Rotational molding process

Details Rotational molding process Rotational molding process

1998-07-14

2001-04-10

Ortiz, Angela (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

To produce composite, plural part or multilayered article

C264S308000, C264S309000, C264S310000

Reexamination Certificate

active

06214272

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to rotational molding methods and particularly to a rotational molding which create a shiny and hard surface layer for a molded part.
2. Description of Related Art
Rotational molding, which is often referred to as “rotomolding”, is a process where a powdered thermoplastic is poured into a mold and the mold is suspended in an oven. The material utilized for the powdered thermoplastic is often polyethylene, while the mold is ordinarily constructed from metal. Once suspended in the oven, the metal mold is heated beyond the melting point of the powder and rotated to evenly coat the inner surface of the mold with the melted plastic. Once properly heated, the mold is then cooled and the part is demolded.
The mold serves two purposes: (1) to transfer heat to the resin to establish the shape to be formed and (2) to provide a cavity in which to cool the material. Typically, a pre-measured resin is loaded into the mold and the mold is disposed within an oven and rotated on both its vertical and horizontal axis. As the resin melts it sticks to the inner surface of the mold, coating such surface evenly. Usually, the rotation of the mold continues during the cooling cycle to allow the intended part to maintain an even wall thickness. Once the part is cooled, it is removed from the mold.
Rotomolding has been utilized, amongst other items, for the manufacture of boats. Some advantages to rotomolding, as compared to other molding techniques is the relatively low cost for molds, ease in adaptation for short production runs, the elimination of secondary tooling and the minimal amount of scrap which is produced from the process. Rotomolding also provides for consistent wall thickness and sharp outside corners which are essentially stress free.
Rotomolding has also been traditionally limited to use with a single material. Thus, one specific problem with rotomolded products is that the surface of the product is soft.
When creating a structure comprise of at least two different materials, bonding of the materials is typically an issue, particularly with materials having disparate properties. In the past, rotomolding processes have not been used for marrying materials having separate properties.
It is therefore to the effective resolution of the aforementioned problem that the present invention is directed.
BRIEF SUMMARY OF THE INVENTION
The present invention discloses a process for creating an improved rotomolded products. In a first method the inner mold area is sprayed with a material, such as an acrylic polyethylene alloy, prior to loading the resin into the mold. Once the resin is loaded, the mold is then heated, rotated and cooled to form the molded part. Once cooled the part is then removed from the mold.
In a second method, in lieu of spraying, the acrylic polyethylene alloy is loaded into the mold with the resin. Once loaded, the mold is then heated, rotated and cooled. Once cooled the part is then removed from the mold.
In a third method, a mold is opened and a first resin which is to become the outer surface of the part is added. Once the first resin is added the mold is closed, heated and rotated. Once first resin is sufficiently melted, a “drop” box is opened within the mold to introduce a second resin into the mold. The second resin will become the body or structure of the molded part. After the second resin is introduced by the drop box, the mold is again heated and rotated as described above. As heating begins, the second resin material begins to melt and sticks to the inner surface of the mold over the first resin. Thus, when the mold is cooled, the first resin will form the surface layer of the part. Once cooled the mold is opened and the part is removed.
In a fourth method, a mold is opened and a first resin which is to become the outer surface of the part is added. In this embodiment, the first resin is not an alloy but rather preferably consist of a separate material such as acrylic, polycarbonate, styrenic or other hardened plastic which will create a hardened outer shell. Once the first resin is added the mold is closed, heated and rotated. The mold is heated to a temperature which causes the first resin to melt.
Once the first resin is sufficiently melted, a first drop box is opened within the mold to introduce a second resin which acts as a tie coat or bond layer. The tie coat bonds to the first resin. After the second tie coat resin is released into the mold, the mold is heated and rotated to evenly bond to the first resin which is provided over the inner surface of the mold.
Once the tie coat is properly melted, a second drop box is opened within the mold and a third resin is released. The third resin will become the body or structure of the molded part. After the third resin is released into the mold, the mold is once again heated and rotated, allowing the third resin to become melted and evenly distributed over the inside of the mold. This third resin bonds to the tie coat layer. The previously released tie coat acts as a bridge or intermediary allowing the main body resin to bond to the first layer, thereby bonding two materials which might not normally bond. Thus, when the mold is cooled, the first resin will form the surface layer of the part, the second layer the tie coat and the third layer the main body or substrate of the part. Once cooled the mold is opened and the part is removed.
With all of the methods, a rotomolded product can be constructed having an outer surface with different characteristics and properties as compared to its inner surface or substrate. The methods provided for stronger bonds between the materials as compared to conventional methods, such as lamination. Thus, in one embodiment the molded part can be provided with a shinier and harder surface layer as compared to previous rotational molding methods. Alternatively, the present invention allows for a softer surface layer with a harder substrate layer.
Accordingly, it is an object of the present invention to provide a rotational molding process which yields a harder outer surface of a molded part as compared to conventional rotational molding processes.
It is another object of the present invention to provide a rotational molding process which yields a stronger bond between the surface layer and the substrate of a molded part as compared to conventional rotational molding processes.
In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.


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Database WPI, Section Ch. Week 198623, Jun. 1986 (Jun. 1986)—NIppon Petrochemicals Co. Ltd, Apr. 28, 1986 abstract.
International Search Report—PCT/us99/15889.

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