Abstract:
The cover of a plastic tank suitable for containing water is attached to the tank base by means of alternative joints, including elastically engageable features and lock features to prevent unwanted release. The base of a plastic water tank is comprised of a substrate and a liner and is formed by means which include injection molding into a mold where the core part of the mold is surrounded in part or full by the film which forms the liner.

Description:
[0001]    This application claims benefit of provisional patent application Ser. No. 61/858,757, filed Jul. 26, 2013. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to molded plastic vertical-axis tanks that are useful for storing water and other liquids and methods for making same. 
       BACKGROUND 
       [0003]    It is common to store water in unpressurized vessels from which water from time to time is drawn by means of pump or gravity flow, particularly where the water supply is intermittent, or where the instantaneous demand exceeds the flow rate of the source of supply. It is particularly desirable, to have a vessel which is sealed against egress or ingress of liquids when potable water is being stored. 
         [0004]    Typical water storage tanks that are associated with dwellings and other smaller buildings will have 500 to 1000 liter capacity. A squat cylindrical shape tank is popular for reason of compactness and manufacturability. A representative 500 liter tank might be about 100 cm in diameter and 100 cm high. One approach is to make a one piece tank, such as by blow molding or rotational molding. However, if a whole tank ready for use is shipped, the shipping cost can be an unacceptably high fraction of the total cost of making and delivering the product. 
         [0005]    Thus, one marketplace solution has been to have manufacturing sites dispersed around the country, when it is geographically large as is the U.S. However, for lower volume products with high machinery or tooling costs, the net result of long distance shipping or duplication of manufacturing facilities can be that the price of the product to the consumer is higher than it might be if the product could be made so that it could be shipped in more compact form. 
         [0006]    One solution has been to form a tank from mating identical half tanks which can be nested for shipment and assembled in proximity to the point of use. See U.S. Pat. No. 5,878,907 of Graf. The tank has a mid-elevation joint which is secured by means of clamps or welding. Another approach used in commerce is to make the tank in the form of a bucket like base which is combined with an associated lid. The bases have sufficient draft to enable nesting for shipment. The present invention is concerned with such types of tanks. Further improvements in design and manufacturing are needed for such kinds of tanks, to reduce manufacturing costs while providing a tank which is good for potable water, which has a joint near the top of the tank that is secure and not overly sensitive to small lid-to-base joint imperfections. The tank also should be suited for quick and easy assembly. 
       SUMMARY 
       [0007]    An object of the invention is to provide a plastic water storage tank which can be economically manufactured, shipped and stored. Another object is to provide a two piece water storage tank comprised of components which can be nested for shipment or storage and which have good joint seals when assembled. A further object is to provide a water storage tank made of a lower cost plastic substrate having a thin interior liner made of higher grade material that provides the interior water-contacting surface of the tank. 
         [0008]    In accord with the invention, a plastic water storage tank comprises a tank base which has a tub like interior concavity, and optionally in combination with a cover that is attached to the base. Each base or cover part is nestable within a like part, so the parts can be shipped and stored compactly in kit form. 
         [0009]    In accord with the invention, a tank comprises a base to which is attached a cover or lid by means of to each other by alternative joints, including (a) a joint comprised of elastically engageable circumscribing rims; (b) a joint held together by clips; (c) a joint in which serrations on a skirt of the cover engage mating serrations on a cylindrical portion of the base; and (d) a joint wherein tabs on the rim of the cover part engage lip segments of a flange of the rim of the base, when the parts are mated and rotated relative to each other. A particular flange of the type (d) is configured with openings that enable manufacturing by means of injection molds that do not require slides. 
         [0010]    In further accord with the invention, embodiments of a tank base have a composite wall comprised of two layers. The inner layer, which presents as the surface of the concavity of the base is referred to as the liner. The other layer is referred to as the substrate and it is the structural component of the base. In the methods of the invention the liner is integrally adhered to the substrate layer during the manufacturing process. In preferred embodiments of the invention the substrate is a recycled thermoplastic and the liner is a much thinner layer of virgin plastic. 
         [0011]    In one method of making such tank bases, the liner is provided as a bag-like structure or sleeve which is put around the core of a plastic injection mold, which core defines the interior concavity of the base, prior to positioning of the core within the mold cavity. Plastic is injected into the mold to adhere to the bag or sleeve where it is positioned on the core. A method of so-using a sleeve is two-step. In the first step the plastic injected around the sleeve within the mold to form the substrate of the base; then the core is retracted a bit from the mold to expose the bottom of the substrate where there is no sleeve material, and a further injection of plastic is used to form the liner which surfaces the bottom of the concavity of the base. 
         [0012]    The foregoing and other features of the invention and their advantages will be more fully appreciated from the drawings and description which follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a side elevation view of a cylindrical water tank having a conical base and a detachable conical cover. 
           [0014]      FIG. 1A  is a side elevation exploded view of a cylindrical water tank having a conical cover, spout and cap. 
           [0015]      FIG. 2  is a vertical cross section through the base portion of the tank of  FIG. 1A , showing a composite wall. 
           [0016]      FIG. 3  is a fragmentary vertical cross section through the cover and base of a tank like that shown in  FIG. 1A , illustrating a particular engagement and sealing means between the parts. 
           [0017]      FIG. 4  is a view like  FIG. 3  showing a different means for engagement and sealing. 
           [0018]      FIG. 4A  is a cross section detail showing a modification of the rim of the cover of the tank of  FIG. 4 . 
           [0019]      FIG. 5  is a view like  FIG. 3 , showing another means for engagement and sealing. 
           [0020]      FIG. 6  shows a portion of the upper end of a tank base. 
           [0021]      FIG. 7  shows a fragment of the tank base of  FIG. 6  in combination with a cover, in exploded view. 
           [0022]      FIG. 8  shows is a fragmentary perspective view showing the cover and base of the tank of  FIG. 7  when they are engaged to form a joint. 
           [0023]      FIG. 9  is a vertical cross section of through the joint shown in  FIG. 8 . 
           [0024]      FIG. 10  is a partial view like  FIG. 8 , showing only the upper end of the base with a phantom view of a key which may be an integral part of a cover which locks. 
           [0025]      FIG. 11  is a partial view of the upper end of a tank base showing a particular rim configuration. 
           [0026]      FIG. 11A  is a vertical cross section through the rim of a tank base shown in  FIG. 11  along with parts of the core and cavity mold that are associated with injection molding of the tank base. 
           [0027]      FIG. 12  is a vertical cross section through the side of a tank base showing how a bag made of film is inserted and expanded outwardly to form a base having a two layer wall. 
           [0028]      FIG. 13A  is a vertical cross section through portions of a core and a cavity of an injection mold showing how a liner may be placed around the core before injection molding. 
           [0029]      FIG. 13B  is a view like  FIG. 13B  illustrating how a film sheet may be engaged by a downward moving core and pushed into the cavity of the mold, to form a liner of a tank base. 
           [0030]      FIG. 14  is a vertical cross section through a schematic mold to illustrate how a two layer base is formed, showing a core carrying a film sleeve, positioned within a cavity containing a molded base. 
           [0031]      FIG. 15  is a view of the apparatus in  FIG. 14  after the core of the mold has been retracted and further plastic has been injected to form the bottom of the liner of the base. 
           [0032]      FIG. 16  is a vertical cross section of a schematic injection mold showing a core which has a moveable insert, for use in making a base with a liner in a two-step procedure. 
           [0033]      FIG. 17  is a vertical cross section of a schematic injection mold, illustrating how a two-step injection process may be used to form a two-layer tank base. 
       
    
    
     DESCRIPTION 
       [0034]    In the present invention a water tank is injection molded, preferably made of polypropylene or polyethylene or other thermoplastic material. In an embodiment of the invention, there is a lower bucket-like part called the base, and an optional cover or lid which attaches to the base by means of a joint. The lid may be flat, dished, or as in the preferred embodiment which is described below, it may be generally conical. The lid may have a top portion which is adapted to receive a water pipe or it may have a spout with a cap. 
         [0035]      FIG. 1  is a side view of an exemplary molded tank  720  which has a truncated cone shape base  722  and cover  724  which is mated to the base by cover rim  733 . A circular boss  725  at the cover provides a region where a port may be cut to enable passage of a pipe through the cover. Tank  720  is representative of the kind of tank which may be made within the present invention. In the prior art, five hundred liter capacity commercial tanks having shapes like that of tank  720  may be made by rotational molding of thermoplastic. In the present invention, injection molding is used to obtain more precision of wall thickness and rim definition. For simplicity of illustration, certain exemplary tanks which are pictured in here have near-vertical walls. The features of such simple exemplary tanks may be embodied in tanks with more sloped conical walls, for example like those of tank  720 . 
         [0036]      FIG. 1A  is a side view of molded tank  20  which is comprised of nearly cylindrical base  22  and conical cover or lid  24 . The parts share a common central axis C-C, along which they are aligned. Cap  26  closes the spout  25  of the conical lid  24 . The joint  21  between the lid is only schematically shown in  FIG. 1A ; alternative optional joint configurations are described below. During use, the preponderance of the liquid which is stored in the tank is below the elevation of the joint, thus lessening the hydrostatic pressure which the joint has to resist. 
         [0037]    Tank base  22 , which as the following description indicates is preferably made by injection molding, has a circumscribing side wall  66  and a circular bottom wall  71 . The sidewall  66  of tank base  22  has a draft, typically at least 1-3 degrees, as characterizes typical injection molded products, to enable easy removal from an injection mold. In practice, the draft angle is preferably chosen to be sufficiently large, for instance 6 degrees or more slope per side or more, to enable nesting of bases for shipment or storage. Wall  66  may have a thickness of about 3 to 4 mm. Lid  24  has a wall thickness which is similar to that of the base; the lids are also nestable. The lid and the walls of the base of a tank may be corrugated for strength; alternately the lid and walls may have ribbing, according to what may be accomplished within the limitations of the manufacturing technique. 
         [0038]      FIGS. 3-7  show different means for sealing a joint and means for holding the cover and base together. It will be apparent that the seals and joint locking means may be used in different combinations than the particular embodiments which are pictured. 
         [0039]      FIG. 3  is a partial vertical cross section of tank  220  which has a cover  224  that aligns along tank vertical axis C-C and mates with base  222  as indicated by the arrow. Cover  224  has a circumscribing skirt  34  with an inward facing lip so that in cross section the skirt has a hook-like terminal end. O-ring seal  36  is captured in a groove within the recess within skirt  34 . Base  222  has a rim  32  that comprises an outward facing extending lip, which in cross section provides a hook like terminal end. When the parts  224 ,  222  are mated as indicated by the arrow, skirt  34  of the top edge springs elastically outwardly due to interference-engagement with rim  32  of the base; and, then the skirt  34  springs back inwardly due to the elastic resilience of the cover material; whereupon the hook ends  32 ,  34  are engaged with each other. The dimensions of the skirt  34  and lip of rim  32  are such that when they are engaged, the vertical spacing between the upper surface of the top edge of the base and the base of the groove which circumscribes the edge of the cover is such that the O-ring contained in the groove is compressed by the top edge  33  of rim  32 , thereby accomplishing sealing of the joint. Skirt  34  may have circumferentially spaced part vertical slits to ease its radially outward expansion during the engagement process. 
         [0040]      FIG. 4  is a half cross section view of part of tank embodiment 320, showing how cover  324  mates with base  322 . The rim  38  of the base comprises a shelf upon which rests wedge shape elastomer gasket  42 . As the rim  40  of the cover is engaged with rim  38  of the base, skirt  41  pushes the narrow edge of gasket  42  radially outward and downward. Thereafter, the resilient force of the seal against the skirt  41  both forms a seal at the joint and frictionally retains the cover in place. A multiplicity of C shape clips  39 , made from such as high strength plastic or spring steel, hold the cover in place. The clips clamp rim  40  of the cover to rim  38  of the base by engaging respectively a groove and a lip thereon, as indicated by the dashed arrow lines in  FIG. 4 .  FIG. 4A  shows the outer portion a modified cover  324 A for which a clip  39  is not used. The outer skirt of rim  40 A has a barb or hook-like cross section terminal end  334 , which terminal end resiliently engages the lip  37  on the underside of the rim  38  of base  322 . With respect to the description which follows next, the terminal end  334  can be considered to comprise a circumscribing ridge. 
         [0041]      FIG. 5  is a fragmentary view like  FIG. 3  and  FIG. 4 , showing the outermost portion of a tank  420  which is comprised of base  422  and mating cover  424 , as indicated by the arrow. The interior of the sidewall of the base has a serrated surface comprised of two circumferential Vee shape grooves  50 . The uppermost edge of each Vee groove is nominally horizontal. Optionally there may be fewer or more grooves. The outer surface of skirt  53  of cover  424  is serrated in that it has circumscribing ridges  52 . The two ridges  52  are shaped for mating engagement with the grooves  50 . When the cover and base are pushed together, the wall of the base is deflected outwardly a bit and the skirt  53  of the cover is deflected inwardly a bit. The ridges and grooves likewise elastically deform. When the skirt sufficiently enters the bore of the upper end of the base, ridges  52  snap into grooves  50 . A resilient seal  46  circumscribing the upper end  44  of the base is compressed by contact with the underside of laterally extending flange  48  of the cover, to make a tight joint. Alternatively, the seal may be positioned within a groove (not shown) which may be formed in the underside surface of flange  48  of the cover, and the upper end  44  of the base will then be shaped appropriately for sealing engagement. 
         [0042]      FIG. 6-8  are related.  FIG. 6  shows a portion of tank base  522  and  FIG. 7  shows a portion of cover  524  which engages the base to form a tank  520 . During engagement to form a joint with the base, cover  524  is rotated through a slight arc relative to the base, to reach a final position where the tabs of the lid underlie inward extending lip segments of the rim flange of the base. See  FIG. 8 . Cover  524  is shown in the form of simple lid for simplicity of illustration; it may alternately have a conical shape like covers  24 ,  724 . 
         [0043]    With reference to  FIG. 6 , base  522  comprises sidewall  66  which terminates at upper edge  60 . Rim  55  comprises the upper end of sidewall  66  and flange  54 . Flange  54  has a first portion  67  which extends radially outwardly from the wall  66 . Flange  54  curves upwardly from portion  67  and then runs radially inwardly in the form of lip segments  56  which are spaced apart by cutouts  58 . The lip segments extend inwardly over cavity  73  which is defined by the flange  54 . See  FIGS. 6 and 7 . 
         [0044]    The cover  524 , which is shown in  FIG. 7  as it is being lowered onto the base  522  (see the arrow), has outwardly extending tabs  64  which are shaped to fit into cutouts  58  of the rim of the base. Preferably, tabs  64  have an upper surface which is sloped in the circumferential direction, upwardly from the plane of the circle of the rim of the cover. As indicated by the arrows in  FIG. 7  and  FIG. 8 , the cover is mated with the base so the tabs first pass through cutouts  58 ; the cover is then rotated relative to the base so the tabs  64  move underneath the inward extending lip segments  56  of the rim  55 . Because of the engagement of the sloped upper surfaces of the tabs  64  with the lip segments  56 , the cover is forced closer to the base upon rotation. 
         [0045]    With reference to the perspective view of  FIG. 7  and the cross section view of  FIG. 9 , during the foregoing engagement seal  65  which is contained within a groove in the cover is simultaneously compressed by contact with upper edge  60  of the sidewall of the base—as the cover thrusts downwardly with increasing rotating engagement of the cover with the base. In an alternative embodiment, the tabs  64  have flat top surfaces and the undersides of the lip segments  56  are sloped, to achieve comparable vertical compression of the seal. In still another embodiment, the tabs and undersides of lip segments  56  are flat and there is no downward-thrusting wedging action. 
         [0046]    In one embodiment of tank  520  the cover and base of the tank  520  will stay engaged due to friction between the parts of the cover and base at the joint location. In another embodiment, exemplified by the parts shown in  FIG. 8  and the cross section of  FIG. 9 , cover  524  optionally has a molded-in key  68  which projects radially outwardly from the exterior surface of the cover. There may be more than one key. Exemplary key  68  is circumferentially located relative to the tabs  64  of the cover, so that when the cover is mated with the base so make the tabs slip into cutouts  58 , the bottom side of the key presses downwardly on a lip segment  56  of the rim  55 .  FIG. 9  shows how key  68  presses down on the lip  56 , deflecting it as shown by phantom  56 P in  FIG. 9 . When the cover is then sufficiently rotated, key  68  comes to one of the cutouts and the lip  56  springs upwardly, to thereby limit further rotational motion and potential loosening of the cover. 
         [0047]    In another embodiment of tank  520  which comprises a lock on the lid, illustrated by  FIG. 10 , the inward edge of one or more of lip segments  56 A of the base rim  55 A has a laterally extending wedge portion  72  that causes a lip segments  56 A to be elastically deflected radially outwardly when the cover and integral molded key  68 A (shown in phantom) is rotated as indicated by arrow P. The web shape portions  72  prevent rotation of the cover, in the loosening direction which is opposite to the direction of the arrow P in  FIG. 10 . The wedges on the inner edges of the lip segments which are shown in  FIG. 10  may be used in combination with a cover having tabs  64  that are shown in  FIG. 7 . Such a resultant cover and base combination would comprise a cover which would not be susceptible to easy removal, which may sometimes be desirable. 
         [0048]    It should be appreciated that various of the foregoing joint configurations comprise a structural connection between the parts, and the cover thus provides desirable increased hoop strength to the tank at the joint location. When that is achieved, the wall of a tank part might be made thinner than would otherwise be feasible. 
         [0049]    In general when manufacturing injection molded tanks it is desirable to avoid the use of mold portions which have slides (internal moveable parts). Artisans may consider that making several of the covers and bases according to the particulars of the pictured embodiments here may require molds having slides. In particular the rim  55  and its inward extending lip segments  56  may be thought to require such. Whatever,  FIG. 11  and  FIG. 11A  illustrate how a base having a rim with features of the foregoing embodiments can be made without the use of mold slides. 
         [0050]      FIG. 11  is a partial downward perspective view of the upper portion of the base  622  which is like the base  522  shown in  FIG. 6 , but for the details which are now described. Rim  55  is configured in a way that enables injection molding of the base while using a straight-pull core and cavity mold combination; in particular rim  55  has a plurality of spaced apart holes  57  which underlie the inward extending flange portions  56 B. 
         [0051]      FIG. 11A  is a vertical cross section through the rim portion of base  622 . The Figure also shows a portion of the cavity mold  59  and (in phantom) a portion of the core mold  63  which during opening and closing of the mold move relative to each along line  65 . The arrows indicate the positions to which the mold parts  59 ,  63  move during the plastic injection phase of molding. Referring to both  FIG. 11  and  FIG. 11A , tank base  622  has a wall  66  which terminates at upper end  60 . A plurality of spaced apart inwardly-extending flange lip segments  56 B are shaped to engage the tabs of a tank cover, like the tabs  64  shown in  FIG. 7 . The lip segments  56 B are formed between the top of pedestal  61  of mold cavity part  59  and the underside of mating mold core part  63 . During molding, the top of the pedestal  61  defines the underside of a lip segment  56 B. At the same time a plurality of spaced part holes  57  are (necessarily) created. A benefit of the holes  57  is to allow dirt and water which enters the cavity  73  flange to escape. Cutouts  58 B, which enable tabs  64  of a cover to be vertically slipped downwardly before the cover is rotated, are formed by the mold core part  63 . 
         [0052]    One of the features of the tanks in accord with the present invention is that, if it is necessary to clean the tank, (ignoring that the user has to defeat any lock) the cover can be removed from the base to provide much better access than can be obtained through a port on the cover of the tank. The unique rim and seal configurations which are disclosed above may be used in combination with cover and base tank portions which have flanges that are screwed or bolted to one another. 
         [0053]    The cover and base of a tank may be made of a single composition material. Alternatively, for economic reasons, it may be desirable to fabricate a tank predominately of a structural plastic material which might be less pure than virgin plastic material, and to combine that with a liner made of superior quality material which contacts the water. The aim is to guard against the possibility that potable water in the tank could be influenced by leaching of something from the less than virgin pure material of the substrate of the tank. In another respect, it may be desirable to have a tank with a dark color exterior and a light color interior, so the cleanliness of the interior can be better visually inspected. The following describes how tanks having a two-layer wall, also called a composite wall, may be formed. 
         [0054]      FIG. 2  is a vertical cross section through the base  122  of a tank like tank  20  shown in  FIG. 1 . The tank embodiment has a circumscribing side wall  166  and a bottom wall  171 . The preferred walls comprise first portion or layer  28  and a second portion or layer  30 . The inner layer  30 , which presents on the surface of the concavity of a base, is often referred to here as the liner. The outer layer is often referred to here as the substrate. It will be understood that the term first layer may be used interchangeably with substrate, applying to the same thing. 
         [0055]    The substrate is typically much thicker than the liner. For purposes of illustration the tank is shown in simplified form, the thicknesses of the wall elements are exaggerated, and any familiar but optional substantial angling of the walls is not portrayed. In an exemplary tank, a liner  30  may have a thickness of about 0.01 to about 0.4 mm and the total wall thickness may be about 1.5 to about 3.2 mm. In an exemplary tank, the material of the base substrate may be a recycled polypropylene or polyethylene and the lining  30  may be virgin polypropylene, another polyolefin, or another thermoplastic. 
         [0056]    The base  122  of a tank comprising a liner  30  may be constructed using alternative manufacturing techniques. In one approach, an essential tank base  28  (i.e., a base having a single layer and no liner; the same as a substrate) is molded and removed from the mold. The interior of part  28  is then coated by spraying or deposition, to create on the interior a surface layer or liner having desired chemical properties, e.g., suitable for contact with potable water. For example, a urethane plastic coating or a plastic-adherent paint may be applied. 
         [0057]    In an alternative approach, as illustrated by  FIG. 12  which is a vertical cross section through an essential tank base  28 A, a bag  30 A made of film may be inserted and expanded outwardly to form a two layer wall tank part. Bag  30 A has a shape which is slightly smaller than the interior cavity of part  28 A; the bag is placed within the cavity of part  28 A and is radially expanded, as by gas pressure, preferably while being heated, so that it conforms to and becomes attached to essential base part  28 A, thus becoming lining  30  as shown in  FIG. 2 . Attachment of the lining  30 A to the interior of base part  28 A may be achieved by means of an adhesive applied to the interior cavity surface of part  28 A, or by other treatment of the mating surfaces. 
         [0058]    In another approach, a plastic film may be placed over the core of an injection mold which during molding defines the interior of the tank base part.  FIG. 13A  is a vertical cross section of a largely schematic plastic injection mold comprising a cavity  84  and core  82 . As shown, liner  30  is placed on the core before the core is moved into the cavity  84  part of the mold. The arrow Y shows how the core bearing the liner on its exterior moves into a position (indicated by the phantom  82 P of the core) within the cavity  84  for injection molding. In the molding step, plastic is injected through ports  86  to form the wall and bottom of the base. The plastic flows around the core and becomes adhered to the plastic film  30  carried upon the core. Thus a base  122  like that shown in  FIG. 2  is created. 
         [0059]      FIG. 13B  shows a similar approach to making a base, one in which the liner is not preplaced as a bucket-shape preform on the core, but is provided as a plain sheet.  FIG. 13B  is a vertical cross section of a mold assembly like that of  FIG. 13A  and is very simplified from the complexity of a real apparatus. A sheet  90  of film is drawn from roll  88  and runs between rollers  92 , so that it lies in the path of the core  82  as the core closes (indicated by the arrow Y). As the mold is closed, a portion of film  94  is created with a deformed shape caused by wrapping around the core, as indicated by the arrow pair, and the film portion  94  is pushed into the recess of the mold cavity part  84 . The film portion  94  may be clipped from the rest of the sheet  90  of film. Then plastic is injected through ports  86  to form substrate  28 B shown in phantom; and in doing that the plastic flows around the core and becomes adhered to the plastic film carried by the core. Thus the film portion  94  becomes a layer  30  as shown in  FIG. 2  when the tank base is removed from the mold. Depending on the nature of the film  70 , it is likely that the film which is carried by the core will be wrinkled, particularly at the center of the apex of the core. To address that, the chosen film may be a heat-shrinkable (irradiated) plastic film, and a high heat transfer rate heater may be used to shrink the film after it is gathered on the core, but before the core is inserted into the cavity, given suitable spacing of the core from the cavity and suitable heating means. 
         [0060]    Two other techniques which involve two-step formation of the liner may be used, to lessen any potential film wrinkling issue. A first two step process is illustrated by means of  FIG. 14-15  which are cross sections through an essential base  28 C.  FIG. 14  shows the base as it has been formed by injection molding in a mold comprised of core  82 C and cavity  84 C, shown in phantom. In a first step, a substantially cylindrical sleeve  96  of plastic film is first put around the core  82 C, the core is then positioned within the cavity and plastic is injected into the mold through ports  86 C to form the essential base  28 C which is thereby adhered to the sleeve, as illustrated in  FIG. 14 . Thus, the sleeve forms the circumferential wall portion of a liner  30 . In the second step of the process, as illustrated by  FIG. 15 , the core  82 C is retracted a small distance D from the cavity to create a space between the bottom of the core and the upper surface of the bottom wall of essential base  28 C. Then plastic having desired liner characteristics is injected through port  87 C into the space to form the portion  93  of the liner. The portion  93  covers and adheres to the upper surface of the bottom of the essential base  82 C. When core  82 C is retracted a distance D in step  2 , as above, it is withdrawn relative to sleeve  96 , thereby exposing the bottom end of the sleeve, which bounds the lateral periphery of said space. Thus, when plastic that forms portion  93  is injected, it flows against and adheres to the bottom end of the sleeve; and thus a tank base having a liner  30  like that shown in  FIG. 2  results. 
         [0061]      FIG. 16  is like  FIG. 14 . It illustrates another approach for forming a base comprising a liner. In the method associated with  FIG. 16 , sleeve  96 D is carried by the core  82 D in a way similar to way in which sleeve  96  is carried as on the core, as described in connection with  FIG. 14 . Compare to the simple cylinder of sleeve  96  described above, sleeve  96 D is shaped as a cylinder with a partially closed end  97 . The end  97  is flat disk with a central hole. Core  82 D comprises a moveable insert portion  98 . In a first insert position, the bottom surface of the insert mates aligns with the bottom surface of disk  97  to present a flat surface. In that condition, the essential base  28 D is formed by injection of plastic into the mold space defined by the core  82 D and cavity  84 D; and the foregoing flat surface defines the molded interior surface of the bottom of the essential base  28 D. At the same time, disk portion  97  of sleeve  96 D thereby becomes integral with the essential base  28 D. Next, insert  98  is moved away from the bottom interior surface of the base  28 D (upwardly in the picture) and into insert cavity  95  of the core  82 D. Then further plastic, having a composition consistent with the composition of liner  96 D is injected into the mold through channel  87 D, to fill the space between the retracted insert and the interior bottom of the previously formed essential base  28 D. 
         [0062]    The further plastic contacts and adheres to both the central part of the essential base  28 D and the inner edge of the disk  97 , so that a base with full liner is thereby created. The core is then retracted from the cavity and the base with liner is ejected from the cavity. 
         [0063]    Still another approach is illustrated by  FIG. 17  which is a vertical cross section of a largely schematic plastic injection mold. The mold comprises core  82 B and cavity  84 B. The essential base (substrate)  28 B of a tank is shown after it has been conventionally formed by injection of plastic into the mold. The core  82 B is shown after it has been retracted a distance G from the cavity  84 B. A second shot of plastic material is next injected through the port  87 B in the core, so that the material flows as indicated by the small arrows, to form a second material layer in the space  35 . The second material flows against and adheres to the essential substrate and thus becomes a liner  30  that is integral with the base  28 B, as illustrated by  FIG. 2 . 
         [0064]    Bases made in accord with the invention may have additional layers. For example, a further layer may be applied to the exterior of the substrate to change the color appearance of the tank base. Covers may be fabricated by the same essential processes used for making bases. 
         [0065]    The invention, with explicit and implicit variations and advantages, has been described and illustrated with respect to several embodiments. Those embodiments should be considered illustrative and not restrictive. Any use of words of orientation, such as top, base, horizontal, etc., will be appropriately applied to an object which is oriented differently from the embodiments pictured in the accompanying drawings. Any use of words such as “preferred” and variations suggest a feature or combination which is desirable but which is not necessarily mandatory. Thus embodiments lacking any such preferred feature or combination may be within the scope of the claims which follow. Persons skilled in the art may make various changes in form and detail of the invention embodiments which are described, without departing from the spirit and scope of the claimed invention.