Abstract:
A rod guide fixedly molded around the shank of a sucker rod string with the rod guide including a radially inner non-erodible zone and a radially outer erodible zone. The non-erodible zone includes a radially inner substantially sleeve-shaped portion having an inner cylindrical surface for gripping engagement with the rod. A plurality of flow through channels are spaced outward of the substantially sleeve-shaped portion. Each flow through channel extends axially along the rod guide and has a maximum circumferential width greater than any gap in the radially outer surface of the erodible zone circumferentially aligned with and radially outward of the respective flow through channel. The radially outer surface of the erodible zone may have a cylindrical outer configuration, such that a radially outward substantially sleeve-shaped portion is provided for engagement with the tubing. The upper and lower surfaces of the rod guide may each be inclined such that the radially outer surface of the rod guide extends longitudinally in excess of the inner cylindrical surface of the sleeve-shaped portion and in engagement with the rod. A guided sucker rod includes an elongate rod having threaded end connectors for mating engagement with an adjoining sucker rod and one or more rod guides fixedly molded thereon. According to the method of the invention, left-side and right-side molds are created each including one or more elongate cavity creating member supported in a cantilevered fashion from a supporting end block. A plastic material is injected into the mold cavity, and the supporting end block is then moved longitudinally along the axis of the rod to remove the one or more channel performing members from the molded rod guide.

Description:
This application is a division of Ser. No. 09/023,595 field Feb. 13, 1998, now U.S. Pat. No. 6,065,537. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to rod-pumped oil wells. More specifically, the invention relates to rod guides that centralize sucker rods within tubing and scrape paraffin from the interior wall of tubing. A rod guide having a high erodible wear volume according to this invention has its by-pass area flow channels placed predominately in the non-erodible portion of the guide. 
     BACKGROUND OF THE INVENTION 
     As crude oil is depleted from an underground formation, pressure in the formation decreases to the point that oil must be pumped to the surface. One of several methods for removing crude oil from an underground formation employs a pump-jack located on the surface. The pump-jack is connected via a sucker rod string to a downhole pump at the bottom of the producing oil well. The sucker rod string comprises many sucker rods, each rod connected end-to-end to another rod by a coupling. The entire rod string extends down into a tubing string that is commonly contained within a well casing. The exterior well casing and internal tubing string are permanently installed after drilling the well. The tubing string serves as a conduit for the fluid produced, and the driving force for this production is transmitted to the downhole pump via the sucker rod string positioned within the interior of the tubing string. 
     The sucker rod string commonly reciprocates inside the tubing string as a result of the upward and downward motion of the pump-jack to which the rod string is fastened. Cyclical upward and downward motion of the pump-jack is thus communicated to a downhole pump located at the lower end of the tubing string. In response, the pump forces the produced fluids collected at the bottom of the well up the tubing string to the surface. In other applications, a progressing cavity (PC) pump is used at the bottom of the well, and in these applications power to the pump is transmitted via a rotating sucker rod string. 
     The production fluid in the tubing string typically acts as a lubricant for the sucker rod string. Lubrication is derived from the fluid because it is commonly a mixture which includes crude oil, along with water and natural gas. Typically also included in the production fluids are dissolved and undissolved salts, gases and other formation minerals, such as sand. The recovered crude oil is commonly stored in a tank near the well until it is removed for refining. Natural gas is removed in a pipeline. Water is usually reinjected into the production formation or in a disposal well in another formation close to the production formation. 
     Due to deflections of both the tubing and the rod string, contact may occur between these components. Even though the lubricating bath of the production fluid is present in the tubing, wear is incurred on the rod string and tubing when contact is made. The rod couplings typically have the largest outer diameter of the various components of the rod string and therefore incur, and cause, the most wear. Produced fluids that flow in the rod and tubing annulus also cause wear in the form of abrasion and corrosion. Through time, all these wear factors may lead to parting of the rod string or to the development of holes in the tubing. 
     When a hole develops in the tubing, pressure is lost inside the tubing. Production will then be pumped into the annulus between the tubing and the casing rather than to the surface for collection and storage. When a sucker rod separates, when a rod coupling breaks, or when holes are created in the tubing, the sucker rods and/or tubing must be pulled from the well and inspected in detail for the extent and nature of the damage. Damaged rods and tubing must be replaced. The resultant down-time as well as the workovers are a great expense to the well owners. Therefore, methods and apparatus for reducing or eliminating costs associated with lost production of hydrocarbons, equipment replacements and workovers are of great benefit to the well owners. 
     A well known method of preventing wear to the rods and tubing is the use of rod guides, also known as centralizers and paraffin scrapers. In cases involving a reciprocating rod string, paraffin scrapers may also serve as centralizers to reduce wear, in addition to their implied purpose of removing paraffin from the walls of the tubing. Rod guides have a greater outer diameter than other parts of the rod string. As such, the guides are sacrificial and protective. Rod guides retard rod and tubing wear by incurring most of the wear that does occur. 
     On the average, six rod guides are normally attached at various locations on each sucker rod in the rod string, but as many as ten or more locations per rod or as few as one location per rod may be used. As such, the guides act as a sacrificial and protective buffer between the rod string and the tubing. Wear occurs to the guide as it protects the rod string and the tubing and results in a reduction of the protective thickness of the guide over time. 
     The wearing effects suffered by the rod guides will eventually cause the guides to have an outer diameter which will approach and become similar to the diameter of the couplings or parts of the rod string larger than the shank or body of the sucker rod. When this happens, the guide will no longer buffer the contact between the rod string and the tubing. The rod guides must then be replaced. 
     The general state of the art may be gathered by reference to a Rod Guide/Centralizer/Scraper Catalog published in 1997 by Flow Control Equipment (FCE) Inc. This catalog discusses rod guide material selection, paraffin scrapers; classic rod guide designs such as the standard and slant blade; high performance designs such as the NETB, Stealth and Double Plus; rotating rod guides for PC pumps such as the Spin-Thru and the PC Plus; and field installed guides (FIG&#39;s) such as the Lotus twist-on, NEPG, Lotus Rubber and Guardian polyguides. Also relevant to the general state of the art are patents to rod rotators and stabilizing bars. 
     Many of the design considerations applicable to any rod guide for either rotating or reciprocating sucker rod strings are discussed in a 1993 publication by Charles Hart entitled “Development of Rod Guides for Progressing Cavity (PC) Pumps”, a 1995 publication by Randall G. Ray entitled “Determination of Rod Guide Erodible Wear Volume,” and a 1993 publication by Milton Hoff entitled “Hydraulic Drag Forces on Rod Strings.” The general concept of erodible wear volume EWV and specific formulations as “gross” and “net” EWV are used herein in accordance with the use in these publications. In particular, the portion of a rod guide between the largest outer diameter on the rod string (typically the coupling diameter) and the inner diameter of the tubing string is the volume of the guide which can prevent damaging metal-to-metal contact. This protective volume of the rod guide is referred to as EWV. EWV is an important indicator of rod guide performance. The amount of the rod guide outside the outer diameter of the sucker rod couplings is in general referred to a Gross Erodible Wear Volume or Gross EWV. A more refined concept, which is known as Net Erodible Wear Volume, is that amount of the rod guide material that will erode before the sucker rod coupling contacts the tubing. Net EWV is always less than Gross EWV in conventional rod guide designs when the rod string is reciprocated to drive the downhole pump. Even a reciprocating rod string should be slowly rotated during reciprocation to maximize the useful life of the rod guides. An underlying assumption of both of these EWV definitions is that the rod string is continuously rotated and that the rod guides wear evenly. Also, both definitions are based on the assumption that the rod string is in tension and not in compression. In some rod guide designs, the Gross and Net EWV may be almost the same but as they approach equality, then fluid bypass area decreases and the flow resistance or drag around the guide increases to unacceptable levels. It is the primary objective of the invention presented herein to generate more efficient rod guide designs which have Gross EWV approximating Net EWV without sacrificing the necessary bypass area and geometry necessary to achieve desired levels of flow resistance or drag. 
     For clarity and ease of discussion, a rod guide may be considered to have a radially inner non-erodible zone and a radially outward erodible zone. The boundary line between the two zones, namely the erodible and the non-erodible zones, will be considered to be the projected circumference of the largest outer dimensions of any component anticipated to be on the rod string in the operative region where the respective rod guide is located, which typically will be the rod couplings above and below the respective rod guide. “Operative region” means that section of the rod string close enough to the rod guide so that it may be expected that the rod guide will furnish some protection to the rod and its couplings. It is meant to exclude for definitional purposes couplings or other rod string elements which may be several rod lengths away from the rod guide and which would have no effect on the function or performance of the rod guide, and thus no effect on the guide dimensions at issue. As used herein, the terms “by-pass” and “flow through” are intended to be synonymous and interchangeable. 
     U.S. Pat. Nos. 586,001 and 1,600,577 are directed to a cleaner for oil well tubing and a paraffin scraper, respectively. Both disclosures have a gross similarity to some of the embodiments of the present invention but differ in intent, function, material and design. The same may also be said of U.S. Pat. No. 2,153,787, which is directed to the shrink fitting of a guard by extraction of a plasticizer. A flexible guide is taught in U.S. Pat. No. 2,651,199. A method of on-site molding of scrapers is disclosed in U.S. Pat. No. 3,251,919. 
     U.S. Pat. Nos. 2,863,704 and 4,997,039 disclose a combination rod guide and sand purging device. Several of the embodiments referred to in the materials cited above are disclosed in U.S. Pat. Nos. 4,088,185, 5,115,863 and 5,277,254. Recently disclosed variations of a rod guide are found in U.S. Pat. Nos. 5,358,041 and 5,492,174. 
     None of the above references are directed to the concept of the present invention as set forth and described below. The present invention overcomes the deficiencies of the prior art and achieves its objectives by maximizing EWV while providing adequate flow through paths in and around the guide to both prevent excessive hydraulic drag during movement of the guide with respect to the produced fluid and avoid the creation of an excessive pressure drop as the guide passes through the produced fluid during the downward motion of the sucker rod string. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to maximizing the EWV of the guide while at the same time providing for sufficient flow through and around the guide to achieve the necessary or desired low pressure drop for the particular operating conditions in which the rod guide is used. As will be developed further below, the concept of the present invention calls for maximizing the ratios of the EWV to the total volume (TV) of a rod guide as well as the EWV to the flow resistance or drag of a rod guide. Ideally one of the best designs would have a cross section that resembles a bicycle wheel with as few spokes as possible. 
     The present invention utilizes plastic injection molding technology to secure the rod guide to the sucker rod while also preferably obtaining the formation of the necessary flow passages and open areas in or around the rod guide without resorting to drilling or other subsequent mechanical processes to obtain the desired flow passages. 
     A suitable rod guide according to the invention is secured to a rod string which is then placed in the tubing, with the guide functioning to centralize the rod string in the tubing while it passes through the tubing to the downhole pump and thereby minimizes wear between the rod string and the tubing. The rod guide has a radially inner non-erodible zone available for flow through and a radially outer erodible zone, as defined above. 
     An object of the present invention is to maximize the erodible wear volume of a rod guide while maintaining adequate flow through and around the guide to obtain a desired low pressure drop or drag across the guide. 
     It is an object of the present invention to provide an improved centralizing device which overcomes the deficiencies of the prior art between Gross and Net EWV and at the same time provides for high erodible wear volume consistent with the desired high flow through and low drag characteristics. 
     It is a feature of the present invention to provide for the molding of centralizers on the rod without having to resort to a drilling or similar operation to produce fluid flow paths in the molded guides resulting in the desired flow through for the guide with a high erodible wear volume. 
     It is a feature of the present invention to provide an improved and low cost rod guide which averts contact between the sucker rod string and the tubing of a producing oil well. 
     It is a further feature of the present invention to provide an improved rod guide that may clean mineral scale and paraffin deposits from the interior surface of the tubing when the guide is fixed to a reciprocating rod string. 
     It is another feature of this invention to achieve the above two features with a provision of an EWV which approaches the maximum obtainable in terms of Gros and Net EWV while providing a desired high flow through and low drag characteristics when the rod guide is in a typical application. 
     Still another feature of the invention is a rod guide molded around a rod intended to be placed within the tubing, with the rod guide having a high EWV and flow channels or bypass areas predominately located in the non-erodible zone of the rod guide. 
     A significant advantage of the present invention is that the rod guide may achieve the above objects and features while the guide remains sturdy, compact, durable, simple, ecologically compatible, reliable, and inexpensive and easy to manufacture and maintain. 
     Other objects, features and advantages of the present invention, as well as a fuller understanding of this invention, may be had by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to facilitate the understanding of the present invention, reference will now be made to the appended drawings of preferred embodiments of the present invention. The drawings should not be construed as limiting the invention, but are exemplary only. 
     FIG. 1 is a side view of a typical well having a reciprocating rod string provided with rod guides of the present invention. 
     FIG. 2 is a side view of one embodiment of a rod guide of the present invention. 
     FIG. 3 is a top or end view of the rod guide shown in FIG.  2 . 
     FIG. 4 is an isometric view of the molds for the moving and stationary platens of a molding system used to mold the present invention on a rod. The front right of each side mold supports the cavity rods in a cantilevered fashion. These rods are withdrawn before the mold is opened. The upper side mold is mounted to the stationary platen and is shown positioned adjacent the rod for a molding operation. The lower side mold is mounted on the moving platen and away from the rod. In this view, the moving platen is retracted and the molds are in the open position. The cavity rods are shown partially inserted for clarity. 
     FIG. 5 is an isometric view of the molding apparatus in accordance with the present invention after the separation of the mold from the rod following the molding of a guide on the rod. 
     FIG. 6 is another embodiment of the present invention with an enlarged flow through area creating a rod guide having a generally Maltese cross configuration which can be achieved by changing the configuration and cross-section of the cavity rods. 
     FIG. 7 is an end or top view of another embodiment of the present invention in which the rod guide has expanded internal flow through cavities as well as external flow channels, both of which can be obtained by changing the configuration and cross-section of the cavity rods and mold geometry. 
     FIG. 8 is an end or top view of yet another embodiment of a rod guide in accordance with the present invention in which the generally Maltese cross shaped blades are provided with flow through cavities. The outer surface of the guide is off-set at its center of curvature from the rod center to provide an outer surface conforming to the internal curvature of the tubing. In all cases, the outside diameter of the guide is only slightly less than the inside diameter of the tubing. 
     FIG. 9 is a side cross-sectional view of an embodiment of a rod guide in accordance with the present invention in which the outermost portions of the guide extend longitudinally parallel to the axis of the rod string and in excess of the portion of the guide molded to and in contact with the rod. 
     FIG. 10 is a top or end view of another embodiment of a rod guide of the present invention in which the space between the support arms of the rod guide has been enlarged to provide additional flow through capacity. 
     FIG. 11 is an end or top view of an embodiment of the present invention in which four or more of the two bladed rod guides have been molded on the rod, with each successive guide indexed 45 degrees with respect to the next adjacent rod guide in a nesting approach to concentrate the EWV, which is undesirably low for a single two bladed guide alone but increasingly effective as more two bladed guides are indexed and molded closely together. 
     FIG. 12 is a side view of a portion of the array shown in FIG. 11, illustrates only two of the two blade rod guides indexed at 90°. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is perhaps best understood by reviewing the first principles upon which the invention is based. As has been noted above, it is desired to maximize the EWV relative to the TV of a rod guide and simultaneously, at least to the extent desired or necessary, maximize the fluid flow channels through and/or around the rod guide to minimize the adverse affects of drag, turbulence or pressure drop across the guide. 
     The maximum EWV may be obtained by filling the entire area between the rod coupling outside diameter and the inner surface of the tubing with rod guide material like the rim on a bicycle wheel. By maintaining complete circumferential outer contact of the guide with the tubing inside diameter and also providing flow through the guide in the area between the outer diameter of the rod coupling and the outer diameter of the sucker rod should fluid flow through the guide is provided and erodible wear volume is maximized. As additional flow through holes in the guide are provided, usually in the preferred embodiments in a symmetrical pattern, flow is increased to the desired level with a desired low pressure drop and without decreasing the EWV. Preferably at least three through holes are thus provided in the guide. However, the structural integrity of the rod guide is reduced in the process. The present invention balances these factors in a unique manner to provide a moldable rod guide with a high EWV, a desired structural integrity, and flow through the guide to achieve a low pressure drop or low drag. 
     As holes in the rod guide are enlarged, a Maltese cross configuration such as shown in FIG. 6 may be formed by support arms which interconnect a radially inner substantially sleeve-shaped portion in gripping engagement with the rod with a radially outward sleeve-shaped portion forming a cylindrical outer surface of the rod guide essentially equal to the inside diameter of the tubing. The outer surface may be separated, as shown in FIG. 8 or FIG. 10, to reduce the EWV and provide for greater flow through capacity. Additional flow through capacity (by-pass area) may thus be obtained by increasing the flow through area in the erodible zone of the rod guide. Ideally, the erodible zone of the rod guide is maximized while still providing for high flow through capacity, and the resulting design has a sufficient structural integrity for a molded rod guide. To obtain these objectives, the relatively simple rod guide molding process becomes more complicated. A related concept involves the longitudinal expansion of the radially outer portions of the rod guide as shown in FIG.  9  and will be discussed in greater detail below. 
     Referring to FIG. 1, a pumping apparatus  100  is shown for pumping fluids from a well  102  and through a string of tubing  106  disposed within well casing  108 . Connected to the pumping apparatus  100  is a string of sucker rods  105  connected by coupling, such as typical coupling and pin connector means  104 . The pumping apparatus as shown in FIG. 1 drives the rod string in a reciprocating manner to pump fluid to the surface through the well tubing. The rod string  105  may be rotated by a rod rotator  114 , if desired, to distribute wear more evenly to both the rod guides  107  and the sucker rods  105 . 
     When the pumping apparatus  100  is on the down stroke of its reciprocating action, the string of rods  105  move axially within the tubing  106  to operate the downhole pump (not shown). A plurality of rod guides  107  of the present invention are fixedly engaged around the sucker rods  105  at selected locations throughout the length of the rod string  105 . During this reciprocating movement of the string of sucker rods  105 , the well fluids are caused to flow upwardly in the tubing  106  on the upstroke and the rod guides  107  fall through the fluid on the downstroke. 
     FIG. 2 shows a rod guide  200  molded to a rod  202 . The generally cylindrical rod guide body  204  has a circumferential outer surface  210  and is provided with a plurality of cylindrical holes  206  which end at the top and bottom surfaces  208  and  209  of the rod guide, respectively. As a result of the formation of the holes by cantilevered cavity rods as discussed subsequently, the holes  206  may have excess nipple material  212  at one end, as will be made more apparent by considering the molding process described below. This excess material  212  is shown exaggerated in FIG. 2 for clarity. 
     When seen from a top or end view, the holes  206  appear as holes  306  in FIG. 3 wherein the rod guide  300  is molded about rod  302  and has an outer circumferential surface  304  sized for initial contact (or approximately so) with the internal surface with the tubing (not shown). The outer surface of the couplings in the operative region of the rod  302  is shown in dashed lines and represents the circumferential boundary  308  which defines the inner limit of the erodible wear volume (EWV)  310  which extends to the outer surface  304 . The area between the boundary line  308  and the rod  302  thus defines the non-erodible zone of the rod guide. In general, the rod guide may consist of a radially inner non-erodible zone and a radially outward erodible zone. As noted above, the boundary line between the two zones, the erodible and the non-erodible zone, is the projected circumference of the largest outer dimension of any component anticipated to be on the rod string in the operative region of the rod guide. The boundary line which in this example is equal to the outside diameter of the nearest rod coupling is thus the dashed line  308  shown in FIG.  3 . The erodible zone contains that material in the region between the boundary line  308  and the outer surface of the rod guide  304  which is only slightly less than the inner diameter of the tubing string. The erodible zone includes that volume of material in the rod guide which may be eroded in use before a component on the rod in the operative region of the rod guide contacts the tubing. It is desired for the rod guide to have the maximum amount of material in the erodible zone and thereby to have the maximum EWV for a given length of guide. At the same time, it is desired to provide for adequate flow through capacity (by-pass area) through the rod guide by providing flow channels, holes  306 , through the rod guide. These holes will preferably be located predominately in the non-erodible zone of the rod guide. 
     As the size of the flow through holes is enlarged to provide for a greater volume of fluid flow through the rod guide without increasing drag and pressure drop, a configuration such as shown in FIG. 6 may result, wherein a rod guide  600  is molded on rod  602 . The guide  600  is held in place on the rod by a radially inner substantially cylindrically shaped portion  604  of the non-erodible zone which surrounds the rod  602  and in gripping engagement therewith as a result of the molding process. The enlarged flow through holes  610  form a plurality of support arms  606  in the form of a Maltese cross which connect the radially inner portion  604  with a radially outer cylindrical surface  608  of the erodible zone for complete circumferential contact with the tubing (not shown). 
     As shown in FIG. 7, a rod guide  700  has support arms which include indentations defined by  704  and erodible wear surfaces  702 . Flow through cavities  706  are spaced circumferentially about rod  710 , and additional flow capacity is provided by the circumferential spacing between the indentations  704 . A similar expansion of the flow through area may result in a Maltese cross of the form as shown in FIG. 8, in which a rod guide  800  has an expanded flow through area bounded by surfaces  804  and flow through holes  806 . The outer linear surface  802  has a diameter slightly smaller than the inside diameter of the tubing (not shown). The center  808  of the curved outer surface  802  coincides substantially with the center of the sucker rod  812 . 
     In FIG. 10, a rod guide  1000  is molded about rod  1002  and has flow through areas bounded by  1004  and  1006  which form EWV  1008  bounded on the outside by wear surface  1010 . Support arm extensions  1012  may substantially touch to form a substantially complete circumference of wear surface of the EWV to contact the tubing. 
     The molding operations according to the present invention may be of the type described below in connection with FIGS. 4 and 5. The details of injection molding as employed in the art are well known and, except as expressly noted herein, do not constitute a part of the present invention. A description of the operation and construction of injection molding equipment may be found in a 1962 publication  Manufacturing Processes  by S. E. Ursunoff, American Technical Society, beginning at page 56. A description of the application of molding processes in connection with molded plastic rod guides, centralizers, scrapers and the like may also be found in U.S. Pat. Nos. 3,251,919 and 4,088,185. 
     Among the materials suitable for use in accordance with the present invention are polyphenylene sulfide, polyphthalamide, polyamide (nylon), polyethylene, polypropylene, polycarbonate and polyester. All these thermoplastic resins may also be used with glass, arimide fibers and mineral fillers. Ultra-high molecular weight polyethylene may be employed in circumstances which do not involve injection molding. In general, plastics having suitable shrinkage properties and tensile strengths may be employed if not too brittle on molding, if their abrasion and wear characteristics are satisfactory, and if they can withstand the wide range of temperatures and corrosive conditions found in oil well operations. A more extensive listing of suitable materials may be found in U.S. Pat. No. 4,088,185. 
     It is desirable but not essential according to the present invention to provide the flow through holes in the rod guide without resort to drilling or similar means. The present invention includes the process herein describe of providing such flow through holes as a part of the molding process. As shown in FIG. 4, a two part mold  400  is created or provided consisting of left-side and right-side molds  402  and  404  with a suitably shaped rod guide cavity consisting of left-side and right-side portions  444  and  410 , respectively. The cavity in each half of the mold may be filled with plastic material  408  injected into the mold through tube  406 . Cantilevered within the cavity may be one or more rods, such as, for example, rods  412 ,  414 ,  440  and  458 . These rods may each be cantilevered in the mold cavity  410  and supported by one end of a respective supporting end block member  418 . Each mold half also includes an axially opposing end block member  416 . The connection face between the rods  412  and  414  with the mold half  404  is shown as  462  and  464  in FIG.  4 . 
     The two mold halves  402  and  404  are radially closed about the sucker rod  446  and the end blocks  418  are moved axially with respect to mold portions  402  and  404  to a closed or mold position to provide a totally enclosed cavity into which the plastic material  408  is injected through tubing  406 . Each mold half includes end blocks with substantially semi-circular ports  424  therein for receiving the sucker rod  446  when the mold halves are closed. A suitable face seal  428  is provided on the radially inward face of one or both blocks  416 ,  418  for sealing with the radially opposing block when the mold is closed. Similarly, a seal  430  is provided for sealing engagement between the end blocks  418  and the respective primary left side and right side mold  402  and  404  when the mold is closed. The cantilevered rods  412 ,  414 ,  440  and  458 , which may be of any of many shapes to provide flow through holes of the shape or shapes desired, are further supported in the closed position by insertion of the free or cantilevered end of each rod into shallow pockets  420 ,  422 ,  432 ,  434  in the respective opposing end blocks  416  of mold ports  402  and  404  to support the free ends of the rods. 
     As shown in FIG. 5, after the plastic material  518  has been injected through conduit  520  and through the port  522  in the mold half  516  and into the rod guide cavity  524  formed by the mold halves  515  and  516  which makes up the mold  500 , a rod guide  502  having a desired EWV and outer surface  506  will have been formed about rod  508 . Rod guide  502  contains axially extending flow through holes  504  formed by the cantilevered rod members  412 ,  414 ,  440  and  458 . Also, a plurality of outer flow paths  505  are formed about the outer periphery of the guide  502 , with these axially extending flow paths  505  being formed by the respective generally semi-cylindrical radially inwardly projections  526  provided in each mold half  514  and  516 . The end blocks  509  and  510  are moved longitudinally along the axis of the rod  508 , thereby breaking the seals  530  and removing the rods from the holes  504 . When end blocks  509  and  510  carrying cantilevered rods  412 , 414 , 440  and  458  are clear of the guide  502 , the mold halves which are attached to the moving and stationary platens of the injection molding machine may then be separated. The substantially sideways U-shaped seal  532  comprising end seal  534  and top and bottom legs  536  and  538  will thus be broken during this separation process. Similarly, the face  521  on the end block  510  may be radially separated from the opposing face on the block  509 . In this manner, flow through holes  504  of any desired shape or size may be provided in a single molding operation. The rods  512 ,  514 ,  540  and  558  are cantilevered and fixed to end blocks  509  and  510  and sufficient spacing is provided during the molding operation for the blocks  509  and  510  with their supported rods to clear the molded work piece formed on the sucker rod  508 . In the above fashion, it is possible to provide flow through holes in various pieces of any molded guide in any size or shape. 
     In operation, the mold halves and end blocks are closed about the rod and the plastic material is injection molded around the rod. After the guide is formed, the end blocks with cantilevered rods are moved longitudinally along the axis of the rod until clear of the molded workpiece. The major mold halves may then be opened (moved radially with respect to the rod  508 ) and separated from the molded guide. Those skilled in the art will appreciate that the sucker rods  408  on which the rod guides are molded conventionally have threaded end members  507  as shown in FIG.  5 . During the rod guide molding process, these threaded connections  507  are normally broken and the rod guides are molded at preselected axial locations along the length of a single sucker rod. After the rod guide molding operation, the connections  507  on the rods  508  may be threadedly coupled to comprise a rod guide string which is reciprocated in the well. 
     In a similar fashion to that described above, the dog bone configuration of the centralizer or guide  1100  of FIG. 11 may be molded about rod  1102 . Such guides  1100  may be indexed with respect to each other as shown in FIG. 11 to form a nest of rod guides or a helical array of guides effectively providing complete 360 degree coverage and wear contact area with the tubing. As shown in FIG. 11, guides  1100  may be molded about rod  1102  in an indexed fashion of, for example, 45 degrees from the next adjacent guide. The flared area  1104 ,  1108 , etc. may be as extensive as desired consistent with the needed flow through characteristics to provide the desired wear surface and EWV. Holes for the desired flow through  1106  and  1110  may be provided by the molding techniques described herein. 
     Two of the indexed guides of FIG. 11 are shown in FIG. 12 wherein the array  1200  of guides  1204 , and  1208  with the wear surfaces as described above are molded about rod  1202  in an indexed manner of 90 degrees with respect to the next adjacent guide. If desired, flow through holes  1206  and  1210  may be provided by means of the molding process described above. 
     As shown in FIG. 9, these same techniques may also be applied to mold a guide such as  900  around rod  902  with material in contact with the rod  908  and gripping the rod. The rod guide includes extended longitudinal wings  906  to provide extended wear surface  904  and extended EWV. A multiplicity of flow through holes  910  and  912 , for example, may be provided to permit the necessary and desired flow through capacity. The extended longitudinal wings are a further example of a fundamental concept of the present invention in that such a configuration inherently provides for extra outer material for EWV relating to the total volume of the guide and still maintain the necessary flow through capacity in the non-erodible zone of the rod guide. 
     In most if not all of the configurations shown herein, the circumferential extent of any of the separated arms of the rod guide maybe expanded to any extent desired consistent with the desired flow through characteristics or the need for by-pass area up to and including full circumferential contact with the tubing. 
     While it is preferred to form the flow through channels as described herein, it is within the scope of the claims below describing the present invention to drill some or all of the holes, if desired. The cantilevered rods referred to above may also be suspended by other material supports within the mold cavity. 
     Further embodiments such as the use of a spiral or helical vane may be employed in accordance with the present invention. In such an embodiment, the EWV may be controlled as a function of the pitch and number of leads provided. The flow through capacity may be controlled by the number and position of the holes in the erodible and the non-erodible zones of the rod guide. 
     It will be apparent to one of ordinary skill in the art that the present invention may be modified to employ the principles taught within the scope of the present invention. Various changes and modifications may be effected in the illustrated embodiment of the present invention without departing from the scope and spirit of the invention defined in the appended claims. The embodiments shown and described above are exemplary. Various modifications can be made in the construction, material, arrangement, and operation, and still be within the scope of the invention. The limits of the invention and the bounds of the patent protection are measured by and defined in the following claims.