WO1992001540A1

WO1992001540A1 - Wood composite forming and curing system

Info

Publication number: WO1992001540A1
Application number: PCT/US1991/005054
Authority: WO
Inventors: David J. Parker; Mark T. Churchland; Martin T. Pearson; Walter W. Schilling; Keith Charles Watson; James Douglas Barr; John Leslie Jonas
Original assignee: Macmillan Bloedel Limited
Priority date: 1990-07-23
Filing date: 1991-07-22
Publication date: 1992-02-06
Also published as: CA2025555A1; AU8321591A

Abstract

Adhesive coated wood strands are laid up on a conveyor belt and conveyed thereon to a microwave curing press (104). Due to uneven temperature or moisture profiles of the layup and/or uneven microwave deposition patterns in the press (104), the structural wood products thereby formed in the past had uneven density profiles, which reduces product strength and product properties especially after wetting. Before entering the press (104) the moisture and/or temperature profiles of the layup are controllably altered according to this invention thereby improving the resulting density profiles. Moisture and/or temperature levels of predetermined areas of the mat are increased or decreased to obtain the desired pre-press profiles. The profiles can be adjusted, for example, by the application of hot steam to the lower mat surface directly or the conveyor belt directly and the mat thereby indirectly, and/or the application of a water spray and cooling fan air to the same or different mat layers.

Description

WOOD COMPOSITE FORMING AND CURING SYSTEM

BACKGROUND OF THE INVENTION

The present invention relates to systems for continuously man¬ u acturing wood composite, adhesively bonded products wherein pres¬ sure and microwave energy are simultaneously applied to the curable assemblies. The adhesive bonding agent is thereby cured or set while the product is pressed and/or maintained at the desired dimensions and density. The microwave application cures the resins which are used as binders or adhesives for the composite wood materials, such as wood particles, wood chips, wood wafers, wood strips, wood fibers and wood veneers, used in the production of chipboard, hard board, parti¬ cle board, wafer board, plywood and other composite products. The present invention more particularly relates to processes for curing and pressing layups which are formed by depositing adhesive coated, elongate wood strands oriented substantially longitudinally in a mat, from a plurality of layup feeds.

Detailed discussions of preferred methods of forming adhesive coated, curable assemblies and depositing them on a continuous layup conveyor belt for conveyance to microwave press assemblies are pro¬ vided in copending U.S. application Serial No. 07/555,732 (732), filed July 23, 1990, (Canadian Application Serial No. 2,022,900-4) and in U.S. Patents 4,872,544 ('544), 4,563,237 ('237), 4,706,799 (799), 3,493,021 (*021) and 4,546,886. (These and each of the other applica¬ tions and patents mentioned anywhere in this disclosure are hereby incorporated by reference in their entireties.) Additionally, products containing oriented elongate strands such as could be used herein are disclosed in U.S. Patent 4,061,819 (which was reissued as Re. 30,636) and the '021 patent. The *544, 799 and '237 patents remedy the "card decking" orientation problems of the system of the '021 patent. In these three patents the product is formed from elongate members at least about a foot in length which are oriented, compressed and bonded together. The elongate members are deposited on a moving carrier and oriented substantially parallel to the direction of move¬ ment of the carrier and on the carrier over a length thereof that is at least as long as about one and a half times the length of the elongate members and is at least as long as about thirty times the final thick¬ ness of the compressed, composite product.

When two or more layup systems have been used to form a sin¬ gle layup mat the interfaces between the layups are prone to strand alignment inconsistencies, reducing the strength properties across the beam thereby produced. A system for remedying this problem, that is, insuring that the final compressed product has consistent mechani¬ cal properties throughout its cross-sectional depth, is disclosed in the copending 723 application. The system(s) described in that applica¬ tion include at least first and second simultaneously formed layups, overlapping one on top of the other in zig-zag patterns on a longitudi¬ nal, side-to-side moving conveyor trough. The top of the bottom layer and the bottom of the top layer are formed at the same time, and the mat thereby formed has a continuous average strand angle throughout the layer interfaces.

Before laying the strands in a side-by-side lengthwise dimen¬ sion in the trough layup they are coated with a curable adhesive, such as a standard phenol-formaldehyde glue having a small wax compo¬ nent. The adhesive strand layup mat is then conveyed to a press where the arranged strands are heated by microwave energy and com¬ pressed in a converging belt press to thereby form a high strength dimensional composite product. Examples of belt presses are those disclosed in U.S. Patent 4,517,148 and copending U.S. application Serial No. 07/456,657 ('657), filed December 29, 1989. Microwave applicators for these continuous presses are disclosed in U.S. Patent 4,456,498 and copending U.S. application Serial No. 07/557,652 ('652), filed July 27, 1990 (Canadian application 2,022,945-4). In the press a pair of endless belts converge to a press chamber at their nip region. At opposite side walls of the press chamber are the microwave appli¬ cators, and windows at both of the side walls form parts of the side walls of the press chambers and also block the layup from entering the applicators as it is being pressed. The windows are transparent to the microwave energy from the applicators which passes therethrough to the layup as it is being pressed in the belt press.

A problem in the past has been that the final structural wood product has an uneven density profile. If the temperature or moisture contents of the mat are not consistent within a few degrees, instabili¬ ties in the change of temperature development, that is uneven heat¬ ing, occur in the microwave press for two reasons. First the dielec¬ tric constant epsilon, both its real and imaginary parts, increases as temperature increases, which means that more energy is deposited into areas that are already warmer. This has a multiplier effect; that is, the warmer these areas get, the more they will attract energy, and so forth. A second factor is that these layups are comprised of wood fiber, and wood is compressed as it is microwave heated. The wood is softer when it is warmer, and the warmer part is more easily com¬ pressed in the microwave field. As it compresses, the warmer areas take up more of the compression, soften and compress to a higher density sooner, which again increases their dielectric absorption. That is, more energy focuses into those areas, and as this happens they become softer and compress more readily and instability again results. Both of these factors work against even heating and even final product density in wood composite products.

For example, in a wood product with an 11.4 inch by 14.75 inch cross-section, the top and the bottom of the mat can be about 25 ° to 30 °C, while the center two-thirds of the mat can be about 35° to 50 °C, due to the natural progression of the water uptake. The center heats up since the water chemically binding to the cellulose lignin structure in the mat is an exothermic activity. As the mat progresses through the press, the top and bottom of the mat can have specific gravities of .5 gram per cubic centimeter, while that of the center two-thirds can be .6 to .65 gram per cc. Further, the moisture con¬ tent of the .5 gram per cc top and bottom areas is about 12 to 13% of the dry basis of wood, while the .6 to .65 gram per cc has about 9 to 10% moisture content. The density gradient is important to these parameters, since whenever there is a density gradient there is also a strength parameter gradient. Accordingly, there is a different ther¬ mal normalizing effect on the compressed mat and thus a different moisture response. The cooler areas tend to expand more rapidly, more readily and more permanently on wetting, which can lead to bowing or splaying of the final product. The resulting density gradi¬ ent is thus due to two factors. The first is the uneven temperature and moisture profile of the mat as it enters the microwave press, and the second is the uneven microwave deposition pattern of the micro¬ wave applicator. Although the more significant of these appears to be the uneven pre-press profile, the uneven microwave pattern is also a substantial factor.

One solution to this density gradient problem might be to vary the microwave patterns, but they are difficult to control and can never be made precisely even. Referring to the '652 application, the applicator horn expands out towards the layup thereby producing microwave patterns which are difficult, if not impossible, to avoid.

A prior art attempt to remedy this density gradient problem has been to raise the temperature of the entire mat. The temperature was raised by insulating the top and the bottom of the mat and then providing an oil heating system around the conveyor itself. More par¬ ticularly, oil heating lines were positioned along the sides of the trough and heating devices underneath the bottom and insulation cov¬ ers placed on top of the mat as soon as the last strands were depos¬ ited. The mat was thereby lifted out of the very sensitive operating range where these control parameters have their biggest effect. A mat that is entering the press with a 50 to 60 °C temperature has already experienced the bulk of its softening. Thus, even though it still has a temperature gradient of five to ten degrees, this gradient has less of an effect on the final product.

When the temperature of the entire mat is raised sufficiently, the mat behaves reasonably consistently in the microwave heating process. It does not compensate for inadequacies in the evenness of the microwave heating, however. The prior art system of heating the entire layup to make it hotter was thus not an attempt to control either the moisture or temperature beyond an even mat nor did it achieve an even mat temperature. A benefit of raising the tempera¬ ture of the entire mat is that the effects of the ambient temperature are reduced but not eliminated. In other words, on hot summer days there is a different mat self-heating profile than on cold winter days, and these effects are reduced to a certain extent by heating the entire mat.

A significant disadvantage of this "whole mat" heating tech¬ nique, however, is that if the temperature gets too high, for example to 60 or 70 °C, then the glue in the mat can be precured, making the product useless. The product may still look good, consolidated and strong but if the glue was even partially cured before the final com¬ pression and microwave heating, there is little left to hold the wood strands or composite assemblies together. Aside from the precuring problem, there is also the problem that the entire mat as a practical matter is difficult to heat evenly since there are differential chemical reactions occurring with this water uptake. The mat is simply not stable enough to be totally heated to an even, higher temperature.

A prior art technology in the board industry for reducing press curing times is to radio frequency (RF) preheat the mat before it reaches the press. That is, an RF field is applied to the uncompressed mat to raise the temperature thereof to 50 to 70 °C or even higher before final compression and heating with a hot press. This board forming technique is usually a batch and not a continuous process, however, and the purpose of the RF preheating is to shorten the pressing time in the hot press. This mat is also formed very thin so that there is no significant steam transport within it. Further, this prior art board forming process does not involve any significant or positive compression of the mat. Thus, any small irregularities in preheating will not magnify during the process. When a mat is to be simultaneously heated and compressed (such as in the present process described in detail below) compressibility during heating is very important and any instabilities tend to magnify. SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to provide an improved process for forming wood composite structural products in a microwave press wherein the resulting product has improved density gradients and is not chemically changed.

Directed to achieving this object, an improved process (and system) for forming wood composite products is herein disclosed. These products are microwave heated, relatively thick (thickness gen¬ erally greater than two inches) and have improved cross-sectional density profiles due to the subject process. The process includes forming a layup or mat of composite assemblies, and generally before passing it through a microwave curing press assembly, significantly altering the temperature and/or moisture level contents in predeter¬ mined localized areas thereof. The temperature of the localized areas will be changed by at least 5°C and the moisture content will be changed by up to 10% over other areas of the mat in the same cross- section thereof. Whereas the density gradients of the final wood product without this altering step would vary by as much as 50%, with this step they will vary only by as much as 10% or less. While simply heating the mat has little effect on the moisture content variability, applying a water mist spray allows for a wide variability change. Two or three percent changes, for example, which is a gross average though since ten percent might be added to the mat surface and noth¬ ing to the center, can be made without difficulty. Since only heat and/or moisture have been added to or subtracted from the composite mat, there is no chemical difference in the final structural wood product. Further, no change is advantageously needed or made in the microwave applicators.

A number of different methods of determining the amount and area of the changes of the altering step are included herein. One method measures the profile before and after the conveyance through the microwave press, varies the profile entering the press and observes the resulting profile and then calculates the amounts and locations of the alterations needed. Another method provides a mat with very even moisture and temperature profiles, conveys it through the press and then measures the resulting heating and density profiles. This shows the unevenness of the microwave deposition pattern(s) and thus the areas where additional moisture and/or heat are needed.

A number of different methods for affecting this alteration in temperature and/or moisture level in selected areas are within the scope of this invention. These include adding moisture directly, pre¬ venting evaporation, providing cooling air or other fluid, adding hot water in a jet, mist or steam to the mat, heating the mat with infra¬ red energy or radiant heat, or by significantly affecting the tempera¬ ture of adjacent conveyor belt structure which temperature is then conveyed to the mat. These techniques can be used once the layup has been deposited in the conveyor trough. It is also within the scope of the invention to affect the resulting moisture and/or temperature profiles of the resulting mat by acting upon the composite assemblies before they are deposited in the conveyor trough. One way of doing this, when the layup in the mat is fed from a plurality of different layup systems, is to heat (or cool) the glue differently in one or more of the layup systems before they are deposited on the respective com¬ posite assemblies.

Other objects and advantages of the present invention will become more apparent to those persons having ordinary skill in the art to which the present invention pertains from the foregoing description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram of a system of the present invention.

Figure 2 is a perspective view of a portion of a first embodi¬ ment of the system of Figure 1.

Figure 3 is a perspective view of a portion of a second embodi¬ ment of the system of Figure 1.

Figure 4 is a perspective view of a portion of a third embodi¬ ment of the system of Figure 1. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring to Figure 1, a system of the present invention is shown diagrammatically at 100. System 100 includes a longitudinal conveyor 102 which conveys a mat of composite material into a microwave press assembly shown generally at 104. Details of a pre¬ ferred microwave press assembly 104 are shown in the '657 and '652 applications. The composite assemblies are deposited on the conveyor 102 by a depositing system shown generically at 106. An example of a preferred depositing system 106 is that disclosed in the '000 applica¬ tion wherein four generally separate layups feed simultaneously onto a single conveyor trough (102). Generally before the mat enters the microwave press assembly 104, the localized areas thereof have their temperature and/or moisture contents controllably altered by an alteration system shown generically at 108, such that the density gra¬ dients of the structural product leaving the press assembly 104 are significantly improved. This altering step (108) can occur before the composite assemblies are deposited on the conveyor belt 102, as they are deposited on the conveyor belt, after they have been deposited and have traveled a distance on the conveyor belt, or any combination thereof, as would be apparent to those skilled in the art from the sub¬ ject disclosure. That is, the altering system 108 can precede, follow and/or be part of the layup depositing system 106, and/or even occur within the microwave press assembly 104 (as discussed in detail later). Thus, to compensate for inadequacies in the evenness of the micro¬ wave heating the mat temperature and/or moisture content profiles are altered by the altering system 108 before entering the microwave press assembly 104. The microwave press assembly 104 itself and its microwave deposition patterns are unchanged. These profiles can be measured by measuring systems before and after the press assembly 104 as shown generically in Figure 1 by measuring systems 110 and 112, respectively.

A number of techniques or altering systems 108 of the present invention, seven of which are listed below, allow for the heating pat¬ terns of the mat entering the press assembly 104 to be tailored and therefore the final density patterns of the cured compressed product to be more markedly improved as can be measured by the measuring system 112. (1) A first example is that moisture can be added to the dry wood mat; this creates an exothermic reaction as the water becomes chemically bound to the wood and that is why changes in the temperature of the wood can occur with changes in moisture content. (2) Steam can be added to physically heat one or more portions of the layup. (3) Similar to (2) hot water is added to physically heat prede¬ termined areas of the mat. (4) Moisture is added locally to change the microwave absorption properties locally; this can be done by spraying a fine mist of water anywhere in the mat as it is being formed, on the bottom or the top or on the sides after it has been formed. (5) The mat can be cooled with air, or any other suitable fluid, to lower the temperature in predetermined parts of the mat; this can be used in conjunction with the addition of water to the wood to lower the tem¬ perature of the mat created from the exothermic reaction from the added water. (6) The mat can be insulated, as on the top and/or bot¬ tom and/or sides thereof, to control the rate of change of tempera¬ ture and the moisture can be contained so that it is given off to lesser degrees by natural evaporation; this makes the moisture more readily available for exothermic chemical addition to the wood and therefore heating the wood. (7) The platens of the microwave press 104 can be heated prior to and/or during microwave heating actually within the microwave press.

Heating the platens raises the temperature on the very surface of the mat, and therefore creates an area of higher energy absorption through extra densification and higher initial loss tangent. Since there is a higher initial loss tangent to the wood when there is a higher temperature, heating the surface of the mat with the platens affects that kind of a change. Although heating the platens directly heats only a relatively small depth of the mat, perhaps a quarter to a half of an inch thick surface layer, this heat propagates a significant distance into the mat, as by steam generation from excess microwave heating. The one-half inch heat propagation allows greater amounts of microwave energy to be absorbed in the area that is heated by the steam, and the microwave absorption effects of this heating propa¬ gate a distance into the product.

In the embodiment of Figure 2 steam 120 is injected over the segmented metal transport belt 122 (or conveyor 102) underneath the mat, thereby heating the belt. The steam 120 can be injected through large orificed tee connectors 126 of a steam header applicator 128, adjacent the tail sprocket 130 of the belt 122. The bottom of the mat is thereby heated since it is placed directly onto the hot surface of . the belt 122 as the mat is laid up in the layup trough 132. This uneven heat formation is troublesome in part due to the exothermic reaction, which is extremely temperature dependent in its own right. Thus, if a mat area cools locally, the exothermic reaction progresses more slowly and therefore the area stays cooler yet, creating another insta¬ bility. By heating (with steam 120) the steel chain or belt 122 under¬ neath the mat, heat is being added to the bottom of the mat without adding moisture to it.

The heat (and moisture) according to another technique of this invention can be injected directly on the mat, and a system embody¬ ing this technique is illustrated in Figure 3. As shown therein a steam supply pipe 140 is positioned in a space shown generally at 142 in the conveyor 102 directly below the layup mat 144. A cross-section of the mat 144 held in the layup trough 146 on both sides of the conveyor 102. The conveyor 102 includes a pair of aligned layup belts 148, 150, each having its own drive roll 152, 154, respectively. The end of the steam pipe 156 within the space 142 has a plurality of upwardly dis¬ posed apertures 158, out through which steam 160 passes to heat (and moisten) the bottom surface of the mat 144 as it is conveyed across the space 142 between the layup belts 148, 150. Direct steam 160 is thereby used to increase the temperature of the bottom of the mat 144 and potentially the moisture content thereof.

As a specific example, where the bottom of the mat (144) is to be raised from 25 to 30 °C, a 50 to 55 °C jet of wet steam (160) can be directed directly on the bottom of the mat. The change of tempera¬ ture with steam can increase moisture content. The density of the mat can be raised from its nominal point, .5 gram per cc, to about .65 to .7 gram per cc. By exceeding the temperature or by matching the temperature and exceeding by a small amount the moisture content of the rest of the mat, its density is raised to above that of the densest part of the mat. Thus, final density profile of the product is directly controlled by the local application of steam. The final moisture con¬ tent gradient of the product is a strong function of the final tempera¬ ture gradient of the product. Similarly, the final density gradient is a function of the final temperature gradient. If the temperature is even and consistent in the final product coming out of the press, the moisture and density are also even; the three are directly related. It is difficult to obtain an even temperature profile and an uneven mois¬ ture profile in a well formed product, or an even temperature profile and an uneven density profile. Thus, by carefully tailoring the mois¬ ture and temperature profiles to compensate for the natural heating patterns an even density results. An even moisture pattern also can result and the moisture gradient in the final product is important as it affects its proper use. The more even the moisture, the less likely it is for the product to tend to bow or splay.

Proper control of the alteration of the temperature and mois¬ ture contents of the layup results in no history of the control remain¬ ing in the product, other than the resulting even density and moisture content. If an area was properly made wetter or hotter to increase microwave absorption, the results will be even moisture, not higher moisture, and even density, not higher density. This is because if a mat area is wetter it absorbs more microwave energy as it reaches the desired temperature and this drives the extra moisture away. There is thus no evidence of what was done to obtain the even mois¬ ture and density in the product. Nothing was added chemically that is different than what normally exists in the product. Accordingly, the final density can be advantageously controlled without adding chemi¬ cals, absorbers or anything else aside from heat and moisture to the layup.

The temperature in the mat can be monitored as it is being formed by measuring system 110 and compared with the development of the temperature profile in the mat compared to the profile of the final product density as measured by measuring system 112. This allows for a quick determination of where the extra heat and/or mois¬ ture will benefit the final density profile. For example, one monitor¬ ing or measuring system 110 includes a series of thermocouples (not shown) added in the cross-section of the mat and run down the mat formation. For every two inches, for example, in the depth of the mat another thermocouple is added across the mat as well as verti¬ cally throughout the mat. As many thermocouples are added as needed to obtain the desired resolution. For example, a high resolu¬ tion can be obtained where the thermocouples are positioned every half-inch; if the temperature in the bottom of the mat is the subject of the modification then this one-half inch positioning in the bottom six inches of the mat can be used to provide a higher resolution in monitoring the temperature of the bottom of the mat. Before the mat enters the press 102 the thermocouples are usually removed so they do not interfere with the microwave heating patterns. The ther¬ mocouples thereby provide an excellent history of the progression of the temperature of the mat on the way to the press 102.

Another monitoring procedure or measuring system 110 is to actually stop production and take moisture contents samples from the mat. This can provide both temperature and moisture profiles of the mat prior to pressing. After pressing, the profiles of the final density and temperature profile can be obtained by measuring system 112; the temperature profile can be obtained with infrared cameras and the density profiles by actually cutting out density samples and measuring them.

If the microwave press 102 is heating quite evenly, then even temperature and moisture profiles entering the system (or press) are desirable so that the energy is absorbed evenly. There is an inherent imbalance in all microwave application systems, however. By trial and error techniques, for example, the temperature/moisture profile needed to create an even density resulting profile can be determined. The entering temperature moisture profile will be essentially an inverse of the heating profile. If the entry moisture and temperature profiles are even and the heating profile pattern and the final density profile are known, then the heating pattern of the microwave system and its unevenness can be determined. The simple observation of where insufficient heat was applied evidences where moisture or higher temperatures in the mat going into the press are needed.

By injecting steam on the bottom very close to the press, such as in the system of Figure 3, the density is also improved markedly. The density can actually be increased beyond that of the average which shows an ability to manipulate the density to any desired amount by controlling temperature.

Further improvements to this process involve monitoring (110) moisture and temperature during mat formation, carefully controlling and monitoring the energy and moisture application during the mat layup process, to understand precisely the profile going into the press and then comparing it with the exact profile exiting the press 102. The process of the alteration can then be fine tuned. Further improvements involve experiments to determine whether heating or moisture is the predominant or better parameter to manipulate. A disadvantage with the present process though is that it is a dynamic process such that monitoring it can actually change it. In other words, by changing the operating speed or stopping and starting oper¬ ation to monitor the process, the process is itself altered. Control is lost over the temperature and moisture profiles by stopping and start¬ ing the system. Even during constant operation the transit time between mat formation and microwave heating varies between one- half hour and one hour depending on where the strand is deposited in the one hundred and twenty foot long layup trough (102). The exo¬ thermic reaction time thus varies through the cross-section. The temperature can initially rise and begin to fall again. In some instances, it continues to rise and in others it falls, depending on the location in the mat and the extent of insulation, or whether water is continuing to be applied from the atmosphere, a jet or a spray.

Ideally, a system (110, 112) would be developed to accurately monitor the temperature, for example, of the bottom of the mat after some of the techniques described above have been used to manipulate the temperature and moisture content. This can be done with infra¬ red surface measurements. Moisture meters working on differential infrared and optical properties, that is, visible light techniques, can be adapted for use herein. Either absorption or reflectivity of the light is measured, and it represents a relatively sensitive function of moisture content. This type of moisture meter has been used in the past in the particle board industry to determine moisture content, wherein the conveyor is flowed past a sensor and the moisture con¬ tent determined by infrared invisible light techniques. Infrared heat¬ ing techniques can also be used to alter or monitor the temperature directly.

The very bottom of the mat can be as low as 20 or 25°C, basi¬ cally ambient temperature, whereas the middle of the mat can go up to a hot 40 °C. This is a self-generated heat often just from the mois¬ ture uptake after glue application. Thus, to heat the surfaces, a small amount of moisture is added or the surface is insulated to prevent natural evaporation and heat loss therefrom; insulation from both heat loss and moisture loss can thereby be made. When the wood heats, it tends to give off a bit of moisture as well as to chemically absorb it into its structure. Proper application of the present process allows essentially any desired temperature profile to be obtained. For example, with the center at 45 or 50 ° , the outer surfaces have been with the process of this invention heated to as high as 35 or 40 ° . This is essentially within five degrees of the center temperature, which is usually sufficient for the currently used apparatus.

This process can be used to directly apply heat and steam to the very bottom of the mat, as previously described with respect to Figure 3, and thereby gain higher than normal density in that area, that is, along the very bottom face or the bottom couple of inches. Thus, the temperature profile can be inverted and thereby the density profile inverted. Although the main objective of this invention is to obtain a homogeneous wood product, there may be instances where a different profile is desired. For example, if a relatively thin board is made, which is not going to be remanufactured, a relatively high density surface may be desirable. Certain kinds of thin (about .5 inch) panel products have a high surface density and a low core density which give strength properties that are more beneficial and thereby give it a higher moment of inertia for the total mass and the same thickness. These boards are typically used in furniture where the high density surface is usually a better consolidated surface and takes fin¬ ishes better. In the present case this process could make stiffer scaf¬ fold planks or more stable or stiffer beams (e.g., 3.5 inches by sixteen inches) with denser faces at the extremes of the sixteen inch dimen¬ sion. The processes of the present invention are particularly directed to wood products having thickness between generally two and twelve inches.

Another embodiment of the altering system 108 is illustrated in Figure 4 wherein it is primarily the bottom and middle portions of the mat 172 whose temperature and moisture content are being altered. The basic layup system, such as shown in the 723 application, includes the layup trough 174 associated with the conveyor 102 and in which the mat 172 is conveyed and the oscillating table 176 and chute 178 for directing and depositing the wood strands of the mat 172 within the trough 174. Additional layup systems coordinated therewith for this same layup trough and as would be understood from the 732 application can be provided. The temperature and moisture content profiles of the mat 172 are controUably altered as the mat is being formed (as the strands are being laid on the trough 174) by spraying a fine mist 180 of water out through a spray nozzle 182 on the bottom strands and then blowing, by fans 184 positioned adjacent the layup trough 174, cooling air to cool the strands which in this case are in the center of the mat. The fans 184 are preferably relatively large fans with fractional horsepower motors, such as standard room fans used by painters to ventilate rooms which are being painted. A disad¬ vantage of this technique though is that the resulting mat is cold and thereby resists compression and is difficult to pull through the press (104).

For the bottom steam heating technique, as in Figure 3, the amount of steam 160 will vary as needed to raise the temperature and can be measured by flow meters (not shown). The amount of steam used depends on a number of factors, such as how well the steam jet is aimed and focused, how hot the steam is and how much the steam is diluted by the surrounding air. True live steam directly impinging upon the wood mat would probably be too hot and could at least par¬ tially precure the glue. Again, care must be taken, under any of the techniques of this invention, to not precure the glue, such as might happen if the line stops moving and live steam continues to impinge on the product for even a very few additional seconds.

Referring again to Figure 2 wherein a steam jet 120 is used to heat a metal belt 122, the belt is heated so that its temperature rises by generally twenty degrees which brings it up to a 30 to 50° temper¬ ature range. It thereby has energy to give up to the product. If the object is to overcompensate — to provide a hotter base of the mat than center — then a hotter steel belt is probably needed. The com¬ posite assemblies or strands do not rest entirely on the steel belt 122 though, since they sit on little ridges of the steel belt. Thus, the very bottom surface of the mat is taking radiant or convective, rather than conductive, heat from the steel arid thereby can tolerate a higher temperature in the steel belt 122 without precuring. By heating the steel belt 122 to a temperature at least as hot as the lower surface of the mat, the steel then does not act as a heat sink. If the steel is very cold, the strands next to it will never go through the chemical pro¬ cesses of absorbing water fast enough to raise the temperature of the wood. Thus, by having a bed of hot steel close by, the progression of chemical reaction of water uptake is encouraged.

Where a multi-layup head system is used, such as described in the 732 application, the temperature and/or moisture contents of one or more of the layup heads can be varied to thereby locally affect the temperature and/or moisture contents of the mat thereby produced in the conveyor belt trough. Either the moisture and/or the temperature of the strands can be differentially altered in one or more of the layup heads. For example, the adhesive or glue used can be hotter with one layup head than with the other to heat the top and bottom stranders. Warm glue can be used on the faces and thereby give the chemical reaction a kick start. The glue can be gently and delicately heated with industrial heat exchange units (not shown) and without curing the glue. The glue pot life is dramatically affected if the glue is heated much above 20 °C. For example, its pot life can be reduced by a factor of approximately ten (depending on the resin) if the glue is heated to 40 °C.

From the foregoing detailed description, it will be evident that there are a number of changes, adaptations and modifications of the present invention which come within the province of those skilled in the art. However, it is intended that all such variations not departing from the spirit of the invention be considered as within the scope thereof as limited solely by the claims appended hereto.

Claims

WHAT IS CLAIMED IS;

1. A layup process, said process comprising the steps of: forming a layup, during said forming step altering at least one of temperature or moisture levels in resulting localized areas of a wood adhesive composite layup thereby forming the layup; and thereafter, pressing and microwave curing the formed layup and thereby creating a structural wood based product; said altering step improving the density gradient of the structural wood based product.

2. The process of claim 1 wherein said altering step heats the composite layup without precuring the adhesives thereof.

3. The process of claim 1 wherein said altering step raises the temperature of the localized areas and thereby causes water to be absorbed faster through a process, which is a chemical process.

4. The process of claim 1 wherein said forming step includes generally separately forming first and second layups and overlapping the first and second layups to form the composite layup.

5. The process of claim 4 wherein said altering step includes the first and second layups having different temperatures immediately prior to said overlapping.

6. The process of claim 5 wherein said different tempera¬ tures differ by at least 15 °C.

- 7. The process of claim 5 wherein said first and second layup forming steps include using a first adhesive in the first layup and a second adhesive substantially different in temperature than the first adhesive in the second layup.

8. The process of claim 4 wherein said altering step includes the first and second layups having different moisture con¬ tents immediately prior to said overlapping.

9. The process of claim 8 wherein said different moisture contents differ by at least two percent.

10. The process of claim 1 wherein said forming step includes forming the composite layup as a multi-layer strand layup from a plurality of strand layups at least one of which is at least 10 °C warmer than another.

11. The process of claim 1 wherein said pressing and micro¬ wave curing step includes conveying the formed layup on a continuous belt through a microwave curing press assembly.

12. The process of claim 1 wherein said adjusting step includes heat insulating the localized area.

13. The process of claim 1 wherein the final product density gradient is an even density gradient varying by less than 5% from one product cross-sectional area to another.

14. The process of claim 1 wherein said pressing and micro¬ wave curing step includes conveying the formed layup through a microwave press assembly.

15. The process of claim 14 further comprising determining a physical characteristic profile differing from the preferred and resulting in a layup conveyed through the microwave press assembly, and said altering step compensating at least in part for the deter¬ mined uneven density profile.

16. The process of claim 14 further comprising comparing physical characteristic profiles of a layup before entering the micro¬ wave press assembly, the resulting product exiting therefrom, and a preferred resulting product profile, and said altering step taking into account the differences of the profiles.

17. The process of claim 14 wherein said altering step takes into account uneven heating patterns of the microwave press assembly.

18. The process of claim 1 wherein said altering step includes injecting fluid on portions of the layup.

19. The process of claim 18 wherein said fluid is substan¬ tially hotter than the layup portions.

20. The process of claim 18 wherein said fluid is a liquid.

21. The process of claim 18 wherein r;aid fluid is a mist.

22. The process of claim 18 wherein said fluid is a gas.

23. The process of claim 1 wherein said altering step includes heating the localized areas with radiant or convective heat.

24. The process of claim 1 wherein said altering step includes heating the localized areas with conductive heat.

25. The process of claim 1 wherein said pressing and curing step includes utilizing a microwave press assembly, and said forming step includes conveying the layup to the microwave press assembly on a heat conductive conveyor belt.

26. The process of claim 25 wherein said altering step includes heating the conveyor belt.

27. The process of claim 26 wherein said heating step includes heating by at least 10 °C the layer of layup adjacent the heated conveyor belt.

28. The process of claim l wherein said altering step is con¬ trolled to compensate for uneven microwave deposition patterns dur¬ ing said pressing and microwave curing step.

29. The process of claim 1 further comprising controlling said altering step to thereby compensate for uneven microwave depo¬ sition patterns during said microwave curing step.

30. A layup microwave curing process, said process com¬ prising the steps of: adjusting the level of at least one of moisture or temper¬ ature levels in at least one preselected and localized area of a wood based layup; thereafter, pressing and microwave curing the adjusted layup; and said adjusting step improving the density of the pressed and cured layup product.

31. The process of claim 30 wherein said adjusting step includes altering the moisture level in the localized area.

32. The process of claim 31 wherein said altering step includes increasing said moisture level.

33. The process of claim 31 wherein said altering step includes decreasing said moisture level.

34. The process of claim 31 wherein said adjusting step includes altering the temperature of the localized area. 92/01540

35. The process of claim 30 wherein said adjusting step includes altering the temperature of the localized area.

36. The process of claim 35 wherein said altering step includes increasing said temperature.

37. The process of claim 35 wherein said altering step mcludes decreasing said temperature.

38. The process of claim 30 wherein said localized area includes the bottom layer of the layup.

39. The process of claim 30 wherein said localized area includes the top layer of the layup.

40. The process of claim 30 wherein said localized area includes a central layer of the layup, spaced from the top and bottom thereof.

41. The process of claim 30 wherein said pressing and microwave curing step includes continuously conveying the layup through a microwave press assembly.

42. The process of claim 30 further comprising conveying the layup on a conveyor belt to the location of said pressing and microwave step.

43. The process of claim 42 wherein said adjusting step includes altering the temperature of the conveyor belt and thereby of the adjacent area of the layup.

44. The process of claim 43 wherein the conveyor belt com¬ prises a segmented metal transport belt.

45. The process of claim 44 wherein said altering step includes heating the segmented metal transport belt.

46. The process of claim 42 wherein said altering step includes subjecting the conveyor belt to a jet of hot steam.

47. The process of claim 46 wherein the belt includes a plu¬ rality of spaced support ridges, and said altering includes transmitting heat from the belt to the layup via the ridges.

48. The process of claim 42 further comprising heating the belt so that it is at least as hot as the adjacent layup surface and thereby does not function as a heat sink for the layup.

49. A wood composite microwave curing process, comprising: microwave heating a composite wood mat; and before said heating step, manipulating the temperature and moisture content profile of the composite wood mat, and thereby improving the density gradient of the mat after being microwave heated in said microwave heating step.

50. A wood base layup pressing and curing process, comprising; controUably adjusting the level of at least one of mois¬ ture and temperature in at least one preselected and localized area of a wood based layup to thereby improve the density of the pressed and cured layup product; thereafter, pressing and microwave curing the adjusted layup.

51. A layup process, comprising: altering at least one of the cross-sectional temperature, moisture and density profiles of a curable wood strand layup; and thereafter, pressing and microwave curing the altered layup to form a structural wood product; wherein said altering step results in an improved density profile of the wood product.

52. A composite wood forming system, comprising: forming means for forming an adhesive, wood composite layup mat; a microwave curing press; and conveying means for conveying the mat to said micro¬ wave curing press; wherein said forming means includes altering means for altering at least one of the temperature and moisture cross-sectional profiles of the mat generally before it enters said microwave curing press and thereby improving the density gradient of the resulting pressed and cured wood product to a coefficient of variation of at least ten percent.

53. The system of claim 52 wherein said altering means includes changing means for controUably changing the temperature in at least one preselected area of the mat.

54. The system of claim 52 wherein said altering means includes changing means for controUably changing the moisture con¬ tent in at least one preselected area of the mat.

55. The system of claim 52 wherein said conveying means includes a conveyor belt, and said altering means includes changing means for changing the temperature of said conveyor belt and thereby of the bottom layer of the mat.

56. The system of claim 55 wherein said changing means includes steam heating means for steam heating said conveyor belt generally before the mat is deposited thereon.

57. The system of claim 52 wherein said altering means includes blowing means for blowing cooling air across the top surface of the mat during mat formation.

58. The system of claim 52 wherein said altering means includes blowing means for blowing cooling air across the top surface of the mat after mat formation.

59. The system of claim 52 wherein said altering means includes spraying means for spraying a water mist on the top surface of the mat being formed.

60. The system of claim 52 wherein said conveying means includes a conveyor trough, said forming means includes depositing means for depositing wood strands on said conveyor trough to thereby at least in part form the mat, and said altering means alters at least one of the temperature and moisture content of some of the wood strands before said depositing means deposits them on said conveyor trough.

61. The system of claim 60 wherein said altering means selectively heats only some of the wood strands.