a more particular description of certain embodiments of reduced density thermoplastics may be had by references to the embodiments described below , and those shown in the drawings that form a part of this specification , in which like numerals represent like objects . in one embodiment , a set of plastic blanks may be created by either cutting from a roll of plastic or by injection molding of the shapes . each blank may be shaped in such a manner that it provides the material for a small number of parts , such as one to 100 . the blanks , which may be of any shape or size , may then be loaded into a rack for ease of saturation . the rack may then be loaded into a pressure vessel for saturation . the plastic may be allowed to saturate partially or fully , possibly resulting in a high gas concentration at the surface of the blanks . the blanks may then be removed , and the plastic may be transferred into a heating rack . the heating rack may be fed through a conventional thermoformer , where the blanks may be heated and stretched into shape . during heating and stretching , a solid state foam structure may be created . a variation of the above embodiment is to injection mold blanks instead of extruding them . injection molded parts may be formed of varying thickness , allowing different levels of saturation and the parts may be shaped in such a manner as to require little post - forming after heating . the injection molded parts may also be directly molded into a pressure vessel and allowed to cool , eliminating the need for saturation time . fig1 illustrates plastic blank 110 and mask device 170 , according to one embodiment . mask 120 and mask 140 may be coupled by coupler 130 to form mask device 170 . coupler 130 may be a hinge , one or more clips , a bend in the material forming mask 120 and mask 140 , or other means to couple them . plastic blank 110 may be placed between mask 120 and mask 140 , which may fold at coupler 130 to seal plastic blank 110 in mask device 170 . plastic blank 110 may be any type of thermoforming plastic . mask 120 may have an opening 150 , and mask 140 may have an opening 160 , which may allow portions of plastic blank 110 covered by mask 120 and mask 140 to be exposed to a gas foaming agent , while protecting other portions of plastic blank 110 from the gas foaming agent . one having skill in the art will recognize that various ways of masking plastic blanks may be used , and that masks may have different sizes and shapes . mask device may also be configured to hold several plastic blanks at the same time . fig2 illustrates a plastic blank enclosed in mask device 170 , according to one embodiment . mask device 170 may close tightly against plastic blank 110 , forming a gas - proof or near gas - proof seal . mask device 170 may have sufficient thermal shielding to prevent masked portions of plastic blank 110 from becoming heated enough to foam if mask device 170 is used during heating . fig3 illustrates placing mask device 170 holding plastic blank 110 into rack 310 , according to one embodiment . rack 310 may be configured to hold multiple mask devices 170 , with spacing between mask devices 320 sufficient to allow plastic blank 110 to be exposed to a gas foaming agent . fig4 illustrates a plastic sheet and mask according to one embodiment . large plastic sheet 420 from which many parts may be formed , may be loaded into saturating and heating trays 410 , 430 . trays 410 , 430 , ( serving as masks ) may contain a plurality of holes , the shape of which is similar to the exterior profile of a plastic part to be formed , for example , round for a cup or square for a tray . the plastic sheet 420 may then be loaded into a pressure vessel and saturated for a period of time . the holes present in the trays may allow the plastic sheet to be saturated in areas in which plastic parts will be formed . the plastic may then be removed from the pressure vessel , heated , and formed using conventional thermoforming equipment . the plastic may be transported through the thermoforming machinery utilizing the same masking tray as was utilized for saturation or a different masking tray . it may also be desirable to eliminate the use of trays in the saturation process , and use rails on the side of the pressure vessel and forming equipment to transport the plastic without the use of trays . fig5 illustrates rack 310 being placed into pressure vessel 510 , according to one embodiment . rack 310 may hold at least one mask device 170 with plastic blank 110 . pressure vessel 510 may be pressurized , to allow a gas foaming agent to penetrate exposed portions of plastic blank 110 . in another embodiment , sheet may be extruded directly into a pressure vessel . the extruded sheet may be allowed to cool into the pressure vessel , and loaded into racks . this variation eliminates the need for saturation time . in yet another embodiment , gate - style valves and pressure locks may be used at the inlet and exit to all pressure vessels , and a conveyance system may be configured within the pressure vessel . the pressure locks may allow sheet , blanks , or preforms to enter a pressure vessel which contains other plastic . a material conveyance system within the pressure vessel may move items through the pressure vessel . all materials in this arrangement may receive precisely the same amount of time in the pressure vessel . various parameters such as temperature , pressure , gas foaming agent , and timing , may be adjusted to provide various characteristics of resulting foamed plastic . saturation time and desorption time variations have been shown to be able to produce novel structures within the polymer . higher gas concentration can lead to increases in crystallinity , and also to increases in bubble nucleation rates . lower gas concentrations can lead to increases in density reduction . combining these two elements can result in parts with high density exteriors and low density interiors , a structure that leads to optimal mechanical properties for a given part weight . it may be desirable to add a solvent to the pressure vessel during the saturation process . when saturating pla , it has been found that water induces a change in the surface of the sheet , creating a uniform white surface during heating . when saturating pet , it has been found that acetone will induce crystallization on the surface of the part . controlled heating of the plastic may be used to induce specific properties in foam parts . heating a single side of the plastic sheet may cause an asymmetric density and foam cell distribution . asymmetric density may optimize mechanical properties , such as creating a cup that is resistant to external crushing forces while bending outward easily for removal from a stack . colorant usage may be optimized by creating a colored , high density exterior ( colorants are dispersed by reducing density ) while minimizing density in the remainder of the part . it may be advantageous to partially distort the plastic blanks , or plastic exposed in holes during heating . this may be done to increase the uniformity of heating , or begin stretching into shape before beginning final shape stretching into a mold . this may be performed by increasing air pressure on one side of the sheets , decreasing air pressures on one side of the sheets , utilizing high pressure air jets , or by inducing stretching by direct mechanical force through the use of temperature controlled plugs . upon completion of the foaming operation , rack 310 may be removed from pressure vessel , and mask device 170 may be extracted . plastic blank 110 may then be heated through a thermoforming machine . plastic blank 110 may be held by rack or chain rails until heated . portions of plastic blank 110 exposed by the mask may become foamed while portions protected by the mask may remain unfoamed . for some products , plastic blank 110 may be pushed into a mold using , for example , a mechanical plug . plastic blank may be formed into a final desired shape using vacuum ( reduced pressure ), positive pressure , mechanical coercion , or combinations of these techniques . for example , plastic blank 110 may be moved via a material conveyance system until it is over a mold to form a drinking cup . a clamp ring which may aid in the forming of a lip may hold plastic blank 110 in the proper position as an upper portion of a mold lowers to hold plastic blank 110 in place . positive pressure may be applied from the top , while vacuum may pull plastic blank 110 into the mold . the mold may allow a lip to be formed where the top and bottom of the mold hold plastic blank 110 securely in place . fig6 illustrates an example of a cross - section of a plastic blank after foaming , according to one embodiment . in this example , pressure , temperature , timing , and gas foaming agent selections have produced a plastic with large bubbles 610 near one side of the cross - section , with small bubbles 620 adjacent to large bubbles 610 , and small bubbles 540 near an opposite side . between small bubbles 620 and small bubbles 640 is unfoamed polylactic acid ( pla ) 630 . this example illustrates the results of heat applied at a higher rate on one side than the other during thermoforming . fig7 illustrates an example of a cross - section of a plastic blank after foaming , according to another embodiment . in this simplified scanning electron microscope image of a sample , large bubbles 720 start near one side and extend to near the center of the blank . pla 710 may be unfoamed , while smaller bubbles 630 are near an opposite side from bubbles 720 . fig8 is a flow chart of creating a pla product , according to one embodiment . create blank 810 may include cutting a blank from a sheet , extruding a sheet , or other ways to obtain a suitable blank of plastic . for place blank in mask device 820 , the mask device may be configured to allow a gas foaming agent or heat to be exposed to portions and not exposed to others portions of the blank . place mask device in rack 830 may allow for multiple blanks to be processed at a time . in other embodiments , racks may not be required . place rack in pressure vessel 840 may be a manual or automated process , moving the rack to a position where saturate blank 850 may occur . saturation parameters may include gas foam - agent used , pressure , temperature , and time . varying these parameters may provide reduced density thermoplastics with differing properties , including strength , insulation value , and flexibility . the rack may be removed from the pressure vessel , and mask devices may be removed from a rack , if one was used . selectively heat blank 860 may allow exposed portions of the blank to be heated in a thermoformer . by masking portions of the blank , heating and foaming may be restricted to what is required for the desired product . edges of the blank may be constrained to prevent the blank from warping , and may facilitate draw blank into shape 870 . draw blank into shape 870 may use pressure and vacuum to cause the blank to conform to a mold . hold part in mold 880 may allow the part to cool and retain a shape . trim and remove part 890 may include trimming excess material and removing the part from the mold . at recover scrap 895 , portions of the blank that were not exposed to gas foaming agent and heat may then be reused to create another blank . fig9 is an image showing plastic blank 110 in mask device 170 , placed between two heating elements 910 . heating elements 910 may be radiative , conductive , or convective heating elements . by changing the relative temperature of these heating elements , asymmetric heat can be applied to each side of plastic blank 110 . fig1 is a flow chart for extruding plastic sheet directly into a pressure vessel . extrude polymer sheet into pressure vessel 1010 may provide a polymer sheet from which to create the saturated sheets . allow polymer sheet to cool 1020 may include reducing temperature inside the pressure vessel . segment polymer into sheets 1030 may include cutting the sheet into appropriate sizes for an intended product . the resulting polymer sheets may be removed 1040 through a series of pressure locks , which may allow the pressure inside the pressure vessel to be maintained . fig1 is a flow chart for injection molding polymer blanks directly into a pressure vessel . injection mold polymer blanks into pressure vessel 1110 may provide polymer blanks from which to create the saturated blanks . allow polymer blanks to cool 1020 may include reducing temperature inside the pressure vessel . the resulting saturated polymer blanks may be removed 1130 through a series of pressure locks , which may allow the pressure inside the pressure vessel to be maintained . fig1 is a flow chart of introducing solvent into a pressure vessel . in this example , a supply valve for a high pressure gas source may be opened 1210 . the high pressure gas may be routed 1220 through a porous membrane saturated with solvent , or past a nozzle ejecting solvent . the high pressure gas containing solvent may then fill pressure vessel 1230 . the following conditions have been used to produce a drinking cup . a sheet of natureworks 4032 d , 0 . 024 inches in thickness , may be saturated in a co 2 pressure vessel for 10 minutes , at a pressure of 600 pounds per square inch ( psi ), with water vapor introduced during the fill cycle of the pressure vessel . the sheet was allowed to lose gas for 5 minutes prior ( desorption time ) to heating and stretching , and then heated while constrained within 0 . 2 inches of the cup perimeter until reaching approximately 90 degrees celsius . the sheet may then be forced into a cup mold using an aluminum plug at 7 degrees celsius , and set against a cup mold at 110 degrees celsius for 15 seconds . pressure ranges from 400 - 1000 psi for pla and saturation times of 5 to 45 minutes may be useful for producing products . desorption times of 5 to 30 minutes may be used to produce products . plug temperatures may be varied from − 5 c to 90 celsius , and different materials may result in different cup wall thickness distributions . cooler mold temperatures down to 0 celsius may allow more rapid processing , although less foaming will be induced . while the detailed description above has been expressed in terms of specific examples , those skilled in the art will appreciate that many other configurations could be used . accordingly , it will be appreciated that various equivalent modifications of the above - described embodiments may be made without departing from the spirit and scope of the invention . the foregoing description of various embodiments of the invention has been presented for the purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed . it is intended that the scope of the invention be limited not by this detailed description , but rather by the claims appended hereto . the above specification , examples and data provide a complete description of the manufacture and use of the invention . since many embodiments of the invention can be made without departing from the spirit and scope of the invention , the invention resides in the claims hereinafter appended .