Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Composite article making
Reexamination Certificate
2001-08-20
2003-09-16
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Composite article making
C264S257000, C264S258000, C264S102000
Reexamination Certificate
active
06620357
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for producing a product comprising plastic foam in an automatic molding machine.
2. Background Information
Beads are foam particles of small dimensions. Various plastics can be employed in this context, e.g. polystyrene (PS), polyethylene (PE) or polypropylene (PP). The following area of application is envisaged in European Patent No. EP 0664313 of Sep. 9, 1998, for example: polypropylene as well as ethylene and propylene copolymers, comonomers, e.g. alpha alkenes such as propylene, butene, pentene, hexene, octene; furthermore vinyl esters such as vinyl acetate, acrylic acid esters, methacrylic acid esters, maleic acid esters or fumaric acid esters of alcohols.
The beads can have a spherical or some other shape, e.g. a cylindrical shape. The shapes of the beads are determined in part by the way in which they are produced. Irrespective of their shape, the bead diameter is generally used to define the size of the beads. Diameters of 0.5 to 6 millimeters are customary. This does not exclude smaller or larger beads.
A fundamental distinction is drawn, in producing beads, between production in an autoclave and production by extrusion. After production, the beads are held in silos or in some other suitable way before being used.
The beads are joined together in a mold, referred to as an automatic molding machine. This is accomplished by heating the beads at the surfaces until there is a greater or lesser degree of softening, and by compression. Given sufficient melting of the surface and pressure, the beads weld together. Adequate bonding/sintering may occur with less melting and a lower pressure.
Heating not only causes melting of the surface but also expansion. The expansion depends on the type and amount of gas trapped in the beads. The expansion contributes to the build-up of pressure.
Otherwise, the pressure is normally built up by virtue of the fact that the beads are fed into the mold cavity of the automatic molding machine under pressure and that the cavity is completely filled. Even if the mold is ventilated after filling, the beads introduced remain under pressure because the beads compressed by the filling pressure cannot expand in the mold cavity.
A gaseous medium, in particular conveying air, is suitable for feeding in the beads. Conveying air is available in unlimited supply. One blower is sufficient for producing conveying air. The blower draws in ambient air and forces this air through a pipe into the mold cavity of the automatic molding machine. On the way to the mold cavity, the conveying air draws the desired quantity of beads out of a silo, preferably from the bottom.
In the mold cavity, the beads are separated from the conveying air. While the conveying air is supposed to flow into the mold cavity and then back out of the mold, the beads are supposed to remain in the mold cavity. The beads are retained in the automatic molding machine because the exhaust-air opening is of appropriately narrow configuration. That is to say, the diameter of the exhaust-air opening is smaller than the diameter of the beads. In order, at the same time, to allow a relatively large conveying air rate, it is customary to provide a plurality of exhaust-air openings. In practice, it is customary to distribute a multiplicity of openings over the inflow area of the automatic molding machine. EP 0664313 describes an automatic molding machine of this kind. Here, a perforated mold is referred to. That is to say there are a sufficient number of openings to allow the use of the term “perforated surface”.
Once sufficient filling has been achieved, the feed opening of the mold is closed. Superheated steam is then blown through corresponding nozzles into the mold cavity. The superheated steam flows between the beads to the opposite side of the mold cavity and flows out there, like the conveying air before it. During this process, the superheated steam displaces the air which remains between the beads and heats up the surface of the beads. The steam pressure is chosen so that it will overcome the flow resistance of the beads in the mold cavity. The temperature of the steam is furthermore chosen so that within a steam application time (generally up to five minutes) the beads are heated and melt at the surface to an extent such that the bead pressure leads to welding of the beads at the contact surfaces. The necessary pressure is brought about by the pressure filling of the mold cavity and/or by the pressure of the superheated steam and/or by the heating and associated expansion of the beads. The surface temperature of the beads should exceed the so-called glass transition temperature but should remain below the melting temperature of the plastic.
During the process, the openings in the mold are generally replicated on the surface of the moldings produced.
EP 0664313 proposes a concept by means of which replication of the openings is avoided. In this case, a woven wire fabric or a perforated plate is placed against at least one mold wall on the inside, its thickness being 0.2 to 5, preferably 0.5 to 2, millimeters. Metal, such as aluminium or stainless steel, or temperature-stable plastic is to be used as the material for the wire or plate. The woven wire fabric or perforated plate are to be laid loosely in the mold, pressed in, fastened by means of clips or screws or spot-brazed or-spot-welded in. As is customary, the walls of the mold can be provided uniformly with perforations, allowing the superheated steam to spray the interior of the mold uniformly. Since a certain clearance is produced between the surface of the molding and the inner wall of the mold by the inserts, the nozzles cannot be copied on the surface of the molding. On the other hand, the particles resting on the surface are pressed partially into the holes in the fabric or plate, with the result that their structure is impressed upon the surface of the molding. The result is that, instead of receiving irregular impressions, it has essentially a completely regular structure imparted to it.
Structured surfaces are not always advantageous. In the food industry, for example, smooth, easily cleaned surfaces are required for reasons of hygiene. The moldings and automatic molding machines known from EP 0664313 cannot achieve this.
OBJECT OF THE INVENTION
The object on which the invention is based is to improve the surface finish. In this context, the invention sees a conceptual starting-point in the woven fabric lining of the mold or automatic molding machine of the type disclosed by EP 0664313 and also known from earlier publications, even though an unwanted surface structure is produced there.
SUMMARY OF THE INVENTION
The invention teaches that this object can be accomplished by a method for producing a product comprising plastic foam in an automatic molding machine with a mold cavity in conformity with the configuration of the product, said method comprising the steps of: providing a woven fabric on at least one surface of said mold cavity, said woven fabric comprising a fabric having a plurality of plies, with a fine-mesh woven fabric ply that faces the interior of said mold cavity upon assembly and a coarse-mesh woven fabric ply that is disposed adjacent said at least one surface of said mold cavity and beneath said fine-mesh woven ply upon assembly to support said fine-mesh woven ply; introducing beads comprising plastic foam into said mold cavity; contacting said beads with a gaseous heat transfer medium, such as superheated steam; said step of contacting comprising: introducing said gaseous heat transfer medium into the mold cavity; flowing said gaseous heat transfer medium along a first surface of said mold cavity; and removing said gaseous heat transfer medium along a second surface of said mold cavity; and removing the product from the mold.
According to the invention, in accordance with one aspect, the desired smooth surface structure is achieved with the aid of a lining comprising a plurality of interconnected woven fabric plies, the foam s
Bruning Jürgen
Lang Eberhard
Gefinex Polymerschaume GmbH
Kuhns Allan R.
Nils H. Ljungman & Associates
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