Abstract:
A dual constituent container includes a compartment for liquids and an elongate inwardly directed recess open to the container exterior and isolated from the compartment. The container is formed by blow molding, using a mold that incorporates a shaping feature projected longitudinally into the mold cavity. The shaping feature incorporates an arrangement of longitudinal channels having a channel width such that when a thermoplastic preform is expanded into contact with the mold cavity walls and the shaping feature, portions of the expanded preform span the channels and cooperate with the channels to provide passages that accommodate pressurized air to facilitate separation of the expanded preform from the shaping feature. The same portions of the expanded preform can partially protrude into the channels, thus to form longitudinal ribs along the recess which aid in frictionally holding a secondary constituent.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority of Provisional Application No. 60/938,373 entitled “Container Fabrication Process,” filed May 16, 2007, which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to containers that incorporate separate compartments for different constituents, products, or elements, and more particularly to a blow molding process for fabricating such containers, and containers formed by the process. 
         [0003]    As shown in U.S. Pat. No. 6,989,168 (Fahey), hereby incorporated herein by reference, a beverage container can be formed with a substantially centered upright cylindrical wall to divide the container into a primary compartment for the beverage, and an interior compartment open to the container bottom for containing a secondary food product, for example a package of cookies, food, a food bar, medicine, tablets, water purification tablets, crackers, or other products. To provide a more secure hold on the secondary product, the cylindrical wall forming the recess is stepped near the interior end of the recess, as perhaps best seen in  FIGS. 2 and 6  of the patent. While this is considered to be an effective approach to providing the desired frictional engagement with the secondary product, it can be difficult to form the interior cavity when using a blow molding process to fabricate the containers. 
         [0004]    Another processing difficulty, occasioned by the inward extension of the secondary compartment (i.e. upward in an upright container), is the tendency of the blow molding step to create a partial vacuum that holds the expanded thermoplastic firmly against the molding feature used to form the secondary compartment. This can prevent withdrawal of the molding feature, and thus prevent removal of the bottle from the mold. 
         [0005]    An alternative approach for blow molding the containers was considered, namely to introduce a slight taper or incline in the recess to gradually reduce its diameter in the upward or inward direction. Then, the reduced-diameter region near the interior of the recess would provide the desired frictional fit. A sufficient degree of incline along the surfaces of the molding feature and the resulting compartment would have the additional advantage of facilitating removal of the expanded container from the mold after blow molding. 
         [0006]    While workable, this approach created a conflict between two objectives: a secure frictional hold on the secondary product, aided by an axial or near-axial orientation of the secondary compartment wall; and enhancing the blow mold process, facilitated by providing a tapered secondary compartment wall. 
       SUMMARY OF THE INVENTION 
       [0007]    A first aspect of the invention is a dual constituent container having a container wall shaped to provide a compartment for containing a first constituent. The container wall includes a bottom section and an outer wall section extending generally upwardly from the bottom section to determine an outer periphery of the container. The container wall further includes an inner wall section spaced apart radially inwardly from the outer wall section and extending upwardly from the bottom section to define a cylindrical recess elongate in a longitudinal direction and open at a bottom thereof to an exterior of the container. The container wall further includes an interior-end wall section disposed at a top of the inner wall section and cooperating with the inner wall section to isolate the compartment from the recess. An arrangement of elongate ribs is disposed axially along an inside surface of the inner wall section, and projects radially inward from the inside surface extending along at least a portion of a longitudinal length of the inner wall section. The first ribs are adapted to exert a radially inward force upon a second constituent removably inserted into the recess, thus to frictionally engage the second constituent while allowing the second constituent to slide longitudinally relative to the inner wall section. 
         [0008]    Another aspect of the invention is a process for forming a container with a compartment and inwardly projecting recess isolated from the compartment, including the following steps: (a) providing a plurality of mold sections including a selected mold section with an elongate recess shaping feature; (b) assembling the mold sections into a mold in which the recess shaping feature projects longitudinally into a mold cavity of the mold, wherein the recess shaping feature has a smooth outer surface and includes at least one channel along the outer surface, the channel having a predetermined channel width and being recessed inwardly from the outer surface to an inner surface of the channel by at least a predetermined channel depth; (c) mounting a thermoplastic preform to the mold such that an expandable part of the preform projects into the mold cavity; (d) with the preform so mounted and with the expandable part heated to a processing temperature above a glass transition temperature of the preform, introducing a fluid under pressure into the preform to cause the expandable part of the preform to undergo a plastic expansion into surface contact with a cavity wall of the mold cavity and with the outer surface of the recess shaping feature to form an expanded container structure; (e) cooling the container structure to a temperature below the glass transition temperature to solidify the container structure; (f) after cooling the container structure, supplying a fluid under pressure via the at least one channel to an interface between the container and the shaping feature while simultaneously moving the selected mold section longitudinally relative to the container structure to separate the shaping feature from the container structure; and (g) removing the container structure from the mold cavity. 
         [0009]    A further aspect of the invention is a system for molding a container with a compartment and a cylindrical recess separate from the compartment. The system includes first and second opposing mold sections having respective first and second confronting surface regions and respective first and second cavity-forming walls each recessed from its associated surface region. The opposing mold sections are adapted to be mounted in confronting relation for lateral movement between an open position in which the first and second mold sections are laterally spaced apart and a closed position to form a mold in which the first and second confronting surface regions are contiguous and the first and second cavity-forming walls cooperate to form a mold cavity. A third mold section includes an elongate shaping feature with an axially extending outer wall, supported for longitudinal movement relative to the first and second mold sections between: (1) an advanced position, in which the shaping feature extends longitudinally into the mold cavity; and (2) a retracted position, in which the shaping feature is spaced apart from the first and second mold sections. A support structure is adapted to support a thermoplastic preform with an elongate expandable part of the preform extending longitudinally into the mold cavity when the first and second mold sections are in the closed position. A first fluid conduit is adapted to conduct a fluid under pressure into the preform to cause the expandable part of the preform, when heated to a processing temperature above the glass transition temperature, to undergo a plastic expansion into contact with the cavity-forming walls and the outer surface of the shaping member to form an expanded container structure. The third mold section incorporates a fluid passage for conducting a fluid under pressure from outside the mold to an interface between the outer wall of the shaping feature and the expanded structure. The fluid passage includes a plurality of channels formed along the outer wall of the shaping feature and recessed inwardly by a predetermined channel depth. A second fluid source is adapted to supply a fluid under pressure to the interface via the passage simultaneously with a longitudinal retraction of the third mold section from the advanced position, to facilitate separation of the shaping feature from the container structure. 
         [0010]    To achieve the foregoing objects, the shaping feature used to form the internal compartment is provided with a series of channels or vents that extend in the axial direction. In one particular embodiment, the channels extend over substantially the complete length of the shaping feature, are spaced apart angularly from one another and distributed substantially symmetrically about the shaping feature. In one embodiment, the channels are substantially uniform in transverse profile, i.e., in profiles taken in planes perpendicular to the axial direction. 
         [0011]    The channels are selectively sized, particularly in terms of their transverse profiles to perform several useful functions during the molding process. First, the channels provide passages for air between the mold cavity wall and the thermoplastic preform to escape from the mold cavity as the preform is expanded toward contact with the mold cavity wall. Second, the channels provide air passages between the expanded container and the cavity wall. This is accomplished by forming the channels with a radial depth sufficiently large to promote airflow, and an angular or circumferential width sufficiently narrow to prevent the expanding thermoplastic material from completely entering or “filling” the channels. In one embodiment, the channels remain open to receive air under pressure from outside the mold cavity, and to separate the expanded preform from the shaping feature used to shape the interior compartment. This enables retraction of the shaping feature from the container, and thereby enables removal of the container from the mold. 
         [0012]    In one example, the channel width, while sufficiently narrow to prevent the expanding thermoplastic from completely entering the channel as just noted, also is sufficiently large to allow a degree of thermoplastic penetration into the channel. In particular, the thermoplastic tends to form a smooth, rounded protrusion directed radially into the channel, substantially uniform in cross-section or profile taken transversely of the channel length. These protrusions form ridges or ribs that run axially along the wall section forming the interior compartment and extend radially a slight distance (e.g. several thousandths of an inch) into the interior of the compartment. The ribs cooperate to provide axially-extending, angularly-spaced-apart regions of concentrated frictional holding force between a secondary constituent in the compartment and the cylindrical compartment wall section surrounding the constituent. 
         [0013]    Thus in accordance with the invention, a molding feature used to form an internal compartment in a container is provided with channels that facilitate the blow molding process by enabling convenient removal of the expanded container from the shaping feature after the blow molding step. The channels further form axially directed ribs along the secondary compartment interior for an improved frictional hold on the secondary constituent. 
     
    
     
       IN THE DRAWINGS 
         [0014]      FIG. 1  is a sectional view of a bottle fabricated in accordance with the invention; 
           [0015]      FIG. 2  is a bottom view of the bottle; 
           [0016]      FIG. 3  is an elevation of a thermoplastic preform used to fabricate the bottle; 
           [0017]      FIG. 4  is a schematic view of a system for fabricating the bottle; 
           [0018]      FIG. 5  is a more detailed elevation showing one of two opposing mold sections of the system; 
           [0019]      FIG. 6  is an elevation of a base mold section of the system; 
           [0020]      FIG. 7  is a top plan view of the base section; 
           [0021]      FIG. 8  is a partially sectioned elevation showing the base section advanced into a cavity formed by the opposing mold sections; 
           [0022]      FIGS. 9-13  illustrate stages of the molding process; and 
           [0023]      FIGS. 14-16  are schematic views illustrating inflation of the thermoplastic material into contact with a part of the mold cavity wall defined by the base mold section. 
           [0024]      FIG. 17  is a schematic view illustrating an alternative example of inflation of the thermoplastic material into contact with a part of the mold cavity defined by the base mold section. 
           [0025]      FIG. 18  is a cross-sectional view of the base mold section. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]      FIGS. 1 and 2  illustrate a dual compartment beverage container  16  in the form of a bottle symmetrical about a vertical axis  18 . Over the lower portion of its length, container  16  has a vertical outside profile, followed by an upward and inward taper to a neck  20 . The neck includes an annular rim  22  and threads  24  used to secure a cap to the bottle. 
         [0027]    Container  16  is formed of a single continuous container wall  26  including an outside wall section  28  that determines the container profile, an annular bottom wall section  30 , a substantially vertical and cylindrical inside wall section  32 , and an upper or inner end section  34 . The bottom wall section is toroidal, or a section of a toroid, in one embodiment. The term toroidal and toroid refers to a surface generated by the revolution of a closed plane contour about an axis lying in its plane. For the bottom wall section, the closed plane contour can be a circle, but more likely is a different type of contour having a flattened portion corresponding to the bottommost surface of the bottle. 
         [0028]    The inside wall section and the end section cooperate to define a recess or elongate recess  36  open to an exterior of the container through an access aperture  38 . Thus, the container forms two compartments: a compartment  40  for containing a beverage which can be considered the “primary” constituent, and a compartment  42  corresponding to recess  36  for containing a secondary constituent, typically cookies or another secondary food product. Examples of the secondary constituent are food, a food bar, medicine, tablets, water purification tablets, and crackers. Container  16  is formed of a material that is capable of stretch blow molding. For example, container  16  is formed of a polymeric material, e.g. a thermoplastic such as polyester, polyethylene, polyethylene terepthalate (PET), high density polyethylene, polyethylene naphthalate, polylactic acid (PLA), or polypropylene. 
         [0029]    Six elongate ridges or ribs  44  run axially (i.e. vertically) along an interior surface of inside wall section  32 , then extend generally radially inward along end section  34  to converge at the center of the end section. Near the bottom of container  16 , lower end regions of ribs  44  extend radially outwardly to a circular, annular rib or ridge  46  formed along bottom section  30 . Upper end regions of the ribs extend radially inwardly along end section  34 . In other embodiments, different numbers of ribs  44  are provided, such as any number of ribs in the ranges one to five or seven to ten ribs. 
         [0030]    The term elongate is intended to mean that the overall structure of each rib is long in the axial direction compared to its width in a radial direction. In one embodiment, the ribs  44  extend axially over the entire length of the recess. In another embodiment, the ribs extend over only a portion of the recess. In one embodiment, one or more ribs are made up of a series of protrusions in an axial arrangement, so that the overall structure of the series of protrusions is elongate. 
         [0031]    A secondary constituent, e.g. a cartridge indicated in broken lines at  48 , is removably maintained in compartment  42  at least partially by friction. More particularly, the cartridge typically is formed of a compliant polymeric thin film or sheet material such as cellophane or metalized polypropylene, wrapped in close surrounding relation about the accessory constituent. The cartridge is compliant and tends to conform to the shape of the cookies or other constituent. This provides a degree of flexibility and enhances the frictional hold on the cartridge. Ribs  44 , by virtue of their extension away from inside wall section  32  radially toward the center of the recess, provide six substantially linear regions of contact between the cartridge and the inside wall section. Although other regions of the cartridge and of the inside wall section can be contiguous, the linear contact regions tend to concentrate the radial forces that the cartridge and inside wall section exert on one another. In the embodiment shown, the distance by which the ribs project radially inward is substantially uniform over the inside wall section, although in at least one embodiment, ribs can project radially inward at varying distances over the inside wall section. For example, in one embodiment the wall of the recess is a truncated conical section, and the ribs extend farther from the wall at the bottom region than at the top of the recess. As a result, a more uniform hold on the secondary constituent is possible. 
         [0032]    The container  16  has a circular cross-section in the embodiment of  FIG. 1 . However, it is also possible for the container to have an oval cross-section or a square cross-section with rounded corners. 
         [0033]    The recess  36  is a substantially cylindrical recess in one embodiment. By cylindrical, it is meant herein that for every point of the surface there is a straight line that lies on the surface, and the surface is spanned by a one-parameter family of parallel lines. According to this definition, the cylindrical recess is a circular cylindrical recess in one particular embodiment and an elliptic or oval cylinder in other particular embodiments. It is also possible for the recess to have a square cross-section with rounded corners. 
         [0034]    In some embodiments, the recess is a truncated conical section, where the surface of the cone forms an angle of 0.5 degrees to 5 degrees to a vertical line. In one embodiment, the angle of the surface is 1 degree from vertical. 
         [0035]    In various embodiments, the diameter of the recess  36  near its opening is at least 1 cm and not more than 5 cm. In one embodiment, the diameter of the recess  36  near its opening is 3 cm. In various embodiments, the diameter of the recess  36  near its top is at least 0.5 cm and not more than 5 cm. In one embodiment, the diameter of the recess  36  near its top is 2.3 cm. The height of the recess  36  is at least 3 cm and not more than 10 cm, or 7 cm in various embodiments. The height of the container  16  is at least 10 cm and not more than 30 cm, or 18 cm in various embodiments. In some embodiments, the ratio of the inner cavity or recess diameter to the container diameter is at least 25% and not more than 50%, while in one embodiment it is 35%. In some embodiments, the ratio of the inner cavity height to the container height is at least 10% and not more than 60%, or at least 35% and not more than 55%, or 40%. 
         [0036]    As compared to the stepped compartment interior surface shown in the aforementioned 6,989,168 patent, inside wall section  32  through its substantially vertical orientation affords the advantage of maintaining a frictional hold of the cartridge over substantially the entire length of the cartridge. As compared to a cylindrical inside surface that is smooth, i.e. without ribs  44 , inside wall section  32  provides a superior frictional hold of the cartridge. This is due to the relatively higher radial force levels along the linear contact regions, and also due to an improved capacity to accommodate irregularities in cartridge  48 , as well as any variance among the different cartridges. It is also possible for container recess and rib structure to be configured to concentrate the hold on the leading edge of the cartridge  48 . 
         [0037]    In some embodiments, the diameter of the recess  36  narrows toward the top of the recess, either due to a stepped configuration with discrete sections having different diameters or inclined walls. 
         [0038]    Container  16  is formed preferably by a blow molding process in which a parison or preform  50  (see  FIG. 3 ) is heated, then expanded through high pressure gas into contact and conformity with a mold cavity. Preform  50  resembles a test tube, with a support ledge  52  and threads  54  near its top, corresponding to rim  22  and threads  24  of container  16  as shown in  FIG. 1 . The preform body  55  is the portion of the preform  50  below the support ledge  52  in the orientation of  FIG. 3 . The preform body  55  is the expandable part of the preform that is heated. 
         [0039]      FIG. 4  schematically shows a system for forming containers such as container  16  from preforms like preform  50 . At the heart of the system is a container cavity mold consisting of three separate mold sections: a first cavity mold section  56 ; a second cavity mold section  58 , opposing and confronting mold section  56 ; and a base mold section  60 . Opposing sections  56  and  58  are supported for horizontal reciprocal motion to alternatively open and close the mold. Base section  60  is supported for vertical reciprocal motion relative to mold sections  56  and  58 , between a raised or advanced position (in which the base section protrudes into the cavity formed by mold sections  56  and  58 ), and a lowered or retracted position. 
         [0040]    A blow nozzle  62  includes a gripping device  64  for picking up the heated preform and placing it in position with respect to the mold cavities. The blow nozzle is movable to position the preform for fixation between mold sections  56  and  58  to extend in a downward direction into the cavity when the mold is closed. For a stretch blow molding process, blow nozzle  62  incorporates a vertically reciprocal stretch rod  65  which is not illustrated in  FIG. 4 . 
         [0041]    A source  66  of pressurized air is fluid connected to blow nozzle  62 , and is operable to provide pressurized air through a passage in the blow nozzle to expand or inflate preform  50 . A source  68  of pressurized air is fluid coupled to base section  60 , and is operable to supply air under pressure to the cavity when the mold is closed and when the base section is in the raised position. The pressurized air is released from the nozzle surrounding the stretch rod  65 . 
         [0042]    With reference to  FIG. 5 , mold section  56  includes a main body  70  formed of any material known in the art such as, for example, aluminum or steel. An interior surface of the body, i.e. the surface that confronts opposing mold section  56 , has a substantially planar region  72  and a rounded cavity region  74  shaped to define the outer profile of container  16 , specifically the radially outward portion of bottom wall section  30  and outside wall section  28  except for the top of the container including threads  24  and rim  22  (see  FIG. 1 .) A circular recess  76  at the top of mold section  56  accommodates support ledge  52  of preform  50  to center the preform relative to the mold cavity (See  FIG. 4 .) 
         [0043]    The mold sections  56  and  58  close around the preform  50  and the gripping device is released, so that the support ledge  52  rests on the circular recess  76 .  FIG. 5  illustrates the circular recess  76  with exaggerated dimensions so that it is visible in the Figure. FIGS.  4  and  9 - 11  do not illustrate the circular recess. It is also possible for the support ledge  52  to simply rest on the top surface of mold sections  56  and  58 , as illustrated in  FIGS. 9-11 . However, the circular recess provides the function of centering the preform body within the mold cavity. 
         [0044]    Corresponding planar and cavity surface regions of opposing mold section  58  are substantially identical, so that when these sections are brought together to close the mold, the respective planar surfaces are contiguous and the respective cavity surfaces cooperate to provide a cylindrical, continuous cavity surface. Further, mold sections  56  and  58  when closed provide a top opening for the top of the preform  50 , and a bottom opening to receive base section  60  into the cavity, as shown in  FIG. 4 . After the bottom opening receives the base section  60 , the base section locks into place. 
         [0045]    As seen in  FIGS. 6 and 7 , base section  60  includes a platform  78 , a pedestal  80  immediately above the platform, and an elongate recess shaping feature  82  extending upwardly from the pedestal. At the top, the shaping feature is rounded to provide an upper surface  84 , which at its center is substantially horizontal. 
         [0046]    Base section  60  includes a series of channels or vents, three of which appear in  FIG. 6  at  86 ,  88  and  90 . The remaining channels are shown in  FIG. 7  at  92 ,  94  and  96 . Each channel runs radially along pedestal  80  beginning at its outer edge, proceeds axially or vertically along feature  82 , then extends radially inward along upper surface  84 , to a point where the channels converge. A shallow recess  98  is formed at the top of the shaping feature, centered on the point of channel convergence. Channels  86 - 96  are equally angularly spaced apart from one another, symmetrically arranged about the base section. 
         [0047]    The complete blow molding cavity is formed by advancing base section  60  upwardly into the space between opposing mold sections  56  and  58  when these sections are closed, an arrangement illustrated in  FIG. 8 . An annular gap  101  between the base section and the other mold sections is in fluid communication with channels  86 - 96 . The presence of the annular gap  101  causes formation of an annular rib  46 , in some embodiments. The base section is removably fixed to an upper section  100  of a support device  102 , through an arrangement of locking rings  104  and  106 . Upper section  100  and a medial section  108  of the support device are coupled through a cylinder  110  and piston  112  operable to reciprocate the upper section relative to the medial section and a bottom section  114  of the support device. 
         [0048]    An elongate insert  116  is contained within a central recess formed axially along base section  60 . The insert is shaped to provide a passage with helical and linear portions to accommodate a flow of water or another suitable liquid to cool shaping feature  82 . Opposing mold sections  56  and  58  likewise are provided with cooling liquid passages. 
         [0049]    Mold sections  56 ,  58  and  60  are formed of any material known and used in the art, which can include aluminum, steel, or other appropriate material. Upper section  100  is preferably formed of stainless steel, as are locking rings  104  and  106 . Sections  108  and  114  of the support device preferably are formed of aluminum, while cylinder  110  and piston  112  are formed of stainless steel. Insert  116  preferably is formed of ABS or another suitable thermoplastic. 
         [0050]      FIGS. 9-13  illustrate the use of the system to fabricate container  16 . While these figures do not depict certain aspects of container formation, for example injection molding and other treatment of preform  50 , these aspects are known in the art and not particularly germane to the present invention. In an embodiment, the interior mold surfaces are lubricated before the molding process is started in order to help the molded product to be released from the mold. 
         [0051]      FIG. 9  illustrates an initial stage in which opposing mold sections  56  and  58  are brought together and base section  60  is upwardly advanced, thus to close the mold about preform  50 . An upper portion of the preform including support ledge  52  and threads  54  (see  FIG. 3 ) remains outside the mold cavity. At this stage preform body  50 , having been heated just before its insertion into the mold, has a processing temperature above the glass transition temperature of the thermoplastic. The upper region of preform  50  is supported by its support ledge  52 . An expandable part of the preform extends downwardly into the mold cavity. 
         [0052]    With the system employing a stretch blow molding process as illustrated in  FIG. 10 , a stretch rod  65  is extended downwardly from blow nozzle  62  to encounter the bottom of preform  50 , and then extended further to axially stretch the preform. Axial elongation continues until the preform encounters the upper surface of shaping feature  82 . Specifically, the extended tip of the preform encounters recess  98  in upper surface  84  ( FIG. 7 ), which facilitates an accurate centering of the preform relative to base section  60 . The almost fully stretched preform is shown in  FIG. 10 . 
         [0053]    At this stage, air from source  66  is introduced into the preform through blow nozzle  62 . Air may be provided at a relatively low pressure for initial expansion (e.g. about 100 psi) followed by a much higher pressure (e.g. about 600 psi) to complete inflation of the preform. Alternatively, the air may be introduced at a single, high pressure (e.g. about 550 psi). In either event, the preform is expanded into contact with the cavity surface, including the cavity surface regions  74  and  118  (as depicted in  FIG. 4 ) of mold sections  56  and  58  and an outer surface  120  of shaping feature  82 . This forms a thermoplastic wall  122  that substantially conforms to the cavity surface, as shown in  FIG. 11 . For purposes of clarity, the thermoplastic wall  122  is depicted with an exaggerated wall thickness in both  FIG. 11  and  FIG. 12 . Due to the circulation of water through mold sections  56 ,  58  and  60 , the thermoplastic wall cools rapidly upon contact with the mold cavity to form a stable expanded container structure. 
         [0054]    Following expansion formation, the stretch rod  65  is retracted and support device  102  (see  FIG. 8 ) is operated to retract base section  60  relative to mold sections  56  and  58  and the container. Simultaneously, air at a relatively low pressure (e.g. 70 psi) is provided to base section  60  from source  68  (see  FIG. 4 ). The pressurized air exits base section  60  from an opening  121  in the center of the top of the shaping feature  82 . The location of the opening  121  is indicated in  FIGS. 6 ,  8  and  12 , although the opening itself is not visible in the perspective of these drawings.  FIG. 7  also shows the location of the opening  121 , though the scale of the drawing does not permit showing the opening itself. A channel in the wall of the shaping feature leads to the opening  121  and is connected to the source  68  of pressurized air.  FIG. 18  is a cross-sectional view of the shaping feature  82  that shows the channel  123  and the opening  121 . The pressurized air flows through the channels  86 - 96  to facilitate separation of the shaping feature  82  from the adjacent, surrounding thermoplastic material corresponding to inside wall segment  32  of the finished container. The channel  123  and the channels  86 - 96  together are a fluid passage of the third or base mold section for conducting a fluid under pressure from outside the mold to an interface between the outer wall of the shaping feature and the expanded container structure. 
         [0055]    After retraction of the base section, opposing mold sections  56  and  58  are separated from one another to allow removal of the expanded container, as shown in  FIG. 13 . At this point the container can be extracted from blow nozzle  62 . If desired, extraction can be facilitated by providing a low pressure burst of air from air source  66 . 
         [0056]      FIGS. 14-16  show the interaction of preform thermoplastic wall  122  with the mold cavity surface, specifically part of shaping feature  82  near channel  86 . During preform expansion, the pressurized air provided through the blow nozzle acts against wall  122  to drive it toward the cavity surface, as indicated by the arrows above wall  122 . During expansion, air inside the cavity and outside the preform is compressed and evacuated from the cavity. Some of the air inside the cavity and outside of the preform enters the channels and flows out of the mold cavity via the channels and a narrow annular gap between base section  60  and opposing mold sections  56  and  58 . In connection with channel  86  in  FIG. 14 , this is indicated by the arrows below wall  122 . 
         [0057]    Continued internal pressure drives wall  122  of the preform against the cavity surface as shown in  FIG. 15 . Conductive cooling of the preform wall begins immediately upon contact with the aluminum mold sections. Nonetheless, wall  122  retains its formable character for a brief time after contact (e.g., about 0.5 seconds), especially along regions  124  aligned with and spanning the channels, where conduction can occur only through wall  122  rather than through the aluminum body of one of the mold sections. During this brief time, wall  122  remains subject to the pressurized air force. 
         [0058]    As a result, region  124  of the preform wall undergoes a plastic deformation to form a protrusion  126  that partially enters channel  86 , as shown in  FIG. 16 . Because the wall of the preform tends to have a substantially uniform thickness over the extent of its contact with the mold cavity, and because the pressure inside the preform is substantially uniform throughout the preform interior at any given point in time, the protrusions tend to be uniform along the lengths of the channels, and replicate the pattern formed by the channels to produce ribs  44 . 
         [0059]    With reference to  FIGS. 6 ,  7  and  16 , protrusion  126  represents a partial entry or penetration of the thermoplastic material into channel  86  and the other channels, leaving a substantially open volume  128  adapted for accommodating pressurized air between preform wall  122  and outer surface  120  of the shaping feature. As best seen from  FIG. 7 , channels  86 - 96 , and accordingly their associated open volumes, are not confined to the axial section of shaping feature  82  but extend along upper surface  84  as well. Thus, pressurized air in the channels exerts a lifting force along substantially the entire outer surface of shaping feature  82  that tends to separate wall  122  from the shaping feature  82  (see  FIG. 14 ). This facilitates its retraction from the expanded preform and mold sections  56  and  58 . 
         [0060]    In one embodiment, illustrated in  FIG. 17 , a container wall  222  includes a protrusion  226  that enters farther into the channel  86  of mold section  82  than shown in  FIG. 16 . In this embodiment, the protrusion  226  makes contact with the channel inner surface. An open volume  228  is still present on either side of the protrusion  226  in the embodiment. It is also possible for a protrusion of the container wall to completely fill the channel. 
         [0061]    A feature of the invention resides in providing channels  86 - 96  with a profile suitable for forming ribs  44 . As seen in  FIG. 14 , the transverse profile of channel  86 , i.e. the profile transverse to the axial or length direction of the channel is characterized by a circumferential or angular width w and a radial depth d. The width is the more critical dimension. If the channel is too narrow, no discernable protrusion will form during cooling, and result is a smooth, non-ribbed surface of the inside wall section. Conversely, if the channel is too wide, the internal pressure during preform expansion drives wall  122  completely into the channels. In such case, channels  86 - 96  would fail to provide air passages for use in separating inside wall section  32  from shaping feature  82 , rendering the expanded container more difficult or impossible to remove from the mold. 
         [0062]    The most suitable channel width w varies in accordance with several factors, including the thermoplastic material involved, the thickness of expanded preform wall  122  as it engages the cavity surface, the preform expansion pressure, and the temperatures of the preform and the mold sections. In fabricating PET containers, a suitable width w has been found to be 0.015 inches or 0.038 centimeter. The width may vary within a range of 0.01 inches to 0.04 inches, or 0.03 centimeter to 0.1 centimeter. 
         [0063]    The channel depth d is selected to ensure that protrusion  126  only partially enters the channel, remaining spaced apart from an inner surface  127  of the channel and leaving sufficient open volume to accommodate passage of air or another gas through the channels. The channel depth may vary within a range of 0.001 inches to 0.040 inches, or 0.003 centimeter to 0.1 centimeter, and more preferably is 0.005 inches or 0.01 centimeter. The protrusion amount can vary within a range of 0.001 to 0.005 inches or 0.003 to 0.01 centimeter. 
         [0064]    With reference to  FIGS. 6 and 7 , the symmetrical arrangement of six axially extending channels spaced apart angularly from one another in sixty degree increments has been found effective, both to accommodate pressurized air for separation of the expanded container from the shaping feature, and to provide a suitable arrangement of ribs along inside wall section  32  to enhance the frictional retention of the secondary constituent. Other channel arrangements can be employed, involving more or fewer channels, or an alternative channel direction such as a helical winding about feature  82 . A primary consideration is that the surface area of the ribs, in proportion to the total outer surface area of the feature, is sufficient so that air provided at a relatively low pressure exerts a separation force to allow retraction of the feature after the expanded container is cooled. 
         [0065]    Thus, the arrangement of channels  86 - 96  over the shaping feature surface enhances the blow molding process in several respects. During expansion of the preform toward the inside surface of the cavity, the channels assist and guide the flow of air out of the cavity from the region inside the cavity yet outside of the expanding preform. After the fully expanded container has cooled and stabilized, the open volumes remaining in the channels provide passages for pressurized air used to separate the expanded preform from the mold feature. As a result, the mold feature and the resulting inside wall segment of the finished container can be axial, i.e. vertical in an upright container, or tapered. 
         [0066]    Finally, the channels are selectively sized to promote formation of radially inwardly extending ribs along the inside wall section that replicate the pattern of the channels in the shaping feature. The ribs provide regions of concentrated retaining force between the inside wall section and a secondary constituent surrounded by the inside wall section, for more effective frictional retention. 
         [0067]    While specific examples of the invention are described in detail above to facilitate explanation of various aspects of the invention, it should be understood that the intention is not to limit the invention to the specifics of the examples. Rather, the intention is to cover all modifications, embodiments, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.