Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Forming articles by uniting randomly associated particles
Reexamination Certificate
1999-11-05
2002-04-09
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Forming articles by uniting randomly associated particles
C219S679000, C219S756000, C264S489000, C264S490000, C264S122000, C425S174400
Reexamination Certificate
active
06368544
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the manufacture of three-dimensionally molded particleboard parts.
BACKGROUND OF THE INVENTION
The raw materials used in the manufacture of molded particleboard parts consist of mixtures of lignocellulosic materials combined with heat-curable binders whose rate of cure is accelerated by the application of heat. The mixtures may take the form of loose collections of particles, fiber bundles, flakes, slivers, or shavings. The mixtures may also be compacted to some extent to create molded preforms. The various forms of the raw material for molded particleboard parts will be collectively referred to as the “furnish” throughout this disclosure.
Many different lignocellulosic materials, particle types, and particle sizes are used in the furnish. For example, wood-based furnish can take the form of wood chips, wood-fiber bundles, wood flakes, wood shavings, wood slivers, wood flour, or a variety of wood residues. The most common binders used in the furnish are urea-formaldehyde and phenol-formaldehyde, while isocyanate resins are used only occasionally. These binders are all synthetic heat-curable binders in which curing is accelerated by the application of heat. In most common furnishes, the binder constitutes approximately 2-10% of the furnish, although higher binder percentages are occasionally used.
The furnish is compressed into final shape between matched male and female metal dies that are pressed together and simultaneously heated in a conventional heated press. The female die is known as the cavity and the matched male die is known as the punch. In operation, the shape of the facing surfaces of the punch and the cavity impart three-dimensional features to the finished particleboard parts. Molded particleboard parts are to be distinguished from standard flat-panel composition boards that could be characterized as being only two-dimensional. The present invention does not pertain to two-dimensional flat-panel composition boards, such as these, since these products are not normally molded using cooperating metal dies.
A simple particleboard molding process is used to produce shallow draws on embossed products, such as door skins, cabinet fronts, and embossed wall panels. More sophisticated methods have been developed to mold parts having deep draws, such as cores for upholstered furniture, tabletops with profiled edges, irregular boxes, curved drawer fronts, beverage cases, and toilet seats. Many of the initial steps in producing molded particleboard parts are similar to the initial steps for producing standard flat composition boards. For example, furnishes are prepared in very similar ways. Important differences occur primarily at the forming station, where the furnish is consolidated into its finished form.
All of the current processes for making molded particleboard products require very long furnish dwell times within the heated press, due to the low heat transfer rates from the heated molds to the furnish being compressed. Furnish dwell times vary between approximately one minute to more than 10 minutes, depending upon the part thickness. Since profitability of a particleboard molding operation is closely related to product throughput, these long furnish dwell times within the heated press limit production rates and continue to be a major economic concern to the industry.
At first glance, it may seem possible to remedy this situation and decrease furnish dwell times within the heated press by further heating the fanish with radiowave energy as the furnish is being consolidated in the press. This process is known as in-press radiowave heating. The term, radiowave, by standard definition, is an electromagnetic wave having a frequency between 10 kilohertz up to about 30 gigahertz. Using supplementary radiowave heating, decreased furnish dwell times within the heated press would be expected because radiowaves are known to propagate throughout the volume of dielectric materials, thereby producing rapid volumetric heating in radiowave-absorptive dielectrics, such as wood/binder compositions.
In practice, for most molded particleboard sizes and shapes, radiowaves applied within the forming mold during the hot-pressing operation do not establish uniform electric field distributions throughout the volume of the furnish contained within the mold. This is because only a relatively small number of microwave modes can be established within the restrictive confines of the mold interior. In addition, because the thickness of the molding space between the punch and the cavity bottom is usually much less than a wavelength at commonly used microwave frequencies, the microwave electric fields are polarized primarily in only a single direction roughly normal to the base of the part.
With few microwave modes and only a single field polarization, strong electric field concentrations form at various standing-wave maxima, and microwave fields are attenuated within intricate mold features. High microwave field levels will also be produced near the point where microwaves are fed into the mold interior. Since the parts are held in a stationary position within the mold cavity, these field variations lead to strong non-uniform heating of the compacted furnish and non-uniform curing. In addition to producing non-uniform heating and curing of the furnish, application of microwaves to the mold interior during hot-pressing would require complex new mold designs and replacement of existing molds, making retrofitting of microwave-heated mold cavities to existing molding equipment impractical and uneconomical.
Radiowave frequencies below the microwave range have also been considered for in-press radiowave heating. For the most common industrial frequency bands in this range, wavelengths are much greater than mold dimensions. Radiowave system construction in this frequency range differs considerably from construction in the microwave range. Compared to microwave heating, these relatively low frequencies produce much less heating power within the furnish for a given electric field level, since the power density scales primarily with frequency. To match microwave power densities, much higher electric fields are required at lower frequencies, which can lead to more frequent arcing.
Non-uniform in-press radiowave heating can also occur in this low frequency range just as it does for in-press radiowave heating in the microwave range. For low frequency radiowave in-press heating, non-uniform heating can occur because of fringing fields, field concentrations at sharp mold features, and variations in field levels due to variations in the thickness of the parts. Because the use of low frequency radiowaves for in-press heating also requires replacement of existing molds, the same problems with retrofitting existing particleboard molding operations exist, as were described for in-press microwave heating schemes.
As a consequence of the problems that occur when radiowaves are applied within the mold cavity as the mold is hot-pressed, another solution to the problem of long hot-press dwell times and long production cycles for molded particleboard parts is needed.
SUMMARY OF THE INVENTION
The essence of the invention is a heretofore unsuggested use of radiowave energy to speed production of molded particleboard parts by preheating the furnish prior to the application of heat and pressure within the forming mold during the hot-pressing operation. Because radiowaves propagate readily throughout the volume of the furnish, rapid heating is obtainable even for a loose, uncompacted particulate furnish that has very poor thermal conductivity. By utilizing a radiowave preheating scheme, rather than applying radiowaves simultaneously with hot pressing, relatively simple yet highly effective radiowave applicators may be devised. In many cases, standard applicators disclosed in the prior art may be used.
An important precept of the invention is that the temperature of the preheated furnish is kept low enough that the dry-out point of the binder is not reached, and it is still possibl
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