Plastic and nonmetallic article shaping or treating: processes – Vacuum treatment of work
Patent
1996-06-26
2000-10-24
Silbaugh, Jan H.
Plastic and nonmetallic article shaping or treating: processes
Vacuum treatment of work
264112, 264115, 264120, 264128, B29C 4322
Patent
active
061362395
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the manufacture of board products from lignocellulosic material.
BACKGROUND OF THE INVENTION
Methods of manufacturing board from raw materials which are based on lignocellulose are well-known and widely applied. These manufacturing processes generally comprise the following steps: disintegration of the raw material to particles and/or fibers of a suitable size, drying to a definite moisture ratio and gluing the material prior or, subsequent to the drying, forming the glued material into the form of a mat, which can be built up of several layers, possibly cold prepressing, preheating, surface nozzle-spraying in some instances, and hot pressing simultaneously with pressure and heat applied in either a discontinuous or a continuous press, to produce a finished board.
During conventional hot pressing, the pressed material is primarily heated by means of thermal conduction from the adjacent heating plates or steel belts which have a temperature of 150.degree. to 250.degree. C., depending on the type of product being pressed, the glue used, the desired capacity, and other such factors. The moisture of the material which is closest to the heat source is thus evaporated, whereby as the pressing continues a dry layer develops and a steam front successively moves from each side inward to the center of the board. The temperature in this developing layer rises to at least 100.degree. C., which causes normal glues to cure. When the steam front has arrived at the center of the board, the temperature there has risen to at least 100.degree. C. and the board begins to harden, even at the center at which point pressing can be terminated within a number of seconds. This applies to the use of conventional urea formaldehyde glue (UF) and similar glues, such as melamine-fortified glues (MUF). When other glues with higher curing temperatures are used, a higher temperature and a higher pressure must develop in the board before curing can take place. In connection with conventional hot pressing, methods have been developed to control the density profile of the board in the thickness direction. In most cases it is desired to achieve a high density in the surface layers in order to improve paintability, strength and the like, and a reasonably low density in the central layer, i.e., as low as possible for holding the board weight and cost down, but sufficiently high for achieving an acceptable internal bond strength and the like. During the manufacture of particle board, more finely disintegrated particles with a slightly higher moisture content in the surface layers often have been used, for example, in order to achieve a higher density in the surface layers of the board. During the manufacture of MDF (Medium Density Fiberboard), which have a homogeneous material structure, methods have been developed by utilization of a controlled distance between the heat sources in order to approach the final position successively in a predetermined manner as the steam front moves inward towards the center. See, for example, Swedish patent No. 469,270 for a continuous press, and Swedish pat. appln, No. 93 00772-2 for a single opening discontinuous press. These methods, which were developed for MDF, are now at least partly used for other types of board.
In order to achieve the desired density profile, a press must be capable of applying a high surface pressure at high temperatures. This in itself is no problem for a discontinuous press. However, such a press has other disadvantages, such as, for example, inferior thickness tolerances. For continuous presses the required high surface pressure and simultaneously high temperature have resulted in the need for expensive precision solutions for the roller table between the steel belt and the underlying heating plate. The method of supplying heat to the board by means of thermal conduction further entails heating which takes a relatively long time, which thus results in great press lengths (large press surfaces). Presses up to about 40 m in len
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Lundgren Goran
Schedin Kurt
Sislegard Lars-Otto
Thorbjornsson Sven-Ingvar
Jones Kenneth M.
Silbaugh Jan H.
Valmet Fibertech Aktiebolag
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