EP0567870A1

EP0567870A1 - Short-stroke hydraulic press for compression moulding

Info

Publication number: EP0567870A1
Application number: EP93106227A
Authority: EP
Inventors: Giampaolo Spanio; Sergio Fumagalli
Original assignee: Tcs Molding Systems SpA
Current assignee: Tcs Molding Systems SpA
Priority date: 1992-04-22
Filing date: 1993-04-16
Publication date: 1993-11-03
Also published as: ITMI920958A0; ITMI920958A1; IT1254901B

Abstract

Hydraulic press for the compression-moulding of plastic materials; the press comprises a lower fixed platen (10) and upper movable platen (12) sliding along guide columns (11) extending from and movable respect to the lower platen (10); sensing means (25, 26) are provided to control the downwards movement and velocities of the upper platen (12) and guide columns (11), sending control signals to a logic control unit (27) programmed to start the descending movement of the columns (11) during the downward movement of the upper platen (12) and to activate clamp means (16) to lock the upper platen (12) to the moving columns (11) when the relative speed between said upper platen (12) and columns (11) is zero.

Description

The present invention relates to a short-stroke hydraulic press for the compression-moulding of plastic materials having reliably control means to precisely control the movement velocities of the movable platen towards a closed condition of the press to reduce the power consumption and the length of the working cycle.
A hydraulic press usually is provided with a lower fixed platen for supporting a first mould member, and an upper movable platen for supporting a second mould member, in which the upper platen is movable along vertical guide columns upwardly extending from the fixed lower platen; long stroke and short-stroke hydraulic actuators are separately provided to selectively effect the fast downward movement of the movable platen from an upper position, where the mould is open, to a lower intermediate position adjacent to the fixed platen where the movable platen is locked to the guide columns by actuating suiplaten hydraulic or mechanical locking devices, after which a short working stroke is performed to close the mould of the press. Presses of this kind are known for example from US-A- 4,304,540 and US-A 4,457,684.
In hydraulic presses of the kind mentioned above, in particular in the case of large-size presses, the upper movable platen must be locked to the guide columns in order to perform the closing of the mould and the working operation of the same press; however, locking to the columns may be performed only after the movable platen has been securely stopped with respect to the lower platen of the press, before proceeding with the closure of the mould for the final working operation; this is mainly due to particular working organisation of the press and structural disposition of the actuators.
In presses of the kind mentioned above, the intermediate operation for locking the movable platen to the columns of the press involves therefore the interruption of the press's working cycle, resulting in dead times between the fast advancing movement of the movable platen and the next working operation; having to wait for the movable platen to be completely stopped before locking it to the columns and before starting the working stroke, inevitably the duration of the entire press's working cycle is extended, resulting in loss of productivity.
A scope of present invention is to provide a hydraulic press particularly, but not exclusively for the compression-moulding of plastic materials, according to the preamble of the main claim, by means of which it is possible to reliably control the locking of the movable platen to the guide columns resulting in continuity of movement of the upper platen to reduce the time length of the entire operating cycle of the press.
A further scope of present invention is to provide a hydraulic press, of the kind mentioned above, provided with a simplified and economical structure allowing a reduction of power consumption in respect to prior known presses.
The above is achieved by a hydraulic press according to the main claim comprising a fixed base platen having columns slidingly extending from the same, an upper platen movably supported by said columns, long stroke drive means to reciprocate the movable platen between an upper position and an intermediate position along said columns, and short-stroke drive means to move said columns with respect to the fixed base platen; the press furthermore comprising control means to sequentially operate said drive means and clamping means to lock the movable platen to the guide columns while the latter are moving for the closure of a mould, and when the relative speed between the parts reaches zero, while maintaining the descending movement of the guide columns, to close the mould. In this way the movable platen may be securely locked to the guide columns while the same platen is downwardly moving, thus resulting in time saving and in increased productivity for the working cycle of the same press.
The invention will be described in greater detail hereinbelow, with reference to the accompanying drawings which illustrate, in schematic form, a hydraulic press with its associated actuating system.
In these drawings:

Fig. 1: is a general view of the press with the movable platen in its upper position where the mould is open;
Fig. 2: is a view similar to that of Figure 1, with the movable platen in its lower intermediate position where it is locked to the moving guide columns;
Fig. 3: is a diagram showing the comparison between the absolute velocities and the relative velocities of the moving parts of the press, during their descending movements.

As shown in Figure 1, the hydraulic press substantially comprises a base frame and lower fixed platen 10 from which there four parallel columns 11 extend (only two shown in Figure 1) for slidingly supporting and guiding an upper platen 12, to move the latter therealong towards and away from the fixed platen 10. References 13 and 14 in figure 1 denote, respectively, two mould members removably fixed to the platen 10 and platen 12 respectively, for compression-moulding for example of sheet plastic materials 15 in a manner known per se.
The columns 11 are mounted to the lower platen 10 for an axially guided movement by guide means 19, and are rigidly connected at their top ends by cross-pieces 18 or frame members as schematically shown.
Reference 16 in figure 1 denotes moreover hydraulically actuated locking devices for locking the movable platen 12 to the columns 11 of the press; in substitution of said locking devices any other known locking means may be used which can be actuated by a control system.
The fast descending movement of the platen 12, from the upper position shown in figure 1, in which platens 10 and 12 are spaced apart to lower intermediate position shown in figure 2, in which the upper movable platen 12 is closed position to the fixed platen 10 and in which the movable platen 12 may be locked to the columns 11 of the press, occurs as a result of gravity and is suitably braked by a long-stroke, hydraulic piston cylinder assembly 17, which also effects the movement of platen 12 upwards again.
As previously referred to, the columns 11 for guiding the movable platen 12 are in turn longitudinally sliding inside the bushes 19 defining guide means provided in the fixed platen 10 of the press, so as to allow a working short stroke of the movable platen 12 to be performed by a downwards movement of the same columns 11 as explained below.
Each column 11 protrudes underneath the fixed platen 10 and is connected to the piston of a respective double-acting hydraulic cylinder 20 which controls its descending and rising movements in synchronism with the piston-cylinder assemblies of the other column 11 of the press, and sequentially to the cylinder assembly 17 under the control of a programmable processing unit 27.
In the abovementioned figure, reference 21 denotes moreover the servo valves of a control system for supplying pressurised oil to the hydraulic cylinders 20 of the column, 22 denotes a servo valve for feeding pressurised oil to the locking devices 16, whereas 23 denotes the servo valve for the long-stroke cylinder assembly 17 which controls the braked descending movement of the movable platen 12 and which effects its return upward movement during the press operating cycle; lastly reference 24 denotes a general hydraulic control unit for supplying pressurised oil to cylinder assemblies of the press.
As referred to above, the present invention reside in sequentially controlling the speed of descent movements of the movable platen 12 and columns 11 of the press by suiplaten sensing means which feed control signals to the programmable control unit 27, for starting the descending movement of the columns 11 when the upper platen 12 is still moving along the same columns 11, and in actuating the locking devices 16 when the relative velocity of the movable platen 12 in respect to said columns 11 is equal to zero, while the assembly consisting of the columns 11 and mobile platen 12 continues its downward movement to the closed position of the mould.
Therefore according to the present invention, the movable platen 12 is provided with a stroke transducer or encoder 25 for monitoring the descending movement and velocity of the platen 12, provided between the frame member 18 connected to the columns 11 and the same movable platen 12; therefore the encoder 25 supplies to control unit 27, control signals indicative of the position of the platen 12 and its velocity with respect to the columns 11 of the press, irrespective of the movement or resting condition of the columns 11.
Similarly, each cylinder 20 of the guide columns 11 is provided with a stroke transducer or encoder 26 for monitoring the descending movement and velocity of columns 11 and to supply corresponding control signals to control unit 27 indicative of the position signals to control unit 27 indicative of the position and absolute velocity of the columns 11 with respect to the fixed platen 10 of the press. The encoders 25 and 26 send their signals to the control or processing unit 27 which controls the overall operating cycle of the press and the various hydraulic cylinders via the servo valves 21, 22 and 23 referred to above.
As already mentioned, the control unit 27 may be programmed to calculate, by means of a algorithm, the absolute displacements and relative displacemen ts of the movable platen 12 with respect to the columns 11 in order to initiate the descending movement of the same columns and effect subsequent locking action of the platen 12 to the columns themselves, while the latter perform their downward travel.
The above may be understood more clearly with reference to Figure 3 which shows the graphs for the absolute displacement and relative displacement which the moving parts of the press perform over time. In Figure 3 S1 indicates the displacement of the movable platen 12 with respect to the columns 11 of the press; this displacement is detected by the encoder 25. S2 on the other hand indicates the displacement of the columns 11 of the press, detected by the encoders 26, while S3 indicates the absolute displacement of the movable platen of the press, from the instant P when the descending movement of the columns 11 is initiated; this displacement is obtained by adding together the displacements S1 and S2 for each instant, after the instant P. If the corresponding velocities are associated with the displacements, it will be noted that from the instant P to the instant P_o where the relative velocity between the movable platen 12 and the columns 11 reaches zero, the two velocities added together algebraically; from the instant P_o the platen 12 is locked to the columns 11 and continues its downward travel, at the same speed as the guide columns.
Therefore, the logic control unit 27, in addition to controlling the position of the platen 12 and columns 11 via the signals received from the various encoders, calculates the velocity of translation of the moving parts and is programmed so as to activate the locking of the platen 12 to the columns 11 at an instant T lying between P and P_o, before the relative velocity of the two parts reaches zero, in accordance with the reaction times of the entire system.
More precisely, with reference to the figures of the accompanying drawings, the press is initially in the open condition shown in Fig. 1 where the columns 11 are at a standstill and raised position and where the movable platen 12 is at a standstill in its upper position. Starting from this position, initially the command is given for fast descending of the platen 12, which rapidly moves towards a lower intermediate position. When the movable platen 12 during its downward movement reaches a predetermined point P, the processing unit 27 commands actuation of the working cylinders 20 so as to initiate the slow descending movement of the columns 11 controlled by the encoders 26, while continuing the braked descending movement of the movable platen 12 controlled by the encoder 25.
After the columns 11 have started their downward movement, the platen 12 moves with respect to the columns 11 at a relative speed which gradually decreases due to braking action of the cylinder 17; at the point P_o the speed of the platen 12 coincides exactly with the speed of the columns 11. Therefore, an instant T before the point P_o is reached, still under the control of the program of the logic unit 27, locking of the platen 12 to the columns 11 is initiated by actuating the locking devices 16 so as to ensure that the various movable parts of the press are substantially rigidly locked or connected to one another. This condition of the press is shown in Figure 2 where it can be noted that the pistons of the cylinders 20 of the columns 11 have performed part of their descending stroke; after the movable platen 12 has been locked to the columns 11, from this movement the assembly consisting of the columns 11, the upper cross-pieces 18, the long-stroke cylinder 17 and the movable platen 12 will continue to move downwards as one unit at the descending velocity of the columns 11 to close the mould, thus completing the working stroke of the press.
From the above and the illustrations in the accompanying drawings, it will therefore be understood that a hydraulic press has been provided for the compression-moulding of plastic materials, which is substantially innovative in that it permits an operative cycle controlled and managed by a suitably programmed logic unit so as to effect simultaneously movement of the columns 11 and lockage of the movable platen 12 to the column themselves, while the latter have already started their downward movement. In this way, during the working cycle of the press, the intervening dead times which normally occurs during the locking operation of the movable platen are eliminated, thus resulting in increased productivity.

Claims

A hydraulic press comprising: a base frame and a lower platen (10) fixed to said base frame, movable columns (11) upwardly extending from said fixed platen (10) and mounted on said base frame for a sliding movement thereto, and short-stroke cylinder assemblies (20) axially aligned and connected to the lower ends of the columns (11) to reciprocate the same in respect to said fixed platen (10); a movable platen (12) sliding along said columns (11) between an upper position and a lower position, to open and close mould members (13, 14) disposed between the platens (10, 12) mentioned above, a long stroke cylinder assembly to reciprocally slide said movable platen (12) along said columns (11) between said upper position and an intermediate position spaced apart from the fixed platen (10) of the press, clamp-means (16) selectively actuable being provided to lock said movable platen (12) to the movable columns (11) of the press, and programmable control means (21, 22, 23, 24, 27) sequentially actuable to move said columns (11) and to drive said movable platen (12) between the open and the intermediate positions mentioned above, said control means comprising reference signal generators (25) for monitoring the position and the velocity of said movable platen (12) when sliding along the columns (11) of the press; characterised in that said movable columns (11) are rigidly connected by a frame member (18) at their top ends; said control means furtherly comprising reference signal generators (26) for monitoring the position and velocity of the columns (11) in respect to the fixed platen (10) of the press; said control means (21, 22, 23, 27) programmed:
- to drive said movable platen (12) from the upper position to said intermediate position adjacent to said fixed platen (10), by said long-stroke cylinder assembly (17);

- to sequentially start the downward movement of the columns (11) by said short-stroke cylinder assemblies (20) when the movable platen (12) is approaching said intermediate position along said columns (11); respectively

- to actuate said clamp means (16) to lock said movable platen (12) when the relative velocity of the same movable platen (12) in respect to the slowing movement of the columns (11), as monitored by said signal generators (25, 26), reach zero; and

- successively to move the assembly of said columns (11) and said movable platen (12) in the locked condition, towards the closed position of the mould (13, 14), by continuing the downwardly movement of said short-stroke cylinder assemblies (20).
A hydraulic press according to Claim 1, characterised in that said long-stroke cylinder assembly (17) is movably supported with said frame member (18) connecting the columns (11) at their top ends.
Hydraulic press according to the preceding claim 1, characterised in that said reference signal generators comprise a first long-stroke transducer (25) between the upper frame member (18) connecting the columns (11), and the movable platen (12) of the press, as well as a short-stroke transducer (26) between the piston member of short-stroke cylinder assembly (20) and the base frame of the press.
A hydraulic press according to Claim 1, characterised in that the fixed lower platen (10) comprises guide means (19) for the movable columns (11).
A hydraulic press according to preceding claim 1, characterised by comprising hydraulically actuable clamp means (16) for locking the movable platen (12) to the columns (11).