The predominant blowing agent for the generation of thermoplastic foam extrudates on a commercial basis has been fluorocarbons, for example, trichlorofluoromethane and dichlorodifluoromethane.  As a result of ecological studies, these fluorocarbons have been determined to be an undesirable pollutant for the atmosphere.  Accordingly, ways are being sought to eliminate or materially reduce the utilization of fluorocarbons for all purposes wherever possible.  A significant area in which to eliminate fluorocarbons from manufacturing processes is in the generation of thermoplastic foam extrudates such as thermoplastic foam sheeting and the like. 
In the process of eliminating fluorocarbons, it becomes apparent that if conventional chemical blowing agents also can be successfully eliminated and only harmless ingredients utilized as the blowing agents, then the manufacturing processes for foamed thermoplastic extrudates would become more ecologically sound. 
Natural gases such as air, nitrogen or carbon dioxide, all of which have predictable and acceptable effects on the ecology, would be the most desirable types of blowing agent.  Water is also a prime candidate for an ecologically sound blowing agent. 
In addition to overcoming the above-noted ecological problems, it has been desirable to provide a means and method for agitating the thermoplastic melt to promote better mixing and to eliminate dead regions or stagnation during the mixing process.  Previous attempts at improving the mixing conditions for thermoplastic melt have included a plasticizer screw for advancing molten plastic to an injection mold which has a circulating row of balls behind the flights of the screw for agitating the melt to promote better mixing as disclosed by U.S. Pat.  No.  3,944,192 to Farrell.  Another attempt at achieving better mixing conditions for thermoplastic melt is disclosed by U.S. Pat.  No.  3,530,534 to Pomper wherein an extruder includes a screw having a thread structure interrupted with a recess which includes two rows or rings of spherical balls.  An attempt at improving mixing conditions for powdery rubber mixtures is disclosed by U.S. Pat.  No.  4,416,543 to Brinkmann which describes an apparatus for treating powdery rubber mixtures which includes a toothed central spindle and toothed planetary spindles disposed symmetrically around the central spindle such that the planetary spindles rotate and orbit the central spindle.  This system has been in commercial use for thermoplastics as well as where simultaneous mixing and cooling is required.  An attempt at improving mixing conditions for processing viscous fluids is disclosed in U.S. Pat.  No.  3,443,798 to Overcashier et al wherein a fluid processing device includes a rotor centrally mounted in a cylindrical shell and a plurality of sets of axially staggered orbital rollers located in the annulus between the rotor and the inner wall of the shell.  Another approach by Overcashier suggests also the use of making gear teeth on rotor, shell and staggered orbital rollers.  Although the devices disclosed by these patents have included gears, balls, spindles or rollers in addition to a centrally rotating core in order to improve mixing conditions, further improved mixing conditions are still sought which provide for more intensive mixing with less shear and less stagnation, as well as the capability of using a variety of blowing agents when foamed thermoplastic materials are to be produced, especially with gases. 
It is therefore an object of the present invention to supplant fluorocarbons as a blowing agent for foamed thermoplastic extrudates and the like. 
Another object of the present invention is to provide a new and novel method and apparatus for mixing air, nitrogen, carbon dioxide, water and/or combinations thereof in an extrusion system as blowing agents to produce foamed thermoplastic extrudate. 
A further object of the invention is to provide a mixing system with minimized plastic inventory so as to reduce residence time after the addition of blowing agents, particularly when air (with oxygen) is used. 
Another object of the present invention is to reduce the cost of producing foamed thermoplastic extrudates. 
Still another object of the present invention is to provide foamed thermoplastic extrudates of enhanced homogeneous quality. 
Yet another object of the present invention is to provide an apparatus which utilizes a plurality of mixing balls which rotate in a planetary manner to intensively mix thermoplastic melt with other ingredients. 
Another object of the present invention is to provide a method and apparatus employing mixing balls for use in intensive mixing operations wherein parts of the apparatus are not subjected to disadvantageous wear, as is inherent with systems employing gears and bearings. 
It is yet another object of the present invention to provide a method for intensively mixing ingredients with thermoplastic melt wherein a plurality of mixing balls which rotate in a planetary manner are disposed in a staggered arrangement about a spindle rotatably disposed within a barrel so as to impart multiple divisions in a flowing mass of thermoplastic melt which is caused to pass between the surfaces of the mixing balls, spindle and barrel. 
An important object of this invention is to provide an effective mixing system with minimum shear in order to avoid heat generation at the same place the temperature of the melt is being reduced. 
These and other objects of the present invention will become more fully apparent with reference to the following description and drawings which relate to several preferred embodiments of the invention.