Everything You Need to Know About Die Casting:
A typical hot-chamber die consists of two sections: the cover half and the ejector half, which meet at the parting surface. The orientation of this surface and the direction that the die moves relative to the casting must be recognized in the product design. The cover half is secured to the front or stationary platen of the machine. The sprue, which directs the molten metal towards the die cavity is in this half, and it is aligned with the nozzle of the casting machine. It contains the ejector mechanism and, in most cases, the runners.
The cold-chamber die is arranged in essentially the same manner. The die cavity, which forms the component to be cast, is machined into both halves of the die block or into inserts that are installed in the die blocks. The die and the casting are so designed that the casting remains in the ejector when the die opens. The casting is then pushed out of the cavity by the ejector pins that come through holes in the die and are actuated by the ejector plate, which in turn is powered by the machine. Guide bars or leaders pins, extending from one die half, enter holes in the other half as the die closes to ensure alignment of the halves.
Dies may contain fixed and/or movable cores in either half in order to produce complex shapes. Fixed cores are anchored in the die; consequently, the casting must be designed to permit their movement parallel to the direction of the die opening. Movable cores, which are locked in place when the die closes, are actuated by the cam pins or hydraulic cylinders. They may be incorporated into either half, but the best location from the diecasters viewpoint is in the die parting. Movable cores add to die fabrication and maintenance costs, and may increase cycle time. However, they can be employed to advantage when they allow features to be cast, eliminating subsequent machining steps.
Whenever there is relative motion between die members, there must be clearance, and all such clearances tend to increase due to wear. Molten metal may be forced into these clearances, leaving flash on the casting, which often must be removed. The designer should visualize the die relationship to the casting, so that potential flash can be predicted, and provision made for removal where necessary.
Multiple-cavity dies--Whenever possible, particularly when production volumes are high, dies may contain more than one cavity so that several castings are made on each shot. A die with identical cavities is usually referred to as a multiple cavity die. A die with cavities of different shapes is referred to as a combination die or family die. Combination dies are frequently used to make sets of components for an assembly.
Unit dies--A class of dies widely used for high volume production is the unit die, which consists of a die holder into which a number of standard size blocks are fitted. Each die block contains one or more cavities. Unit dies provide for quick changeover and a high degree of flexibility. However, they are additional pieces to be fitted, and thus constitute an additional tolerance in the die alignment (ejector side to cover side) stackup.
Die rill--Much of the recent research and development work designed to improve die casting technology has focused on proper filling of the die with molten metal. In the ideal case, the molten metal should move in a solid wave front through the metal delivery system, through the gates (orifices that connect the runner to the cavity), and into the cavity. The air in the system should be pushed out of the cavity ahead of the molten metal. In the actual case, the solid wave front does not fully develop. The metal flow is somewhat turbulent, so that air is entrained, possibly forming porosity in the casting. The metal solidification process tends to drive this porosity away from the surface into the core of the section.
Die castings normally exhibit smooth surfaces with no visible defects, but the metal removal operations that cut deep enough to penetrate this skin may expose the porosity which could lie beneath. Where product applications require a smooth surface, or where the porosity is interconnected and pressure tightness is required, the presence of such porosity would call for impregnation of the die casting.
The last area of the die cavity to fill tends to exhibit the poorest quality. This area generally has the coolest metal and potentially the highest porosity. It is therefore common practice to locate overflow cavities at the die parting plane in those areas. The overflows receive the poor quality metal, raising the quality of adjacent metal in the die cavity. Overflows are carefully sized, because they constitute additional extraneous metal, or scrap, which must be recycled.
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