CA2093535A1 - Combination method and apparatus for heating wood composites - Google Patents
Combination method and apparatus for heating wood compositesInfo
- Publication number
- CA2093535A1 CA2093535A1 CA 2093535 CA2093535A CA2093535A1 CA 2093535 A1 CA2093535 A1 CA 2093535A1 CA 2093535 CA2093535 CA 2093535 CA 2093535 A CA2093535 A CA 2093535A CA 2093535 A1 CA2093535 A1 CA 2093535A1
- Authority
- CA
- Canada
- Prior art keywords
- heating
- assembly
- high frequency
- wood
- heat
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27N—MANUFACTURE BY DRY PROCESSES OF ARTICLES, WITH OR WITHOUT ORGANIC BINDING AGENTS, MADE FROM PARTICLES OR FIBRES CONSISTING OF WOOD OR OTHER LIGNOCELLULOSIC OR LIKE ORGANIC MATERIAL
- B27N3/00—Manufacture of substantially flat articles, e.g. boards, from particles or fibres
- B27N3/08—Moulding or pressing
- B27N3/086—Presses with means for extracting or introducing gases or liquids in the mat
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27D—WORKING VENEER OR PLYWOOD
- B27D3/00—Veneer presses; Press plates; Plywood presses
- B27D3/04—Veneer presses; Press plates; Plywood presses with endless arrangement of moving press plates, belts, or the like
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27M—WORKING OF WOOD NOT PROVIDED FOR IN SUBCLASSES B27B - B27L; MANUFACTURE OF SPECIFIC WOODEN ARTICLES
- B27M1/00—Working of wood not provided for in subclasses B27B - B27L, e.g. by stretching
- B27M1/02—Working of wood not provided for in subclasses B27B - B27L, e.g. by stretching by compressing
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27M—WORKING OF WOOD NOT PROVIDED FOR IN SUBCLASSES B27B - B27L; MANUFACTURE OF SPECIFIC WOODEN ARTICLES
- B27M3/00—Manufacture or reconditioning of specific semi-finished or finished articles
- B27M3/0013—Manufacture or reconditioning of specific semi-finished or finished articles of composite or compound articles
- B27M3/0026—Manufacture or reconditioning of specific semi-finished or finished articles of composite or compound articles characterised by oblong elements connected laterally
- B27M3/0053—Manufacture or reconditioning of specific semi-finished or finished articles of composite or compound articles characterised by oblong elements connected laterally using glue
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27N—MANUFACTURE BY DRY PROCESSES OF ARTICLES, WITH OR WITHOUT ORGANIC BINDING AGENTS, MADE FROM PARTICLES OR FIBRES CONSISTING OF WOOD OR OTHER LIGNOCELLULOSIC OR LIKE ORGANIC MATERIAL
- B27N3/00—Manufacture of substantially flat articles, e.g. boards, from particles or fibres
- B27N3/08—Moulding or pressing
- B27N3/20—Moulding or pressing characterised by using platen-presses
- B27N3/203—Moulding or pressing characterised by using platen-presses with heating or cooling means
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B37/00—Methods or apparatus for laminating, e.g. by curing or by ultrasonic bonding
- B32B37/06—Methods or apparatus for laminating, e.g. by curing or by ultrasonic bonding characterised by the heating method
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B37/00—Methods or apparatus for laminating, e.g. by curing or by ultrasonic bonding
- B32B37/10—Methods or apparatus for laminating, e.g. by curing or by ultrasonic bonding characterised by the pressing technique, e.g. using action of vacuum or fluid pressure
- B32B37/1027—Pressing using at least one press band
Abstract
ABSTRACT OF THE INVENTION
This invention pertains to a novel method and apparatus for the manufacture of wood composite products. A method of eliminating arcing at the edges of a wood composite assembly constructed of wood elements bonded with a heat curable glue and heated by high frequency heating comprising: applying high frequency energy to an interior part of the assembly between and removed from the exterior edges of the assembly, and heating the peripheral parts of the assembly between the interior part and the exterior edges of the assembly with heat transferred from the interior part of the assembly.
This invention pertains to a novel method and apparatus for the manufacture of wood composite products. A method of eliminating arcing at the edges of a wood composite assembly constructed of wood elements bonded with a heat curable glue and heated by high frequency heating comprising: applying high frequency energy to an interior part of the assembly between and removed from the exterior edges of the assembly, and heating the peripheral parts of the assembly between the interior part and the exterior edges of the assembly with heat transferred from the interior part of the assembly.
Description
R ~ ~ 9 3 ~ 3 5 ~, COMBINATION METHOD A~ APPARATUS
FOR XEATING WOOD COMPOSITES
's !' ;~ This invention pertains to a novel apparatus and method for the manufacture and heating of wood composite products. More particularly, this invention relates to a novel high frequency apparatus and method of using the high frequency apparatus for the manufacture of wood composite products without having arcing occur 10 at the exterior exposed areas of the wood composite products.
Use of high frequency (HF) heating in the manufacture of wood composite materials can provide a significant improvement over conventional heating methods. The HF heating methods (radio frequency heating or microwave heating) can produce a relatively uniform temperature throughout the cross-section of the heated composite assembly and can potentially achieve higher heating rates than are possible with other heating methods.
The HF technique generates heat by agitating dipole molecules of the heated substance in an oscillating electrical field. The dipole position of a molecule flips with the changing electrical field and the resulting alternating motion o the molecules produces heat. The oscillating HF field has the advantage that it affects the molecules deep inside the assembl~, as well as those at the surface. Conduction of heat to the interior of the heated mass is not necessary. It is the important advantage of the high , frequency heating method that the heat is generated everywhere within the cross-section of the composite assembly exposed to HF
power. High generator power outputs can heat the wood composite '~ rapidly and uniformly throughout the mass of the wood composite.
,...
/l .
~ 93~35 f ~ Conventional external heating methods require that the heat $ from the surface in contact with the source of heat be conducted to ~.j the interior of the assembly. Since wood is a poor conductor of heat, this process is slow, especially when larger cross-sections are heated. A given volume of wood can be heated to a given temperature by HF heating in a much shorter period of time than by a conventional external heat method. However, the maximum HF power that can be applied to a wood composite assembly is limited by several factors, one of which is arcing at the exposed surfaces of the wood composite.
Efficiency of the HF heating method depends on the ability of the heated substance to absorb the HF energy and on tuning of the , HF system to the particular dielectric characteristics of the heated substance. The sensitivity to the applied energy, or the loss factor of the material, is generally related to its electrical , conductivity. Unfortunately, the potential for arcing also increases with the conductivity of the material. Depending on the physical arrangement of the heating system, the maximum power 3 20 density that can be applied is limited. Above this limit, an increase in the power density will result in arcing and possibly destructive burning of the exterior surfaces of the product.
/
The binding systems (a phenol formaldehyde glue, for example), commonly used in the manufacture of wood composites, are usually applied in liquid form and in such quantities that some of the glue may be squeezed out of the wood composite assembly during compression. The glue that is squeezed out accumulates on the edges of the wood composite billet and other surfaces. If the glue has high conductivity, as is the case with the common phenolic resins, accumulations of the conductive glue substance within a strong electrical field may cause arcing. The arcing occurs when the ~ 3~35 buildup of glue material on the edge of the assembly, or the surface of the HF applicator develops sufficiently high potential.
The developed electrical potential is discharged through an electric arc between the conductive mass and a location of opposite i`
; potential. This is precipitated by dielectric breakdown of the media (usually air) separating these locations. Extremely high I localized temperatures may exist. In most cases the arcing takes place on the edges where there is oxygen available for combustion.
Burning of the product may result. Furthermore, arcing may cause 10 erosion of the surface of the equipment and may interfere with the process control systems.
The conditions affecting arcing include the magnitude of the electrical potential and the dielectric strength of the media separating the two locations. The magnitude of the charge that develops in a particular location depends on the conductivity and quantity of the substance in that location. Controlling these variables is the basis of the methods that have been applied by the prior art in order to limit, or prevent the incidence of arcing.
One industry practised method of suppressing the tendency to ~ -arc in high frequency electric fields is to lower the conductivity1 of the glue. This is generally done at the expense of efficiency of heating and increased cost of the glue. Another method is to lower the power density. The penalty in this case is a slower heating rate and increased production costs. Accumulation of the conductive glue material can be prevented by minimizing the amount of glue used, and by lowering the mobility of the resin. The down side of this approach is lower quality and strength of the wood composite 30 product.
jJ
Othex prior art focuses on ensuring a sufficient gap between $~
~, ~93~35 the possible glue accumulations and the high frequency applicators, because higher power densities are required to induce arcing over larger distances. Some applied methods involve changing the dielectric characteristics of the material separating the applicators from the gluelines. A common method is to insert an additional dielectric material between HF applicator and the wood composite assembly.
Several high frequency heating methods applicable for the production of wood composites that recognize the problem of arcing are described in certain patents.
Mann et al., U.S. Patent No. 2,434,573, granted ~anuary 13, 1948, disclose a radio frequency heating method for manufacturing plywood. The problem of arcing is resolved by the use of caul boards between the HF applicators and the assembly. A parallel heating method is used.
Coulter et al., U.S. Patent No. 3,232,811, issued February 1, 201966, disclose a process of bonding lumber by parallel high frequency heating. Pxevention of arcing is achieved by creating an air gap that separates the glue from the high frequency applicators.
Parker, U.S. Patent No. 3,474,209, granted October 21, 1969, discloses microwave heating and glue curing of wood products. The apparatus and method of this invention involves continuous dielectric drying and curing of thin wet products, such as paper, plywood and adhesives.
Fraser et al., U.S. Patent No. 3,888,715, issued June 10, 1975, disclose a method of improving the uniformity and efficiency ~93~35 , of a parallel high frequency heating system. This is achieved by ; incorporating conductive elements in the gluelines. To prevent arcing, it is recommended that the maximum HF power be limited to safe levels.
Lamberts et al., U.S. Patent No. 4,221,950, granted September 9, 1980, describe an electrode system for high frequency heating.
The substance of the invention deals with an apparatus useful for tuning of high frequency system to changing conditions in the continuous production of wood composites.
Neubauer et al., U.S. Patent No. 4,420,357, issued December 13, 1983, disclose a method and apparatus for the continuous manufacture of particleboard, including a radio frequency tunnel heater. The method uses a resin system of low moisture content and ~,~
mobility to minimize squeezed-out glue.
This invention relates to a novel combination heating method useful for the manufacture of wood composite products, consisting of heating a portion of the wood composite assembly with high frequency energy and transferring the generated heat to areas not directly exposed to the high frequency energy. More specifically, the high frequency energy is used to heat the interior parts of the ~3 wood composite assembly and using the interior gen~rated heat to heat the peripheral parts of the wood composite assembly. In the preferred embodiment of the invention, the heat transfer to the peripheral areas is achieved through migration of steam. The high frequency applicators are arranged in relation to the heated i composite assembly to decrease or eliminate the impediment of JJ~ 30 arcing in the wood composite production.
. i ;~ The invention is directed to a method of heating a wood ,,~
~i - 5 -~i composite combining high frequency heating and steam injection, the wood composite assembly being constructed of wood elements such as sheets of veneer, strands, wafers, chips or particles bonded with a heat curable glue comprising: a) preparing a wood composite assembly constructed of a plurality of wood elements and thermosetting resin between the respective elements, the resin between wood elements forming a plurality of gluelines within the composite wood assembly; b) compacting the wood composite assembly and subjecting the said assembly to heating by a high frequency heat applicator having effective heating and non-heating zones, the area of the effective heating zone of the applicator being smaller than the surface area of the composite assembly in contact with the i applicator, the part of the assembly within the effective heating zone being the interior part, the gluelines within the interior :`
part being the interior gluelines, the gluelines outside of the effective heating zone being the peripheral gluelines within the peripheral part of the assembly, the distance between the external edge of the assembly and the effective heating zone of the applicator being between zero and to about 25% of the total distance between opposite free edges of the assembly; c~ heating the interior gluelines by applying high frequency heating within the effective heating zone; d) heating the peripheral gluelines by heat transferred into the peripheral part of the assembly from outside of the peripheral parts. The heat transfer may be by injected steam generated within the effective heating zone by high frequency heating of the moisture within interior gluelines.
~1 This invention further pertains to a combination heating method wherein the heating of the interior part of the assembly is by high frequency heating and heating of the peripheral parts is by steam injection wherein the steam is injected from a source exterior to the wood composite assembly.
, - 6 -J
~0~3~3~
The high frequency heating in the method according to the present invention can be radio frequency heating or microwave heating.
The wood elements from which the wood composite assembly is formed can be sheets of veneer, strands, wafers, chips or particles.
This invention further pertains to an apparatus for high freq~ency heating of a wood composite assembly comprising a pair of opposing and spatially located high frequency heating applicators each comprising: a) a heating electrode having an effective heating width and length that is equal to or smaller than the corresponding width and length of the surface of the heated wood composite assembly in contact with the said heating electrode; b) dielectric pressing plates continuously disposed beside at least two opposing sides of the heating electrode, the heating electrode and the dielectric pressing plates forming a common pressing surface of the applicator, the pressing surface of the applicator being larger than the surface of the composite assembly in contact with the pressing surface of the applicator in at least one direction; c) a generator of high frequency heating energy connected to the heating applicators; d) a means of imposing pressure between the high frequency applicators and the wood composite assembly.
The pressing surfaces of the pressing high frequency heat applicators can be larger than the pressed surfaces of the wood composite assembly in all directions while the corresponding pressing surfaces of the high frequency heating electrodes are smaller than the pressed surfaces of the said wood composite assembly in all direction.
Although a specific arrangement of radio frequency heating applicators is depicted in the drawings, it will be apparent to ~ ~ ~ 3 ~ 3 ~
those skilled in the art that other application systems, such as those used for the application of microwave heating, may be employed without departing from the scope of this invention.
Figure 1 is a schematic view of a cross-section of a wood composite assembly located within and pressed by a high frequency heat applicator according to the present invention, the heating electrodes being connected to a source of high frequency energy.
Figure 2 depicts a schematic partial section view of a typical platen press equipped with high frequency heating applicator.
Figure 3 is a schematic view of a continuous process for wood composites that utilizes high frequency heating applicators according to the present invention.
The present invention combines two heating methods, high frequency (HF) heating and steam injection heating, in one process.
Utilizing this combination minimizes the potential for arcing at the exposed surface areas of the wood composite assembly and at the same time enables an efficient and high rate heating process to be used. The solution to the problem of arcing is based on heating only the interior of the wood composite directly with HF power, thereby separating the critical external edges of the wood composite assembly from the direct effect of the HF electrical field and heating the peripheral parts by heat transfer through steam injection and heat conduction.
The high frequency applicators are arranged relative to the wood composite assembly so that the electrical field heats the interior zone of the wood composite assembly only. The effective heating zone of the high frequency energy applicators may not ~093 ~)3.~
extend to the edges of the composite billet. The peripheral parts of the wood composite assembly in the vicinity of, but outside the effective HF heating zone are not fully heated by the high frequency energy. Instead, steam migration and heat conduction are utilized to transfer the heat to these areas, thereby achieving a consistent cure throughout the whole wood composite mass.
The distance between the edge of the effective heating zone and the edge of the wood composite billet must be sufficiently large to ensure low power densities in the proximity of the billet's edges, but sufficiently small to ensure that the steam, together with the other heat transferring mechanisms within the composite billet has sufficient heat capacity to heat and cure these billet's peripheral parts at the required rate.
If an external steam source is used to heat the peripheral parts of the wood composite assembly, the maximum practical ¦distance between the edge of the composite billet and the edge of the effective HF heating zone will be determined by the required ~ 20 rate of penetration of the steam into the assembly and heating of ¦the peripheral parts of the assembly. At any rate, for economy and ~efficiency reasons, it is desirable to use the smallest practical ¦and safe separation of the HF heating zone and the billet's edges.
¦The description of the preferred embodiments of the present invention is in reference to steam and heat generated internally by the high frequency heating energy during the heating process.
However, it should be understood that a substitution, or ¦supplementation, of the internally generated steam and heat by steam from an external source or by heat transferred through conduction from the exterior of the assembly, are considered to be included within the scope of this invention and are useful in _ g _ ,, ~0~35~5 carrying out the dual heating principle of this invention.
The present invention is useful in manufacturing processes for composite materials or other materials that utilize high frequency heating to generate the heat needed for the process. The specific description disclosed here uses a particular type of wood composite that is formed from sheets of veneer and thermosetting glue between each pair of sheets forming the gluelines. Such a product is regarded as plywood when the veneer sheets are grain cross-layered, 10 or as laminated veneer lumber if all the veneer sheets are layered with the grain direction coinciding with the longitudinal direction of the billet. However, the benefits of the present invention can be realized in processes that are using other wood elements such as ¦ strands of wood, wafers, particlesl or other forms.
j The preferred solution to the arcing problem disclosed herein3 is specifically adapted to the novel manufacturing method for ! making wood composites, described in concurrently co-pending Canadian patent application Serial No. 609,460, filed August 25, ~0 1989, entitled "Wood Composite", which is incorporated herein for reference. During the HF heating cycle, as disclosed in that application, the composite assembly is under low external pressure, selected such as to balance the target saturated steam pressure that is to be internally generated within the assembly by the applied high frequency energy. When the steam pressure approaches the magnitude of the opposing external pressure between the assembly and the applicator, it migrates easily along the gluelines to the exterior of the wood composite assembly. The steam escapes from the interior of the heated assembly through the peripheral 30 parts and transfers heat to the areas unheated by the HF energy. At the end of the heating cycle, the temperature of the peripheral regions of the assembly will approach the temperature of the \
~093535 interior of the assembly. This process is in principle a steam injection heating process where the injected steam is generated from the moisture within the heated assembly.
The above mentioned co-pending patent application entitled "Wood Composite" discloses a method of preparing a wood composite material comprising the steps of: (a) preparing a wood composite assembly constructed of a plurality of moisture containing wood elements having a thermosetting liquid resin between the respective elements; ~b) placing an external pressure on the composite assembly and subjecting the composite assembly to heat treatment that generates saturated steam from the moisture within the ~., composite assembly, the steam being at a pressure that is about equal to the external pressure on the composite assembly but at a temperature below the curing temperature of the liquid resin, the liquid resin and the moisture within the composite assembly being at a boiling point for at least a portion of the heating process, the generated steam forcing the liquid resin to fill hollow imperfections in the wood elements in the composite assembly; (c) heating the restrained composite assembly to a temperature at which the resin is in a thermoplastic state and the composite wood is in plasticized state, the heating being terminated when the resin is about to undergo ~ curing reaction; (d) compressing the plasticized wood composite assembly at a rate which minimizes damage to the wood elements to thereby form a compressed wood composite; and (e) heating the compressed wood composite to a temperature required to cure the resin in the compressed wood composite to a thermoset ;~ state. Alternatively, if necessary or desirable, the steam may be i ,, supplied from an external source.
In principle, the present invention combines two heating "
~ methods. The primary method is high frequency heating. It is , .~
;
,:
,, ~935~5 applied in those areas of the curable assembly where the potential for arcing is low or nonexistent. The regions where the potential for arcing is high, i.e. the exterior edges or peripheral regions, are heated by steam injection. In the preferred embodiment of this invention the steam injected into the peripheral regions is generated within the interior HF heating zone of the assembly and migrates along the gluelines to the exterior edges of the assembly.
, .
Figure 1 illustrates a schematic view of a cross-section of wood composite assembly 11 compressed between a set of dielectric pressing plates 14 and two HF electrodes 12 and 13 which are electrically connected by means of conductors 18 and 19 to a high frequency generator 20. The pressing surface of the dielectric ,~ pressing plates 14 and the pressing surface of the corresponding HF
electrodes 12 and 13 form a common pressing surface of the applicator. In other words, the dielectric pressing plates 14 extend the pressing surface of the HF electrodes 12 and 13. The electrodes 12 and 13 are supported by upper press platen 21 and lower press platen 22. The upper electrode 1~ is shown as being electrically isolated from the platen 21 by means of spacers 15.
In the arrangement shown in Figure 1, the electric field lines between the electrodes 12 and 13 are oriented in a direction perpendicular to the gluelines of the wood composite assembly 11.
However, the principles of the invention apply to other heating ; arrangements as well. The composite assembly 11 is located within , the HF applicator and is under external pressure imposed by the HF
applicator. The effective heating zone of the applicator is defined as the space between the electrodes 12 and 13 which are the same in size and vertically aligned. The effective heating zone between the electrodes 12 and 13 is smaller than the composite assembly 11 in at least on~ direction, as indicated in the Figure 1. The effective .
., ! ~
` ~093535 heating zone may be smaller than the composite assembly in all directions. The distance between the exterior edge 16 of the ~, assembly 11 and the corresponding boundary of the effective heating zone identified in the Figure 1 by the edges 17 of the electrode 12 should be selected such that the possible accumulation of glue on the edges 16 is not exposed to significantly high electric field.
The pressing plates 14 are made of dielectric, that is electrically non-conductive material. Such material can be a ceramic or other suitable material. The pressing plates 14 are placed as an 1~ extension of the electrodes 12 and 13 and provide pressing surfaces for the peripheral parts of the wood assembly outside of the HF
heating zone.
The peripheral regions of the wood composite assembly 11 outside of the effective HF heating zone are not heated by the high frequency energy to the same extent as the material within the effective heating zone. Some HF heating will take place within the peripheral region due to fringing electrical fields at the edge of the electrodes. In large part however, the heat to the peripheral ~0 parts of the assembly is transferred by steam. In the preferred embodiment of this invention the steam is generated within the effective HF heating zone from the moisture within the interior part of the composite assembly and is allowed to migrate to the peripheral parts for heat transfer. For the steam to migrate in sufficient quantities and transfer adequate amounts of heat to the peripheral regions, the steam pressure should be at about the same pressure as the external compacting pressure acting on the assembly j 11 during the heating period. The method of producing the desirable pressure and temperature conditions within the assembly 11 and 3, 30 achieving the required steam migration is disclosed in the above-~ mentioned Canadian patent application Serial No. 609,460, I'Wood ,1 Compositell.
,, . . .
~.' .,, ~, ,, ~9~5~5 A number of possible equipment arrangements for practising the , present invention are possible. Figure 2 shows a side partial section two-stage view of a platen press 21 equipped with high frequency heating apparatus 22. The right side of Figure 2 illustrates the composite 11 before pressing with glue between the wood layers, while the left side illustrates the assembly 11 in ~, compressed condi~ion after pressing. The high frequency is applied according to the principles discussed in the co-pending patent application Serial No. 609,460, mentioned previously. The wood composite assembly 11 can be manufactured in such a press in either discrete (shown in Figure 2), or continuous length. As seen in Figure 2, the edge of the wood composite assembly 11 extends beyond the edge of the high frequency electrode which defines the effective heating zone.
Figure 3 depicts a schematic side view of a continuous manufacturing system according to the invention discussed in the co-pending patent application Serial No. 609,460, that uses an initial independently located preheat tunnel heater 31, which heat-treats the composite assembly 11, an intermediate continuous press33 which continuously compresses the assembly 11, and an after heat tunnel heater 32 where the final thermoset cure of the resin is achieved. Both tunnel heaters utilize the principles according to the present invention.
'I
¦ As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
. - 14 -
FOR XEATING WOOD COMPOSITES
's !' ;~ This invention pertains to a novel apparatus and method for the manufacture and heating of wood composite products. More particularly, this invention relates to a novel high frequency apparatus and method of using the high frequency apparatus for the manufacture of wood composite products without having arcing occur 10 at the exterior exposed areas of the wood composite products.
Use of high frequency (HF) heating in the manufacture of wood composite materials can provide a significant improvement over conventional heating methods. The HF heating methods (radio frequency heating or microwave heating) can produce a relatively uniform temperature throughout the cross-section of the heated composite assembly and can potentially achieve higher heating rates than are possible with other heating methods.
The HF technique generates heat by agitating dipole molecules of the heated substance in an oscillating electrical field. The dipole position of a molecule flips with the changing electrical field and the resulting alternating motion o the molecules produces heat. The oscillating HF field has the advantage that it affects the molecules deep inside the assembl~, as well as those at the surface. Conduction of heat to the interior of the heated mass is not necessary. It is the important advantage of the high , frequency heating method that the heat is generated everywhere within the cross-section of the composite assembly exposed to HF
power. High generator power outputs can heat the wood composite '~ rapidly and uniformly throughout the mass of the wood composite.
,...
/l .
~ 93~35 f ~ Conventional external heating methods require that the heat $ from the surface in contact with the source of heat be conducted to ~.j the interior of the assembly. Since wood is a poor conductor of heat, this process is slow, especially when larger cross-sections are heated. A given volume of wood can be heated to a given temperature by HF heating in a much shorter period of time than by a conventional external heat method. However, the maximum HF power that can be applied to a wood composite assembly is limited by several factors, one of which is arcing at the exposed surfaces of the wood composite.
Efficiency of the HF heating method depends on the ability of the heated substance to absorb the HF energy and on tuning of the , HF system to the particular dielectric characteristics of the heated substance. The sensitivity to the applied energy, or the loss factor of the material, is generally related to its electrical , conductivity. Unfortunately, the potential for arcing also increases with the conductivity of the material. Depending on the physical arrangement of the heating system, the maximum power 3 20 density that can be applied is limited. Above this limit, an increase in the power density will result in arcing and possibly destructive burning of the exterior surfaces of the product.
/
The binding systems (a phenol formaldehyde glue, for example), commonly used in the manufacture of wood composites, are usually applied in liquid form and in such quantities that some of the glue may be squeezed out of the wood composite assembly during compression. The glue that is squeezed out accumulates on the edges of the wood composite billet and other surfaces. If the glue has high conductivity, as is the case with the common phenolic resins, accumulations of the conductive glue substance within a strong electrical field may cause arcing. The arcing occurs when the ~ 3~35 buildup of glue material on the edge of the assembly, or the surface of the HF applicator develops sufficiently high potential.
The developed electrical potential is discharged through an electric arc between the conductive mass and a location of opposite i`
; potential. This is precipitated by dielectric breakdown of the media (usually air) separating these locations. Extremely high I localized temperatures may exist. In most cases the arcing takes place on the edges where there is oxygen available for combustion.
Burning of the product may result. Furthermore, arcing may cause 10 erosion of the surface of the equipment and may interfere with the process control systems.
The conditions affecting arcing include the magnitude of the electrical potential and the dielectric strength of the media separating the two locations. The magnitude of the charge that develops in a particular location depends on the conductivity and quantity of the substance in that location. Controlling these variables is the basis of the methods that have been applied by the prior art in order to limit, or prevent the incidence of arcing.
One industry practised method of suppressing the tendency to ~ -arc in high frequency electric fields is to lower the conductivity1 of the glue. This is generally done at the expense of efficiency of heating and increased cost of the glue. Another method is to lower the power density. The penalty in this case is a slower heating rate and increased production costs. Accumulation of the conductive glue material can be prevented by minimizing the amount of glue used, and by lowering the mobility of the resin. The down side of this approach is lower quality and strength of the wood composite 30 product.
jJ
Othex prior art focuses on ensuring a sufficient gap between $~
~, ~93~35 the possible glue accumulations and the high frequency applicators, because higher power densities are required to induce arcing over larger distances. Some applied methods involve changing the dielectric characteristics of the material separating the applicators from the gluelines. A common method is to insert an additional dielectric material between HF applicator and the wood composite assembly.
Several high frequency heating methods applicable for the production of wood composites that recognize the problem of arcing are described in certain patents.
Mann et al., U.S. Patent No. 2,434,573, granted ~anuary 13, 1948, disclose a radio frequency heating method for manufacturing plywood. The problem of arcing is resolved by the use of caul boards between the HF applicators and the assembly. A parallel heating method is used.
Coulter et al., U.S. Patent No. 3,232,811, issued February 1, 201966, disclose a process of bonding lumber by parallel high frequency heating. Pxevention of arcing is achieved by creating an air gap that separates the glue from the high frequency applicators.
Parker, U.S. Patent No. 3,474,209, granted October 21, 1969, discloses microwave heating and glue curing of wood products. The apparatus and method of this invention involves continuous dielectric drying and curing of thin wet products, such as paper, plywood and adhesives.
Fraser et al., U.S. Patent No. 3,888,715, issued June 10, 1975, disclose a method of improving the uniformity and efficiency ~93~35 , of a parallel high frequency heating system. This is achieved by ; incorporating conductive elements in the gluelines. To prevent arcing, it is recommended that the maximum HF power be limited to safe levels.
Lamberts et al., U.S. Patent No. 4,221,950, granted September 9, 1980, describe an electrode system for high frequency heating.
The substance of the invention deals with an apparatus useful for tuning of high frequency system to changing conditions in the continuous production of wood composites.
Neubauer et al., U.S. Patent No. 4,420,357, issued December 13, 1983, disclose a method and apparatus for the continuous manufacture of particleboard, including a radio frequency tunnel heater. The method uses a resin system of low moisture content and ~,~
mobility to minimize squeezed-out glue.
This invention relates to a novel combination heating method useful for the manufacture of wood composite products, consisting of heating a portion of the wood composite assembly with high frequency energy and transferring the generated heat to areas not directly exposed to the high frequency energy. More specifically, the high frequency energy is used to heat the interior parts of the ~3 wood composite assembly and using the interior gen~rated heat to heat the peripheral parts of the wood composite assembly. In the preferred embodiment of the invention, the heat transfer to the peripheral areas is achieved through migration of steam. The high frequency applicators are arranged in relation to the heated i composite assembly to decrease or eliminate the impediment of JJ~ 30 arcing in the wood composite production.
. i ;~ The invention is directed to a method of heating a wood ,,~
~i - 5 -~i composite combining high frequency heating and steam injection, the wood composite assembly being constructed of wood elements such as sheets of veneer, strands, wafers, chips or particles bonded with a heat curable glue comprising: a) preparing a wood composite assembly constructed of a plurality of wood elements and thermosetting resin between the respective elements, the resin between wood elements forming a plurality of gluelines within the composite wood assembly; b) compacting the wood composite assembly and subjecting the said assembly to heating by a high frequency heat applicator having effective heating and non-heating zones, the area of the effective heating zone of the applicator being smaller than the surface area of the composite assembly in contact with the i applicator, the part of the assembly within the effective heating zone being the interior part, the gluelines within the interior :`
part being the interior gluelines, the gluelines outside of the effective heating zone being the peripheral gluelines within the peripheral part of the assembly, the distance between the external edge of the assembly and the effective heating zone of the applicator being between zero and to about 25% of the total distance between opposite free edges of the assembly; c~ heating the interior gluelines by applying high frequency heating within the effective heating zone; d) heating the peripheral gluelines by heat transferred into the peripheral part of the assembly from outside of the peripheral parts. The heat transfer may be by injected steam generated within the effective heating zone by high frequency heating of the moisture within interior gluelines.
~1 This invention further pertains to a combination heating method wherein the heating of the interior part of the assembly is by high frequency heating and heating of the peripheral parts is by steam injection wherein the steam is injected from a source exterior to the wood composite assembly.
, - 6 -J
~0~3~3~
The high frequency heating in the method according to the present invention can be radio frequency heating or microwave heating.
The wood elements from which the wood composite assembly is formed can be sheets of veneer, strands, wafers, chips or particles.
This invention further pertains to an apparatus for high freq~ency heating of a wood composite assembly comprising a pair of opposing and spatially located high frequency heating applicators each comprising: a) a heating electrode having an effective heating width and length that is equal to or smaller than the corresponding width and length of the surface of the heated wood composite assembly in contact with the said heating electrode; b) dielectric pressing plates continuously disposed beside at least two opposing sides of the heating electrode, the heating electrode and the dielectric pressing plates forming a common pressing surface of the applicator, the pressing surface of the applicator being larger than the surface of the composite assembly in contact with the pressing surface of the applicator in at least one direction; c) a generator of high frequency heating energy connected to the heating applicators; d) a means of imposing pressure between the high frequency applicators and the wood composite assembly.
The pressing surfaces of the pressing high frequency heat applicators can be larger than the pressed surfaces of the wood composite assembly in all directions while the corresponding pressing surfaces of the high frequency heating electrodes are smaller than the pressed surfaces of the said wood composite assembly in all direction.
Although a specific arrangement of radio frequency heating applicators is depicted in the drawings, it will be apparent to ~ ~ ~ 3 ~ 3 ~
those skilled in the art that other application systems, such as those used for the application of microwave heating, may be employed without departing from the scope of this invention.
Figure 1 is a schematic view of a cross-section of a wood composite assembly located within and pressed by a high frequency heat applicator according to the present invention, the heating electrodes being connected to a source of high frequency energy.
Figure 2 depicts a schematic partial section view of a typical platen press equipped with high frequency heating applicator.
Figure 3 is a schematic view of a continuous process for wood composites that utilizes high frequency heating applicators according to the present invention.
The present invention combines two heating methods, high frequency (HF) heating and steam injection heating, in one process.
Utilizing this combination minimizes the potential for arcing at the exposed surface areas of the wood composite assembly and at the same time enables an efficient and high rate heating process to be used. The solution to the problem of arcing is based on heating only the interior of the wood composite directly with HF power, thereby separating the critical external edges of the wood composite assembly from the direct effect of the HF electrical field and heating the peripheral parts by heat transfer through steam injection and heat conduction.
The high frequency applicators are arranged relative to the wood composite assembly so that the electrical field heats the interior zone of the wood composite assembly only. The effective heating zone of the high frequency energy applicators may not ~093 ~)3.~
extend to the edges of the composite billet. The peripheral parts of the wood composite assembly in the vicinity of, but outside the effective HF heating zone are not fully heated by the high frequency energy. Instead, steam migration and heat conduction are utilized to transfer the heat to these areas, thereby achieving a consistent cure throughout the whole wood composite mass.
The distance between the edge of the effective heating zone and the edge of the wood composite billet must be sufficiently large to ensure low power densities in the proximity of the billet's edges, but sufficiently small to ensure that the steam, together with the other heat transferring mechanisms within the composite billet has sufficient heat capacity to heat and cure these billet's peripheral parts at the required rate.
If an external steam source is used to heat the peripheral parts of the wood composite assembly, the maximum practical ¦distance between the edge of the composite billet and the edge of the effective HF heating zone will be determined by the required ~ 20 rate of penetration of the steam into the assembly and heating of ¦the peripheral parts of the assembly. At any rate, for economy and ~efficiency reasons, it is desirable to use the smallest practical ¦and safe separation of the HF heating zone and the billet's edges.
¦The description of the preferred embodiments of the present invention is in reference to steam and heat generated internally by the high frequency heating energy during the heating process.
However, it should be understood that a substitution, or ¦supplementation, of the internally generated steam and heat by steam from an external source or by heat transferred through conduction from the exterior of the assembly, are considered to be included within the scope of this invention and are useful in _ g _ ,, ~0~35~5 carrying out the dual heating principle of this invention.
The present invention is useful in manufacturing processes for composite materials or other materials that utilize high frequency heating to generate the heat needed for the process. The specific description disclosed here uses a particular type of wood composite that is formed from sheets of veneer and thermosetting glue between each pair of sheets forming the gluelines. Such a product is regarded as plywood when the veneer sheets are grain cross-layered, 10 or as laminated veneer lumber if all the veneer sheets are layered with the grain direction coinciding with the longitudinal direction of the billet. However, the benefits of the present invention can be realized in processes that are using other wood elements such as ¦ strands of wood, wafers, particlesl or other forms.
j The preferred solution to the arcing problem disclosed herein3 is specifically adapted to the novel manufacturing method for ! making wood composites, described in concurrently co-pending Canadian patent application Serial No. 609,460, filed August 25, ~0 1989, entitled "Wood Composite", which is incorporated herein for reference. During the HF heating cycle, as disclosed in that application, the composite assembly is under low external pressure, selected such as to balance the target saturated steam pressure that is to be internally generated within the assembly by the applied high frequency energy. When the steam pressure approaches the magnitude of the opposing external pressure between the assembly and the applicator, it migrates easily along the gluelines to the exterior of the wood composite assembly. The steam escapes from the interior of the heated assembly through the peripheral 30 parts and transfers heat to the areas unheated by the HF energy. At the end of the heating cycle, the temperature of the peripheral regions of the assembly will approach the temperature of the \
~093535 interior of the assembly. This process is in principle a steam injection heating process where the injected steam is generated from the moisture within the heated assembly.
The above mentioned co-pending patent application entitled "Wood Composite" discloses a method of preparing a wood composite material comprising the steps of: (a) preparing a wood composite assembly constructed of a plurality of moisture containing wood elements having a thermosetting liquid resin between the respective elements; ~b) placing an external pressure on the composite assembly and subjecting the composite assembly to heat treatment that generates saturated steam from the moisture within the ~., composite assembly, the steam being at a pressure that is about equal to the external pressure on the composite assembly but at a temperature below the curing temperature of the liquid resin, the liquid resin and the moisture within the composite assembly being at a boiling point for at least a portion of the heating process, the generated steam forcing the liquid resin to fill hollow imperfections in the wood elements in the composite assembly; (c) heating the restrained composite assembly to a temperature at which the resin is in a thermoplastic state and the composite wood is in plasticized state, the heating being terminated when the resin is about to undergo ~ curing reaction; (d) compressing the plasticized wood composite assembly at a rate which minimizes damage to the wood elements to thereby form a compressed wood composite; and (e) heating the compressed wood composite to a temperature required to cure the resin in the compressed wood composite to a thermoset ;~ state. Alternatively, if necessary or desirable, the steam may be i ,, supplied from an external source.
In principle, the present invention combines two heating "
~ methods. The primary method is high frequency heating. It is , .~
;
,:
,, ~935~5 applied in those areas of the curable assembly where the potential for arcing is low or nonexistent. The regions where the potential for arcing is high, i.e. the exterior edges or peripheral regions, are heated by steam injection. In the preferred embodiment of this invention the steam injected into the peripheral regions is generated within the interior HF heating zone of the assembly and migrates along the gluelines to the exterior edges of the assembly.
, .
Figure 1 illustrates a schematic view of a cross-section of wood composite assembly 11 compressed between a set of dielectric pressing plates 14 and two HF electrodes 12 and 13 which are electrically connected by means of conductors 18 and 19 to a high frequency generator 20. The pressing surface of the dielectric ,~ pressing plates 14 and the pressing surface of the corresponding HF
electrodes 12 and 13 form a common pressing surface of the applicator. In other words, the dielectric pressing plates 14 extend the pressing surface of the HF electrodes 12 and 13. The electrodes 12 and 13 are supported by upper press platen 21 and lower press platen 22. The upper electrode 1~ is shown as being electrically isolated from the platen 21 by means of spacers 15.
In the arrangement shown in Figure 1, the electric field lines between the electrodes 12 and 13 are oriented in a direction perpendicular to the gluelines of the wood composite assembly 11.
However, the principles of the invention apply to other heating ; arrangements as well. The composite assembly 11 is located within , the HF applicator and is under external pressure imposed by the HF
applicator. The effective heating zone of the applicator is defined as the space between the electrodes 12 and 13 which are the same in size and vertically aligned. The effective heating zone between the electrodes 12 and 13 is smaller than the composite assembly 11 in at least on~ direction, as indicated in the Figure 1. The effective .
., ! ~
` ~093535 heating zone may be smaller than the composite assembly in all directions. The distance between the exterior edge 16 of the ~, assembly 11 and the corresponding boundary of the effective heating zone identified in the Figure 1 by the edges 17 of the electrode 12 should be selected such that the possible accumulation of glue on the edges 16 is not exposed to significantly high electric field.
The pressing plates 14 are made of dielectric, that is electrically non-conductive material. Such material can be a ceramic or other suitable material. The pressing plates 14 are placed as an 1~ extension of the electrodes 12 and 13 and provide pressing surfaces for the peripheral parts of the wood assembly outside of the HF
heating zone.
The peripheral regions of the wood composite assembly 11 outside of the effective HF heating zone are not heated by the high frequency energy to the same extent as the material within the effective heating zone. Some HF heating will take place within the peripheral region due to fringing electrical fields at the edge of the electrodes. In large part however, the heat to the peripheral ~0 parts of the assembly is transferred by steam. In the preferred embodiment of this invention the steam is generated within the effective HF heating zone from the moisture within the interior part of the composite assembly and is allowed to migrate to the peripheral parts for heat transfer. For the steam to migrate in sufficient quantities and transfer adequate amounts of heat to the peripheral regions, the steam pressure should be at about the same pressure as the external compacting pressure acting on the assembly j 11 during the heating period. The method of producing the desirable pressure and temperature conditions within the assembly 11 and 3, 30 achieving the required steam migration is disclosed in the above-~ mentioned Canadian patent application Serial No. 609,460, I'Wood ,1 Compositell.
,, . . .
~.' .,, ~, ,, ~9~5~5 A number of possible equipment arrangements for practising the , present invention are possible. Figure 2 shows a side partial section two-stage view of a platen press 21 equipped with high frequency heating apparatus 22. The right side of Figure 2 illustrates the composite 11 before pressing with glue between the wood layers, while the left side illustrates the assembly 11 in ~, compressed condi~ion after pressing. The high frequency is applied according to the principles discussed in the co-pending patent application Serial No. 609,460, mentioned previously. The wood composite assembly 11 can be manufactured in such a press in either discrete (shown in Figure 2), or continuous length. As seen in Figure 2, the edge of the wood composite assembly 11 extends beyond the edge of the high frequency electrode which defines the effective heating zone.
Figure 3 depicts a schematic side view of a continuous manufacturing system according to the invention discussed in the co-pending patent application Serial No. 609,460, that uses an initial independently located preheat tunnel heater 31, which heat-treats the composite assembly 11, an intermediate continuous press33 which continuously compresses the assembly 11, and an after heat tunnel heater 32 where the final thermoset cure of the resin is achieved. Both tunnel heaters utilize the principles according to the present invention.
'I
¦ As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
. - 14 -
Claims (8)
1. A method of heating a wood composite combining high frequency heating and steam injection, the method comprising the steps of:
a) preparing a wood composite assembly constructed of a plurality of wood elements and thermosetting resin between the respective elements, the resin between wood elements forming a plurality of gluelines within the composite wood assembly;
a) compacting the wood composite assembly and subjecting the said assembly to heating by a high frequency heat applicator having effective heating and non-heating zones, the area of the effective heating zone of the applicator being smaller than the surface area of the composite assembly in contact with the applicator, the part of the assembly within the effective heating zone being the interior part, the gluelines within the interior part being the interior gluelines, the gluelines outside of the effective heating zone being the peripheral gluelines within the peripheral part of the assembly, the distance between the external edge of the assembly and the effective heating zone of the applicator being between zero and to about 25% of the total distance between opposite free edges of the assembly;
b) heating the interior gluelines by applying high frequency heating within the effective heating zone;
c) heating the peripheral gluelines by heat transferred into the peripheral part of the assembly.
- Page 1 of Claims -
a) preparing a wood composite assembly constructed of a plurality of wood elements and thermosetting resin between the respective elements, the resin between wood elements forming a plurality of gluelines within the composite wood assembly;
a) compacting the wood composite assembly and subjecting the said assembly to heating by a high frequency heat applicator having effective heating and non-heating zones, the area of the effective heating zone of the applicator being smaller than the surface area of the composite assembly in contact with the applicator, the part of the assembly within the effective heating zone being the interior part, the gluelines within the interior part being the interior gluelines, the gluelines outside of the effective heating zone being the peripheral gluelines within the peripheral part of the assembly, the distance between the external edge of the assembly and the effective heating zone of the applicator being between zero and to about 25% of the total distance between opposite free edges of the assembly;
b) heating the interior gluelines by applying high frequency heating within the effective heating zone;
c) heating the peripheral gluelines by heat transferred into the peripheral part of the assembly.
- Page 1 of Claims -
2. A method according to claim 1 wherein the heating of the peripheral gluelines is by transfer of heat generated within the effective heating zone by high frequency heating of the moisture within interior gluelines.
3. A method according to claim 2 wherein the transfer of heat is by steam.
4. A method according to claim 1 wherein the heat transfer is by steam injected from the exterior to the composite assembly.
5. A method according to claim 1 wherein the heat applicator is a radio frequency heat applicator.
6. A method according to claim 1 wherein the heat applicator is a microwave heat applicator.
7. A method according to claim 1 wherein the wood elements are sheets of veneer, strands, wafers, chips or particles.
8. An apparatus for high frequency heating of a wood composite assembly comprising a pair of opposing and spatially located high frequency heating applicators each comprising:
a) a heating electrode having an effective heating width and length that is equal to or smaller than the corresponding width and length of the surface of the heated wood composite assembly in contact with the said heating electrode;
b) dielectric pressing plates continuously disposed beside at least two opposing sides of the heating electrode, the heating electrode and the dielectric pressing plates forming a common - Page 2 of Claims -pressing surface of the applicator, the pressing surface of the applicator being larger than the facing surface of the composite assembly in at least one direction;
c) a generator of high frequency heating energy connected to the heating applicators;
d) a means of imposing pressure between the high frequency applicators and the wood composite assembly.
- Page 3 of Claims -
a) a heating electrode having an effective heating width and length that is equal to or smaller than the corresponding width and length of the surface of the heated wood composite assembly in contact with the said heating electrode;
b) dielectric pressing plates continuously disposed beside at least two opposing sides of the heating electrode, the heating electrode and the dielectric pressing plates forming a common - Page 2 of Claims -pressing surface of the applicator, the pressing surface of the applicator being larger than the facing surface of the composite assembly in at least one direction;
c) a generator of high frequency heating energy connected to the heating applicators;
d) a means of imposing pressure between the high frequency applicators and the wood composite assembly.
- Page 3 of Claims -
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2093535 CA2093535A1 (en) | 1993-04-06 | 1993-04-06 | Combination method and apparatus for heating wood composites |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2093535 CA2093535A1 (en) | 1993-04-06 | 1993-04-06 | Combination method and apparatus for heating wood composites |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2093535A1 true CA2093535A1 (en) | 1994-10-07 |
Family
ID=4151425
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2093535 Abandoned CA2093535A1 (en) | 1993-04-06 | 1993-04-06 | Combination method and apparatus for heating wood composites |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2093535A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2179827A3 (en) * | 2008-10-23 | 2012-10-24 | Forestry And Forest Products Research Institute | Heat pressing apparatus with puncture prevention function and method for producing woody material |
-
1993
- 1993-04-06 CA CA 2093535 patent/CA2093535A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP2179827A3 (en) * | 2008-10-23 | 2012-10-24 | Forestry And Forest Products Research Institute | Heat pressing apparatus with puncture prevention function and method for producing woody material |
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