Patent Application
20120052236
This invention relates to methods for producing structural foam and to products containing structural foam.
The term “structural foam” is used to denote molded parts having a cellular interior structure. Structural foam can be made from such materials as polyethylene and polypropylene. Structural foam products have a low density, cellular interior structure sandwiched between two superficial high density layers. The cellular structure is produced during injection molding by generation of gas bubbles within the polymer melt when the melt is exposed to heat during the plasticizing process. The gas bubbles may be generated either chemically or mechanically. In chemically blown foam, a powdered blowing agent is mixed into the polymer melt together with an adhesive oil. Mechanically blown foam is produced by introducing pressurized nitrogen or carbon dioxide directly into the melt during the plasticizing process.
In comparison to a solid panel made from the same material, a structural foam panel has a significantly lower density but is only slightly weaker than the solid panel of the same thickness. Thus, a structural foam panel has to be slightly thicker than the solid panel of comparable strength. Nonetheless, a structural foam panel will still have a significantly lower weight than a solid panel of comparable strength made from the same material. Use of structural foam panels thus allows a significant savings in the polymer material while a structure, such as a cabinet, having structural foam panels is significantly lighter than a cabinet made from solid panels of the same material. It is known to use structural foam panels in cabinets and similar structures having a density of about 0.9 gr/cm3. Structural foam panels of lower density have not found use in large structures such as cabinets, cupboards and shed due their tendency to buckle under the weight of the structure and any contents inside the structure.
In its first aspect, the present invention provides a method for producing structural foam. In accordance with this aspect of the invention, plasticizing granules are heated causing plasticizing of the plastics. A gas or a powdered blowing agent is introduced into the melt which is then injected into a mold. Initially, the mold is closed to an initial volume V1 as a volume V2 of the melt is injected into the mold, where V2<V1. The mold is then compressed from the volume V1 to the volume V2. As the volume of the mold is decreased to the volume of the melt in the mold, the melt is pressed to completely fill the mold. The mold is then expanded from the volume V2 to a final volume V3. As the volume of the mold increases, the pressure in the mold decreases, and the melt expands by the formation of gas bubbles inside the melt so that the melt completely fills the expanded mold. The already solidified surface layers of the melt cannot foam up, but the liquid melt of the core section does foam up and expands. As the core expands, it pushes the surface layers apart causing the melt to fill the previously unoccupied mold volume until both surface layers are pressed against the mold walls. A structure is generated in the molded part 18 in which in the surface skin layers the plastic are dense, while in the interior of the molded part, a microcellular foam core is formed.
In accordance with the invention, the volumes V1, V2, and V3 are selected to yield a structural foam product having a density in the range of 0.4-0.75 gr/cm3. For example, the inventors have found that, in order to produce a sheet having a specific gravity in the range of 0.4-0.75 gr/cm3 the following procedure may be used:
Initial thickness of the mold during injection of the melt (corresponding to the volume V1): 1.8-4 mm.
Second thickness of the mold to press the melt and to completely fill the mold (corresponding to the volume V2): 1-2 mm.
Final thickness of the mold (corresponding to the volume V3): 1.5-6 mm.
In its second aspect, the invention provides a panel. The panel of the invention comprises a sheet of structural foam. In accordance with the invention, the sheet of structural foam has a density in the range of 0.4-0.75 gr/cm3. The sheet may be made from any material known to be used in the manufacture of structured foam such as polyethylene and polypropylene. The sheet of the panel is surrounded by a frame. The inventors have found that a panel of the invention can be constructed having a strength comparable to that of prior art structural foam panels of higher density. The low density of the panel allows a significant savings in the amount of polymer in the panel, and hence allows a significant reduction in the weight of and manufacturing cost of a structure comprising the panels.
In its third aspect, the present invention provides a structure comprising a plurality of panels of the invention the structure may be, for example, a cabinet, cupboard, shed, or a piece of furniture.
Thus, in its first aspect, the present invention provides a method for generating structural foam having a density in the range of 0.4-0.75 gr/cm3 comprising:
(a) plasticizing plastics granules to form a polymer melt;
(b) generating a gas in the melt under conditions inhibiting bubble formation in the melt;
(c) closing a mold to an initial volume V1;
(d) injecting a volume V2 of the melt into the mold, where V2<V1;
(e) compressing the mold from the volume V1 to the volume V2; and
In its second aspect; the invention provides a sheet of structural foam having a density in the range of 0.4-0.75 gr/cm3 produced by the method of the invention.
In it third aspect, the invention provides a panel comprising a sheet of structural foam of the invention.
In its fourth aspect, the invention provides a structure comprising two or more panels of the invention.
In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:
In the second phase, (
In the third phase, shown in
In the fourth phase (
In the fifth phase, (
In accordance with the invention, the volumes V1, V2, and V3 are selected to yield a structural foam product having a density in the range of 0.4-0.75 gr/cm3. For example, the inventors have found that, in order to produce a panel having a specific gravity in the range of 0.4-0.75 gr/cm3 the following procedure may be used:
Initial thickness of the mold during injection of the melt (corresponding to the volume V1): 1.8-4 mm.
Second thickness of the mold to press the melt and to completely fill the mold (corresponding to the volume V2): 1-2 mm.
Final thickness of the mold (corresponding to the volume V3): 1.5-6 mm.
b show two perspective views of a panel 30 in accordance with one embodiment of the invention. The panel 30 may be used, for example, in a structure such as a cabinet, cupboard or shed. The panel 30 comprises a sheet 32 of structural foam. In accordance with the invention, the sheet of structural foam 32 has a density in the range of 0.4-0.75 gr/cm3. The sheet 32 may be made from any material known to be used in the manufacture of structured foam such as polyethylene and polypropylene.
In the panel 30, the sheet 32 is surrounded by a frame comprising a first lateral support 34 and a second lateral support 36, a top support 38 and a bottom beam 40. At the bottom of the first and second lateral supports 34 and 36 are tenons 42 and 44, respectively. At the top of the first and second lateral supports 34 and 36 are mortices 46 and 48 that are shaped to snugly receive the tenons 42 and 44, respectively, of an identical panel when two or more panels 30 are to be joined together into a larger structure, as explained below.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL10/00352 | 5/4/2010 | WO | 00 | 11/3/2011 |
