Abstract:
A casting is made from a low melting point alloy having a solidus temperature in the range of 35°-300° C. by subjecting a charge of molten alloy to pre-pressurization then delivering the charge at a flow rate of 0.1 to 1 kg/sec into a die and maintaining the metal in the die under pressure for a time in excess of that required to fill the die.

Description:
BACKGROUND OF THE INVENTION 
     The use of a melt-out metal core of complex shape to provide a detailed internal configuration to a subsequently moulded part of plastic material is an area of developing technology, especially in the automobile industry. Such cores are made of a low melting point alloy and are removed from the moulded component by melting. 
     The prime requirement for these metallic cores is that they should provide accurate dimensional forms, as well as predetermined surface finishes. The metals from which such cores can be made have solidus temperatures in the range of 35° to 300° C. 
     There are a number of established methods of casting such alloys, ranging from simply pouring the liquid metal into a suitable metallic or non-metallic mould, either by hand or mechanically, through a range of various pressure devices to introduce the metal into the mould cavity, examples of which are centrifugal rubber mould casting, low pressure gravity diecasting, high pressure diecasting, and the Durville casting method. 
     For the present application it has been found by experience that none of the available techniques provides castings with specific required characteristics of dimensional tolerance, surface finish and lack of internal porosity or cavitation. While diecasting as normally practised produces a good surface finish, there is a tendency to provide porosity in the castings which is unacceptable in the above-mentioned cores. 
     The object of the invention is to enable low melting point alloys with solidus temperatures in the range 35°-300° C. to be accurately and reproduceably cast. 
     SUMMARY OF THE INVENTION 
     The invention accordingly provides a method of producing a casting from a low melting point alloy having a solidus temperature in the range of 35° to 300° C., which comprises subjecting a charge of the molten alloy to pre-pressurisation, then delivering the charge at a flow rate of 0.1 to 1 kg/sec. into a die, and maintaining the metal in the die under pressure for a period longer than that required to fill the die. 
     The invention also relates to an apparatus for carrying out this method comprising a die, a tank to contain the molten alloy to be cast, a cylinder immersed in the metal in the tank and having at one end an inlet to enable it to fill with the metal, a piston in the cylinder, an outlet valve leading to the die and communicating with an outlet at the other end of the cylinder, and a control system operable to impart, in successive operating cycles, a preliminary stroke to the piston sufficient for it to close the inlet while the outlet valve is closed and then to open the outlet valve and thereafter to impart a further stroke to the piston to deliver molten metal at a flow rate of 0.1 to 1 kg/sec from the cylinder and through the outlet valve into the die, the outlet valve remaining open to maintain the metal within the die under pressure for a period longer than that required to fill the die and then closing to allow the piston to be returned to its initial position in readiness for a further cycle of operations. 
    
    
     DESCRIPTION OF THE FIGURE 
     The accompanying FIGURE diagrammatically depicts a preferred embodiment of apparatus for producing a metal alloy casting according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The apparatus depicted in the FIGURE includes a tank 8 containing liquid metal and a dispensing cylinder 11, having an inlet 10 for liquid at its upper end and an outlet 16 at its lower end which is connected to a lock-off valve 12. The upper end of the cylinder 11 is disposed below the level 8A of liquid in the tank. Operating within the dispensing cylinder 11 is a piston 9 connected to a piston rod 4, carrying a stop bracket 6, which rod is actuated by a pneumatic or hydraulic cylinder 3. 
     The cylinder 3 is movable by means of a toggle system 2 actuated by a pneumatic or hydraulic cylinder 1 and constrained to move vertically by a guide 5. 
     The outlet 16 controlled by the valve is connected to a nozzle 13, to the outlet 13A of which, before commencement of a casting cycle, a die 15 is brought into sealing engagement. When the die 15 is to be filled from the side or from below, it is fitted with a valve 14 for retaining liquid metal within it. When provided the valve 14 is opened and closed simultaneously with the valve 12. A microprocessor 17 is provided for effecting sequential operation of the cylinders 1,3 and the valves 12 and 14. Alternatively the cylinders 1, 3 and the valves 12,14 may be actuated by a pneumatic control system including solenoids. 
     At the start of the casting cycle the valve 12 is closed. The microprocessor 17 first causes the cylinder 1 to close the toggle system 2 and move the piston 9 downwardly to an extent sufficient to cover the liquid metal inlet 10 of the cylinder 11. This serves to effect pre-pressurisation of liquid metal in the cylinder and thus avoids any gravitational surge of metal into the die 15 at a later stage. The valve 12 is then opened and the cylinder 3 is actuated to cause metal to be dispensed into the die 15 by means of the piston 9 until the stop bracket 6 contacts a fixed stop bracket 7. 
     As the flow rate of metal is critical, it is important that the valve 12 should not open until the piston 9 has closed the inlet 10. This prevents any free fall of metal once the valve 12 is opened. Typical pressures exerted on the column of metal to be delivered are 0.25-3.0 bar. 
     The volume of liquid metal delivered to the die depends on the position of adjustment of the stop 6 on the rod 4. 
     After the piston 9 has completed its downward stroke, the valve 12 is held open for a dwell time exceeding the time required for the delivery stroke of the piston 9, so maintaining the metal in the die 15 under pressure until solidification. 
     The valve 12 then closes and the piston 9 is returned to its initial position in preparation for the next casting cycle. 
     The die 15 is normally maintained in sealing engagement with the injection mechanism for a time after the valve 12 has closed, to ensure that the still molten inner portion of the casting does not melt its way out. However, it may be required in some cases to cast a hollow core for special conditions of the subsequent plastic moulding. In this case, the seal may immediately be broken to allow part of the molten metal to drain out of the casting. 
     The stops 6 and 7 need not necessarily be a single mechanical device but may include a proximity switch and/or electro optical technique. 
     A &#34;swan-neck&#34; 13B in the through passageway of the nozzle 13 ensures that at the end of the stroke of piston 9 and the closing of the valve 12, the liquid metal runs out until the &#34;knife edge&#34; of the &#34;swan-neck&#34; is reached at which point no more metal is released and there is a positive cut off with no dripping. 
     The apparatus described may constitute an adjunct to a plastic moulding machine, the core metal melted out after the plastic moulding operation being returned to the tank 8, the level in which is maintained high enough to cover the inlet 10. 
     In one example of use of the apparatus for casting a core of a plastic automobile pump, the composition of the metal in the tank 8 was 56% tin, 3% antimony, the balance lead, the tank was maintained at a temperature of 200°-230° C. and the die 15 at a temperature of 50°-70° C. The weight of each cast core was 0.6 kg. The duration of the second and delivery stroke of the piston 9 was 3 seconds and the dwell time after delivery and before closing of the valve 12 was 7-12 seconds. 
     In another example of use of the apparatus for casting a core of an automobile injection manifold the metal in the tank 8 was a eutectic alloy of bismuth and tin, the tank was maintained at a temperature of 180° C. and the die at a temperature of 35° C., the weight of each cast core was 20 kg, the duration of the delivery stroke of the piston 9 was 35 seconds and the dwell time was 8 seconds. 
     It is useful in some cases, e.g. the casting of a core for a plastic automobile intake manifold, to use in the tank 8 a number of injection cylinders 11 and pistons 9 operating as described above to deliver molten metal simultaneously, each to the inlet of a different die.