BACKGROUND ART The injection moulding of large plastics articles such as large tanks, portable buildings, boat hulls, etc., poses problems in the handling of the dies and also in the removal of product from the injection moulding machine.

Conventional injection moulding machines commonly comprise two fixed platens connected by tie bars with a third intermediate movable platen located between the fixed platens and moved by a hydraulic ram operable between one fixed platen and the movable platen. In this manner the tie bars are tensioned to clamp together two die members located on the movable platen and the fixed platen (remote from the hydraulic ram) respectively. The tie bars however cause considerable difficulty in changing large dies in a machine of this type as it can be very difficult, or impossible with some proposed dies, to manoeuvre the dies between the tie bars when changing dies. It will be appreciated that dies of this nature are extremely heavy and normally need to be supported by cranes or other heavy duty handling apparatus. It can also be a problem when moulding large plastics objects to remove the object from the dies after injection, and after the dies have been moved apart for product removal. Particularly in deep draw dies, the platens must be moved apart a significant distance in order to enable the product to be stripped from the dies and this can cause problems with the necessity for lengthy tie bars which may be difficult to support, and which are expensive to manufacture and provide in a large injection moulding machine. Furthermore it can be difficult to remove the injected product between the tie bars which extend between the platens in the machine.

DISCLOSURE OF INVENTION The present invention therefore provides a die press for injection moulding comprising a first fixed platen adapted to mount a first die member, a second platen adapted to mount a second die member, the second platen being movable toward and away from the first platen bringing the second die member into and out of engagement with the first die member, coupling means comprising a plurality of first coupling members mounted on and extending from the first platen toward the second platen, and corresponding second coupling members mounted on and extending from the second platen toward the first platen in register with the first coupling members, arranged such that the second coupling members are engageable with the first coupling members as the second platen approaches the first platen at the commencement of each injection cycle and disengageable as the second platen moves away from the first platen after injection leaving an open area surrounding the two die members for product removal or die changes, and clamping means comprising hydraulic piston and cylinder assemblies connected one to each of either the first or second coupling members and arranged to tension the coupling means after engagement of the first and second coupling members, clamping the first and second die members together for injection.

Preferably the clamping means are connected to the first coupling members on the fixed platen.

Preferably the coupling members comprise male and female bayonet members engageable by inserting the male bayonet member into the female bayonet member and turning one member relative to the other.

Preferably the female bayonet member is incorporated in the first coupling member and is coupled to the piston in the clamping means. Preferably the female bayonet member has a stem extending through the centre of the hydraulic piston to rotation means arranged to rotate the female bayonet member

through a predetermined arc for engagement and disengagement with the male bayonet member.

BRIEF DESCRIPTION OF THE DRAWINGS Notwithstanding any other forms that may fall within its scope, one preferred form of the invention will now be described by way of example only with reference to the accompanying drawings, in which:-

Fig. 1 is a diagrammatic side view of an improved die press according to the invention showing the press in the open or demoulding position;

Fig. 2 is a similar view to Fig. 1 showing the press with the coupling means engaged ready for clamping;

Fig. 3 is a similar view to Figs. 1 and 2 showing the press in the clamped position ready for injection; Fig. 4 is a view on the arrow IV of Fig. 3 showing the optional use of a central coupling member;

Fig. 5 is a partially cut-away elevation to an enlarged scale of the clamping means used in the improved die press according to the invention; and Fig. 6 is an end view of the apparatus shown in Fig. 4.

MODES FOR CARRYING OUT THE INVENTION In the preferred form of the invention a die press is constructed having a first fixed platen (1) and a second movable platen (2) which is conveniently mounted on slide ways or tracks (not shown) at its lower point (3) and is movable toward and away from the fixed platen (1) as shown by arrow (4) under the influence of transport means (5). In Figs. 1, 2 and 3 the transport means is shown as a long stroke hydraulic piston and cylinder assembly having a piston (6) connected to a piston rod (7) and operable in an hydraulic cylinder (8). It will be appreciated however that alternative means of moving the platen (2) may be utilised, such as a rack and pinion drive mechanism.

In the fully opened position, as shown in Fig. 1, there is an open space (9) between the platens and the first and second die members (10 and 11 respectively) allowing easy changing of die members even when the die members are very

- A - large in size and difficult to manoeuvre and requiring the use of overhead cranes or other materials handling equipment.

The first platen is provided with a plurality of first coupling members (12), and typically four such coupling members arranged adjacent each corner of the platen. The second platen (2) is provided with corresponding second coupling members (13) arranged in register with the first coupling members so that as the second platen (2) is moved toward the first platen as shown in Fig. 2, the second coupling members (13) align with, and are engaged with the first coupling members (12) .

The coupling members may be provided in any appropriate form but typically comprise bayonet couplings wherein a male bayonet member (14) is provided on each of the second coupling members (13), and a female bayonet member (15) on each of the first bayonet members (12). The construction and operation of the bayonet members will be described later with particular reference to Figs. 4 and 5.

The die press is further provided with clamping means in the form of hydraulic piston and cylinder assemblies (16) connected to each of the first coupling members (12) . The hydraulic piston and cylinder assemblies are operable to pull the first coupling members (12) away from the second platen (2) after engagement of the bayonet members (14) and (15) in order to securely clamp the two die members (10) and (11) together for the injection process.

The cycle of operation of the die press according to the invention can clearly be seen in the sequence of Figs. 1 to 3 wherein Fig. 1 shows the machine in the fully open position at the commencement of the cycle. The transport ram (5) is then actuated moving the movable platen (2) toward the fixed platen (1) until the coupling means (12) and (13) are engaged as shown in Fig. 2 with the die members located a short distance apart. The bayonet members (14) and (15) are then engaged and hydraulic pressure applied to the piston and cylinder assemblies (16) causing the pistons (17) therein to move from right to left in the sense shown

in the accompanying drawings moving the two die members (10) and (11) into contact and applying a clamping pressure to the die members for the injection process.

After the injection process has been completed, reverse pressure is applied to the cylinders (16), moving the pistons (17) from left to right in the sense shown in the accompanying drawings until the dies are in the position shown in Fig. 2. In this manner considerable hydraulic pressure can be applied in order to break the suction or seal caused between the dies during the injection process. The bayonet fittings (14) and (15) are then disengaged, and the transport ram (5) actuated to return the movable platen to the position shown in Fig. 1 for product removal and repetition of the injection cycle. Because there are no conventional tie bars permanently extending between the first and second platens, it is extremely easy to remove large product from the dies (10) or (11) when the press is in the position shown in Fig. 1. Due to the large open area surrounding the dies, and the absence of tie bars, it is also extremely easy to change the die members (10) and (11) on the platens (1) and (2) when it is desired to change to a different product run.

Because it is possible to use extremely large dies with the die press according to the invention, the clamping loads are extremely high and the distances between the coupling means can be quite large. This combination leads to very high forces on the platens (1) and (2) tending to bend the platens and requiring massive platen reinforcement and/or extremely heavy dies (10) and (11) to overcome this problem. To ameliorate this effect it is possible to provide the die press with a central set of coupling members in the position shown at (13A) in Fig. 4. (This coupling is omitted from Figs. 1 to 3 for clarity). The coupling (13A) is of exactly the same configuration as the couplings (12, 13) previously described but is located in the centre of the platens (1) and (2) and therefore in the centre of the dies (10) and (11). Many products have a natural aperture or

opening at or about the central region permitting the location of such a coupling and in other products, design variations may be made to allow the central coupling of the die platens to be incorporated. Although this necessitates forming a hole or aperture through the middle of the product, such aperture can later be filled in by a cover plate in the actual finished product, where desired. It is only by the use of the detachable couplings as described for this invention, that a central coupling member of this type can be utilised as the disconnection of the coupling in the open or demoulding position as shown in Fig. 1 allows the product to be removed from the dies. Large die presses using conventional tie bars, cannot of course incorporate such a central coupling as the moulded product could never be removed from the dies at the end of the moulding cycle. The use of the central coupling (13A) resists the bowing forces in the platens (1) and (2) during the injection process and therefore allows either lighter and less expensive platens to be used in the machine, or enables even larger product to be manufactured. Although only a single central coupling (13A) has been shown and described, it will be appreciated that numbers of such couplings could be utilised in very large dies distributed over the area of the die support platen. The construction and operation of the hydraulic pistons and cylinders (16), (17) and the coupling means (12) and (13) will now be described with reference to Figs. 5 and 6.

In Fig. 5, which is cut-away in section in the upper part of the figure, the hydraulic cylinder (16) is mounted on the platen (1) and contains the piston (17). The piston is located in a chamber (18) to which hydraulic pressure can be applied on either side of the piston (17) by hydraulic pumping apparatus and/or accumulators (not shown) . The piston (17) may be prevented from rotating in the cylinder (16) by way of a dowel pin (19).

The piston (17) has a hollow centre in which is located a tension shaft (20) having a flange (21) at one end. The

piston (17) is arranged to tension the shaft (20) by applying force from a shoulder (22) via a bearing surface (23) to the flange (21). In this manner the piston (17) may apply considerable force and tension to the shaft (20) while allowing the shaft (20) to rotate relative to the piston (17).

At the opposite end of the tension shaft (20) from the flange (21) the female bayonet member (15) is provided in the form of a substantially cylindrical collar (16) having inwardly flanged ends (24). Each inwardly flanged end is provided with castellations, and the end of the shaft (20) is provided with a corresponding male bayonet fitting (25) which is inserted into the collar (16) through the castellations at the left hand end (as shown in Fig. 5) and then rotated to firmly lock the male bayonet member (25) in place in the collar. This configuration is for ease of assembly only, allowing the male bayonet member (25) to be inserted through the hollow end in the piston (17) and then locked in place in the collar (16) by set screws (26). In this manner the female bayonet fitting (15) is formed having castellated openings (27) leading to an interior chamber (28) behind which a male bayonet fitting may be locked and engaged against shoulders (29) for the application of tensioning force to the coupling means. Turning now to Fig. 6, it will be seen that the piston assembly is mounted on the fixed platen (1) by way of a mounting plate (30) to which is attached a radial arm (31) . The outer end of the radial arm is provided with a mounting bracket (32) forming a support mounting for a hydraulic piston and cylinder assembly (33) used to rotate the shaft

(20) within the piston (17) . To this end the flange (21) on the shaft (20) is coupled to an eccentric arm (34) which in turn is engaged with a clevis (35) located at the end of the piston rod (36) . In this manner operation of the hydraulic piston and cylinder assembly (33) causes the eccentric arm

(34) to rotate about the centre (37) of the shaft and flange (20, 21) so causing the shaft to rotate within the piston

(17) . The location of the shaft within the piston can be set by proximity switches (38) which are in turn used to control the feed of hydraulic fluid to the piston and cylinder assembly (33). In this manner the shaft (20) and hence the female bayonet member (15) may be rotated through a predetermined arc for engagement of the female bayonet member with the male bayonet member (14) located on the second coupling members (13).

The male bayonet member (14) is simlar in configuration to the bayonet member (25) and is arranged to be axially movable through the castellations (27) when the female bayonet member is rotated to the unlocked position, and to be held behind the castellations (24) in the chamber (28), abutting the shoulders (29), when the female bayonet member is rotated to the locked position.

Each male bayonet member (14) is mounted on the corresponding coupling member (13) which is in turn secured to the movable platen (2) . This mounting is adjustable for length to enable dies of different thicknesses to be mounted between the platens (1) and (2).

Operation of the transport ram (5), the rotation of the female bayonet members (15), and operation of the clamping cylinders (16) are all controlled by logic circuitry under the influence of suitably placed proximity switches in order to safely achieve the operating sequence previously described.