Pressing Control Device

Abstract

A pressing device for the pressing of synthetic resin materials in a mould, comprising an hydraulic motor which is provided with a piston which is movable in a cylinder, a piston rod being connected to the said piston. Hydraulic pressing liquid can be supplied to the spaces on both sides of this piston by means of a servo valve. The piston rod is provided with a force detector and a displacement detector. The signals supplied by these detectors are compared with adjusted values. As long as the pressing force remains below the adjusted value, the control unit controls the servo valve such that the piston moves at an adjusted speed. When the adjusted pressing force is reached, the control unit controls the servo valve such that the piston rod does not exceed the adjusted pressing force.

Description

The invention relates to a pressing device, in particular for the pressing of synthetic resin materials in a mould, provided with an hydraulic motor which comprises a piston which is movable in a cylinder, a piston rod being connected to said piston, it being possible to supply or withdraw hydraulic pressure liquid to or from the spaces on both sides of said piston by means of a servo valve.

The products which are pressed of synthetic resin materials by means of suitable known pressing devices often have defects. These defects can be inter alia excessive burrs, air-inclusions, poor surface condition, poor material distribution, premature hardening etc. All these defects are an indication of insufficient control of the pressing process which in known pressing devices is controlled mainly by adjustment of the pressure difference across the piston.

The invention has for its object to provide an improved pressing device of the kind set forth. So as to achieve this object, the pressing device according to the invention is characterized in that the piston rod is provided with a force detector and a displacement detector, the signals supplied by these detectors being applied to a control unit in which these signals are compared with adjusted values, the control unit controlling the servo valve such that the piston moves at an adjusted speed as long as the pressing force remains below the adjusted value, the control unit controlling the servo valve, when the adjusted pressing force is reached such that the piston rod does not exceed the adjusted pressing force.

Because the force detector measures on the piston rod, always the pressing force which is actually exerted on the synthetic resin material is measured as well as possible. The effect of, for example, the piston friction in the cylinder is then completely eliminated.

By effecting the control in reaction to the displacement detector and by imparting to the piston a speed which corresponds to an adjusted value during the time that the pressing force is lower than the adjusted force, the plunger will introduce the material to be pressed in the mould at such a speed that the risk of air-inclusions, premature hardening and poor surface conditions is very small.

If the pressing force increases, a change-over will be made, when the adjusted pressing force is reached, from control of the piston speed to control of the pressing force which now will not exceed the adjusted value, so that burrs are prevented.

By selective feedback of a displacement and a pressing force signal to the servo valve, the signals corresponding as well as possible to the speed and pressing force experienced by the synthetic resin material, a pressing device is thus obtained in which the drawbacks of the known pressing devices are eliminated.

So as to eliminate the adverse effect of any misalignment of the hydraulic piston with respect to the mould, a further preferred embodiment of the pressing device according to the invention is characterized in that the piston rod is connected to a pressing plunger via a thrust bearing, the said thrust bearing allowing the pressing plunger some play in the transverse direction with respect to the piston rod.

The invention will be described in detail with reference to the drawing which is a diagrammatic representation of an embodiment of a pressing device.

The reference numeral 1 in the drawing denotes a cylinder. Accommodated in this cylinder is a piston 2 which is provided with a piston rod 3.

The cylinder 1 is supported by a plate 4, which is connected to the frame 6 of the pressing device via a number of columns 5.

Arranged on the frame 6 is a mould 7 in which one or more cavities are present in which synthetic resin material can be pressed. Pressing is effected by means of a plunger 8, the upper side of which is provided with a structural part 9 which grips about a plate 10 which is connected to the piston rod 3.

Part 9 and plate 10 together constitute a hydrostatic thrust bearing. To this end, the plate 10 is provided with a number of pressure chambers 11 which receive liquid from the space 14 in cylinder 1 via a duct 12 incorporating restrictions 13. The space 14 communicates, via a duct 15, with a liquid supply duct 16 which communicates with the outlet side of a liquid pump 17. The liquid pump is driven by an electric motor 18 and derives its liquid from liquid reservoir 19.

The space 21 can communicate, via duct 22 and servo valve 20, with the liquid supply duct 16 or with the liquid outlet duct 25. Liquid from the bearing chambers 11 of the hydrostatic bearing can reach outlet duct 25 via duct 24.

The plunger 8 is provided with a force detector 30 which supplies its signal to control unit 31. Also provided is a displacement detector 32 which detects the displacement of the piston rod 2 and which applies a corresponding signal to the control unit 31. The control unit 31 controls the control unit 33 of the servo valve 20. The operation of the device set forth is as follows. When the matrix is filled, an hydraulic force is exerted on the piston 2 such that the piston with the plunger starts to move in the downward direction. This is achieved by supplying liquid to space 21 by means of servo valve 20.

The displacement of piston 2 is detected by displacement detector 32, whilst the delivered pressing force is measured by the pressure detector 30. The measured values are compared in the control unit 31 with adjusted values, the control unit 31 comparing the value measured by the displacement detector 32 with the adjusted desired piston speed when the force measured by the pressure detector 30 is below the adjusted value. Any difference between adjusted and measured value produces a signal which is applied to the control unit 33 which adjusts the control valve 20 so that the supply of liquid to the space 21 is changed such that the desired piston speed is obtained.

As soon as the pressure detector 30 measures a pressing force which corresponds to the adjusted value, the control unit 31 switches over from speed control to force control, the signal of pressure detector 30 then being compared with the adjusted pressing force, and any difference producing a signal by means of which the control unit of the servo valve 20 is controlled such that the liquid flow applied to the space 21 is so large that the adjusted value of the pressing force is exactly maintained. If the speed increases beyond a given value again, a change-over takes place to speed control.

Instead of the measuring of the force on the piston rod, the pressure in the mould can also be used as an control quantity in given circumstances.

A pressing device is thus obtained in which the pressing process can be very well controlled. Control is effected in reaction to signals which are measured very near to the synthetic resin mass on which the process is performed. This means that very few secondary effects such as, for example, friction of piston 2 in cylinder 1 etc. can adversely influence the measurement.

Due to the use of an axial bearing 9, 10, 11, 12, 13 between piston rod 3 and plunger 8, any misalignment will not give rise to excessive friction of the plunger 8 in mould 7.

It will be obvious that instead of an hydrostatic bearing, another axial bearing can be used for this purpose.

The force and displacement detectors, the control unit and the servo valve are only diagrammatically shown, but a variety of types which are known to those skilled in the art can be used for these purposes. Of importance is only that the piston displacement can be controlled both as regards speed and force.

Claims

What is claimed is:

1. A pressing device, in particular for the pressing of synthetic resin materials in a mould, provided with an hydraulic motor which comprises a piston which is movable in a cylinder, a piston rod being connected to said piston, it being possible to supply or withdrawn pressure liquid to or from the spaces on both sides of said piston by means of a servo valve, characterized in that the piston rod is provided with a force detector and a displacement detector, the signals supplied by these detectors being applied to a control unit in which these signals are compared with adjusted values, the control unit controlling the servo valve such that the piston moves at an adjusted speed as long as the pressing force remains below the adjusted value, the control unit controlling the servo valve, when the said pressing force is reached, such that the piston rod does not exceed the adjusted pressing force.

2. A pressing device as claimed in claim 1, characterized in that the piston rod is connected, via a thrust bearing, to a pressing plunger, the said thrust bearing allowing the pressing plunger some play in the transverse direction with respect to the piston rod.