A method and apparatus for the pressure die-casting of articles from thixotropic aluminium

Abstract: Thixotropic aluminium is injected into a pressure die (4) comprising a fixed die (6) and a movable die (8) in the molten state by means of an injection piston (20). During the injection step, the injection piston (20) is stopped at predetermined instants and a variable, pulsed thrust force is applied thereto. At the end of the injection step, a variable pulsed thrust force is applied to the closure piston (9) of the movable die (8).

Description: The present invention relates to a method and apparatus for the pressure die-casting of articles from thixotropic aluminium. Pressure die-casting is known to be a process which enables articles to be obtained with very complicated geometries, normally with the use of aluminium alloys. The process is carried out by the injection of aluminium alloy in the molten state into a closed die cavity, the presses used having a closure force which may vary between 294°300 Nw (300t) and 1°569°600 Nw (1600t). The articles thus obtained are of good quality but, if thixotropic aluminium alloys were to be used, the quality of articles manufactured by the usual pressure die-casting techniques would be considerably improved. The present invention relates to method for the pressure die-casting of articles from thixotropic aluminium and to apparatus for carrying out the method, which enable the quality of the articles produced to be improved significantly. The present invention achieves this object by means of a method for the pressure die-casting of articles from thixotropic aluminium having the characteristics claimed specifically in the claims set out below. The invention further comprises a pressure die-casting press for carrying out the method. Further characteristics and advantages of the present invention will become apparent from the following detailed description, given purely by way of non-limitative example, with reference to the appended drawing which is a sectional view of pressure die-casting apparatus according to the invention. With reference to the drawing, pressure die-casting apparatus according to the invention is generally indicated 2. The apparatus 2 comprises a vertical die-casting press 4 having a fixed die 6 and a movable die 8 slidable vertically under the action of a closure piston 9 towards the fixed die 6 along four guide pillars 10, 10'; 12, 12', fixed to the floor 13 on which the press 4 is fixed. The fixed die 6 and the movable die 8 are heated electrically by means of electrical resistances (not shown in the drawing). An injection assembly 14 has wheels R whereby it is movable on the floor 13 on which the press 4 rests between the four guide pillars indicated above and includes a crucible 16, an injection nozzle 18 and an injection piston 20 for injecting the thixotropic aluminium into the assembly constituted by the fixed die 6 and the movable die 8 when these are closed. Control units 22 and 24 are connected to the closure piston 9 and to the injection piston 20 respectively. In use, the press 4 is closed by the lowering of the movable die 8 onto the fixed die 6 and the closure force required is exerted on the movable die 8 through the control unit 22 which acts on the closure piston 9. When the die cavity is closed, the control unit 24 causes the injection piston 20 to effect its injection travel and this starts to inject the thixotropic aluminium into the die 8, 6 through the injection nozzle 18. The speed of the injection piston 20 is between 0.7 and 1.3 m/sec. At predetermined instants, always by means of the control unit 24 the injection travel of the injection piston 20 is interrupted and a pulsed force is applied to the piston, the magnitude of which varies between the minimum and maximum thrust forces exertable by the injection piston 20 itself, at a frequency of between 5 and 10 Hz and for a period of between 1 and 2 seconds. This operation is repeated for a predetermined number of times during the injection of the thixotropic aluminium. At the end of the injection step, a pulsed thrust force is applied to the closure piston 9 of the movable die 8 by means of the control unit 22 of a magnitude which varies between the minimum and maximum thrust forces exertable by the closure piston 9 of the press 4 and at a frequency of between 10 and 20 Hz for a period of between 1 and 5 seconds. The manufactured articles obtained from the thixotropic aluminium by means of the process just described have mechanical and physical characteristics which are equal to or better than those of articles made by conventional injection methods. The control units 22 and 24 may, for example, be realised by means of microprocessors which, through fluid pumps, control valve assemblies connected to actuators connected to the injection piston 20 or to the closure piston 9, in the manner explained above. Microprocessors as indicated above are fully described in EP 0 613 740 A in the name of the present applicant. Reference may be made to this application for fuller structural details which are, however, outside the scope of the present invention.

Claims: A method for the pressure die-casting of articles from thixotropic aluminium with the use of a pressure die (4) including a fixed die (6) and a movable die (8) driven by a closure piston (9), an injection duct (18) for supplying the thixotropic aluminium to the pressure die (4) by means of an injection piston (20), including the steps of: closing the pressure die( 4) by means of the closure piston (9), and injecting the molten thixotropic aluminium into the pressure die (4) characterised in that it includes the further steps of: interrupting the travel of the injection piston (20) at predetermined instants during the injection of the thixotropic aluminium, applying pulsed thrust force injection piston (20) while it is stationary at the said predetermined instants, the thrust force varying between the minimum and maximum thrust forces that can be exerted by the injection piston (20), at a frequency of between 5 and 10 Hz and for a duration of between 1 and 2 seconds. A method according to Claim 1, characterised in that, at the end of the step of injecting the thixotropic aluminium into the pressure die (4), it includes a further step of applying to the closure piston (9) of the movable die (8) a thrust force which is variable between the maximum and minimum thrust forces exertable by the closure piston (9) on the pressure die (4) at a frequency of between 10 and 20 Hz for a duration of between 1 and 5 seconds. A pressure die-casting press for carrying out the method of Claims 1 or 2, comprising a pressure die (4) constituted by a fixed die (6) and a movable die (8), the movable die (8) being closable on the fixed die (6) by means of a closure piston (9), the pressure die (4) having an injection duct (18) for the supply of thixotropic aluminium by an injection piston (20) when the die cavity is closed, characterised in that the fixed die (6) and the movable die (8) of the pressure die (4) are heated by means of electrical resistances and in that the injection piston (20) is controlled by a first thrust control unit (24) capable of applying a pulsed thrust force to the injection piston. A pressure die-casting press according to Claim 3, characterised in that the closure piston (9) is controlled by a second thrust control unit (22), capable of applying a pulsed thrust force to the closure piston.

Inventors: DA RE MARIO, DA RE', MARIO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1996
Main classification: B22D17
IPC codes: B22D17