WO1992003027A1 - Microwave curing system - Google Patents

Abstract

A system for the simultaneous application of pressure and microwaves to a thick mat of curable assemblies. As the curable assemblies are conveyed through a belt press, microwaves are applied to them through an adjacent, rapidly expanding applicator horn. A microwave-transparent ceramic dam or window across the outlet of the horn blocks the compressed assemblies from entering the horn. To minimize any heat cracking of the window it is formed of a number of pieces. These pieces are held in place at the horn outlet by fins secured within the horn and protruding into the window. The fins extend across the interior of the horn and divide it into a number of different microwave paths. The ceramic window pieces at the ends of the paths are carefully shaped to act as lenses and to act on the microwave energy from each of the different paths so that the microwaves from all of the paths are in phase at the product interface. Air is pumped through a serpentine path in the window to cool it and help prevent cracking thereof. Applicator horns having different heating patterns can be advantageously used at different locations along the press bed.

Description

MICROWAVE CURING SYSTEM

BACKGROUND OF THE INVENTION

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, however, 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.

Numerous attempts have been made to use radio frequency (R.F.) energy, that is, dielectric heating, to cure the resin. Where R.F. heating techniques were used and especially where the phenolic resin layer was thick, arcing and tracking in the resin resulted. This undesirable phenomena appears to be due to the relatively high activity of some resins which leads to breakdowns when subjected to R.F. fields of the necessary field strength. Although the arcing and tracking problem can be reduced significantly if the R.F. field is applied transverse to the glue line, transverse application reduces the efficiency of the process.

Application of 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. For example, U.S. patents 4,018,642 and 4,020,311 disclose techniques for simultaneously applying microwaves and pressure to curable assemblies. These and each of the other patents, publications and applications mentioned anywhere in this disclosure are hereby incorporated by reference in their entireties.

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.

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.) Referring thereto, it is seen that 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. 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. This means for the heating temperature in the microwave product 122 to even out that there must be a steam transport of approximately seven inches from the higher temperature to the lower temperature, and this is too great a transport distance. In other words, in the hot areas of the compressed mat or product 122 a considerable amount of the steam is generated due to the boiling of the water in the mat. Since boiling is an expansion process, a large volume of gas is created from the small volume of liquid which then tends to flow under pressure gradients to smooth out the low and high temperature spots. A signif icant evening of the high and low temperatures can only take place, however, if the highs and lows are not spaced too far apart, which is not the case with the "prior art" of Figure 2. In fact, if the product of Figure 2 were allowed to sit for ten or fifteen minutes, a chain saw cut then made through it and a thermograph (not shown) taken of the resulting cross-section, the temperatures in the cross-section would vary from thirty to forty degrees.

The main effect of 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.

Similar problems have been experienced in other work and corrected using a steam post treatment of wafer board. Some times wafer board is hot pressed too quickly in order to speed the press cycle and the center of the board does not reach a sufficient temperature but rather only a point where it cures enough to hold itself together. Consequently, if the wafer board is later wetted it can spring up to approximately double its thickness, which is unacceptable for most end uses.

The present composite wood product (122) as described, for example, in copending U.S. application Serial No. 07/555,000 ('000), filed July 23, 1990, and entitled "System for Oriented Strand Layup," (and in Canadian application Serial No. 2,022,900-4), if cured correctly, has only about a two or three percent retained spring back in the compression direction after wetting and drying and in the other directions behaves similar to natural wood. In other words, if the present product is wetted and dried it will return to its original dimensions except in the compression dimension where after drying it will be two to three percent thicker than it originally was. If the higher temperatures are not obtained consistently, then a five or even ten percent increase in thickness after drying is experienced.

This swelling is undesirable for nearly every application since wood that is dimensionally stable is easier to engineer and to service and better able to survive a change in the elements. Often during construction, wood becomes quite wet and only by the fact that it has been placed inside a building that is eventually enclosed does natural evaporation dry the wood to an equilibrium of from about six to sixteen percent.

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. In other words, 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.

As previously mentioned, 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. As 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. SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to provide an improved system for simultaneously exposing curable assemblies to pressure and microwave energy in a continuous process.

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.

Directed to achieving these objects, an improved microwave curing assembly for a continuous press is herein provided. The 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.

To prevent cracking of this large window, it is formed of a number of window pieces held together by the metal horn assembly. More particularly, 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 TE₀₁ in the horn. Since the paths have different lengths and the microwave energy must be in phase at the interface with the curable assemblies at the outlet of the window, the window pieces are configured to delay the phases of the microwaves in one or more of the paths as needed. In other words, the window pieces are curved and dimensioned to form lenses for the microwave paths. To cool the window a serpentine channel is formed therethrough and cooling fluid, such as air at plant pressure, is pumped therethrough.

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

Figure 1 is a 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.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The usual desired mode of propagation in a waveguide is the TE₀₁ 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. Thus, 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, as will be described below, 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.

It was determined that two geometries of the system 100 of Figure 1 were preventing that objective from being attained. First, the rapidly expanding horn 102 created a cylindrical shape to the wave front which thereby distorts and becomes more complex in shape as it bounces off the parallel press belts. Second, the structure of the quarter wave traps 109 (as reported in the prior art) created a step discontinuity between the expanding horn 102 and the press belt which added further complexity to the shape of the electric fields. The complex shape of the electric fields changes the field strength dramatically along the field lines. Further, since heating is proportional to the square of the electric field, the effect of uneven fields on heating is even more pronounced. Significant modifications to the waveguide 102 of Figure 1, and particularly additional structure added thereto, were thus determined to be needed.

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. Details of preferred arrangements for the belt assembly 202 are shown in copending U.S. application Serial No. 07/456,657, filed December 29, 1989, (and in Canadian application No. 2,006,947-3) and. in U.S. patents 4,508,772 and 4,517,148. A detailed discussion of preferred methods of forming adhesive coated, curable assemblies and depositing them on a continuous lay-up conveyor belt for conveyance to compressing and curing systems, such as system 200, is found in the '000 application, which is related to U.S. patents 4,872.544, 4,563,237. and 4.706,799, and in U.S. patents 3,493,021 and 4,546,886. Additionally, products containing oriented elongate strands such as might be used herein are disclosed in U.S. patent 4,061,819, which was reissued as Re. 30,636.

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. Also, 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.

Electric fields, as a law of physics, attach themselves perpendicularly to metallic surfaces. 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. With the power flowing in the TE₁₀ mode, 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 TE₀₄, because the size of each horn is below the cutoff for the TE₀₂ 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. In contrast, 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. In other words, 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. Thus, when the microwave fields in a wide open horn are perturbed, an unstable oscillating and undulating of those fields results. In contrast, 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 TE₀₁ 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. Alternatively, 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. Bolts hold the fin in position, and the shoulder of the one-inch half cylinder 280 bears the load. The fins 244, 246, 248 are bolted into place, and the surfaces thereof are then machined for final fitting of the ceramics. Each fin is thus uniquely assigned in the applicator 208 and not interchangeable with any other fin. In other words, 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. Alternatively and in lieu of this individual fit, 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 (the right hand tips as depicted in Figure 4), are not permanently welded but rather are floating or cantilevered out.

Referring to Figure 4, at the left-hand side of the right-hand part of the fins, there is a step 292 at the top and a shoulder 294 at the bottom of that step that supports the ceramic. In fact, there is a step at the outer edges of the product at the top and at the bottom. The plane 296 of those five steps is exactly parallel to the window face 260 and defines the shape of the window 210 and the position in which it is held. 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. Accordingly, the reflection caused by the first discontinuity is cancelled by the reflection caused by the second. As shown in Figure 4, 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. Referring to Figures 3 and 11, 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. As can be understood from the geometry, 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.

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 Al₂O₃ 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.

It is seen in Figure 11 that the cylindrical radiused electric field travels through the applicator 208, strikes the ceramic window 210 and is effectively straightened out. Although it is theoretically possible to make a perfect vertical line out of those fields, in reality there is some degree of electric field variation. There is also a reflection from this surface that is a complex function of many things including the incident angles and relative dielectric constants, so there is some distortion of that field caused by the reflection. Similarly, there is a reflection of the interface between the product and the ceramic 210, and it is the interaction of those interfaces that determines how perfectly this electric field enters the product.

As shown in Figure 5, at the top and the bottom of the product in the applicator 208 are 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.

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. Alternatively, if more air flow is beneficial the air can be fed to each ceramic bore from a manifold and collected at the other end reducing the resistance to air flow.

Thus, to develop an even heating pattern from top to bottom in the 11.4 dimension of the composite microwave-curable product, 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.

Figure 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. As can be seen, 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. There is thus only a distance of a little more than an inch between the highs and lows compared with the nearly seven inches of the prior art product as shown in Figure 2. Accordingly, not only do the highs have lower temperatures and the lows have higher, but the highs and lows are spaced closer - - close enough for effective steam transport between them. Thus, with the product of Figure 11 and after about ten or fifteen minutes, a 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 TE₁₀ 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. Similarly, the 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. These angles and line segments are selected so that the rear surface 290 approximates a cylinder.

The embodiment of Figures 4 and 12 have first and second angles 302, 304 of ii and 9º, respectively. For these angles and this three fin arrangement the computer model temperature profiles are shown in Figures 11 and in Figure 12 at 306. 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. If they were allowed to diffuse, remarkably similar sets of hot and cold spots will result. The 155º hot spot of Figure 11 will sit on top of the 128° hot spots of Figure 12 and the cold spots of 80° of Figure 11 will sit on top of the 100° cold spots of Figure 12. Although the relatively cool spots at the base of the windows are moved slightly and modified slightly, they function the same. From the product's point of view, these are similar heating patterns.

Recent tests have shown that a better configuration of the rear surface of the lens is to have angle 302 be 3° and angle 304 be 6°. The heating pattern resulting from this configuration is shown in Figure 13 generally at 312, which is apparently the most even and thus best heating pattern. The lowest or coolest point is in the center the 111ºC band going vertically from top to bottom with little deviation. There is a large area around 120° on either side of it and the 125° band is very close to the surface, with the exception of the very tiny corners which are at 145° no other areas exceed 135°. In other words, the two hot lobes of 128° in the profile 306 of Figure 12 have been eliminated. The fact that the small fringes have higher temperatures is inconsequential because the corners of the product are cooled in operation by steam escaping from the product. In other words, they influence only a very small volume of the product and tend to diffuse to a more even profile.

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. It has angles 302 and 304 of 3º and 6°, respectively, and the resulting temperature profile is shown in Figure 15 generally at 320. It is seen therein that hot spots of 135º are formed on the top and bottom surfaces and cold spots of 95° in the central area. To some effective degree the cold and hot spots of the profile of Figure 15 correspond with those of Figure 13, that is, the 120° hot spots of Figure 13 would sit on top of the 95º spots of Figure 15.

A belt press with microwave applicators typically uses a number of pairs of applicators along the length of the press chamber. Thus as 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), 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. It is seen therein that 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). While the curable assemblies 336 are under the compression in this press section, the microwaves are directed from a plurality of microwave applicator horns into the curable assemblies. After passing out the end of the press section 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

2,025,555-2, and International Application No.

, filed July 22,

1991 and entitled "Wood Composite Forming and Curing System"). For example, 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. (Only three applicators or horns 348, 350, 352 are shown though in Figure 14 for illustrative purposes.) 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°.

When using applicator horns having Figure 13 and 15 profiles in a single continuous press, 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.

The actual precise positioning of the 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 4½-9° 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. In contrast, 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. Thus, 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. The present invention creates a single phase front and where it is not perfect it will have very high order modes, on the order of five or higher. From the foregoing detailed description, it will be evident that there are a number of changes, adaptations and modifications of the present invention which come within the province of those skilled in the art. However, it is intended that all such variations not departing from the spirit of the invention be considered as within the scope thereof as limited solely by the claims appended hereto.

Claims

WHAT IS CLAIMED IS:

1. A microwave curing system, comprising:

belt means for conveying and compressing curable assemblies;

an expanding horn having a horn inlet and a horn outlet, said horn opening up towards the curable assemblies conveyed past said horn outlet by said belt means;

a source of microwave energy communicating with said horn inlet and creating in said expanding horn an expanding cylindrical wave; and

phase delay means generally at said horn outlet for phase delaying the front of the expanding cylindrical wave to a plane wave at the interface with the curable assemblies at said horn outlet.

2. The system of claim 1 wherein said phase delay means comprises a plurality of layers of microwave transparent material of known dielectric properties attached to the inside of said expanding horn.

3. The system of claim 2 wherein said microwave transparent material comprises alumina ceramic.

4. A microwave curing system, comprising:

a microwave guide applicator horn having a horn outlet; at least one fin extending across said horn and dividing said horn into at least first and second generally independent and different microwave paths;

supplying means for supplying microwave energy into said horn and to said first and second paths; and

lens means for delaying the phase of the microwaves along at least part of one of said first and second paths so that the microwaves from said first and second paths are in a near phase plane relation at said horn outlet for entry into curable assemblies thereat.

5. The system of claim 4 wherein said lens means comprises a first lens generally at the end of said first path and a second lens generally at the end of said second path.

6. The system of claim 5 wherein at least one of said first and second lenses is secured to said dividing fin and is thereby at least in part supported by, in and relative to said horn.

7. The system of claim 5 wherein said first and second lenses are formed generally together as one continuous lens structure.

8. The system of claim 5 wherein said first and second lenses are formed of ceramic material.

9. The system of claim 4 wherein said horn has an outwardly expanding horn portion and an angled neck, said horn portion has an inlet and an outlet, said lens means is secured at said horn portion outlet, said angled neck communicates said supplying means with said horn portion inlet, and said dividing fin is mounted generally in said horn portion outlet.

10. The system of claim 4 wherein said lens means forms a secure dam at said horn outlet blocking the compressed curable assemblies pressing thereagainst and passing thereacross from entering said horn.

11. The system of claim 10 wherein said lens means is separate from said secure dam at said horn outlet.

12. The system of claim 4 further comprising belt conveying means for conveying the curable assemblies past said horn outlet.

13. The system of claim 4 further comprising belt compressing means for compressing the curable assemblies generally as they are being subjected to microwave energy from said supplying means.

14. The system of claim 4 wherein said horn has a horn inlet and said fin includes a center fin which extends to said horn inlet.

15. The system of claim 4 wherein said horn outlet has an opening height of at least 7.5 inches.

16. The system of claim 4 wherein said horn outlet has an opening height of generally 11.4 inches.

17. The system of claim 4 further comprising bolt securing means for securing said at least one fin in and to said applicator horn.

18. A microwave curing system, comprising:

a microwave waveguide having a waveguide inlet and a waveguide outlet; introducing means for introducing microwave energy into said waveguide inlet for passage through said waveguide, out said waveguide outlet and to curable assemblies generally at said waveguide outlet;

dividing means for dividing said waveguide into generally independent first and second waveguide paths both communicating with said waveguide inlet and outlet; and

coordinating means for coordinating the flows of microwave energy along said first and second waveguide paths so that the two flows are in phase when entering the curable assemblies.

19. The system of claim 18 wherein said phase delay means comprises a microwave lens at the end of said first waveguide path.

20. The system of claim 18 wherein said phase delay means comprises a first microwave lens at the end of said first waveguide path and a second microwave lens optically different from said first microwave lens and at the end of said second waveguide path.

21. The system of claim 20 wherein said first and second microwave lenses form at least part of a dam blocking the entry of the curable assemblies into said waveguide.

22. The system of claim 21 wherein said lens is separate from said dam.

23. The system of claim 21 wherein said first and second lenses form generally a continuous window.

24. The system of claim 18 wherein said introducing means introduces the microwave energy at a frequency of approximately 915 MHz and 25 kilowatts of power.

25. A microwave curing system, comprising:

a microwave applicator having an applicator inlet and an applicator outlet;

conveying means for compressing curable assemblies and conveying them past said applicator outlet;

microwave means for passing microwave energy through said applicator and into the cur c.e assemblies as they are conveyed by said conveying means past said applicator outlet; and dam means at said applicator outlet for blocking the flow of compressed curable assemblies into said applicator, said dam means comprising adjacent microwave transparent first and second dam pieces and supporting means between said first and second dam pieces and secured to said applicator for supporting said first and second dam pieces in said applicator and relative to one another.

26. The system of claim 25 wherein said supporting means comprises a plate positioned between said first and second dam pieces and secured to and within said applicator.

27. The system of claim 25 wherein said first and second dam pieces are formed as ceramic horizontal bars, one above the other.

28. A microwave curing system, comprising:

a microwave applicator having an inlet communicating with a source of microwave energy and an outlet out through which microwave energy from the source passes into compressed curable assemblies conveyed past said outlet; and

dam means at said outlet for blocking the flow into said applicator of the compressed curable assemblies conveyed past said outlet, said dam means comprising a sheet of microwave transparent material covering said outlet and having an inward face and slot on said inward face generally dividing said sheet into first and second dam pieces, and a plate member inside said applicator, fitted in said slot and at least in part securing said first and second dam pieces in and to said applicator.

29. The system of claim 28 wherein said slot comprises a first slot, and said sheet includes second and third slots dividing said sheet into further third and fourth dam pieces.

30. The system of claim 29 wherein said plate member defines a first securing plate member, and further comprising a second securing plate member inside said applicator and fitted in said second slot and a third securing plate member inside said applicator and fitted in said third slot.

31. The system of claim 30 wherein said second and third securing plate members are disposed on opposite sides of said first securing plate member.

32. The system of claim 31 wherein said first, second and third plate securing plate members extend generally horizontally rearwardly from said sheet.

33. The system of claim 31 wherein said applicator defines an expanding horn and the ends of said second and third securing plate members are angled in towards the centerline of said horn.

34. The system of claim 28 wherein said applicator defines a horn whose inside surfaces define a thirty degree angle with a vertical cross-sectional plane.

35. The system of claim 28 wherein said sheet forms a secure side wall of a compression chamber for the curable assemblies.

36. The system of claim 35 wherein said dam means has a height of at least generally 11.4 inches and a width of generally at least ten inches.

37. A method for curing curable assemblies, said method comprising the steps of:

introducing microwave energy into an expanding waveguide horn;

suppressing the formation of modes of the microwaves other than TE₀₁ in the horn; and

creating a microwave plane wave at the entry interface with curable assemblies at the outlet of the horn.

38. The method of claim 37 further comprising, providing a dam at the interface and through which the introduced microwave energy passes into the curable assemblies, and passing fluid through at least one channel in the body of the dam and thereby cooling the dam.

39. A method of microwave curing product, said method comprising the steps of:

dividing microwave energy propagated from a microwave source along at least first and second generally independent paths within a waveguide; and phase matching the microwaves from the first and second paths at the entry interface with microwave-curable product at the outlet of the waveguide.

40. The method of claim 39 further comprising, compressing the curable assemblies while conveying them past the horn outlet, and blocking the compressed curable assemblies at the entry interface from entering the waveguide outlet as they are conveyed therepast.

41. The method of claim 40 wherein the microwave-curable product at the waveguide outlet has a depth greater than 7.5 inches.

42. The method of claim 41 wherein the depth is 11.4 inches.

43. A method of microwave curing compressed product, said method comprising the steps of:

conveying and compressing microwave-curable product; propagating microwave energy through an expanding horn to the compressed microwave-curable product at the outlet of the expanding horn as it is conveyed therepast; and

phase delaying the cylindrical microwave field resulting in the expanding horn generally at the outlet so that the field enters the product in a wave plane.

44. For curing curable assemblies, a microwave assembly comprising:

a microwave waveguide having a microwave inlet, communicable with a microwave source, and a microwave outlet;

a microwave transparent dam secured at said microwave outlet so as to prevent curable assemblies conveyed therepast and pressed thereagainst from entering said waveguide, said dam having a rear surface configured to approximate a cylindrical surface; and

a fin extending rearwardly from said cylindrical dam rear surface.

45. The microwave assembly of claim 44 wherein said rear surface comprises a plurality of straight surfaces defining angles at their junctures.

46. The microwave assembly of claim 45 wherein said straight surfaces include an outer edge surface, a planar center surface and a middle connecting surface directly connecting said edge and center surfaces, such that a first angle is defined between said center and connecting surfaces and a second angle is defined between said center and edge surfaces.

47. The microwave assembly of claim 46 wherein the second angle is twice as large as the first angle.

48. The microwave assembly of claim 46 wherein the second angle is nine degrees and the first angle is four and a half degrees.

49. The microwave assembly of claim 46 wherein the second angle is six degrees and the first angle is three degrees.

50. The microwave assembly of claim 46 wherein the second angle is between five and a half and six and a half degrees and the first angle is between two and a half and three and a half degrees.

51. The microwave assembly of claim 46 wherein said fin extends rearwardly from said center surf ace.

52. The microwave assembly of claim 51 further comprising outer fins extending rearwardly at the juncture of said edge and connecting surfaces.

53. The microwave assembly of claim 52 wherein said outer fins are held in slots in said rear surface.

54. A microwave curing and pressing system for curable assemblies, said system comprising:

a press chamber including a side wall assembly;

press belt means for continuously advancing curable assemblies through said press chamber;

first microwave applicator means for applying, through said side wall assembly, microwaves in a first heating pattern to the curable assemblies in said press : r.amber and as they are advanced therethrough at least in part by said press belt means; and

second microwave applicator means for applying, through said side wall assembly and downstream of said first microwave applicator means, microwaves in a second heating pattern to the curable assemblies in said press chamber and as they are advanced therethrough, the second heating pattern being different than the first heating pattern to accommodate the fact that being downstream the curable assemblies have already been partially heated by the first heating pattern.

55. The system of claim 54 wherein the first heating pattern has average top and bottom surface temperatures at least ten degrees Fahrenheit greater than those of the second heating pattern compared to their respective averages.

56. The system of claim 54 wherein the first heating pattern has average top and bottom surface temperature which are thirty degrees Fahrenheit greater than those of the second heating pattern compared to their respective averages.

57. The system of claim 54 wherein the first heating pattern has average temperatures in generally the middle thirds of the top and bottom surfaces thereof at least ten degrees Fahrenheit greater than those of corresponding locations of the second heating pattern, each compared to their respective averages.

58. The system of claim 57 wherein the first and second heating patterns are spaced approximately eighteen inches apart along the press bed.

59. The system of claim 54 wherein said press belt means defines a nip region and said second microwave applicator means is downstream of the nip region.

60. The system of claim 59 wherein said first microwave applicator means is downstream of the nip region.

61. The system of claim 59 wherein said first microwave applicator means is upstream of the nip region.

62. The system of claim 54 wherein the microwaves from said first and second microwave applicator means have the same frequency and power.

63. The system of claim 62 wherein the frequency is 915 MHz and the power is 25 KW per applicator.

64. The system of claim 54 wherein said side wall assembly includes first and second side walls on opposite sides of said press belt means.

65. The system of claim 64 wherein both said first and second microwave applicator means apply their microwaves through said first side wall.

66. The system of claim 64 wherein said first microwave applicator means applies microwaves through said first side wall and said second microwave applicator means applies microwaves through said second side wall.

67. The system of claim 64 wherein said first microwave applicator means includes a first applicator horn in said first side wall and a second applicator horn in said second second wall, opposed to and aligned with the first applicator horn.

68. The system of claim 54 wherein the second heating pattern has an average temperature in the central region thereof which is. ten degrees greater than that of the first heating pattern with respect to their average temperatures.

69. The system of claim 54 wherein said first and second applicator means include first and second, respective, microwave- transparent dams in said side wall assembly.

70. The system of claim 69 wherein said first microwave applicator means includes a single fin only assembly extending rearwardly from said first dam and dividing all of the microwaves passing through said first dam into a pair of wave paths, and said second microwave applicator means include three spaced fins extending rearwardly from said second dam and dividing all of the microwaves passing through said second dam Into four wave paths into the curable assemblies.

71. The system of claim 69 wherein both said first and second dams have the back surfaces thereof shaped as cylindrical lenses.

72. The system of claim 54 wherein the first heating pattern is more uneven than the second heating pattern.

73. The system of claim 72 wherein the difference in unevenness between the two patterns is greater than ten degrees Fahrenheit.

74. The system of claim 72 wherein the first heating pattern has an unevenness of approximately 20ºF and the second heating pattern has an unevenness of approximately 20°F.

75. A method of pressing and curing curable assemblies, said method comprising the steps of:

continuously conveying curable assemblies into and through a press chamber;

pressing the curable assemblies in the press chamber; using a first applicator, directing microwaves into the curable assemblies at a first area in the press chamber with a first heating pattern;

using a second applicator, directing microwaves into the curable assemblies at a second area, downstream of the first area and, in the press chamber, with a second heating pattern, the second heating pattern being different from the first heating pattern to accommodate for the fact that the first heating pattern did not heat evenly.

76. The method of claim 75 wherein the first heating pattern has greater surface effective temperatures than that of the second heating pattern.

77. The method of claim 75 wherein microwave energy is provided to the first and second applicators from different first and second sources.

78. The method of claim 75 wherein microwave energy is provided to the first and second applicators from a single common source.