Plastic and nonmetallic article shaping or treating: processes – Reactive gas or vapor treatment of work – Work is organic material
Reexamination Certificate
1998-06-23
2001-02-13
Theisen, Mary Lynn (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Reactive gas or vapor treatment of work
Work is organic material
C264S102000, C264S109000
Reexamination Certificate
active
06187234
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to methods for making a composite board, such as, particleboard, fiberboard, chip board or the like, and more particularly to a method for making composite board having at least one finished surface, e.g. embossed, smooth or patterned. The board is made from a mat composed of wood particles, chips and/or fibers and a curable or hardenable binder, for example, a resin.
BACKGROUND OF THE INVENTION
Composite wood products, such as board may be formed by consolidating a loose mat of lignocellulosic materials under heat and pressure, until the materials adhere together to form a solid wood-like product. The lignocellulosic materials may take the form of wood materials, such as, particles, chips, fibers and/or the like and it will be understood that these terms are used interchangeably herein. Although it is possible to bind lignocellulosic materials under suitable heat and consolidation conditions without additional treatments, typically, the materials forming the mat are treated with a binder, such as a resin, before heat and consolidation are applied, to enhance adherence of the materials and improve the resulting properties of the finished product.
Consolidation of the mat is generally conducted in a press. A conventional press for consolidating a binder treated wood composite mat to a particular molded shape, such as, for example, a board, includes two opposing press platens spaced to define a mold cavity. Typically, at least one platen is heated through conduction, such as through the use of electric heating coils or by passing a heated fluid or gas medium, such as steam, through conduits located in the platen body. Upon contact with the mat, heat is transferred from the platen to the mat by conduction. The press platens used in a conventional press, i.e., conventional press platens, generally have a surface for contacting the mat which is free of openings or ports. Such openings in the contact surface of a platen would cause imperfections in the surface of a finished product. Thus, conventional platens are suitable for pressing boards having a “finished” surface, e.g., a surface which does not require further mechanical working or machining in post-press operations such as sanding or planing to arrive at a market ready surface. Because post-press finishing operations such as sanding or planing are not necessary for boards produced in a conventional press, the conventional press platen may be adapted to provide a smooth “finished” surface, or an embossed or patterned “finished” surface. Subsequent to removal from the press, the board may be sold as is, or the “finished” surface of the board may be treated with a protective and/or decorative coating, such as paint or stain, to yield an enhanced market ready product.
Presses using conventional press platens have several drawbacks. Presses using conventional press platens may be unsuitable for curing certain high temperature curing resins because heat transfer from a conventional platen to the inside of a mat may be slow, thus causing temperature differentials across the thickness of the mat that are unsuitable for proper curing. For example, materials near the surface of the mat may be exposed to excessive heat, causing resin to cure too quickly and composite materials to burn, thus negatively effecting such properties as appearance and bond strength. Conversely, the inside of the mat may be exposed to insufficient heat, such that the composite material does not sufficiently consolidate and the resin does not fully cure, thus weakening the internal board strength. For the foregoing reasons, i.e., heating differentials across the thickness of a mat during consolidation and/or curing resulting in negative effects on board properties, conventional press platens are unsuitable for curing relatively thick board products.
Also, although conventional presses have been successful in making fiberboard products using only conduction heat (hot pressing), today's manufacturing demands require faster cycle times on the press and the use of stronger high-temperature resins to produce highly detailed, higher density, and, at times, thicker fiberboard products. It is known that the disadvantages of conventional platens can be overcome by supplying, or injecting, steam directly into a mat through modified press platens provided with steam injection ports for that purpose. This is generally known as “steam injection” pressing. The steam passes from the injection ports into interstitial spaces between the wood particles, chips and/or fibers forming the mat, thus carrying heat quickly and uniformly to the inside of the mat. Steam injection pressing has several advantages. Steam injection pressing speeds the curing of typically dimensioned boards using conventional resins, thus significantly shortening press cycles. Steam injection pressing also permits the use of high temperature curing resins, which are not typically suitable for use in conventional pressing, and which may be cheaper, safer and/or result in a stronger bonded product. And steam injection permits consolidation and curing of relatively thick composite boards, which either do not properly cure in a conventional press or do not cure quickly enough to provide a cost competitive product. Thus, steam injection is known to speed curing of composite product, improve product quality and shorten production time for wood composite products, particularly products having thick dimensions.
The benefits and advantages of steam injection can be significantly enhanced by conducting the injection in a sealed press, i.e., a press that isolates the press cavity from the surrounding atmosphere. This can be accomplished by sealing the perimeter of the cavity. Alternatively, the entire press can be isolated in a sealed chamber. A sealed press significantly reduces or eliminates the loss of valuable steam and facilitates the injection of steam into the mat at elevated temperatures.
Steam injection pressing is generally considered unsuitable for producing a “finished” surface on board products because, as noted above, ports in a press platen typically cause imperfections in the surface of the molded product. Surface imperfections must be machined or mechanically removed, by, for example, sanding or planing, in post-pressing manufacturing steps, thus adding to the cost and complexity of manufacture. In addition to steam injection ports, steam injection platens may have channels in the mat contact surface to direct the flow of injected steam to various parts of the mat.
In a process referred to herein as “single-sided” steam injection, a mat is pressed between a single steam injection platen (a platen having steam injection ports) and a conventional platen free of steam injection ports. Steam injected through the single steam injection platen speeds curing of the mat and shortens press cycles. The conventional platen in a single sided steam injection process avoids undesirable imperfections in one surface of the molded product that would typically result from the steam injection ports of a steam injection platen. However, in single-sided steam injection, intersticial air in the uncured mat is pushed by a steam front moving from the steam injection platen towards the conventional platen free of steam injection ports. Simultaneously, a second steam front from moisture converted to steam by the conduction heat of the conventional platen may push trapped air towards the steam injection platen. Thus, the air is trapped in the core of the mat, generally closer to the conventional platen than the steam injection platen because the steam is injected under pressure. The air is unable to vent or escape through the conventional platen, which has no ports, and is trapped between the injection steam front and the conventional platen, or between the injection steam front and the second steam front. If the process is conducted in a sealed press, the problem is compounded by the inability of the trapped air to escape through the edges of the mat. The trappe
Bonomo Brian
Merrell Michelle
Seifert Kelly
Vergara Alex
Walsh Pete
Darby & Darby
Masonite Corporation
Theisen Mary Lynn
LandOfFree
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