, such as sand casting, investment casting, and others, where the mold is made of nonmetallic materials. Metal mold casting is the predominant way to manufacture shape castings. Specifically, about 90% of all aluminum castings produced are die cast -- including gravity fed, low pressure, and high pressure die castings.

Metal mold casting technologies are classified as gravity, pressure, squeeze, and specialized processes.

Examples of gravity metal mold casting processes are semipermanent mold and permanent mold technologies. Pressure die casting processes have a variety of regimes -- high pressure, medium pressure, and low pressure. In the high pressure process, the liquid metal is fed into the die cavity and fills it very quickly (fast fill). Low pressure castings are carried out either in semipermanent molds or permanent molds. Medium pressure metal mold castings lie between these two ranges. The relation between the casting pressure and the velocity of the metal at the gate is given by Fig. 4.1, where gate velocity is given in meters/second and is plotted against casting pressure given in bars. One will note that gravity die castings are carried out at both low casting pressures and low gate velocities, whereas medium pressure die castings are carried out at gate velocities between 1-5 m/sec, and high pressure die castings are carried out between 40-70 m/sec. Squeeze casting, on the other hand, is a hybrid: the cavity is filled very slowly (slow fill), and after the cavity is filled, the metal is solidified under high pressure, on the order of 100 bars and above. Specialized processes refer to the most recent metal mold casting technology, semisolid processing. Here the paradigm is totally changed: instead of "pouring" liquid metal into the die cavity, a slug of reheated rheocast billet is emplaced in the die cavity, and when the ram is injected, the thixotropic medium flows and fills the mold. Semisolid processing has many advantages because there is no handling of liquid metal and because the flow of the metal into the die cavity is more akin to Bingham flow rather than to Newtonian flow. Figure 4.2 is a schematic of the opportunities in the semisolid processing market made by comparing this new unexploited market to castings and forgings. The numbers shown in Fig. 4.2 refer to annual U.S. sales for castings and forgings. Note that the viscosity of the starting materials increases from liquid at 10-3 to semisolids at 102 all the way to solid materials at 108.


Fig. 4.1. Casting processes (Buhler).


Fig. 4.2. Schematic of the opportunities for the semisolid processing market.
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Published: March 1997; WTEC Hyper-Librarian