Horizontally supported planar surfaces – Industrial platform – Having plastic load-contacting surface
Reexamination Certificate
2001-01-12
2003-12-09
Chen, Jose V. (Department: 3637)
Horizontally supported planar surfaces
Industrial platform
Having plastic load-contacting surface
C428S004000
Reexamination Certificate
active
06659020
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to extrusion and calibrating or sizing methods for formable materials. Also in a particular non-limiting aspect, it relates to products including materials which have been manufactured according to these extrusion and calibration methods.
2. Background of the Invention
There are a number of known methods of calibrating or sizing a thermoplastics melt, where applicable, after it leaves the extrusion die or orifice. Without some form of sizing, the extruded melt would generally deform in a manner which would make the extrudate unacceptable in shape or dimensions, unless the application is undemanding with regard to its dimensional accuracy or the plastics raw material is one which holds its hot shape quite well, for example, flexible polyvinyl chloride (PVC). This property is known as melt strength or hot strength. Details of common methods of sizing or calibration of pipes and profiles are given in a chapter devoted to that subject in the book, Extrusion Dies for Plastics and Rubber by Walter Michaeli, Hanser Publishers Second Edition 1992. As this invention is particularly directed to methods involving the production of calibrated extrusions having one or more ribs which may serve to provide strength, the term profile as used throughout the specification and claims relates to the production of extrusions having one or more ribs. The one or more ribs may take the form of one or more corrugations.
For products that are tubular there are a number of known methods of sizing or calibrating the hot viscous extrudate, the two most common being internal pressure sizing and external vacuum sizing, these being often referred to as simply internal or external sizing or calibration. In the case of internal pressure sizing, air pressure is used to inflate the molten extrudate to force its external surface against a fixed cooled internal tubular surface in order to size or calibrate the extrudate. With vacuum sizing the reverse applies where vacuum is employed to pull the external surface of the molten extrudate to a cooled surface or surfaces and thus to size or calibrate the extrudate. Sometimes there is also direct contact of the hot extrudate with water between, for example, a series of cooled plates with positive air pressure inside the extrudate or vacuum applied externally.
A further known method of sizing an extrudate is that of drawing the molten extrudate through a series of fixed plates which may be water or air cooled and thus assist in the extrudate retaining its desired shape. This method again is only applicable to those plastics which exhibit good melt strength and relatively low shrinkage during cooling, for example, PVC ABS, celluosic materials, to name only some. Such a method is not readily applicable to those materials which have neither high melt viscosity or good melt strength, for example, polyethylene (PE), polyethylene terephthalate (PET) or polypropylene (PP), to name just three common thermoplastics materials. Hence these materials are not frequently employed to manufacture extruded profiles.
Another known method for making somewhat more complex generally essentially tubular shapes from thermoplastics materials employing extrusion sizing, involves the use of a series of generally water cooled female dies mounted on a pair of opposing caterpillar tracks. Michaeli chapter 11.1.6 refers to this process as “movable calibrators”. “If the calibrators move continuously with the extrudate, profiles with a changing cross section (in the direction of the take-up) can be produced.” The matching female dies mounted on caterpillar tracks usually above and below the extrudate, accept the molten extrudate which is usually expanded with internal air pressure to the dies, where it is then cooled to shape. An example of this technique is the production of corrugated drainage pipe. A similar technique has been employed for manufacture of blow moulded containers. This too employs matching female dies generally mounted on caterpillar tracks or other longitudinally movable means, to produce hollow bodies rather like a string of sausages.
Many extruded profiles or sections also have vacuum calibration or sizing applied to them in order to have them conform to the required shape and dimensions. A common example of this is the extrusion of Window frame profiles, generally from rigid (unplasticised) PVC.
All of these methods have the objective of enveloping the lateral outside directions of the extrudate (i.e., the cross sectional shape of the section lateral to the direction of extrusion) and providing contact by pressure or suction of the outside surfaces of the molten extrudate with an appropriately shaped means of cooling, for a time which is sufficient to cool the surfaces of that extrudate to an extent that it will maintain its shape and dimensions to an acceptable degree. In other words to achieve and maintain the desired dimensional accuracy.
In sheet extrusion the melt generally passes through a multi-roll stack which somewhat cools the product and makes its thickness uniform. Further functions of this stack are to impart a surface finish to the extrudate which may be gloss, matte, embossed or otherwise textured. In some instances another sheet or film may be laminated to the extrudate through the roll stack. The method of cooling the sheet after the roll stack is generally air cooling.
It is also known to produce corrugated sheet by the extrusion process from thermoplastics materials such as unplasticised (rigid) PVC, polycarbonate, acrylic etc. Some of these methods involve drawing a hot extruded sheet over rolls or bosses in order to form the corrugations in the longitudinal direction with respect to the direction of extrusion. Another known method involves rolling in the corrugations in the lateral direction with respect to the direction of extrusion. These methods are generally only applicable to plastics materials which have a relatively high melt viscosity and/or good melt strength.
The use of belts to form a plastics product in a dynamic manner is known to those skilled in the art. Potchen et al in U.S. Pat. No. 2,866,730 uses flat metallic continuous belts to form laminated panels employing polyurethane foam. Wood metal or other panels previously produced by other methods are fed into the machine of this invention to form the outer skins of the laminated panels. This avoids the liquid, foaming polyurethane with its adhesive tendencies, coming in contact with the flat belts (not profiled). However, the use of a separate skin formation step with subsequent lamination introduces complexity and cost.
Kornilak in U.S. Pat. No. 3,914084 in a process used to produce profiled foam products from thermosetting materials, uses thin profiled rubber continuous belts. These profiled belts only have the required profile shape when they are travelling along a straight run and are deformed to a more planar configuration during the action of passing around the drive and driven rolls. The thin profiled rubber belts are supported by a series of matching profiled dies mounted on caterpillar tracks. Release agents are used in order to prevent the plastics materials from adhering to the rubber belts.
Kemerer et al in U.S. Pat. No. 4,128,369 disclose a process for making panels from thermoplastics materials having three-dimensional patterns and surface textures. In this process opposing flat, thin, steel belts have cured silicone elastomeric continuous forms adhered to those opposing flat thin steel belts in order to form a complementary pair of continuous or occasionally separated silicone elastomeric moulds. Silicone RTV's in their cured state are known for their release properties with hot thermoplastics materials and indeed are used in the plastics industry as coatings on rolls, etc. which come in contact with hot extrudates in order to avoid adhesion to those rolls, etc. Again the elastomeric moulds must deform in order to pass around the drive and driven rolls, but in a different manner to that of Kornilak,
Chen José V.
Cohen & Grigsby P.C.
Starlon Pty Ltd
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