fig1 discloses a shopping cart 10 . the shopping cart 10 includes a frame 14 , a basket assembly 18 , a handle 22 , a rear gate 26 , a lower tray 30 , and multiple wheels 34 supporting the frame 14 for movement over the ground . in the embodiment shown in fig1 - 4 , the frame 14 forms a u - shaped member including a base 38 , uprights or arms 42 extending upwardly from the base 38 , and a cover 46 connected to each arm 42 . as best shown in fig4 , each arm 42 includes an upper end 50 positioned away from the base 38 . the arms 42 taper inwardly from the upper ends 50 , such that the distance between the arms 42 proximate the base 38 is less than the distance between the upper ends 50 of the arms 42 . the cart 10 defines a front end 52 , a rear end 54 opposite the front end 52 , and a horizontal cart axis 56 extending between the front end 52 and the rear end 54 . as used herein , the term “ rearward ” refers to a direction that extends toward the rear end 54 , and the term “ forward ” refers to a direction that extends toward a front end 52 . the frame 14 is formed by a combination of over - molding and gas - assist plastic forming processes . as shown in fig5 a , a substrate 58 such as a piece of metal is over - molded by plastic material 60 . although the substrate 58 in fig5 a has a cross - section that is u - shaped , the substrate 58 can be formed into various other shapes , including a round tube ( fig5 b ) or an enclosed piece that is partially hexagonal and partially rectangular ( fig5 c ). in the illustrated embodiments , the plastic material 60 near the substrate 58 is molded as a lattice structure having ribs 62 for increased rigidity . in one embodiment , the substrate 58 is made from steel , although in other embodiments the substrate could be made from another material . the plastic material 60 may include any type of injection - molded plastic . the gas - assist process creates at least one hollow cavity ( not shown ) in the plastic material 60 . this combination process yields a frame 14 that is both strong and low weight . the substrate 58 and the geometric relationship between the substrate 58 and the plastic 60 increases rigidity over a simple injection - molded plastic . the combination process satisfies the load and durability standards , is less expensive than conventional glass - filled nylon processes , and provides a frame 14 having a lower density than the glass - filled nylon frames . in one embodiment , a frame 14 made using the combination process costs about $ 0 . 60 per pound and has a lower density , whereas a frame made from glass - filled nylon costs about $ 2 per pound and has a higher density . referring to fig2 , the basket assembly 18 is removably coupled to the frame 14 and includes a basket frame 74 and a pair of slots 78 on either side of the basket frame 74 . the slots 78 receive the upper ends 50 of the arms 42 . the walls and bottom surface of the basket assembly 18 have been removed for purposes of clarity ; the basket assembly 18 also includes basket walls similar to those shown in fig1 - 13 and fig2 . the basket assembly 18 is moved onto the arms 42 and slides down along the arms 42 until the basket assembly 18 reaches a predetermined position . in the illustrated embodiment , the predetermined position is the point at which the slots 78 engage the arms 42 in an interference fit due to the inward taper of the arms 42 . in other embodiments , the predetermined position may be defined by a detent that locks the basket assembly 18 in place relative to the arms 42 , or the basket assembly 18 may be secured relative to the arms 42 by fasteners . referring to fig1 and 2 , the handle 22 includes a generally u - shaped member having two ends 86 that are adapted to engage the upper ends 50 of the arms 42 . after the basket assembly 18 is secured relative to the frame 14 , the handle 22 is coupled to the upper ends 50 of the arms 42 , further locking the basket assembly 18 in position . the handle 22 also includes a pair of openings 90 ( fig2 ), each of which is positioned proximate one of the handle ends 86 . in the illustrated embodiment , the handle ends 86 are secured to the arms 42 by a snap fit or interference fit . the rear gate 26 includes a wall 94 and a pair of gate arms 98 and is positioned proximate the rearward end of the cart 10 . the gate arms 98 are positioned on opposite edges of the wall 94 and extends away from the wall 94 . in the embodiment shown in fig1 and 2 , each gate arm 98 is positioned within one of the openings 90 on the handle 22 . the rear gate 26 is thus pivotably coupled to the handle 22 and pivots about a gate axis 100 transverse to the cart axis 56 . when a second cart ( not shown ) is pushed into the rear of the shopping cart 10 , the rear gate 26 pivots upwardly , toward the front of the basket assembly 18 in order to enable the basket assembly of the second cart to nest within the basket assembly 18 . in other embodiments , the gate arms 98 may engage openings positioned on the basket assembly 18 . the rear gate 26 engages a stop 102 ( fig7 a ) positioned on the basket assembly 18 to prevent the rear gate 26 from pivoting beyond the rearward end of the basket frame 74 , i . e ., to limit pivoting of the rear gate 26 in a clockwise direction as shown in fig7 a . the frame 14 is capable of supporting multiple types of basket assemblies 18 . for example , in an alternative embodiment shown in fig6 - 8 , a second basket assembly 218 includes multiple basket frames 274 having different sizes and characteristics from the first basket assembly 18 . the basket assembly of fig6 - 8 includes a first or lower basket 274 a and a second or upper basket 274 b . in addition , the handle 222 is pivotably coupled to blocks 224 that are coupled to the arms 242 . in the illustrated embodiment , the handle 222 is formed as a u - shaped member that is pivotably coupled to the cart 10 at each end 286 . the handle 222 is pivotable about a handle axis 228 ( fig7 b ) transverse to the cart axis 256 . in other embodiments , the handle 222 may be coupled to the basket assembly 218 or the handle axis 228 may be oriented in a different direction . referring to fig7 b , the upper basket 274 b is substantially above the lower basket 274 a . the lower basket 274 a includes a first or lower gate 226 a proximate the rear end 254 of the cart 210 . the lower gate 226 a is pivotable about a lower gate axis 300 a transverse to the cart axis 256 between a raised position and a lowered position . the lower gate 226 a is similar to the rear gate 26 described above with respect to fig1 and 2 . the lower gate 226 a includes a wall 294 a and a pair of arms 298 a for coupling the lower gate 226 a to the lower basket 274 a . the wall 294 a extends across the rearward end of the lower basket 274 a when the lower gate 226 a is in the lowered position . as shown in fig7 a , the wall 294 a has an upper end 296 below an upper end 276 of the lower basket 274 a when the lower gate 226 a is in the lowered position . as shown in fig7 b , the upper basket 274 b includes a second or upper gate 226 b proximate the rear end 254 of the cart 210 . the upper gate 226 b is pivotable about an upper gate axis 300 b transverse to the cart axis 256 between a raised position and a lowered position . the upper gate 226 b includes a wall 294 b ( fig7 b ) and a pair of arms 298 b for coupling the upper gate 226 b to the upper basket 274 b . the wall 294 b extends across a rearward end of the upper basket 274 b when the upper gate 226 b is in a lowered position . both the lower gate 226 a and the upper gate 226 b pivot upwardly toward the front of each basket 274 a , 274 b to allow the lower basket 274 a and the upper basket 274 b to receive a lower basket and upper basket , respectively , of another cart ( not shown ). the stops 102 ( fig7 a ) prevent the gates 226 a , 226 b from pivoting backward beyond the rearward end of each basket 274 a , 274 b . the lower basket 274 a is configured to be loaded from the rear . as best illustrated in fig7 a and 7b , the lower basket 274 a is positioned partially behind the upper basket 274 b , enabling a user to load and unload the lower basket 274 a from behind the cart 210 and beneath the handle 222 . stated another way , a rearward end of the upper basket 274 b is positioned forward of a rearward end of the lower basket 274 a to provide easy access to the contents of the lower basket 274 a from the rear of the cart 210 . in addition , because the upper end 296 of the wall 294 a is below the upper end 276 of the lower basket 274 a when the lower gate 226 a is in the lowered position , it is easier for a user to load and unload the lower basket 274 a . the handle 222 is moveable about the handle axis 228 between at least a first or rear position ( fig6 ) and a second or forward position ( fig7 a ). as shown in fig6 , in the rear position the handle 222 extends toward the rear end 254 of the cart 210 . this configuration allows the user to push the cart 210 with the handle 222 . referring to fig7 b , in the forward position the handle 222 extends toward the front end 252 of the cart 210 . in the forward position , the handle 222 is substantially aligned with the upper gate 226 b ( see fig7 b ) and forward of the rearward end of the lower basket 274 a . this position of the handle 222 improves access to the lower basket 274 a , allowing the user to more easily load or unload the lower basket 274 a from the rear . the frame 14 is configured to support many types of basket assemblies 18 , so the manufacturer can select a basket assembly 18 from multiple types of basket assemblies 18 in order to customize the cart 10 as necessary . this reduces the need to manufacture many types of carts 10 individually , instead providing a modular approach for manufacturing the carts 10 from similar parts . in other embodiments , shown in fig9 , multiple basket assemblies 418 may be positioned on the arms 442 , including one or more rear - facing baskets 474 c . in this embodiment , the handle 422 includes ends 486 that are coupled to the arms 442 by fasteners such as bolts . alternatively , as shown in fig1 - 11 , the arms 642 can be removably received on the frame 614 , and the basket assemblies 618 include brackets 620 for coupling each basket assembly 618 to the arms 642 . the bracket 620 can be secured to the arms 642 by a snap fit or by fasteners . the alternative embodiments of fig1 - 14 also illustrate that the basket assembly 818 can be directly fastened to the frame 814 , and the handle 822 may be formed integrally with the basket assembly 818 or the frame 814 . in addition , fig1 shows a cart 810 in which the lower tray 830 includes an open - ended basket . as shown in fig1 - 17 , the frame 14 is formed in a mold 106 . the mold includes a removable first module or insert 110 . in the illustrated embodiment , the first insert 110 forms a portion of the frame base 38 . the first insert 110 may be removed from the mold 106 and replaced with a second insert 114 that forms the same portion of the base 38 , but the portion of the base 38 has a shorter dimension than the portion that is formed by the first insert 110 . in other embodiments , the second insert 114 could provide a different shape for the portion of the base 38 . in still other embodiments , the inserts 110 , 114 could form a different portion of the frame 14 or another component of the cart 10 . the removable inserts 110 , 114 enable the operator to readily customize the size and shape of the manufactured frame 14 by changing the configuration of the mold 106 . because the mold 106 is capable of producing various frame configurations , the forming process requires only a single large mold 106 with several smaller inserts like the first insert 110 and second insert 114 , rather than multiple large molds for each desired configuration . in this way , tooling costs are reduced . as shown in fig1 - 21 , the frame 14 also includes a caster lift mechanism 122 to raise the wheels 34 when the cart 10 is nested or stacked with another cart . the caster lift mechanism 122 includes a ridge 126 formed on an upper surface 130 of the base 38 , a housing 134 formed on an inside surface 138 of at least one of the arms 42 , and an insert 142 ( fig1 ) positioned within the housing 134 and behind the cover 46 . in one embodiment , the insert 142 is made of a wear - resistant plastic material . in the illustrated embodiment the housing 134 extends inwardly or toward the center of the base 38 and forms a cylindrical recess 146 . the recess 146 is open at the inner end of the housing 134 and the housing 134 has a longitudinal slot 150 communicating with the bottom of the recess 146 . the insert 142 is cylindrical and is inserted into the inner end of the recess 146 such that the lower portion of the insert 142 extends through the slot 150 . referring to fig2 , when a second cart is nested into the rear of a first cart 10 , the insert 142 from the first cart 10 engages the ridge 126 and causes the rear of the first cart 10 to lift away from the ground . lifting the rear end of the cart 10 insures that the rear wheels 34 do not contact the ground , thereby reducing the friction between the nested carts 10 and the ground . this reduced contact makes it easier for the nested carts 10 to be pushed and maneuvered together . over time , the repeated engagement between the ridge 126 and the insert 142 will cause the insert 142 to wear down . the insert 142 provides a replaceable wear surface such that once the insert 142 wears down to a predetermined point , the user can replace the insert 142 to extend the working life of the caster lift mechanism 122 . in an alternative embodiment , shown in fig2 , the cart 1210 includes a basket frame 1274 , a bottom surface 1276 , and multiple walls 1280 . the basket frame 1274 is configured to support multiple types of basket walls 1280 that can be removably coupled to the basket frame 1274 . in one embodiment , the basket frame 1274 is molded separately from the walls 1280 , and the walls 1280 are coupled to the basket frame 1274 by a snap fit . in other embodiments the walls 1280 are coupled by fasteners . the basket frame 1274 is configured to support many types of basket walls 1280 , so the manufacturer can select a basket wall 1280 from many basket wall types in order to customize the cart as necessary . for instance , various walls 1280 may define different mesh patterns or wall cross - sections . this reduces the need to manufacture many types of baskets individually . instead , the manufacturer can create the same basic frame 1274 and then select a particular wall 1280 , thereby implementing a modular approach to manufacturing the basket frames 1274 from similar parts . the wall 1280 of the basket frame 1274 may also include one or more billboard panels 1284 for displaying graphics or other information . the panels 1284 can be molded with an in - mold graphic foil 1288 ( fig2 ) and changed as desired ( for example , seasonally ). fig2 a - 23 d provide various examples of billboard panels 1284 . as shown in fig2 a , the graphic foil 1288 may be slidably received through the basket frame 1274 and positioned within the billboard panel 1284 . alternatively , fig2 b and 23c show that the panel 1284 may include a transparent window 1292 that is pivotable relative to the basket frame 1274 to allow the graphic foil 1288 to be positioned behind the window 1292 . the panel 1284 may also be removably fitted within the basket frame 1274 , as shown in fig2 d . fig2 shows a nesting flat - bed cart 1610 . the flat - bed cart 1610 includes a frame 1614 , a pivoting load bed or deck 1616 , at least one spring - loaded slow - close hinge 1620 , and a latch mechanism 1624 . in one embodiment , the deck 1616 is made from a non - slip plastic material . the frame 1614 includes a base 1628 having a handle 1622 , multiple wheels or casters 1634 coupled to the base for supporting movement of the frame over the ground , and a cross piece 1636 . in the illustrated embodiment , the rear wheels 1634 can swivel , while the front wheels 1634 are fixed . the frame 1614 defines a longitudinal axis 1640 . the deck 1616 includes a hand grip 1644 and a ribbed underside 1648 ( fig2 ) to provide increased rigidity . the deck 1616 is pivotably attached to the base 1638 by the spring - loaded slow - close hinge 1620 . the deck is pivotable about a pivot axis 1652 transverse to the longitudinal axis 1640 . the slow - close hinge 1620 is similar to those used for toilet seats . the spring action of the hinge 1620 biases the deck 1616 toward an upright position as shown in fig2 . the latch mechanism 1624 is located on the cross piece 1636 extending between the front wheels 1634 . when the deck 1616 is moved to a lowered or use position , the latch mechanism 1624 locks the deck 1616 . in one embodiment , the latch mechanism 1624 includes a rotating hook ( not shown ). the hook may be spring - biased to automatically engage and lock the deck 1616 in place when the deck 1616 is moved to the lowered position . when the latch mechanism 1624 is unlocked , the slow - close hinge 1620 moves the deck 1616 to the upright position . the hand grip 1644 enables the user to assist in lifting the deck 1616 to the upright position . in other embodiments , a heavy spring or gas strut may be used in addition to or in place of the slow - close hinge 1620 . with the decks 1616 in the upright position , multiple flat - bed carts 1610 can be stacked or nested together in a manner similar to the cart 10 of fig1 . that is , the frame of a second flat - bed cart 1610 can be pushed into the first flat - bed cart 1610 from the rear , such that the frames 1614 of the two carts 1610 nest together the flat - bed cart 1610 of fig2 can be made with a frame 1614 that is similar to the frame 14 of fig1 except with a different handle . in addition , the flat - bed cart 1610 of fig2 can include the caster lift mechanism 122 described above with respect to fig1 - 21 . in the embodiment shown in fig2 , the flat - bed cart 1610 further includes a small basket 1618 that can be removably attached to the handle 1622 , such as by a snap fit or by fasteners . the basket 1618 may also be integrally formed with the handle 1622 or frame 1614 . fig2 - 27 disclose a nesting flat - bed cart 1810 with a folding deck 1816 . the flat - bed cart 1810 of fig2 - 27 is similar to the flat - bed cart 1610 of fig2 , except that it is designed to support greater weights or loads . the deck 1816 includes a first or rear section 1844 pivotably coupled to the frame 1814 , and a second or front section 1848 pivotably connected to the rear section 1844 by a hinge 1852 . the deck 1816 folds along a lateral break line that is transverse to the longitudinal axis 1840 of the frame 1814 , in a manner similar to a folding banquet table . the deck 1816 can pivot about the hinge 1852 when in a lowered position , which allows the flat - bed cart 1810 to accommodate bumps or rough surfaces over which the cart 1810 travels . in the illustrated embodiment , the front section 1848 and rear section 1844 include at least one hand grip 1856 for moving the deck 1816 between the upright and lowered positions , and the front section 1848 includes one or more casters 1860 . in one embodiment , a single caster 1856 is used for supporting up to 2000 pounds and two casters 1856 are used for supporting up to 3000 pounds . various features of the invention are set forth in the following claims .