Abstract:
A molded product is made comprising a low-density product having a porous volume including cells and an essentially non-porous layer adjacent an external surface of the product. Essentially none of the cells are exposed at the external surface. The external surface is sufficiently tough, smooth, strong and durable to be suitable as a substrate for silk-screen printing.

Description:
[0001]    This patent application is a continuation of U.S. application Ser. No. 08/929,938, filed Sep. 15, 1997 and now allowed, which is a divisional of U.S. application Ser. No. 08/734,900, filed Oct. 22, 1996 and now U.S. Pat. No. 5,736,075. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates generally to molded objects and methods of molding. It particularly concerns an object molded of foam material which has many of the characteristics of a solid molded object, and molding methods for achieving such characteristics.  
         BACKGROUND OF THE INVENTION  
         [0003]    Toys and other objects which are fabricated from porous materials such as foamed plastics have certain advantages: they are light in weight, owing to their low density; and they are safe for children, owing to their compressibility. Such lightness, safety and compressibility makes these materials especially suitable for bouncing balls, flying toys such as gliders, and throw-toys such as darts. But these materials have not generally produced very good results when used for other types of toys and other products.  
           [0004]    In the past it has been difficult or impossible to fabricate such materials in a form which is acceptable to the marketplace. Foam materials are usually supplied in sheet form, making it difficult to fabricate them into objects of non-uniform thickness. By comparison, injection-molded plastic objects can be made in a wide variety of shapes of non-uniform thickness. Injection molding also has been favored for the inclusion of molded-in-place inserts of various types. Historically, foam materials have not held such inserts well.  
           [0005]    In addition, the toy marketplace favors objects with a shiny surface, preferably decorated with a variety of bright colors. Foam materials have heretofore had a dull, porous surface which is unsuitable for finishing with brightly colored paints, silk screens, tampo printing and decals. Such surfaces are also unsuitable for the molding of fine surface detail, which lends a desirable realism to toy objects. Another problem encountered with foam objects of the prior art is their vulnerability to tearing because of the softness of the surface layer.  
           [0006]    The present invention, however, contemplates the molding of toys and other objects from foam materials, yet provides such objects with a tough, smooth, shiny surface which is similar to that which results from insert molding. As a result, the molded foam objects of this invention combine foam&#39;s advantages of lightness, safety and compressibility with a superior ability to hold molded-in-place inserts and to accept various types of surface decorations, preferably brightly colored, plus fine surface detail. The molded foam objects of the invention are also more durable, because the tough surface layer resists tearing.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    These desirable results are achieved by a method of molding a low-density object having an impermeable surface, from porous material, comprising the steps of preparing a mold having a cavity the outline of which defines the outline of the object, stamping from the porous material a blank having substantially the outline of the object, inserting the blank into the cavity with the outline of the blank substantially congruent with the outline of the cavity, heating the mold with the blank therein to form the blank permanently into a product, and then removing from the mold and freezing the product, the durations and temperatures of the heating and freezing steps being chosen such that an impermeable surface is permanently formed on the product, whereby the product has the appearance of having been formed from non-porous material.  
           [0008]    In a most preferred form of the invention, after heating the mold is quenched in cold water, after which the molded product is removed and subjected to freezing.  
           [0009]    The result of this molding method is a low-density product having an impermeable surface and a foam interior integral with the impermeable surface. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0010]    [0010]FIG. 1 is an exploded perspective view of a sheet of foam plastic material, from which a number of mold blanks have been cut and removed;  
         [0011]    [0011]FIG. 2 is a perspective view of a double mold designed to receive two of said blanks;  
         [0012]    [0012]FIG. 3 is an elevational view of a heating chamber through which a number of said molds are passed, thereby molding the blanks into desired objects;  
         [0013]    [0013]FIG. 4 is a vertical sectional view of a cold-water-filled quenching chamber used for subsequent rapid chilling of said molds with the molded products therein;  
         [0014]    [0014]FIG. 5 is a vertical sectional view of a freezer used for subsequently freezing the molded products;  
         [0015]    [0015]FIG. 6 is a front elevational view of one of the finished molded products;  
         [0016]    [0016]FIG. 7 is an enlarged sectional view of a portion of the surface layer of the finished molded product.  
         [0017]    [0017]FIG. 8 is a perspective view of another type of finished product which can be made in accordance with the present invention, which is in the form of a hollow shell;  
         [0018]    [0018]FIG. 9 is a perspective view of a mold used for making the product of FIG. 8 in accordance with this invention;  
         [0019]    [0019]FIG. 10 is a vertical sectional view of the mold of FIG. 9; and  
         [0020]    [0020]FIG. 11 is a perspective of a conveyer oven used in making the product of FIG. 8.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The molding process of this invention begins with conventional plastic material formed of foamed poly-ethylene (PE) or foamed poly-ethylene vinyl acetate (EVA), both of which are commercially supplied in the form of a flat sheet  10  of uniform thickness (i.e., in the vertical direction, as seen in FIG. 1). The standard commercially available sheet size is 100 cm. by 200 cm.; and the available thicknesses range from 3 to 100 mm., the thickness being substantially uniform across the length and breadth of any particular sheet. The density of such materials is comparable to that of rubber at ordinary ambient temperatures.  
         [0022]    The objective of the process is to produce a toy or other end product  12  (FIG. 6) which in general has a non-uniform thickness in all dimensions, notwithstanding the uniform thickness of the commercially available sheet stock.  
         [0023]    The first step in the process is to die-stamp from the foam sheet  10  one or more mold blanks  12 A, B, C, etc. which have an outline roughly congruent with the intended outline of the final product  12 . Because the stamping die cuts through the entire thickness (in the vertical direction) of the foam sheet  10 , no flash or parting line results from the die-cutting process. In FIG. 1 the first blank  12 A is shown after having been lifted out of the foam sheet  10 . At this stage of the process, the blanks  12 A, B, etc. have the same uniform thickness (in the vertical direction) as the sheet  10  from which they came.  
         [0024]    The next step is to put one or more blanks  12 A, B; etc. into suitable half cavities such as  14 A, B formed in the lower half  14  of a metal mold. Connected to the lower half  14  by hinge  16  is an upper half  18  of the metal mold, which is formed with complementary half cavities  18 A, B. After the foam blanks  12 A, B, etc. are inserted into the lower cavities  14 A, B, the upper half  18  is rotated about the hinge  16  to close the mold, and the mold is then clamped closed by means of a conventional clamping mechanism, part of which is visible at  20 .  
         [0025]    When the mold is closed, the upper half cavities  18 A, B cooperate with the lower half cavities  14 A, B to form complete molding cavities which completely enclose the foam blanks  12 A, B, etc. The outlines of the upper and lower half cavities are congruent with each other, and with the desired outline of the final product  12 .  
         [0026]    The concave surfaces of the half cavities have the external surface shapes of the upper and lower portions respectively of the desired final product, and thus serve to mold the blanks  12 A, B into products  12  of the desired final shape. That shape will generally have a non-uniform thickness, even though the foam blanks previously were of uniform thickness, like the foam sheet  10  from which they were stamped. In addition, the concave surfaces of the half cavities are engraved with fine surface relief detail which is to be impressed upon the surface of the final product  12 .  
         [0027]    The closed mold  14 ,  18 , with the foam blanks  12 A, B, etc. inside the cavities  14 A,  18 A and  14 B,  18 B is then passed through an oven  22  mounted on a table  24 . Movement of the molds through the oven may be either intermittent or continuous, and may be accomplished either manually or by means of a conventional automatic conveyer mounted on the table  24 . The temperature inside the oven  22  is in the range from 200 to 300° C., and the duration of exposure to the oven temperature is in the range from about 5 to about 15 minutes.  
         [0028]    After emerging from the oven, the molds  14 ,  18 , with the foam blanks  12 A, B, etc. still inside, are immersed in a vat  26  of cold water  28  for the purpose of quickly chilling the surface of the foam blanks where they contact the concave surfaces of the cavities  14 A,  18 A and  14 B,  18 B. The relatively high heat conductance of the metal mold  14 ,  18  facilitates rapid cooling of the surface layer of the mold blanks  12  where they contact the mold. The temperature of the cooling water is in the range from about 5 to about 20° C., and the duration of exposure to the chilling water is from about 5 to about 15 minutes.  
         [0029]    Subsequently the molds  14 ,  18  are removed from the water bath  28 , opened by releasing the clamps  20  and rotating the upper mold half  18  away from the lower mold half  14  about hinge  16 , and the molded objects  12  are then removed from the molds and placed in a freezing compartment  30  by inserting the objects  12  through a door  32  and placing them on shelves  34  inside the compartment  30 , after which the door  32  is closed. The freezer temperature is from about −5 to about −15° C., and the duration of exposure to the freezing temperature is in the range from about 30 to about 45 minutes.  
         [0030]    The EVA foam starting material used in the present invention is classified as “closed cell,” which means only that the cells do not connect with each other; the cells do, however, open through the external surface of the sheet  10 , making the surface dull-looking and porous. The PE starting material is classified as “open cell,” and also has a dull and porous external appearance because of the fact that the cells open through the surface.  
         [0031]    The process of this invention produces a substantially lower reject rate than prior processes for molding foam products, which have tended to suffer from irregularities in shape and finish, and incorrect and inconsistent control of softness and density. In addition, prior art products molded from foam materials were characterized by dull and porous surfaces which have defied efforts to apply bright and colorful finishes; those finishes which were applied tended to peel off easily and did not achieve a bright, shiny or colorful effect. The surface detail was also poor. But, as seen in FIG. 7, the result of the successive hot molding, cold quenching and freezing steps of this invention is to create a layer  12 L at the surface of the final product  12  in which the cells or pores  12 P are sealed and do not open up through the surface of the final product  12 . Indeed, the surface now consists of a tough, smooth, impermeable skin  12 S, which has substantially the same characteristics as the surface of an injection-molded product: i.e., it is tough, smooth, shiny, durable, and able to receive and hold various types of decorative finishes.  
         [0032]    The finished product  12  is also better able to hold molded-in-place inserts, and is less likely to be torn in use. The surface  12 S also has a smooth, shiny finish which is inherently more attractive than the dull, porous look of untreated foam materials; which holds fine detail in a way which has not previously been accomplished with foam materials; and which also lends itself to various finishing processes involving bright and decorative colors.  
         [0033]    In particular, the finishing processes which have been found to work well in the method of this invention are color die spraying, tampo printing, silk-screening, and the use of a non-porous PE film pre-printed with graphics or a solid color and laminated with a glue which causes the films to adhere to the surface of the product  12 . It is a particular feature of the invention that such films can be applied to the mold blank  12  before the heating step of FIG. 3, in which case the film is permanently molded in place on the surface of the product  12 .  
         [0034]    At the same time, the bulk of the product  12 , below the surface layer  12 L, retains the original porosity and hence the low density of the original foam sheet  10 . As a result, the final product remains soft, light, flexible, and therefore safe for use as a glider, dart toy, bouncing ball, or the like. In addition, the final product  12  can have any desired non-uniform thickness, despite having been formed from a sheet  10  of uniform thickness.  
         [0035]    In accordance with an additional aspect of this invention, it is suitable for molding shapes having the general configuration of a hollow shell, such as the toy car body  40  illustrated in FIG. 8, which features a hollowed-out interior space  40 A. This type of configuration is produced by means of the mold  42 , which has a lower mold half  44  formed with a convex core  44 A and an upper mold half  46  formed with a complementary cavity  46 B. A hinge  48 , guide pins  50  on the lower mold half  44 , complementary guide holes  52  on the upper mold half  46 , and a latching mechanism  54 ,  56  complete the mold assembly.  
         [0036]    The sectional view of FIG. 10 illustrates the hollow shell-like shape  40  being formed within the closed mold  42 . In order to permit a bulky mass of foam material cut from the sheet  10  to be more easily compressed between the core  44 A and cavity  46 A for molding purposes, it is preferable to heat the foam material at a temperature of about 200° C. for about 3 to about 5 minutes in order to soften the material before beginning the molding process. For this purpose, an oven  60  (FIG. 11) is provided, and is equipped with a conveyor belt  62  which transports sheets of foam material  10  through the oven prior to their undergoing the process steps described above. Alternatively, this pre-heating procedure may be performed on the foam blanks after they are stamped from the sheet  10 ; but the stamping is facilitated if the pre-heating step is done first, so that the sheet material  10  is softer at the time the blanks are stamped out.  
         [0037]    The ability to mold hollow shell-like configurations as just described opens up the possibility of using the process of this invention to manufacture a wider variety of products, not all of which are toys. For example, inexpensive soft-sided luggage-like containers with attractive exterior finishes may be manufactured in this manner, as well as protective shock-absorbing housings, cases or covers for small electronic appliances, books, and a wide variety of other objects.  
         [0038]    Despite somewhat higher raw material costs, the present invention achieves certain significant economies as compared to other molding processes. First, the molds employed in injection molding processes are very expensive. In order to withstand the high temperatures employed in injection molding, the molds must be made of expensive steel; whereas in the present process a much less expensive aluminum alloy material is adequate. In addition, injection molding requires numerous conduits and fittings to conduct the molten plastic resin into the mold cavity; all of which is unnecessary with the process of this invention.  
         [0039]    As a result, the mold cost, which can run around $40,000 per cavity for an injection mold, is reduced to the neighborhood of only $2,500 per cavity. This permits the construction of a greater variety of molds for a given cost; which is advantageous generally, but particularly in the toy field because it permits a manufacturer to satisfy consumer desires for greater product variety.  
         [0040]    Second, the fact that products manufactured in accordance with this invention can be finished with multi-colored pre-printed PVC film offers a significant saving over other finishing processes requiring more individualized attention to each manufactured piece.  
         [0041]    Accordingly, the present invention, in both its process and product aspects, economically produces a flexible, light, shiny, brightly and colorfully finished, durable product of any desired cross-sectional shape. While these features and advantages of the invention have been described in connection with a preferred illustrative embodiment, the scope of protection to which the invention is entitled is to be determined by the appended claims, interpreted in light of the novel principles disclosed herein.