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1、Unit 4 Pressure Die-Casting Die Design,4.1 Lesson 9 The Assembly of Pressure Die-Casting Dies,A pressure die-casting die is an assembly of materials, mostly ferrous metals, each of which plays a part in a mechanism which will operate under conditions of rapidly changing temperatures, as the molten m
2、etal is injected under pressure and then immediately cooled.1 Some parts of the die merely act as holding elements; others, such as the die inserts and the cores, have to withstand the impact and high temperature of the molten metal. The mechanisms for moving the ejectors and cores must work smoothl
3、y in the constantly changing temperature of a die. Parts which act as bearings are made either of non-ferrous metals such as phosphor bronze, or the steel surfaces are given a nitride or other treatment to resist wear.,The die insert for producing of the shape of the casting has to be machined accur
4、ately and heat-treated to provide the best possible mechanical properties at high temperature. Most die-casting dies operate automatically and, while this leads to high productivity when there are no delays, the cost of breakdowns is correspondingly high.2 During the past decade the manufacture of d
5、ies has been revolutionized by sophisticated methods ranging from improved spark erosion to computer-controlled die making. Among other efforts the Science Research Council is supporting a large scale research programme to include die heat treatment and surface coatings, computer-aided die design, s
6、pecial machining processes and the economics of die manufacture.,A single die may contain l0 or more different steels plus several non-ferrous metals and special heat resisting alloys. Figures 4-1 and 4-2 illustrate schematically a die which typifies the various components, each of which involves de
7、sign, engineering and metallurgical problems.,4.2 Reading Materials 9 The Die Insert,The die insert is the heart of the die, since it keeps the outside shape of the component, forming the cavity into which the molten metal will be injected. Die inserts, together with the cores which form recesses, m
8、ust endure the effects of temperature and injection pressure of the molten metal. Each injection is a step towards the thermal fatigue which will eventually cause deterioration of the die. The cycle of stresses in the die insert results in a sudden increase of temperature as the molten metal enters,
9、 followed by a rapid decrease. Often the die insert is built up from several pieces, either to save material when large blocks incorporate small projections, or to facilitate replacement of parts, or to ensure that an available heat treatment furnace will accommodate the die block.,Spark erosion tec
10、hniques, widely used nowadays to manufacture small or medium sized die components in one piece, avoid the danger of molten metal being forced down joins between separate parts. Cast-to-form is now a well established procedure for making cast iron permanent dies, of sufficient accuracy for the requir
11、ements of that process. Die inserts for pressure die-casting need to be made to an accuracy of the order of 0.002 mm. Furthermore, the inserts are usually to be made of 5% chromium steel and, so far, the difficulties of casting such a material to a sufficient accuracy have prevented “cast-to-from” f
12、rom being used for pressure die-casting dies.,Where deep inserts are required, clearances to assist fitting are provided and radii on the corners are included to improve strength and remove stress raisers, as indicated in Figure 4-3. For very deep inserts, fitting at two levels is advisable, both for ease of assembly and to assist heat transmission from the insert to the die block. The select of steel and its subsequent heat treatment are vital factors in ensuring optimum life and accuracy of the die-casting die.,
