Static molds – Container-type molding device – Plural article forming mold – or molds with community feature
Reexamination Certificate
2000-02-02
2001-09-18
Nguyen, Nam (Department: 1722)
Static molds
Container-type molding device
Plural article forming mold, or molds with community feature
C249S139000, C249S171000, C425S186000, C425S453000, C425SDIG002
Reexamination Certificate
active
06290202
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the molding of thin sheets of a resin composition and, more particularly, to methods and apparatus for molding thin resin sheets of substantially constant thickness.
BACKGROUND OF THE INVENTION
Countertops for use in laboratories must withstand severe environmental conditions. They must be resistant to strong chemicals such as solvents, acids and corrosive compositions, and they must also withstand severe physical conditions such as impacts and localized heating without breaking or cracking. The countertop must have a smooth, impermeable surface which is easy to clean. To meet these demanding performance requirements, many laboratory countertops have been formed from an epoxy composition. Typically, the countertop comprises a thick slab of cured epoxy resin containing a mineral filler. The slabs are cast in thicknesses of approximately 1 inch to 1½ inches, in lengths of up to 8 feet and in widths of up to 4 feet. Epoxy countertops of this general type have performed quite well under the demanding environmental conditions encountered in laboratories, and have been used extensively. Indeed, this type of countertop is used in most academic and industrial laboratory countertop installations. However, a drawback to this type of countertop is that it is quite heavy. A typical epoxy countertop slab may weigh 10 pounds or more per square foot. Thus, the material cost and shipping expense is significant and the weight also makes handling and installation difficult.
Thinner sheets of an epoxy composition on the order of about ¼ to ⅜ inch thick have been produced for use in less demanding installations, for example as liners for fume hoods, by casting in an open horizontal mold. However, this method is incapable of meeting the exacting dimensional tolerances and flatness requirements of countertop applications. Also, the requirements for impact resistance and heat resistance are less severe in fume hood liner applications than in countertop applications.
Other methods which have been proposed for producing thin resin sheet materials include pressure gelation and compression molding. However, these methods have drawbacks or limitations which make them impractical or uneconomical for countertop applications.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for molding thin sheets of a thermosetting resin composition in relatively large sizes and with substantially constant thickness so as to be suitable for use as upper exposed surfaces of laboratory countertops or the like. In accordance with one broad aspect of the present invention, the thin resin sheet is produced in a closed vertical mold. The method comprises forming a castable liquid composition comprising a thermosetting resin, positioning a pair of generally planar mold plates in a vertical orientation in opposing parallel relationship and narrowly spaced apart from each other to form a narrow vertical mold cavity therebetween, and introducing the castable liquid composition into the mold cavity and allowing the composition to flow by gravity to fill the mold cavity. The castable liquid composition is cured and hardened in the mold cavity to produce a resin sheet and the sheet is then removed from the mold cavity. In a preferred embodiment of the invention, the castable liquid composition is cured and hardened by heating the composition in the mold at an elevated temperature. For example, the heating may be accomplished by placing the mold in an oven at a temperature of at least 250° F.
In a further and more specific aspect, the method of the present invention entails positioning a pair of planar rigid generally vertically oriented walls in opposed relationship narrowly spaced apart from one another to form a narrow generally vertical closed mold cavity, forming a mixture of a liquid epoxy resin, mineral filler and epoxy resin hardener, and filling the mold cavity with the mixture. Curing of the resin mixture is initiated by heating the mixture in the mold cavity, thereby producing a hardened resin panel. Preferably, to facilitate filling of the narrow mold cavity without voids, the mold walls are preheated prior to filling the mold cavity. This reduces the viscosity of the epoxy resin mixture and allows it to more readily fill the mold cavity.
An apparatus for molding thin resin sheets in accordance with the invention comprises a mold including at least first and second generally planar mold plates which are oriented generally vertically and spaced apart to define a generally vertical mold cavity between the plates. More specifically, the mold plates have opposing generally planar surfaces that are parallel to each other and narrowly spaced apart from one another. The mold plates are supported by a mold support, the second mold plate preferably being movably supported so as to be movable into an open position permitting a hardened resin sheet to be removed from the mold. The mold preferably also includes a spacer strip which projects outwardly from the generally planar surface of one of the plates and abuts the generally planar surface of the other plate when the mold is closed. The spacer strip extends along at least lower and side edges of the one of the plates so as to define a substantially closed mold cavity bounded by the spacer strip, while accurately and uniformly holding the mold plates in spaced apart relation.
In one embodiment, the apparatus includes a third mold plate, the first mold plate being disposed between the second and third mold plates. The third mold plate is oriented generally vertically and narrowly spaced from the first mold plate so as to define a second narrow generally vertical mold cavity therebetween. Both the second and third mold plates preferably are pivotally connected to the mold support so as to be movable into open positions. The second mold cavity also includes a spacer strip which bounds the cavity. The spacer strips of the two mold cavities preferably are integrally formed with the second and third mold plates.
For molding thin resin sheets having integral edge flanges, which give the appearance of a solid slab having the thickness of the flange, the apparatus preferably includes a recess in at least one of the generally planar surfaces of the second and third mold plates, the recess extending along at least one of the edges of the respective mold cavity. The recess has a bottom surface which is generally planar and parallel to the opposing generally planar surface of the first mold plate. The recess thus defines an enlarged mold cavity which adjoins the respective narrow mold cavity. When the mold is filled with a curable composition, the enlarged mold cavity is also filled, such that the resulting resin sheet has an integral edge flange.
In accordance with another aspect of the invention, an apparatus for molding thin resin sheets includes a mold having a pair of generally planar mold plates which are oriented generally vertically and parallel to each other and spaced apart to form a generally vertical mold cavity therebetween. The mold includes an opening into the mold cavity along an upper edge thereof for pouring heat-curable composition into the mold cavity. The apparatus further includes a heater for heating the mold after the heat-curable composition has been poured into the mold. The heater is adapted to heat the mold while the mold is in a generally vertical orientation. The heater may be an oven into which the generally vertical mold is placed to effect heating of the mold to cure the composition. Alternatively, the heater may comprise an infrared heater, an electrical resistance heater, or heat transfer fluid circulated through fluid passageways in the mold plates. The apparatus may also include a vacuum mixer adapted to mix the heat-curable composition while under vacuum so as to remove gases from the composition prior to pouring into the mold.
REFERENCES:
patent: 327546 (1885-10-01), Hansen
patent: 415332 (1889-11-01), Payen
patent: 1430763 (1922-10-01), Sivertson
patent: 206
Foote David K.
Rindoks Kurt P.
Alston & Bird LLP
Kewaunee Scientific Corporation
Nguyen Nam
Nguyen Thu Khanh T.
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