Metal founding – Process – Shaping liquid metal against a forming surface
Patent
1995-01-20
1997-03-18
Hail, III, Joseph J.
Metal founding
Process
Shaping liquid metal against a forming surface
164313, B22D 2709, B22D 1708
Patent
active
056113870
DESCRIPTION:
BRIEF SUMMARY
CROSS-REFERENCE TO RELATED APPLICATION
This application is A 371 OF PCT/GB92/01350, filled Jul. 23, 1992.
FIELD OF THE INVENTION
The present invention relates to moulding devices, in particular, high pressure die casting machines and is particularly, although not exclusively, concerned with the production of metal castings having low porosity.
BACKGROUND OF THE INVENTION
There are a number of high pressure die casting devices/machines in use but many produce cast articles which have a high degree of porosity. If the level of porosity is allowed to rise above certain limits, the pores can adversely affect the properties of the component so as to lead to the failure or deterioration of the cast article during use (most porosity remains internal; only if the die castings are machined is the porosity usually exposed). Therefore, in any high pressure die casting device, an object will be to reduce the porosity levels as close to zero as possible.
A high pressure die casting (hpdc) machine which is used extensively is the "cold chamber" hpdc machine which manufactures aluminium alloys. This machine operates by transferring molten metal from a shot sleeve into a die cavity by means of a high velocity piston or plunger. The molten metal is forced along a series of channels or a runner system and through a fixed, narrow gate or opening into the die cavity. The liquid metal is then effectively sprayed through the gate to produce a first coating over the surface of the die cavity and then the remainder of the liquid metal is introduced into the cavity to complete the cast article. The first coating of liquid metal commonly produces a very fine grain surface layer having a very smooth surface finish. However, the cold chamber hpdc process suffers from two major disadvantages. First, because the molten metal has to flow through a runner system of channels, its temperature will fall by the time it reaches the narrow gate and consequently, it will freeze around the narrow gate which reduces the pressure which can be transmitted effectively from the runner and gate onto the metal in the die cavity. The reduction in pressure transmission will produce die castings which are notoriously porous and may, therefore, not be heat treatable for fear of blistering. Furthermore, subsequent machining operations will expose the porosity which causes a high rejection rate. Secondly, the cold chamber hpdc process is commonly only about 33% efficient because approximately 50% of the cast metal (i.e in the runner and gate sections) needs to be removed from each casting for remelting. Moreover, since there is an additional casting scrap rate of 5-15% the efficiency of the cold chamber hpdc process is rarely greater than 25% in material utilisation and considerably less than 20% in energy utilisation.
Several solutions have been proposed to reduce porosity. One such solution involves evacuating the die set prior to casting with a view to reducing gas entrapment. However, the casting still freezes or solidifies remote from the point of application of pressure and therefore, the solidifying casting cannot be fed from the reservoir of metal in the runner and wad. Hence, contraction cavities arise in the casting. A further solution proposed was to purge the die set with oxygen or another suitable gas which would combine spontaneously with the liquid metal to remove gas from the die set. However, contraction cavities are still formed. The mould is sprayed with a lubricant prior to the casting which evaporates on contact with the hot metal so that gases are still present in the mould. A slightly different approach was to apply enhanced pressure on the wad by a smaller secondary piston but porosity still exists in the casting due to remote application of the pressure and freezing off at the gate.
In order to attempt to limit the amount of porosity, another approach to high pressure die casting has been devised which is more accurately described as a "squeeze" casting. Squeeze casting is the term used to denote processes in which liquid meta
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patent: 2669760 (1954-02-01), Venus
patent: 4436140 (1984-03-01), Ebisawa et al.
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patent: 4860818 (1989-08-01), Dannoura
patent: 5188165 (1993-02-01), Ivansson
Patent Abstracts of Japan, vol. 6, No. 267, M-182, abstract of JP,A,57-159251 (Toyota Jidosha Kogyo K.K.) 1 Oct. 82.
Hail III Joseph J.
Hi-Tec Metals Limited
Lin I.-H.
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