Metal founding – Process – Shaping liquid metal against a forming surface
Patent
1993-05-26
1995-02-14
Bradley, P. Austin
Metal founding
Process
Shaping liquid metal against a forming surface
164137, 164122, 164342, 164348, 164401, 164DIG8, B22D 1502, B22D 2904
Patent
active
053886321
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for gravity die-casting articles particularly, pistons for internal combustion engines and compressors.
One method for the production of aluminium alloy pistons for internal combustion engines and compressors is the well known gravity die-casting technique, carried out in permanent, multi-piece dies, usually constructed of steel.
Pistons vary greatly in design, but generally comprise a skirt portion, having relatively thin metal section thicknesses; and a crown portion, having relatively thicker metal section thicknesses. The relative thicknesses of the skirt and crown metal sections also vary depending upon the type of piston in question. The differences in thicknesses between the two sections is a minimum for a small, monometal piston for a gasoline engine; and is a maximum for a large diesel engine piston which also embodies a combustion bowl in the crown and, for example, a cast-iron piston ring groove reinforcement.
Conventional die construction utilises a die body comprising two halves having a split line on a diametral plane of the die. The die halves form the generally cylindrical outer features of the piston; a multi-piece collapsible internal core forms the piston internal features; and a top plate or core, which rests on the top surface of the die halves, forms the piston crown surface features. Other die components such as core pins to form the bores of the gudgeon pin bosses may also be present. Further, encast items such as piston ring groove reinforcements also may be present, and if present are located in the closed die body halves before closure by the top plate.
Subsequently in this specification, and in the accompanying claims, reference is made to the die having a crown forming part, such as a top plate or core; and a skirt forming part. However, the crown forming die part also may be employed to form regions of the skirt contiguous with the crown; or the skirt forming die part also may be employed to form regions of the crown contiguous with the skirt. In particular, for convenience in this specification and in the accompanying claims, the term "skirt forming die part" is employed to refer to the constituent die part forming at least the major region of the piston skirt; and the term "crown forming die part" is employed to refer to the die part forming at least the major region of the crown.
Because of the differences in metal section thicknesses between the skirt and crown, the time to achieve complete solidification of each such piston portion varies. The relatively thin section skirt portion freezes first whilst the thicker section crown portion takes longer to solidify. Previously the rate of piston production from a conventional casting die has been governed by the time it takes for the crown portion to solidify, the casting not being able to be removed from the die until complete solidification has occurred.
A further disadvantage of conventional dies is that the top plate or core which closes the upper end of the die often becomes misaligned on die closure due to entrapped debris such as die flash or misalignment of die halves, for example. A consequence of this may be that the top surface of the as-cast piston is positionally inaccurate with respect to other piston features such as an encast ring groove reinforcement and gudgeon pin bosses, for example. Since the as-cast crown surface is often used as a datum for machining the piston, such casting inaccuracies can result in dimensional inaccuracies in the finish machined piston.
In order to improve the strength properties of cast pistons, they are sometimes subjected to a quenching operation immediately on removal from the die. Because it has been necessary to wait for sufficient time so as to ensure complete solidification of all parts of the piston before removal of the casting from the die, the temperature of at least a part of the casting has fallen to a level where such quenching operations are not as beneficial as they might otherwise have been.
It is an object of th
REFERENCES:
patent: 3472308 (1969-10-01), Lauth
Bradley P. Austin
Puknys Erik R.
T & N Technology Limited
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