Patent Publication Number: US-7210606-B2

Title: Web conveyor for a thermoforming apparatus, web support apparatus for a thermoforming apparatus, and thermoformable web support apparatus

Description:
RELATED PATENT DATA 
     This continuation application claims the benefits of U.S. patent application Ser. No. 10/460,933, entitled “Web Conveyor and Web Supporting Apparatus”, which was filed Jun. 12, 2003 now U.S. Pat. No. 7,040,519, and which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This invention pertains to fabrication of plastic products from plastic webs using differential pressure thermoforming apparatus. More particularly, the present invention relates to a web support apparatus and method for limiting sagging of a plastic web of thermoformable material when processing the material through a heating station and into a thermoforming station. 
     BACKGROUND OF THE INVENTION 
     Thermoforming lines are used to manufacture and form a variety of plastic, thin-walled articles by processing a continuous web or sheet of thermoformable plastic material. One particular technique involves the use of continuous web, differential pressure, thermoforming machines which encounter a problem wherein the web of thermoformable material is heated after which the material sags at a heating station before it reaches a molding station. When a thin web of thermoformable plastic material is heated in a heat tunnel, the thin web of material has relatively little “hot strength”. Typically, a thin web of thermoformable plastic material is clamped along its edges as it is conveyed using a thermoforming conveyor through a heat tunnel and into thermoforming machine. However, the strength of the heated, web of plastic material is typically insufficient to fully support the mid portion of the web. 
     One technique for limiting sagging of a heated web or sheet of thermoformable plastic material entails the use of longitudinally extending, endless sag bands that are configured to support a mid portion of the web, as disclosed in U.S. Pat. No. 2,967,328. However, sag bands are made from spring steel, and they have been known to break. When a sag band breaks, the steel band creates a risk to downstream machinery as the broken band can be fed into a downstream thermoforming press resulting in damage to the press. Additionally, stationary sag bands and wires have also been utilized to support a web of thermoformable plastic material within an oven. However, stationary sag wires have been known to slightly melt into the plastic web of material, leaving blemishes in the surface of the material, which can affect the finished quality of articles formed in the web. Furthermore, the wires have also been known to break, similar to the bands. 
     Another previously known technique involves the utilization of integral, transversely extending support strips that are formed via a cooling operation in a sheet or web of thermoformable plastic material, as disclosed in U.S. Pat. No. 3,664,791. However, the incorporation of such integral support strips complicates the manufacturing process and can slow it down. Additionally, the strips, which are rigidified, are not completely effective at eliminating sag in all cases. 
     Even another previous system for inhibiting sagging comprises a sheet support apparatus that utilizes air under pressure within a box that is provided beneath the sheet of thermoformable material in order to float the sheet above the box, as disclosed in U.S. Pat. No. 4,101,252. However, the introduction of a high volume of air against an underside of a web will complicate the uniform thermal heating of the web to a particular desired and uniform temperature. Furthermore, the supply of heated air is interrupted when the web (or sheet) of material is stationary; otherwise, the air might cause chilling of the overlying sheet portion that is being supported thereabove. Such an operation can have a significant negative effect on operating speed because air is compressible and intermittent interruption of the supply of air will take time to support and unsupport the web as the air is supplied and interrupted, respectively. 
     Accordingly, further improvements are needed to provide a more efficient and effective web support apparatus for delivery of a web of heated thermoformable material through a heat tunnel (or oven) and into a thermoforming machine. 
     SUMMARY OF THE INVENTION 
     A web support apparatus and a web conveyor are provided for supporting a relatively thin sheet of continuous, heated thermoformable plastic material where the continuous web (or sheet) is intermittently delivered to an oven (or heat tunnel) and into a thermoforming machine such that the web is intermittently moved from stationary positions that correspond with a footprint within the thermoforming machine to form an array of articles within the continuous web of material. Accordingly, the web moves and stops intermittently, which can tend to increase the friction by imparting static friction between the heated web and the underlying web support apparatus. Accordingly, the web support apparatus incorporates a friction-reducing material along a top edge of the web support apparatus. Secondly, the web support apparatus incorporates a discrete geometry that supports the web at discrete locations. Furthermore, the web support apparatus incorporates a temperature-regulating system within the web support apparatus for regulating temperature of the support structure and friction-reducing material within a desired operating range. 
     According to one aspect, a web conveyor is provided having a frame, a pair of conveyor rails, and at least one sag rail. The pair of conveyor rails is carried by the frame in laterally spaced-apart relation. The pair of conveyor rails is configured to support and convey respective edges of a thermoformable web of plastic material. The at least one sag rail includes a friction-reducing material provided along at least a portion of a top edge of the sag rail. The sag rail is provided between the conveyor rails and extends longitudinally along a web travel path. The sag rail is configured to support a web of material intermediate the conveyor rails. 
     According to another aspect, a web support apparatus is provided with a support frame and a sag rail. The sag rail is carried by the frame and includes a friction-reducing material provided along at least portions of a top edge of the sag rail. 
     According to yet another aspect, a web support apparatus is provided with the sag rail. The sag rail has a top edge with local, raised portions intermittently spaced along the top edge to provide an air flow gap between adjacent pairs of the raised portions. 
     According to even another aspect, a web support apparatus is provided having a sag rail including a temperature regulator provided in the sag rail. 
     According to yet even another aspect, a support device is provided for a heated plastic sheet. The support device includes a thermally-regulated sag rail. 
     The present invention provides an advantage by reducing static, or start-up, friction between a heated web of thermoformable material and a web support structure when intermittently delivering the web so that the web is stationary, then moving, during an intermittent motion thermoforming operation. 
     The present invention provides another advantage by increasing uniformity of heat delivery to a web in an oven because the friction-reducing material is elevated above a top edge of a sag rail to provide gaps between the intermittent insert pieces of friction-reducing material. The gaps enhance delivery of heat to the web of material while the web is moved through an oven, or heat tunnel, so as to impart more uniform delivery of heat across and along the web of material. 
     The present invention provides yet another advantage in that the web is more uniformly heated by enhancing temperature control of the web supporting apparatus. To achieve this, a temperature regulating system is provided for controlling temperature of a friction-reducing material and an accompanying web support rail to maintain the friction-reducing material within a desired and safe temperature operating range within an oven, or heat tunnel, and to regulate temperature of the rail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are described below with reference to the following accompanying drawings. 
         FIG. 1  is a schematic side view representation of a web support apparatus and web conveyor incorporated into a thermoforming line; 
         FIG. 2  is a left side and vertical view of the thermoforming line of  FIG. 1  taken along line  2 — 2  of  FIG. 1 , between a web rotary unwind machine and a web conveyor, looking towards the web conveyor to illustrate the web conveyor and web support apparatus, but omitting the heat tunnel; 
         FIG. 3  is an enlarged view of the encircled region  3  of  FIG. 2 , illustrating in greater detail the web conveyor and web support apparatus of  FIGS. 1–2 ; 
         FIG. 4  is a vertical sectional view taken along line  4 — 4  of  FIG. 3  and illustrating a sag rail for the web support apparatus within the web conveyor of  FIGS. 1–3 , and further illustrating the heat exchanger and control system for controlling temperature of the sag rails; 
         FIG. 5  is a partial plan view of the web conveyor and web support apparatus taken along line  5 — 5  of  FIG. 2 ; 
         FIG. 6  is a vertical view of one sag rail taken along line  6 — 6  of  FIG. 5 , but eliminating remaining portions of the web conveyor; 
         FIG. 7  is an enlarged isometric view taken within the encircled region  7  of  FIG. 6  illustrating insert pieces of friction-reducing material supported in spaced-apart relation along the sag rail; 
         FIG. 8  is a vertical sectional view taken along line  8 — 8  of  FIG. 7  illustrating a supported configuration of a sag rail within a rail clamp as taken along line  8 — 8  of  FIG. 6 ; 
         FIG. 9  is an enlarged, partial isometric view taken adjacent an upstream end of a web support apparatus having a plurality of laterally spaced-apart sag rails as provided in the conveyor of  FIGS. 1–8 ; 
         FIG. 10  is an enlarged perspective view taken within the encircled region  10  of  FIG. 9  further illustrating the mounting configuration of a sag rail on a conveyor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8). 
     Reference will now be made to a preferred embodiment of Applicant&#39;s invention. An exemplary implementation is described below and depicted with reference to the drawings comprising a web support apparatus and a conveyor having a web support apparatus according to one aspect of the present invention. However, alternative embodiments will be understood and described (where appropriate) with reference to the figures. 
     While the invention is described by way of the preferred embodiment, it is understood that the description is not intended to limit the invention to this embodiment, but is intended to cover alternatives, equivalents, and modifications which may be broader than this embodiment, such as are included within the scope of the appended claims. 
     Furthermore, in an effort to prevent obscuring the invention at hand, only details germane to implementing the present invention will be described in great detail. Presently understood peripheral details will be incorporated by reference, as needed, as being presently understood in the art. 
     A preferred embodiment web support apparatus is first described with reference to  FIGS. 1–10  and is identified by reference numeral  18 . As shown in  FIG. 1 , web support apparatus  18  forms one component of a thermoforming line  10 . Thermoforming line  10  starts with a delivery roll  12  of thermoformable web material that is carried on a delivery roll support frame (not shown), commonly referred to as a rotary unwind machine. A web (or sheet) of relatively thin, thermoformable plastic or foam material is unrolled from delivery roll  12  as it is conveyed by a chain conveyor  16  through a thermoforming oven (or heat tunnel)  24  where heat is delivered to web  14  to prepare the web to be formed into articles  28  via a thermoforming machine (or former)  26 . 
     Conveyor  16  includes a pair of chain conveyor rails  20  and  22  that each cooperate with a conveyor frame  23  and a former frame  27  to form a frame. The overall frame is carried in laterally spaced-apart relation by frame  27  of former  26  and frame  23  of conveyor  16  in order to support and convey respective lateral edges of thermoformable web  14  of plastic (or foam) material. 
     As shown, conveyor  16  delivers a web  14  of thermoformable material in a supported manner, via web support apparatus  18 , through adjustable thermoforming oven  24  and through thermoforming machine (or former)  26 . Oven  24  and former  26  are configured for cycle-based operation, with conveyor  16  delivering web  14  in intermittent increments from delivery roll  12  through oven  24  and into former  26 . Web  14  is held stationary during each forming operation within former  26  which causes the intermittent motion. Conveyor  16  is driven by a servo drive motor whose operation is controlled by a system operating computer (not shown). 
     A machine control system (not shown) is provided in the form of a combination of software and hardware typically provided on a dedicated system control computer to coordinate and control the operation of conveyor  16 , oven  24 , and former  26 . Additionally, a trim press (not shown) and a web recycling machine (not shown) can also be controlled via the same machine control system, and are placed downstream of former  26  to separate formed articles  28  from web  14  and to recycle the remaining web. Likewise, an article stacking and packaging apparatus (not shown) can also be provided downstream of former  26  for stacking and bagging articles  28 . It is presently envisioned that any of a number of presently available machine control systems can be utilized for controlling thermoforming line  10 , including “Ballerina”, presently commercially available from Irwin Research and Development, Inc., of Yakima, Wash. However, alternative machine control systems can include combinations of purely mechanical kinematic linkages. Additionally, conveyor  16  can be optionally constructed and controlled to continuously deliver web  14  through a processing machine at a constant line speed. For example, conveyor  16  can deliver web  14  through a pair of rotary forming and cutting dies which enables continuous feeding of web  14  during forming and cutting operations. 
     Thermoforming machine (or former)  26  is essentially a thermoforming rotary-driven press. Former  26  is illustrated here in simplified form since the actual construction and operation is not important to operation or implementation of web support apparatus  18 , as long as frame  27  of former  26  supports an exit end of conveyor  16 . Former  26  of  FIG. 1  has an upper and a lower kinematic drive linkage assembly (not shown) that is configured to support and drive associated upper and lower platens  34  and  36  for forming articles  28  in web  14  after web  14  has been used within oven  24 . Upper platen  34  includes a plurality of article cavities  30 , and lower platen  36  includes a plurality of complementary, corresponding article plugs  32  that cooperate to form articles  28  as plugs  32  are drawn into complementary cavities  30  during a forming operation. Such details of thermoforming machines (or formers) are presently understood in the art and additional features are not important to the implementation of the present invention. Accordingly, further details have been omitted in order to simplify description of the thermoforming line  10  in order to focus on description of conveyor  16 . 
     In operation, conveyor  16  unwinds and delivers web  14  from storage roll  12 , through thermoforming oven  24 , and through former  26  where web  14  is molded into articles  28 , as depicted in  FIG. 1 . Former  26  is opened and closed onto heated plastic web  14  by one or more rotary electric servo motor drives configured to drive associated kinematic linkages (not shown) and accompanying dies  34  and  36 . In this manner, the plugs  32  and cavities  30  of former  26  cooperate to impart molded features into web  14 . In operation, it becomes necessary to choreograph movement of former  26  with conveyor  16  in order to optimize production rate of articles  28  being formed in former  26 . For example, conveyor  16  is operated so as to feed web  14  when dies  34  and  36  are separated (or open), allowing feeding of new material from web  14  to be formed within former  26 . However, conveyor  16  is stopped when dies  34  and  36  of former  26  are closed together, or nearly closed, during an actual thermoforming step. 
     Further details of one exemplary construction for conveyor  16  (but omitting the web support apparatus of the present invention) are disclosed in U.S. Pat. No. 5,806,745, entitled “Adjustable Conveyor for Delivering Thin-Web Materials”, issued to Jere F. Irwin on Sep. 15, 1998. This U.S. Pat. No. 5,806,745 is incorporated herein by reference. 
     One suitable construction for thermoforming oven (or heat tunnel)  24  is disclosed in U.S. Pat. No. 5,893,994, entitled “Adjustable Length Heat Tunnel for Varying Shot Lengths”, issued to Jere F. Irwin, et al., on Apr. 13, 1999. This U.S. Pat. No. 5,893,994 is incorporated herein by reference. 
     One suitable construction for thermoforming machine  26  is disclosed in U.S. Pat. No. 5,773,540, entitled “Mold Assembly for Thermo-Forming Machine”, issued to Jere F. Irwin, et al., on Jun. 30, 1998. This U.S. Pat. No. 5,773,540 is incorporated herein by reference. 
       FIG. 2  illustrates in end view the configuration of web support apparatus  18  as it is supported and mounted integrally within conveyor  16 . More particularly, web support apparatus  18  comprises three individual sag rails  38 – 40  which are provided in laterally spaced-apart and longitudinally extending relationship between, and parallel with, chain conveyor rails  20  and  22 . The relative position can be adjusted by unclamping, moving, and reclamping each sag rail  38 – 40 . A plurality of insert pieces  80  are provided along a top edge of each sag rail  38 – 40 , elevated above the remaining portion of each respective sag rail, and are constructed of a friction-reducing material to reduce friction between a web of material being supported by and traveling therealong as conveyed along lateral edges by conveying chains of chain conveyor rails  20  and  22 . 
     As shown in  FIG. 2 , conveyor chains within rails  20  and  22  draw material along and over sag rails  38 – 40  to maintain contact with discrete insert pieces  80  so as to reduce contact friction therealong as a web of material (not shown) is conveyed through an oven (not shown) for heating and into thermoforming machine  26  for molding of articles. As shown in enlarged elevational view in  FIG. 3 , sag rails  38 – 40  cooperate to provide web support apparatus  18  in a configuration substantially parallel and equally spaced apart between chain conveyor rails  20  and  22 . Sag rails  38 – 40  are each supported at intermittent locations along their length by pairs of retainer bars  42  and  43 . Adjacent pairs of retainer bars  42  and  43  are secured together at each end by an end plate  74  that is affixed at each end to a respective one of the retainer bars  42  and  43  using a threaded bolt  72 . Further details of such construction for retainer bars  42  and  43  are illustrated in FIGS.  5  and  9 – 10 . Each sag rail  38 – 40  is affixed in the longitudinal position relative to retainer bars  42  and  43  with a retainer lock  76  and a rail clamp  78  that each clamp in a desired longitudinal position on each sag rail  38 – 40 , respectively, adjacent an upstream end. 
     As shown in  FIGS. 3 ,  5  and  9 – 10 , pairs of retainer bars  42  and  43  are secured to chain conveyor rails  20  and  22  using chain rail end clamp plates  62  and  64 , respectively. As shown in  FIG. 5–6  and  8 – 10 , individual rail clamps  78  position each sag rail  38 – 40  in a specific lateral position between rails  20  and  22  by securing in rigid engagement between retainer bars  42  and  43 . However, each sag rail can axially slide within additional, similar rail clamps  178  which are provided in spaced-apart relation downstream from rail clamps  78  at an upstream end. Rail clamps  178  differ from upstream rail clamps  78  in that rail clamps  78  have a T-shaped slot that compresses and secures to clamp onto a corresponding base flange on the sag rail  38 – 40  to affix rail clamp  78  at a specific axial location along the sag rail. In contrast, rail clamps  178  have a slightly larger sized T-shaped slot which, after bolting together the rail clamp  178 , the remaining T-shaped slot enables axial sliding of the sag rail  38 – 40  within the rail clamp  178  so as to enable thermal expansion and contraction of the rail member. Accordingly, remaining rail clamps  178  are spaced apart downstream of rail clamps  78  so as to laterally affix and position each respective sag rail  38 – 40 , but allow for axial sliding to provide for a thermal expansion and contraction of rail member  120 . 
     As shown in  FIG. 6 , a plurality of retainer locks  76  are also clamped securely onto the base flange of rail member  120  of the sag rail  38  to further secure the sag rail along a conveyor within an oven. More particularly, a retainer lock  76  is clamped onto rail member  120  immediately adjacent and upstream of each respective rail clamp  78  and  178 . In this manner, if rail clamp  78  loosens and rail member  120  is inadvertently allowed to slide axially downstream, retainer locks  76  will prevent rail member  120  from migrating in a downstream direction (which will tend to happen because of frictional forces acting on insert pieces  80  as a web is transferred there across). Such additional fixation provides a safety feature because migration of rail member  120  in a downstream direction would result in rail member  120  being deposited between dies of a thermoforming machine which could result in a significant amount of damage and destruction to the thermoforming machine and thermoforming line. Hence, retainer locks  76  provide a safety feature which prevents inadvertent downward migration of rail members  120  on each sag rail  38 – 40 . 
     As shown in  FIGS. 3–4  and  6 , sag rails  38 – 40  each receive a flow of temperature-regulated fluid in order to regulate temperature for the respective sag rail, and more particularly, to control the maximum operating temperature for each low-friction insert piece  80  when operating within an oven. As shown in  FIG. 3 , inlet hoses  52 – 54  and outlet hoses  56 – 58  are connected to an upstream end of each sag rail  38 – 40 , respectively. Inlet hoses  52 – 54  receive a supply of temperature-regulating fluid from a manifold  60 ; whereas outlet hoses  56 – 58  deliver the fluid back to manifold  60  where the fluid is ejected via an elbow  92  through an outlet muffler  94  (where the fluid is air) in response to opening of a solenoid  96 . A fluid supply line  88  provides a supply of temperature-regulated fluid, such as cooled (or heated) air, to manifold  60  for delivery through inlet hoses  52 – 54  down sag rails  38 – 40 , respectively, and back through outlet hoses  56 – 58  for ejection through outlet muffler  94  to ambient atmospheric pressure. 
     In one case, the supply of temperature-regulated fluid is air that is heated to a specific temperature. Such temperature-regulated fluid is pumped through the respective sag rail in order to elevate the temperature of the sag rail at start-up of the thermoforming line and oven. Once the oven has reached a desired operating temperature, the temperature-regulated fluid (or air) may be cooled air that is delivered in a metered manner through the sag rail in order to maintain the sag rail temperature within a desired range which is below the operating temperature of the oven. Hence, in this case the air is used as a cooling fluid to reduce the temperature of the sag rail and to particularly reduce the operating temperature of the friction-reducing insert pieces. For the case where the friction-reducing insert pieces  80  comprise polytetrafluoroethylene (or Teflon™), the Teflon™ has a desirable maximum operating temperature, such as 350 degrees Fahrenheit. However, a thermoforming oven may have heater elements that run in the 600–900 degree Fahrenheit temperature range. Accordingly, once the oven and conveyor are up to an operating temperature and desired steady state operating speed, it may be desirable to effectively cool the sag rail and insert elements to a desired acceptable operating temperature range particularly if the oven temperature exceeds a maximum allowable temperature for Teflon™. 
     Also shown in  FIG. 3 , a web of thermoformable plastic material (not shown) is edge-supported and moved by chains  44  and  46  within chain conveyor rails  22  and  24 , respectively, so as to be drawn taut across insert pieces  80  of sag rails  38 – 40 . In this manner, a heated web (or sheet) is supported to travel through an oven and into a former. Chains  44  and  46  have intermittent perforating fingers that pierce the web at intermittent locations so as to hold and convey the web therealong. Chains  44  and  46  are also tightened utilizing a chain tightening system (not shown) that is supplied with hydraulic fluid via chain tension or hydraulic feed lines  82  and  84  that are commonly fed from a main hydraulic feed line  86 . A common hexagonal shaft  48  is configured to drive chains  44  and  46  via sprockets (not shown), as taught in U.S. Pat. No. 5,806,745, previously incorporated herein by reference. 
     A thermocouple (or temperature sensor)  89  is also provided within manifold  60  for detecting temperature of fluid leaving sag rails  38 – 40  via outlet hoses  56 – 58 . Optionally, thermocouple  89  can be located within one or more sag rails  38 – 40 . A thermocouple lead  90  from thermocouple  89  provides an input signal to a rail temperature control system  98  of  FIG. 4 . 
     As shown in  FIG. 4 , sag rails, such as sag rail  39 , receive an inlet supply of temperature-regulating fluid via lower inlet hose  53  and ejects the fluid, after temperature regulating the sag rail  39 , via upper outlet hose  57 . In this manner, low-friction insert piece  80  is kept within a desired temperature range at its mounting location on sag rail  39 . 
     Each inlet hose, such as inlet hose  53  (of  FIG. 4 ), receives a supply of temperature-regulating fluid in a controlled and regulated manner via fluid supply line  88 . According to one construction, where the temperature-regulating fluid is cooled, a heat exchanger  100  is used to cool the supply of fluid, such as air, in response to computer-controlled rail temperature control system  98 . Alternatively, the fluid is heated (e.g., at start-up). Control system  98  receives input from thermocouple  89  which senses temperature of fluid leaving the respective sag rail. According to one construction, a fan is used to drive air through the heat exchanger to transfer heat between the heat exchanger and the air. In one case, the heat exchanger cools the air to a desired temperature. In another case, the heat exchanger heats the air to a desired temperature. 
     Control system  98  of  FIG. 4  includes processing circuitry  104  and memory  106 . A control algorithm  108  is stored within memory  106  and processed via processing circuitry  104  in order to implement control of heat exchanger  100  to deliver a desired temperature of fluid via supply line  88  to each sag rail, such as sag rail  39 . In this manner, a desired cooling (or heating) is provided to sag rail  39  and insert pieces  80 . Optionally, the rate of fluid flow can be adjusted to control the temperature of the sag rails. Further optionally, the flow can be pulsed between “on” and “off”. For example, the flow of room temperature air can be intermittently “pulsed” through each sag rail. Even further optionally, the fluid can be water, oil or even radiator fluid. In this case, a closed-loop fluid circuit is used, and heat exchanger  100  is provided in the circuit. 
     Control system  98  regulates the operation of heat exchanger  100  via control algorithm  108 , either based upon a feedback signal from thermocouple  89  or based upon a predetermined value stored in memory, to regulate temperature of temperature-regulating fluid within a desired range. In one case, the fluid temperature is controllably regulated between a minimum and maximum value. In another case, the temperature of the fluid is regulated below a maximum value. In yet another case, the temperature is regulated above a minimum value. In an even further case, the temperature is held at a target value, within a predetermined differential temperature tolerance range. For example, the fluid can be held within a range of 300–375 degrees Fahrenheit. Other examples are also possible. 
     As shown in  FIG. 6 , each sag rail  38 – 40  is similarly constructed and sag rail  38  is illustrated for purposes of showing the flow of temperature-regulated fluid that is delivered by way of the control system of the present invention to regulate temperature of sag rail  38 . More particularly, inlet hose  52  delivers fluid into a lower cooling cavity  112  of an extruded aluminum rail member  120  of sag rail  38 . By way of a bridge cooling cavity  116 , fluid travels from lower cooling cavity  112  up to upper cooling cavity  114 . Accordingly, a temperature-regulating (either cooling or heating, or both) fluid circuit  110  is provided by the combination of lower cooling cavity  112 , bridge cooling cavity  116 , and upper cooling cavity  114  extending as a circuit through extruded rail member  120 . To facilitate construction, a pair of threaded and sealed plugs  118  and  119  are provided at the downstream ends of cooling cavities  112  and  114 , respectively, adjacent bridge cooling cavity  116 . 
     As shown in  FIG. 5 , a single pair of retainer bars  42  and  43  are shown affixed between rails  20  and  22 . However, it is understood that a plurality of pairs of retainer bars  42  and  43  are spaced apart longitudinally along rails  20  and  22  in order to support sag rails  38 – 40  in multiple locations. Typically, pairs of retainer bars  42  and  43  are spaced approximately eight feet apart along the length of conveyor  16 . More particularly, an upstream end of sag rails  38 – 40  are each supported by way of rail clamp  78  which laterally and longitudinally affixes each sag rail  38 – 40 . Additionally, retainer lock  76  is rigidly affixed at an axial location on each sag rail  38 – 40  and prevents axial movement of the respective sag rail as it abuts against rail clamp  78 . Similar rail clamps  78  are provided upstream of rail clamps  178 . 
     Retainer bars  42  and  43  are held in a precise spaced-apart relation by securing an end plate  74  at each end using a pair of threaded bolts  72  that thread into a threaded bore in each end of each retainer bar  42  and  43 . A respective clamp plate  62  and  64  is provided adjacent each end. Clamp plate  62  supports bars  42  and  43  by receiving a retainer bar clamp plate  66  therebelow via a pair of threaded fasteners (not shown) (see  FIG. 10 ) that pass through a clearance bar in clamp plate  62  and thread into a hidden, threaded bore in each clamp plate  62  and  64 . Each plate  62  and  64  is secured to a respective rail  20  and  22  using a pair of recessed-head fasteners  70  and nut  71 . Such mounting is also illustrated in  FIG. 10 . When the distance between rails  20  and  22  is adjusted laterally, clamp plate  66  is loosened from clamp plate  62  (by loosening the threaded fastener) such that rail  20  can be moved outwardly along retainer bars  42  and  43  (as shown in  FIG. 10 ) and reclamped. Accordingly, clamp plate  66  provides a lower clamp plate that cooperates with clamp plate  62  that provides an upper clamp plate. 
     As shown in  FIG. 10 , clamp plate  66  is constructed similar to retainer bar clamp plate  132  of  FIGS. 6 and 8 , wherein a fastener similar to fastener  138  is used to clamp and unclamp clamp plate  66  relative to plate  62  so as to rigidly secure and release rail clamps  78  and  178  against retainer bars  42  and  43 . 
     As shown in  FIG. 8 , the spaced-apart configuration of sag rails  38 – 40  and the longitudinally spaced-apart configuration of low-friction insert pieces  80  provide support for a web of thermoformable material as it passes through an oven and into a former for processing of articles in the web. 
       FIG. 6  also illustrates the placement of an upstream rail clamp  78  and an adjacent, upstream retainer lock  76  along extruded rail member  120 . At least one additional rail clamp  178  is provided at a downstream location along extruded rail member  120  for clamping to a similar pair of retainer bars (not shown). Optionally, another retainer lock  76  is provided adjacent and upstream to rail clamp  178  at the downstream location(s). Rail clamps  78  are rigidly affixed at longitudinal locations along rail member  120 ; whereas rail clamps  178  laterally secure member  120 , but enable axial sliding therealong. 
     As shown in  FIG. 8 , rail clamp  78  comprises a pair of rail clamp members  134  and  135  and a lower clamp plate  132 . Rail clamp  78  is released for adjustment along the retainer bars (not shown) by separating clamp plate  132  from rail clamp member  134 . Separation is achieved by releasing fasteners  138  which pass through a clearance bar in clamp plate  132  and thread into a hidden, threaded bore in rail clamp member  134  contained therebetween. By tightening such fasteners, clamp plate  132  is driven toward rail clamp members  134  and  135  so as to trap (or clamp) retainer bars that are provided therebetween at either end. 
       FIG. 7  illustrates in greater detail the placement and mounting of individual low-friction insert pieces  80  along extruded rail member  120 . More particularly, a plurality of serpentine receiving apertures  122  are machined transversely through extruded rail member  120 , along a top edge. Serpentine receiving aperture  122  is formed in complementary relation to receive a serpentine-shaped bottom portion provided on the bottom of insert piece  80 . Accordingly, insert piece  80  is received in a lateral direction into serpentine receiving aperture  122 , after which a pair of elliptical, or oblong, plates  124  and  125  are mounted together on either side of insert piece  80 . More particularly, aluminum rivets  126  are used to entrap insert piece  80  on either side by plates  124  and  126 . Plates  124  and  126  are slightly larger than a base portion of insert piece  80  so as to expand the outer bounds of receiving aperture  122 . Accordingly, low-friction insert piece  80  is retained within receiving aperture  122  by the sandwiched (and assembled) action of plates  124  and  125 . 
     Also shown in  FIG. 7 , each low-friction insert piece  80  extends elevationally above a top edge of rail member  120  so as to elevate a web thereabove. Elevated insert pieces  80  provide for passage of hot air and gases between adjacent insert pieces  80  along a common rail member  120  so as to more evenly and fully heat a sheet of thermoformable material being supported thereabove. In this manner, rail members  120  do not provide a complete lateral baffle under the web that prevents lateral transfer of heat from one side of rail member  120  to another side of rail member  120  as a web is provided thereabove. 
     According to one construction, low-friction insert piece  80  is formed from polytetrafluoroethylene (otherwise known as Teflon™, of E.I. du Pont de Nemours and Company). Optionally, other relatively low-friction insert pieces can also be provided as long as a coefficient of friction for insert piece  80  is less than that for rail member  120 . Optionally, rail member  120  can be partially or completely coated or encased n such a friction-reducing material. Further optionally, extruded rail member  120  can be formed with elevated bumps, after which a polytetrafluoroethylene (or other low-friction) coating is applied there atop. The material of the insert piece  80  has a low coefficient of static and dynamic friction than does the underlying material of extruded aluminum rail member  120 . 
     Even though it is advantageous to provide insert pieces  80  elevationally above rail member  120 , it is not necessary. Accordingly, rail member  120  can have a relatively low-friction coating or insert piece provided there atop which has an elevationally uniform configuration extending along a length of rail member  120 . 
       FIG. 8  further illustrates the entrapped mounting of insert piece  80  atop extruded rail member  120  using steel (or metal) plates  124  and  125  as entrapment washers. Plates  124  and  125  are slightly larger than a base portion of serpentine receiving aperture  122  which traps insert piece  80  atop rail member  120 . Additionally, lower cooling cavity  112  and upper cooling cavity  114  are shown extending axially through rail member  120 . 
     Also shown in  FIG. 8 , rail clamp  78  includes two rail clamp members  134  and  135  that cooperate, when assembled together, to form a T-shaped elongated slot  128  sized to receive a base flange  130  of rail member  120  in relatively conforming, but slidable axial relationship. A threaded fastener  136  secures rail clamp members  134  and  135  together a fixed distance so as to provide a slightly under-sized T-shaped slot  128  that longitudinally entraps base flange  130  of extruded rail member  120 . More particularly, a clearance bore is provided through rail clamp member  134  for fastener  136 . Likewise, a hidden, threaded bore is provided into rail clamp member  135 . Accordingly, threaded engagement of fastener  136  with the threaded bore in rail clamp member  135  drives clamp members  134  and  135  together so as to longitudinally affix extruded rail member  120  along rail clamp  78  (and associated retainer bars). In contrast, rail clamps  178  are machined to have a slightly wider T-shaped slot  128  than is found in rail clamp  78 . Accordingly, extruded rail member  120  is secured laterally from movement by rail clamps  178 . However, axial translation is allowed between extruded rail member  120  when rail clamp  178  is assembled thereabout because base flange  130  is slightly smaller than T-shaped slot  128 . Hence, thermal expansion and contraction of member  120  can be accommodated while still laterally retaining member  120  along a conveyor at downstream locations from rail clamp  78 . Optionally, rail clamp  78  can be provided at a midstream location or downstream location along extruded rail member  120 , with remaining locations utilizing rail clamps  178 . Accordingly, thermal expansion will be allowed in different directions according to such alternative placements of rail clamps  78  or  178 . 
       FIG. 9  illustrates in partial isometric view a breakaway portion of conveyor  16  showing a web  14  being conveyed downstream along a web travel path  150  through an oven (not shown). As shown herein, web support apparatus  18  comprises laterally spaced-apart and axially extending sag rails  38 – 40 . Sag rails  38 – 40  extend within an oven relative to oven heating rods  140  which can be arranged either axially or transversely (or both) of sag rails  38 – 40 . 
     Although a single pair of retainer bars  42 – 43  is shown in  FIG. 9 , it is understood that additional pairs are provided downstream along sag rails  38 – 40  and between rails  20  and  22  (see  FIG. 5 ). In operation, a respective chain  44  on each rail  20  (and  22 , see  FIG. 5 ) pierces a respective edge on a web of thermoformable material, carrying the web in a downstream direction over elevated low-friction insert pieces  80  of sag rails  38 – 40 . Accordingly, individual gaps  148  are provided between adjacent insert pieces  80  along each respective sag rail  38 – 40  so as to provide for transverse passage of gases and heat which traverse over the respective sag rails  38 – 40 . Hence, more uniform heating is provided to a web within an oven in which web support apparatus  18  is provided. Furthermore, the low-friction nature of insert pieces  80  reduces static friction which intermittently occurs as a web of thermoformable material is stopped and moved intermittently during a thermoforming operation. Hence, the static friction that occurs at start-up when the web is again moved intermittently is significantly reduced by the relatively low friction provided by insert pieces  80 . Hence, an accelerated operating speed is provided for a thermoforming line and more controllable moving conditions are imparted to a web when using web support apparatus  18 . 
     During development, testing was performed on a sag rail having a substantially uniform elevational top surface, but without having a friction-reducing, or relatively low friction top portion. During such testing, a shear effect was created between the chain rails  20  and  22  (also see  FIG. 2 ) when a heated web of plastic material is stopped during an intermittent thermoforming operation, after which chain rails  20  and  22  are suddenly driven forward. Static friction buildup between the heated web and the intermediate sag rails was found to cause a shearing effect within the web which tended (in some cases) to produce wrinkles and undesirable surface finish effects in the web of heated thermoformable plastic (or foam) material. Accordingly, testing and development led to the improvements presented herein. 
       FIG. 10  illustrates in greater detail the mounting of one selected sag rail  38  to a pair of retainer bars  42  and  43  within an oven. As shown in  FIG. 10 , rail clamp members  134  and  135  are joined together via a pair of threaded hexagonal-head fasteners, or bolts,  136 , each with a lock washer  137 . Fasteners  136  extend through a clearance bore in member  134  and into a threaded hidden bore in member  135  so as to draw together members  134  and  135  to laterally and axially affix rail member  120  relative to retainer bars  42  and  43 . Similarly, retainer bar clamp plates  66  and  132  each have a pair of vertical clearance bores that align with hidden, threaded bores in clamp plate  62  and clamp member  134 , respectively, which enables clamping of clamp plates  66  and  132  toward claim plate  62  and clamp member  134  so as to entrap retainer bars  42  and  43  therebetween in rigidly affixed engagement. Additionally, retainer lock  76  rigidly clamps to the base flange of rail member  120  by securing a pair of threaded fasteners  146  (and lock washers  147 ) through a pair of clearance holes in retainer lock member  142  into blind, threaded bores provided in retainer lock member  144 . Accordingly, threading of fasteners  146  into the bores of member  144  drive together members  142  and  144  so as to rigidly clamp onto the bottom flange of rail member  120 . In this manner, retainer lock  76  provides a safety lock that prevents downstream migration of rail member  120  that might otherwise be caused by frictional drag of the web as it moves over the sag rail in a downstream direction. More particularly, retainer lock  76  will physically abut against rail clamp  78  so as to prevent further downward migration of member  120 . 
     When mounting retainer locks  76  along member  120 , it is not necessary that retainer locks  76  be provided in direct physical abutment with rail clamps  78  (as well as rail clamps  178 ). There can be provided slight gaps which further provide room for expansion and contraction of the rail member  120  as it passes through a heated oven which can impart dimensional changes to related components within the web support apparatus and conveyor. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.