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CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 09/306,506, filed May 6, 1999 now U.S. Pat No. 6,367,226. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to structural members, and in particular to plastic structural members for utility pole components, railroad ties and other applications. 
     2. Description of the Prior Art 
     Structural members have been made from various materials in order to withstand the environments in which they are installed. For example, utility poles are in widespread use for suspending utility lines, including electrical power, telephone/data, etc., at safe distances above the ground. Utility pole structural members have traditionally been manufactured predominately of wood, which has the advantages of being relatively ubiquitous, inexpensive, nonconductive, and generally at least adequate as a structural material with desired strength characteristics. 
     Other structural compositions for utility pole structural members include reinforced concrete, prestressed concrete, steel, aluminum, fiberglass and rigid plastic. For example, the Papin U.S. Pat. No. 5,775,035 discloses a power pole composed of rigid plastic structural members. The Sakai et al. U.S. Pat. No. 5,725,940 discloses a structure and method for making composite molded articles. The structures disclosed therein are generally molded, pulltruded and cast. They can be constructed with hollow, foam-filled and fiberglass honeycomb cores. Such structures and their manufacturing techniques tend to be relatively expensive or suffer from other drawbacks and disadvantages. 
     Disadvantages of wood include its susceptibility to damage from insects, birds, termites, etc. Wood is also subject to attack by biological organisms, particularly in humid environments. Still further, wood tends to deteriorate when exposed to the elements, such as ultraviolet radiation, precipitation, humidity, temperature cycles, etc. These and other factors have the cumulative effect of reducing the useful lives of wooden structural members which are exposed to the elements. 
     Disadvantages are also associated with other structural materials. For example, metals are subject to rust and corrosion. Moreover, their electrical conductivity generally makes them unsuitable for electrical utility pole construction requiring insulative properties. Materials which are hollow or porous are often unsuitable for exterior construction because they tend to admit water which can damage the structural members. Moreover, water permeation tends to increase electrical conductivity, making such materials unsuitable for electrical utility structures. 
     Other disadvantages associated with prior art structural materials include susceptibility to deformation and breaking when subjected to significant loads, both static and dynamic. Special tools and assembly techniques associated with some structural members can further increase the cost of construction and maintenance. Still further, a common practice involves coating exposed wooden structural members with a preservative, such as creosote. However, environmental laws and regulations significantly limit the permitted uses of wood preservatives, particularly those that contain toxins. 
     Plastic is often used as a replacement material for wood. For example, recycled plastic/composite railroad crossties have been substituted for wooden railroad crossties. 
     Although plastic materials tend to repel or resist water and are nonconductive, their disadvantages include vulnerability to ultraviolet radiation, higher densities as compared to wood, and cost. The present invention addresses some or all of the disadvantages and limitations associated with prior art structural members, including wooden and plastic utility pole crossarms and crossarm assemblies. 
     Heretofore there have not been available structural members or methods of manufacturing same with the advantages and features of the present invention. 
     SUMMARY OF THE INVENTION 
     In the practice of the present invention, a structural member, such as a utility pole crossarm, is manufactured from a plastic material and has a relatively dense outer surface and a less dense core. A crossarm assembly includes a plastic crossarm and a pair of diagonal braces for supporting the crossarm on the utility pole. Other utility pole components adapted for plastic construction according to the present invention include cross braces, insulators and the poles themselves. The structural members can be homogenous throughout with respect to their material composition. Other structural member applications include railroad ties. 
     A method of manufacturing the structural members includes extruding a continuous band comprising a polypropylene base material, a fiber reinforced plastic fill material, and a blowing or foaming agent. Additional materials can include chopped glass fibers, UV inhibitors, light stable pigments, antioxidant additives, processing aids, recycled materials, and endothermic gas producing agents. The materials are combined and extruded to form a continuous band, which is shaped and cooled in several stages and cut to predetermined lengths to form the structural members. 
     OBJECTS AND ADVANTAGES OF THE INVENTION 
     The principal objects and advantages of the present invention include: providing plastic structural members; providing such structural members which are adaptable to various applications; providing such structural members which are usable in constructing a wide variety of external structures; providing such structural members which are resistant to static and dynamic loads; providing such structural members which are resistant to environmental damage; providing such structural members which can be cut, drilled and assembled in the field using common tools and procedures; providing such structural members which meet all applicable regulatory requirements; providing such structural members which are adaptable for use as railroad ties; providing a plastic crossarm for utility poles; providing such a crossarm which is resistant to the elements; providing such a crossarm which is resistant to pest damage; providing such a crossarm which meets or exceeds the strength specifications for wooden crossarms; providing such a crossarm which weighs approximately the same amount as a comparable wood crossarm; providing such a crossarm which can be cut, drilled, etc. with tools used for working on wooden crossarms; providing such a crossarm which utilizes recycled plastic; providing a crossarm assembly with a plastic crossarm and plastic braces; providing such a crossarm assembly which is capable of passing lightning impulse tests associated with utility pole applications; providing such a crossarm assembly which meets or exceeds electrical insulation levels required for utility pole applications; providing such a crossarm which is economical to manufacture, efficient in operation, capable of a long operating life and particularly well adapted for the proposed usage thereof; and providing a method of manufacturing plastic structural members. 
     Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. 
     The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary, vertical, elevational view of a utility pole assembly including a crossarm and crossarm assembly embodying the present invention; the pole is shown in broken lines. 
     FIG. 2 is a top plan view thereof. 
     FIG. 3 is a top plan view of a crossarm embodying the present invention. 
     FIG. 4 is an elevational view thereof. 
     FIG. 5 is a vertical, cross-sectional view thereof, taken generally along line  5 — 5  in FIG.  4 . 
     FIG. 6 is a block schematic of a system for practicing a method of manufacturing a plastic structural member according to the present invention. 
     FIG. 7 is a perspective view of a plastic railroad tie comprising a first modified embodiment of the present invention. 
     FIG. 8 is a cross-sectional view of a hollow structural member comprising a second modified embodiment of the present invention. 
     FIG. 9 is a cross-sectional view of a filled-core structural member comprising a third modified embodiment of the present convention. 
     FIG. 10 is a cross-sectional view of a structural member with fiberglass reinforcing rods comprising a fourth modified embodiment of the present invention. 
     FIG. 11 is a cross-sectional view of a structural member with fiberglass reinforcing rods comprising a fifth modified embodiment of the present invention. 
     FIG. 12 is a schematic diagram showing a system for practicing a modified method of the present invention whereby fiberglass reinforcing rods are co-extruded into the structural member. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     I. Introduction and Environment 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. 
     Referring to the drawings in more detail, the reference numeral  2  generally designates a crossarm assembly embodying the present invention and mounted on a utility pole  4 . “Crossarm” as used herein includes a wide variety of structural members mounted on utility poles, including buckarms, twinarms, dead ends, etc. The crossarm assembly  2  generally includes a crossarm  6 , a pair of diagonal braces  8  and insulators  25   a,b.    
     II. Crossarm  6   
     The crossarm  6  includes opposite ends  10 , opposite side faces  12 , and top and bottom faces  14 ,  16 . The crossarm  6  includes an outer surface  18  and an inner core  20 . The core  20  includes entrained voids which are formed by a foaming or blowing agent introduced into the plastic and fiber reinforced plastic base and fill materials in the manufacturing process, as described below. The core  20  is thus less dense than the outer surface  18 . A medial, horizontal bolt or pin hole  22  extends between and is open at the side faces  12 . Multiple lateral, horizontal holes  24  also extend between and are open at the side faces  12 . Each lateral hole  24  is located between a respective crossarm end  10  and the medial bolt hole  22 . Vertical holes  29  can be provided at suitable locations in the crossarm  6 , for example, at spaced locations for mounting electrical insulators  25   a,b,  hangers, etc. The crossarm  6  includes radiussed upper and lower edges  21   a,b.    
     III. Braces  8   
     Each brace  8  includes inner and outer ends  26 ,  28 . The brace inner ends  26  are  9  mounted on the utility pole  4  by a brace/pole mounting bolt  29 . The brace outer ends  28  are mounted on the crossarm  6  by brace/crossarm mounting bolts  30  extending through brace outer ends  28  and respective lateral bolt holes  24 . 
     IV. Crossarm Manufacturing Method 
     FIG. 6 is a schematic diagram showing a system for practicing a method of manufacturing structural members, such as the crossarm  6  and the crossarm assembly  2 . The method includes the steps of providing a source  32  of plastic pellets. Without limitation of the generality of useful plastic base materials for the crossarm  2 , polypropylene homopolymer base material (e.g., NT-418.T001-8000) with 20% to 30% chopped glass fibers has been found to be particularly suitable for use in the manufacture of the crossarm  6 . Additives for the manufacturing process include pigment  34   a,  UV inhibitors  34   b,  and antioxidant additives  34   c.  A foaming agent source  34   d  is also provided and introduces a suitable endothermic foaming or blowing agent, such as Rowa Tracel P02201-P, into the pellet stream from the pellet source  32 . Another additive source comprises processing aids  34   e.    
     The combination of plastic pellets and foaming agent is introduced into an extruder  36  which can apply mechanical energy and/or heat to the raw material mixture which is forced through a forming dye  38  mounted on the extruder  36 . From the dye  38  a continuous band  40  of crossarm stock emerges and enters a vacuum tank which includes a sizer. The stock band  40  is formed to a predetermined size with relatively constant thickness and height dimensions in the vacuum tank  42 . 
     Upon exiting the vacuum tank  42 , the band  40  is subjected to an annealing step whereafter it enters a spray cooling tank  44 . Upon exiting the spray cooling tank  44 , the band  40  is again subjected to an annealing step and enters a second cooling process in a water cooling bath  46  wherein the band  40  is submerged. In the spray cooling tank  44  the band  40  generally floats on the surface of the water and is subjected to continuous spray. In the second water cooling bath  46  the band  40  is submerged. The cooling water is provided by a refrigerated water source  48  whereby its temperature is lowered to approximately 55°. A puller  50  is positioned downstream of the water cooling bath  46  and pulls the band  40  through the production process. Upon exiting the puller  50 , the band  40  is cut to predetermined lengths by a cutoff saw  52 . 
     The following test results (Table 1) were obtained in load/deflection testing in accordance with Rural Utility Services (RUS) test requirements. The test procedure involved placing the crossarm in a rigid test frame and securing it at a point fourteen inches from the outermost hole. Upward pulling forces were applied at the outermost hole and deflection measurements were recorded in increments up to a load of 1000 pounds. Loading was then continued until failure occurred. The procedure was performed on both ends of the crossarm. The results of these tests are summarized as follows: 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Test #1/ 
                 Test #2/ 
               
               
                 Applied Load (lbs) 
                 Deflection (inches) 
                 Deflection (inches) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 250 
                 {fraction (7/16)} 
                 {fraction (5/16)} 
               
               
                 500 
                 {fraction (13/16)} 
                 ⅞ 
               
               
                 750 
                 1 ¼ 
                 1 ⅜ 
               
               
                 1000 
                 1 ¾ 
                 1 {fraction (15/16)} 
               
             
          
           
               
                 Ultimate Load (lbs) 
                 1925 
                 1675 
               
               
                   
               
             
          
         
       
     
     V. First Modified Embodiment Railroad Crosstie Structural Member  102   
     A first modified embodiment structural member comprising a railroad crosstie  102  is shown in FIG.  7 . The railroad crosstie  102  can be cut off from a continuous length of plastic structural member extruded and processed according to the method described above. The railroad crosstie  102  is provided with holes  104  which can be predrilled for receiving the rail-mounting plates. 
     VI. Second Modified Embodiment Hollow Structural Member  202   
     A second modified embodiment structural member is shown in FIG.  8  and is generally designated by the reference numeral  202 . The structural member  202  includes a side wall  204  enclosing a hollow passage or core  206 . The dimensions and proportions of the side wall  204  with respect to the passage or core  206  can vary considerably. For use as a utility pole crossarm, the thickness of the sidewall  204  can be approximately one inch. 
     VII. Third Modified Embodiment Filled-Core Structural Member  302   
     A structural member  302  comprising a third modified embodiment of the present invention is shown in FIG.  9  and includes a side wall  304  generally enclosing a core  306  comprising a suitable fill material. The fill material comprising the core  306  is preferably chosen for the particular application of the structural member  302 . For example, closed-cell foam or expanding foam would be suitable for many applications and would provide a relatively lightweight but strong structural member  302 . 
     VIII. Fourth Modified Embodiment Structural Member  402   
     A structural member  402  comprising a fourth modified embodiment of the present invention is shown in FIG.  10 . The structural member  402  includes a wall  404  with a generally rectangular cross-sectional configuration defined by upper, lower, first side and second side sections  404   a,b,c,d  respectively. A passage or bore  406  extends longitudinally and can either be hollow (similar to the structural member  202  described above) or filled (similar to structural member  302  described above). The wall sections  404   a,b,c,d  intersect at respective pairs of upper and lower edges  408   a,b.    
     In proximity to each edge  408   a,b  a fiberglass reinforcing rod  410  is embedded in the wall  404 . The fiberglass reinforcing rods  410  provide tensile strength in proximity to the wall upper and lower sections  404   a,b.  Thus, the reinforced structural member  402  can possess enhanced strength and stiffness as compared to unreinforced structural members. 
     IX. Fifth Modified Embodiment Structural Member  502   
     A structural member  502  comprising a fifth modified embodiment of the present invention is shown in FIG.  11  and includes a wall  504  with upper, lower, first side and second side sections  504   a,b,c,d  respectively. The lower wall section  504   b  is thicker than the other sidewall sections  504   a-c.  Fiberglass reinforcing rods  510  are embedded within the wall lower section  404   b  and provide greater strength and rigidity. In particular, he reinforcing rods  510  provide stiffness and strength to resist bending loads applied perpendicularly to the upper and lower wall sections  504   a  and  504   b  respectively. 
     X. Modified Embodiment Method Utilizing Co-extrusion 
     FIG. 12 shows a system  602  for practicing a modified method of the present invention. In addition to the other components of the system shown in FIG. 6, The system  602  includes an additional extruder  604 . The additional extruder  604  extrudes fiberglass reinforcing rods  410  or  510 , which enter a joiner  606  for embedding same in the structural member walls  404   a-d  or  504   a-d.  The system  602  can be configured in various ways to produce reinforced plastic structural members of various desired configurations. 
     It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown. For example, it will be appreciated that a wide variety of other structural members can be formed with and according to the present invention.

Summary:
A structural member comprises a plastic base material with fiber reinforced plastic fill material. A gas producing substance or foaming agent is mixed with the base material and provides a lower density within the interior of the member. The structural member can comprise one or more of the components of a utility pole assembly, such as the crossarms, bracing, insulators and pole. A method of producing the structural member includes the steps of mixing additives with plastic pellets for feeding into an extruder. The extruded structural members are formed, cooled, pulled and cut to desired lengths.