CA1249115A - Method and apparatus for manufacturing composite wood panels - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An improved hot gas pressing system for use in manufacturing wood-based composite panels reduces in-press time substantially while reducing blistering, pitting, and warping in the final panel. Condensable steam as the preferred gas is injected into both faces of the mat after the press closes to an intermediate position compressing the mat to an inter-mediate density. After the steam is applied for a predetermined time period at the intermediate density quickly raising the mat temperature, a steam-through step is applied after which the press is closed to its final position. Steam is reapplied to both surfaces of the densified mat to maintain temperature further reducing cure time of the adhesive after which venting and vacuum steps are applied to both surfaces of the mat to reduce internal pressure and remove moisture from the mat prior to opening of the press.

Description

?RESSIN~ PROÇESS i~'~R COI~1POSITE ~\IOOD PAi~ELS

BACKGROUND OF THE INVENTION
This invention relates ~enerally to the pressing process for manufacturing composite wood-hascd panel prDducts. ~lore particularly, it rclutes to un impro~ed pressing process wherein steQm or other suitable condensable hot gas is injected into a wood furnish-~dhesive ,~t during the pressing cycle, and apparatus for oper~ting th~ process.
In the manufacture of composlte wood-oased panel products, wood pQrticles in various forms are combined with thermosetting or sometimes thermoplastic binder systems and formed into loosely compacted nats. The mat is then pressed to final thickness and density under ?ressure I0 and elevated temperatures while the adhesive is cured. Thc wood particles can be in fiber form, flake form, particulate form, strand form and other forms that are known in the industry. The generic end products that result are referred to by a variety of names such as fiberboard, hardboard, flakeboard, strandboard, particleboard, and waferboard and indicate the constituent type particulate material within the product. In the case of hardbo~rd or medium density fiberboard, there is also an indication of the produet density- Each pr~duct however, is characterized by being ~n~n~
factured ~Jith wood particulate material and an adhesive system to bind the wood together- ~hese panel products h~ve a variety of well-known end u~ses.
u~ a typic~1 m~nufacturin~ process, using fiberbo~rd ~s an example, a refining station reduces the incoming wood raw material to fiber îorm. The fiber is then dried ~nd directed to a blending station where the thermosetting resin Ss added in a controlled manner and from there to a forming station where the fiber-resin m~xture is formed into loosely compacted mats. The mats can be forrned individually top cauls, although more typically the mat is continuously formed atop a rnoving supporting structure such as an endless belt. ~fter the mat is formed, it must 'oe comp~cted and the fiber-resin mixture pressed to thickness and final density ~t the pressing station. A prepressing station is normally employed to initially reduce the mat thickness and density to manageable levels prior to entry into the final pressing station. Typic~lly, individual ~ats are then loaded into a 2laten ~ot press which is then closed and the resin allowed to '~

, .

~ure. `~ior~ recently ihlgle openin~, ~ua~ ntinuous presses luve been ~tili~ed to ~ress long ~ats of the Nood-resin .ni.Yture. ~he cure time ~an vary depending upon resin type, final p~nel thickiless, and densitJ, but ~or typical ,nedium density ~iberboard panel product having a thickness of l9mm (3/4"), the cure time is approximately 7-8 minutes.
~ he final board or panel product shoLIld have properties falling within the predetermined ranges for all panel characteristica under control.
The density should be controlled as ,hould the panel thic!cness. The surface should be smooth, uniform, and free from blemishes.
In typical prior art pressing systems utilizing hot pla~ens, the resin cure time is determined, in part, by the heat transfer into the mat once the platens compress the mat. Heat must be distributed throughout the fiber-resin mixture in order to bring the entire volume of material up to the desired cure temperature. When only the conductive heat transfer 15 vehicle is utilized, the time required to uniformaly heat the mat and cure the resin is significant.
It has been proposed in the past to use hot gases, such as steam, as a heat transfer medium to bring the unconsolidated or partially consoli-dated mat temperature up to the desired curing temperature quickly and to 20 reduce consolidation pressures. For e~cample, U.S. Patent 3,280,237 assigned to the assignee of the present invention discloses the use of a superheated steam injection method to improve the pressing process in the manufacture of composite wood panel products. By utilizing superheated steam injected into the porous mat, the cure times were reduced significantly.
The process, as disclosed in U.S. Patent 3,280,237, while having pressing times significantly lower than state-of-the-art press cycles, did not become commercially feasible primarily because of problems with product quality but also because of the requirement for superheated steam whicin is expensive to generate. It was found that an unacceptable number of p~nels 30 coming out of the press were affected with blistering, surfsce pitting, ~md panel warping. It was determined the blisters were caused by incomplete steam penetration. This effect results in uncured resin and therefore structurally ~eak or unsound areas in the panel. Such panels are either unacceptable entirely or they must be degraded into a less valuable product 35 going to different end uses.
Surface pitting was found to be caused by the impuct of the steam flGw as it ~as injected into the mat thr~ugh holes in the platen. The ~ss dnd velocity of the ~team flow waS found to disturb the .ioer-rcs mixture in its uncured form directly under the ste~m injection ~oles. Such surface pitting is undesirable snd can result in degradin~ a panel product into a lower gr~lde.
Finally, the panel warping was the result of steam injection from one platen only. This resulted in the p~nel surfaces not having equal physical properties or uniform moisture leYels after pressing. While press cycle time was reduced, the product quality w~s generally unacceptable and therefore the ~team pressing process as disclosed in V.S. Patent 3,280,237 did not become commercially viable. It has as a disadvantage the requirement ~or superheated steam which is expensive as a heat transfer medium in Q steam pressing process. Ideally, although not a requirement for practicing the present invention, saturated steam of high quality should be used as its heat of condensation can be used effectively in quickly raising the temperature of the m~t and because it is less costly to generate than superheated steam.
While the potential benefits to be derived through the use of hot gases injected into a wood-resin mixture during the pressing cycle were known, a process had not been developed to successfully reduce press cycle times while producing acceptable panels of the desired grade. An improve-ment in the pressing system was needed to make it commercially feasible for implementation.
Accordingly, from the foregoing~ one objective of the present inYentiGn is to reduce or eliminat~ blister, pitting and warping problems when utilizing hot gas injection to reduce press cycle timesO
Another objective of the present invention is a methodology to predict the appropriate parameter vPlues for hot gas pressing cycles for p~nels of various thicknesses and final densities.
These and other objectives o the present invention will become more apparent upon reading the description of the preferred embodiment in conjunction with tlle attached drawings.
SUMMARY OF THE INVENTION
The present ir.Yention is pr~cticed in one form by placing a wood particulate adhesive ~nixture in mat form b~t~een hot F~S injection press p]atens. The hot gas is p~eferably a vapor such as saturated steam.
The mat will be pressed to a predetermined intermedia~e density, and then an initial period of hot ,:

- 4 ~

gas injection will be applied to koth mat surfaces at a predetermined pressure and temperature in order to quickly raise the temperature of the wood-adhesive mixture. As the hot gas is applied, the press may continue to close at a slow rate. Af-ter a time period to allow for substantially complete permeation of the ~at with hot gas during which time heat transfer is taking place, and while still in the intermedia-te density range, a gas-through step is conducted passing the hot gas through the mat from one platen surface to the other to complete permeation. The mat then will be xapidly pressed to its final density and thickness.
Before opening the platens, hot gas application may be con-tinued to both mat surfaces at a predetermined pressure and temperature and for a time period to allow for substantially complete cure of the adhesive in the mat. A venting and vacuum step may also be carried out to reduce moisture and reduce internal pressure so the press can be opened and the consolidated panel removed.
DESCRI~TION OF THE DRAWINGS
Figure~l is a schematic view showing the elements in the hot gas pressing system.
Figure 2 is a cutaway perspective view depicting -the structure of a representative press platen usable in the pressing metho~.
Figure 3 is a graph visually depicting exemplary press cycle variables as they change over the steps of the present invention.
Figure 4 is another graph depicting the mat pressure response as it changes over a representative press cycle.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring fir~t to Figure 1, a schematic depiction of the pressing system shows a pair of press platens 10, 12 spaced from each other with an opening 14 therebetween. The platens are constructed and incorporated into a hot pressing system substantially according to kncwn methods with modifications to carry out the process of the present invention. Typically, press platens are large substantially flat metal plates fixed to a supporting structure and have internal conduits for flow of a heating medium. One or both platens of an opposed pair are moveable toward and away from the other in order to open and close the press. When the press is open, the mat of wood-adhesive mixture is inserted into the press through a known loading meanS (not shown) and deposited at~pthe bottom platen. Typically, platens are opened and closed through suitable closure means using a hydraulic cylinder. In Figure 1 a cylinder and ram assembly is indicated at 16.

.

~t ~n \IDstrellm fo~ming station (not ~nown~. the lvood-~idhe~ive ~ixture is formed into ~ mat h~ving the ?redetermined basis veight in order to provide a loosely colnpacted rnat IYith the right bulk density fol~ pressing into panels that will have a predetcrrnined thickness ~nd density. ~here are many well~known forming m~thods and any one is suitahle for use with the present invention, provided it supplies the pressing system with uniform mats of wood-adhesiYe ;nixture having the correct predetermined weight.
Platens 10, 12 ~re modified compared to staIldard press platens by the addition of internal conduit 18 as shown in Figure 2. A plurality of perforations 20 in the platen surfaces llre connected to the cross boring 22.
Pl~ten heating conduits 24 are also located within each platen 10, 12 ~nd serve to carry the platen heating medium. Conduit 18 carries the hot gas injection nedium .vhich in the preferred embodiment is high quality saturated steam from the incoming supply lines 26 and serves ss ~ manifold to distribute the injection medium to crossboring 22. As indicated in the Figures, the injection medium is steam, although superheated steam is suitable and other hot condensable gases may be suitable. When steam is supplied to conduit 18, it will flow outwardly through perforations 20. The pattern of perforations is uniform to ensure uniform distribution of steam into the mat when, during those portîons in the pressing cycle, it is injected into the mat. As an example, from development work conducted using a small 0.5xO.S meter press, it has been found that for a wood fiber-resin mixture 2.4 mm holes in a square pattern on 25.4 mm centers are accept~ble for carrying out the process. After further development work using a large 1~22x2.44 meter press, it was found that a pattern cf 23x27 mm with the perforations being offset to yield a triar~ular p~ttern produced ~ood results.
In order to properly diffuse the steam or hot gas, slow its velocity, and more evenly distribute it over the sur~aces of the mat, ~as velocity reducing Qnd diffusing mear~ such ~s a pair of wire screen*s 28,30 (a suitable commercially availeble screen material is KPZ 80/6 ~nanufactured by the Peter Villsorth Company, West Germany, commonly used to transport mats into the open press) or porous rnetal plates are inserted between a platen and the mat. The incoming supply lines 26 of each platen are connected to a powered valvin~ system as depicted in Figure 1 that allows the incomin~
steam supply lines 26 to be in one of four states: closed, connected to the steam source 32, open to atmosphere 34, Ol open to a v~cuurn source 36.
* Tra~emark ~2~

The ~alving svstem the press closure ~e~ns, ;Ind ~he stcam ;,ystem pre~sure can be controlled ~nd p~ogrammed through a sm~ll computer or ~ith the use of several rnicroprocessors. The press closure ~neans is ~unctional to move the platens 10, 12 in a controlled manner from the open position to the fully closed position with the ability to hold positions and v~ry closure r~tes in ordér to carry out the steps of the present invention. The closed position is that position when the mat has been compressed to its final predetermincd in-press thickness.
The valving system serves to control the application of steam, 10 its pressure at the mat surfaces, and time duration. The valving system also controls the venting and application of the vacuum to the surfaces o~ the mat. A steam supply control valve 38 allows steam at a suitable predeter-mined temper Iture and pressure to enter the pressing system from souree 32 through line 40. A suitable measurement device in line 42, indicated 15 schematically at 44, serves to detect the pressure and temperature in order to properly control the steam source 32. ~low measurement rneans ~6 detects the flow rate of the steam in line 42. At the T-joint in line 42, a steam line 48 is directed to the top platen 10 and a steam line 50 is directed to the bottom platen 12. Steam valves 52, 54 serve to open and close lines 20 48, 50 respectively as stearn is caIled for by the process control systen controlling the pressing process. Line 48 is then divided at another T-joint and lines 56, 58 are directed to opposite sides of upper platen 10. The top platen steam inlet temperature and pressure are measured by any suitable means (not shown) and signals directed to the process control system. The 25 platen temperature is als~ measured and controlled since it is consistently maintained a few degrees hotter than the maximum injection steam temperature to prevent steam condensation in the platen. All steam lines between s~earn valve 38 and press platens 10, 12 are also heat traced and insulated to prevent steam condensation within the lines. It is the purpose 30 to have the condensable hot gas give up-its heat of condensation to the wood ~iber-resin mixture thereby quickly raising its temperature to the desired curing temperature of the adhesive.
Steam line iO is likewise divided into separate ~lo-v lines 60, 62 which are directed to opposite sides of bottom platen 12. Similarly, as with 35 upper platen lO, the inlet steam temperature, pressure and platen tempera-ture are detected for monitoring control purposes and suitable ~ignals sen~

to the process control syste:n. ExhalJst val~/es 64, 6fi ilre controllable, -~ndwhen open, connect the platens to e~chaust line 68 which is directed o a three-way valve 70 which is either open to vacuum source 36 or to atmosphere 34.
Line 72 serves to divert condensate developed in the ~team lines ahead of valves 52, 54. Branching from line 42 after supply valve 38, is line 74 which leads to a pressure safety valve 76.
Having structurally described a pressing systern capable of carrying out the process steps of the present invention, definitions of a 10 general set of process parameters will now be given to be followed by an exemplary set of parameter v~lues for pressing a particular panel. By being specific to a particular wood composite panel manufacturing process, it is not intended that the scope of the invention be limited, but rather that those skilled in the art understand a particular embodiment of the invention.
The process parameters are divided into two groups: "pressing"
parameters that control press actions of closing and holding position; and 7'steaming" parameters that control the injection valving, SteQm, and ~acuum. Figures 3 and 4 show the curves ~or the exemplary press cycle and provide a viSuP1 reference for the parameters. The table followin~ the 20 parameter definitions is of the process parameters as they appear for computer programming. I~he pressing parameters are~
Pn ~ Press position or mat thickness, three positions are used in the press cycle design: Pl, P2, P3. Units = mm.
Ptn - Time at press position n; only one position time, Pt1, is used and is used to improve transition from R1 to R2. Units = s.
Rn ~ Press closing rate to press position n, three rates: R11 R2, R3 are used. Units = mm/s.
Dr~ - Mat denisty at press pos;tion n; Dl, D2, D3 are used in calculation of the position paramaters. Units = kg/m3.
30 k _ A proportionality parameter used in c~lculat;on of Stl. Un;ts = s/m m .

The steaming parameters are:
P1 - Press pos;tion 1 as defined in the pressing parameters is used again in the steam control to initiate the sequence of events.
Stn - Time duration of steaming event n, four steam times are used:
St1, St2, St3, St4. Units = s.

SP Ste/~ ressure used during steam ing event n, four ~team pres-sures are used: SPl, SP2, SP3, SP~,s. Units = kPa.
Vent~ ~ime duration of opening the platens to atmosphere after the ste~ming sequence und ;?rior to opening the platens to the ~acuum source.
Vact - Time duration of opening the platens to vacuum.

The defini~ions for the valving codes in the Table below are as follows:
20 = Both pl~tens closed to steam, ~tmosphere ;lnd vacuum.
10 11 = Both platens open to steam 13 = Top platen open to steam, bottom platen open to atmosphere.
18 = ~oth platens open to atmosphere.
19 = Both platens open to vacuum.

PRESS CYCLE TABLE

Pressing Seq~lence 20 ~ Rate or Position Switch Point .

2 P1 UNTIL Pt

3 R2 UNTIL 2 P3 UNTIL PressOpens Steaming Seguence 30 ~Valvin~Code Switch PointPressure 2 11 UNTIL St1 SP
3 13 UNTIL St2 SP2 35 4 11 UNTIL St3 SP3 11 UNTIL St4 SP4 6 18 UNTIL Vent Atmospheric 7 19 UNTIL Vact -30 8Press Opens ;:

. , . .

~L24~ 5 I`he .ollowing process example is for !producing a ~nediurn dcnsity fiberbourd .~rjth the furnish being red ~lder wood ibers produced in l ~ypical pressuri~ed refiner ~1nd having a moisture content of 1l6 dry basis ?rior to pressing. The adhesive used is a commercially ~v~ilable urea formuldehyde resin and is added to the fiber using conventionlll blending dt a rate of 9%
resin solids on a dry wood weight basis. Addition~lly, 0.25~ parafin wax solids are added to the fiber. At the forming station, the proper amount of fiber-resin mixture is deposited in mat form on the bottom screen to yield a predetermined panel thickness (21.1 mm~ and density (700 kg/m3) after 10 pressing. The top screen is placed on the mat after îorming.
Following are specific parameter values for pressing the abolfe de~cribed medium density fiberbourd. ~igures 3 and 4 depict press position, steam pressures, and mat pressure as they occur during the tot~l pressing period of 43 seconds.
Following the table of press cycle parameters, physical proper-ties of the finished panel are ~i~en~ Also listed are the properties for the same panel formulution pressed in a conventionally heated press for 475 seconds using a platen temperature of 171C.
Press Cycle Table for Medium Density F erboard Example Pressure Sequence ~Rate or Position Switch Point 25 110 mm/s UNTIL ~2.8 mm 2 32.8 mm UNTIL 2.0 s 3 0 . 8 mm/s UNTIL29. 5 mm

4 4.2 mm/s UNTIL 21.1 mm 3û 5 21.1 mm UNTILPRESS OPENS

'~

~teaming Sequence Step Valvin~ode Switch Point Pressure 1 2û UNTIL 32 . 8 mm 150 kPa 2 11 UNTIL 4.1 s lS0 kPa 3 13 UNTIL2. 0 s 150 kPa 4 11 UNTIL3 . 0 s 200 kPa 11 UNTIL10. 0 s 200 kPa 10 6 18 UNTIL2 . 0 s 0 kPa 7 19 UNTIL15. 0 s -90 kPa 8(PRESS OPENS) Base Parameters:
D1 = 450 kglm3 D~ = 500 kg/m3 Panel Properties 24-Hour ~` 20 Water Soaking InternalModulusModulus ~Y~ter Thick-Sanded Bond of of Absor~ ness Thickness Density Strength ~ Elasticity tionSwellin~
___ _ tmm) (kg/m~) (kPa) ~MPa) (~;Pa) (~6) (%) Sterm Pressed 18.7 7121,1~0 22.8 2.3~ 48 8.2 30 Conventional1y Pressed ' :
` 19.3 705 783 32.9 3.13 5B 9.7 Having described the general process parameters and given 35 specific values for a particular exemplary panel, the following describes thefunctions of process steps and the method for determining parameter values for other mat basis weights and wood particulate geometries Conceptually, four process requirements were needed in order to eliminate blisters, surface pitting and panel warpin~. First, the mat should ~0 have the steam or other selected hot gas completely penetrate the volume of mat material in order to effect complete heat transfer raising the mat temperature quickly and uniformly throughout. Second, the pl~ten pressure on the mat surfaces should be relatively low during the portion of ihe cycle - : .
, " .
., - ' ' .: -~hen ,team ~enetration occurs ,uch that surfac~ consolidation does not prevent flow to the core. Third, steam vclocity at the injection locations hould be relatively low thereby eliminating disturbances of the wood-resin miYture over the surface, and fourth, steam treatment of the surfaces should be substantiaUy equal.
Starting with the need for low steam velocity to reduce pitting, two methods are used. First, the wire screens 28, 30 are used on both sides of the mat to create channeLs for lateral steam flow at the platen surface.
This allows the steam to spread to a uniform ~ront over the entire mat 10 surface, rather than being concentrated at points directly under the platen perforations 20. Second, the total steam flow rate is controlled by initially steaming at low stsam pressures and with ~ preselected intermediate mat density (Dl) that also serves to control steam flow.
To meet the requirement of ~ow platen pressure during steam 15 penetration, the press closing rate is reduced after reaching the inter-mediate mat density (D1) to a slow rate (R2) until steam penetration is substantially complete and the en~ire mat substantially saturated with steam. The selection of the intermediate mat density is again important as a density too high will result in excessive platen pressure.
The function of second closing rate ~R2) is to maintain contact between the mat surface, screen and top platen. The mat may shrink in thickness sUghtly during the first two steam periods ~St1 and St2~. Suffi-cient mat contact minimi~es steam loss at the mat edges and maintains steam flow into the mat core. No edge sealing apparatus around the platen 25 perimeter is required in this process. However, the pressing screen edges are filled with silicone rubber to prevent stearn leakage through the screen edges. ThSs ~illed band at the screen edge is covered by several centimeters of the particle mat.
The result of the low steam velocity and low platen pressure 30 requirements is that there ~ust be an intermediate density or r~nge that satisfies both of these potentially exclusive conditions; that is, a density high enough to slow steam flow and avoid surface pitting, yet low enough to avoid platen pressure le~els that cause blisters Experiments ha~e shown that an acceptable range of densities exists ~or wood particles~ generally 35 between 300 kg/m3 and 550 kg/m3. The optimum values vary with vvood species and particle geometry, and generally must be determined by 1~

e~periment~tion. It ~as been Lound that density variation of + 15,o can readily be accommodated by the process.
The third requirement is to ensure complete steam penetrQtion of the mat core. Because steam injection is initiated ~rom both platens,

5 areas of localized hi~h mat weight or pockets of air t,rithin the mat may restrict stearn flow. To assure complete penetration after Initially steaming for a time (~t1) from both platens, the steam valve on one platen is closed and switched to venting moàe while steam is applied through the other platen. This allows steam flow through the mat for a time (St2) from one 10 surface to the other and produces complete steam saturation of the mat core. This is done after the surfaces have received steam, so no dissimilar consolidation or treatment of the mat surfaees will occur. A short (approximately 2-20 second) push from one side was found to eliminate any unsteamed pockets. This time period appears to be adequate for any mat 15 weight. On completion of the second steam period, the press is closed to final thickness (P3) at rate R3. The temperature of the mat during St1 and St2 is quickly elevated and established at a point corresponding to the saturation temperature of the condensable gas. The rate R3 is not critic~l to the process but should be relatively rapid to minimize press cycle time.
20 As depicted in Pigures 3 and 4, the positiorl vs. time curve has a step shape.
After reaching the final thickness (P3~ and density (D3) in the cycle, the purpose changes from avoiding the steam pressing problems to meeting the temperature requirements of adhesive cure and pressure and moisture requirements for press opening. The balance of the in-press time 25 generally depends upon panel thickness, particle type and adhesive. As may be seen in Figure 3 for the fiber furnish where urea formaldehyde is the adhesive, steam pressure ~ill be maintained on both faces of the mat after the final position is reached. During final press closing, the steam ?reSsure is brought up to the final curing temperature and pressure for a period (St4 30 as seen on the curves. In the example, approximately ten seconds of steam application i5 needed to establish the temperature to cure the resin, after which the press is first vented (Vent) for approximately 2 seconds and then a vacuum drawn (Vact) to evacuate the ~ases and moisture from the pressed panel. After a suitable time, approximately 15 seconds in the example, and 35 preferably after releasing the vacuum the press is opened and the board removed from the press, completing the pressing operation.

rhe îourth ~equire~ent o~ equ~l surîace treatment is essenti~lJy met by initially injecting the gas or steam through both surfaces until the mat is substantially suturated. ~fter the initial steam saturation step (Stl), the short steam through step (St2) can be carried out Nithout affecting the 5 surface. Additionally, the steam-through step is done before fin~l panel consolidation (R3) and the final steaming step tSt4) where steam is ag~in applied to both surfaces.
The press cycle parameters associated with the first two steam-ing periods càn be calculated for other mat basis weights based on two 10 relationships. One, total steam requirements are proportional to the nat mass or basis weight; and two, steam flow rate into the mat is a function of furnish geometry, mat density and steam pressure, and is independent of time or mat thickness.
From known thermodynamic equations, the theoretical steam 15 mass flow requirement to bring a mat to saturated steam temperatures can be calculated given the mat mass, mat heat capacity, and heat of condensa-tion of the steam. This assumes the mat temperature change is the result of steam condensation only. The second relationship was suggested by experi-mental steam flow vs. time data that shows steam flow to reach steady 20 state almost within the first second after initiation. The steam flow slows when mat saturation occurs. Given this steam flow rate, the required initial steam time to heat the mat to steam saturation temperature is proportional to the mass of the mat or basis weight.
The steam flow rate during the first steaming period also varies 25 with the initial mat steaming densities (D1 and D2) and steam pressure (SP1)~ and furnish geometry. Once the steaming densities (Dl and D~) and s.eam pressure tSPl) are selected for a particular furnish within the limits established by the surface pitting and blister problems, the initial steam time (St1) varies only with mat basis ~reight. This is ;nathematic~ly 30 equivalent to: St1 = k x P1 as the intitial steaming po~ition (P1) varies proportionally to mat basis weight. The proportionality parameter (k~ may be determined from calculated steam requirements and steam flow rate data. The proportionality parameter tk) must yield an initial steam period (St1) of sufficient length to substantially saturate the ~at with steam. The 35 position paramaters, Pl and P2, are calculated by generally known formulas to yield densities, D1 and D2, for the mat b~sis Neight to be pressed.

,., .
' '~ ; : ' ~2~ 5 1~
-In the rnedium densit~ fiberboard e:campIe, the Pl position is maintained ~or a period (Ptl) of two seconds- This allows the press controI
means to accurately achieve the first position (Pl) before beginning the second press cIosiJIg rate (R2).
;, The second press closing rate (R2) continues through the second steaming period (St2). As in the example for fiberboard, the second steaming period (St2) is about two secondsO The second rate may therefore be calculated:
3~2 = Pl - P2 The third press closing rate is not critical to the process, but should be rapid15 to minimize pressing times. Steaming is continued from both platens during this period (St3) and may be used as a transition period to the final steaming conditions necessary to reach and maintain uniform temperature for adhesive cure.
The press cycle parameters that follow the first three steaming 20 periods (Stl, St2 and St3) have the functions of affecting adhesive cure and degassing and drying the panel for press opening. The curing steam perio (St4) must be determined experimentally for a given adhesilre according to desired physical properties. For example, phenol formaldehyde adhesives generally reqwre ~ longer ~eriod (5t4) and higher steam pressures (SP4) than 25 urea formaldehyde adhesives. The ~ent period serves to relieve the panel of stearn or other gases under high pressure. The vacuum period removes steam or o~her gases not expelled by their pressure and dries the board. The degassing periods (Vent and Vact) generally must be varied ~ith final panel thickness (P3), density (D3), and furnish geometry.
In addition to use of aIternate types of adhesive, it has been previously pointed out that wood furnishes other than fiber can be utilized and the broad pressing process- may still be employed as the pressing cycle although the parameter values may vary dependent upon adhesive, furnish, and final density and thickn0ss desired. One having ordinary skill in 35 pressing technology for composite panels will readily understand how e~ch particular press cycle will be derived for the variables. When using saturated steam as the hot condensable gas and when manufacturing conventional wood based panels using conventional adhesives, as a general ~2~

statement of -the approxirnate allocation of time to the various steps in the pressing process it may be stated that: (1) the period from beginning of press closure to reaching Dl should be about 15% or less of the time to press opening/ (2) the period of St1 should be about 3-15% oE the total ti~e, (3) the period of St2 should be from akout 5-25% of the total -time, (4) the period for S-t3 cmd St4 cornbined should be from about 10-60% of the total time, and (5) the period for Vent and Vact combined should be fron about five tirr.es the length of t~he Vent step.
While a detailed description has been given of the improved hot gas passing process and apparatus, one that will enable those skilled in the art to both m~ke and use the invention, it may occur to other that modifications may be rnade withou-t departing from the broad scope of the invention. Pll such mcdifications are intended to be included within the scope of the following claims.

Claims (27)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of forming a panel or the like from a mat of lignocellullosic material and a curable binder, comprising the steps of:
compressing the mat between a pair of heated press platens to a first density within an intermediate-density range which is less than a final density and to a thickness within an intermediate thickness range which is greater than the final thickness, injecting steam into both major surfaces of the mat while the mat is within the intermediate density and thickness ranges for a period of time sufficient to substantially saturate the mat with steam while allowing excess steam to exhaust through the edges of the partially compressed mat, passing steam substantially through the mat from one major surface to the other while the mat is still within the intermediate density and thickness ranges to assure complete saturation, and compressing the mat to a higher density and a lower thickness to consolidate the mat and cure the binder, and opening the platens after curing the binder and removing the so formed panel.

2. The method as in claim 1 including the step of finally curing the binder after the mat is compressed to its final density and thickness by again injecting steam into both major surfaces of the mat before opening the platens.

3. The method as in claim 2 including the step of venting the mat after the mat has reached its final density and thickness and after the binder has been substantially cured.

4. The method as in claim 3 including the further step of drawing a vacuum over both major mat surfaces after the venting step.

5. The method as in claim 1 including the step of continuing to compress the mat when it is within the intermediate density and thickness ranges and while the steam is being injected into the mat.

6. The method as in claim 1 in which the steam is saturated steam.

7. The method as in claim 1 in which the steam is superheated steam.

8. The method as in claim 1 in which the time far compressng the mat to the first density is about 15% or less of the period from beginning of platen closure to platen opening.

9. The method as in claim 8 in which the time for injecting the steam into both major surfaces is from about 3-15% of the period from beginning of platen closure to platen opening.

10. The method as in claim 9 in which the time for passing steam through the mat is from about 5-25% of the period from beginning of platen closure to platen opening.

11. The method as in claim 2 in which the time for further injecting steam into both surfaces of the mat is from about 10-60% of the period from beginning of platen closure to platen opening.

12. The method as in claim 4 in which the venting and vacuum steps combined are from about 5-45% of the period from beginning of platen closure to platen opening.

13. The method as in claim 12 in which the vacuum step is about five times the length of time of the venting step.

14. A method of producing a wood board from a mat of a material to be compressed made up of chip and/or fibre material and curable binder such as a synthetic resin binder, by: pressing for a predetermined pressing time subdivided into a number of pressing time stages and curing by means of steam between heated press platens of a platen press, said press platens having means for supplying steam from a steam generator and, on the side adjacent the mat, a plurality of steam apertures which are distributed over the press and from which the steam can emerge, the mat being introduced between the press platens and the press platens then being moved towards one another into a precompression position, the mat being pre-compressed in a first precompression pressing time stage, the steam then being introduced into the mat through the steam apertures of the two press platens during a steaming time in-terval, the press platens also being moved towards one another into an end position defining the wood board thickness during a final compression pressing time stage during which the mat is further compressed, and the finally compressed mat being cured in this end position of the press platens during a final pressing time stage without any further steam supply, characterised in that the steaming time interval is interrupted by a scavenging pressing time stage, during which the steam emerges from the steam apertures of one press platen, flows through the precompressed mat in the direction of the thickness and is discharged through the steam apertures of the other press platen disconnected from the steam source, and in that the press platens are then moved into the end position and the steam supply to the finally compressed mat is continued via the steam apertures of the two press platens for the remainder of the steaming time interval, and then during the final pressing time stage the mat is subjected to a vacuum via the two press platens and their steam apertures and thus dried, the steam supply means being connected to a vacuum source for this purpose.

15. A method according to claim 14, characterised in that during the first stage of the steaming time interval and during the scavenging pressing time stage the press platens are slowly moved on towards one another while maintaining contact with the mat.

16. A method according to claim 14, characterised in that the steam supply is continued continuously via the steam apertures of the two press platens during the final compression pressing time stage.

17. A method according to any one of claims 14 to 16, characterised in that the mat is connected to atmosphere via the two press platens and their steam apertures during an expansion time stage before being subjected to the action of the vacuum in the final pressing time stage.

18. A method according to any one of claims 14 to 16, characterised in that during the remainder of the steam time interval the finally compressed mat is treated with steam of a pressure higher than that of the steam supplied during the first stage of the steaming time interval.

19. A method according to any one of claims 14 to 16, characterised in that condensable steam is used and during the first stage of the steaming time interval and during the scavenging pressing time stage the steam is brought to condensation in the mat and thus the temperature of the mat is set to about 100 to 135°C.

20. A method according to claim 14 in the embodi-ment for the production of wood boards in the form of chip-boards or fibreboards with urea-formaldehyde glue, humidity of raw mixture about 8%, raw density about 390 kg/m3, characterised in that a pressing time of about 20 to 50 seconds is used, the steaming time interval is set to a maximum of two-thirds of the pressing time, the first stage of the steaming time interval until the start of the scav-enging pressing time stage is less than 10 seconds, prefer-ably about 5 seconds, the remainder of the steaming time interval is less than the difference between the steaming time interval and the first stage of the steaming time interval, and the scavenging pressing time stage has a duration of less than 5 seconds, preferably about 2 seconds.

21. A method according to claim 20, characterised in that the final compression time stage is added to the remainder of the steaming time interval.

22. A method according to claim 20, characterised in that the steam used has a temperature of about 110° to 175° C.

23. Apparatus for performing the method according to claim 1 comprising;
a platen press with two steaming press platens having a plurality of steam apertures distributed over their facing surfaces, a steam generator, means for supplying the steam to the steaming press platens, said means being provided with valves, a press platen control means for accurate control of the position of the closing movement of the steaming press platens with a defined precompression position and a defined final compression position, and steaming regulating means for the supply of steam, characterised in that the steam supply means comprises a branch line adapted to be connected to the two steaming press platens via separate outlet valves, so that steam can be supplied simultaneously to both platens or to only one platen, and that each platen is connected by a separate exhaust valve to an exhaust line itself selectively connectible via a distributor valve system to the ambient atmosphere or to a vacuum source, so that during a scavenging pressing time stage one steaming press platen can receive steam while the other platen is vented to atmosphere, and so that both steaming press platens may be connected to said vacuum source during a final pressing time stage.

24. Apparatus according to claim 23, characterised in that the press platen control means and the steaming regulating means are coupled.

25. Apparatus according to claim 23, characterised in that the steaming regulating means is arranged to control the steam supply during the scavenging pressing time stage and the vacuum treatment during the final pressing time stage.

26. Apparatus according to any one of claims 23 to 25, characterised in that the press platen control means is arranged so that when the precompression position is reached and when the steaming press platen reaches the final compression position the said control means deliver control signals to the steaming regulating means to carry out the indicated steam supply steps respectively for the scavenging pressing time stage and during the final pressing time stage.

27. Apparatus according to any one of claims 23 to 25, characterised in that the press platen control means and the steaming regulating means are adapted to be controlled by a program between the precompression position and the final compression position of the steaming press platens and, if required, after reaching the final compression position.