Abstract:
A laminate is prepared, which comprises a base layer of a fibrous material mixed with a synthetic resin and its cross linking agent, a cover layer of unvulcanized rubber and an intermediate layer of thermoplastic resin. The laminate is shaped into a desired shape by the application of heat and pressure at such an elevated temperature as to cause the cross linking reaction of the synthetic resin in the base layer, the vulcanization reaction in the cover layer and a change of the thermoplastic resin into a soft and adhesive condition to take place simultaneously.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for making various covers to be used in connection with an engine, and more specifically to a method for making such covers which are mainly made of a fibrous material having a good damping capacity. 
     Various covers are used for internal combustion engines. For example, there are rocker covers (cylinder head covers), front covers, dust covers, undercovers, engine air filter casings and their covers, noise reducing covers located in the vicinity of an engine, oil pans and hood insulators. These covers must be water-proof, oil-proof, light in weight, hard to some extent, and shape-retentive to some extent. 
     These covers are usually made of sheet metal, plastic or other hard material. Covers made of such a hard material are easily excited by the vibrations of an engine, and they tend to become a secondary source of noises which increases the engine noise. 
     Recently, there have been proposed covers which are made of a fibrous material having an excellent capability in absorbing sound and mechanical vibration. For example, Japanese Patent provisional publication Sho 54-80387 discloses such covers. Covers of this kind are made by mixing a synthetic resin and its cross linking agent to a fibrous material, and pressing the resultant mixture into a desired shape and a densely compressed state by the application of heat and pressure. 
     In order to make such covers impervious to rain water and engine lubricating oil, it is necessary to cover one or both of the surfaces of the fibrous material layer with a rubber layer having a water-proofness and an oil-proofness, because the layer of the fibrous material is not impervious in itself. It is, however, desirable to make the rubber layer as thin as possible because the rubber layer increases the weight of a cover, and raises the cost of material. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a convenient method which makes it possible to easily produce a cover of a laminated structure having a base layer of a fibrous material providing the cover with a good capability in absorbing noise and vibration, and a water-proof layer of a rubber material which is uniform in thickness and made as thin as possible. 
     According to the present invention, a fibrous sheet, an unvulcanized rubber sheet and a thermoplastic resin film are prepared first. The fibrous sheet is made of a fibrous composition comprising fiber material, synthetic resin and its cross linking agent. The unvulcanized rubber sheet is made of an unvulcanized rubber composition comprising unvulcanized rubber. After that, a laminate is made, which comprises the fibrous sheet, the unvulcanized rubber sheet and the thermoplastic resin film placed between the fibrous sheet and the unvulcanized rubber sheet. The thus made laminate is shaped into a desired shape by applying pressure and heat at such an elevated temperature as to cause the cross-linking reaction of the synthetic resin in the fibrous sheet, the vulcanization reaction of the unvulcanized rubber in the unvulcanied rubber sheet, and a change of the thermoplastic resin film to a soft and adhesive condition, to take place simultaneously. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration for showing the producing process according to the present invention; and 
     FIG. 2 is a sectional view of a cover made by the process of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the present invention is shown in FIGS. 1 and 2. FIG. 2 shows an engine dust cover 10 by way of example, and FIG. 1 show the process for making the dust cover 10. The dust cover 10 is made of a laminated plate consisting of a base layer 11, an impermeable layer 12, and an adhesive layer 13 placed between the base layer 11 and the impermeable layer 12. First, a fibrous material 1 for forming the base layer 11 is prepared by mixing synthetic resin such as phenol resin and its cross linking agent such as formalin, to organic fibers such as wool, cotton, acetate fibers, polyester fibers, or inorganic fibers such as glass wool and rock wool. For convenience of transport and handling, the fibrous material 1 is formed into a sheet having a thickness of about 25 mm by application of heat for a short time at a temperature ranging from 150° C. to 170° C. On the other hand, an unvulzanized rubber material 2 is prepared for forming the impermeable layer 12. For example, nitrile rubber, acrylic rubber, or epichlorohydrin rubber is used as the unvulcanized rubber material 2. After mixing ingredients to the unvulcanized rubber material 2, the unvulcanized rubber material 2 is formed into a sheet having a thickness of about 0.3 mm by calender. A thermoplastic resin film 3 for forming the adhesive layer 13 is tightly overlaid on one side of the sheet of the unvulcanized rubber 2 by slightly pressing, and then the unvulcanized rubber sheet covered with the resin film 3 is cut into a desired shape. The resin film 3 overlaid on the unvulcanized rubber sheet serves to prevent the unvulcanized rubber sheet from sticking to another unvulcanized rubber sheet during storage or transportation. As the thermoplastic resin film 3, polyvinyl chloride, nylon 12, acetate, polypropylene or the like is used. The thickness of the resin film 3 is about 0.05 mm, by way of example. 
     The fibrous material sheet 1 and the unvulcanized rubber sheet 2 covered with the resin film 3 which are thus prepared separately, are arranged in a laminate, and positioned between an upper press die 5 and a lower press die 6 of a press machine 4. The upper die 5 and lower die 6 have, respectively, faces 5a and 6a having configurations for forming the shape of the dust cover 10. The upper die 5 (or the lower die 6) may have a cutting edge 5b for trimming, on the outer periphery of the face 5a if the shape of products permits such trimming. The upper and lower dies 5 and 6 are, respectively, provided with heating elements 7 and 8. During operation, the lower die 6 to be in contact with the fibrous material sheet 1 is heated to a temperature from 150° C. to 230° C. by the heating element 8, and the upper die 5 to be in contact with the unvulcanized rubber sheet 2 is heated to a temperature of from 140° C. to 170° C. Under such heated conditions, the laminate of the fibrous material sheet 1, the unvulcanized rubber sheet 2 and the thermoplastic resin film 3 is formed into the shape of the dust cover 10 by applying pressure, as shown in FIG. 1(C). In this case, a relatively high pressure of about 20 to 50 kg/cm 2  is applied, and the thickness of the three-layer laminate is compressed to about 2 mm. 
     By the application of heat and pressure, the cross linking reaction of the contained synthetic resin takes place in the fibrous material sheet 1, and the sheet 1 is formed into the base layer 11 which is densely compressed, and has a sufficient capability in retaining its shape. At the same time, in the unvulcanized rubber material sheet 2, the vulcanization reaction takes place by the application of heat, and the impermeable layer 12 having the configuration of the face 5a of the upper die 5 is formed. The impermeable layer 12 made by this method is very thin and uniform. In parallel with the cross linking reaction and the vulcanization reaction, the thermoplastic film 3 softens and becomes adhesive. Thus, the thermoplastic resin film 3 firmly bonds the base layer 11 and the unpermeable layer 12, and becomes the adhesive layer 13. 
     The thermoplastic resin film 3 prevents the unvulcanized rubber material 2 from passing into the fibrous material sheet 1, and by so doing, serves to make the impermeable layer 12 uniform and thin. The thermoplastic resin film 3 is shaped simultaneously with the fibrous material sheet 1 and the unvulcanized rubber sheet 2, so that cracks and other irregularities hardly occur in the thermoplastic resin film 3 even if the shape to be formed is complex. Because the thermoplastic resin film 3 is almost free from defect, the impermeable layer, too, is almost free from a defect which would be caused by the rubber material 2 permeating into the fibrous material sheet 1 through a defect of the thermoplastic film 3. Besides, the defect-free adhesive layer can provide a satisfactory adhesion of the impermeable layer. 
     One example is shown in Table 1, in which the thicknesses before and after the forming process are listed. As evident from Table 1, the impermeable layer 12 is made very thin. 
     
                       TABLE 1______________________________________Thickness of Laminated Layers (mm)            Before                  After            shaping                  shaping______________________________________Impermeable layer  0.3     0.22(rubber sheet)Adhesive layer     0.05    0.05(resin film)Base layer         25      1.73(fibrous sheet)Total                      2.00______________________________________ 
    
     A cover having two impermeable layers covering both surfaces of the cover can be made in a similar manner. If a cover requires a decorative appearance as in the case of a rocker cover, it is possible to make a cover having such a decorative appearance. In this case, laminated sheets including a decorative sheet made of thermoplastic resin, non-woven fabric or the like, placed on the outer surface of the unvulcanized rubber sheet are shaped simultaneously by the application of heat and pressure. One example is shown in Table 2 in terms of thickness. 
     
                       TABLE 2______________________________________Thickness of Laminated Layers (mm)            Before                  After            shaping                  shaping______________________________________Non-woven fabric   0.16    0.16Impermeable layer  0.3     0.22(rubber sheet)Adhesive layer     0.05    0.05(resin film)Base layer         25      1.57(fibrous sheet)Total                      2.00______________________________________ 
    
     According to the present invention, covers having a good damping capacity can be made easily and inexpensively by a single step. The fibrous sheet and the rubber sheet are shaped and bonded together simultaneously without any additional bonding step which would be very troublesome especially when the shape of a cover is complicated. Besides, the impermeable rubber layer made by this method is almost free from wrinkles or other defects, and its thickness can be made very thin and uniform. Accordingly, covers made by this method are not only excellent in damping capacity, but also light in weight and inexpensive.