Abstract:
A method is disclosed for high volume production of molded dimensionally accurate polymer composite articles comprising three-dimensional carbon fiber reinforcement in high volume. Carbon fiber cloth infiltrated with a liquid thermosetting polymer precursor is shaped by application of pressure to urge the cloth against the surface of a mold and then heated while under pressure to cure the polymer precursor into a solid polymeric matrix for the carbon fiber article. Mold tooling is formed of suitable iron-nickel alloys and provided with a hard electroless nickel coating on the mold cavity surface. The iron and nickel alloy provides thermal expansion properties generally matched to those of the articles for the molding of dimensionally accurate parts. The nickel alloy coating provides a durable surface to resist abrasion by the carbon fiber cloth without adversely affecting the expansion properties of the mold, and remains adherent to the base metal during the repeated thermal cycling encountered in high volume production.

Description:
TECHNICAL FIELD 
     This invention pertains to the molding of a succession of many like carbon fiber reinforced polymer parts such as in the production of light weight automotive vehicle body panels. More specifically, this invention pertains to the preparation and use of durable tooling for such repetitive molding operations on abrasive materials. 
     BACKGROUND OF THE INVENTION 
     Carbon fiber composite parts comprise carbon fiber cloth impregnated with a matrix polymer such as a suitable epoxy resin composition. While epoxy resins are commonly used matrix polymers, other polymers such as polyester resins, vinyl ester resins, polypropylene resins, or polyamide resins are also suitable. In low volume production of a given part, a thermosetting composite article is usually produced by layering sheets of carbon fiber cloth onto a mold cavity surface that defines the shape of the desired product. The alignment and weave of the carbon fibers in the cloth may be important in the mechanical performance of the resulting product. The layered sheets of carbon fiber cloth are carefully infiltrated and impregnated with uncured liquid polymer material. The liquid material is carefully flowed around each fiber or strand of the cloth to form a fiber and liquid composite with minimal air or gas voids. The liquid is uncured precursor material for the thermosetting polymeric material that will constitute a solid matrix around each fiber of the carbon cloth or other fibrous carbon reinforcing material. Impregnation with the liquid polymer precursor may be accomplished before or after the layered sheets are placed in the mold. There the layers are compacted into a desired shape and the liquid polymeric precursor matrix is heated and cured to a solid matrix. The cured polymer-carbon fiber composite product is light in weight and very strong. 
     For low volume production liquid polymer precursor (sometimes called “resin”) impregnated carbon cloth sheets (called “prepreg”) may be laid up by hand on a single-sided tool. Prepreg sheets about 0.2 mm thick and comprising, for example, 40 weight percent liquid epoxy resin precursor and 60 weight percent of a single layer of carbon cloth are cut to a desired shape and laid up in multiple layers on a mold surface to obtain a part shape. The assembly is then placed in a vacuum and compressed and heated to promote resin flow to eliminate voids in the molding. Heating at a suitable temperature cures the epoxy resin (or other resin material) to form a strong carbon composite product characterized by reinforcing layers of carbon fiber cloth in a polymer matrix. This composite is very strong and its constituents are of relatively low specific gravity. Compared to other candidate materials of construction, carbon fiber composites provide a unique combination of stiffness and low weight. 
     High value carbon fiber composite parts are used, for example, in aerospace applications, high performance sail boats and bicycles, and in racing vehicles or unique light-weight super cars. These “high value” applications permit the use of high labor cost, hand lay-up molding practices. And the low production volumes permit the use of molding tool materials that do not provide hard surfaces for resisting wear from the abrasive composite materials. The high volume production, for example, of automobile body panels requires molding tools in which many identical panels may be successively formed to high dimensional accuracy. 
     Invar tooling has been used for molding precision carbon fiber composite panels with complex geometry in both aircraft and automotive industries. The alloy, Invar-36, consists, by weight, of 36% nickel and 64% iron. It became the preferred tooling material because of its unique thermal expansion behavior: a coefficient of linear thermal expansion (CLTE) of 
     2×10 −6  per ° C. in the temperature range of 25° C.-150° C., very close to that of the carbon fiber composites. The tooling material is considered to be too soft for production of a large number of composite parts. The hardness of Invar-36 (80 HRB) is significantly lower than that of the P20 tool steel (50 HRC), suggesting that Invar tooling will not be sufficiently wear-durable by automotive standards even for low volume production. 
     It is an object of this invention to provide a suitable tool material for molding liquid polymer precursor infiltrated, carbon fiber composite parts in high volumes where the tool surface is continually and repeatedly exposed to abrasive contact with the composite material. An example of such a manufacturing situation is the molding of automotive body panels at automotive production volumes. 
     SUMMARY OF THE INVENTION 
     It is found that Invar-36 type tooling may be made more durable for high volume production of carbon cloth (or other fiber structures) reinforced polymer matrix parts without adversely affecting its desirable thermal expansion properties in relation to the thermal expansion properties of the molded composite material. In other words, this invention retains the dimensional accuracy of the Invar-36 type mold material while providing a hard, wear resistant surface. Carbon fiber-containing composite parts with good dimensional accuracy may be repetitively made in large volume production using the improved tooling. 
     In accordance with this invention, a single mold tool or a set of complementary mold tools are fabricated from slabs, sheets or other shape of the Invar-36 nickel-iron composition or a like nickel alloyed, iron based composition having generally equivalent thermal expansion properties for molding carbon fiber composite articles. The mold cavity surfaces are machined slightly oversized and the article shaping surfaces are then provided with a uniform layer of electroless nickel. The electroless nickel coating on the tool surface(s) is suitably formed by an autocatalytic chemical reduction of nickel ions in aqueous solution using a phosphorous-containing or boron-containing catalyst. In general, a phosphorus-containing catalyst, such as hypophosphite ions, is preferred because of the hardness of the resulting nickel-phosphorus deposit. The thickness of the coating is suitably, for example, about 60-90 micrometers. 
     It is found that the electroless nickel plated nickel-iron alloy (e.g., Invar-36) has a suitably durable surface for the repetitive molding of many carbon fiber composite parts to high dimensional accuracy. Moreover, the electroless deposited nickel layer retains a smooth molding surface and remains adherent to the ferrous alloy substrate after hundreds of laboratory accelerated thermal cycles as experienced in repetitive loading of composite materials, heat curing of the composite materials, and unloading of the cured composite product. 
     The use of the electroless nickel coated, nickel-iron tooling of this invention is applicable to the many various molding practices for the forming of carbon fiber composite parts. The hard surface provides abrasion resistance against the composite materials. The adherent coating and underlying substrate cooperate to retain good dimensional properties for the repetitive molding of substantially identical parts as required in the automotive and other industries. 
     The tooling is useful in the conduct of liquid molding processes for the composite parts such as resin transfer molding and reaction injection molding in which a preform of the reinforcing cloth is placed between complementary tools. The tools are closed and liquid resin is injected into the cavity to infiltrate the reinforcing cloth, and the tools are heated to cure the resin impregnated carbon fiber composite. The tooling is also useful in compression molding of carbon fiber epoxy prepregs, carbon fibers containing vinyl ester resins, and various carbon fiber reinforced thermoplastic polymers. And the nickel coated tooling is useful in hand lay-up carbon fiber composite laminate moldings that are autoclave cured. 
     Other objects and advantages of the invention will be apparent from a description of illustrative preferred embodiments which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic flow diagram of a resin transfer molding process for producing carbon cloth or carbon fiber containing composites using nickel-coated tools in accordance with this invention. 
         FIG. 2  is an enlarged view of a section of the molding tool shown in  FIG. 1  showing its electroless nickel coating layer on an Invar-36 substrate. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with this invention, Inver-36 alloy specimens have been plated with a thin layer of hard electroless nickel for evaluation and use as automotive production tooling. The surface layer as deposited comprised an alloy of nickel and phosphorus because the reduction catalyst for the deposition was a hypophosphite salt. The evaluation focused on examining the thermal cycling stability of the nickel alloy coatings since composite molding tools are constantly subjected to heating and cooling cycles during their service life and the mismatched CLTE between the nickel coatings (about 13×10 −6  per ° C.) and the Invar substrates could cause significant stress in the coatings. The study was carried out by monitoring the hardness, roughness, and morphological changes of the coating surface and the coating/substrate interface at various predetermined heating and cooling cycles. 
     Preparation of Samples and Testing 
     Four Invar-36 plaques (152 mm×102 mm×9.5 mm) supplied by Re-Steel (Eddystone, Pa.) were used in this study. Before plating, the surfaces of the plaques were polished or polished and textured: two plaques were polished to 600 grit finish and then textured by blasting with a mixture of aluminum oxide and glass bead such as for a panel outer surface, and the other two were polished to 400 grit without texturing for a panel inner surface. 
     The polished plaques were plated with two proprietary commercial electroless nickel coatings: TM103 and E-Krome by Techmetals, Inc. of Dayton, Ohio. Electroless nickel coating processes generally employ sodium hypophosphite as a reducing agent; consequently, the deposits produced are nickel-phosphorus alloys rather than pure nickel. This allows the composition to be varied (normally between about 2 wt % and about 13 wt % phosphorus) thereby providing a range of useful properties, including extreme hardness, good resistance to corrosion, and attractive appearance. A comparison of the supplier&#39;s specifications of these two coatings is shown in Table I. The E-Krome coating had lower phosphorus content, a higher melting temperature, and was harder than the TM103 coating. However, the salt spray corrosion performance of the E-Krome coating was not as good as the TM103 coating, possibly due to the lower phosphorous content. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Properties comparison between TM103 and E. Krome coatings 
               
             
          
           
               
                 Properties 
                 TM103 
                 E. Krome 
               
               
                   
               
               
                 Phosphorus content (wt. %) 
                 10.5-13.0 
                 4.0-7.0 
               
               
                 Melting point (° C.) 
                 880 (eutectic) 
                  960-1205 
               
               
                 Hardness (HRC) 
                 45-50 
                 60-64 
               
               
                 Corrosion resistance (ASTM-B117) 
                 1000 hrs 
                 300 hrs 
               
               
                   
               
             
          
         
       
     
     The nickel alloy plated Invar plaques were subjected to laboratory accelerated thermal cycling according to the following procedure of heating and cooling. The plaques were heated for 1 hour in a 177° C. hot air oven, removed from the oven and allowed to cool at room temperature for one-half hour. This cycle was repeated 7 times a day until a desired total number of cycles were completed. This laboratory thermal cycling procedure is patterned after molding cycles used in low volume production (by automotive industry standards) of Corvette® carbon fiber composite hoods. 
     The stability of the nickel coatings was investigated by monitoring the hardness, roughness, and surface and interface morphologies of the plated plaques during the thermal cycling test. The micro-hardness was measured using Matsuzawa MXT70 with a Knoop Indenter. The hardness value was then converted to the Rockwell scale using ASTM E140-05 for comparison with conventional tooling materials. The surface roughness of the coatings was measured using a Wyko 3-D Profilometer. The surface morphology of the plated samples was examined using a Zeiss Evo 50 scanning electron microscope (SEM). An elemental detector (EDS) attached to the SEM was used to examine the approximate compositions of the coatings. For monitoring the interface morphology, cross-sections of each sample were mounted in Lucite and polished to a mirror finish. 
     Results and Discussion 
     Surface Hardness 
     As mentioned earlier, Invar-36 alloy has a HRB hardness of 80, significantly softer than the conventional P20 steel often used for automotive molding tools. The measured HRC hardness values of the four electroless nickel plated Invar plaques, both as plated and thermal cycled, are shown below in Table 2 as a function of thermal cycles. The grit numbers, 400 and 600, refer to the grit polishing of the Invar substrate prior to the deposition of the electroless nickel alloys. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Thermal Cycles 
               
             
          
           
               
                   
                 0 
                 100 
                 200 
                 300 
               
               
                   
               
             
          
           
               
                   
                 TM 103, 400 
                 40 HRC 
                 36 HRC 
                 36 HRC 
                 36 HRC 
               
               
                   
                 TM 103, 600 
                 35 HRC 
                 40 HRC 
                 40 HRC 
                 40 HRC 
               
               
                   
                 E-Krome, 
                 45 HRC 
                 50 HRC 
                 50 HRC 
                 50 HRC 
               
               
                   
                 400 
                   
                   
                   
                   
               
               
                   
                 E-Krome, 
                 50 HRC 
                 52 HRC 
                 50 HRC 
                 52 HRC 
               
               
                   
                 600 
               
               
                   
               
             
          
         
       
     
     The data indicate that both TM103 and E-Krome electroless nickel coatings successfully hardened the Invar substrates; although the resulting hardness values are approximately 15%-25% lower than those given in the coating specifications shown in Table I. The as-plated HRC hardness values of the two E-Krome coatings are higher than that of the TM103 coatings and are comparable to the hardness of P20 tool steel, 50 HRC. The TM103 coatings failed to meet the target hardness value. Consequently, they are significantly less preferable as a surface material for tools for significant production of carbon fiber composite parts. 
     Table 2 also shows that the hardness values of both TM103 and E-Krome nickel coatings were not much affected by the heating and cooling cycles. The hardness of E-Krome 400 showed a minor increase at the very beginning of the test and leveled off after 100 thermal cycles. The results suggest that the hardness of the coatings will be stable in the operating temperature range of the molding tools. Additionally, the initial surface finish of the Invar substrates, 400 grits and 600 grits, did not have any influence on the hardness of the electroless nickel coatings before and after the thermal cycling. 
     Surface Roughness 
     The arithmetic mean roughness values, Ra, of the nickel coated Invar plaques indicate that the as plated E-Krome coatings had a smoother surface than the as-plated TM103, 0.7 μm versus 1.3 μm, respectively. The surface roughness values of both coatings were not affected by the thermal cycling tests, suggesting again that the coatings will be stable in the operating temperature range of the molding tools. Similar to the hardness data, the surface roughness of the coated samples was not much influenced by the initial surface finish of the Invar substrates. The difference in roughness of TM103 and E-Krome samples apparently is a result of the coating characteristics, at least when the substrates are reasonably finished as those used in the study. 
     Coating-Substrate Interface 
     To examine the interface between electroless nickel coatings and Inver substrates, optical micrographs were taken from the cross-sections of coated samples, both as plated and thermal cycled. For all the samples examined, the micrographs revealed no signs of delamination at the coating-substrate interfaces after 300 cycles of accelerated heating and cooling. The result provides strong evidence that the electroless nickel coatings will be stable for use as composite molding tools, consistent with previous suggestions made based on hardness and roughness data. 
     Furthermore, the cross-sectional optical micrographs showed that the average coating thickness is about 75 μm (3 mils) for both TM103 and E-Krome plated samples. This coating thickness is effective to dampen or remove any substrate influence to the coating properties as seen by the insensitivity of the hardness and surface roughness data with respect to the initial surface condition of the substrates. The absence of change in coating thickness during thermal cycling is also consistent with the finding that the electroless nickel coatings on Inver substrates are stable. 
     SEM Analysis of E-Krome Coatings 
     The above hardness and stability studies have clearly identified that the E-Krome plated Invar-36 alloy is a feasible tooling material for automotive composite molding applications. The E-Krome coatings, both as plated and thermal cycled, were therefore further characterized using SEM analysis to determine their elemental compositions and surface morphologies as described in the following. 
     The results of the SEM-EDS analysis of the elemental compositions of the E-Krome 600 coatings before and after the heating and cooling cycles are shown in Table 3. The phosphorus contents of the coating samples are in the range of 6-7 wt % and remain practically constant throughout the thermal cycling test The elemental composition of the as plated E-Krome 600 coating, 7.6 wt % phosphorus and 92.4 wt % nickel, is consistent with the coating specification given in Table 1. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Elemental compositions of the E-Krome 
               
               
                 600 coatings as plated and thermal cycled 
               
             
          
           
               
                 Composition 
                 0 cycles 
                 100 cycles 
                 200 cycles 
                 300 cycles 
               
               
                   
               
             
          
           
               
                 Phosphorus (wt. %) 
                 7.6 
                 6.6 
                 6.5 
                 6.3 
               
               
                 Nickel (wt. %) 
                 92.4 
                 93.4 
                 93.5 
                 93.7 
               
               
                   
               
             
          
         
       
     
     The results of the SEM surface microscopy revealed no noticeable surface morphological changes for the nickel plated samples before and after thermal cycling. The alloys containing lower phosphorus content were characterized by the presence of crystalline and microcrystalline nickel, which indicates that the number of phosphorus atoms is not sufficient to distort the nickel lattice to an extent where amorphous nickel can be obtained. The nodular structure of nickel-phosphorus deposits decreases with increasing phosphorus content. The structure of the E-Krome coatings remained the same during the cycling test. 
     Electroless nickel plated Invar tooling is suitable for high volume production of carbon fiber polymer composite parts made by molding and curing of the materials. Following are examples of molding practices for such parts. 
     EXAMPLE 1 
     Autoclave Cured, Hand Lay-Up, Carbon Fiber Composite Laminate Molding 
     This process is currently being used to make automotive vehicle body panels in very low volume. Substantially unidirectional carbon cloths sheets (about 0.2 mm thick) impregnated with a liquid epoxy resin precursor composition (40% by weight of the composite) are cut into predetermined patterns. The sheets are laid-up as plies on the cavity surface of a single Invar-36 tool. For example, six plies are laid-up to a total thickness of about 1.2 mm, with successive layers of carbon fibers oriented in different directions to obtain suitably isotropic properties in a molded panel. The laid-up sheet body is covered with a plastic film and sealed around its perimeter for removal of air by vacuum. The entire assembly is loaded into an autoclave for compression against the tool surface and heating to cure the epoxy resin matrix. The tool and cured composite material are removed from the autoclave and cooled, and the film is stripped from the formed panel. 
     The tool is subject to wear by the abrasive carbon-filled polymer composition during the lay-up process in which hand and hand tools are used to push and force the carbon sheets to conform to the tool surface. The tool is also subject to wear during compression and de-molding. In addition, material builds up on the tool surface and the tool is periodically cleaned using a solvent wash of the mold surface and/or by mechanical blasting of the surface using a relatively soft particle mix. 
     The process is quite slow. But in the case of, for example, the automotive industry, the process could produce about 2,000 parts a year per tool with three shift operation and durable tooling. For a typical 5-year car program, the tool will need to produce 10,000 parts. The nickel coated tooling of this invention will provide the durability for such production requirements. 
     EXAMPLE 2 
     Liquid Molding Process—Resin Transfer Molding (RTM) and Reaction Injection Molding (RIM) 
     Liquid molding is capable of producing carbon fiber composites up to 100,000 parts per year per tool, depending on the resin chemistry used in the process. There are various forms of liquid molding process and the most widely used are RTM and RIM. The process is illustrated with reference to the schematic flow diagram of  FIG. 1 . 
     Referring to  FIG. 1 , a sheet-like or slab-like multi-layer body  10  (also identified as Fiber Reinforcement in  FIG. 1 ) of carbon fiber cloth is placed between opposing and complementary preforming tools consisting of a lower (as illustrated in the drawing figure) male preforming tool  12  and an upper female preforming tool  14 . The set of tools  12 ,  14  is also identified as Preforming Tool in the drawing figure. At the stage of illustration in  FIG. 1 , the tools  12  and  14  are vertically separated in their open position and the body  10  of carbon fiber cloth may be laid on lower tool  12 . The tools are then closed to impart, under pressure, a preform shape  16  to the original body of reinforcing carbon fibers. This preform shape  16  (also labeled as Preform in the molding process flow sequence of  FIG. 1 ) is illustrated after removal from tools  12 ,  14 . This preforming of the reinforcing material is optional, depending on the complexity of the shape of the article to be formed, and is usually conducted at a slightly elevated temperature at the manufacturing location. The tools  12 ,  14  are opened and the preform shape  16  removed for the liquid polymer molding step. 
     The preforming tools  12 ,  14  shown in  FIG. 1  can be made of the durable tooling of this invention if the dimensional accuracy of the preform is critical to the quality of the final part. 
     The preform shape  16  of multilayer carbon fiber cloth is then placed between a set opposing and complementary carbon composite molding tools comprising lower male molding tool  18  and upper female molding tool  20 . The set of molding tools  18 ,  20  is labeled as Molding Tool in  FIG. 1 . Molding tools  18  and  20  are formed of a nickel-iron alloy (suitably Invar-36) with a suitably durable layer of electroless nickel alloy  26  on their respective mold cavity defining surfaces  22 ,  24 .  FIG. 2  is an enlarged schematic view of a portion of the mold tool  18  cavity surfaces  22  showing the layer  26  of electroless nickel alloy. The nickel alloy layer  26  is suitably applied to a uniform thickness of about seventy micrometers and comprises, by weight, about five to about seven percent phosphorus. 
     Molding tools  18 ,  20  are closed on preform shape  16  which is confined between cavity defining surfaces  22 ,  24 . The defined cavity including the carbon fiber preform  16  reinforcing material are commonly evacuated of trapped air through an air-venting passage, not shown, in one of molding tools  18 ,  20 . A liquid two-part epoxy resin precursor composition is pumped into the mold cavity (as indicated in the Injection step of  FIG. 1 ) through a suitable resin charging conduit  28 , infiltrating and filling voids in the carbon cloth reinforcing material preform shape  16 . Thus, a liquid resin-filled carbon fiber composite  30  is created with in the cavity defined by the closed molding tools  18 ,  20 . The liquid and carbon fiber composite is compressed against the nickel alloy coating layers  26  on cavity surfaces  22 ,  24  of tools  18 ,  20 . At this stage of the molding process the liquid resin portion of composite  30  is uncured. 
     The molding tools are heated, such as with internal electrical resistance heating elements (not illustrated) or, simply, hot fluid, to cure the void-free injected resin material and form the solid matrix phase of the carbon cloth-reinforced molded part  32 . This is illustrated at the Cure stage of  FIG. 1 . 
     The molding tools  18 ,  20  are opened, as illustrated in the Demold stage of  FIG. 1 , and the molded product  32  is removed from the molding surfaces of the tools. 
     EXAMPLE 3 
     Compression Molding 
     Compression molding is the most traditional process to produce both thermosetting and thermoplastic composite parts. Ni plated Invar tools can be used to mold carbon fiber composite parts for certain applications such as automotive inner parts. The materials suitable for this type of molding are carbon fiber epoxy prepregs (same as the materials used for autoclave molding), carbon fiber sheet molding compound, SMC (chopped carbon fibers in vinyl ester resin, for example), and various carbon fiber containing thermoplastic polymers such as polypropylene, polyamide, etc. The layers or bodies of prepreg material or SMC are loaded into the cavity of the opened, opposing complementary molding tools. The tools are closed to compress the uncured resin impregnated carbon fiber body in the cavity formed between them. Again, the tools are heated to cure the matrix resin material and then opened for removal of the hot molded part. For thermoplastic molding, the tools are closed to compress the hot resin coated fiber mats to produce shaped final parts 
     A common feature of each of the above described molding processes (and others) is that abrasive carbon fiber reinforcing material is repeatedly brought into compressive and frictional sliding contact with the electroless nickel alloy coated nickel-iron alloy molding tool. The nickel alloy surface is durable despite such repeated abrasive contact and the repeated thermal cycling inherent in the oft-repeated molding cycles required for high volume production of such parts. Moreover, the coated mold tool retains its desirable thermal expansion compatibility with the molding compositions to obtain dimensional accuracy in the molded carbon fiber composite parts. 
     The practice of the invention has been illustrated by some specific examples which are not intended to limit the scope of the invention.