Abstract:
A mold assembly is provided for injection molding a plastic container that is adapted to releasably interlock with a lid for sealing off the container. The mold assembly is arranged to mold a container that is characterized by a rim section that projects inwardly of the inner surface of the container&#39;s side wall and has an annular locking channel for receiving a locking rib on the lid. The mold assembly comprises a core member, a first cavity member that cooperates with the core member to form a first mold cavity section that is shaped to mold the bottom and side walls of the container, and a second cavity member that cooperates with the core member and the first cavity member to form a second mold cavity section that is an extension of said first mold cavity section and is shaped to mold the rim section of the container.

Description:
This application is a continuation-in-part of copending application Ser. No. 09/281,367, filed Mar. 30, 1999. 
    
    
     The present invention relates generally to manufacture of plastic containers for containing paint or other materials, and more particularly to a new apparatus and method for injection molding plastic containers that are adapted to be sealed off by removable lids. 
     FIELD OF THE INVENTION 
     As is well known, the ordinary one-gallon paint can has been made of steel and is provided with a friction fit lid that also is made of steel. In the paint industry prevention of leakage is important since paint cans frequently encounter rough handling while being transported or stacked for storage or retail display. Consequently a substantially hermetic seal is required between the paint can and its lid. This is achieved by a friction fit air-tight engagement between the lid and container which is such as to permit the lid to be removed manually using a suitable prying tool. In addition, the standard metal paint can lid does not protrude beyond the perimeter of the paint can so that as to prevent accidental disengagement of the lid. The configuration of the interlocking connection between the standard metal paint cans and their metal lids is such that the lids remain tightly in place even when subjected to the action of paint shaking machines or to other severe handling or shock conditions. Further the lids can be re-attached to again provide a fluid tight seal with the container. However standard metal paint cans have certain shortcomings, one of which is the tendency to corrode. 
     In the past, efforts have been made to provide containers for paint that are made of plastic. For example, U.S. Pat. No. 5,097,977, issued Mar. 24, 1992 to R. Straub illustrates a closure assembly for a container that comprises a snap ring connected to the top of the container and a lid that is removably attached to the ring so as to close off the container. A similar arrangement is disclosed by U.S. Pat. No. 4,619,373, issued Oct. 28, 1986 to H. W. Galer. Other plastic paint can designs and/or apparatus for injection molding same are illustrated by the following U.S. Pat. No. 4,777,004, issued Oct. 11, 1988 to H. W. Galer; U.S. Pat. No. 4,619,373, issued Oct. 28, 1986 to H. W. Galer; U.S. Pat. No. 4,349,119 issued Sep. 14, 1982 to I. Letica; U.S. Pat. No. 4,512,494, issued Apr. 23, 1985 to J. W. Von Holdt; U.S. Pat. No. 4,383,519 issued May 17, 1983 to I. Letica; U.S. Pat. No. 4,293,080, issued Oct. 6, 1981 to I. Letica; and U.S. Pat. No. 3,977,563 issued Aug. 31, 1976 to W. G. Holt. 
     However, prior plastic paint can/lid designs have suffered from various limitations, such as need for complex and costly injection molds, not capable of being handled by standard filling, labeling and packaging machinery, inadequate strength, unreliable sealing of lid to container, and/or lack of appeal to prospective customers. 
     A new plastic container/removable lid construction is disclosed and claimed in my copending U.S. application Ser. No. 09/281,367, filed Mar. 30, 1999. The plastic container construction disclosed in my copending application offers numerous advantages. It has a one-piece construction free of any seams or crimps, does not rust internally and requires no internal protective coating, has a higher dynamic compression that metal paint cans, can be manufactured in different colors and surface finishes, weighs less than a metal can of comparable size and volume, can be molded with embossed printing so as to eliminate the need for a subsequent labeling operation, and is adapted to be closed off by a complementary lid that makes an air-tight seal and can be removed and replaced without damage. The container rim and a complementary lid are adapted to interlock in a manner which provides an air-tight friction fit, permits the lid to be easily removed by use of a prying tool, and assures that the lid cannot be accidentally dislodged as a consequence of being subjected to impact, shock or stress in the course of being stacked or transported. 
     SUMMARY OF THE INVENTION 
     The primary object or purpose of the invention is to provide a new and improved injection molding apparatus for use in manufacturing plastic containers that embody the construction disclosed and claimed in said copending U.S. application Ser. No. 09/281,367. 
     A more specific object is to provide an injection mold apparatus for manufacturing one-piece plastic containers having lid-receiving rims that project inwardly of the side walls of the containers. 
     Another specific object is to provide an injection mold assembly for molding plastic containers that does not require a collapsible core. 
     A further object is to provide an improved method of injection molding an improved plastic container for use in storing paint or other material. 
     A further is to provide a novel method and apparatus for manufacturing a plastic container that is adapted to releasably interlock with a lid in a manner that provides positive line contact sealing of the container. 
     Another object is to provide a novel method and apparatus for injection molding a one-piece, substantially straight-sided plastic container for paint or other liquid or particulate material that is characterized by a rim-to-lid interlock which provides an air-tight friction fit, permits the lid to be easily removed by use of a prying tool, and assures that the lid cannot be accidentally dislodged as a consequence of being subjected to impact, shock or stress due to rough handling in the course of being stacked or transported. 
     Still other objects and features of the invention are disclosed or rendered obvious by the following detailed description which is to be considered together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded sectional view in elevation showing a container and a lid therefor that embody the invention disclosed in said copending application Ser. No. 09/281,367; 
     FIG. 2 is an enlarged scale fragmentary sectional view in elevation showing details of the rim on the upper end of the same container; 
     FIG. 3 is a sectional view taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is an enlarged fragmentary sectional view in elevation of the lid; 
     FIG. 5 is a fragmentary sectional view on an enlarged scale showing how the lid interlocks with the rim of the container; 
     FIG. 6A is a schematic sectional view in front elevation of a mold assembly embodying the present invention in fully closed position; 
     FIG. 6B is a schematic sectional view is side elevation of the same mold assembly in closed position; 
     FIGS. 7A to  10 A are additional sectional views in front elevation that illustrate how the mold is operated; 
     FIGS. 7B to  10 B are schematic sectional views in side elevation that illustrate different mold positions correspond to the positions shown in FIGS. 7A to  10 A respectively; 
     FIG. 11 is a fragmentary sectional view on an enlarged scale showing the core plate latching mechanism with the mold assembly in the fully closed position; 
     FIG. 12 is a fragmentary sectional view on an enlarged scale of the components of the mold assembly for molding the rim section and one of the ear sections of the container shown in FIGS. 1-3; 
     FIG. 13 is a fragmentary sectional view similar to FIG. 12 taken at a position that is located approximately 90° away from the viewpoint of FIG. 11; 
     FIG. 14 is a fragmentary sectional view taken along line  14 — 14  of FIG. 12; and 
     FIG. 15 is an enlargement of a portion of FIG.  13 . 
    
    
     DESCRIPTION OF CONTAINER AND LID 
     FIG. 1 illustrates an injection-molded substantially straight-sided container  2  and a lid  60  that embody the invention disclosed and claimed in said copending U.S. application Ser. No. 09/281,367. The disclosure of that copending application is incorporated herein by reference. 
     Container  2  is made of a suitable plastic material that provides an adequate combination of resiliency and strength, e.g., high density polyethylene. Container  2  comprises a side wall  4 , and a bottom wall  6  which preferably is contoured as shown to provide a flat annular downwardly projecting rib  8  for strengthening purposes. Side wall  4  is a substantially constant diameter cylinder. However, if desired, side wall  4  may be tapered so that the upper end has a slightly larger diameter than its bottom end. The bottom end of the paint can also has an axially extending seating flange  10  that forms a continuation of side wall  4 . The side wall also has two diametrically opposed perforated ears  12 . As seen in FIGS. 2 and 3, ears  12  comprise a curved side wall  14  that extends through an angle of at least 180° degrees, preferably about 200°, and a front wall  16  that has a tapered hole  18  for acceptance of one end of a wire handle (not shown) of the kind commonly used on metal paint cans. Hole  18  serves as a pivot point for the wire handle. 
     Referring specifically to FIG. 2, the upper end of the side wall  4  is formed with a split or bifurcated rim, the rim comprising an outer rim section  20 , an inner rim section  22 , and a rim-connecting section  24 . The outer rim section  20  is essentially an extension of side wall  4  and has an outer surface  26  that preferably, but not necessarily, projects radially slightly beyond the outer surface  28  of side wall  4 . Surface  26  may be a straight cylinder or, as shown, may extend at a slight angle to outer surface  28 . Preferably, but not necessarily, the upper end edge of outer rim section  20  is rounded off as shown at  30 . The inner surface of outer rim section  20  is identified generally by numeral  32 . Inner surface  32  extends at a selected acute angle, e.g., an angle between 6 and 7°, to side wall  4  and the longitudinal center axis of the container. Preferably, but not necessarily, the diameter of the upper end of inner surface  32  is enlarged so as to provide an offset or recessed cylindrical surface portion  34  that extends substantially parallel to the longitudinal (vertical) axis of container  2 . The inner surface  32  also is formed with two locking or gripping ribs  36  that are convex in cross-section and preferably extend around the full circumference of the container rim. Alternatively, the ribs  36  could be interrupted at selected points about the circumference of outer rim section  20 . 
     The inner rim section  22  is located inwardly of side wall  4 . Rim section  22  has substantially parallel outer and inner surfaces  40  and  42 , with at least surface  40 , but preferably also surface  42 , extending at a selected acute angle, e.g., an angle between about 9° and 10°, to the side wall  4 . Preferably, but not necessarily, surface  40  of rim section  22  is smooth. However, it could also be provided with gripping ribs similar to ribs  36 . Preferably, but not necessarily, the upper end edge of rim section  22  is rounded as shown at  44 . 
     Preferably but not necessarily, the rim-connecting section  24  is formed with a generally concave upper surface  46 . The bottom surface  48  of section  24  preferably forms a gentle curved transition between the inner surface  42  of inner rim section  22  and the inner surface  50  of side wall  4 . 
     Surfaces  32 ,  40  and  46  together define an annular locking channel for a lid  60  hereinafter described. In this connection, it should be noted that the surface  40  of inner rim section  22  is not parallel to the inner surface  32  of outer rim section  20 ; instead those surfaces are in a converging relation with one another away from rim-connecting section  24 . Preferably they converge on one another at an angle of between about 2° and 4° with increasing distance from bottom wall  6 . In other words, the spacing between surfaces  32  and  40  is greatest near surface  46  and smallest near the top end of rim section  22 . 
     The outer rim section  20  is provided with one or more notches  58  at its upper edge (FIGS. 1 and 2) to facilitate removal of a plastic lid or cover  60 . Lid  60  preferably is made of the same material as container  2 . The lid is circular and comprises a generally flat center or crown section  62  that preferably, but not necessarily, is dimpled at its center as shown at  64 , and a convoluted rim section identified generally by the numeral  66  that is adapted to mate with the bifurcated rim section of container  2 . 
     As seen best in FIG. 4, the convoluted rim section  66  of the lid or cover is characterized by a first upstanding circumferentially-extending rib that comprises an inner wall or leg section  68  that is joined to an outer wall or leg section  70  by a curved connecting wall section  72 . The inner section  68  has an outer peripheral surface  74  that is substantially cylindrical and parallel to the center axis of the lid, while the outer section  70  has an inner circumferentially-extending surface  76  that is canted with the respect to the wall surface  74 . Surface  76  is slanted extending downwardly and inwardly at an angle to the center axis of the lid that is approximately the same as the angle of the surfaces  32  and  40  relative to the center axis of the containers. Preferably, surface  76  extends at an angle of about 7° to 10° to the center axis of the lid. 
     The wall section  70  also forms part of a second downwardly projecting rib that also comprises an outer wall section  80  and a curved connecting wall section  82 . Outer wall section  80  also has an outer surface  84  that extends at an angle that preferably is substantially the same as the angle of the surface  76 . Alternatively, wall section  80  may be formed so that the angle of outer surface  84  relative to the lid&#39;s center axis is slightly greater than the angle of surface  76 , e.g., 1°-3° greater. The upper end of wall section  80  has an outer peripheral surface portion  86  that is essentially cylindrical and is parallel to the center axis of the lid. Surface portion  86  projects outwardly beyond surface  84 , so as to form a shallow shoulder or ledge  88 . Additionally the outer surface  84  is provided with a pair of locking or gripping ribs  90  that preferably are convex in cross-section as seen in FIG.  4 . Ribs  90  are designed to mate and interlock with the similarly shaped ribs  36  formed on the container rim. Ribs  90  preferably extend around the full circumference of surface  84 , but alternatively they could be interrupted at selected points about the circumference of surface  84 . 
     Making the container and lid of a resilient strong material such as a high density polyethylene is advantageous, particularly in the case of making one gallon paint cans, in that the material provides the container with sufficient strength to resist deformation under the weight of one or more like-filled containers. At the same time, the plastic material can flex sufficiently to allow the lid to be secured in place on the container so as to seal off the container&#39;s contents. 
     The downwardly projecting rib on the lid formed by wall sections  70 ,  80  and  82  is designed to make a friction fit in the channel formed between the outer and inner rim sections  20  and  22  of the container. The distance between the surfaces  76  and  84  of the downwardly projecting rib of the rim may be equal to but preferably is slightly in excess of the distance between the surfaces  32  and  40  of container rim sections  20  and  22  respectively. However, that rib is sufficiently resilient as to allow sections  70  and  80  to be forced toward one another under a radial compressing force. Consequently, as shown in FIG. 5, when the lid is attached to the rim section of the container, the depending rib comprising wall sections  70 ,  80  and  82  makes a tight friction fit in the channel between rim sections  20  and  22 , with the gripping ribs  90  interlocking with gripping ribs  36 . 
     When the lid is attached to the container, its periphery is surrounded and protected by the upper end of rim section  20 . The maximum outside diameter of the combined container and lid is essentially the outside diameter of the outer rim section  20  measured at the upper edge of its outer surface  26 . Since that diametrical dimension is nearly the same as that of the outer diameter of wall  4 , the container with the lid attached has an appearance substantially the same as a sealed conventional metal paint can. Removal of the lid from the can is facilitated by the presence of notches  58  in the upper end of rim section  20 . Notches  58  permit a screwdriver or other tool to be engaged with shoulder  88  to pry the lid off of the container. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the mold assembly of the present invention is a mold assembly as shown in the drawings that is designed to produce a one gallon container having the construction shown in FIGS. 1-3. 
     Referring now to FIGS. 6A and 6B, there is shown a mold assembly that comprises a first or front plate  102 , a second or back plate  104 , an ejector plate  106 , a core support plate  108 , and a support plate  110 . Use of the terms “front” and “back” is premised on the fact that in conventional injection molding machines the molds are generally oriented horizontally, i.e., rotated 900° from the position shown in the drawings, and they open and close by relative movement along a horizontal axis. However, the mold assembly is illustrated with a vertical orientation in the drawings for the purpose of making it easier to understand its construction and mode of operation. 
     The front plate  102  is adapted to be securely mounted by threaded bolts  103  to a stationary platen (not shown) of an injection molding machine (also not shown), while back plate  104  is adapted to be mounted by additional threaded bolts  105  to a movable platen (not shown) of the same injection molding machine. Front plate  102  has a sprue hole that is fitted with a hollow sprue bushing  107  for connection to a source of plastic material to be injected into the mold assembly. A plurality of leader pins or guides  112  (only one of which is shown in FIG. 6A) are fixed to and extend between back plate  104  and support plate  110 . A second plurality of leader pins or guides  113  (only one of which is shown in FIG. 6A) are fixed to support plate  110  and mounted in telescoping relation to guide bushings  115  attached to front plate  102 . A support pillar in the form of a solid cylindrical rod  114  is fixed to back plate  104  and extends toward the support plate  110 . Ejector plate  106  and core plate  108  have slide holes  117 ,  119  through which leader pins  112  extend, with the slide holes being sized so that leader pins  112  prevent lateral movement of the ejector and core plates while allowing them to move lengthwise of the leader pins toward and away from support plate  110 . Ejector plate  106  also has a slide hole  121  through which extends support pillar  114 . Pillar  114  terminates a predetermined distance from back plate  104 , and functions as a rear stop member for core plate  108 . A second support pillar in the form of a solid cylindrical rod  116  is attached to and partially overlaps the adjacent end of support pillar  114 . Support pillar  116  extends through a slide hole  123  in core plate  108  and engages support plate  110 . Support pillars  114  and  116  together prevent support plate  110  from collapsing away from front plate  102  under molding pressure when the mold assembly is closed and injected with plastic as described hereinafter. Four pressure pins  139  (only one of which is shown) are slidably mounted in bushings  141  affixed to front plate  102 . Pins  139  and bushings  141  are distributed in a rectangular pattern around cavity member  174  (described hereinafter). A coil compression spring  143  in bushing  141  urges pin  139  toward core support plate  108 . The purpose of pin  139  is to urge plate  108  back away from plate  110  as the mold assembly moves from the fully closed position of FIGS. 6A,  6 B to the open position shown in FIGS. 9A,  9 B and  10 A,  10 B. 
     The ejector plate  106  is adapted to be connected by a threaded member  125  to an operating member of the injection molding machine (not shown) which moves it toward and away from the front plate  2  during the injection molding cycle described hereinafter. Member  125  extends through a hole  127  in back plate  104  that is sized to allow reciprocal axial motion of member  125 . Fixed to ejector plate  106  is a poppet  118  having an enlarged head  120  at its front end. Poppet  118  extends slidably through a hole in core plate  108 . 
     A cam bar  122  is attached to the periphery of support plate  110 . Cam bar  122  projects rearwardly from support plate  110  toward back plate  104 . Attached to ejector plate  106  in line with cam bar  122  is a latch bar  124 . Bar  124  is slotted longitudinally as indicated at  126  to slidingly receive cam bar  122 . The open side of slot  126  faces core plate  108 . Referring now to FIGS. 6 and 11, the inner edge of latch bar  124  is notched as shown at  128 , and the forward end of the latch bar preferably has a projection  130  that extends into a notch  132  in the periphery of core plate  108 . The notch  128  serves to receive a flat detent pin  132  that is mounted in a radially-extending hole  134  in core plate  108 . A threaded lock pin  136  screwed into a tapped hole in core plate  108  extends through an elongate hole  137  in detent pin  132  to limit axial movement of the detent pin in hole  134 . A compression spring  138  in hole  134  urges detent pin  132  into notch  128 . The back edge of notch  128  forms a flat shoulder  140  which is intercepted by the detent pin  132  when the pin is in its extended position (FIG.  11 ). Consequently when ejector plate  106  is moved forward toward front plate  2 , the movable latch bar  124  acts through detent pin  132  to releasably lock core plate  108  to ejector plate  106 , thereby causing the core plate to move forward with the ejector plate. 
     However, cam bar  122  has an inclined inner edge cam surface  142  at its back end. Surface  142  is positioned to engage detent pin  132  when core plate  108  moves with ejector plate  106  towards support plate  110 . As ejector plate  106  and core plate  108  move toward support plate  110 , the slanted edge cam surface  142  of cam member  122  engages detent pin  132  and cams the detent pin into cavity  134  out of engagement with shoulder  140 , thereby freeing movable latch bar  124  (and hence ejector plate  106 ) from its locked connection to core plate  108 . Cam bar  122  and movable latch bar  124  are sized and disposed so that the cam surface  142  engages and pushes the detent pin back into the cavity  134  just as the core plate  108  engages the stationary support plate  110 , thus freeing the ejector plate from the core plate so as to allow the ejector plate to continue moving toward the front plate, carrying with it the poppet  118 . During further movement of ejector plate  106  toward front plate  102 , the inner edge  144  of latch bar  124  holds the detent pin in its retracted position. 
     Support plate  110  has a center hole in which is fixed a ring member  146 . The latter in turn surrounds a core member  148  which is fixed to core plate  108 . Core member  148  has a center hole  150  which slidably receives poppet  118 . The upper end of hole  150  is tapered outwardly, i.e., flared, as shown at  152  (FIG. 7A) so as to nestingly receive the enlarged head  120  of poppet  118 . 
     Turning now to FIGS. 12 and 13, ring member  146  is formed at its front end with a forwardly projecting annular rib  154 . Rib  154  is sized and contoured so as to conform to and mold the surfaces  32 ,  40  and  46  and a part of the surface  26  of rim sections  20 ,  22  and  24  of the container shown in FIGS. 1-3. In this connection, it should be noted that FIGS. 12 and 13 illustrate at  160  the injected plastic material that forms the container. Rib  154  has a pair of grooves  155  (FIG. 13) on its inner surface which are shaped to form the locking projections  36  shown in FIGS. 2 and 5. The inner side of ring member  146  is provided with a tapered surface portion  162  which is joined to a cylindrical surface section  163 . As shown in FIG. 12, the inner diameter of ring member  164  is smallest at cylindrical surface section  163 . 
     Referring now to FIGS. 6A-10B,  12  and  13 , the core  148  is formed with a generally cylindrical outer surface  164  which is joined to a reduced diameter tapered surface  166 . The latter surface joins a surface  168  which is shaped to form the inner surface of rim section  22  of the container. Rearwardly of surface  168  the core  148  has surfaces  170  and  172  which are contoured so as to mate with the surfaces  162  and  164  respectively of ring  146 . 
     Referring again to FIGS. 6A-10B,  12  and  13  a cavity member  174  is secured to front plate  102 . The latter has a cavity defined by a cylindrical side surface  176  and an end surface  178  which are shaped to conform to and mold the outer surfaces of side wall  4  and end wall  6  respectively of the container shown in FIGS. 1-3. The inner end surface of bushing  107  is shaped to conform to and mold the center part of the outer surface of end wall  6  of the same container. Accordingly, bushing  107  may be considered as part of cavity member  174 . 
     Referring to FIGS. 12 and 14, cavity member  174  is formed with two diametrically opposed slots  180  and two circularly curved extensions  182  at the inner ends of slots  180  (for convenience, only one slot  180  and one extension  182  is shown). As seen in FIG. 12, a flat circular groove  184  is formed in cavity member  174  adjacent each extension  182 . Also the side of each slot  180  facing front plate  102  is formed with a semi-cylindrical groove  186  that extends to groove  184 . 
     Disposed in each of the two diametrically-opposed slots  180  is an insert block  188 . The inner face of each insert block  188  is formed with a semicircular slot  190  that complements the adjacent extension  182  of cavity member  174 , but is sized so as to leave a gap therebetween to receive plastic material to form one of the ears  12  on the container. Each insert block  188  also has a semi-cylindrical groove  194  that complements the adjacent groove  186  in cavity member  174 . Each pair of grooves  186  and  194  forms a cylindrical hole in which is located a core pin  196  (FIG.  12 ). 
     Core pin  196  is slidably mounted in a bore  198  in a block  200  that is an extension of insert block  188  and is affixed to support plate  110 . A spring  202  surrounds the shaft of each pin  196  in an enlarged part of bore  198  and acts against the pin head  204  to urge the pin away from the core  48 . The inner end of each core pin  196  is tapered (beveled) to conform to the tapered openings  18  in ears  12 . Core pins  196  are moved toward core  148  by means of two cam bars  206  that are attached to and extend rearwardly from front plate  102 . Cam bars  206  occupy diametrically opposed positions relative to the axis of core member  148 . Each cam bar  206  is aligned with one of the blocks  200 , and each block  200  is slotted fore and aft (vertically as viewed in FIGS. 6A,  7 A and  12 ), with that slot being sized so that the associated cam bar  206  makes a close sliding fit therein. The inner end of each cam bar  206  has a slanted cam surface  208  that is located so that it can engage the head  204  of the adjacent core pin  196  when the mold assembly is closed (FIG.  6 ). In this connection it should be noted, as shown in FIG. 12) that the outer end surface of each core pin head  204  is slanted at substantially the same angle as cam surface  208 , so as to facilitate camming of core pin  196  by cam bar  206  in the manner hereinafter described. When the mold assembly is moved to its closed position (FIGS. 6A,  6 B), blocks  200  move with back plate  104  and support plate  110  toward front plate  102 , causing core pin heads  204  to engage cam surfaces  208  of cam bars  206 , whereupon the core pins  196  are cammed inwardly toward core  148 . The cam surfaces  208  force core pins  196  inward to a limit position in which their tapered inner ends are spaced from curved cavity extensions  182  by an amount equal to the desired thickness of walls  16  of ears  12 . 
     Referring now to FIGS. 7A,  7 B and  10 A,  10 B, it is to be noted that the end surface  129  of head  120  of poppet  118  forms a mirror image of a major portion of the inner end surface  178  of cavity member  174 , and the inner end surface of bushing  107 , and that the corresponding annular end surface  149  of core member  148  is the mirror image of the remainder of surface  178 , i.e., the front end surface  129  of head  120  of poppet  118  and the surrounding end surface  149  of core member  148  cooperate with inner end surface  178  of cavity member  174  and the inner end surface of bushing  107  to define the container bottom wall section of the mold cavity in which the container is molded. Also, ring member  146  acts as an auxiliary cavity member since it forms an extension of cavity member  174  and coacts with core member  148  to determine the shape of the rim section of the formed container  160 . Accordingly when the mold is closed, the confronting and mutually spaced surfaces of core member  148  and ring member  146 , cavity member  174  and insert blocks  188  coact to define the container side wall section and the container rim section of the mold cavity in which the container is molded. 
     Operation of the above-described mold assembly is straightforward. Assume that the mold assembly is mounted in an injection molding machine, with front plate  2  and back plate  4  secured to a fixed platen and a movable platen respectively of the machine. Assume also that ejector plate  6  is attached to a mechanical operator (not shown) that forms part of the same injection molding machine and is adapted to move the ejector plate toward and away from front plate  2  at predetermined times during the operating cycle of the machine. The sprue hole bushing  107  is connected to a source of plastic (not shown) which is to be injected into the closed mold assembly via a suitable injection pump (also not shown). Assume also that the machine has just completed its operating cycle, so that (1) the mold is in its fully closed position (FIGS. 6A,  6 B), with ejector plate  106  engaging or located adjacent to back plate  104 , and core plate  108  locked to ejector plate  106  by latch bar  124  and spaced back from support plate  110 ; and (2) a formed plastic container  160  occupies the mold cavity defined by core  148 , cavity member  174 , sprue bushing  107 , ring  146 , insert blocks  188  and core pins  196 . The machine is programmed so as to automatically and repeatedly execute an operating cycle which comprises the following steps starting with the mold in the closed position shown in FIGS. 6A and 6B. 
     1. The mold is opened by moving back plate  104  and ejector plate  2106  together away from front plate  102  (FIGS. 7A,  7 B). When the mold is opened, the back plate  104  is moved away from front plate  102  a distance that exceeds the longitudinal dimension of the cavity of cavity member  174  by an amount sufficient to permit subsequent removal of the formed container  160  (FIGS. 10A,  10 B). The rearward movement of back plate  104  away from front plate  102  causes blocks  188  and  200  to move clear of cam bars  206 , freeing core pins  196  and allowing springs  202  to move those core pins outwardly away from the curved extensions  182  of cavity member  174 . It should be noted that during the rearward mold-opening movement of back plate  104  and ejector plate  106 , the core plate  108  remains locked to ejector plate  106 . As the mold is opened, the formed container  160  remains in place because of its interlocking engagement with ring member  146  and core member  148 . Spring  143  acts to extend pressure pins  139  as the mold is opened, causing the pins to exert a force on core plate  108  so as to prevent the latter from moving away from back plate  104  in the direction of plate  102 . 
     2. Immediately after the mold has been opened, the machine moves ejector plate  106  (and hence poppet  118 ) a selected distance away from back plate  4  (FIGS. 8A,  8 B). By way of example but not limitation, this movement is about 2 inches in the case of molding a one gallon container for paint. During this movement, core plate  108  is locked to ejector  106  plate and hence it and core  148  move with the ejector plate. As seen in FIGS. 8A,  8 B, this initial movement of ejector plate  106  moves core plate  108  into contact with or immediately adjacent to support plate  110 . This joint movement of poppet  118  and core  148  is sufficient to strip the molded container free of ring member  146 . It also is sufficient to move the rim portion of the formed container beyond the insert blocks  188 , thereby allowing for lateral expansion of the formed container  160  as it is freed subsequently from core  148 . 
     In this connection it should be noted that the wall-molding surface  164  of core member  148  has a larger diameter than its surfaces  166  and  168  which help mold the inner rim section  22  of the container. Accordingly the rim end of the formed container needs to expand outwardly as it is being forced off of the core member by relative movement of poppet  118  (see step 3 below). The molded container  160  has sufficient flexibility and resiliency to permit it to expand radially enough to fit over and slide along the core member under the driving influence of the poppet. In this connection it should be appreciated that this radial expansion could not occur without the prior limited movement of core member  148  by ejector plate  106 , that limited movement being sufficient to move the formed container away from the ring member far enough to prevent the insert blocks  188  from restricting expansion of the rim section of the formed container as its rim section moves axially from the reduced diameter portion (surfaces  166  and  168 ) to the increased diameter portion (surface  164 ) of the core member. 
     3. Thereafter, as core plate  108  engages support plate  110 , cam bar  122  cams pin  132  inward of hole  134 , thereby unlocking ejector plate  106  from core plate  108 , and the machine continues to move the ejector plate further toward front plate  102 . Preferably, as shown in FIGS. 9 a ,  9 B, the machine moves ejector plate  106  into face-to-face contact or near face-to-face contact with core plate  108 . This action achieves the result of moving the poppet relative to the core plate in a forward direction toward front plate  102 , thereby forcing the formed container  160  off of core member  148 . 
     4. Once the poppet has moved the formed container free of the core member, the container is removed from the poppet (FIGS. 10A,  10 B). This may be done manually, in which case the machine is programmed to stop indefinitely to allow safe removal of the formed container, after which the machine can be commanded manually to resume its operating cycle. Preferably, however, the machine is provided with means (not shown) for automatically removing the formed container from the machine, with the machine being programmed to resume operation automatically immediately after removal of the molded container. 
     5. Following removal of the formed container, ejector plate  106  is retracted away from front plate  102  back to the position shown in FIGS. 8A,  8 B. At the beginning of this retracting movement, core  148  remains stationary and latch bar  124  moves relative to cam bar  122  away from front plate  102 . However, after the ejector plate has moved back a limited distance, e.g., about 6 inches, projection  130  of latch bar  124  engages the core plate at notch  132 . Substantially simultaneously shoulder  140  moves past detent pin  132 , whereupon spring  138  pushes that pin into slot  128 . As a result, core plate  108  is again locked to the ejector plate. 
     6. Ejector plate  106  completes its rearward movement back to its original position (FIGS. 6A,  6 B), carrying core plate  108  with it. As a result, when ejector plate  106  again rests against or adjacent to back plate  104 , core plate  108  will be stopped by pillar  114  a limited distance from support plate  110 , as shown in FIGS. 7A,  7 B. 
     7. Thereafter back plate  104  and ejector plate  106  (and also core plate  108 ) are moved back toward front plate  102  far enough to cause core member  148  to mate with cavity member  174  (FIGS. 6A,  6 B). As this occurs, cam bars  206  will re-engage core pins  196  and force them inward to molding position. 
     8. The cycle of operation is completed by again injecting molten plastic material into the formed cavity via sprue bushing  107 . It is to be understood that the mold assembly stays in its closed position (FIGS. 6A,  6 B) long enough to allow the injected molten plastic material to cool and solidify, after which the mold assembly is opened according to step (1) above. 
     Mold assemblies embodying the present invention may be provided for molding containers in sizes larger or smaller than the conventional one-gallon size commonly used by American paint manufacturers. Although the illustrated mold assembly was designed to mold containers with substantially straight side walls, it is contemplated that the cavity-defining components may be modified so as to provide for injection molding of containers that have a tapered side wall, with the containers having their maximum outer diameter at the top ends and their minimum outer diameter at their bottom ends. Also the mold assembly may be modified so as to eliminate formation of the strengthening rib  8 , and/or to form other strengthening contours, recognizing that the need or desire for such feature may result from one or more factors or functions, e.g., container size, overall weight of the contents of the container, and the material of which the container is made. The mold assembly also may modified to vary the number of gripping ribs  36  on the rim section of the container. Also the mold assembly may be modified to totally eliminate formation of locking ribs  36 , in which case the lid may be locked to the container rim solely as a result of the rib sections  70  and  80  being compressed together between and gripped by surfaces  32  and  40 . Although it is preferred to make the containers and lids of a high density polyethylene, the mold assembly of the present invention may be used to injection mold containers of other plastics materials known to persons skilled in the art, e.g., polypropylene. Colored, clear or translucent plastic may be used in molding containers. The mold assembly also can be modified so as to mold the container with embossed printing on its side wall so as to eliminate the need for a subsequent labeling operation. The mold assembly also may be modified to mold containers having a rim section that is shaped differently from the rim section of the container shown in FIGS. 1-3. Still other changes will be obvious to persons skilled in the art from the foregoing description and the drawings. 
     The invention offers a number of advantages. Perhaps the most important advantage is that the invention provides a mold assembly for forming a container wherein the rim section extends inwardly of the inner surface of the container, and accomplishes this without having to use a collapsible core which is expensive to make and maintain. Another important advantage is that the invention makes it possible to manufacture a plastic container for use in holding paint or other products in liquid or particulate form that has sufficient strength to allow it to be filled, capped, labeled, and stacked or packaged using conventional filling, labeling and packaging machinery. Still other advantages provided by this invention are that the formed containers have a one-piece construction and, if desired, free of any seams or crimps.