CA1220406A - Method and apparatus for press position control - Google Patents
Method and apparatus for press position controlInfo
- Publication number
- CA1220406A CA1220406A CA000458155A CA458155A CA1220406A CA 1220406 A CA1220406 A CA 1220406A CA 000458155 A CA000458155 A CA 000458155A CA 458155 A CA458155 A CA 458155A CA 1220406 A CA1220406 A CA 1220406A
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- period
- pressing
- sheets
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Abstract
ABSTRACT OF THE DISCLOSURE
A method and apparatus for pressing plywood is disclosed. A stack of wood veneer sheets having a layer of adhesive arranged between successive sheets is pressed together between a pair of pressing platens with a given pressure for a first period of time. The platens are then held in a first relative platen press-ing position for a second period of time. The first relative position corresponds with the position reached by the pressing platens at the end of the first period of time. As a result of the pressing method, compression of the wood veneers is minimized, thereby permitting the use of thinner veneers. A transducer is provided for sensing the first relative pressing position of the platens at the end of the first period of time and for producing a signal during the second period of time corresponding with the deviation of the platens from the first relative position. In response to the deviation signal, the pressing platens are returned to the first position. In order to enhance the adhesive bond between successive sheets of the stack, heat is applied to the stack during the first and second periods of time.
A method and apparatus for pressing plywood is disclosed. A stack of wood veneer sheets having a layer of adhesive arranged between successive sheets is pressed together between a pair of pressing platens with a given pressure for a first period of time. The platens are then held in a first relative platen press-ing position for a second period of time. The first relative position corresponds with the position reached by the pressing platens at the end of the first period of time. As a result of the pressing method, compression of the wood veneers is minimized, thereby permitting the use of thinner veneers. A transducer is provided for sensing the first relative pressing position of the platens at the end of the first period of time and for producing a signal during the second period of time corresponding with the deviation of the platens from the first relative position. In response to the deviation signal, the pressing platens are returned to the first position. In order to enhance the adhesive bond between successive sheets of the stack, heat is applied to the stack during the first and second periods of time.
Description
~2~
METHOI~ hND APPARATUS ~OR PRESS POSITION CONTROT, ~ _ ____ _ _ _ACKGROUND OF THE _VENTION
Owing to the relatively high cost of lo~s and the low prices for commodity wood products such as plywood, every improvement in the efficiency of converting logs to wood products and every incremental reduction in the volume of wood required ~o manufacture a square foot of product resul*s in a substantial cost savings in the manufacture of the product. The preser.t invention relates to a new method and apparatus for pressing a stack of wood veneers into a sheet of pl~ood wherein compression and crushing of the wood veneers is mini~
mized. With the present invention, the input veneer thickness may be r~duced by three to four percent relative to conventional input thicknesses with no loss in the dimension or stren~th of the finished product.
BRIEF DESCRIPTION OF THE PRIOR ART
According to conventional plywood pressing tech-niques, a stack of wood veneers is pressed with asteady pressure of between lQ0 and 200 psi in a hot press at a temperature of about 300F for a given pL~riod of tim~ of between 2 and 15 minutes for curing. The resulting plywood panel has a loss in thickness ranging from six to eight percent relative to the combined thickness of the original softwood veneers.
This loss in thickness is a result of permanent crushing or densification of the wood which takes place in the hot press as the adhesive between the 3~ veneers ~ecomes fully cured under heat and pressure.
The~reasons for the crushing have been well established by microscopic examination of the cross-section and ' ~
surfaces of the veneer before and ~fter hot pressing.
Specifically, when the metal platens of the hot press close on an assembly of dried veneers, any remaining moisture in the wood is quickly turned to steam beginning at the surfaces against the hot platens and progressing toward the center of the panel. Vnder the influe~ce of this steam heat and the continuous 100 to 200 psi pressure of the platens, the wood becomes slightly plastic and "flows" into the voids within its surface and interior.
The amount of ~low and densification increas~s with in-creases in the pressure and temperature of the press~
the length of the press time, and the moisture content of the wood. Densification is also greater among woods of lower density and in veneers of rougher surfaces.
Thus in order to minimize panel thickness loss, low pressures and temperatures, short press times, and smoothly peeled dense, dry veneer should be used in the plywood manufacturing process. Owing to the different species of logs from which veneer is cut and to ~he necessity of achieving a strong adhesive bond between successive ~eneer sheets, there are limits to the degrees to which the above ~ac~ors may be reduced.
Because of these constraints, the minimal loss in panel thickness using conventional pressing techniques
METHOI~ hND APPARATUS ~OR PRESS POSITION CONTROT, ~ _ ____ _ _ _ACKGROUND OF THE _VENTION
Owing to the relatively high cost of lo~s and the low prices for commodity wood products such as plywood, every improvement in the efficiency of converting logs to wood products and every incremental reduction in the volume of wood required ~o manufacture a square foot of product resul*s in a substantial cost savings in the manufacture of the product. The preser.t invention relates to a new method and apparatus for pressing a stack of wood veneers into a sheet of pl~ood wherein compression and crushing of the wood veneers is mini~
mized. With the present invention, the input veneer thickness may be r~duced by three to four percent relative to conventional input thicknesses with no loss in the dimension or stren~th of the finished product.
BRIEF DESCRIPTION OF THE PRIOR ART
According to conventional plywood pressing tech-niques, a stack of wood veneers is pressed with asteady pressure of between lQ0 and 200 psi in a hot press at a temperature of about 300F for a given pL~riod of tim~ of between 2 and 15 minutes for curing. The resulting plywood panel has a loss in thickness ranging from six to eight percent relative to the combined thickness of the original softwood veneers.
This loss in thickness is a result of permanent crushing or densification of the wood which takes place in the hot press as the adhesive between the 3~ veneers ~ecomes fully cured under heat and pressure.
The~reasons for the crushing have been well established by microscopic examination of the cross-section and ' ~
surfaces of the veneer before and ~fter hot pressing.
Specifically, when the metal platens of the hot press close on an assembly of dried veneers, any remaining moisture in the wood is quickly turned to steam beginning at the surfaces against the hot platens and progressing toward the center of the panel. Vnder the influe~ce of this steam heat and the continuous 100 to 200 psi pressure of the platens, the wood becomes slightly plastic and "flows" into the voids within its surface and interior.
The amount of ~low and densification increas~s with in-creases in the pressure and temperature of the press~
the length of the press time, and the moisture content of the wood. Densification is also greater among woods of lower density and in veneers of rougher surfaces.
Thus in order to minimize panel thickness loss, low pressures and temperatures, short press times, and smoothly peeled dense, dry veneer should be used in the plywood manufacturing process. Owing to the different species of logs from which veneer is cut and to ~he necessity of achieving a strong adhesive bond between successive ~eneer sheets, there are limits to the degrees to which the above ~ac~ors may be reduced.
Because of these constraints, the minimal loss in panel thickness using conventional pressing techniques
2~ i5 between six and eight percent.
One technique of reducing permanent densification of plywood panels is to reduce the pressure applied to a panel during its cure without changing the bond quality of the adhesive. This has been accomplished by way of a dual pressure curing cycle wherein an initial high pressure is applied to a veneer stack during the first half of the curing cycle to provide adequate contact between successive sheets, followed by a second reduced pressure .
~,~
36 ~) applied during the sècond half of the cycle to limit compression of the wood. Typically, a dual pressure cycle for a ~ inch thick softwood ply~ood panel would comprise the application of 175 psi for t~o minutes followed by the ~lpplication of 100 psi for 1.5 n,inute~.
More recently, fo~lr stage pressing cycles have bee~ developed ~herein during each successive pressing stage, the pressure is sequentially reduced so tha~
the effects oE each pressure reduction on contact and adhesion are minimized.
Using the four stage pressure reduction technique, the loss in panel thickness has been reduced from betwee~
six and eight percent -to between four and five percent.
The present invention was developed in order to lS further minimize losses in panel thickness resulting from compression of the wood during the formation of plywood panels.
SUMMARY OF T~E INVENTION
Accordingly, it is a primary object o~ the present invention to provide a method and apparatus for pressing plywood from a stack of veneer sheets with minimal compression of the panel thickness, thereby permitting the use of thinner veneer sheets A plurality of veneer sheets are arranged in stacked relation with a layer of adhesive between sl~ccessive sheets. The stack of sheets is pressed together between a pair of pressing platens under a given pressure for a first period of time to establish a first relative pressing posilion.
The pressing platens are then held in the first relative position for a second period of time. Any deviation of the pressing platens ~rom the first relative position during the second period of time is autoTnatically corxected.
According to a n~ore specific object of the invention, heat is applied to the stack o~ sheets during both of the first and second periods of time.
According to another object of the inventi~n/ a transducer is p~ovided for sensing the first relative position of the pl~t~ns. The transducer produce~ a signal corresponding ~ith the deviatiorl of the pressing platens from the sensed posi~ion which is used to return the platens to the first relative position.
According to a further object of the invention, a micrcprocessor controlled hydraulic circuit is provided for applying the given pressure to the pressing platens during,the first period of time and for repositioning the platens to the first relative position during the second period of time.
RIEF DESCRIPTION OP THE FIGUR~S
Other objects and advantaqes of the present invention will become apparent from a study of the following speci~ic~tion when viewed in the light of the accompany-ing drawing, in which:
Fiy. 1 is a block diagram of the presspositioning system; and P~g. 2 is a detailed schematic diagram of the press posit.ion hydraulic circuit.
DETA}LED DESC~IPTION
Referring first to Fig. 17 the press positioning system for pressing plywood with a minimal amount of compression is shown. A plurality of wood ~eneer sheets 2 are arranged in a stack 4 between a pair of parallel ~z~
pressing platens 6, 8. The veneer sheets are cut rom a suitable softwood log and dried in a dryer prior to being arranged in stacked relation. Each veneer sheet has a thickness of approximately 0~097 inches which is approximately 0.003 inches thinner than the veneer used in convention~l ply~ood pressing techniques.
In order to join the veneer sheets -together, a layer of glue or adhesive such as phenol-formaldehyde resin is arranged between successive sheets of the stack.
The pressing platens 6, 8 are heated to approxi-mately 300~F in order to assist in curing the plywood during pressing. The upper pressing platen 8 is fixed and the lower platen 6 is displaceable relative thereto by a press ram 10 under the control of a press prefill valve 12 driven by a hydraulic circuit~ The hydraulic circuit is in turn controlled by a microprocessor 16.
A timer 18 is connected with the microprocessor and a three~position switch 20 is connected with the timer.
In accordance with the in~ention, the plywood curing cycle comprises two periods of time. During the first period of time Tl, a given pressure is applied to the stacX of wood veneer sheets 4 through displacement of the lower pressing platen 6 toward the upper platen 8. The given pressure may be in the range of 150 to 250 psi and preferably is 200 psi. During the second period of time T2, the pressing platens are held in a first relative platen pressing position attained at the end of the irst period of time.
As shown in the preferred embodiment of Fig. 1, the first period of time Tl lS selected by the three-position switch 20, each position corresponding with a different length of time in accordance with the thickness of the plywood panel being pressed~ For example, whe~e a one-half inch plywood panel is to be .:, ~2Z(3~6 pressed, the s~i~ch may be posikioned at setting Tla to set the timer 18 at 25 seconds. Setting Tlb would set the timer at 40 seconds where a five-eighths inch ply~ood panel is to be pressed, and setting TlC would set the timer at 60 seconds where a three-quarter inch paneL ls to be pressed. Of course, other settings and times may be pro~ided for establishing the firs-t period of time in the timer.
Upon the actuation of a press operation switch 21 by the press operator the press is closed and pressure accumulates on the lower pressing platen 6. When the given pressure is attained, the timer 18 begins timing the first period of time Tl set by switch 20 and the press cycle timer 23 begins timing the plywood curin~
cycle. During the first period of time, ~he microprocessor 16 con-trols the hydraulic circuit to displace the lower pressing platen 6 upwardly to press the stack of veneer sheets with the given pressure.
At the end of the first period of time Tl, the timer 18 signals the microprocessor 16 to begin the secand period of time during which the pressing platens are maintained in a ~irst relative platen pressing position carresponding with the position attained by the lower platen 6 at the end of the first period of time Tl.
A transducer position encoder device 22 is connected with the pressing platens and is operable to sense the first rela$ive position of the platens at the end of the first period of time Tl. It will be appreciated by those skllled in the art that the transducer 22 may 30 ~ be connected directly with the displaceable lower plaken 6 to sense its displacement relative to the fixed upper platen 8, thereby to determine the position :
:) of the lower pla~en at the end of the first perio~
of time. Alternatively, the transduce~r 22 may sense the ~istance X bet~een the pressing platens.
Owiny to excess pressure frvm the hy~a~lic circuit or bclck pressure from ~he veneer stacX 4/ there rnay be c~ slight ~eviation of the lower p~-essing platen 6 frotn the first relative position during the second perio~ of ti~ne T2. The ~eviation from the first relative position is ~etected by the trans~ucer 22 which pro~uces an output deviation signal D which is delivere~ to the microprocessor 16. In response to the ~eviation signal, the microprocessor delivers a correction signal C to servo valve 24 which is operable to meter flui~3 between the hydraùlic circuit an~ the press prefill valve 12 to reposition the lower pressing platen 6 to the fixed position.
As noted above, the plywoo~ curing cycle consists of the sum of the first and second periods of time.
A typical curing cycle would be approximately 5 minutes which is set or proqrammed in the press cycle timer 23.
With the pressing platens open, a stack of veneers is arranged therebetween. The first perio~ of time Tl is selecte~ via the three-position selector switc~ 20 in accor~ance with the thickness of the plywood panel being produced. After the press hydraulic circuit reaches the given pressure, the timer 18 begins counting the set time T1~ the press cycle timer 23 begins counting the plywood curing cycle, and the microprocessor 16 controls the hydraulic circuit to displace the lower pressing platen 6 towar~ the fixed platen 8 to apply a given pressure to the ~eneer stack~ The timer 18 indicates the end of the first period of time Tl to the microprocessor at which time the position of the lower -~z~
pressing plate~ G i5 .sense~ by the transducer 22 and fixe~ by the hy~raulic circuit via the xam 10 and s~rge v~lve 12. D~lring the second perio~l o~E time T2, the trans~lucer 22 senses all positive ~isplace1nent of the lvwer pressing platen v to~ar~ the upper platen 8 relative to the first relative platen position, as wel~
as all neyative ~isplacement of the lower pre~sing platen 6 away from the upper platen 8 relative to the first position. In response to the sensed displacement, the trans~ucer sen~C a deviation signal D to the microprocessor 16 which in turn sends a correction signal C to the servo valve 24. Where the sensed ~eviation is posi.tive, the servo valve is operable to meter hydraulic fluid from the press prefill valve 12, thereby reducing hydraulic pressure to the press ram 10 resulting in movement of the lower pressing platen 6 away from the upper platen 8 to the first relative position. On the other hand, where the sensed deYi3tion is negative, the servo valve 24 is operable in response to the correction signal C to meter hydra~lic fluid from the hy~raulic circuit to the press prefill valve 12, thereby to increase the pressure on the press ram 10 to move the lower pressing platen towar~ the upper platen to the first relati.ve position. It is apparent that the greater the ~eviation of the lower plate from the first relative platen position, the greater the correction provi~e~ by the servo valve.
In Fig. 2, a preferre~ embo~iment of the hydraulic circuit for positioning the Iower press platen in respo~se to signals from the microprocessox 16 is shown in detail. The hy~raulic circuit includes a variable ~elivery pump 26, a three-way pu~p control loop valve 28, a three-way servo loop valve 30, a three-way press :
_9_ direction control valve 32, a prefi.ll valve operator 34, and a plurality of hy~raulic lines fo:r i.ntegrat:ing the hydra~lic circuit with the press ~rarns 10, the pre~il.l valve 12, an~ the servo valve 24. As will be ~le~eloped bel.ow, an electrical le~ 36 ~e~i.vers the correction signal C from the microprocessor 16 to the ~rive solenoid 38 of the servo va:L~e, and an electrical lead 40 ~elivers a control signal to the solenoi~ 44 of the se~vo loop valve 3~.
Prior to operation of the press, the pump 26 is activated to deliver hydraulic flui~ such as oil through ports P an~ A of the servo loop valve 30.
Because the servo valve 24 is closed at this time, the oil is ~elivered to ports P an~ T of the press direction control. valve 32.
After a stack of veneer sheets has been arrange~
between the pressing platens, the lower pressing platen 6 is ~isplace~ upwardly to close the press. This is accomplished ~y shifting the press direction control valve 32. ~ore particularly, a signal from the press operation switch 21 through the electrical lea~ 42 actuates the press ~irection control valve first solenoi~ 46 to shift the valve to the left to its crossed position.
With the press ~irection control valve 32 shifte~
to its crosse~ position, oil flows from port P to port B thereof. A portion of the oil flows to close~ port 3 of the prefill valve 12, a portion of the oil is delivere~ to port 6 of the prefill valve operator 34, an~
the remainder of the oil is delivered ~irectly to a jack ram 48 which rapidly ~isplaces the lower press platen 6 upwar~ly to a closed position.
~Z~6 The prefill ~alve operator 34 includes a control valve 50 h~vinq a preset pressure setting. As oi.l is delivered to the port 6 oE the prefill val~e operator 39, pressure accumulate.s until the preset pressure oE
-the control va].ve 50 is reached. Once the preset pressu~e of the control valve 50 is reached, the preill valve operator 34 shifts the prefill val~e 12 to the left, thereby connecting ports 2, 3, and 5. With the prefill valve closed, oil from the pump is delivered to the main ram 52 to fully close the press, followinc~ which pressure is built up. A pressure swi.tch is provided in the hydraulic line to the main ram 52. ~hen the given pressure (i.e., 200 psi) is reached, the pressure switch. 54 delivers a first slgnal to the microprocessor which activates the timer18 to hegin countin~ the first period of time Tl estahlished by the switch 20 and a second signal to the press cycle timer 23 to begin counting the plywood curing cycle. During th~ ~irst period of time, the givPn pressure is continuously applied to the stack of veneer sheets via the lower pressing platen 6.
At the end of the first period of time Tl, the microprocessor 16l in response to a signal from the timer 18, energizes the solenoid 44 on the servo loop valve to shift the valve to the left to its crossed position. Accordinglyr oil from the pump 26 and port P
oE the servo loop valve will be diverted from port A to port B for delivery to p~rt 1 of the servo valve 24 via a high pressurefilter 56. Simultaneously, the pump control loop valve 28 is shifted in response to hydraulic fluid supplied from the servo loop valve 30 via the pilot line 60 to shift to the left., crossed position.
,~
:~2~
Accor~ingly, ~uring the secon~ period of time T2, the microprocessor, the hy~raulic systern, and the press are iJI the servo positioning mo~le wherein the Lo~er pressing platen ~ is fi~e~ in a first relati~e pla~en pressiny position achieve~ a~ the elltl of the firs~
perio~ of time Tl. With the pump contro~ loop ~Jalve ~8 in the crosse~ position, the pump i5 lOOp~ lore par~Licul~rly, the line from port 5 of the prefill valve 12 is connected to ports T and A o the pump control loop valve 28, with port A being blocke~l. Ports B and P of the pump control loop valve axe connecte~ to circu'ate oil back to a pump pressure control valve 62 which acts to limit the pump ~6 to its prese~ pressure.
As noted earlier, the transducer 22 senses the first relative platen pressing position of the lower pressing platen 6 at the end of the first perio~l of time T1 and ~elivers a dev.iation signal D to the microprocessor 16 ~uring the second period of time T~ correspon~ing with the ~eviation of the lower pressing platen from the first relative position resulting from excessive pressure, back pressure, or the like. In response to the deviation signal, the microprocessor ~elivers a correction signal C to the solenoi~ 38 of the servo valve 24 to control the me-terin~ of hy~raulic flui~ -to reposition the lower pressing platen 6 to the first relative position. The greater the ~e~iation signal, the greater the correction signal to correct -the press position.
Where the ~eviation si~nal is negative, indicatin~
that the lower pressing platen has backed off from the first relative position, the servo valve is operable to ~eliver hy~raulic fluid from the pump to the press rams via the prefill valve to raise the Iower pressing platen back to the first relative position. More particularly, the servo valve 24 is shifte~ to the left by the solenoi~
38 to ~eliver hy~raulic flui~ from the purnp 26 through ports l an~ 3 of the servo valve to the press ~irection control v~lve 32 via the check val~e 64. The fluid passes through ports P an~ ~ of the press ~lirectio~
control valve 32 for ~elivery ~o the jack ram 48 an~
to the main ram 5~ via the close~ prefil.l ~alve 12.
The greater the correction signal C, the greater the ~isplacement of the servo valve 24 to meter a greater amount o hy~raulic flui~ from the pump to the press ram for repositioning of the lower p~essing platen.
Where the deviation signal is positive, in~icating that the lower pressing platen has move~ upwar~ly beyon~
the first relative position, the servo valve is operable to blae~ hy~raulic fluid from the press rams to the tank 66. Specifically, the servo valve i9 shifte~ further to the left to close port l an~ to open a metered path between ports 3 an~ 4 thereof. Oil flows from the main ~0 press ram 5~ into port 2 of the prefill valve 12, out port 3 of the closed prefill valve, through ports B an~
P o the press ~irection control valve 32, through a high pressure filter 68 an~ to port 3 of th~ servo valve where the oil is metere~ to the tank 66 via port 4 of the servo valve. The greater the cor:rection signal, the greater the volume of oil metere~ to the tank to lower the lower pressing platen to the first relative position.
Repositioning of the lower pressing platen to the first relative position sensed by the transducer occurs continuously throughout the second~perio~ of time T2.
, ~.22~
At the en~3 of the secon~ perio~ of time T2, an~
accordingly, the end of ~he plywoo~ cu~inq cycle, the p~ess cycle timer 23 ~lelivers a signal to the microprocessor 16 to ~le-energize the ser~o vaL~e solenoi~ 38 an~ the se~vo loop valve solenoi.l 44 ~o close the servo ~ialve 24 an~l to reposition ~he servo loop valve 30 to its center positio~ shown in Fig. 2.
Repositioning of the servo loop valve 30 removes h~draulLc flui~ from the pilot line 60 to the p~1mp loop control valve 28 which accor~ingly is also repositione~ to its center position. The press cycle timer 23 also simultaneously delivers a signal to the secon-3 solenoi~ 70 of the press ~irection control valve via a lead 72 to shift the valve to i~s far right position connecting port P with port A an~ port T with port B.
With the servo valve 24 now close~, oil from the pump 26 i5 delivere~ through ports P an~ A of the press ~irection control valve 32 to port 4 of the prefill valve operator 3g to open the prefill valve 12. Oil from the jack ram 48 flows through ports B an~ T of the press ~irection control valve 3~ to the tank 72~ Oil from the main ram 52 Plows through port 2 an~ port 1 of the prefill valve 12 to the tank 74, thereby allowing the lower pressing platen to ~rop to open the press for removal of the cured plywood and insertion of the next stack of veneer sheets to be presse~.
While only a single press has been ~escribe~ in connection with the invention, it will be apparent to those skilled in the art that a plurality of parallel presses may be controlle~ by the invention. Similarly, while the press ram is shown comprising a jack ram an~ a main ram, which are preferably single acting , c rams, any number of press rams may be provi~e-l for displacem~nt of the lower pressing platen.
Other suitable control tneans for controlling the operation of the hydraulic pressing circuit may be use-l in place of the microprocessor/timer. For example, the hydr~ulic system co~l~ be manually controlle~ to provide a given press~re to the press for a first period of time, following which the entire system could be locke~ to maintain the press position constant ~uring a second period of time. Alterna~i~ely, an electromagnetic sensor ana control may be use~ to control the repositioning of the press to a first relative platen pressing position.
One advantage realize~ by the preferre~ embo~iment of the invention illustrate~ in Figs. l an~ 2 is that the f irst relative position of the press may be correcte~
by the servo valve to within a toleranc~ of 0~001 inches.
With this accurate repositioning of the press, the press is in effect maintaine~ in a constant position~
thereby reducing densification of the woo~ venesrs ~urin~ the curing process to between two an~ three percent an~ thus permitting the use of thinner veneers.
While in accor~ance with the provisions of the Patent Statutes the preferred forms and e~bo~iments of the invention have been illustrated an~ ~escribe~, it will be apparent to those skilled in the art that various changes and modifications may be made without ~eviating from the inventive concepts set forth above.
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: .
One technique of reducing permanent densification of plywood panels is to reduce the pressure applied to a panel during its cure without changing the bond quality of the adhesive. This has been accomplished by way of a dual pressure curing cycle wherein an initial high pressure is applied to a veneer stack during the first half of the curing cycle to provide adequate contact between successive sheets, followed by a second reduced pressure .
~,~
36 ~) applied during the sècond half of the cycle to limit compression of the wood. Typically, a dual pressure cycle for a ~ inch thick softwood ply~ood panel would comprise the application of 175 psi for t~o minutes followed by the ~lpplication of 100 psi for 1.5 n,inute~.
More recently, fo~lr stage pressing cycles have bee~ developed ~herein during each successive pressing stage, the pressure is sequentially reduced so tha~
the effects oE each pressure reduction on contact and adhesion are minimized.
Using the four stage pressure reduction technique, the loss in panel thickness has been reduced from betwee~
six and eight percent -to between four and five percent.
The present invention was developed in order to lS further minimize losses in panel thickness resulting from compression of the wood during the formation of plywood panels.
SUMMARY OF T~E INVENTION
Accordingly, it is a primary object o~ the present invention to provide a method and apparatus for pressing plywood from a stack of veneer sheets with minimal compression of the panel thickness, thereby permitting the use of thinner veneer sheets A plurality of veneer sheets are arranged in stacked relation with a layer of adhesive between sl~ccessive sheets. The stack of sheets is pressed together between a pair of pressing platens under a given pressure for a first period of time to establish a first relative pressing posilion.
The pressing platens are then held in the first relative position for a second period of time. Any deviation of the pressing platens ~rom the first relative position during the second period of time is autoTnatically corxected.
According to a n~ore specific object of the invention, heat is applied to the stack o~ sheets during both of the first and second periods of time.
According to another object of the inventi~n/ a transducer is p~ovided for sensing the first relative position of the pl~t~ns. The transducer produce~ a signal corresponding ~ith the deviatiorl of the pressing platens from the sensed posi~ion which is used to return the platens to the first relative position.
According to a further object of the invention, a micrcprocessor controlled hydraulic circuit is provided for applying the given pressure to the pressing platens during,the first period of time and for repositioning the platens to the first relative position during the second period of time.
RIEF DESCRIPTION OP THE FIGUR~S
Other objects and advantaqes of the present invention will become apparent from a study of the following speci~ic~tion when viewed in the light of the accompany-ing drawing, in which:
Fiy. 1 is a block diagram of the presspositioning system; and P~g. 2 is a detailed schematic diagram of the press posit.ion hydraulic circuit.
DETA}LED DESC~IPTION
Referring first to Fig. 17 the press positioning system for pressing plywood with a minimal amount of compression is shown. A plurality of wood ~eneer sheets 2 are arranged in a stack 4 between a pair of parallel ~z~
pressing platens 6, 8. The veneer sheets are cut rom a suitable softwood log and dried in a dryer prior to being arranged in stacked relation. Each veneer sheet has a thickness of approximately 0~097 inches which is approximately 0.003 inches thinner than the veneer used in convention~l ply~ood pressing techniques.
In order to join the veneer sheets -together, a layer of glue or adhesive such as phenol-formaldehyde resin is arranged between successive sheets of the stack.
The pressing platens 6, 8 are heated to approxi-mately 300~F in order to assist in curing the plywood during pressing. The upper pressing platen 8 is fixed and the lower platen 6 is displaceable relative thereto by a press ram 10 under the control of a press prefill valve 12 driven by a hydraulic circuit~ The hydraulic circuit is in turn controlled by a microprocessor 16.
A timer 18 is connected with the microprocessor and a three~position switch 20 is connected with the timer.
In accordance with the in~ention, the plywood curing cycle comprises two periods of time. During the first period of time Tl, a given pressure is applied to the stacX of wood veneer sheets 4 through displacement of the lower pressing platen 6 toward the upper platen 8. The given pressure may be in the range of 150 to 250 psi and preferably is 200 psi. During the second period of time T2, the pressing platens are held in a first relative platen pressing position attained at the end of the irst period of time.
As shown in the preferred embodiment of Fig. 1, the first period of time Tl lS selected by the three-position switch 20, each position corresponding with a different length of time in accordance with the thickness of the plywood panel being pressed~ For example, whe~e a one-half inch plywood panel is to be .:, ~2Z(3~6 pressed, the s~i~ch may be posikioned at setting Tla to set the timer 18 at 25 seconds. Setting Tlb would set the timer at 40 seconds where a five-eighths inch ply~ood panel is to be pressed, and setting TlC would set the timer at 60 seconds where a three-quarter inch paneL ls to be pressed. Of course, other settings and times may be pro~ided for establishing the firs-t period of time in the timer.
Upon the actuation of a press operation switch 21 by the press operator the press is closed and pressure accumulates on the lower pressing platen 6. When the given pressure is attained, the timer 18 begins timing the first period of time Tl set by switch 20 and the press cycle timer 23 begins timing the plywood curin~
cycle. During the first period of time, ~he microprocessor 16 con-trols the hydraulic circuit to displace the lower pressing platen 6 upwardly to press the stack of veneer sheets with the given pressure.
At the end of the first period of time Tl, the timer 18 signals the microprocessor 16 to begin the secand period of time during which the pressing platens are maintained in a ~irst relative platen pressing position carresponding with the position attained by the lower platen 6 at the end of the first period of time Tl.
A transducer position encoder device 22 is connected with the pressing platens and is operable to sense the first rela$ive position of the platens at the end of the first period of time Tl. It will be appreciated by those skllled in the art that the transducer 22 may 30 ~ be connected directly with the displaceable lower plaken 6 to sense its displacement relative to the fixed upper platen 8, thereby to determine the position :
:) of the lower pla~en at the end of the first perio~
of time. Alternatively, the transduce~r 22 may sense the ~istance X bet~een the pressing platens.
Owiny to excess pressure frvm the hy~a~lic circuit or bclck pressure from ~he veneer stacX 4/ there rnay be c~ slight ~eviation of the lower p~-essing platen 6 frotn the first relative position during the second perio~ of ti~ne T2. The ~eviation from the first relative position is ~etected by the trans~ucer 22 which pro~uces an output deviation signal D which is delivere~ to the microprocessor 16. In response to the ~eviation signal, the microprocessor delivers a correction signal C to servo valve 24 which is operable to meter flui~3 between the hydraùlic circuit an~ the press prefill valve 12 to reposition the lower pressing platen 6 to the fixed position.
As noted above, the plywoo~ curing cycle consists of the sum of the first and second periods of time.
A typical curing cycle would be approximately 5 minutes which is set or proqrammed in the press cycle timer 23.
With the pressing platens open, a stack of veneers is arranged therebetween. The first perio~ of time Tl is selecte~ via the three-position selector switc~ 20 in accor~ance with the thickness of the plywood panel being produced. After the press hydraulic circuit reaches the given pressure, the timer 18 begins counting the set time T1~ the press cycle timer 23 begins counting the plywood curing cycle, and the microprocessor 16 controls the hydraulic circuit to displace the lower pressing platen 6 towar~ the fixed platen 8 to apply a given pressure to the ~eneer stack~ The timer 18 indicates the end of the first period of time Tl to the microprocessor at which time the position of the lower -~z~
pressing plate~ G i5 .sense~ by the transducer 22 and fixe~ by the hy~raulic circuit via the xam 10 and s~rge v~lve 12. D~lring the second perio~l o~E time T2, the trans~lucer 22 senses all positive ~isplace1nent of the lvwer pressing platen v to~ar~ the upper platen 8 relative to the first relative platen position, as wel~
as all neyative ~isplacement of the lower pre~sing platen 6 away from the upper platen 8 relative to the first position. In response to the sensed displacement, the trans~ucer sen~C a deviation signal D to the microprocessor 16 which in turn sends a correction signal C to the servo valve 24. Where the sensed ~eviation is posi.tive, the servo valve is operable to meter hydraulic fluid from the press prefill valve 12, thereby reducing hydraulic pressure to the press ram 10 resulting in movement of the lower pressing platen 6 away from the upper platen 8 to the first relative position. On the other hand, where the sensed deYi3tion is negative, the servo valve 24 is operable in response to the correction signal C to meter hydra~lic fluid from the hy~raulic circuit to the press prefill valve 12, thereby to increase the pressure on the press ram 10 to move the lower pressing platen towar~ the upper platen to the first relati.ve position. It is apparent that the greater the ~eviation of the lower plate from the first relative platen position, the greater the correction provi~e~ by the servo valve.
In Fig. 2, a preferre~ embo~iment of the hydraulic circuit for positioning the Iower press platen in respo~se to signals from the microprocessox 16 is shown in detail. The hy~raulic circuit includes a variable ~elivery pump 26, a three-way pu~p control loop valve 28, a three-way servo loop valve 30, a three-way press :
_9_ direction control valve 32, a prefi.ll valve operator 34, and a plurality of hy~raulic lines fo:r i.ntegrat:ing the hydra~lic circuit with the press ~rarns 10, the pre~il.l valve 12, an~ the servo valve 24. As will be ~le~eloped bel.ow, an electrical le~ 36 ~e~i.vers the correction signal C from the microprocessor 16 to the ~rive solenoid 38 of the servo va:L~e, and an electrical lead 40 ~elivers a control signal to the solenoi~ 44 of the se~vo loop valve 3~.
Prior to operation of the press, the pump 26 is activated to deliver hydraulic flui~ such as oil through ports P an~ A of the servo loop valve 30.
Because the servo valve 24 is closed at this time, the oil is ~elivered to ports P an~ T of the press direction control. valve 32.
After a stack of veneer sheets has been arrange~
between the pressing platens, the lower pressing platen 6 is ~isplace~ upwardly to close the press. This is accomplished ~y shifting the press direction control valve 32. ~ore particularly, a signal from the press operation switch 21 through the electrical lea~ 42 actuates the press ~irection control valve first solenoi~ 46 to shift the valve to the left to its crossed position.
With the press ~irection control valve 32 shifte~
to its crosse~ position, oil flows from port P to port B thereof. A portion of the oil flows to close~ port 3 of the prefill valve 12, a portion of the oil is delivere~ to port 6 of the prefill valve operator 34, an~
the remainder of the oil is delivered ~irectly to a jack ram 48 which rapidly ~isplaces the lower press platen 6 upwar~ly to a closed position.
~Z~6 The prefill ~alve operator 34 includes a control valve 50 h~vinq a preset pressure setting. As oi.l is delivered to the port 6 oE the prefill val~e operator 39, pressure accumulate.s until the preset pressure oE
-the control va].ve 50 is reached. Once the preset pressu~e of the control valve 50 is reached, the preill valve operator 34 shifts the prefill val~e 12 to the left, thereby connecting ports 2, 3, and 5. With the prefill valve closed, oil from the pump is delivered to the main ram 52 to fully close the press, followinc~ which pressure is built up. A pressure swi.tch is provided in the hydraulic line to the main ram 52. ~hen the given pressure (i.e., 200 psi) is reached, the pressure switch. 54 delivers a first slgnal to the microprocessor which activates the timer18 to hegin countin~ the first period of time Tl estahlished by the switch 20 and a second signal to the press cycle timer 23 to begin counting the plywood curing cycle. During th~ ~irst period of time, the givPn pressure is continuously applied to the stack of veneer sheets via the lower pressing platen 6.
At the end of the first period of time Tl, the microprocessor 16l in response to a signal from the timer 18, energizes the solenoid 44 on the servo loop valve to shift the valve to the left to its crossed position. Accordinglyr oil from the pump 26 and port P
oE the servo loop valve will be diverted from port A to port B for delivery to p~rt 1 of the servo valve 24 via a high pressurefilter 56. Simultaneously, the pump control loop valve 28 is shifted in response to hydraulic fluid supplied from the servo loop valve 30 via the pilot line 60 to shift to the left., crossed position.
,~
:~2~
Accor~ingly, ~uring the secon~ period of time T2, the microprocessor, the hy~raulic systern, and the press are iJI the servo positioning mo~le wherein the Lo~er pressing platen ~ is fi~e~ in a first relati~e pla~en pressiny position achieve~ a~ the elltl of the firs~
perio~ of time Tl. With the pump contro~ loop ~Jalve ~8 in the crosse~ position, the pump i5 lOOp~ lore par~Licul~rly, the line from port 5 of the prefill valve 12 is connected to ports T and A o the pump control loop valve 28, with port A being blocke~l. Ports B and P of the pump control loop valve axe connecte~ to circu'ate oil back to a pump pressure control valve 62 which acts to limit the pump ~6 to its prese~ pressure.
As noted earlier, the transducer 22 senses the first relative platen pressing position of the lower pressing platen 6 at the end of the first perio~l of time T1 and ~elivers a dev.iation signal D to the microprocessor 16 ~uring the second period of time T~ correspon~ing with the ~eviation of the lower pressing platen from the first relative position resulting from excessive pressure, back pressure, or the like. In response to the deviation signal, the microprocessor ~elivers a correction signal C to the solenoi~ 38 of the servo valve 24 to control the me-terin~ of hy~raulic flui~ -to reposition the lower pressing platen 6 to the first relative position. The greater the ~e~iation signal, the greater the correction signal to correct -the press position.
Where the ~eviation si~nal is negative, indicatin~
that the lower pressing platen has backed off from the first relative position, the servo valve is operable to ~eliver hy~raulic fluid from the pump to the press rams via the prefill valve to raise the Iower pressing platen back to the first relative position. More particularly, the servo valve 24 is shifte~ to the left by the solenoi~
38 to ~eliver hy~raulic flui~ from the purnp 26 through ports l an~ 3 of the servo valve to the press ~irection control v~lve 32 via the check val~e 64. The fluid passes through ports P an~ ~ of the press ~lirectio~
control valve 32 for ~elivery ~o the jack ram 48 an~
to the main ram 5~ via the close~ prefil.l ~alve 12.
The greater the correction signal C, the greater the ~isplacement of the servo valve 24 to meter a greater amount o hy~raulic flui~ from the pump to the press ram for repositioning of the lower p~essing platen.
Where the deviation signal is positive, in~icating that the lower pressing platen has move~ upwar~ly beyon~
the first relative position, the servo valve is operable to blae~ hy~raulic fluid from the press rams to the tank 66. Specifically, the servo valve i9 shifte~ further to the left to close port l an~ to open a metered path between ports 3 an~ 4 thereof. Oil flows from the main ~0 press ram 5~ into port 2 of the prefill valve 12, out port 3 of the closed prefill valve, through ports B an~
P o the press ~irection control valve 32, through a high pressure filter 68 an~ to port 3 of th~ servo valve where the oil is metere~ to the tank 66 via port 4 of the servo valve. The greater the cor:rection signal, the greater the volume of oil metere~ to the tank to lower the lower pressing platen to the first relative position.
Repositioning of the lower pressing platen to the first relative position sensed by the transducer occurs continuously throughout the second~perio~ of time T2.
, ~.22~
At the en~3 of the secon~ perio~ of time T2, an~
accordingly, the end of ~he plywoo~ cu~inq cycle, the p~ess cycle timer 23 ~lelivers a signal to the microprocessor 16 to ~le-energize the ser~o vaL~e solenoi~ 38 an~ the se~vo loop valve solenoi.l 44 ~o close the servo ~ialve 24 an~l to reposition ~he servo loop valve 30 to its center positio~ shown in Fig. 2.
Repositioning of the servo loop valve 30 removes h~draulLc flui~ from the pilot line 60 to the p~1mp loop control valve 28 which accor~ingly is also repositione~ to its center position. The press cycle timer 23 also simultaneously delivers a signal to the secon-3 solenoi~ 70 of the press ~irection control valve via a lead 72 to shift the valve to i~s far right position connecting port P with port A an~ port T with port B.
With the servo valve 24 now close~, oil from the pump 26 i5 delivere~ through ports P an~ A of the press ~irection control valve 32 to port 4 of the prefill valve operator 3g to open the prefill valve 12. Oil from the jack ram 48 flows through ports B an~ T of the press ~irection control valve 3~ to the tank 72~ Oil from the main ram 52 Plows through port 2 an~ port 1 of the prefill valve 12 to the tank 74, thereby allowing the lower pressing platen to ~rop to open the press for removal of the cured plywood and insertion of the next stack of veneer sheets to be presse~.
While only a single press has been ~escribe~ in connection with the invention, it will be apparent to those skilled in the art that a plurality of parallel presses may be controlle~ by the invention. Similarly, while the press ram is shown comprising a jack ram an~ a main ram, which are preferably single acting , c rams, any number of press rams may be provi~e-l for displacem~nt of the lower pressing platen.
Other suitable control tneans for controlling the operation of the hydraulic pressing circuit may be use-l in place of the microprocessor/timer. For example, the hydr~ulic system co~l~ be manually controlle~ to provide a given press~re to the press for a first period of time, following which the entire system could be locke~ to maintain the press position constant ~uring a second period of time. Alterna~i~ely, an electromagnetic sensor ana control may be use~ to control the repositioning of the press to a first relative platen pressing position.
One advantage realize~ by the preferre~ embo~iment of the invention illustrate~ in Figs. l an~ 2 is that the f irst relative position of the press may be correcte~
by the servo valve to within a toleranc~ of 0~001 inches.
With this accurate repositioning of the press, the press is in effect maintaine~ in a constant position~
thereby reducing densification of the woo~ venesrs ~urin~ the curing process to between two an~ three percent an~ thus permitting the use of thinner veneers.
While in accor~ance with the provisions of the Patent Statutes the preferred forms and e~bo~iments of the invention have been illustrated an~ ~escribe~, it will be apparent to those skilled in the art that various changes and modifications may be made without ~eviating from the inventive concepts set forth above.
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Claims (12)
1. A method for making plywood from a stack of wood veneer sheets having a layer of adhesive arranged, between suc-cessive sheets of the stack, comprising the steps of (a) pressing the stack of veneer sheets together be-tween a pair of pressing platens under a given pressure for a first period of time (T1), thereby to establish a first relative platen pressing position (X), said given pressure and said first period of time being less than that required to produce complete densification of the sheets; and (b) continually adjusting said pressing platen to main-tain said first relative position for a second period of time (T2), whereby the reduction in thick-ness of the veneer sheets is minimized, thereby permitting the use of thinner veneer sheets.
2. The method as defined in claim 1, and further including the steps of (c) sensing a deviation of said platens from said first relative position; and (d) returning said platens to said first relative position.
3. A method as defined in claim 2, and further comprising the step of continuously heating the stack of veneer sheets at a given temperature during both said first and second periods of time.
4. Apparatus for producing plywood from a stack of wood veneer sheets having a layer of adhesive arranged between successive sheets of the stack, comprising (a) press means including a pair of relatively movable initially-spaced parallel pressing platens for receiving the stack of sheets therebetween;
(b) means for displacing said platens together under a given pressure for a first period of time (T1), thereby to establish a first relative platen press-ing position (X), said given pressure and said first period of time being less than that required to produce complete densification of the sheets; and (c) control means for continually adjusting said platens to maintain said first relative position for a second period of time (T2), whereby the re-duction in thickness of the veneer sheets is minimized, thereby permitting the use of thinner veneer sheets.
(b) means for displacing said platens together under a given pressure for a first period of time (T1), thereby to establish a first relative platen press-ing position (X), said given pressure and said first period of time being less than that required to produce complete densification of the sheets; and (c) control means for continually adjusting said platens to maintain said first relative position for a second period of time (T2), whereby the re-duction in thickness of the veneer sheets is minimized, thereby permitting the use of thinner veneer sheets.
5. Apparatus as defined in claim 4, wherein said control means comprises (d) sensing means for sensing the deviation of said platens from said position; and (e) means for returning said platens toward said first relative position.
6. Apparatus as defined in claim 5, wherein said displacing means includes (1) press ram means connected with at least one of said platens;
(2) pump means for supplying hydraulic fluid at a given fluid pressure; and 13) prefill valve means normally operable to supply fluid at said given fluid pressure from said pump means to said press ram means.
(2) pump means for supplying hydraulic fluid at a given fluid pressure; and 13) prefill valve means normally operable to supply fluid at said given fluid pressure from said pump means to said press ram means.
7. Apparatus for producing plywood from a stack of wood veneer sheets having a layer of adhesive arranged between successive sheets of the stack, comprising (a) press means including a pair of relatively movable initially-spaced parallel pressing platens for receiving the stack of sheets therebetween;
(b) means for displacing said platens together under a given pressure for a first period of time (Tl), thereby to establish a first relative platen pressing position (X), said given pressure and said first period of time being less than that required to produce complete densification of the sheets, said displacing means including (1) press ram means connected with at least one of said platens;
(2) pump means for supplying hydraulic fluid at a given fluid pressue: and (3) prefill valve means normally operable to supply fluid at said given fluid pressure from said pump means to said press ram means; and (c) control means for continually adjusting said platens to maintain said first relative position for a second period of time (T2), said control means including (1) sensing means for sensing the deviation of said platens from said first position; and (2) means for returning said platens toward said first relative position, including (i) servo valve means for metering fluid between said prefill valve means and said pump means; and (ii) microprocessor means for controlling said servo valve means in response to the deviation sensed by said sensing means, thereby to return said platens to said first relative position, whereby the reduction in thickness of the veneer sheets is minimized to permit the use of thinner veneer sheets.
(b) means for displacing said platens together under a given pressure for a first period of time (Tl), thereby to establish a first relative platen pressing position (X), said given pressure and said first period of time being less than that required to produce complete densification of the sheets, said displacing means including (1) press ram means connected with at least one of said platens;
(2) pump means for supplying hydraulic fluid at a given fluid pressue: and (3) prefill valve means normally operable to supply fluid at said given fluid pressure from said pump means to said press ram means; and (c) control means for continually adjusting said platens to maintain said first relative position for a second period of time (T2), said control means including (1) sensing means for sensing the deviation of said platens from said first position; and (2) means for returning said platens toward said first relative position, including (i) servo valve means for metering fluid between said prefill valve means and said pump means; and (ii) microprocessor means for controlling said servo valve means in response to the deviation sensed by said sensing means, thereby to return said platens to said first relative position, whereby the reduction in thickness of the veneer sheets is minimized to permit the use of thinner veneer sheets.
8. Apparatus as defined in claim 7 , and further including pump loop control valve means for controlling said pump means to maintain said given fluid pressure.
9. Apparatus as defined in claim 7 , and further including servo loop valve means for controlling the flow of fluid from said pump means to said servo valve.
10. Apparatus as defined in claim 9, and further including press direction control valve means for opening and closing said press means and for controlling the supply of a fluid to said prefill valve means.
11. Apparatus as defined in claim 10, wherein said microprocessor means controls the operation of said servo loop valve means.
12. Apparatus as defined in claim 11, and further including timer means connected with said microprocessor means for establishing said first period of time.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US56974684A | 1984-01-10 | 1984-01-10 | |
| US569,746 | 1984-01-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1220406A true CA1220406A (en) | 1987-04-14 |
Family
ID=24276679
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000458155A Expired CA1220406A (en) | 1984-01-10 | 1984-07-05 | Method and apparatus for press position control |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1220406A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102632525A (en) * | 2012-04-25 | 2012-08-15 | 北京满融建筑材料技术研究所 | Preparation method of tissue wood grain paper decoration polyester plywood |
| CN113799223A (en) * | 2021-10-20 | 2021-12-17 | 惠州市盛源木业有限公司 | Cold press is used in plank production |
-
1984
- 1984-07-05 CA CA000458155A patent/CA1220406A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102632525A (en) * | 2012-04-25 | 2012-08-15 | 北京满融建筑材料技术研究所 | Preparation method of tissue wood grain paper decoration polyester plywood |
| CN113799223A (en) * | 2021-10-20 | 2021-12-17 | 惠州市盛源木业有限公司 | Cold press is used in plank production |
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