Chapter 15 Introduction to Nonferrous Metals 337 The steel dies for die casting require considerable machining and heat-treating, so only large production runs are die cast. The dies are saved by the casting firm for repeat orders. To produce consistently uniform cast products, the melt temperature, die temperature, temperature distribution across the die, plunger speed near the start and end of the stroke, and ram force must all be monitored carefully. Automation makes it easier to maintain the desired conditions, but the automatic machine does not always monitor all the required factors. Opening a shop window on a spring day can change the airflow in the room, and thus change the temperature profile in the die. Liquid metal dissolves gases from the air, so if the metal sits in a holding chamber for too long, such as hours, the porosity can increase. Large amounts of remelt from gates and runners can add impurities, and of course different alloys must be kept separate to maintain the composition of each remelt. Two pressure die casting methods are used for nonferrous metals, cold-chamber die casting and hot-chamber die casting. Cold-Chamber Die Casting In cold-chamber die casting, liquid metal is poured into a shot chamb er, or shot sleeve, manually or by machine. A plunger then forces the liquid metal into a die, where it freezes rapidly, Figure 15-17. Aluminum die casting is usually done using cold chambers. Hot-Chamber Die Casting In hot-chamber die casting, a tube is immersed in liquid metal, and a v ertical plunger forces the liquid up through a “gooseneck” (a feeding spout) and into the die cavity, Figure 15-18. The liquid metal is not exposed to air, so no oxide “skull” enters the die, and any cover flux cannot enter the die, so the part has higher integrity. Only alloys that do not erode steel can be used with hot-chamber machines, because the plunger and gooseneck are all continuously exposed to the liquid metal. Magnesium and zinc alloys work well with hot-chamber casting. Liquid metal poured in Die half locked during injection Plunger direction Liquid metal Shot chamber Goodheart-Willcox Publisher Figure 15-17. With cold-chamber die casting, a horizontal plunger forces liquid metal into the die cavity. The liquid metal solidifies rapidly and takes on the shape and surface texture of the die cavity. Copyright Goodheart-Willcox Co., Inc.