Arrangement for controlling a board press

Abstract

 An arrangement for controlling the thickness of board products (4) during the pressing process in a board press with a first and a second press table (5,2) where the pressing force is transferred by several hydraulic press cylinders (1). A number of hydraulic position valves (7) are arranged in spaced relationship on both sides of the press on the first press table (5). Corresponding push-rods (9) are arranged on the second press table (2). The push-rods are adjustable, and a common shifting motor (12) is provided for simultaneous adjustment of the push-rods (9).

Description

[0001] This invention relates to an arrangement for controlling the thickness of board products during the pressing in board presses, particularly presses with few openings.

[0002] At the manufacture of board-shaped products where the starting material is resilient mats loosely bound together, the mats are compressed under pressure while simultaneously heat is supplied from the press surfaces consisting of the heating plates of the press. The mats are compressed to a final thickness whereafter in an application at the manufacture of fibre board according to the dry method or of particle board, a hot-setting glue cures and provides the board with sufficient strength for the pressing to be completed. At other applications, for example at the manufacture of fiber board according to the wet method, natural glue substances in the fiber mat yield the necessary strength after heating and drying. Other types of products pressed under heat and pressure can also be a possible choice.

[0003] At the manufacture of the aforesaid products, it is known to control the final thickness in several different ways, by means of spacing strips, electrical position scanners or hydraulic direct acting valves. Each of these means has its special disadvantages.

[0004] Spacing strips in most cases are attached along the long sides of the press, outside the press material, or in the vicinity of press cylinders, placed outside the press material. Spacing strips, as the term indicates, can be used only for controlling a certain predetermined final thickness. The thickness of the spacing strip has been determined at the manufacture and cannot be changed thereafter, if the boards would show to be thinner or thicker than desired, even if the spacing strips happen to have the ideal thickness. Such deviations can occur for several reasons. One reason is that spacing strips and countercurrent surfaces after some time of operation are coated with material from the press material. Thickness deviation also can occur when the hydraulic system of the press and the press tables are designed so that loads arise which are greater than required by the mat, resulting in deflections across the press and thereby in thickness deviations.

[0005] In addition to the aforesaid drawbacks with fixed predetermined thickness, production losses also occur with spacing strips when the strip thickness is changed.

[0006] Spacing strips, as mentioned, can be used only for controlling the final thickness. The movement of the press toward final thickness cannot be affected by this equipment.

[0007] Electric position scanners can be either of limit position type or position measuring type. The simplest one is the limit position type where the position must be changed for each thickness to be pressed. Due to hysteresis in the limit position proper and to errors arising at position adjustment, these systems are not sufficiently accurate, either, and show most of the weaknesses of spacing strips. When using transmitters of position measuring type, of the type linear potentiometers or the like, the adjustment of nominal value thickness can be made simplified from a central control panel. The accuracy of this equipment is not at optimum, either, for presses with few openings. One weakness of the electric systems for position scanning is, that the signal, which normally must be amplified, secondarily controls a solenoid of a hydraulic valve controlling the pressure in the cylinder. There are, thus, several steps between measurement and action. Another disadvantage, especially with limit positions, is that only the final position can be controlled.

[0008] For the position control of the final board thickness, also hydraulic direct acting position valves can be used, whereby a very accurate direct thickness control is obtained. One disadvantage is, thst thickness adjustments, either fine adjustment or change to another thickness, must be carried out at the press by changing a small spacer for every valve. Another disadvantage is, that only the final thickness can be controlled. Hydraulic direct acting position valves have been used with great success for achieving narrow thickness tolerances.

[0009] The known systems mentioned above control only the final position. It has been found, however, that not only a well-controlled final thickness is necessary, but also that an accurate control of the press table position or mat thickness in all mat sections, also before the final thickness has been attained, is important both for the spring back of the board after pressing and thereby the real thickness after pressing, and for achieving a desired density profile in the thickness direction. The appearance of the density profile affects essential board parameters such as internal bond strength, bending strength and paintability. A system for controlling the press table position at varying positions, besides, renders it possible to improve and shorten the pressure reduction process at the end of the pressing cycle, which is critical with regard to the risk of disc bursting due to enclosed steam.

[0010] The present invention has the object to improve the pressing process, so that the distance between the heating plates can be maintained equal at all position valves, not only at the final position, but also both during the compression process toward the final position and during the pressure reduction process, to control these processes in a predetermined way and also to improve the position accuracy in all positions.

[0011] This is achieved according to the invention, which has the characterizing features defined in the attached claims.

[0012] Of particular advantage is a system with ball-screws, which are operated simultaneously, controlled from an electronic control system, and by using very accurate position valves with double seat function. In order to achieve a successful result, the position valves should be designed with moderate control forces suitable for the ball-screw system.

[0013] The invention is described in greater detail in the following, referring for reasons of clarity to a single opening press with only four cylinders, and with reference to the accompanying drawings, in which Fig. 1 is a lateral view of a press with the arrangement according to the invention, Fig. 2 is a view from above of the press, Fig. 3 is a cross-section of the position valve in starting position, Fig. 4 is a cross-section of the position valve in pressure stop position, Fig. 5 is a cross-section of the position valve in pressure reducing position, Fig. 6 shows a press sequence, Fig. 7 shows an unfavourable density profile of a pressed disc, Fig. 8 shows a favourable density profile.

[0014] In Figs. 1 and 2 a µress is shown with cylinders 1,upper press table 2 and lower press table 5 with upper and, respectively, lower heating plates 3, mat 4 to be pressed and a force absorbing structure 6.In the vicinity of each cylinder a position valve 7 is mounted on the lower press table 5. The position valves 7 are actuated mechanically by ball-screws 8 with push-rod end 9,attached via holds 10 on the upper press table 2. (The position valves and holds can be mounted directly on the respective heating plate). All ball-screws can be operated simutaneously via the link arrangement 11 located on the upper press table 2 and driven by a shifting motor 12 (stepping motor, servo-motor or the like) via a reduction gear 13. The system also comprises a stroke limiter 14, miter gears 15 and 16 and a suitable number of couplings 17. The position valves 7 are connected via lines 31 each to a cylinder 1 and receive pressure supply from a common line system 18 receiving its pressure from a pressure source 19. The position valves 7 are also connected to a common line system 20 connected to a tank 21. The system further includes a position transmitter 22,preferably designed as an absolute-coded (GREY-coded) encoder.

[0015] In Figs. 3,4 and 5 the position valves are shown in greater detail, with stop end 30,inset 23, piston 24,spring 25, piston 26,piston 27, spring 28 and intermediate piece 29. In Fig. 3 the starting position is shown where the spring 25 has its greatest length. The connection between the pump 19 here is open via the line 18 for pressure supply at the line 31 to the cylinder 1. In Fig. 4 the position is shown when the piston 24 just gets into contact with the abutment plane on the piston 26 whereby the connection between the pressure supply 18,19 and the cylinder 1 is closed. In Fig. 5 the position is shown where the piston 24 still has contact with the abutment plane on the piston 26, and where the piston 26 has lost contact (excessively large movement in the Figure) with the abutment plane on the intermediate piece 29, whereby the connection between the cylinder 1 and tank 21 via the lines 31,20 is opened. The piston 26 normally is held at the top in contact with the intermediate piece 29 by the spring 28 and the servo-pressure acting beneath the piston 27.

[0016] Upon closing the press, the pressure liquid is directed from the common line system 18 via all of the position valves 7, which then are in open position according to Fig. 3, to the cylinders 1, whereby the upper press table 2 together with the ball-screws 8 move downward. The ball-screws have been set by the shifting motor 12 so that they are in a suitable position. When the respective push-rod end 9 meets the stop end 30 on its position valve 7, the piston 24 moves down to the abutment plane on the piston 26. Upon its arrival at the abutment plane, according to Fig. 4, the connection to the pressure system 18,19 is broken, and the press table 2 stays in this position. The heat from the heating plates 3 penetrates successively into the mat and softens it, thereby reducing the resistance force of the mat. When the ball-screws 8 are not adjusted, the piston 26 moves a fraction of a millimeter downward and opens the connection to the tank 21 according to Fig. 5, whereby the pressure in the cylinder 1 is self-adjusted so that the desired position is maintained. If for some reason the mat would expand so that the distance between the heating plates 3 increases, the tank connection is closed and the connection to the pump 19 is opened, so that the pressure is increased to the required level in order to maintain the correct thickness.

[0017] The system includes safety means against erroneous settings and operating manners. Firstly, the position valves 7 are constructed so that they allow for a relatively large overtravel past the intended control position, whereby the system is protected against breakdown if a wrong board thickness has been set from the control system, or if a wrong mat has been fed into the press. Secondly, the stroke limiter 14, which is equipped with a threaded-on, nut running against a mechanical stop in the end position, has such a strong structure that the torque of the shifting motor 12 in the end position is not propagated to the ball-screw system.

[0018] In Fig. 6 a typical pressing process is shown, where time is the abscissa and table position and pressure are the ordinate. The table position farthest to the left and, respectively, farthest to the right corresponds to open press. It can be assumed, that the position described above of the ball-screws 8 corresponds to the table position L1 in the diagram. It is intended to move the press table 2 from L1 with a ramp (linear or curved) to L2 on time T2. In subsequent periods of time the press table is advanced successively to the final position L4, which is maintained while the glue hardens. The pressure increases first to a maximum pressure, Pmax, and then varies responding to the pressing conditions, for example as shown in Fig. 6. The control instructions for achieving this motion pattern is programmed in a programmable control syetem, a so-called PLC-equipment, microcomputer or the like. As soon as position L1 has been reached, instructions are given to the shifting motor 12, so that it changes the position of the ball-screws 8 simultaneously to desired values. The system is fed-back thanks to the position transmitter 22. By means of these adjustment possibilities practically all desired position ramps can be effectuated, the object being to optimize the density profile of the boards, so that a suitably high density is obtained in the surface layer, so that density of a suitable level is obtained at the centre, and so that a suitable transition between these portions is obtained. These density values are of essential importance for the properties of the boards, and a well-balanced density profile according to Fig. 8 can save raw material etc., because a lower mean density compared with Fig. 8 can be accepted at disc properties being equal in other respects. This is of great economic value. At a single opening press for particle board with a capacity of 400 m³/day an annual saving of SEK 1 million for raw material, glue handling, painting and drying can be made when the mean density is reduced by 10 units, for example from 650 to 640 kg/m³.

[0019] Compared with previous equipment, the invention renders it possible to change a desired boardthickness in a stepless manner from the switchboard without production loss. Besides, a given thickness can be finely trimmed by changing the nominal value for all ball-screws 8 from the switchboard and/or changing the individual position valves by changing the insets 23 at the valve in question. The position of the ball-screws can be adjusted very accurately, within 0.01 mm, by a suitable gear change. (The gear ratio of the system with the ball-screws can be chosen so that a turning of the motor shaft by 70 degrees corresponds to a movement of the push-rod end of 0.01 mm). In combination with the highly accurate position adjustment of the valve proper, the board thickness can hereby be held at a very narrow tolerance. Compared with previously, a gain of the magnitude 0.1 mm can be expected, which at the same production as above corresponds to an annual saving of SEK 500.000.

[0020] A great advantage of position valves with seat function, compared with conventional sliding valves, is that a set position is held constant for a very long time, while the edges of a sliding valve are eroded away, whereby the slide is displaced, which in its turn results in wrong board thickness.

[0021] A further advantage of the invention is that pressure reduction and degassing of the boards according to times T6, T7 and T8 in Fig. 6 can be controlled very precisely and uniformly about the entire press, whereby the degassing process is softened in spite of the fact that the times can be reduced. Time gains of the magnitude 2-4 seconds have been observed at pressings in full scale. Every second the pressing time is shortened corresponds at the same production as above to an annual saving of SEK 150.000.

[0022] The invention is not restricted to the embodiment described, but can be varied within the scope of the invention idea. A position valve 7,for example,can be used for two cylinders 1,if the rigidity of the press tables 2,5 is deemed sufficient. A second variant is to let the ramps move to a thinner than final thickness and thereafter move back. A third variant is to use traditional pressure levels or ramps alternatingly with position control according to the invention. A fourth variant is to utilize the invention for continuous presses. In this case a variable gear in the link system can be required, so that the admission key of the press gets the intended openings. The design of the equipment, of course, can also be varied within the scope of the invention. The piston 26 of the position valve 7,for example, can for practical reasons be designed to consist of several piston portions, which does not affect the function or above description.

Claims

1. An arrangement for controlling the thickness of board-shaped products (4) during the pressing process in a board press with a first and a second press table (5,2) where the pressing force is transferred by several hydraulic press cylinders (1), characterized in that a number of hydraulic position valves (7) are arranged in spaced relationship on both sides of the press on the first press table (5) and corresponding push-rods (9) are arranged on the second press table (2), which push-rods (9) are simultaneously adjustable by a common shifting motor (12).

2. An arrangement as defined in claim 1, characterized in that ball-screws (8) are used for adjusting the push-rods (9).

3. An arrangement as defined in claim 1 or 2, characterized in that a link system (11) is provided to transfer the adjustment of the shifting motor (12) to adjustment of the push-rods (9).

4. An arrangement as defined in any one of the preceding claims, characterized in that the position valves (7) are designed as double seat valves with restricted adjustment force.

5. An arrangement as defined in any one of the preceding claims, characterized in that the shifting motor (12) is preprogrammed with a control program for adjusting the push-rods (9) during the pressing process.

6. An arrangement as defined in claim 5, characterized in that the control programm is built up of ramps.

7. An arrangement as defined in any one of the preceding claims, characterized in that each position valve (7) is connected to a press cylinder (1).

8. An arrangement as defined in any one of the preceding claims, characterized in that all position valves (7) are supplied with pressure from a common pressure source (8).

9. An arrangement as defined in any one of the preceding claims, characterized in that a stroke limiter (14) is provided as overload protection between the shifting motor (12) and push-rods (9).

Drawing