Abstract:
A composite part made from a sheet molding compound is disclosed as having improved characteristics over traditional sheet molding compound composite parts. The composite part may be made from a unique sheet molding compound material having a resin impregnated filamentized fiber layer and a resin impregnated fiber layer prior to compaction. The resin impregnated filamentized fiber layer side prevents the movement of partially filamentized or unfilamentized fibers to the visible surface of the composite part when the part is molded. The resin impregnated filamentized fiber layer may be contain a conductive filamentized fiber such that the surface of a sheet molding compound may be conductive and be capable of being electrostatically sprayed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present invention claims priority from Provisional Application Serial No. 60/328,860 entitled “Sheet Molding Compound Having Improved Characteristics, filed Oct. 12, 2001, which is incorporated herein by reference. 
   TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION 
   The present invention relates generally to reinforced fiber composites and more specifically to sheet molding compounds having improved conductive and/or surface characteristics. 
   BACKGROUND OF THE INVENTION 
   In the manufacture of fiber reinforced resin products, sheet molding compounds are frequently used. Sheet molding compounds offer an appealing solution for the production of Class A surface parts compared to steel both in terms of cost and coefficients of thermal expansion. 
   Sheet molding compounds consist of a mixture of a liquid thermosetting resin, particulate filler and chopped reinforcement fibers, such as glass fibers. In most cases, the resin and chopped fibers are sandwiched between films of plastic material to form a laminated sheet that is wound in rolled form or festooned for storage. The laminated sheet is stored under conditions that will not result in final curing of the resin, but will allow the paste to thicken to a desired molding viscosity range, typically between 30,000 and 50,000 centipoise (MilliPascal seconds). At the time of use, the protective carrier film is removed and the laminated sheet is cut into blanks, or plies, of a desired shape and size. The plies are then molded to form a cured composite part. In most applications, multiple plies of the laminated sheets are used in the composite structure and typically comprise between 25 and 50% of the die/tool&#39;s surface area. When the laminated sheets are molded, the resin and glass flow within the mold under heat and pressure to cover the entire surface of the mold. Sheet molding compounds are used in a variety of applications that require aesthetic appeal, corrosion resistance, lighter weight dimensional control and high strength. 
   One deficiency with currently available sheet molding compounds is that the charge typically does not form a Class A type surface parts when cured. This is due to the fact that the chopped fibers move to the surface of the sheet molding compound to form surface imperfections. Further, the fiber used in some sheet molding compounds typically does not flow well in the mold, and this creates surface imperfections such as surface pores. Thus, sheet molding compounds require sanding and polishing, or otherwise reworking to be used in applications requiring a desired surface appearance. 
   Yet another problem with surface characteristics occurs when these composite parts formed from the sheet molding plies are painted. Paint pops may be caused by the release of volatile liquids (such as water, styrene or di-vinyl benzene monomer) from the sheet molding paste or by the release of moisture or solvents contained within fiber bundles during the curing process are quite common, typically affecting 5–10% or more of painted SMC composite parts. This leads to substantial cost in terms of rework and waste. 
   In addition, to allow a long and uniform flow that will produce a wavefree surface, the fibers used in sheet molding compounds are typically provided by the glass manufacturer as bundles or “splits” of multiple filaments. The act of impregnating the bed of chopped fibers between two layers of sheet molding compound paste often leaves air trapped within the composite sheet, most often besides the bundles where small differences in surface tension adversely affects the wetting of the bundles or splits. Unfortunately, this bundling may also include entrapped air which, when released during the flow, produces tiny bubbles which travel slowly under a pressure gradient. To evacuate these bubbles, it is useful to have the molding compound flow to fill out the tool to allow the action of the pressure gradients to move those air bubbles towards the edge of the flow front and thus towards the edge of the part. Such large flow typically calls for loading the tool by a charge representing 50% or less of the area of the part 
   It is therefore highly desirable to improve the surface characteristics of sheet molding compound. This would allow sheet molding compound parts to be used in a wider variety of composite applications wherein surface quality is a concern. 
   SUMMARY OF THE INVENTION 
   It is thus an object of the present invention to improve the physical and surface characteristics and electrostatic sprayability of composites parts made of sheet molding compound composite sheets. 
   The present invention addresses the above object and comprises a composite part may be made from a unique sheet molding compound composite sheet having a thin resin surface layer, a resin impregnated filamentized fiber layer, a resin impregnated unfilamentized or partially filamentized fiber layer, and a second resin paste layer. The resin impregnated filamentized fiber layer side acts as a barrier to prevent the movement of partially filamentized or unfilamentized fibers into the thin resin surface layer of the composite part when the part is compacted, molded and cured. This presents a visible surface that is resin rich and porous free, which is shown to improve surface characteristics of the composite part without adversely affecting strength and stiffness characteristics. 
   Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a cross-sectional view of a sheet molding compound having improved surface characteristics according to one preferred embodiment of the present invention; 
       FIG. 1B  is a cross-sectional view of a prior art sheet molding compound; 
       FIG. 2A  is a schematic diagram for making the sheet molding compound of  FIGS. 1A and 1B ; 
       FIG. 2B  is an alternate schematic diagram for making the sheet molding compound of  FIGS. 1A and 1B ; 
       FIG. 2C  is a close-up view of the funnel shaped dispensing device of  FIG. 2B ; 
       FIG. 2D  is an enlarged partial cross sectional view of the funnel shaped dispensing device shown in  FIG. 2B . 
       FIGS. 3A and 3B  are a perspective view of a sheet molding compound having improved surface characteristics according to another preferred embodiment of the present invention; 
       FIG. 4A  is a schematic diagram for making the sheet molding compound of  FIG. 3A ; 
       FIG. 4B  is a schematic diagram for making the sheet molding compound of  FIG. 3A ; 
       FIG. 4C  is a schematic diagram for making the sheet molding compound of  FIG. 3B ; 
       FIG. 5A  is a schematic diagram for making a sheet molding compound according to another preferred embodiment of the present invention; 
       FIG. 5B  is a schematic diagram for making a sheet molding compound according to another preferred embodiment of the present invention; 
       FIG. 6A and 6B  are a close-up view of the volumetric paste extrusion device used in  FIGS. 5A and 5B ; 
       FIGS. 7A and 7B  are a close-up view of the bulk molding volumetric extrusion device of  FIGS. 5A and 5B ; 
       FIG. 8  depicts another preferred embodiment of a slit extrusion die of  FIGS. 5A and 5B  having a deformable lip; and 
       FIG. 9  depicts another preferred embodiment of a volumetric paste extrusion device that can be used in  FIGS. 5A and 5B . 
   

   DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION 
   Referring now to  FIG. 1A , a sheet molding compound made in accordance with a preferred embodiment of the present invention is generally shown as 10 prior to compaction. The sheet molding composite (SMC) sheet  10 , from top to bottom, is shown having an upper carrier film layer  12 , a top resin paste layer  14 , a resin impregnated unfilamentized or partially filamentized fiber layer  16 , a resin impregnated filamentized fiber layer  18  (including paste  18 ′ and filamentized fibers  57 ) and a bottom carrier film layer  20 . Of course, the order of the layering from top to bottom for a finished composite part made from this sheet molding compound is reversed, as the resin impregnated filamentized fiber layer  18  forms the class A surface side desired when the SMC sheet  10  is made into a finished part. The process for forming the SMC sheet  10  of  FIG. 1A  is shown below in  FIGS. 2A and 2B . Additionally, the cross-section of  FIG. 1B  may be made using this device, provided the paste  18 ′ does not include filamentized fibers  57 , and therefore the shown bottom layer  13  of  FIG. 1B  does not include these fibers as does the layer of paste  18  of  FIG. 1A  (one skilled in the art appreciates that the layer of paste  18  (and the paste  18 ′) in the remaining figures could be replaced by a nonfilamentized paste, or layer of nonfilamentized paste  13 , where appropriate). 
   The compositions of the resin-containing layers  14 ,  18  used in the present invention are variations to formulations currently used for molding Class A surface and/or structural parts. In addition to the polyester resin (which may include the thermoplastic, thermoset, reactive monomer, etc. as known to one skilled in the art), the formulation contains fillers such as calcium carbonate, a resin inhibitor and initiator (catalyst), an alkaline earth oxide or urethane thickening agent, and an internal mold release agent. Of course, other additives may be added depending upon the desired characteristics of the paste and finished composite part. 
   For the unfilamentized or partially filamentized fiber layer  16  formulation, chopped fibers (shown as  58  in  FIG. 2A ) are also introduced to the formulation. These chopped fibers  58  preferably comprise between approximately 0.25 and 60 percent by weight of the formulation. Any suitable chopped fiber may be used in the invention. Preferably, the chopped fiber  58  is be selected from fibrous materials that are commonly known in the art, such as glass, carbon, natural fibers, polymers and other fiberizable materials known in the art, or mixtures thereof. 
   For the resin-impregnated filamentized fiber layer  18  formulation, chopped and filamentized or milled fibers are mixed between about 0.25 and 30% by weight in the formulation without fillers. Any suitable fiber that may be filamentized, flaked or milled can be used in the invention. 
   A preferred composition for the resin paste layer  14  and the resin impregnated filamentized fiber layer  18  are shown below in Table 1 and 2, respectively. As noted herein, the glass fibers shown in table 2 may be supplemented with, or replaced by, other fibers, such as carbon fibers or flakes, preferably in the range of about 0.1 to 10% by weight of the filamentized paste  18  and replacing a portion of the glass fiber content. One preferable composition for a conductive resin impregnated fiber layer  18  is shown in Table 3 below. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               SMC RESIN PASTE LAYER 14 
             
           
        
         
             
               INGREDIENTS 
               WEIGHT % 
               MANUF. NAME 
               DESCRIPTION 
             
             
                 
             
           
        
         
             
               T341 
               16.95 
               AOC/Alpha 
               Thermosetting 
             
             
                 
                 
               Owens Corning 
               Polyester Resin in 
             
             
                 
                 
                 
               styrene 
             
             
               T154 
               7.24 
               AOC/Alpha 
               Thermoplastic 
             
             
                 
                 
               Owens Corning 
               Polyester resin in 
             
             
                 
                 
                 
               styrene 
             
             
               Styrene 
               3.13 
               Ashland 
               Styrene monomer 
             
             
               DVB 
               1.33 
               Dow 
               Divinyl benzene 
             
             
               P710 
               0.88 
               BASF 
               Polypropylene 
             
             
                 
                 
                 
               oxide 
             
             
               PBQ 
               0.008 
               Aldrich 
               P-benzoquinone 
             
             
               CBA-60 
               0.88 
               Witco 
               Non-ionic 
             
             
                 
                 
                 
               surfactant 
             
             
               1300x40 
               0.59 
               B. F. Goodrich 
               Hycar Rubber in 
             
             
                 
                 
                 
               styrene 
             
             
               TBPB 
               0.53 
               Atofina 
               T-butyl 
             
             
                 
                 
                 
               perbenzoate 
             
             
                 
                 
                 
               catalyst 
             
             
               Cal St 
               1.18 
               Mallinckrodt 
               Mold Release Agent 
             
             
               Huber W-4 
               62.02 
               Huber 
               Calcium carbonate 
             
             
               RP510 
               1.83 
               AOC/Alpha 
               Thermoplastic 
             
             
                 
                 
               Owens Corning 
               polyester resin in 
             
             
                 
                 
                 
               Styrene 
             
             
               Zn St 
               0.15 
               Mallinckrodt 
               Mold release agent 
             
             
               PDI-1805 
               0.03 
               Ferro 
               Iron pigment 
             
             
               Huber W-4 
               2.66 
               Huber 
               Calcium carbonate 
             
             
               CaO 
               0.53 
               C. P. Hall 
               Alkaline earth 
             
             
                 
                 
                 
               oxide thickener 
             
             
               Water 
               0.05 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               FILAMENTIZED LAYER 18 PASTE 
             
           
        
         
             
                 
               Function 
               Component 
               Weight percent 
             
             
                 
                 
             
             
                 
               Table 1 Resin 
               Sheet Molding 
               72–73% 
             
             
                 
               Paste 14 
               Resin 
             
             
                 
               Formulation (may 
             
             
                 
               be less some or 
             
             
                 
               all fillers) 
             
             
                 
               Owens Corning 
               E-type glass 
               27–28% 
             
             
                 
               R25H 
               reinforcement 
             
             
                 
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               CONDUCTIVE FILAMENTIZED LAYER 18 
             
             
               PASTE 
             
           
        
         
             
                 
                 
                 
               Weight 
             
             
                 
               Function 
               Component 
               percent 
             
             
                 
                 
             
             
                 
               Table 1 Resin Paste 14 
               Sheet Molding Resin 
                72–73% 
             
             
                 
               Formulation (may be 
             
             
                 
               less some or all 
             
             
                 
               fillers) 
             
             
                 
               Toray 300 
               Conductive Carbon Fiber 
               0.1–10% 
             
             
                 
               Owens Corning 954 or 
               E-type glass 
               0.1–15% 
             
             
                 
               973 
               reinforcement 
             
             
                 
                 
             
           
        
       
     
   
   Referring now to  FIG. 2A , one preferable assembly process is shown for making a compacted SMC sheet  77  from the SMC sheet  10  of  FIG. 1A  is generally shown as  50 . The assembly process begins by unrolling the bottom carrier film layer  20  from a roll or reel  52  and transporting it across a carrier belt  54 . Of course, in other embodiments, the carrier belt  54  is not necessary where the tensile strength of the carrier film  20  is sufficiently strong to hold the entire SMC sheet  10 . The resin impregnated filamentized fiber layer  18  is then introduced onto the film layer  20  in the form of a wet paste  18 ′ from a traditional dispensing device  17 . The device  17  preferably meters the paste  18 ′ using a doctor blade  56 , generally an upside down weir blade. The device illustrated here is illustrated with an arcuate rear side opposite the doctor blade  56 , but one skilled in the art appreciates a rectangular doctor box may be used as well. 
   Partially filamentized or unfilamentized fiber  58  is then chopped using a chopper  60  onto the resin impregnated filamentized fiber layer  18 . An upper carrier film layer  12  is unrolled from a roll or reel  62  and a second resin paste layer  14  is deposited onto the second carrier film  12  using another traditional dispensing device  17 . The second resin paste layer  14  is deposited as a wet paste  14 ′ onto the inner side of the upper carrier film  12 . The thickness of the second resin paste layer  14  is controlled using a doctor blade  66 . The upper carrier film layer  12  and second resin paste layer  14  is then rolled around a roller  68  and laid on top of the chopped glass fiber  58  such that the second resin paste layer  14  is below the upper carrier film layer  12 . This forms the SMC sheet  10  shown in  FIG. 1A . A wire mesh belt  70  compacts the SMC sheet  10  to form a compacted SMC sheet  77  of a desired area weight prior to rolling onto a take up roll  72 . By controlling the amount of pastes  14 ′ and  18 ′ deposited by the respective doctor blades  66 ,  56 , by a simple gap adjustment, one skilled in the art can control both the overall compacted sheet  77  weight and the percentage of filamentized reinforcement material that is contained within each compacted composite sheet  77 . 
   As the film layer  20  passes under the device  17 , the film layer  20  pulls the bottom of the viscous paste  18 ′, and may form a puddle of paste within the device  17 . The viscous paste  18 ′ may therefore move in a circular pattern, thereby causing a meniscus to form between the back edge of the paste puddle and the film (a void exists between the rear wall of the paste adjacent the film due to a radius formed on the puddle of paste). Depending upon the film speed and the paste viscosity, the meniscus may be as long as a few centimeters. Periodically, possibly due to film stretching, the meniscus collapses and air is trapped within the paste  18 ′. This trapped air exits underneath the doctor blade  56  in the form of bubbles, resulting in a resin film layer having non-uniform thickness, or can form fisheyes in the final surface. This can cause local regions having higher or lower glass content within the paste film, which in turn can cause unwet regions in the molding compound. 
   In an alternative embodiment, as shown in  FIG. 2B , a funnel-shaped dispensing device  17 B replaces one or more of the traditional dispensing devices  17 . In one embodiment, both illustrated devices comprise such funnel-shaped devices, although not illustrated as such in  FIG. 2B . The funnel-shaped dispensing device  17 B helps to prevent the entrapment of air that is common in traditional dispensing devices such as device  17 , as described in the preceding paragraph. To control viscosity in the dispensing device  17 B, heat or cooling may be applied to the device  17 B to maintain a constant temperature, and thereby better enable and control the viscosity and flow therefrom. Heat may be applied to the device  17 B near the exit  22  of the device  17 B to decrease the viscosity of the paste  18 ′ and thereby improve wet-out of the glass fibers or mat within the sheet  10 . In this regard, the heat source (not shown) could be applied against or within a wall  24  of the device  17 B near the exit  22  or by using the dividing plate  23  as a heat source within the vertical feeding slot  21 . Alternatively, a volumetric paste extrusion device may be employed. 
   The output of the device  17 B is controlled by controlling the viscosity of the paste  18 ′ through composition and temperature, plus controlling the pressure within the feeding slot  21 , which may be accomplished by controlling the height of the paste in the device  17 B, and may include an optional pressurization of the device  17 B through known mechanical means (not shown). Accordingly, by controlling the pressure, underfeeding and overfeeding of the doctor blade may be avoided, thereby avoiding too thin, or too thick application (or a mess), respectively. Further, one skilled in the art appreciates that more than one funnel may be provided in series, and accordingly more than one type of paste may be deposited onto the sheet; for example, the first funnel may include a non-filamentized paste to provide a resin rich layer on the outside of the part, and a filamentized paste may be deposited by a second funnel adjacent the nonfilamentized paste. 
   As shown in  FIG. 2B , the new funnel-shaped dispensing device  17 B is preferably attached to the doctor box, and more preferably the lip of a conventional doctor blade, as shown in  FIG. 2A . The height of the paste  18 ′ is controlled via a float valve or similar device so one does not overfeed the doctor blade  56  and force excessive paste underneath the blade  56 . The device  17 B contains a foot  19  that extends out into the main dispensing area that ensures that the film layer  20  does not catch when the machine is in operation. The foot  19  is adjusted to have a small gap between it and the film layer  20  that is sufficient to prevent the paste  18 ′ from flowing out the back of the dispensing device  17 B. Further, the dispensing device  17 B has a vertical feeding slot  21 , which is preferably divided into two or more narrower slots  21 ′ by one or more dividing plates  23 , each such slot  21 ,  21 ′ forming a column of paste. The first such slot  21 ′ nearest the foot  19  first contacts the carrier film  20 . In the event that the paste deposited from the first such slot  21 ′ includes air bubbles or incompletely coats the film  20 , each subsequent slot  21 ′ will help to coat over any imperfection in the paste from the preceding slots  21 ′. Thus, the multi-slotted funnel-shaped dispensing device  17 B is engineered to reduce trapped air or voids within the paste  18 ′, and to form a layer  18  with uniform weight and thickness. Preferably, the width of the slots  21  and  21 ′ are adjustable by either installing fewer/additional, thinner/thicker plates  23 , and/or adjusting walls  24 . In a preferred embodiment the foot  19  is positioned between about 0.03–0.25 inch above the film  20 , and the tip of the doctor blade  56  is positioned between about 0.05–0.125 inches above the film  20 , however these gaps will depend upon the paste composition, viscosity, and overall operating conditions. 
   Each batch of compacted SMC sheet  77  is then allowed to mature and thicken thereby increasing viscosity at approximately thirty-two degrees Celsius for approximately one to fourteen days prior to any molding application. The batch may then be further processed by cutting the SMC sheet  10  to an appropriate ply or laminate size, removing the upper and lower carrier films  12 ,  20 , molding the remaining material to an appropriate shape in a heated matched metal or composite die, and curing it under heat and pressure to make a finished composite part (not shown). Preferably, the curing step is done at approximately 5–10 MPA (750–1500 psi) at about 140–163 degrees Celsius (280–325 Degrees F.) for about one-half to three minutes. 
   During the compaction step described above, excess resin from the resin paste layer  14  and resin impregnated filamentized fiber layer  18  penetrates within and through the partially filamentized or unfilamentized fiber  58  to form the discrete resin impregnated unfilamentized or partially filamentized fiber layer  16 . 
   However, the filamentized fibers  57  within the resin impregnated filamentized fiber layer  18  generally do not significantly penetrate within this fiber layer  16  during compaction. The compacted SMC sheet  77 , when cured, forms a composite part in which visible surface layer forms a resin rich and nearly porous free layer that has improved surface characteristics with less surface pores as compared with traditional sheet molding compound composites. 
   While the above example indicates only one ply of SMC sheet  77 , it is understood that more than one ply is typically used to form a composite part. The number of plies of the SMC sheet  77  used to form the composite article varies as a function of the thickness (i.e. volume) of the composite part desired and the weight per square meter of the SMC sheet  77 , but typically ranges from two to four plies. In a preferred embodiment, a top ply of the SMC sheet  77  and one or more plies of conventional SMC, such as those produced in  FIG. 1B , made according to the prior art are placed in the mold. This forms a composite part having a Class A surface side on a visible side of the composite part, and a non-class A surface that is usually found on the non-visible side. If both the top and bottom surface of the composite part formed need Class A surfaces, then a top ply and bottom ply of the compacted SMC sheet  77  may be used, with one or more plies of sheet molding compound made according to the prior art contained within these sheets  77 . In another preferred embodiment, the ply has a sheet weight adequate to form the composite part with a single ply. 
   In addition, if conductive materials such as carbon or nickel coated carbon or glass fibers are used in the filamentized fiber layer  18 , a cured composite part having improved electrostatic sprayability characteristics may be realized. Such conductive fibers may be used in addition to, or instead of, glass fibers. Similarly conductive flakes, fibrils, powders, or carbon or nickel coated carbon or glass fibers or conductive particles may be used in the resin impregnated filamentized fiber layer, each of which is to be considered as conductive fibers for the purposes of this disclosure. Further, by concentrating the conductive materials within the fiber layer  18  at a location which is very close to the surface of the composite part, less conductive material is needed within the composite part as compared with traditional sheet molding compound composite parts having conductive material, which reduces raw material costs. 
   While the invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings.