CA2322067A1 - Method and apparatus for post-production dimensional stabilization of lignocellulosic or cellulosic fibre-based composite products using high-frequency heating and pressure - Google Patents

Abstract

A process for post-formation treatment of composite wood products formed from lignocellulosic or cellulosic material to improve dimensional stability. The process involves raising the moisture content of the product after forming of the product, applying pressure to the product, and then subjecting the compressed product to high frequency heating. Apparatus for carrying out the process is also disclosed. The process and apparatus create a finished composite wood product that has significantly improved dimensional stability without adversely affecting other physical and mechanical properties of the product.
In particular, thickness swelling of the composite product when exposed to moisture is significantly reduced. In testing, a 60%
reduction in thickness swell is observed at all points measured across a test sample.

Description

METHOD AND APPARATUS FOR POST-PRODUCTION DIMENSIONAL
STABILIZATION OF LIGNOCELLULOSIC OR CELLULOSIC
FIBRE-BASED COMPOSITE PRODUCTS USING HIGH-FREQUENCY
HEATING AND PRESSURE
FIELD OF THE INVENTION
This invention relates generally to the field of composite fibre-based wood products, and in particular, to a method and apparatus for treating finished products to improve their dimensional stability.
BACKGROUND OF THE INVENTION
Plywood is manufactured from layers of wood or plies that are cut or peeled from logs. Each layer has a grain, and in the assembled plywood board, adjacent layers having grains running in different directions are stacked and glued together to maximize the strength of the board. Plywood panels have good dimensional stability, that is, they tend to maintain their shape and size when exposed to moisture which makes them the preferred panel for load bearing components in wood frame construction or for panels that will be exposed to the environment. In particular, plywood panels are preferred for flooring and sub-flooring applications.
Plywood panels tend to be more expensive to manufacture in view of higher raw material costs for logs suitable for conversion into plies. In addition, plywood manufacturing process tends to be labour intensive with resulting increased labour costs.
In view of the relatively high cost of plywood panels, alternative less expensive wood based panel products have been developed. In general, these alternative panels are composite products that are manufactured from lignocellulosic or cellulosic materials such as chips, strands or flakes. The chips, strands or flakes are mixed with glue or resin in a homogenous mass and then pressed and heated at high temperatures and pressures to form structural panels. Oriented strand board (OSB) is an example of such a composite wood panel.
While conventional composite panels are equivalent to plywood panels in terms of structural strength and tend to be less expensive, they currently suffer from the drawback that they lack dimensional stability when exposed to humidity or water. In other words, composite wood products tend to increase significantly in thickness and to a lesser degree in length when they become wet.
Under normal manufacturing conditions, wood composite products are highly compacted. When exposed to moisture, the product tends to swell and to a greater extent than for the original wood material due to the compaction. For example, the thickness swell for oriented strand board is 10-15~ compared to 4-7~ for solid wood or a plywood panel of an identical original thickness. Such thickness swell makes composite wood panel products less desirable for certain structural or exposed applications where plywood still dominates.
Considerable efforts have been made in the composite wood products industry to address the problem of swelling, particularly thickness swelling. Commonly used method include using greater amounts of resin and wax. Unfortunately, this approach increases the cost of the final product with the result that specially treated composite wood products approach or exceed the cost of plywood panels thereby defeating the purpose of a lower cost alternative to plywood panels.
To reduce the cost of adding additional resins or waxes, heat treating methods have been proposed which involve compacting the wood particles under higher temperatures and for longer periods of time. These solutions have turned out not to be practical as they cause charring or discolouration or a reduction in productivity.

Other heat treating methods interfere with the normal production process as they require pre-treatment of wood furnish or post-treatment of individual panels in special presses or post-treatment of multiple boards in a conditioning chamber which significantly lowers productivity and raises costs.
Examples of alternative processes known to the applicant include that disclosed in Japanese Kokai Patent Application No. 6-238615. This reference discloses a method for treating woody material to improve dimensional stability. The process can be used with lumber or with composite panels. A special jig for controlling the thickness of the product is provided and the process requires a sealing material about the peripheral edges of the product. Pressing (compacting) and treating are done in a single step which requires major modifications to existing presses. Productivity suffers because the process is carried out on a single panel at a time with a longer pressing time than is conventional.
United States Patent No. 5,028,286 to Hsu discloses a method of making dimensionally stable composite board that involves a steam pre-treatment and then heating and compressing with a hot platen press. In practice, the Hsu method is limited to treating one panel at a time because the required platen heating of panels would take too long if multiple panels were stacked together.
moue et al. in the paper "Stabilization of compressed wood using high frequency heating" discloses a treatment method for densifying and stabilizing solid wood (lumber). The process involves pressing and treating a single wood piece at the same time and relies on a special jig that restrains the edges of the wood under treatment. Also, the process requires feeding of cold water into the press platens at the end of the treatment to cool the board before the press opens. This water cooling method cannot be applied to treating multiple large sized boards.

SUI~~iARY OF THE INVENTION
Accordingly, there is a need for a treatment method for composite wood products that addresses the shortcoming described above.
Applicant has developed a method and apparatus for efficient treatment of composite wood boards to improve dimensional stability without significantly increasing costs. The process of the present invention is carried out after the conventional manufacturing process on multiple boards at a time so that the treatment process does not interfere with the normal manufacturing process.
Accordingly, the present invention provides a process for post-formation treatment of composite wood products formed from lignocellulosic or cellulosic material comprising the steps of:
raising the moisture content of the product after forming of the product;
applying pressure to the product;
subjecting the compressed product to high frequency heating to create a finished product having improved dimensional stability.
The present invention also provides apparatus for post-formation treatment of composite wood products formed from lignocellulosic or cellulosic material comprising:
a moisture application station to increase the moisture content of the formed wood product;

a press to apply pressure to the wood product after the moisture application station; and high frequency heating apparatus to generate heat in the wood 5 product while in the press whereby a wood product of improved dimensional stability results.
The apparatus and method of the present invention allow for production of a composite wood product that has significantly improved dimensional stability. In particular, thickness swelling of an oriented strand board (OSB) product processed according to the process of the present invention was significantly reduced.
In testing, a 60~ reduction in thickness swell is observed at all points measured across the O$B test sample.
BRIEF DESCRIPTION OF THE DRAWINGS
Aspects of the present invention are illustrated, merely by way of example, in the accompanying drawings in which:
Figure 1 is a schematic view of a preferred embodiment of the apparatus of the present invention;
Figure la) is a detail view of a water bath that can be used in an alternative arrangement of the apparatus of the present invention;
Figure lb) is a detail view of a high humidity chamber that can be used in an alternative arrangement of the apparatus of the present invention; and Figure 2 is a graph showing the difference in thickness swelling of oriented strand board treated according to the method of the present invention and as manufactured conventionally.

DESCRIPTION OF THE PREFERRED ~ODIMBNTS
Referring to Figure 1, there is shown schematically a preferred embodiment of the apparatus of the present invention for carrying out a method that improves the dimensional stability of composite wood products. The apparatus is positioned at the output end 10 of a conventional press 12 for manufacture of composite wood products. Composite wood products include any products formed from lignocellulosic or cellulosic material such as particles, fibres, flakes or strands, that are mixed with a binder material and compressed and heated to create a finished article. In many cases, the finished article is a board for use in building construction. An example of a composite wood product is oriented strand board (OSB). Other products include particleboard, medium density fibreboard (MDF), TimberstrandTM, ParallamTM, and strawboard. Under normal manufacturing conditions, wood composite products are highly compacted at high temperature for a short time in press 12, and are glued with as little resin as possible. When exposed to humidity of water, the product tends to swell to a greater extent than lumber due to the compaction process. For example, the thickness swell for OSB is 10-15 ~ compared to only 4-7~ for solid wood.
The apparatus and method of the present application have been developed to significantly improve the dimensional stability of composite wood products. In Figure 1, as finished boards 14 are produced by the conventional manufacturing process, they are delivered to conveyor 16 for transport to a moisture application station 18 to increase the moisture content of the board. For example, manufactured OSB boards usually have a very low internal moisture content ranging from between 2~ to 6~ moisture by weight.
Moisture application station 18 is necessary to increase the moisture content of the OSB board prior to further treatment according to the method of the present invention. Various methods for increasing the moisture content are possible. For example, as illustrated in Figure 1, one or more spray nozzles can be provided to direct water to the surface of board 14. Alternatively, the moisture application station can include a bath 22 (Figure la) into which the boards are submerged or a chamber 24 (Figure lb) having an internal environment at a higher relative humidity than the moisture content of the board. The spray nozzle and bath arrangements provide the quickest and most efficient method for increasing the moisture content of the board. In prototype testing, the board was immersed in water for at least one minute.
When the moisture content of a board is increased by spray nozzles or immersion in a bath, it is preferable to allow for a period during spraying or immersion for the moisture to further penetrate into the board. In addition, it is preferable that the board be stacked with others into piles 28 so that subsequent processing steps can be carried out on multiple boards at a time to increase the efficiency of the process. Stacking of the boards provides a convenient opportunity for additional penetration of moisture. Stacking is performed at station 25 (Figure 1) and can be handled by conventional stacking equipment or done manually.
Preferably, stacked boards are left for at least 15 minutes to allow for further penetration of moisture into the interior of the boards.
In the case of boards that are treated in a high humidity chamber, raising the moisture content of the boards is a much slower process. In testing, conditioning periods of 3 weeks are used to increase the moisture content of the boards to over 10~ by weight.
It is preferable that the moisture content of a board be increased to at least 10~ prior to further processing.
After increasing the moisture content and stacking, the piles of boards are transported to a press 30 to apply pressure to the boards. Press 30 is preferably a conventional platen press having spaced platens 31 and 32 that are movable relative to each other to compress boards 14 introduced between the platens.
While the boards are compressed, high frequency heating apparatus 36 is used to generate heat in the boards. Preferably, the high frequency heating apparatus comprises a conventional radio frequency (RF) heating unit that is able to generate heat quickly and uniformly in the boards. High frequency heating units work by creating an alternating electric field between two spaced electrodes 36a and 36b. The material to be heated, in this case the boards, is introduced between the electrodes where the alternating electrical field causes polar molecules in the material to continuously reorient themselves with respect to the electrodes. The friction resulting from the molecular movement causes the material to rapidly heat throughout its entire mass.
The amount of heat generated is determined by the frequency, the applied voltage, the dimensions of the material and the dielectric loss factor which is essentially a measure of the ease with which the material can be heated by the high frequency waves. Microwave heating units are also suitable for use with the process and apparatus of the present invention as they operate according to the same concept but at much higher frequencies.
The increased moisture content of the boards significantly increases the heating effect of the high frequency heating units.
External pressure applied by the press acts to seal any steam generated by the heating of natural or added water inside the board. The press also prevents the board structure from changing due to heat or steam expansion.
As best shown in Figure 1, the spaced press platens 31 and 32 define generally parallel planes above and below the pile of boards 14. The stacked pile of boards 14 are introduced into the press such that the boards are parallel to the platens to ensure that uniform pressure is applied to the boards. Based on testing, press 30 preferably operates to apply pressure in the range of 150-250psi. Pressure is applied while the boards are heated for a period of at least 4.5 minutes.
In a preferred embodiment, the electrodes 36a,36b of the high frequency heating unit extend transverse to the planes of the platens. Alternatively, as shown by dashed lines, the heating unit electrodes can extend parallel to the platen planes. In the cases where the electrodes are parallel, they can be incorporated into the press platens. In testing, a radio frequency heating unit of lOkW and 6.13 MHz was used. The power output of the unit was kept constant throughout heating by gradually increasing the current applied across the electrode plates. Normally, as the boards dry, the power decreases and the current must be increased to compensate. High frequency heating for at least 4.5 minutes is required to enjoy the improved dimensional stability benefits of the method of the present invention.
After the stacked boards 14 are subjected to pressure and heating for a predetermined period, the high frequency heating unit is stopped and the press is released. Testing has shown that at this point, the boards emerging from the press having significantly improved dimensional stability. In particular, thickness swelling tends to be reduced by about 60~ at all test points measured across the surface of OSB test samples.
The step of releasing the pressure of the platen press can be done rapidly or gradually. The treated boards are allowed to cool (stage 40 in Figure 1)and then subjected to sanding and other conventional finishing steps (stage 42 in Figure 1) to produce finished composite wood products 45 suitable for sale.
The following specific examples will further illustrate the practice and advantages of the present invention:

Example 1 Commercially manufactured OSB panels (4ft by 8ft by 5/8 5 inches) were cut into sample pieces of 6 inches by 48 inches for fitting within a prototype press and heating unit according to the present invention. The sample pieces were conditioned to an equilibrium moisture content of 15% and then subjected to compression and radio frequency heating (lOkW and 6.13MHz).

10 During treatment, 6 samples were stacked in parallel between aligned press platens. The electrode plates were perpendicular to the sample boards. The press was operated at a pressure of 190psi. Heating time was 4.5 minutes and the press was opened immediately without holding or cooling.
After treatment, samples were conditioned to the same moisture and temperature as the control samples which were cut from the panel source. Then both treated and control samples underwent 24 hr. water soak test according to Canadian Standard Association (CSA 0437 Series -93). Figure 2 is a graph showing the difference in thickness swell between the untreated OSB board samples and the treated samples. The graph indicates that the treatment of the present invention results in a 60% reduction in the thickness swell for an OSB board at all points measured across the tested samples.
Example 2 Conventional heat treatment of OSB can cause strength reduction due to thermal degradation. This example shows that there is no significant negative effects on the physical and mechanical properties of OSB board treated according to the present invention.

Commercial OSB panels were cut into 6 inches by 40 inch samples. To quickly increase panel moisture content, 6 samples were soaked in a water bath and then stacked before treatment.
The initial water content of the boards was 6~ by weight and the boards were soaked for one minute and left in a stack for 15 minutes.
The stacked samples were then compressed at 190 psi and subjected to RF heating for 15 minutes. The press was opened gradually over 5 minutes to allow the compression pressure to drop to zero.
After treatment, the samples were conditioned to the same moisture content and temperature as the control samples and then tested according to CSA standards. The results are tabulated in Table 1 which indicates that while thickness swell is significantly reduced, other mechanical and physical properties such as internal bond strength (MPa), bending modulus of elasticity (MPa), modulus of rupture (MPa) and linear expansion (~) in both the parallel to strand orientation and perpendicular to strand orientation are substantially the same for treated and untreated samples. In Table 1 and subsequent tables, the following abbreviations are used:
TS - thickness swell IB -- internal bond strength MOE~~ and MOR~~ -- bending modulus of elasticity and modulus of rupture in the parallel to strand orientations MOE1 and MOR1-- bending modulus of elasticity and modulus of rupture in the perpendicular to strand orientations LE~~ and LE1 -- linear expansion in both directions Table 1. Comparing mechanical and physical properties between treated and untreated OSB panels TS IB MOEii MORiiLEii MOE1 MORl LEl (%) (MPa)(MPa) (MPa)(%) (MPa) (MPa)(%) Control 9.8 0.42 5735 32.070.20 2327 16.850.31 Treated 4.7 0.39 5618 33.340.19 1827 14.820.30 Example 3 One of the key steps in the process of the present invention is the increasing of the moisture content of the board.
Manufactured OSB panels are usually very dry with a moisture content that ranges from 2~ to 6~ by weight. Low moisture content affects the polarity of OSB panels and thus reduces the efficiency of high frequency heating. This example will demonstrate this effect and will serve to demonstrate that soaking boards in a bath tends to be more effective than conditioning in a high humidity chamber.
OSB samples from the same source as in Examples 1 and 2 were cut into samples having dimensions of 6 inches by 40 inches. The samples were divided into three groups. Group 1 samples had a moisture content of 6~ under ambient conditions. Group 2 samples had a moisture content of 10~ after conditioning for 4 weeks in a humidity chamber. Group 3 samples had a moisture content of 10~
after a one minute soaking in a water bath. All three groups were then pressed at 190psi and heated using a RF heating unit for 15 minutes. The press was gradually opened over a five minute period.
The results are tabulated in Table 2. Comparison between Group 1 and 2 samples clearly indicate the advantage of adding water for high frequency heating. The thickness swell was reduced from 7.7~ to 5.7~ without significantly affecting panel mechanical properties. It is also worth noting that the thickness swell was further reduced by 1~ to 4.7~ if moisture was added through a water bath. This unexpected advantage probably results from moisture being concentrated in the higher density surface layers in the water soaking process. The higher density surface layers are where most of the thickness swelling occurs.
In addition, during the treatment test, it was found that the internal panel temperature was much higher for Group 3 samples than for Group 2 samples even though the moisture content for both groups was 10~s. Once again, this would appear to result from the concentration of moisture in the higher density surface layers due to soaking of the Group 3 samples.
Table 2. The effect of water addition (WAl on dimensional stability of l5mm OSB panels WA TS IB MOEii MORii LEii MOEl MORl LEl (%) (%) (MPa) (MPa) (MPa) (%) (MPa) (MPa) (%) 2 0 Group 1 6.0 7.7 0.37 5415 35 0.17 2183 16 0.29 Group 2 10.0 5.7 0.36 / / / 1746 15 0.28 Group 3 10.1 4.7 0.39 5618 33 0.19 1827 15 0.30 Example 4 This example demonstrates that maintaining pressure during heating is important to minimize thickness swell and to maintain the structural properties of the treated panels.
Once again, commercially manufacture OSB panels of dimensions Oft by 8ft by % inches were cut into 6 inch by 40 inch sample pieces. The samples were divided into three groups as follows:
Group 1 is a control group Group 2 is heated using RF heating without pressure Group 3 is heated using RF heating and compressed at 190psi.
Other treatment conditions were as follows:

Initial panel moisture content: 6~
Water soaking time: 1 minute Stacking time: 15 minute RF heating time 15 minutes Press Opening time: 5 minutes(time during which pressure drops to zero) The results are shown in Table 3. While both Group 2 and 3 samples enjoyed significantly less thickness swell than control group 1, further thickness swell reduction is achieved by applying pressure during treatment. This is most likely due to the fact that compressing the panels with the press platens minimizes the loss of generated steam to the atmosphere and retains the steam in the panels resulting in accelerated stress relaxation needed for dimensional stabilization. Applying pressure during treatment also yields the advantage of minimizing the reduction of bending stiffness parallel to the strand direction(compare MOE~~ between Group 3 and Group 2 samples).
Table 3. Comparing 19 mm Panels properties between untreated and treated with and without pressure TS IB MOE// MORN/ MOEl MOR1 (%) (MPa) (MPa) (MPa) (MPa) (MPa) Group 1 (Control) 9.1 0.22 4203 22 / /
Group 2 5.5 0.19 3323 25 1838 13 3 0 Group 3 4.0 0.17 3903 23 1808 15 Example 5 This example demonstrates the effect of sudden or gradual press opening on the thickness swell and other properties of the processed boards.

Commercially manufactured OSB panels were separated into two treatment groups: Group 1 panels were treated according to a process that involves rapid openings of the press to suddenly release pressure and Group 2 panels were subjected to a gradual 5 five minute opening of the press to slowly release the pressure on the boards. Other treatment parameters are as follows:
Initial panel moisture content: 6~
Water soaking time: 10 minutes 10 Stacking time: 15 minutes RF heating time: 10 minutes Platen pressure: 190 psi The results of physical properties testing on the treated 15 samples are listed in Table 4. Press opening time appears to have little effect on the thickness swell and physical properties of the panels. A possible explanation is that toward the end of treatment, the steam pressure generated by heating of the internal water is not high enough to break the glue bonds already formed in the manufacturing process. This provides the advantage that it is not necessary to build press opening controls for automating gradual opening of the press. As well, the overall treatment time is reduced by eliminating press opening time.
2 5 Table 4. Comparing properties of 19 mm panels treated with sudden and gradual press opening TS IB MOE// MORN/ MOEl MORl (%) (MPa) (MPa) (MPa) (MPa) (MPa) 3 0 Group 1 5.0 0.20 3456 19 1640 11 Group 2 4.4 0.19 3582 21 1657 12 35 Example 6 This example demonstrates the effect of the configuration of the boards to be treated and the high frequency heating units.

Commercially manufactured OSB panels with size of 4 ft by 8 ft and ~; inches were cut into 6 inches by 40 inches samples and the samples were broken into three groups. Group 1 was a control with no treatment. Group 2 (parallel configuration) boards were heated by RF heating electrode plates arranged parallel to the stacked boards. Group 3 (perpendicular configuration) boards were heated by electrode plates extending transversely to both the press platens and the stacked boards. In both cases, the platen pressure was 190 psi. Other treatment conditions were as follows:
Initial panel moisture content: 6~s Water soaking time: 1 minute Stacking time: 15 minutes RF heat treatment time: 15 minutes Press opening time: 5 minutes (time during which pressure drops to zero) .
The treated and control sample boards were then conditioned to the same moisture and temperature and exposed to a water soak test. The thickness swell for the perpendicular configuration boards was 4.0~ compared to 6.1~ for the parallel configuration boards and 9.2~ for the control boards. The results indicate that the optimal configuration of the apparatus is for the electrode plates of the heating unit to be arranged transverse to the plane of the boards and for the press platens to be arranged in parallel to the plane of the boards. It is speculated that in this configuration, the moisture in the surface layers of the boards is optimally exposed in parallel to the electrical field of the high frequency heating unit. In the configuration when the boards and the heating unit electrodes are parallel, the drier internal core of the boards are exposed at right angles to the electrical field with the result that less heating of the boards occurs and the reduction in thickness swelling is not as significant. Note, however, that both configurations using high frequency heating resulted in improved dimensional stability of the treated boards as compared to a non-treated control sample.
Although the present invention has been described in some detail by way of example for purposes of clarity and understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims.

Claims (36)

1. A process for post-formation treatment of composite wood products formed from lignocellulosic or cellulosic material comprising the steps of:
raising the moisture content of the product after forming of the product;
applying pressure to the product;
subjecting the compressed product to high frequency heating to create a finished product having improved dimensional stability.

2. A process as claimed in claim 1 including the additional step of collecting multiple products in a stack and applying the steps of the process to the stack of products.

3. A process as claimed in claim 1 in which the step of raising the moisture content of the product involves spraying the product with water.

4. A process as claimed in claim 1 in which the step of raising the moisture content of the product involves submerging the product in a water bath.

5. A process as claimed in claim 1 in which the step of raising the moisture content of the product involves exposing the product to a high humidity environment.

6. A process as claimed in claim 1 in which the step of raising the moisture content of the product is performed to raise the moisture content to at least 6% by weight.

7. A process as claimed in claim 1 in which the step of high frequency heating of the product involves radio frequency (RF) heating.

8. A process as claimed in claim 1 in which the step of high frequency heating of the product involves microwave heating.

9. A process as claimed in claim 1 in which the step of high frequency heating of the product is performed for at least 4.5 minutes.

10. A process as claimed in claim 1 in which step of applying pressure to the product includes the step of introducing the product to a platen press having upper and lower platens that compress the product.

11. A process as claimed in claim 1 in which the step of applying pressure to the product involves applying pressure in the range of about 150-250 psi for a period of at least 4.5 minutes.

12. A process as claimed in claim 1 including the additional step of immediately releasing pressure from the product after high frequency heating.

13. A process as claimed in claim 1 including the additional step of gradually releasing pressure from the product after high frequency heating over a period of at least one minute.

14. The composite board product manufactured according to the process of claim 1.

15. A process for making composite board comprising the steps of:
forming a board from lignocellulosic or cellulosic material by mixing with a binder material and compressing with heating;
raising the moisture content of the formed board;
applying pressure to compress the board;

subjecting the compressed board to high frequency heating to create a processed board having improved dimensional stability.

16. A process as claimed in claim 15 including the additional step of collecting multiple boards in a stack after raising the moisture content and applying all subsequent steps to the stack of boards.

17. A process as claimed in claim 15 in which the step of raising the moisture content of the board involves spraying the board with water.

18. A process as claimed in claim 15 in which the step of raising the moisture content of the board involves submerging the board in a water bath.

19. A process as claimed in claim 15 in which the step of raising the moisture content of the board involves exposing the board to a high humidity environment.

20. A process as claimed in claim 15 in which the step of raising the moisture content of the board is performed to raise the moisture content to at least 6% by weight.

21. A process as claimed in claim 15 in which the step of high frequency heating of the board involves radio frequency (RF) heating.

22. A process as claimed in claim 15 the step of high frequency heating of the board involves microwave heating.

23. A process as claimed in claim 15 in which the step of high frequency heating of the product is performed for at least 4.5 minutes.

24. A process as claimed in claim 15 in which step of applying pressure to the board includes the step of introducing the board to a platen press having upper and lower platens that compress the board.

25. A process as claimed in claim 15 in which the step of applying pressure to the board involves applying pressure in the range of about 150-250 psi for at least 4.5 minutes.

26. A process as claimed in claim 15 in including the additional step of immediately releasing pressure from the board after high frequency heating.

27. A process as claimed in claim 15 in including the additional step of gradually releasing pressure from the board after high frequency heating over a period of at least one minute.

28. Apparatus for post-formation treatment of composite wood products formed from lignocellulosic or cellulosic material comprising:
a moisture application station to increase the moisture content of the formed wood products;
a press to apply pressure to the wood products after the moisture application station; and high frequency heating apparatus to generate heat in the wood products while in the press whereby a wood product of improved dimensional stability results.

29. Apparatus as claimed in claim 28 in which the moisture application station comprises a spray nozzle to spray the products with water.

30. Apparatus as claimed in claim 28 in which the moisture application station comprises a bath into which the products are submerged.

31. Apparatus as claimed in claim 28 in which the moisture application station comprises a chamber having an internal environment at a higher relative humidity than the moisture content of the products.

32. Apparatus as claimed in claim 28 in which the high frequency heating apparatus comprises a radio frequency (RF) heater.

33. Apparatus as claimed in claim 28 in which the high frequency heating apparatus comprises a microwave heater.

34. Apparatus as claimed in claim 28 in which the press comprises a platen press having spaced platens that are movable relative to each other to compress products introduced between the platens.

35. Apparatus as claimed in claim 34 in which the spaced platens define parallel planes and the high frequency heating apparatus includes spaced electrode plates that extend transverse to the planes of the platens.

36. Apparatus as claimed in claim 34 in which the spaced platens define parallel planes and the high frequency heating apparatus includes spaced electrode plates that extend parallel to the planes of the platens.