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WO1992003027A1 - Systeme de cuisson par micro-ondes - Google Patents

Systeme de cuisson par micro-ondes Download PDF

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Publication number
WO1992003027A1
WO1992003027A1 PCT/CA1991/000264 CA9100264W WO9203027A1 WO 1992003027 A1 WO1992003027 A1 WO 1992003027A1 CA 9100264 W CA9100264 W CA 9100264W WO 9203027 A1 WO9203027 A1 WO 9203027A1
Authority
WO
WIPO (PCT)
Prior art keywords
microwave
horn
applicator
outlet
dam
Prior art date
Application number
PCT/CA1991/000264
Other languages
English (en)
Inventor
Mark T. Churchland
Original Assignee
Macmillan Bloedel Limited
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Macmillan Bloedel Limited filed Critical Macmillan Bloedel Limited
Publication of WO1992003027A1 publication Critical patent/WO1992003027A1/fr

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Classifications

    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B6/00—Heating by electric, magnetic or electromagnetic fields
    • H05B6/64—Heating using microwaves
    • H05B6/78—Arrangements for continuous movement of material
    • 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
    • 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
    • B27N3/10—Moulding of mats
    • B27N3/14—Distributing or orienting the particles or fibres
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B6/00—Heating by electric, magnetic or electromagnetic fields
    • H05B6/64—Heating using microwaves
    • H05B6/80—Apparatus for specific applications
    • H05B6/806—Apparatus for specific applications for laboratory use

Definitions

  • the present invention relates to systems for continuously manufacturing composite, adhesively bonded products in which pressure and microwave heat are applied simultaneously to curable assemblies.
  • the adhesive bonding agent is thereby cured or set while the product is pressed and/or maintained at the desired dimensions and density.
  • the invention further relates to microwave methods for curing resins used as binders or adhesives for materials, such as wood particles, wood chips, wood wafers, wood strips, wood fibers and wood veneers, used in the production of chip board, hard board, particle board, wafer board, plywood and other composite products.
  • wood products of this type have been subjected in the past to heat and pressure in hot presses.
  • Wood is a relatively poor conductor of heat, and the heat from the platens of the hot press can only be directed against the outer surfaces of the wood product being formed. Consequently, considerable time was required for the necessary heat to penetrate to the center of the wood product and to cure the resin therein. If the temperature was increased beyond a certain amount to reduce the curing time required, scorching or charring of the outer surfaces of the wood product resulted. These higher temperatures also were difficult and expensive to attain since they required greater steam pressure and additional equipment. Additionally, at higher temperatures, water which may be entrapped can result in steam explosion in the product.
  • R.F. radio frequency
  • microwave energy has been used in recent years to cure, in composite masses, adhesives which have cure rates which are accelerated by the application of heat.
  • Microwave heating can be more rapid, that is, it can provide a shorter cure time, than conventional heating or hot press processes, and therefore allows for a continuous production technique as compared to batch processes.
  • Arcing and tracking common with the R.F. technique are also not a problem.
  • U.S. patents 4,018,642 and 4,020,311 disclose techniques for simultaneously applying microwaves and pressure to curable assemblies.
  • U.S. patent 4,456,498 0498 An improved microwave applicator for continuous presses is disclosed in U.S. patent 4,456,498 0498), and the present invention is an improvement thereon.
  • the '498 patent was also cited in recent U.S. patents 4,609,417, 4,879,444 and 4,906,309.
  • the '498 patent shows a pair of endless belts forming a nip region, a press chamber defined by the belts in the nip region and by two side walls, a means for applying microwaves to the curable assemblies through a waveguide which forms an interface with the press chamber located in an opening in the side wall, and a window or dam at the interface between the waveguide and the press chamber and having sufficient strength to withstand the lateral pressures exerted thereon by the curable assemblies as they are being pressed and to thereby block the entry of the assemblies into the waveguide.
  • This window was constructed of a material which is strong, rigid, abrasion resistant, impermeable to adhesives and transparent to microwave energy. Ceramic materials are examples of such materials, and a preferred ceramic material is aluminum oxide (alumina).
  • the applicator system as disclosed in the '498 patent works well on product depths of four inches or less.
  • FIG. 1 An example of a more recent applicator is shown in Figure 1 generally at 100.
  • This applicator is prior art for U.S. patent practice, since it has been in secret commercial use for more than one year.
  • the applicator waveguide 102 is shaped with a fifteen degree angle as shown by reference numeral 104 in a rapidly expanding horn to define an opening 106 having a depth or height of 7.6 inches as shown by dimension 108.
  • the location of the quarter wave trap of this waveguide is shown at 109.
  • the inlet to the horn or waveguide 102 as shown by dimension 110 is 2.17 inches.
  • a piece of Teflon 114 Positioned behind the ceramic window 112 is a piece of Teflon 114 which is about two inches thick and 9.75 inches wide.
  • the front face of the ceramic window 112 is ten inches wide and has a rectangular configuration. This design for a 7.6 inch depth product worked relatively well and did not generally present any tremendous heating pattern problems. Some degree of uneven heating and occasional product browning was experienced with the 7.5 inch depth product using the applicator or waveguide 102 of Figure 1, but when the size of this applicator was increased for an 11.4 inches opening, to produce a desirably larger product, the uneven heating patterns worsened and became unacceptable.
  • Figure 2 shows generally at 120 a temperature profile using a rapidly expanding horn similar to that of Figure 1 and for a product depth or window opening of 11.4 inches.
  • This is a temperature profile for dielectric conditions of epsilon prime equaling three and epsilon double prime equaling 0.3.
  • the temperature profile in the product 122 shows the extremes of upwards of 170oC at the edge of the ceramic window 112 and window 112a (of a similar microwave system on the other belt side) and a low temperature of 67o in the center.
  • the profile thus comprises one massive low in the center of the product and two significant highs on the edges, with significant distances between them. Since the product 122 is fourteen inches wide, there is approximately seven inches from one edge or hot center to the middle or colder center.
  • an inconsistent temperature profile 120 is the resulting inconsistent normalization bf the product 122. If the product has been heated to about 100o, for example, and then wetted, it will later spring back to the remembered prior size. On the other hand, if it is heated above 120 °, not only has the glue been cured but the lignin has also softened and actually melted and the wood fibers caused to slide internally.
  • the (wood) product 122 resulting from the temperature profile 120 of Figure 2 would most likely be a non-functioning beam.
  • a temperature of 100oC is needed to cure the glue. It is unlikely that the 120° area and the 140° area would have enough energy to bring the 67° center area to a full 100° temperature before the outside, currently at 170°, overheats to the point of browning the wood, which destroys the lignin cellulose matrix.
  • under cured centers and edge browning result from the Figure 1 applicator 100 when adapted and used on thicker products. A more even energy flow through this thick product is accordingly needed.
  • a microwave transparent dam 112 has been secured in a microwave curing system 100, such as that of Figure 1 or of the '498 patent, to the outlet end of the waveguide to prevent the mat, as it is being conveyed and compressed thereagainst and therepast, from entering the waveguide.
  • the dam thus must be strong enough to resist pressures of many hundreds of pounds per square inch.
  • the dam or window is made larger, for example ten inches across and 11.4 inches in depth to accommodate the larger or thicker product, thermal cracking thereof often occurs.
  • Another object of the present invention is to provide a microwave curing system which can simultaneously compress and cure masses of curable assemblies having larger depths of greater than seven and a half inches and more particularly 11.4 inches and with an even resultant heating pattern.
  • a further object of the present invention is to provide an improved window dam for a microwave applicator for curing larger depths of curable assemblies as they are conveyed therepast and compressed thereagainst and which window dam is less susceptible to thermal cracking.
  • an improved microwave curing assembly for a continuous press includes a pair of endless metal press belts forming a nip region, the belts converging to apply pressure to the curable assemblies conveyed between them.
  • a press chamber is defined by the two opposing press belts and by two side walls.
  • Microwaves are applied to the curable assemblies within the chamber via a microwave applicator communicating at one end with a microwave generator and at the opposite end thereof with one of the side walls.
  • a dam or blocking window is mounted at the interface of the outlet of the applicator and the press wall.
  • This press can handle a larger depth of product than previously possible, on the order of 11.5 inches, without undercured centers or browned product edges resulting.
  • the window then must have an overall height of 11.5 inches and the applicator is shaped as a rapidly expanding horn, due to the press configurations, expanding out to this 11.5 inch dimension and with a width of approximately ten inches.
  • the window is formed with three spaced horizontal slots on its inside surface, and three fins are fitted into the slots and secured within the horn.
  • the middle of the three fins extends a further distance back in the horn generally to the end thereof, and the upper and lower fins are shorter and angle inwardly a distance towards the middle fin.
  • the fins extend the entire width of the horn and thereby define three generally independent microwave paths for the microwave energy entering the microwave horn. These paths help suppress the formation of modes of the microwaves other than TE 01 in the horn.
  • the window pieces are configured to delay the phases of the microwaves in one or more of the paths as needed.
  • the window pieces are curved and dimensioned to form lenses for the microwave paths.
  • cooling fluid such as air at plant pressure
  • Figure 1 is a cross-sectional view of a "prior art" (as previously defined) microwave curing applicator.
  • Figure 2 is a computer-generated isothermic drawing of the compressed curable assemblies immediately after exposure to the microwaves from an applicator similar to that of Figure 1.
  • Figure 3 is a cross-sectional view through a microwave curing and press assembly of the present invention.
  • Figure 4 is an enlarged view of the (three fin) applicator horn of the assembly of Figure 3 illustrated in isolation.
  • Figure 5 is an enlarged view of the quarter-wave trap area of the assembly of Figures 3 and 4.
  • Figure 6 is a rear elevational view of the ceramic window, illustrated in isolation, of the applicator horn of Figure 3 with portions thereof broken away for illustrative purposes.
  • Figure 7 is a side elevational view of the window of Figure 6 and in the same orientation of that in Figure 3 and 4.
  • Figure 8 is a side elevational view of the other side of the window of Figure 7.
  • Figure 9 is a partial top view of the window.
  • Figure 10 is a perspective view of the exit end of the applicator horn of Figure 1, with the ceramic window omitted for purposes of illustrating the fin assembly in the horn.
  • Figure 11 is a computer-generated temperature profile in the product and the electric field in the applicator using the system of Figure 3.
  • Figure 12 is a computer model temperature profile similar to that of Figure 11 except taken in a different place in the dielectric spectrum.
  • Figure 13 is a profile similar to that of Figure 12 except for a three-six degree lens configuration instead of a four and a half-nine degree lens configuration.
  • Figure 14 is a view similar to that of Figure 4 of an alternative (single fin) horn of the present invention.
  • Figure 15 is a computer model temperature profile similar to that of Figure 13 using applicator horns of Figure 14 from both sides of the mat.
  • Figure 16 is a side elevational cross-sectional view of a continuous press of the present invention using at least first and second different applicator horns, such as those of Figures 4 and 14.
  • the usual desired mode of propagation in a waveguide is the TE 01 mode where the electric field is everywhere normal to the broad face of the guide, for example, a WR975. guide, or a nine and three-quarter inch by four and three-quarter inch waveguide, at nine hundred and fifteen megahertz.
  • This field pattern is optimal for even heating but was not possible for larger products in the past as explained above.
  • an objective of this invention is to regain as much as possible that purely transverse, straight electrical field propagating through the product.
  • the microwave curing system of the present invention provides an even heating pattern (see Figure 11) from top to bottom of a larger depth of product, on the order of at least 11.4 inches. In terms of the electric field patterns of the microwaves, this is the equivalent of the electric vector being vertical and straight from the top press belt through the product to the bottom press belt, as will be explained in conjunction with Figure 11.
  • a system of the present invention for compressing and microwave curing curable assemblies on a continuous basis is shown generally at 200 in Figure 3.
  • One-half of the system 200 is shown in Figure 3, with the other half being a mirror image thereof and on the opposite side of the center line. (This may be made more apparent when considered in conjunction with Figure 11.)
  • the system 200 basically comprises an endless belt assembly shown generally at 202 and defining upper and lower surfaces of a press chamber shown generally at 204, a microwave generator shown schematically at 206, a microwave waveguide shown generally at 208' into which microwaves from the generator are applied and opening up into a side wall of the press chamber, a multi-piece dam 210 positioned at the end of the waveguide and interfacing with the product in the press chamber, a fin assembly shown generally at 212 mounted inside of the waveguide horn and extending into the dam and whose purpose and construction will be described later, and a quarter wave trap as shown generally at 214.
  • the belts of the belt assembly 202 in normal operation travel at speeds of about three to ten feet per minute. They apply a pressure of about three hundred to about nine hundred psi on the composite material in the press chamber 204.
  • the waveguide 208 is illustrated in its extreme inward position in Figure 3 and is normally drawn closer to the edge of the belts, which position is partially shown by the dotted lines.
  • the width of the press can thereby be adjusted between twelve and seventeen inches, for example, to accommodate different products. One preferred setting is 14.75 inches but this depends on the desired end product.
  • the press belt assembly 202 comprises a press belt, shown in Figure 3 at 218 as a thin layer extending underneath and on top of the applicator assembly.
  • a side dam 220 is removable from the press, backing plates 222 fixed permanently to the press hold the side dam to the press and a donut-shaped wheel 224 sits in a V-shaped trough 226 on the side dam.
  • a cam follower 228 on top and in the other orientation holds or pulls (to the left) the side dam 220 by a small hydraulic cylinder 230 to which the cam follower 228 is mounted.
  • the cylinder 230 pulls the side dam 220 up against the backing plate 222 and makes a seal such that the microwaves from the generator 206 are caused to flow, without leaking, up the channel 231 at an angle, make the turn and flow through towards the window dam 210.
  • a series of roller chains 232 immediately above and below the belt 218 comprise the platen, friction reducing means between the belt and the press itself.
  • the return lines 234 for the roller chains 232, which are the bearing surfaces, are shown in the drawing by the small staggered rectangles. In Figure 3 two are represented, and immediately above and below the ceramic window 210 small rectangles 236 which perfectly match the size of those holes are shown.
  • the holes are staggered in that platen to provide for mechanical structure for applying and supporting the platens through a metal plate that is also returning the chains. All of the stress of the pressure of the platens follows a zig-zag path through that chain return block.
  • a large "O" frame 242 then completely surrounds the platen belt window assembly.
  • the hydraulic cylinders and their piston rods 239 are all on about eighteen inch centers, are about ten or twelve inches in diameter castings in series of two cylinders per block. They are thus only inches apart, and there is no room between them for the microwave generator 206 or connecting microwave tube 231.
  • the microwave infeed 231 is thus positioned below the cylinders 239, and the tube 231 angles up to the applicator 208, as can be seen in Figure 3.
  • the applicator 208 has a rapidly expanding horn shape, as opposed to a constant large cross section along its length, so that it is spaced from the hydraulic cylinders 239.
  • the fin assembly 212 comprises three metallic dividers 244, 246, 248 added in the planes normal to the electric vector and dividing the expanding horn 208 into a series of smaller expanding horns 250, 252, 254; 256 as shown in Figure 3.
  • the fins 244, 246, 248 must extend horizontally relative to the incoming waves. If they have any other orientation, they would cause at least a partial short circuit in the waveguide 208 and dramatically perturb the flow of microwaves therethrough.
  • the smaller horns or waveguides 250, 252, 254, 256 thereby formed suppress the formation of modes other than TE 04 , because the size of each horn is below the cutoff for the TE 02 to TE 0N modes.
  • the horn partitions, dividers or fins 244, 246. 248 extend close to and beyond the quarter wave step 214 which thereby reduces the effect that the step has on the greater part of the field. It is important, however, when dividing the horn 208 into the smaller horns 250, 252, 254, 256 to ensure a near perfect phase matching in the plane 260 of the product/window interface at the termination of the horn, and this result is provided by the unique construction of the present window dam 210.
  • the fins 244, 246, 248 extend into the window 210 beyond the one-quarter wave step 214 as far as possible to reduce the distortion of the step on the electric fields.
  • This extension also supports and positions the ceramic window 210, which is herein advantageously comprised of four bar segments 264, 266, 268, 270, as seen in Figures 6-8, for example.
  • the center of a solid alumina ceramic window of the same size would reach 450o Fahrenheit or more.
  • the edges of the window are cooled by cooling air and water or by contact with the water-cooled aluminum or steel frame, which supports the two outside pieces of the four piece ceramic. The expansion of the hot center can thus create a stress on the cold rim resulting in window cracking, which in turn can ultimately cause the window to fail.
  • the fins 244, 246, 248 fit into three slots 272, 274, 276 formed in the rear surface of the window 210 to support the window.
  • These slots 272, 274, 276 define stress relief cracks in the window 210, as can be seen in Figures 7 and 8. They also divide the large window into the four smaller windows 264, 266, 268, 270, and these smaller windows accordingly show a significantly reduced tendency to crack under thermal stress than does a single large window.
  • the window 210 now made in the four pieces 264, 266, 268, 278, is supported by the fins 244, 246, 248 projecting into the ceramic assembly.
  • the fins 244, 246, 248 not only divide the power evenly within the horn 208 but also have sufficient strength to support the three hundred to five hundred psi pressure on the window/product interface from the curable assemblies being pressed thereagainst. It has been determined that each one of the bars or elongated ceramic window pieces 264, 266, 268, 270, each of which has a length of about 9.75 or ten inches, can actually support a three hundred psi pressure in its long beam direction.
  • the fins 244, 246, 248 reduce the formation of secondary modes.
  • the flow of the microwave energy is more stable in the small chambers, horns or waveguides 250, 252, 254, 256 formed by the fins before it reaches the ceramic window 210 than it would be in a similar wide open horn.
  • the small horn chambers 250, 252, 254, 256 formed by the fins of the present invention tend to stabilize the fields. They tend to return to a true TE 01 mode, and higher order modes are thereby suppressed.
  • the fins 244, 246, 248 are made out of aluminum as is the horn 208 and its mount.
  • the center fin 246 goes back approximately six or eight inches, and the upper and lower fins 244, 248 extend back three or four inches at an inward angle.
  • the fins can extend back a much shorter distance and still have a sufficient mechanical spring if they have suitable engineering properties, stiffness and strength.
  • the fins are preferably machined from approximately one inch thick plate. The end of the edges of that section are machined half round, and half-round machining into the block 208 to accept that half round piece is made. This allows the fins 244, 246, 248 with their shaped tips and tapered tails to be accurately positioned to support the tremendous loads which will bear on them.
  • each fin is thus uniquely assigned in the applicator 208 and not interchangeable with any other fin.
  • the fins and their two half cylinders are each machined from a single piece and slid out and removed from the waveguide 208. To assemble them, they are slid back down that half-cylinder channel built into the main block and two bolts are then inserted on either side thereof and tightened to hold them in place.
  • the fins and ceramics can be attached to a framework (not shown) and slipped into the horn.
  • the tips 284, 286, 288 of the fins 244, 246, 248, respectively are not permanently welded but rather are floating or cantilevered out.
  • the ledge also advantageously becomes a quarter wave transformer for the forwardmost tips 300 of the fin.
  • the tips 300 of the fins are discontinuities, and the ledges 292 that support the ceramic are also discontinuities. These two discontinuities are positioned a quarter wave length apart and thereby form a quarter wave transformer.
  • the tip 300 is twice as big as each of the two steps 292, 294; that is, it is as big as the sum of the two edges on either side of a single fin. This forms a step transformer which phase matches the reflections coming off the tips such that the energy flows into the product.
  • the fin assembly 212 is permanently bolted into the waveguide 208, as previously explained, and extends horizontally across the rectangular guide, as can be seen in Figure 10.
  • the center plate fin 246 is ten millimeters thick, runs the length of the applicator and divides the waveguide into two very distinct waveguides shown generally at 302, 304.
  • Each of the outer fins 244, 246 divides the created waveguides 302, 304 into two more waveguides, thereby creating the four waveguides 250, 252, 254, 256 opening to the ceramic window 210.
  • Each ceramic piece 264, 266, 268, 270 then fits into the end of its respective waveguide 250, 252, 254, 256.
  • the ceramics have been sized and shaped to delay the energy flow from each of the four waveguides so that they are all in phase at the front of the window. It is seen in Figure 11 that the electric fields at the face 260 of the window 210, very close to the product, form almost perfect vertical lines going top to bottom, and the degree to which they are perfect is the degree to which the assembly heats evenly and hot spots in the product are less likely to be created.
  • the ceramic window 210 is made in graduated thicknesses to form a plane wave at the product/ceramic interface, and the window pieces or lenses 264, 266, 268, 270 have their curvatures determined using optical type techniques and waveguide calculations.
  • the microwaves propagating through the point at the start of the central plane fin 246 first travel through clean dry air 306 in the horn 208 to the ceramic 210 and then through ceramic to the product.
  • those microwaves that are expanding out along the surface of the horn going upwards and downwards have a longer flight path to reach the front of the ceramic 210.
  • the thickness of the ceramic 210 thus changes gradually to delay the wave arrival times at the front of the ceramic so that all wave components arrive at the same time.
  • That delay time is accordingly a function of how fast the microwaves travel - - first in air 306 and then in ceramic 210. Since they travel slower in ceramic, the ceramic window 210 is made thicker to slow them down as needed in the center, as shown in the drawings. In other words, the thicker ceramic center phase delays those waves that would be ahead because they are part of a spherical front and would otherwise reach the front of the plane of the window first.
  • the ceramic window 210 As shown in the top view of the ceramic window 210 in Figure 9, there is a 4.25 millimeter radius half-circular cut 308. A corresponding half-circular cut is provided in the aluminum block. After the ceramics 264, 266, 268, 270 are installed, a pin (not shown) is slid down the half circle on either side, and the ceramic window 210 is thereby blocked from falling out the mouth of the applicator 208; the pin thus captures the ceramic pieces in and to the horn.
  • the window pieces 264, 266, 268, 270 are made of ceramic instead of Teflon, which has little strength.
  • the ceramics are preferably a relatively high purity alumina Al 2 O 3 and have a dielectric constant between nine and ten.
  • the dielectric constant determines the thicknesses of the window pieces 264. 266, 268, 270 and along with guide size is directly related to the speed of the microwaves in the ceramic. Since the ceramic window 210 has a high dielectric constant, the lens is relatively flat.
  • steps 310 where the angle portion of the guide steps down and then extends horizontally to the product, or steps up and goes virtually to the top of the product. Those steps are virtually impossible to eliminate because of the need for quarter wave traps 214, and they distort the electric fields.
  • the geometric size of the quarter wave traps 214 has been herein minimized to a degree sufficient to provide even heating.
  • the quarter wave trap 214 has the size of the step from the horn to the belt. This area is herein as small as can be mechanically made, machined and maintained. The step herein is made quite small by making the leading edge 312 of the quarter wave trap where it meets the window 210 at the edge of the aluminum as small as possible, on the order of two or three millimeters. There is also about two or three millimeters of aluminum support - - the mounting frame for the two outside pieces of ceramic 264, 270.
  • FIG. 3 Referring to Figures 3, 7 and 8, six-millimeter bore holes 316 are formed in the ceramic window 210 for cooling it.
  • An air return channel 318 extends between the second and third holes, and the air return channels are staggered as can be understood when considering Figures 7 and 8 together.
  • the air thus enters the top hole in Figure 8, goes down the length and returns to the second block, back to the second block, and returns within the second block and so forth - - zigzagging or serpentining its way through the windows.
  • Small tubes (not shown) are preferably fitted and cemented with silicone in these serpentine bore holes to ensure that the air does not leak out of them.
  • the air is blown into the bore holes 316 from a source of compressed air as shown in Figure 3 at 320, such as air from the mill as would be more apparent from the '000 application, and having a pressure of about one hundred psi, for example.
  • a source of compressed air as shown in Figure 3 at 320, such as air from the mill as would be more apparent from the '000 application, and having a pressure of about one hundred psi, for example.
  • the air can be fed to each ceramic bore from a manifold and collected at the other end reducing the resistance to air flow.
  • structure was added, pursuant to this invention, to the applicator horn 208 and the horn design was altered to create as even an electric field as possible at the product/horn interface and thereafter across the product.
  • This structure includes the window 210— the layers of microwave transparent material of a known dielectric property added to the inside of the horn to phase delay the cylindrical expanding wave.
  • the one quarter wave traps 214 were modified to minimize the discontinuity, to reduce the field distortions in this area.
  • the structure further includes the metallic dividers or fin assembly 212 added in the plane normal to the electric vector to divide the expanding horn 208 into a series of smaller horns 250, 252, 254, 256 and also to secure the pieces 264, 266, 268, 270 of the dam window 210 in place.
  • FIG 11 shows the isotherms and electric fields resulting from the additions and modifications of the present system and the consequent heating pattern for one estimated set of dielectric parameters of an epsilon single prime of three and an epsilon double prime of 0.3.
  • This drawing shows the isotherms in degrees Centigrade, wherein the "H” designates the high temperature areas and the "L” designates the low temperature areas.
  • the temperature ranges from a high of about 155oC to a low of about 80°. This low, however, is sandwiched between two relatively hot areas which are very close together and thus a short time laeer steam transport from the higher temperature into the lower temperature occurs.
  • thermograph (not shown) from a cross section cut of the product would show a difference in temperature of only plus or minus 10°. In other words, the hot and cold spots would have essentially disappeared, and a consistent normalization of the thick product thereby advantageously results.
  • the microwave generator 206 operates preferably at nine hundred and fifteen MHz, which is an Industrial Scientific Medical (ISM) band. These applicators 208 operate in the same electric field mode, the TE 10 mode, as described in the '498 patent.
  • the total power in the system 200 from one window 210 is about twenty-five kilowatts in normal usage, though it can be higher. It is anticipated that there will be sixteen windows in a preferred layup process, as shown in the '000 application, for a total of four hundred kilowatts, which makes about two million cubic feet of product a year. This relationship is effectively linear so that if twice the power were provided, twice the product could be made.
  • the rearward surface 290 of the dam or lens is configured to approximate a cylinder, that is it is a cylindrical surface. It is shaped by a series of connected straight lines or planar surfaces. In a preferred embodiment and as shown in Figures 4 and 12, the surface is comprised of five line segments or surfaces, namely, the central planar (vertical) surface 292, the two surfaces 294, 296 at the outer edges of the lens and the two surfaces 298, 300 between the edge and the central surfaces.
  • the outer fins 244, 248 are positioned between the connecting surfaces and the outer surfaces, respectively.
  • the central fin 246 passes through the center of the center surface 292.
  • the connecting surfaces and the central surface define respective break angles at the junctures. These are shown by angle 302 (four and a half degrees) in Figure 12.
  • connection and the edge surface defines a second break angle 304 (nine degrees) with the central surface or the planar.
  • the second break angle 304 is approximately twice that of the first. In other words, the angle between the connecting and central surfaces equals the first angle 302.
  • the embodiment of Figures 4 and 12 have first and second angles 302, 304 of ii and 9o, respectively.
  • the profile 306 of Figure 12 differs from that of Figure 11 as it is taken at at different place in the dielectric spectrum. It is taken at an epsilon prime of three and an epsilon double prime of three.
  • the profile 306 of Figure 12 is perhaps a better approximation of the actual temperature profile in the product which itself is a function of the glue and moisture contents and is variable. In other words, the profile 306 of Figure 12 represents a better selection of parameters to estimate the actual heating pattern.
  • the profiles of Figures 11 and 12 are very similar though.
  • FIG. 14 An alternative to the horn embodiment of Figure 4 is the horn embodiment of Figure 14 shown generally at 316 wherein the outer fins are not used. This can be done by removing the outer fins and filling in the holes or slots in the back surface of the lens with ceramic to smoothly fill across the gap. Although tiny cracks may result, they are too small to be seen by the relatively long wave length of the microwaves. Instead of forming the lens with four pieces and filling in the two resulting empty slots or gaps, the preferred way is to simply make the lens or dam as two pieces with the central fin 246 positioned between them. This is shown in Figure 14.
  • the window shown in Figure 14 is a larger window on the order of fourteen or fifteen inches high as opposed to 11.4 inches as previously described.
  • a belt press with microwave applicators typically uses a number of pairs of applicators along the length of the press chamber.
  • the curable assemblies such as adhesively bonded interwoven layers of thin wood strands (See U.S. Application 07/555,732, filed July 23, 1990; Canadian application 2,022,900-4; and International Application No. , filed July 23, 1991 and entitled "System for Oriented Strand Lay-up)
  • the curable assemblies are conveyed into the press chamber on the conveyor and between the two converging metal press belts, the curable assemblies are subjected sequentially to microwave energy from a number of different applicators as they are conveyed through the press chamber.
  • An example of a press belt arrangement is shown in Figure 16 generally at 324 and is described in further detail in the '498 patent.
  • the continuous press 324 comprises a pair of steel press belts 326 having belt positioning means including an upper belt and a lower belt, which loop back upon themselves so as to form continuous belts.
  • Pressure transfer means 330 transfer compressive forces to the belts.
  • the belts are driven in the direction of arrow 332 and in operation the curable assemblies move in this direction, enter the nip of the continuous press and are compressed to a maximum degree upon reaching the press section of the press.
  • Examples of belt presses are those disclosed in U.S. Patent 4,517,148 and preferred presses are disclosed in copending U.S. application Serial No. 07/456,657, filed December 29, 1989 (Canadian application 2,006,947-3 and International Application No. PCT/CA90/ 00459).
  • the microwaves are directed from a plurality of microwave applicator horns into the curable assemblies.
  • the cured assemblies 340 are removed from the press.
  • the sidewalls 342 prevent the curable assemblies which are under compression from escaping laterally from the press section.
  • the dams of the applicator horns are secured in openings in the sidewalls 342. In the past, it has been known to use approximately between two and eight applicator horns on each side of the press. Half of these horns were positioned before or upstream of the parallel press region.
  • a preferred continuous press for the present invention takes advantage of the different heating patterns available from using different applicator horn configurations.
  • Use of different heating patterns at different locations along the conveyance travel of the curable assemblies has a number of advantages. It can take advantage of the fact that later or downstream heating patterns are focused on curable assemblies which have been at least partially heated or cured. Further, the different patterns will tend to even out under further compression, curing and subsequent cooling providing with careful control a more evenly heated product.
  • the problems of uneven density profiles of the mat or layup and resultant uneven heating in the microwave press are discussed in copending U.S. Application Serial No. 07/575,007, filed August 30, 1990 (Canadian application
  • the heating profile of Figure 15 can be used for the first three or four applicators or windows followed by the rest of the press comprising four or five applicators using the heating pattern of Figure 13.
  • the first three or four windows then tend to heat the surface near the press belts more than the center to make the compressible assembly slightly more compressible in this area. This tends to compensate for the cold spots in the layup by using differential microwave heating patterns.
  • the applicator horns of Figure 14 can be in the wedged or contracting portion of the sidewalls and thus can be slightly taller on the order of fourteen inches as opposed to 11.4 inches and have a width of approximately ten inches at the front of the window.
  • the heating pattern resulting with the single fin embodiment of Figure 14 and shown in Figure 15 confirms what was expected in that less even heating on the surface of the window results when fins are removed. This is shown by the high density of the isotherms on the surface of the window. This is a more severe and thus generally less desirable heating pattern than that of Figure 13.
  • a result which can be taken advantage of is that hot spots of 135o result on the top and bottom surfaces. In contrast, the warmer spots of Figure 13 are in the center though they are only warmer by less than 10°.
  • the temperature profiles as measured by infrared camera systems are better than that of Figure 13 alone.
  • the reason that they are better is the averaging of the many window heating patterns and also the time between the heating being applied and the observation being taken; there is some diffusion of energy throughout the system which evens the heating.
  • the use of a plurality of heating profiles also allows the system to accommodate different product characteristics. It will be able to tolerate a greater range of moistures, densities and temperature variabilities within the mat.
  • the system can make a better, more evenly heated product over a broader range of glue content variables as well, since glue acts as a strong absorber of the microwaves.
  • break angles 302, 304 and the amounts of these angles can be empirically modified to optimize the evenness of heating. They can be set at an effective radius defined theoretically or empirically by using models to test out shapes and observing heating patterns.
  • the use of three line segments 292, 294, 298 defining two angles is for practical purposes a good solution as the cost of grinding the ceramics (lenses or dams) is not insignificant.
  • the tuning of a lens from a 41 ⁇ 2-9° angular relation to a 3-6° relationship requires that only three millimeters of thickness be ground away from the center of the window.
  • the accuracy of the configuration of the rear surface is within a couple of millimeters which is considerable greater accuracy than would appear to be required from a traditional optical analysis wherein an accuracy of one-twentieth lambda for grinding accuracy of lenses is an optical industry standard.
  • a one twentieth of a wavelength of the microwaves of this invention would be approximately a half centimeter.
  • the present invention carefully controls near field patterns to provide an even distribution of heat.
  • Near field refers to distances of inches or feel in front of the applicator where a very complex distribution pattern for energy density is found.
  • antenna systems are designed to broadcast, and not heat. Their near field heating patterns are thus of no interest to the designer and are usually unsuited since nothing is done to control them. Antennae system designers are only concerned with the far field.
  • the present invention relates to a method of getting a very small guide, on the order of fifty mm, to apply energy evenly across the face of a very large piece of ceramic, on the order of three hundred mm, in a very short distance, on the order of four hundred and fifty mm.

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  • Life Sciences & Earth Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Wood Science & Technology (AREA)
  • Forests & Forestry (AREA)
  • Health & Medical Sciences (AREA)
  • Clinical Laboratory Science (AREA)
  • General Health & Medical Sciences (AREA)
  • Constitution Of High-Frequency Heating (AREA)

Abstract

Système destiné à l'application simultanée de pression et de micro-ondes à une pile épaisse d'ensembles traitables par cuisson. Alors que les ensembles traitables par cuisson sont transportés à travers une presse à courroie, des micro-ondes y sont appliquées par l'intermédiaire d'un cornet applicateur contigu à expansion rapide. Un barrage ou une fenêtre en céramique laisant passer les micro-ondes, ménagé au travers de la sortie du cornet, empêche les ensembles comprimés de pénétrer dans le cornet. Afin de minimiser toute fissure de la fenêtre due à la chaleur, la fenêtre se compose d'un certain nombre de morceaux. Ces morceaux sont maintenus en position au niveau de la sortie du cornet par ailettes fixées à l'intérieur du cornet et faisant saillie dans le fenêtre. Les ailettes s'étendent à travers l'intérieur du cornet et le divisent en un certain nombre de passages de micro-ondes différents. Les parties de fenêtre en céramique se trouvant aux extrémités des passages sont formées avec précision afin de faire fonction de lentilles et d'agir sur l'énergie des micro-ondes provenant de chacun des différents passages de sorte que les micro-ondes venant de tous les passages sont en phase là où elles rencontrent le produit. De l'air est pompé à travers un passage en serpentins ménagé dans la fenêtre afin de refroidir celle-ci et d'en empêcher les fissures. Des cornets applicateurs présentant des configurations de chauffage différentes peuvent être avantageusement utilisés en différents emplacements sur le lit de la presse.
PCT/CA1991/000264 1990-07-27 1991-07-26 Systeme de cuisson par micro-ondes WO1992003027A1 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US55765290A 1990-07-27 1990-07-27
US557,652 1990-07-27
US57500790A 1990-08-30 1990-08-30
US575,007 1990-08-30

Publications (1)

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WO1992003027A1 true WO1992003027A1 (fr) 1992-02-20

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AU (1) AU8195691A (fr)
WO (1) WO1992003027A1 (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8414720B2 (en) 2010-06-21 2013-04-09 Weyerhaeuser Nr Company Systems and methods for manufacturing composite wood products to reduce bowing
EP3461231A1 (fr) * 2017-09-26 2019-03-27 HOMAG GmbH Applicateur destiné à l'activation thermique d'une couche fonctionnelle d'un matériau de revêtement

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4456498A (en) * 1982-08-10 1984-06-26 Macmillan Bloedel Limited Microwave applicator for continuous press
FR2650775A1 (fr) * 1989-08-09 1991-02-15 Poudres & Explosifs Ste Nale Dispositif de mise en forme sous presse au moyen de microondes

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4456498A (en) * 1982-08-10 1984-06-26 Macmillan Bloedel Limited Microwave applicator for continuous press
FR2650775A1 (fr) * 1989-08-09 1991-02-15 Poudres & Explosifs Ste Nale Dispositif de mise en forme sous presse au moyen de microondes

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8414720B2 (en) 2010-06-21 2013-04-09 Weyerhaeuser Nr Company Systems and methods for manufacturing composite wood products to reduce bowing
EP3461231A1 (fr) * 2017-09-26 2019-03-27 HOMAG GmbH Applicateur destiné à l'activation thermique d'une couche fonctionnelle d'un matériau de revêtement

Also Published As

Publication number Publication date
AU8195691A (en) 1992-03-02

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