Molded table and its method of manufacture
View Patent ↗A plastic table structure includes a tabletop comprising a thin polymer shell layer surrounding an expanded foam core. The multiple layers of the finished tabletop are molded concurrently in a one-pass biaxial or centrifugal rotational molding process, and cured in an oven. The method includes the steps of introducing polymeric material into a mold, and rotating the mold while heating it to a first temperature so as to form a molten shell of a portion of the polymeric material on the inside of the mold. A foaming agent is introduced within the mold and mixes with a second unmelted portion of the polymeric material and distributes throughout the mold. When the mold reaches a second higher decomposition temperature, the foaming agent causes expansion of the second portion of the polymeric material, forming an expanded foam core inside the shell.
1. A method for making a molded article; comprising the steps of:
a) introducing polymeric material without a foaming agent into a mold having an inside surface;
b) rotating the mold while heating the material to a first temperature, so as to form a molten shell with molten polymeric material from a portion of the polymeric material on the inside surface of the mold with an unmelted portion remaining inside the molten shell;
c) subsequently causing a foaming agent alone to disperse within the mold and come into contact with the molten polymeric material after the polymeric material has formed the molten shell and mix with the remaining unmelted portion; and
d) rotating the mold while heating the material to a second higher temperature, so as to allow the foaming agent to cause expansion of the remaining unmelted portion of the polymeric material, so as to form an expanded foam core within an interior region of the shell.
2. A method in accordance with claim 1 , wherein the step of causing a foaming agent to disperse within the mold further comprises introducing the foaming agent into an interior of the mold from an external container.
3. A method in accordance with claim 2 , wherein the external container includes an opening in communication with the interior of the mold, the opening being covered by a barrier configured to predictably fluidize to allow dispersion of the foaming agent.
4. A method in accordance with claim 3 , wherein the barrier comprises a meltable barrier of polymeric material configured to melt at a known temperature.
5. A method in accordance with claim 4 , wherein the meltable barrier comprises a polymer material of the same species as the polymeric material of the shell, such that the material of the melted barrier substantially mixes with and becomes an integral and substantially indistinguishable part of the molded article.
6. A method in accordance with claim 2 , wherein the external container is a drop box attached to an exterior of the mold.
7. A method in accordance with claim 1 , wherein the step of causing a foaming agent to disperse within the mold comprises placing a meltable container of foaming agent into the mold prior to rotating the mold and heating it, the meltable container being configured to melt and release the foaming agent within the mold at a predictable temperature.
8. A method in accordance with claim 7 , wherein the step of placing a meltable container of foaming agent into the mold comprises placing a plurality of meltable containers of foaming agent into the mold prior to rotating the mold and heating the polymeric material.
9. A method in accordance with claim 8 , wherein the plurality of meltable containers of foaming agent are of a standard size, such that a discrete number of meltable containers are selected for the mold depending upon a size of the mold.
10. A method in accordance with claim 7 , wherein the meltable container comprises a polymer material of the same species as the polymeric material of the shell, such that the material of the melted container substantially mixes with and becomes an integral and substantially indistinguishable part of the molded article.
11. A method in accordance with claim 1 , further comprising the step of allowing the mold to cool while continuing to rotate it.
12. A method in accordance with claim 1 , further comprising the step of placing a reinforcing member in the mold prior to steps (b) (c) and (d), so as to allow the reinforcing member to become substantially encased within the foam core.
13. A method in accordance with claim 12 , wherein the step of placing a reinforcing member into the mold comprises placing a reinforcing member having a neutral axis substantially along a shrink-neutral axis of the molded article, so as to resist shrinkage-related deformation of the molded article.
14. A method in accordance with claim 13 , further comprising the step of placing attachment point devices in the mold prior to steps (b) (c) and (d), so as to provide attachment points for connecting external structure to the molded article and causing the attachment point devices to become substantially encased within the foam core.
15. A method in accordance with claim 12 , wherein the reinforcing member is configured to resist adhesion with the foam core material.
16. A method in accordance with claim 1 , wherein the article comprises a table top, the mold defining an interior cavity having a shape of a table top.
17. A method in accordance with claim 16 , further comprising the step of placing a structural frame in the mold prior to steps (b) (c) and (d), so as to allow the frame to become substantially encased within the foam core.
18. A method in accordance with claim 17 , wherein the step of placing a structural frame in the mold comprises placing a structural frame having a neutral axis along a shrink-neutral axis of the tabletop, so as to resist shrinkage-related deformation of the tabletop.
19. A method in accordance with claim 16 , further comprising the step of placing attachment point devices in the mold prior to steps (b) (c) and (d), so as to provide attachment points for connecting support structure for the table on an underside of the tabletop to become substantially encased within the foam core.
20. A method in accordance with claim 1 , further comprising the step of introducing a second polymeric material into the mold before causing the foaming agent to disperse within the mold, so as to form a second molten shell of the second polymeric material on the inside of the aforesaid shell of molten polymeric material, the foaming agent coming into contact with the second molten polymeric material.
21. A method in accordance with claim 20 , wherein the second molten polymeric material is of a species different from that of the aforesaid polymeric material.
22. A method in accordance with claim 1 , wherein the steps of rotating the mold while heating it comprise rotating the mold in a multi-axis rotational molding mechanism.
23. A method in accordance with claim 1 , wherein the foaming agent is selected from the group consisting of azodicarbonamide foaming agents and sodium bicarbonate-based foaming agents.
24. A method in accordance with claim 1 , wherein a shape of the mold varies from an intended finished shape of the molded article, such that post-molding shrinkage-related deformation of the molded article will cause the shape of the molded article to substantially conform to an intended finished shape.
25. A method for making a molded article, comprising the steps of:
a) placing polymeric material in particulate form and without a foaming agent into a mold having an inside surface;
b) rotating the mold while heating the polymeric material to a first elevated temperature, so as to melt a portion of the polymeric material and form a molten shell of the polymeric material on the inside surface of the mold with an unmelted portion of the polymeric material remaining inside the molten shell;
c) dispersing a foaming agent alone, in particulate form, within the mold subsequent to the polymeric material forming the molten shell;
d) rotating the mold to cause the foaming agent to spread and adhere to and mix with the remaining unmelted portion of the polymeric material; and
e) rotating the mold while heating the polymeric material and foaming agent to a second higher temperature, so as to cause the remaining unmelted portion of the polymeric material that is mixed with the foaming agent to expand to form an expanded polymer foam core inside the shell.