Rapid rotational foam molding process

Abstract

The nature of the rotational molding process is cyclic. It requires the temperature of the rotating mold and the plastic it is charged with to be elevated from room temperature to beyond its melting temperature and then cooled back to room temperature. Consequently, rotational molding cycle times are lengthy, which is often considered as the fundamental drawback of this plastic fabrication process. The motivation and objectives of this disclosure are twofold. The presently proposed invention focuses on developing an innovative extrusion-assisted rotational foam molding processing technology for the manufacture of integral-skin cellular composite moldings having adjacent, but clearly distinct, layers of non-cellular and cellular structures, consisting of identical or compatible polymeric grades. Its primary goal is to significantly reduce the processing cycle time in comparison with respective currently implemented technologies by employing melt extrusion in order to maximize the speed of controlled polymer melting.

1 . A molding process including the steps of:
 precharging a mold with a predetermine amount of non-foamable plastic powder;   rotating and heating the mold in an oven until the plastic powder melts and coats the inside of the mold thereby producing a solid skin;   removing the mold from the oven;   injecting extruded plastic foam into the mold and rotating the mold; and   cooling the mold to produce a foamed product with an outer skin.   
   
   
       2 . The molding process as claimed in  claim 1  wherein the extruded plastic foam includes heat activated blowing agent. 
   
   
       3 . The molding process as claimed in  claim 2  wherein the heat activated blowing agent is a chemical blowing agent. 
   
   
       4 . The molding process as claimed in  claim 1  wherein the foamed product includes a foamed core with the outer skin. 
   
   
       5 . The molding process as claimed in  claim 1  wherein in the rotating and heating step the mold is rotated biaxially and then the rotation is converted to uniaxial rotation prior to the foam injecting step. 
   
   
       6 . The molding process as claimed in  claim 1  wherein the extruded foam is prepared in an adjacent extruder during the steps of precharging, rotating and heating the mold and removing the mold. 
   
   
       7 . The molding process as claimed in  claim 1  wherein the extruded foam is injected via an insulated injection port serving as an extruder to mold interface. 
   
   
       8 . The molding process as claimed in  claim 1  wherein the mold is separable. 
   
   
       9 . The molding process as claimed in  claim 8  wherein the mold is a metal mold. 
   
   
       10 . The molding process as claimed in  claim 1  wherein the mold has a shape that is chosen from the group consisting of cylindrical, flat plate or box like shapes. 
   
   
       11 . A molding process including the steps of:
 precharging a mold with a predetermine amount of non-foamable plastic powder;   rotating and heating the mold in an oven until the plastic powder melts and coats the inside of the mold thereby producing a solid skin;   removing the mold from the oven;   injecting foamable extrudate including a heat activated blowing agent into the mold and rotating the mold;   reinserting the mold into the oven in order to induce decomposition of the heat activated blowing agent contained in the foamable extrudate and produce a foamed layer inside the solid skin; and   cooling the mold to produce a foamed product with an outer skin.   
   
   
       12 . The molding process as claimed in  claim 11  wherein the heat activated blowing agent is a chemical blowing agent. 
   
   
       13 . The molding process as claimed in  claim 11  wherein in the rotating and heating step the mold is rotated biaxially and then the rotation is converted to uniaxial rotation prior to the injecting step. 
   
   
       14 . The molding process as claimed in  claim 11  wherein the foamable extrudate is a non-chilled extrudate. 
   
   
       15 . The molding process as claimed  11  wherein the foamable extrudate is simultaneously prepared in an adjacent extruder. 
   
   
       16 . A molding apparatus comprising:
 a mold having and an interface device;   an arm adapted to rotate the mold;   an extruder operably connectable to the interface device of the mold; and   an oven adapted to heat the mold.   
   
   
       17 . The molding apparatus as claimed in  claim 16  wherein the arm is capable of being switched from rotating the mold uniaxially to rotating the mold biaxially. 
   
   
       18 . The molding apparatus as claimed in  claim 16  whereby the extruder is capable of working while the mold is being rotated in one axis. 
   
   
       19 . The molding apparatus as claimed in  claim 17  whereby the extruder is capable of working while the mold is being rotated in one axis. 
   
   
       20 . The molding apparatus as claimed in  claim 17  wherein the extruder is one of a single screw extruder and a double screw extruder. 
   
   
       21 . The molding apparatus as claimed in  claim 17  wherein the extruder includes a nozzle and the mold includes an interface port assembly adapted to receive the nozzle. 
   
   
       22 . The molding apparatus as claimed in  claim 16  wherein the oven and the extruder are fixed and the arm and the mold moves between the oven and the extruder via translational movement. 
   
   
       23 . The molding apparatus as claimed in  claim 21  wherein the interface port assembly is an insulated interface port assembly including an insulation cap, a mold interface and a mold connection adaptor. 
   
   
       24 . The molding apparatus as claimed in  claim 21  wherein the interface port assembly includes a valve made from resiliently deformable metal having a plurality of radial slits formed therein. 
   
   
       25 . The molding apparatus as claimed in  claim 24  wherein the resiliently deformable metal is spring steel.