in accordance with the present invention , it has been found that a multiple layered sheet or film may be produced from two polymeric materials which have hitherto not been successfully united by conventional techniques . specifically , the sheeting or film of the present invention consists of a layer of polystyrene having a thickness substantially corresponding to the final composite sheeting thickness desired , and adhered to either one or both sides of the polystyrene layer , a layer or layers of a polycarbonate resin . therefore , there is provided a composite sheeting having the desirable economic characteristics of polystyrene and concurrently possessing the highly desirable surface and structural properties of polycarbonate resins . such composite materials characterized by the desirable surface and structural properties of the polycarbonates are well suited for the fabrication of various appliances and in aircraft and automotive applications such as finish items , bumpers , headlight bezels , fender extensions and the like , and in various sports equipment and recreational vehicles such as football , baseball , motorcycle and snowmobile helmets , hoods , consoles , backrests , clutch guards and the like . the term polystyrene , as employed herein , includes both homopolymers of styrene and copolymers of styrene with other polymerizable and polymerized monomers . included within the latter category are impact polystyrenes which comprise graft copolymers of styrene with conjugated diene backbone polymers such as polybutadiene , butadiene - styrene copolymers , butadiene - acrylonitrile copolymers , natural rubber , and the like . also included in this category are normal copolymers of styrene with other well - known conventional monomers . the polycarbonate resins useful are those having the structural formula ## spc1 ## where x is 100 to 400 units . such polycarbonate resins are formed by the reaction of phosgene and bis phenol a . these polycarbonate resins possess uncommonly high impact strength , not only at temperatures up to 280 ° f , but also at temperatures as low as minus 100 ° f . additionally , the polycarbonate natural resins have water - clear transparency . also , the polycarbonate resins possess a wide temperature range at which their properties remain useful . polycarbonate performs well down to minus 100 ° f and displays heat distortion temperatures above 280 ° f at 264 psi . loading . these resins have a very predictable and low shrinkage characteristic and polycarbonate exhibits very little creep . a particularly useful group of polycarbonate resins are marketed by general electric under the trade name &# 34 ; lexan &# 34 ;. the present extrusion process has been found operable to produce multi - layered sheeting having extremely thin surface layers of polycarbonate , e . g ., on the order of 1 mil . or less , in comparison to the total sheet thickness , e . g ., 10 to 12 mils . this is achieved by providing relative feed rates of the polystyrene to the polycarbonates such that the volume of polycarbonates generally does not exceed about 25 %, and preferably is less than about 20 % of the total resin feed . relative feed rates for the polycarbonates of less than about 15 % of the total resin feed are , typically , preferred even more . the principles of the present invention generally are applicable to the manufacture of either multi - layer polymeric &# 34 ; film &# 34 ; ( less than 10 mils . in thickness ) or &# 34 ; sheeting &# 34 ; ( 10 mils . or thicker ). however , polystyrene based film has not found widespread popularity because of the difficulties encountered in adapting this material to blown film apparatus conventionally employed together with extrusion equipment in the manufacture of film products . therefore , while it is possible to produce a composite film having a 1 mil . or thinner surface layer of polycarbonate on a 4 or 5 mil ., or even a 1 mil . base layer of polystyrene , practically speaking , the products of most interest are multi - layer sheets of from 10 mils . up to 3 / 8 inch in thickness having polycarbonate resin surface layers measuring from a fraction , e . g ., 1 / 4 to 1 / 2 mil . to several mils . accordingly , it will be appreciated that the volumetric feed rates for the polycarbonate component are frequently very small , e . g ., less than 1 %, when thicker sheeting is desired , and likewise that rates in excess of 20 % or 25 % are contemplated when very thin film is produced . referring now to the drawings , in fig1 there is schematically illustrated an apparatus , generally designated by the reference numeral 10 , particularly adapted for the process of the present invention . the apparatus 10 comprises in cooperative combination a first extruder 11 for the extrusion of molten polystyrene , and connected thereto , a discharge conduit 12 . a second extruder 14 having a discharge conduit 15 is adapted for providing a minor stream of molten polycarbonate resin . conduit 15 terminates at conduit 12 at a point upstream of the sheeting die 17 which is in operative communication with conduit 12 and receives the flow therefrom . sheet 19 is formed at the die lips 18 and thereafter carried from the die by means of polished cooling rollers 21 , 22 and 23 . in fig2 there is illustrated a sectional view of the discharge conduits 12 and 15 , extrusion die 17 and cooling roller arrangement 21 , 22 and 23 . this view illustrates the intersection of conduits 12 and 15 , and the configuration of the polystyrene resin 31 and polycarbonate resin 32 as they are combined in conduit 12 to form a stratified stream having an interfacial juncture plane 25 . stratification at the intersection point of the two resins is aided by metal plate 34 positioned in conduit 12 adjacent the entry point of coextruder discharge conduit 15 . similarly , there is illustrated the passage of the stratified resin stream into the die manifold 27 , past restricter bar 28 and ultimately through the extruder die lips 18 to the chrome or cooling roller assembly . throughout this entire traversal of the extrusion equipment , it is noted that the individual layers of resinous material maintain their stratified relationship , despite the minute proportion of polycarbonate , to form a final product having an essentially uniform surface layer of this material . in fig3 is illustrated a top view of the conduit 12 and extrusion die 17 to demonstrate the lateral flow pattern of molten polymeric material as it passes into and through the extrusion die . fig4 illustrates more clearly the preferred configuration of metal plate 34 positioned in conduit 12 . to produce a multi - layered sheet or film having a satisfactory degree of adherence between layers , it is necessary to observe certain process limitations during the extrusion procedure . while it is of course desirable to provide relatively steady flow of polystyrene and polycarbonate resin through the apparatus and to avoid any turbulence therein , these limitations are rather typical in most extrusion processes employing highly viscous synthetic resins . more important limitations ( although not wholly unrelated to the foregoing ) reside in the extrusion temperatures for the polystyrene and polycarbonate , and the temperature maintained in the extrusion die . the extrusion of the polystyrene from extruder 11 into conduit 12 should be carried out at a temperature of from about 350 ° f to about 500 ° f , preferably 360 ° f to 450 ° f , whereas the polycarbonate should be discharged from extruder 14 at a temperature of from about 550 ° f to about 600 ° f . this range is not intended to be an absolute limitation on the invention , but rather indicates only the median range wherein the melt viscosity of the polycarbonate resin may be rendered proximate to the melt viscosity of polystyrene so as to provide strong adherence between layers in the extruded sheet . it is to be understood that the invention would likewise embody within its scope the use of polycarbonate resins which might be extruded at even higher temperatures to produce satisfactory adherence to the polystyrene substrate co - extrudate . as indicated , die temperature also is an important process variable . this parameter particularly affects the surface characteristics , e . g ., gloss , surface smoothness , etc ., of the extruded sheet . die temperatures should be maintained relatively constant during extrusion , with typical values thereof for coextrusion of polycarbonate and polystyrene ranging between about 460 ° and 490 ° f . the best characteristics are attained with temperatures above about 480 ° f . in the coextrusion of polystyrene and polycarbonate the conditions maintained at the cooling roller assembly also affect the properties of the final product . the manufacture of smooth sheeting normally requires the use of highly polished rollers , e . g ., chrome , typically three in number , each being approximately 12 inches in diameter and being adapted for internal circulation of cooling water . when polycarbonate is coextruded as a surface layer , however , it has been found that a slightly higher than normal top roll temperature , e . g ., 265 ° f , as well as a lower than normal top roll pressure , e . g ., approximately enough to overcome upward spring tension on the rollers , are required to achieve optimum characteristics . other processing variables of lesser importance include the pressures at which the polystyrene and polycarbonate are separately extruded before combination . these pressures typically fall within the range of 1900 and 2100 p . s . i . for the polycarbonate stream . of course , the downstream pressure at the point where the streams intersect is equal in both streams . a main resin stream of impact polystyrene ( cosden oil & amp ; chemical co . 825d pellets ) is extruded from a 41 / 2 inch diameter two stage vented extruder containing a 4 : 1 compression ratio screw . a 11 / 4 inch diameter 24 : 1 single stage side extruder also having a 4 : 1 compression ratio screw is arranged as illustrated in fig1 and 2 and supplies a second stream of a medium viscosity polycarbonate resin marketed by general electric as lexan 103 , having a specific gravity at 73 ° f of 1 . 2 , heat defection temperature at 264 p . s . i . of 270 ° f . the polystyrene is extruded at a temperature of 450 ° f . and a feed rate of approximately lbs ./ hr . the polycarbonate side stream is discharged from the 11 / 4 inch extruder at 550 ° f . and at a feed rate of approximately 18 lbs ./ hr . the two resinous streams are then combined in the discharge conduit of the 41 / 2 diameter extruder utilizing a baffle plate arrangement illustrated in fig4 . there results a single stratified bi - component stream having a horizontal juncture plane between the polystyrene and polycarbonate . the bi - component stream is conducted to a center - fed sheet - form extrusion die which opens to an extrusion slit 37 inches wide with its lips set at approximately 59 mils . the die temperature averaged about 460 °- 465 ° f . upon leaving the die lips , the extruded sheet passes through a series of three 12 inch polished chrome cooling rolls , the top roll maintained at 265 ° f ., middle roll at 170 ° f . and bottom roll at 140 ° f . roll pressures were maintained at about 80 p . s . i . for top roll , 70 p . s . i . for middle roll and 40 p . s . i . for bottom roll . the roll gap setting was 55 mils . the polishing and subsequent rubber rolls are operated at about 10 % and 8 %, respectively , excess speed to stretch the extruded sheet to a final thickness of 55 mils . examination of the final sheeting product evidences an essentially uniform layer of polycarbonate approximately mils thick firmly adhered to the impact polystyrene base layer . this thickness corresponds closely with the relative feed rates for the two resins . thus , there has been provided according to the invention a process for co - extruding multi - layered sheeting of polystyrene and polycarbonate wherein at least one polycarbonate layer is firmly adhered to the polystyrene base layer . moreover , the subject process enables the production of multi - layered film or sheeting of polystyrene having very thin , e . g ., less than 1 mil , surface layers of polycarbonate . while the fundamental novel features and advantages of the invention have been pointed out in connection with a single illustrated embodiment thereof , it will be appreciated that various obvious modifications of the co - extrusion process will suggest themselves to one of ordinary skill in the art . therefore , it is intended to be limited only by the scope of the following claims .