FIELD OF THE INVENTION The present invention relates to tie-bar less molding machines, such as injection presses for plastics and metal molten materials; more particularly concerns a method and a device for tightly closing a mold positively using the elastic features of the frame of the press in order to generate thrusting actions to keep close the mold members during injection of the molten material into a shaped cavity of a mold.

PRIOR ART Hydraulic, pneumatic, mechanical, electrical or different type of actuators are generally known, and used in several technical field in order to operate a movable member or to exert thrusts on tools or machine members, by mechanical connections.

In the case where forces with a substantially high value, of the order of a few tonnes, tens or hundreds of tonnes are required, the conventional actuators must be designed and dimensioned such as to be heavy and extremely strong to withstand the high forces generated by the reaction of the tool or the machine; such problems particularly occurs in the closure of a mold for plastics injection presses and die-casting machines.

All this results in the use of very complex and costly mechanical solutions which are subjected to several drawbacks, such as a high degree of frictional forces between the moving parts, which adversely affects the efficiency and the performance of the molding press or die-

casting machine.

The molding and die-casting machines can be traditionally classified in two main categories which comprise vertical- stroke presses and horizontal-stroke presses, respectively, of tie-rod or tie-rodless type; the present invention is particularly but not exclusively directed to improve the mold-closure in tie-rodless presses.

In the injection molding of molten plastic or metal material, a tie-rodless press typically comprises a base frame which extends in a longitudinal direction parallel to the axis of a mold, provided with a stationary platen rigidly connected to the base frame, for supporting a first member of the mold, and a movable platen for supporting a second mold member; the movable platen is suitably connected to a drive unit which can be actuated so as to firstly move and approach the movable mold member to the stationary mold member, and secondly to tightly close the mold by a thrust necessary for withstanding the high internal pressures exerted by the molten material during the injection.

In general, to prevent the two mold members from becoming misaligned during the injection of the molten material, causing a leakage at the high injection pressures, it is necessary to exert clamping down forces for sealingly closing the mold, which are very high i. e. of the order of several tens, hundreds or thousands of tonnes, the reaction of which must be fully withstood by the same closure device. Presses of this kind are shown, for example, in US 5,297,925, US 5,332,385, WO 98/02291, WO 97/21529, WO 96/05041 or in EP-A-0,311,133 and EP-A-0,554,068.

In injection-molding machines or presses of the type

referred to, both of the vertical and of the horizontal type, entirely conventional operating systems of hydraulic, mechanical or electrical type are therefore used, having several drawbacks in that they require closing actuators or toggle mechanisms which are extremely complex and bulky, in order to approach the mold members and at the same time to exert the high forces required for tightly clamping down the mold, as well as to withstand the high reaction stresses caused by the closure of the same mold.

Moreover, the conventional closing systems used hitherto, in particular of the toggle-joint type, use of a large amount of energy is required owing to the articulations or joints structure which must be particularly strong and complex in view of the high internal frictional forces to be overcome and the high thrusts to be exerted on the mold.

There exists, moreover, an increasingly greater need to facilitate the access to the working zone of a press for an operator, both for maintenance operations and/or replacement of the molds, as well as for automation purposes of the working cycle, making the same press more accessible for demolding.

In this connection, by way of an alternative to the conventional presses described for example in US 4,545,757 or US 4,276,013 and US 4,773,845, tie-rodless presses, or so-called C-shaped or open frame presses have been varyingly proposed, said presses having to be designed with a particularly strong frame to withstand the high forces and reactions generated upon closing of the mold and during the molding process.

Although this type of press allows for good accessibility to the molding zone and its use is becoming increasingly

widespread, it nevertheless has certain limitations and drawbacks mainly due to the intrinsic structure of the frame and to the closing procedure of the mold.

In particular, with the open-frame presses of the conventional type, it is not possible to exert high closing forces while maintaining a simple and easily accessible configuration of the press, since forces arising at the closure tend to cause bending and distortion of the press frame, causing in turn a misalignment between the two mold support platens; in this type of press it is therefore necessary to prevent or compensate for the deformations of the frame.

In EP-A-0,726,131 therefore has been proposed to react or compensate for the deformation of the press frame, and misalignment of the mold members during the clamping down of the mold, by means of a compression device comprising a cylinder acting at the same time and in opposition to the cylinder of the mold closing device, so as to restore the parallelism condition between the movable platen and the stationary platen, namely to correct or avoid misalignment of the mold. As stated at column 5 line 37 onwards of EP 0.726.131, the compression system is merely used and suggested, conjointly to the mold clamping down cylinder, to compensate the deformation of the frame and maintain a parallelism between the stationary and movable platens of the press; therefore in EP 0.726.131 the compression system is not suggested or used to positively cause the clamping down of the mold. All this involves considerable constructional and functional complications with consequent increased costs, without however entirely overcoming the drawbacks of said conventional press.

OBJECTS OF THE INVENTION There exists, therefore, the need to find new solutions which allow for presses of simple design by the use of hydraulic or different type of actuators to provide high forces using an extremely small amount of energy.

Therefore the general object of the present invention is to provide a method and a mold-closing device which are suitable for solving the technical problems mentioned above.

A further object of the invention is to provide a mold closing device of the type mentioned above, which is structurally simple and able to exert high clamping forces for the mold, by using in a unique manner the same structure of a press.

Another object of the present invention is to provide a tie-rodless press both for plastic and for molten metal, which makes use of a new closure for the mold, by means of which the disadvantages previously referred to are significantly reduced.

A further object of the present invention is to provide a molding press or apparatus as defined above, provided with a closing system for the mold, by means of which it is possible retain an extremely simplified structure of the same press, devoid of counteracting elements, while keeping the mold members in an aligned and parallel condition.

Yet another object of the invention is to provide a molding apparatus of the kind mentioned above comprising a mold closing system of simplified constructional features, and of comparatively low cost.

A further object of the invention is to provide an injection molding apparatus both for plastics and metal, of the tie-rodless type which, in addition to offering greater constructional simplicity and ease of access to the working zone, also allows a substantial reduction in energy consumption, since the thrust for clamping down and sealingly closing the mold is obtained by a fluid pressure actuator of special design.

BRIEF DESCRIPTION OF THE INVENTION These objects are achieved by means of a method according to claim 1, and a mold closing device for a molding press according to claim 2.

The innovative principle of the invention resides in the generation of thrusts or pushing forces for closing mold members of a mold disposed between thrust members or posts of an elastically deformable structure, by providing an expander between spaced apart abutting surfaces of the deformable structure, and positively causing an elastic deformation of the structure by the expander, at the closure of the mold, to generate oppositely directed clamping forces by the thrust members to tightly close and clamping down the mold.

For the purpose of the present invention"closure of a mold"means a condition in which the two mold members, after being approached and brought slightly in contact each other, are clamped down and tightly urged against each other, in a sealed condition, to allow injection of molten of liquid materials preventing any leakage.

As stated above, unlike the molding apparatus of the known type, where the bending or deflexion of the frame is

regarded as a negative phenomenon to be counteracted, the novelty and originality of the present invention substantially consists in positively using the elastic bending or deflection or the machine frame, which is deliberately caused so as to generate high thrust forces, oppositely directed on the platens supporting the mold members, to tightly close the mold; in this way any defect in the parallel alignment of the mold members may be automatically compensated for by controlling the deflection degree of the frame of the press.

Therefore, according to a first aspect of the invention, a method has been provided for clamping and tightly closing a mold comprising: movable and stationary mold members disposed between thrust exerting shoulder members, on a side of an elastically deformable structure of a press frame; one of said mold member being movably supported from an advanced and a retracted position in respect to the other stationary member of the mold, the press frame longitudinally extending in the direction of the thrust axis of the mold, characterised by the steps of: providing at least one thrust generating expander between spaced apart abutting surfaces of a seat on at least one side of the press frame; approaching said movable mold member to the stationary mold member by an actuator; and urging on the abutting surfaces of the press frame, to cause an elastic deformation of the press frame, by the same expander, to positively generate oppositely directed thrust forces through the shoulder members on the both mold members, to tightly clamp the mold.

According to another aspect of the invention, in a molding press a mold-closing device has been provided, suitable for generating high clamping forces along a thrust axis or the center line of a mold provide between thrust exerting members on a side of an elastically deformable frame structure of a press, characterised by comprising: at least one thrust generating expander; at least one seat having open side edges, for housing said expander on at least one side of the press frame; said open side seat comprising spaced apart abutting surfaces for the expander; and control means to actuate said expander to urge against both abutting surfaces with thrusting forces suitable for positively causing an elastic deformation of press structure to generate opposite thrusting forces by said thrust members along said thrusting axis to tightly clamp together both members of the mold.

According to another aspect of the invention, the expander is in the form of a pressure-actuated expandable membrane, having a pressure chamber connectable to a hydraulic pressure source; the membrane may be provided by a flattened tubular element sealingly welded to side support bars having an inlet port for a pressurised fluid, and an outlet port for venting air, or by facing steel sheets suitably welded along their peripheral edges and/or to the above mentioned side bars.

According to another aspect of the invention, the membrane may comprise two parallely arranged facing plates which are movable with respect to each other, to define a pressure

chamber closed by a peripheral seal.

According to a further aspect of the invention, the expander is in the form of an elastically expandable flat membrane, manufactured from a sheet-steel blank comprising a rectangular or square central panel defining a first side wall of the membrane, and shaped panels on the peripheral sides inwardly foldable in respect to the central panel, said central and peripheral panels being sealingly welded along mating edges to form the other side wall of the membrane; in this way the welds are provided on one side wall of the membrane to adhere and be supported by the facing thrust receiving or abutting surface of the seat housing the membrane of the mold closing device.

According to a fourth aspect of the invention, the expandable membrane comprises a flat plate on a side defining a rigid wall, and an elastically flexible steel sheet on the other side of the membrane, spaced apart and suitably welded flush and along the peripheral edge of the facing plate.

According to another aspect of the invention, a tie-barless press for molding plastic or metal pieces has been provided, of the type comprising: an elongated frame having a longitudinal deflexion axis, and a thrust exerting or shoulder member on each end side thereof; a stationary mold support platen fastened to a shoulder member for supporting a first stationary member of the mold; a movable mold support platen for supporting a second

movable member of the mold, said first and second mold members being axially aligned on said side of the press frame; and drive means to displace the movable platen to firstly approach the movable mold member to the stationary mold member, and thrust generating means successively actuated to the drive means for clamping and tightly closing the mold, characterized in that said thrust generating means comprise: at least one pressure-actuable expander between abutting surfaces of a seat of the press frame, said expander and said seat being constructed and positioned to cause an elastic deformation of the press frame and generate clamping forces on the sides of the mold members, to close the mold; and fluid pressure control means for connecting said thrust generating expander to a fluid pressure source.

According to a preferential embodiment of the invention, the thrust generating expander is in the form of a double walled expandable membrane having a pressure chamber, fitted into a open seat of the press frame, or between abutting surfaces transversally extending to the frame of the press.

In all cases the expansion of the membrane should be such as to cause a small displacement of the abutting surfaces or of the force applying points, having the same order of the admissible elastic deformations for the press frame, for example of some millimetres, depending on the features

and dimension of the press; the quantity of hydraulic fluid to be supplied to the membrane being therefore entirely negligible and the work necessary for generating the thrusts for the clamping and the closure of the mold, being very small or comparatively small in respect to the prior know closing devices for molding presses.

Since the opposite thrusts for closing the mold are now generated by the elastic deformation of the press frame, after the two mold members have been approached using a minimum amount of energy, it is no longer necessary to design particularly strong and bulky closing devices as per prior art molding presses, since the control means for positioning of the movable platen, no longer has to generate very high closing forces, as occurs in the conventional presses, but must only be able to displace the movable platen on respective guides and passively opposing the reaction forces which arise at clamping of the mold.

BRIEF DESCRIPTION OF THE FIGURES These and further features and advantages of the invention will emerge more clearly from the description which follows with reference to the examples of the accompanying drawings in which: Fig. 1 is a schematic view of a general thrust generating device comprising an expandable membrane, to explain the principle of the invention; Fig. 2 is a cross-sectional view along the line 2-2 of Figure 1; Fig. 3 is a plan view of the membrane of the thrust generating device according to Figure 1; Fig. 4 is a cross-sectional view along the line 3-3 of Figure 3; Fig. 5 is a front view of a second embodiment of a

membrane according to the invention; Fig. 6 is a cross-sectional view along the line 6-6 of Figure 5; Fig. 7 is a front view of a third embodiment of a membrane according to the invention; Fig. 8 is a cross-sectional view along the line 8-8 of Figure 7; Fig. 9 is a top view of a fourth embodiment of a membrane according to the invention ; Fig. 10 is a cross-sectional view along the line 10-10 of Figure 9; Fig. 11 shows an unfolded blank, which can be used to obtain the membrane according to Figure 9; Fig. 12 shows a cross-sectional view of a firth embodiment of an expander membrane according to the invention.

Fig. 13 is a cross-sectional view of a sixth embodiment of an expander for the thrust generating device according to the invention; Fig. 14 shows a first embodiment of a tie-barless molding press comprising a pressure actuable thrust generating device according to the invention; Fig. 15 shows a detail of the left side of a press according to Figure 14, with a different arrangement of a thrusting membrane; Fig. 16 shows a further embodiment of a tie-barless press according to the invention; Fig. 17 is a top view of a tie-barless press comprising a thrust generating device according to the invention.

Fig. 18 shows a sectional view according to line 18-18 of Fig. 17.

DETAILED DESCRIPTION OF THE INVENTION With reference to Figures 1 to 4 we shall now describe the general principles and a first preferred embodiment of the thrust generating device according to the invention.

As shown in Figure 1, a thrust generating device according to the invention comprises a suitably shaped metal structure 10 which may be elastically deformed by the thrusting action of an expander 12 urging on abutting surfaces of a seat 11, to deflect the same structure 10 according to a bending axis orizontally extending and parallely arranged to the axis or center line of two mold members A and B clamped between the shoulder members 10C and 10D.

The frame 10 on the side opposite to the shoulder members 10C, 10D, is provided with an elongated cavity 11 which opens at the bottom and on both sides of the frame 10, to provide an open side seat for housing an expander 12, having side arranged thrusting surfaces urging against corresponding spaced apart abutting surfaces of the seat 11 as shown.

The expander 12 may be of any type or shape, to positively generate high clamping forces for a mold through an elastic deformation of the frame structure 10 caused by the expander 12, without involving any substantial movement for the same expander or part thereof.

Preferably the expander is in the form of a walled pressure-actuable membrane having a pressure chamber 13 of rectangular or square flat shape, and a small deep between the side walls 14,15, in respect to the width and length, substantially corresponding to the deep or spacing between

the abutting surfaces of the seat 11.

A first embodiment for the expandable membrane 12 is shown in the example of Figures 2,3 and 4; the membrane or expander 12, is in the form of a flattened tubular element, each end of which is fitted and welded into a corresponding seat of a support member 16 and 17 such as a square rod, as shown.

The rods 16 and 17 are provided with holes 18 to receive fastening bolts for fixing the membrane 12 into the seat 11 of the elastic structure 10, such as the frame of a press, as well as is provided with holes 19 and 20 opening into the chamber 13 at both ends of the membrane 12. One of the holes 19,20, for example the hole 19 of the rod 16, can be selectively connected and disconnected to a pressure fluid source P, by a solenoid valve 22 or other pressure control device. The hole 20 in the second rod is, on the other hand, closed by a plug member 21, being used to vent air from the chamber 13 of the membrane, when necessary.

Operation of the thrust generating device according to the present invention is essentially as follows: when the control valve 22 is operated to disconnect the chamber 13 of the membrane 12 from the pressure source P, no pressure inside the said chamber 13 will exist, and no thrust will be exerted by the membrane 12 against the abutting surfaces of the seat 11; therefore, the elastic structure 10 will not be subject to any deformation or bending moment. In these conditions, the thrusts S1 and S2 caused by the membrane 12 will be zero.

Differently, when the chamber 13 of the membrane 12 is connected by the control valve 22 to the source P of a fluid under pressure, such as an oil pressure source to

feed the chamber 13 of the membrane with a pressure which may be of the order of tens or hundreds of atmospheres, or higher depending on the thrusting forces S1 and S2 to be produced, a very small quantity of oil under high pressure will be fed into the chamber 13 depending by the small volume of the chamber 13, which will tend to expand or to spread out the side walls 14 and 15 pushing them with high forces S1, S2 against the opposing surfaces inside the seat 11.

Since the membrane 12 may extend over a large flat surface, having a considerable thrusting area, compared to the small deep of the seat 11, the two lower portions 10A and 10B in which the cavity 11 divides the structure 10, will be subjected to high thrusts S1 and S2 which are proportional to the hydraulic pressure inside the chamber 13 and the surface area of the membrane; however, since the expansion of the membrane 12 is extremely small or negligible, in keeping with the maximum deformation permissible for the frame structure 10, the oil quantity under pressure to be supplied into the membrane 12 will also be minimal or entirely negligible compared to conventional hydraulic actuating systems.

The thrusts S1 and S2 of the membrane 12, owing to the elastic deformation which the structure 10 undergoes, may be positively used to generate opposing bending moments M1 and M2 in the structure 10, which cause the shoulder members 10C and 10D of the frame structure 10 to exert opposite thrusting forces S3, S4 against the opposite sides of the pieces A and B such as the two mold members, to clamp and keep the same mold tightly closed.

The novelty and the originality of the invention, compared to conventional molding presses, therefore resides in the

fact of positively using the same capacity of the structure of a press to be elastically deformed, by one or more expanders, such as pressure actuable membranes connectable to a pressure source, fitted into corresponding seats or between opposite abutting surfaces of the structure of a press, so as to generate even considerably high working thrusts to clamp and keep close a mold, using a small amount of energy which is required only in order to operate the expander or to slightly expand the membrane by an amount sufficient to elastically deform the structure of the press. Moreover, since the membrane is wholly supported on the two sides by internal opposing surfaces of the seat or abutting surfaces of the press frame, the membrane may have an extremely simple and lightweight structure even in the case where high thrusts must be generated.

Figures 5 to 13 show further possible solutions for the membrane 12.

In the case of Figures 5 and 6, the membrane 12 again consists of a flattened tubular element having both ends 30 and 31 suitably welded together ; a connector 32 is directly welded to a longitudinal edge of the tubular membrane 12, so as to allow the chamber 13 to be connected to a pressure fluid source. In this case the connector 32 is coplanary arranged to the chamber 13 and the side walls 14,15 of the same membrane 12.

Figures 7 and 8 show a third embodiment of the membrane 12; in this case the side walls 14 and 15 of the membrane 12 consist of two spaced apart suitably shaped steel sheets, which are welded peripherally along their edges 34; the connector 33 in this case is welded to protrude from one side wall of the membrane.

Figures 9 and 11 show a fourth embodiment of an elastically expandable hydraulic membrane 40 for a thrust device according to the invention.

The membrane 12 of figures 9 and 10 is provided starting from a sheet-steel blank shown in Figure 11; the blank is obtained from a sheet 40 of material suitably cut and folded onto itself in the manner shown.

The blank comprises a rectangular central panel 41 defining one side wall of the membrane 12, and four peripheral panels 42,43 and 44,45 having complementary shaped edges and suitably folded inwards towards the panel 41, so as to form the second wall of the membrane 12 which is opposite and parallel to the first one.

More precisely the blank comprises two peripheral panels 42,43 with a triangular shape, along the longitudinal sides of the central panel 41, and two peripheral panels 44,45 with a trapezoidal shape, along the cross sides, the edges of which, after the peripheral panels have been folded inside, met together and are joined by welding.

From Figure 11 it can also be noted that the peripheral panels 42,43,44 and 45 are joined to the central panel 41 by narrow intermediate folding up strips 46,47,48 and 49 having a width corresponding to the thickness or deep of the membrane 12. References numbers 50 and 51 in Figure 9, finally denote two connectors for supplying a pressurised hydraulic fluid and for venting air inside the chamber 13 of the membrane 12.

Figure 12 of the accompanying drawings shows a cross- sectional view of a fifth possible solution. In this case an expander 12 is shown comprising a flat plate 53 defining

a rigid side wall, and a steel sheet 54 defining an elastically flexible facing wall, the peripheral edges 55 of which are suitably folded and turned inside a seating to be welded flush to the peripheral edges of the plate 53.

With reference to Figs. 14 to 18 we shall now describe hereinbelow the invention as applied to a tie-barless injection press of the horizontal type; however, it is understood that the invention is applicable in general to any type of vertical or horizontal press, both for the molding of plastic material and for the die-casting of metallic material in molten state, namely for similar applications where it is required to exert high thrusting forces for closing a mold while maintaining an ample accessibility to the working zone and a simplified construction of mold closing device for a press.

Fig. 14 shows an injection-molding press of the horizontal type, comprising a C-shaped base frame 60 having a deflexion axis which extends in a longitudinal direction of the press; the frame 60 is provided on the upper side with opposite shoulder members 61 and 62 which extend upwards and are rigidly connected to the frame.

The shoulder member 61 is provided with a stationary support platen for a first mold member 63 to co-operate with a second mold member 64 fastened to movable support platen 65 slidably guided along the frame 60 in any suitable manner, for example by means of longitudinal rail guides 66 which extend along the frame 60.

In Fig. 14 reference 67 indicates a three joint toggle mechanism connected between the shoulder member 62 of the frame 60 and the movable platen 65; the toggle mechanism 67 is actuated by a double-acting hydraulic cylinder 67'to

move the platen 65 between a retracted position, with respect to the stationary platen 61, in which the two mold members 63 and 64 are spaced apart, in the open condition of the mold, and an advanced position in which the movable mold member 64 is simply brought alongside the stationary mold member 63, making a light contact, so as to allow subsequent sealed closing of the mold by means of the opposite thrusts produced by an elastic deformation of the frame 60, by the membranes 68, as previously described and as clarified below.

As can be understood by figure 14, when the movable mold member 64 has been approached to the stationary mold member 65, the toggle mechanism 67 is in an extended condition in which the three joints are axially aligned, laying in a plane containing the longitudinal axis center line of the mold 63,64. Therefore no high frictional forces must be overcame inside the joints during approaching of the mold members and the same joints of the toggle mechanism does no more require a strong and complex structure because the toggle-joint mechanism 67, and the cylinder 67', must merely perform the displacement of the movable platen 65 with the movable member 64 of the mold.

Furthermore, in the extended condition of the toggle mechanism 67, the latter must only resist to the reaction forces caused by the closure of the mold, in a steady condition without involving any movement and frictional forces.

In fig. 1 a toggle-joint control unit has been shown for rapid approach of the movable mold member 64 to the stationary mold member 63; instead of the said toggle-joint unit, any other drive unit of the hydraulic, electric, mechanical type, or their combination may be used, without

changing in any way the present invention.

As stated above, a mold closing device for a press according to the invention comprises a first control means, such as the toggle mechanism 67 for rapid approach of the mold members 63,64, lacking the movable mold-supporting platen in the approached condition in respect to the stationary platen of the press, and a second thrust generating unit for tightly urging each other the mold members in the closed condition of the mold, by means of an elastic deformation of the press frame, positively caused by one or more pressure actuated expanders.

In the example under consideration of fig. 14, the second thrust unit for the closure of the mold, comprises two pressure actuated membranes 68 of the type previously referred to, housed in respective open seats 68' symmetrically disposed in the C-shaped frame 60 of the press, at the lower side which is opposite to the upper side for the mold and the closure toggle mechanism; as shown in the same figure 14, the membranes 68 are seated and extending in planes which are orthogonal to the longitudinal axes of the frame 60 and the mold members 63, 64; in this way it is possible to simultaneously exert opposing thrust actions S1, S2 by both membranes 68 which tend to open wide the two abutment surfaces of each seat 68'to produce mutually opposing bending moments M1 and M2 in the frame 60, causing a bending and an elastic deformation of the same frame.

Therefore, the bending and elastic deformation of the frame 60, according to the invention is advantageously and positively used to cause simultaneously inwards rotation of the shoulder members 61,62 rigidly connected to or forming part of the press frame, which rotation is opposed by the

approached mold members 63 and 64 which are therefore pressed against each other; in this way high mold closing forces are produced on both sides of the mold members, and the mold is tightly kept in a sealed condition during the injection of plastic or metal molten material.

As shown in fig. 14 the two thrusting membranes 68 are connected to a fluid pressure source P by a fluid supply circuit 69'and a control solenoid valve 69.

Summarising, according to the present invention, the use of elastically expandable membranes 12,68 and more generally the use of pressure actuated thrust generating devices or expanders for causing an elastic deformation of a frame 10 or 60 of a press, and the generation of opposite thrust forces for closing a mold, is extremely advantageous for the following reason: by positively exploiting the elastic deformation phenomenon of the frame 10 or 60 of a press, which is deliberately produced, it is possible to generate the thrusts necessary for tightly closing the mold after the mold members have been approached, simplifying greatly the structure of the press and facilitating access to the working zone thereof, while limiting as far as possible the power consumption necessary for closing and keeping the mold in a closed condition.

In fact, assuming that the front surfaces of the two mold members 63 and 64 in figure 14 have been brought into contact with each other by the first control unit provided by the toggle mechanism 67,67', the work required for elastically deforming the frame 60 and closing the mold 63, 64, is very small or entirely negligible compared to that required for a conventional press in which high frictional forces have to be overcome, in view of the very small displacement of the two thrust members 61,62 and the small

quantity of pressure oil is to be fed into the thrust membrane, compared to the pressure oil usually required by the hydraulic cylinders of conventional presses. In fact, whereas in a conventional press, the thrust for closing the mold is generated by an axial action of the same control unit which performs the sliding of the movable mold support platen, tending to open the frame or move the two upright ends of the press away from each other, causing a misalignment of the two mold members which must be suitably offset by using a greater amount of energy and a greater closing force, according to the invention the elastic deformation of the frame is positively used to produce bending moments inside the frame structure which tend to cause the two shoulder members at both ends of the frame, to move towards each other instead of spreading out, to generate the closure forces for the mold. The two mold members 18A, 18B or 63 and 64, are therefore pushed against each other with the necessary forces to maintain a correct alignment between them.

The energy required by the closure device according to the invention, in order to produce and maintain the mold closing thrusts, is therefore extremely reduced and solely that required to elastically deform the frame structure 10 or 60 of the press, which is significantly less than that required by conventional closing systems of a conventional press. Therefore, the amount of energy used and the oil to be supplied to the thrust membranes 12 or 68, for each molding cycle, are very limited to that required for generation of the pressure inside the membranes. Lastly, because of the membrane are resting against the thrusting surfaces on both sides of the seating, the structure of the membrane may be extremely simplified in respect to an hydraulic cylinder, saving costs.

In fact, from calculations performed and from some tests carried out, it has been established that the displacements to which the press frame is subject at the various application points of the forces, are of the order of a few tenths of a millimetre for a frame having a length of between 2 or 5 metres, which are well within the range of the elastic deformation permissible for such a structure.

In the case of Fig. 14, the two thrusting membranes 68 are positioned underneath and in planes substantially transverse to the longitudinal axes of the mold members 63, 64 and the same frame 60 of the press.

Other disposition of the thrusting membranes are possible, as shown in the example of Fig. 15, in which only a left- hand side of the press frame 60 according to Fig. 14, is shown.

In the case of figure 15 use is made of a thrusting membrane 68 which is identical to that of the preceding example of figure 14, being arranged in a corresponding seat 68 horizontally extending in a plane underneath and parallel to the longitudinal axis of the mold, at one end 62 of the frame 60.

In view of the arrangement of the membrane 68, in the case of Fig. 15, oppositely directed thrusts S1 and S2 are generated suitable for creating opposite bending moments M3, M4 at the ends of the press frame 60, to cause an elastic deformation of the frame 60 and closure of the mold as described above; in general it is preferable to have a symmetrical disposition of the thrusting membranes, even though it is not entirely necessary.

Instead of the pressure expandable membranes 12,60 it is

possible to use any other type of hydraulic or mechanical expander or thrust device in order to cause elastic deformation of the press frame and generate the mold closing thrusts, provided that they are suitable for the intended purpose.

The same apply for the drive device for approaching the two mold members; in the case of Fig. 16, differently from the example of Figure 14, for the displacement of the movable platen 7, to approach mold members 73,74, use is made of a hydraulic control unit 77 for moving the platen 75 and the mold member 74 towards and away from the stationary mold member 73. However, in the example of Figure 16 as well, the elastic bending or deflexion of the press frame 70 and the thrusting by the shoulder members 71 and 72 is again caused by the pressure actuated membranes 68. In this case it will be necessary to envisage locking means 78 to lock the rod 78 of the cylinder 37 so as to keep the movable platen 75 locked in the advanced mold closing position, so as to withstand the thrusts.

It is understood, however, that stated and shown has been provided purely by way of illustration of the general principles of the invention, which may be applied to any type of press, different from that shown, either of horizontal and vertical type for injection molding or for any other suitable use, without thereby departing from that which is claimed.