Abstract:
A method for preparing a mold form includes forming a mold blank having a surface corresponding to the surface of the article desired and enclosing the mold blank in an open ended enclosure. A thin layer of polymer resin, preferably polyurea is sprayed on the surface of the mold blank and allowed to cure. Heat transfer tubing is inserted over the resin layer. A mixture of metallic pellets and epoxy are inserted over the heat transfer tubing and then another layer of resin is sprayed over the mixture. After the resin has cured, the mold blank is removed to expose the mold form having an outer surface of polyurea.

Description:
RELATED APPLICATION 
     This application is a divisional application of co-pending U.S. patent application Ser. No. 09/211,957, filed Dec. 15, 1999. 
    
    
     The present invention generally relates to the manufacture of molded parts and in particular a thermal cooled mold form. 
     BACKGROUND OF THE INVENTION 
     Conventionally, mold forms are manufactured from such materials as aluminum, steel or wood. In the case of steel and aluminum, expensive casting or cutting machinery is required for producing the final formed shape. After the design is formed, the mold requires hand finishing to finalize certain complex features of the mold. Further, cooling passages must then be installed into the steel or aluminum to provide means for maintaining the mold within a certain temperature range during its use. As a result, a large or complicated mold may require many weeks to complete. Although steel or aluminum molds provide a high quality mold form, the life use of these types of molds may exceed the life of the final product production. Therefore, in many cases a steel or aluminum mold is very expensive relative to the benefits of the final molded piece. 
     Wood molds are undeniably cheaper than the aforementioned steel or aluminum molds, but still requires a labor intensive manufacturing procedure to produce a quality mold. A wood mold involves no cooling system, thereby causing excessive press time in the manufacturing process. Wood tools, without a cooling system, causes unpredictable shrinkage in the manufactured part. The durability of wood tools is very short. 
     It is therefore desirable to provide a mold and a method for constructing the mold which can be manufactured quickly, and at less cost to the manufacturer without measurably affecting durability. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the aforementioned concerns by providing a mold having a mold surface made of a polymer resin, in particular polyurea or a polyurea blend, polyurethanes or a polyurethane blend. The procedure to manufacture the mold piece includes forming a wax or wood pattern having a surface of the shape of the final molded piece, and then building a box structure around the wax or wood pattern. The wax or wood pattern is then sprayed with a polyurea or polyurethane material to completely coat the surface of the wax or wood structure. Cooling lines are placed within the box structure. After the polyurea or polyurethane material has dried/cured, aluminum puffs/pellets and an epoxy material mix is then inserted into the cavities of the mold. Mold support structures and stanchions may be placed within the box to provide strength and stability to the polyurea or polyurethanes mold as well as to provide additional heat transfer material to the mold. Another layer of polyureas or polyurethanes may be sprayed within the mold. After drying/curing, a vacuum plate is placed over the box structure providing exterior connections to the cooling lines as well as connections to the vacuum hole. The original wax or wood structure is then removed leaving a smooth mold surface made out of the polyurea or polyurethane material and/or blends. 
     The resulting mold and its process provides a quick, inexpensive and durable mold. Cooling lines are easily installed and eliminate the need of machining into steel or aluminum. Vacuum holes are able to be drilled throughout the mold at a much faster rate than into steel or aluminum. The support structures provide a heat transfer element during the molding process in addition to providing support to the mold structure. 
     Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein: 
     FIG. 1 is a perspective view of a wax pattern or form contained in a box structure; 
     FIG. 2 is a perspective view showing a polyurea or polyurethane layer being deposited on the wax pattern; 
     FIG. 3 is a perspective view showing cooling lines installed over the polyurea or polyurethane material; 
     FIG. 4 is a perspective view showing a heat transfer and support material being deposited over the cooling lines; 
     FIG. 5 is a perspective view showing another polyurea or polyurethane layer being deposited into the box structure; 
     FIG. 6 is a perspective view showing a covered plate being installed over the box structure; 
     FIG. 7 is a perspective view showing the mold being removed from the wax or wood form; and 
     FIG. 8 is a perspective view showing a completed mold piece. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The intent of the invention is to provide a mold as shown in FIG. 8 for use at a manufacturing facility. The steps to provide the finished mold are shown in FIGS. 1-7. 
     According to one aspect of the invention, the characteristics of the final molded product are stored in a computer. A positive pattern or a mold blank  10  is made based on these characteristics of the finished molded product. These characteristics are retrieved to shape a positive pattern mold blank  10  from a wax or wood block. A box structure  12  is built around the positive mold blank  10  made of wax or wood material such that the positive surface represented as  14  is oriented face up in the interior of the box structure  12 . After the box structure  12  is built, the preferred step is to spray the interior positive surface  14  with the polyurea or other predetermined material. But as an alternative, the positive pattern surface  14  can be prepped for providing vacuum holes to the final mold product before the positive surface  14  is sprayed. That step is described hereinafter. 
     As can be seen in FIGS. 3 and 8, vacuum holes  16  are drilled or otherwise disbursed into the positive surface  14  of the mold blank  10 . The vacuum holes  16  are very small, approximately 0.040 inches diameter. The holes  16  are drilled into the positive surface  14  of the mold blank  10  such that wires  18  may be partially inserted into the holes  16  and extend upwardly into the interior of the box structure  12 , above the positive surface  14  of the mold blank  10 . 
     In FIG. 2, the interior positive surface  14  of the wax pattern is sprayed with preferably a polyurea material  20 . The material may be applied manually or by a robot  22 . Similar material such as a polyurea blend, polyurethane, or a polyurethane or urethane blend may be used. One of the advantages of using polyurea and the other mentioned materials is that the material provides a relatively quick cure time. That is after spraying, the material is dried within 20-30 seconds. Polyurea provides the quickest drying time and the best durability of the aforementioned materials. Further, these materials, and especially polyurea, provide excellent impact strength, stability at high temperature, and surface quality. The spray polyurea or polyurethane material  20  may be tinted to a color such that when the material is manually sprayed onto the wax pattern  10 , the operating personnel knows when the entire positive surface  14  has been covered with the material  20 . The polyurea or polyurethane material  20  is applied as thin as possible, less than one-eighth inch thick and preferably approximately 0.060 inches thick. FIG. 2 shows the positive surface  14  partially covered by the polyurea material  20 . 
     The polyurea material  20  dries within a half a minute so that cooling lines or tubes (heat transfer tubing)  24  may be installed soon after the polyurea material  20  is applied. FIG. 3 shows the cooling lines  24  installed within the box structure  12  over the first polyurea layer  20  and mold blank  10 . The cooling lines  24  are installed according to the shape of the mold blank  10 . The cooling lines  24  are preferably made out of the copper tubing that meet in a pair of manifolds  26  providing an inlet  28  and outlet  30  aperture for later connection to an outside cooling source. The size and configuration of the cooling lines  24  will depend on the mold size and cooling or heating requirements. 
     In addition to the cooling tubing  24 , metal pellets  32  such as aluminum can be disposed within the box structure  12 . Aluminum pellets  32  are preferred because of their light weight and high heat transfer capabilities. The metal pellets or chips  32  are first mixed with an epoxy material  34  to provide some adhesion to the metal pellets  32  and prevent them from freely moving within the box structure  12 . Looking a FIG. 4, the metal pellets  32  and epoxy material  34  can be poured or otherwise disposed into the box structure  12  over the cooling tubes  24  such that the cooling tubes  24  are virtually covered with the pellets  32  and epoxy  34  mixture. The metal pellets  32  and epoxy  34  mixture provides added strength to the final mold, plus provides an additional form of heat transfer. Although the cooling tubes  24  are virtually covered by th metal pellet  32  and epoxy  34  mixture, the inlet an outlet apertures  28 ,  30  respectively of the manifolds  26  remain exposed so that they may be later operatively connected to fluid lines. The metal pellet  32  and epoxy  34  mixture does not provide an impenetrable mixture and therefore many air hole passages are available between the pellets  32  within the mixture. This allows for the vacuum draw to reach the vacuum holes  16  in the first layer of polyurea through the metal pellet  32  and epoxy  34  mixture. 
     After the pellet and epoxy mixture is applied, the epoxy  34  is allowed to solidify. Looking at FIG. 5, it is shown that another layer  36  of polyurea or polyurethane material may be sprayed over the cooling lines  24  and metal pellet  32  and epoxy  34  mixture. Again, the second layer of polyurea  36  or other similar material, as stated above, may be applied thinly over the cooling lines  24  and metal pellet and epoxy mixture. The second layer of polyurea material  36  may also be tinted to indicate that the entire surface has been covered with the polyurea material  36 . 
     As shown in FIG. 6 further supports or stanchions  38  may be inserted into the box structure  12  to provide added strength to the final mold. A vacuum plate or closure top  40  is then inserted and sealed over the box structure  12 . The vacuum plate  40  has apertures  42 ,  44  corresponding to the inlet  28  and outlet  30  apertures of the heat transfer manifolds  26 . Further there is a third aperture  46  for accessing a vacuum source to the interior of the box structure  12 . 
     Generally, after the plate  40  is sealed onto the box structure  12 , the mold blank  10  and wires  18  providing vacuum holes  16 , if applicable, to the polyurea layers  20 ,  36  may be removed as shown in FIG.  7 . The mold blank  10  and wires  18  can be discarded or used again. The box structure  12  in FIG. 7 is turned over 180° to expose the final mold  50  as shown in FIG.  8 . The fluid apertures  42 ,  44  and  46  are positioned at the bottom of the box structure  12  for later connection. If vacuum holes  16  were not provided in the polyurea surface  20  via the wires  18  during the manufacture of the mold, the final mold  50  may be drilled once the mold blank  10  is removed. The drilled vacuum holes  16  are very small, preferably approximately 0.040 inches diameter. Drilling the vacuum holes  16  after the final mold  50  is completed is the preferred method. Drilling the vacuum holes  16  at the end saves steps plus eliminates the need of the wires  18 . 
     The resultant mold  50  provides a cost effective mold that can be manufactured quickly, with a minimum of labor and material expenditures. The resultant mold  50  manufactured by the process as detailed above can be available for use within a couple of days. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.