GB1600226A

GB1600226A - Device machine and method for use in hot chamber pressure die-casting

Info

Publication number: GB1600226A
Application number: GB19598/78A
Authority: GB
Original assignee: Buehler AG
Current assignee: Buehler AG
Priority date: 1977-05-28
Filing date: 1978-05-15
Publication date: 1981-10-14
Also published as: FR2391795A1; BE867472A; IT1095217B; ES469897A1; JPS53147624A; CH619874A5; IT7822633A0; DE2821989C2; DE2821989A1; ES479428A1

Abstract

In order reliably to prevent any unwanted movement of the plunger in the direction of injection after only part of the plunger return stroke has been travelled, the plunger is, after each completed casting operation, locked with the aid of a pressure in time with each step of the stepwise-actuated plunger return. For this purpose, a two-part locking element is provided, the parts (32, 33) of which are each provided with a recess (40, 41) for the plunger rod (24) of the plunger and which are arranged in series along the latter. The two parts (32, 33) can be displaced relative to one another and in a direction normal to the stroke direction of the plunger (23). Upon each step of the plunger return, one part (32, 33) is brought into engagement with the shoulder (241) of the plunger rod (24). <IMAGE>

Description

(54) DEVICE, MACHINE AND METHOD FOR USE IN HOT CHAMBER PRESSURE DIE-CASTING (71) We, GEBRUEDER BUEHLER AG, a Swiss Body Corporate, of CH 9240 Uzwil, Switzerland, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to a device, machine and method for use in hot chamber pressure die-casting.

It is known to provide safety devices to guard against accidental triggering of the casting operation by use of a locking arrangement which prevents the pressing-in travel of a casting piston until a mould closing pressure necessary for the casting operation has been applied to the mould.

According to one aspect the invention provides a hot chamber pressure die casting machine comprising a piston rod for producing forward movement of a casting piston in a casting direction and for retracting it in the opposite direction, and mechanical locking means arranged to block forward movement of the piston rod from at least a first retracted position and then from at least a second further retracted position and to disengage the piston rod for permitting forward movement.

A casting operation initiated in error or by reason of malfunctioning can hence be prevented by such safety device already during the further rearward movement of the casting piston, since the locking effect introduced at the first return travel step remains effective until the instant is reached at which the machine is ready again for the next casting operation.

Preferably the locking means is arranged to block forward movement of the piston rod from the first retracted position in response to a signal in synchronism with a first retraction movement of the piston rod. The locking means may be arranged to block forward movement of the piston rod from every position reached during retraction along at least part of the retraction. An in one preferred form the locking means is arranged to block forward movement of the piston rod from every position between the first retracted position and the second retracted position. In another preferred form the locking means is arranged to block forward movement of the piston rod from the second retracted position in response to a signal in synchronism with a second retraction movement of the piston rod. Desirably the locking means is arranged to disengage the piston rod for forward movement in response to predetermined mould closing pressure and any signals to cause or permit blocking of forward movement are derived from a control unit of the machine controlling in two steps the retraction movement.

The invention also provides a safety device for use in a hot chamber pressure die casting machine the piston rod having a shoulder, the safety device having a two-part locking element whose parts are each constructed with an aperture for the piston rod of the casting piston and are situated one after the other along the said rod, that at least one of the two parts is adapted to be displaced relatively to the other with at least a component of movement directed perpendicularly to the direction of travel of the casting piston, and is adapted to be brought into engagement with the shoulder of the piston rod at least in two locking positions defined at a predetermined spacing from one another in the direction of travel.

This makes it possible to block forward travel of the casting piston in the shot direction, during return movement of the piston, namely from an intermediate position during that return movement, which can take place in steps after a casting operation has been completed. With appropriate measures the blocking effect of the locking elements is not discontinued by untoward absence of pressure for the required relative movement of the parts of the said element: it would be released only by a control signal in conformity with the renewed closing of the casting mould by means of the closing pressure necessary for the casting operation.

In a preferred constructional form of safety device according to the invention the locking element can comprise two plates which can slide on one another and each have a keyhole-shaped hole through which the piston rod extends. The holes are conveniently constructed each with a relatively large circular part the diameter of which corresponds to the diameter of the piston rod, and an elongated restricted part the width of which is equal to the diameter of a reduced portion of the piston rod forming the shoulder on the piston rod. The elongated restrictions of the two holes extend in opposite directions to one another.

It is advisable for one plate to be connected securely to the cylinder and the other plate to the piston of a freely mobile pressure drive, so that by relative movement of piston and cylinder the keyhole holes can be displaced together with the plates over one another and adjusted in accordance with the pressure action of one of the cylinder pressure chambers into three predetermined relative positions with respect to one another. In a first relative position, namely the first locking position corresponding to the first step of the casting piston return travel, the shoulder of the piston rod can be held fast at the restriction of the keyhole-shaped hole in the plate near the casting piston.

In a second relative position, namely the second locking position corresponding to the full casting piston return travel, the same shoulder can be brought into engagement with the restriction of the keyhole-shaped hole in the plate remote from the casting piston.

Finally, in a third relative position, namely the casting position, the piston rod is allowed to travel through in the shot direction through the circular portions of the holes in alignment with one another. In this position it may be found advantageous to limit the movement of the two plates by an abutment in each case.

In another preferred constructional form of safety device according to the invention the locking element is formed of two plates which are wedge-shaped in longitudinal section and which in plan view each comprise a U-shaped recess for the piston rod. It is advisable in this case to adapt the recess of the fixed plate which is nearest the casting piston to be equal to the diameter of the full piston rod and to adapt the recess of the plate which is remote from the casting piston and is displaceable along the common inclined plane of the two plates to the diameter of the reduced portion of the piston rod.

The plate remote from the casting piston should conveniently be connected securely on the one hand to the piston of a pressure drive and on the other hand to a return spring so that it can be displaced by means of this spring along the common inclined plane whereby the locking element is continuously in engagement with the shoulder of the piston rod between two relative positions of the two plates, in accordance with the first step of the return travel of the casting piston and, respectively, the full return travel of the said casting piston.

According to another aspect the invention provides a method of preventing the casting unit of a hot chamber pressure die-casting machine from accidentally releasing the casting operation, the method being as set forth in claim 20.

Two embodiments of the invention will now be described by way of example with reference to the accompanying - drawings wherein: Fig. 1 shows a hot chamber pressure diecasting machine partly in section with a safety device; Fig. 2 is an elevation partly in section of one form of safety device and part of the casting unit of the die-casting machine, the piston rod of the casting piston being locked in its return end position; Fig. 3 is a plan view on to the plates of the locking element of Fig. 2 with the shot cylinder omitted the relative position of the keyhole-shaped holes of the plates corresponding to the locking position shown in Fig. 2; Fig. 4 is a plan view similar to Fig. 3 but wherein the relative position of the keyholeshaped holes corresponds to the locking position after a first step of the casting piston return travel; Fig. 5 is a plan view similar to Figs 3 and 4 in which the relative position of the keyholeshaped holes is shown which allows the casting piston rod to travel through; Fig. 6 shows a front view of a second form of safety device, partly in section, and part of the casting unit in the locking position corresponding to the first step of the casting piston return travel; and Fig. 7 is a section along the plane VII-jVII of Fig. 6.

Fig. 1 shows a hot chamber pressure diecasting machine basically of known construction comprising a hydraulic moulding press 1 and a casting unit 2 associated on a common frame 3.

The moulding press 1 has a two-part casting mould 11 and a mould closing cylinder 12. The mould half 111 of the casting mould 11 which is at the injection side is fast with the casting unit 2, whilst the mobile mould half 112 is mounted for displacement by the pressure piston 122 of the mould closing cylinder 12 by way of a piston rod 121.

The casting unit 2 comprises a melting crucible 21 and a casting sleeve 22 which is arranged therein below the metal bath level 10, with a vertical cylindrical bore 221, a gate passage 222 connecting the bore with the fixed mould half 111, and also a metal supply aperture 223 provided in the wall of the cylindrical bore 221.

A casting piston 23 is mounted for displacement in the cylindrical bore 221 of the casting sleeve 22. This piston is securely connected with a pressure piston, the socalled shot piston 26, by a two-part piston rod 24 and a coupling 25 connecting the parts thereof to one another, the shot piston 26 moving in the so-called shot cylinder 27.

A block drawn in broken lines surrounding the piston rod 24 indicates diagrammatically a safety device 30 acting on the said rod.

This device is operatively connected by way of a line 31 with the machine control unit 60, this being represented by a further block shown in broken lines. From the pressure chamber of the mould closing cylinder 12, which chamber is associated with the full pressure surface of the pressure piston 122, a pressure measuring line 61 leads into the machine control unit 60.

In the position of the casting piston 23 shown in Fig. 1 the molten metal can move from the interior of the melting crucible 21 by way of the metal supply aperture 223 into the casting sleeve 22 so that the casting unit 2 is ready for a casting operation.

The safety device 30 meanwhile holds the casting piston 23 fast by means of the piston rod 24, so that it is not possible to trigger the casting operation by the action of pressure on the shot piston 26 brought about by mistake or because of some error in the hydraulic control system, before the closing of the casting mould 11 with the technically requisite mould closing pressure.

If, after mould closing has been completed, the necessary pressure value is signalled by way of the measuring line 61 to the machine control unit 60, the said unit by way of the line 31 brings about the release of the safety device 30 and then the forward movement of the casting piston and the carrying out of the casting operation.

Each time the casting mould 11 is opened, to remove from the mould a solidified casting, a two-stage casting piston return travel or retraction is carried out. First the casting piston 23 is moved into a first return or retracted position in which the metal supply aperture 223 remains closed, and then the mould 11 is slightly opened.

This procedure is intended to prevent the liquid metal from being drawn by the vacuum which occurs when the mould is opened, out of the crucible 21 by way of the metal supply aperture 223 and the casting sleeve 22, and being sprayed out through the gate aperture of the stationary half 111 of the casting mould 11, which would lead to interruption in working in addition to endangering the operative.

After pressure equilibrium has been brought about in the intermediate position of the casting piston 23, it is returned into the end position shown in Fig. 1, the filling of the casting sleeve 22 with the molten material taking place without the said material issuing from the gate passage 222 so that the machine can be prepared for the next casting operation.

The safety device 30 at the instigation of the machine control unit 60 locks the casting piston 23 or its piston rod 24 against any unintentional movement into the casting sleeve 22 from the attainment of the first return travel position of the casting piston 23 onwards to the release by the machine control unit 60 when the pressure die-casting machine is again ready for carrying out a shot.

In the constructional forms of the safety device 30 which will be described below there is a locking element, in one case a twopart locking element the parts of which are arranged in succession to one another along the rod 24 of the casting piston 23. During the course of their relative movement, carried out at least with a component directed at right angles to the direction of travel of the casting piston 23, they co-operate with a shoulder 241 constructed on the piston rod 24 (compare Figs 2 and 6).

A first constructional form of the safety device 30 is shown in Figs. 2 to 5.

According to Fig. 2 the locking element comprises two plates 32 and 33 which can slide on one another and which are guided in a housing 34 which is connected securely by means of screw bolts 35 by way of a yoke 36 with the shot cylinder 27.

The lower plate 32 is connected securely to a freely mobile drive cylinder 37 and the upper plate 33 by means of a piston rod 38 with the associated drive piston 39.

The plates 32 and 33 each have a keyholeshaped hole 40 and 41 respectively, the rod 24 of the casting piston 23 extending through these holes (see also Figs. 3 to 5). The diameter of the larger circular hole portions 401, 411 corresponds to that of the rod 24 of the casting piston 23 and the width of the elongated restricted hole portions 402, 412 corresponds to the diameter of a reduced portion 242 which forms the shoulder 241 on this piston rod 24. As Figs. 3 to 5 show, the restricted portions 402, 412 extend in opposite directions.

The plates 32 and 33 in Figs. 2 and 3 are situated in a relative position as regards their keyhole-shaped holes 40, 41 which corresponds to the so-called second locking position VS II after the full casting piston return travel. The restrictions 402, 412 of the keyhole-shaped hole 40 and 41 engage about the reduced portion 242 of the casting piston rod 24 and the shoulder 241 is in engagement with the upper plate 33. Movement of the casting piston 23 in the direction of the casting sleeve is not possible.

In Fig. 4 the plates 32 and 33 are in a further relative position as regards their keyhole-shaped holes 40, 41 corresponding to the so-called first locking position VS I after a first step of the return travel of the casting piston has been carried out. In this position, the lower plate 32 is in the same position with its keyhole-shaped hole 40 relatively to the axis designated as O of the casting piston rod 24 as in Fig. 3. On the other hand the upper plate 33 with its keyhole-shaped hole 41 is shown displaced towards the right relatively to the aforesaid axis 0, so that the shoulder 241 of the casting piston rod 24 bears only against the plate 32 and is held back at its restriction. Thus the locking of the casting piston 23 takes place in a position which is spaced from the previously described second locking position VS II at a spacing corresponding to the thickness of the upper plate 33.

Fig. 5 shows the two plates 32, 33 in a third relative position of their keyhole-shaped holes 40, 41 corresponding to the so-called casting position GS.

After a displacement of the lower plate 32 towards the left relatively to the axis 0 of the casting piston rod 24 the circular portions 401, 411 of the two keyhole-shaped holes 40, 31 are situated concentrically with the piston rod 24 and allow it to travel through in the direction of the casting sleeve 22.

In the casting position GS, a projection 331 on the upper plate 33 abuts against an abutment surface 341 formed on the upper portion of the housing 34, and an abutment pin 342 arranged in the lower portion of the housing 34 serves as a fixed abutment for an abutment surface 322 provided in the lower plate 32 at the end of a groove 321 extending in the direction of movement of the said plate. As a result the movement of the upper plate 33 towards the right and the movement of the lower plate 32 towards the left is limited (see Fig. 2), to ensure precise centring of the circular portions 401, 411 of the keyhole-shaped holes 40, 41 relatively to the axis 0 of the casting piston rod 24, so that the piston rod can travel through unhindered to carry out the casting operation.

The pressure operation of the drive cylinder 37 is controlled by means of a magnetically operated valve 42 in accordance with a preset time programme, followed by the machine control unit 60, for the locking which is to be carried out simultaneously with the stepwise casting piston return travel, this programme also including the mould closing pressure necessary for releasing the safety device, i.e. to cause the locking elements to disengage the piston rod for forward movement.

Thus the requirements which are expected of the safety device are met, namely that the casting piston should be released for the pressing-in travel only when a mould closing pressure required for the casting operation becomes effective, and that the locking should be effected in accordance with the piston return travel in two stages, in accordance with a time-distance pattern which is determined in accordance with requirements.

During the return travel, the casting piston rod 24 together with the casting piston 23 is first held fast in the first locking position VS I, in which the metal supply aperture 223 shown in Fig. 1 remains covered, and then in the second locking position VSII which frees the metal supply aperture 223. They then remain in this second position until the plates 32 and 33 take up their casting position GS.

Naturally it is also possible to drive the plates by two individually controlled drive devices independently of one another.

In a second constructional form of the safety device 30 shown in Figs. 6 and 7 the locking element comprises a plate 71 cooperating with a plate 70 both of which are wedge-shaped in longitudinal section, the upper plate 71 being capable of sliding along the common inclined plane 72 of the two plates 70, 71.

The lower plate 70 is connected securely to the yoke 36 and has a U-shaped recess 701, as seen in plan view, the width of which corresponds to the diameter of the full casting piston rod 24. However, the recess 701 could also be a cylindrical hole.

In the upper plate 71 there is also provided a U-shaped aperture 711 with a width adapted to the diameter of the reduced portion 242 of the casting piston rod 24.

The upper plate 72 is connected securely with a return spring 73 and by means of a piston rod 74 with the drive piston 75 of a fixed drive cylinder 76, the direction of the travel of the drive piston 75 being arranged parallel to the common inclined plane 72 of the two plates 70, 71.

The two wedge-shaped plates 70, 71 are so dimensioned that the upper plate 71 with the two prongs of its horizontally arranged Ushaped surface 712 is pushed by the return spring 73 under the shoulder 241 of the piston rod 24 of the casting piston 23 at the instant when the casting piston in its return travel has just reached the first locking position VS I.

Fig. 6 shows this position.

Fig. 7 shows the same position of the device in sectional plan view, the parts thereof above the section plane VIl-VIl indicated in Fig. 6 being omitted.

Under the action of the return spring 73, the upper plate 71 during the course of the subsequent further return travel of the casting piston 23 remains in engagement with the shoulder 241 of its piston rod 24 uninterruptedly until the second locking position VS II is reached.

The spacing of the two locking positions VS I, VS II in this case is defined by the inclination of the common inclined plane 72 of the two plates 70, 71 and the depth of the U-shaped recess 711 of the upper plate 71.

A time programme for the stepwise locking system is not necessary with the arrangement. Furthermore locking takes place even in the event of failure of the control pressure for the drive cylinder 76.

The latter is in this case intended simply to return the upper plate 71 from the region of movement of the casting piston rod 24 when the necessary mould closing pressure is present, and thus to allow the casting operation.

WHAT WE CLAIM IS: 1. A hot chamber pressure die casting machine comprising a piston rod for producing forward movement of a casting piston in a casting direction and for retracting it in the opposite direction, and mechanical locking means arranged to block forward movement of the piston rod from at least a first retracted position and then from at least a second further retracted position and to disengage the piston rod for permitting forward movement.

2. Machine according to claim 1 wherein the locking means is arranged to block forward movement of the piston rod from the first retracted position in response to a signal in synchronism with a first retraction movement of the piston rod.

3. Machine according to claim 1 or claim 2 wherein the locking means is arranged to block forward movement of the piston rod during at least part of the retraction from every position reached during that part of the retraction.

4. Machine according to claim 2 in combination with claim 3 wherein the locking means is arranged to block forward movement of the piston rod from every position between the first retracted position and the second retracted position.

5. Machine according to claim 2 wherein the locking means is arranged to block forward movement of the piston rod from the second retracted position in response to a signal in synchronism with a second retraction movement of the piston rod.

6. Machine according to any preceding claim wherein the locking means is arranged to disengage the piston rod for permitting forward movement in response to predetermined mould closing pressure and any signals to cause or permit blocking of forward movement are derived from a control unit of the machine controlling in two steps the retraction movement.

7. A hot chamber pressure die casting machine comprising a piston rod for producing forward movement of a casting piston, a locking element to engage a shoulder on the piston rod and drive means for displacing the locking element in a direction having at least a component of movement normal to the piston rod axis, the arrangement being such that the locking element when in engagement with the piston rod shoulder blocks the rod against forward movement from each of two locking positions a predetermined distance apart in the direction of travel of the piston.

8. Machine according to claim 7 wherein the locking element is in two parts each having an opening for the piston rod and arranged one after the other along the piston rod and each being adapted to engage a shoulder of the piston rod in the respective locking positions.

9. Machine according to claim 8, wherein the locking element comprises two plates slidable on one another and each having a keyhole-shaped hole through which the piston rod extends, each keyhole-shaped hole having a larger hole portion and a restricted hole portion, the larger hole portion of each plate being of a width corresponding to the diameter of the piston rod and the restricted hole portions of the respective plate extending in opposite directions and corresponding to a reduced piston rod portion which at its junction with the full diameter rod portion forms the shoulder on the piston rod.

10. Machine according to claim 9, wherein one plate is fast with a cylinder of the drive means and the other plate is fast with a piston of the drive means arranged to be freely mobile, and depending on the pressure action of the pressure chambers of the cylinderathe plates assume one of three relative positions of their respective keyholeshaped holes, namely: a first relative position being the first locking position corresponding to the first step of the retraction of the casting piston in which the shoulder of the piston rod is held fast on the restriction of the keyholeshaped hole of the plate nearest the casting piston; a second relative position being the second locking position corresponding to full retraction of the casting piston in which the same shoulder is in engagement with the restriction of the keyhole-shaped hole of the plate remote from the casting piston and a third relative position being the casting position in which the larger hole portions of both keyhole-shaped holes are in alignment

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. device in sectional plan view, the parts thereof above the section plane VIl-VIl indicated in Fig. 6 being omitted. Under the action of the return spring 73, the upper plate 71 during the course of the subsequent further return travel of the casting piston 23 remains in engagement with the shoulder 241 of its piston rod 24 uninterruptedly until the second locking position VS II is reached. The spacing of the two locking positions VS I, VS II in this case is defined by the inclination of the common inclined plane 72 of the two plates 70, 71 and the depth of the U-shaped recess 711 of the upper plate 71. A time programme for the stepwise locking system is not necessary with the arrangement. Furthermore locking takes place even in the event of failure of the control pressure for the drive cylinder 76. The latter is in this case intended simply to return the upper plate 71 from the region of movement of the casting piston rod 24 when the necessary mould closing pressure is present, and thus to allow the casting operation. WHAT WE CLAIM IS:

1. A hot chamber pressure die casting machine comprising a piston rod for producing forward movement of a casting piston in a casting direction and for retracting it in the opposite direction, and mechanical locking means arranged to block forward movement of the piston rod from at least a first retracted position and then from at least a second further retracted position and to disengage the piston rod for permitting forward movement.

and allow the piston rod to move forward to carry out a shot; the movement of the two plates being limited by respective abutments.

11. Machine according to claim 7 wherein the locking element is a plate wedgeshaped in longitudinal section and has a Ushaped recess for the piston rod of a width such that one side of the wedge engages a shoulder on the piston rod, and an abutment intermediate that shoulder and the casting piston for engagement with the opposite side of the wedge.

12. Machine according to claim 11 wherein the abutment is also wedge shaped in longitudinal section and also has a Ushaped recess for the piston rod, the recess in the abutment permitting free passage of the piston rod, the two wedges having a surface lying in a common plane inclined with respect to the casting piston axis and the locking element moving along that inclined plane for engagement with the shoulder of the piston rod.

13. Machine according to claim 12 wherein the locking element is fast with the piston of a drive and with a return spring so as to be displaceable by the said spring along the common inclined plane, whereby the locking element is in constant engagement with the shoulder of the piston rod between the two locking positions namely a first retracted position corresponding to a first stop of the retraction of the casting piston and a second retracted position corresponding to a second stop of the retraction of the casting piston.

14. A hot chamber pressure die casting machine substantially as shown in and hereinbefore described with reference to Figs 1 to 5 or Figs 1, 6 and 7 of the accompanying drawings.

15. Safety device for use in a die casting machine according to any of claims 1 to 6 the piston rod of the machine having a shoulder; the safety device comprising: a locking element to engage such shoulder and drive means for displacing the locking element from a retracted position into an engagement position in which it can engage such shoulder, the locking element being in two parts each having an opening for such piston rod and arranged one after the other, each when in its engagement position being adapted to engage such shoulder, whereby in use of the safety device in such machine engagement of the locking elements with such shoulder blocks such piston rod against forward movement from each of two locking positions a predetermined distance apart in the direction of travel of such piston.

16. Safety device according to claim 15 having the additional features added by claim 9 or claim 10.

17. Safety device for use in a die casting machine according to any of claims 1 to 6 the piston rod of the machine having a shoulder; the safety device comprising: a locking element to engage such shoulder and drive means for displacing the locking element from a retracted position into an engagement position in which it can engage such shoulder, the locking element being a plate wedge shaped in longitudinal section and having a U-shaped recess for such piston rod of such width that one side of the wedge engages such shoulder; and the safety device further comprising an abutment for engagement with the opposite side of the wedge; whereby in use of the safety device in such machine angegement of such wedge with such shoulder blocks such piston rod against forward movement from each of two locking positions a predetermined distance apart in the direction of travel of such piston.

18. Safety device according to claim 17 having the additional features added by claim 12 or claim 13.

19. A safety device for a hot chamber pressure die casting machine, the safety device being substantially as shown in and hereinbefore described with reference to Figs 2 to 5 or Figs 6 and 7 of the accompanying drawings.

20. A method of die casting using a hot chamber pressure die casting machine comprising a piston rod for moving a casting piston, the method including the steps of causing forward movement of the casting piston in a casting direction, retracting it in the opposite direction, and, to prevent accidental forward movement, further including the following steps, viz: when the piston rod has reached a first retracted position in its retraction movement blocking its forward movement from at least that position, and then when the piston rod has reached a second further retracted position blocking its forward movement from that second retracted position, and, when ready for the next casting cycle, disengaging the piston rod to permit forward movement of the piston rod.

21. A method of die casting using a hot chamber pressure die casting machine comprising a piston rod for moving a casting piston, the method including the steps of causing forward movement of the casting piston in a casting direction, retracting it in the opposite direction, and, to prevent accidental forward movement, further including the following steps, viz: when the piston rod has reached a first retracted position in its retraction movement engaging a shoulder on the piston rod with a locking element to block the piston rod against forward movement from each of two locking positions a predetermined distance apart in the direction of travel of the piston, and, when ready for the next casting cycle, disengaging the piston rod to permit forward movement of the piston rod.

22. A method according to claim 21 wherein the locking element is displaced into engagement with the shoulder by drive means causing displacement in a direction having at least a component of movement normal to the piston axis.

23. A method according to any of claims 20 to 22, including the further step of sensing the pressure of pressure fluid used for closing a mould of the machine, and when that pressure has reached a predetermined value disengaging the piston rod in response to a signal indicative that the predetermined value has been reached.

24. A method according to any of claims 20 to 23, wherein the engagement of the piston rod is effected under the control of a timing programme delivered by a control unit of the machine, that control unit causing retraction of the piston to be effected in two steps, viz a first retraction to the first retracted position, and then a second retraction to the second further retracted position.

25. A method of die casting using a hot chamber pressure die casting machine, the method being substantially as herein described with reference to the accompanying drawings.

26. Castings whenever made by a machine according to any of claims 1 to 14 or a method according to any of claims 20 to 25.