Abstract:
A multi element moving vacuum chamber (MEMVC) for rotational casting includes a mold having a predefined shape and an axis of rotation, and a box containing casting material. The MEMVC further includes a first vacuum element mounted to an outer mold surface and a second vacuum element mounted to an outer box surface. The mold and the box are detachably engageable with each other to define an engaged configuration in which air present in an enclosed area defined by the outer mold surface and an inner surface of the first vacuum element, by an inner mold surface and an inner box surface, and by the outer box surface and an inner surface of the second vacuum element, is removed to create a vacuum. Alternatively, the mold and/or the box may be constructed with vacuum loading capability, thereby eliminating the need for the first and/or the second vacuum elements.

Description:
BACKGROUND OF INVENTION 
     a. Field of Invention 
     The invention relates generally to a method and apparatus for plastic casting, and, more particularly to a method and apparatus for powder slush plastic casting using a multi element moving vacuum chamber, surrounding a casting tool and a powder box, for reducing imperfections such as pin holes, surface voiding and skin thickness variations in a finished cast product. 
     b. Description of Related Art 
     A need exists for an improved method and apparatus for powder slush plastic casting, for example in skinned automotive instrument panel manufacturing, whereby common defects such as pin holes, surface voiding and skin thickness variations may be reduced. 
     Powder slush casting (or molding) of thin-walled articles from castable plastic material, such as plastisol and pourable plastic powders, is a well known method of manufacturing molded plastic articles, such as hollow articles, automotive dashboards, door panels and the like. Powder slush casting is particularly useful in the manufacture of hollow articles since the wall thickness of the article can be controlled by adjusting the amount of plastisol or plastic powder used in the casting process. Examples of commonly used plastic powders include Thermo Plastic Urethane (T.P.U.), Thermo Plastic Olefinic (T.P.O.) and Poly Vinyl Chloride (P.V.C.). 
     In conventional slush casting, a mold is first preheated. FIGS. 1A-1F are exemplary diagrams of a related art powder casting cycle illustrating the key stages of a conventional casting cycle. As shown in FIGS. 1A and 1B, a powder box  1 , filled with plastic powder  2  is then brought into contact with pre-heated mold  3  and engaged with it to prevent leakage of plastic powder  2 . As next shown in FIG. 1C, when mold  3  is rotated, plastic powder  2  strikes the heated mold surface  4 . When mold  3  stops rotating, powder  2  on mold surface  4  fuses to form the shape of mold surface  4  (see FIG.  1 D), and any remaining plastic powder  2  drains back into powder box  1  for subsequent casting, or is discarded. As shown in FIG. 1E, powder box  1  is then disengaged from mold  3  and returned to its original location (shown also in FIG.  1 A). Finally, as shown in FIG. 1F, mold  3  is rotated to allow an operator to remove the fused layer (or skin) on mold surface  4 . 
     In the conventional slush casting method and apparatus discussed above, pin holes, surface voiding and skin thickness variations are common defects. In automotive panels for example, pin holes cause urethane foam leakage, and skin thickness variations cause subtle distortions in the finished part&#39;s surface. From a quality stand-point, these defects detract from the product&#39;s appearance, and from a manufacturing stand-point, these defects are the cause of significant scrap levels. 
     In the art, there currently exist various methods and apparatus for slush casting, as disclosed for example in U.S. Pat. Nos. 6,099,771, 6,082,989, 6,019,590, 5,932,162, 5,840,236, 5,580,501, 5,397,409, 5,387,390, 5,290,499, 5,221,539, 4,946,638, 4,898,697, 4,790,510, 4,740,337, and 4,714,424. The slush casting methods and apparatus disclosed in these patents share the disadvantages of pin holes, surface voiding and skin thickness variations in the finished cast product, as discussed above. 
     SUMMARY OF INVENTION 
     The invention solves the problems and overcomes the drawbacks and disadvantages of the prior art by providing a method and apparatus for powder slush plastic casting, which minimizes defects such as pin holes and evens plastic flow to reduce thickness variations in a finished product, and which is relatively simple and inexpensive to install and operate. 
     In particular, the invention accomplishes this by providing three embodiments of a multi element moving vacuum chamber (MEMVC) for rotational casting. The first embodiment of the MEMVC includes a mold having a predefined shape and an axis of rotation. The mold is rotatable about the axis of rotation, and includes an inner mold surface and an outer mold surface. The MEMVC further includes a box containing casting material. The box has an inner box surface for holding the casting material and an outer box surface. A first vacuum element is mounted to the outer mold surface and a second vacuum element is mounted to the outer box surface. The mold and the first vacuum element are detachably engageable with the box and the second vacuum element to define an engaged configuration. When the mold, the box and the first and second vacuum elements are in the engaged configuration, air within an enclosure defined by the outer mold surface and an inner surface of the first vacuum element, by the inner mold surface and the inner box surface, and by the outer box surface and an inner surface of the second vacuum element, may be removed to create a vacuum. 
     In the first embodiment of the MEMVC discussed above (and for the second and third embodiments discussed below), the mold may made of Nickel or another metal. The casting material may be Thermo Plastic Urethane (T.P.U.), Thermo Plastic Olefinic (T.P.O.), Poly Vinyl Chloride (P.V.C.), or a castable plastic. The first and second vacuum elements may be bell shaped, and may each include a vacuum connection for removal of air from an enclosed area defined by the inner surface of the first vacuum element and the outer mold surface, by the inner surface of the second vacuum element and the outer box surface, and further defined by the inner mold surface and the inner box surface. The mold, the first vacuum element, the box and the second vacuum element may further include a vacuum rated seal for providing sealed engagement between each of the mold and the first vacuum element, and the box and the second vacuum element. The mold, the box, the first vacuum element and the second vacuum element may further include a latch (on each) for permitting detachable engagement between the mold and the first vacuum element, and the box and the second vacuum element. 
     The second embodiment of the MEMVC for rotational casting includes a mold having a predefined shape and an axis of rotation. The mold is rotatable about the axis of rotation and further includes an inner mold surface and an outer mold surface. The MEMVC further includes a box containing casting material. The box has an inner box surface for holding the casting material and an outer box surface. The MEMVC yet further includes a vacuum element mounted to either the outer mold surface or the outer box surface, thereby defining a vacuum element mounted component and a non-vacuum element mounted component. Specifically, if a vacuum element is mounted on the mold, the vacuum element mounted component would be the mold with the vacuum element, and if a vacuum element is mounted to the box, then the vacuum element mounted component would be the box with the vacuum element. The non-vacuum element mounted component is capable of vacuum loading. The mold is detachably engageable with the box, and the vacuum element is detachably engageable with the non-vacuum element mounted component, to define an engaged configuration. When the mold, the box, and the vacuum element are in the engaged configuration, air within an enclosure defined by the inner mold surface and the inner box surface, and further defined by an outer surface of the vacuum element mounted component and an inner surface of the vacuum element, may be removed to create a vacuum. 
     The third embodiment of the MEMVC for rotational casting includes a mold having a predefined shape and an axis of rotation. The mold is rotatable about the axis of rotation and has an inner mold surface and an outer mold surface. The mold is also capable of vacuum loading. The MEMVC also includes a box containing casting material. The box has an inner box surface for holding the casting material and an outer box surface, and is also capable of vacuum loading. The mold is detachably engageable with the box to define an engaged configuration. When the mold and the box are in the engaged configuration, air within an enclosure defined by the inner mold surface and the inner box surface may be removed to create a vacuum. 
     In yet another aspect of the invention, the invention solves the problems and overcomes the drawbacks and disadvantages of the prior art by providing a first method for rotatable casting. The method includes providing a mold having a predefined shape and an axis of rotation. The mold has an inner mold surface and an outer mold surface. The method further includes providing a box containing casting material and having an inner box surface for holding the casting material and an outer box surface. The box is located relative to the mold at a first location. The method then includes the steps of mounting a first vacuum element to the outer mold surface and mounting a second vacuum element to the outer box surface. Next, the method includes the steps of heating the mold to a predetermined temperature and moving the mold relative to the box to engage the mold with the box and the first vacuum element with the second vacuum element, thereby defining an engaged configuration. In the engaged configuration, the method includes the step of removing air within an enclosure defined by an inner surface of the first vacuum element and the outer mold surface, by an inner surface of the second vacuum element and the outer box surface, and by the inner mold surface and the inner box surface, in order to create a vacuum in the enclosure. 
     The invention also provides a second method for rotatable casting, which includes providing a mold having a predefined shape and an axis of rotation. The mold has an inner mold surface and an outer mold surface. The method further includes providing a box containing casting material and having an inner box surface for holding the casting material and an outer box surface. The box is located relative to the mold at a first location. The method then includes the step of mounting at least one vacuum element to one of the outer mold surface or the outer box surface, thereby defining a vacuum element mounted component and a non-vacuum element mounted component. The non-vacuum element mounted component is capable of vacuum loading. The method yet further includes the steps of heating the mold to a predetermined temperature, and moving the mold relative to the box to engage the mold with the box, thereby defining an engaged configuration. In the engaged configuration, the method includes the step of removing air within an enclosure defined by an inner surface of the vacuum element and an outer surface of the vacuum element mounted component, and by the inner mold surface and the inner box surface, in order to create a vacuum in the enclosure. 
     The invention also provides a third method for rotatable casting, which includes providing a mold having a predefined shape and an axis of rotation. The mold has an inner mold surface and an outer mold surface, and is capable of vacuum loading. The method further includes providing a box containing casting material. The box has an inner box surface for holding the casting material and an outer box surface. The box is located relative to the mold at a first location and is capable of vacuum loading. The method then includes the steps of heating the mold to a predetermined temperature and moving the mold relative to the box to engage the mold with the box, thereby defining an engaged configuration. In the engaged configuration, the method includes the step of removing air within an enclosure defined by the inner mold surface and the inner box surface. 
     It should be noted that the first through third methods for rotatable casting, discussed above, include all the features of the first through third embodiments of the MEMVC, discussed above. 
    
    
     Additional features, advantages, and embodiments of the invention may be set forth or apparent from consideration of the following detailed description, drawings, and claims. Moreover, it is to be understood that both the foregoing summary of the invention and the following detailed description are exemplary and intended to provide further explanation without limiting the scope of the invention as claimed. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and together with the detail description serve to explain the principles of the invention. In the drawings: 
     FIGS. 1A-1F are exemplary diagrams of a related art powder casting cycle illustrating the key stages of a conventional casting cycle; 
     FIGS. 2A-2F are exemplary diagrams of a first preferred embodiment of a multi element moving vacuum chamber powder slush casting cycle illustrating the claimed apparatus and the key stages of a casting cycle for the first embodiment; 
     FIGS. 3A-3F are exemplary diagrams of a second preferred embodiment of a multi element moving vacuum chamber powder slush casting cycle illustrating the claimed apparatus and the key stages of a casting cycle for the second embodiment; and 
     FIGS. 4A-4F are exemplary diagrams of a third preferred embodiment of a multi element moving vacuum chamber powder slush casting cycle illustrating the claimed apparatus and the key stages of a casting cycle for the third embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First Preferred Embodiment 
     As shown in FIG. 2A, a first preferred embodiment of the Multi Element Moving Vacuum Chamber (MEMVC) assembly generally designated  5  may include mold  6  having mold surface  7 . MEMVC assembly  5  may also include powder box  8  having disposed therein plastic powder  9 . MEMVC assembly  5  may further include a bell-shaped vacuum element  11  mounted onto the outer surface of mold  6  and a similar bell-shaped vacuum element  12  mounted onto the outer surface of powder box  8 . Vacuum element  11  may include vacuum connection  13  for drawing and releasing vacuum by means of a conventional pump (not shown). Similarly, vacuum element  12  may also include a vacuum connection  14  for drawing and releasing vacuum, and a vacuum rated seal  15  for maintaining a seal between mold  6  and powder box  8 . 
     Mold  6  may be made of a metal such as nickel, a metallic material, or any equivalent material known in the art. Plastic powder  9  may be a powder such as Thermo Plastic Urethane (T.P.U.), Thermo Plastic Olefinic (T.P.O.), Poly Vinyl Chloride (P.V.C.), or any equivalent material that may be used with mold  6  to form a cast product. Additionally, plastic powder  9  may have a fine or granulated composition, or may be a liquid. Vacuum elements  11  and  12  may be shaped in any way necessary to partially or fully seal mold  6  and/or powder box  8 , and may be bell shaped (as shown in FIG.  2 A), or any equivalent shape, as would be apparent to a skilled artisan. Vacuum rated seal  15  may be placed on vacuum element  12  (as shown in FIG.  2 A), or may likewise be placed on mold  6 , powder box  8 , and/or vacuum element  11 . 
     As shown next in FIG. 2A, in order to cast plastic powder  9  into the shape of mold surface  7 , mold  6  may first be heated in a hot air furnace (not shown) to a predetermined optimum molding temperature. As shown in FIGS. 2A and 2B, powder box  8  may then be brought into contact with mold  6  and engaged with it in order to prevent leakage of plastic powder  9 . Next, as shown in FIG. 2B, air present in the enclosed area defined by an outer surface of mold  6  and an inner surface of vacuum element  11 , by mold surface  7  and the inner surface of powder box  8 , and by the outer surface of powder box  8  and an inner surface of vacuum element  12 , may be removed through vacuum connections  13  and  14  to simultaneously form a seal between mold  6  and powder box  8  via vacuum rated seal  15 . The vacuum level may range between atmospheric and absolute vacuum. As shown in FIG. 2C, mold  6  may now be rotated, at which state plastic powder  9  strikes heated mold surface  7 . After a predetermined time period, rotation of mold  6  is stopped as shown in FIG.  2 D and vacuum within the enclosed area discussed above is released. At this stage in FIG. 2D, when mold  6  stops rotating, plastic powder  9  on mold surface  7  fuses to form the shape of mold surface  7 . Any remaining plastic powder  9  drains back into powder box  8  for subsequent casting, or may be discarded. As shown next in FIG. 2E, powder box  8  may now be disengaged from mold  6  and returned to its original location (shown also in FIG.  2 A). Finally, as shown in FIG. 2F, mold  6  may be rotated to a predetermined orientation to allow an operator to remove the fused layer (or skin) on mold surface  7 . 
     Second Preferred Embodiment 
     Next, as shown in FIG. 3A, a second preferred embodiment of the Multi Element Moving Vacuum Chamber (MEMVC) assembly generally designated  16  may include mold  17  having mold surface  18 . MEMVC assembly  16  may also include powder box  19  having disposed therein plastic powder  21 . MEMVC assembly  16  may further include a bell-shaped vacuum element  22  mounted onto the outer surface of mold  17 , and powder box  19  may be formed to hold vacuum pressure (i.e. vacuum loading), when engaged with mold  17  (discussed below). Vacuum element  22  may include vacuum connection  23  for drawing and releasing vacuum by means of a conventional pump (not shown). Similarly, powder box  19  may also include a vacuum connection  24  for drawing and releasing vacuum, and a vacuum rated seal  25  for maintaining a seal between mold  17  and powder box  19 . 
     Mold  17  may be made of a metal such as nickel, a metallic material, or any equivalent material known in the art. Plastic powder  21  may be a powder such as Thermo Plastic Urethane (T.P.U.), Thermo Plastic Olefinic (T.P.O.), Poly Vinyl Chloride (P.V.C.), or any equivalent material that may be used with mold  17  to form a cast product. Additionally, plastic powder  21  may have a fine or granulated composition, or may be a liquid. Vacuum element  22  may be shaped in any way necessary to partially or fully seal mold  17 , and may be bell shaped (as shown in FIG.  3 A), or any equivalent shape, as would be apparent to a skilled artisan. Vacuum rated seal  25  may be placed on powder box  19  (as shown in FIG.  3 A), or may likewise be placed on mold  17  and/or vacuum element  22 . 
     As shown next in FIG. 3A, in order to cast plastic powder  21  into the shape of mold surface  18 , mold  17  may first be heated to a predetermined optimum molding temperature. As shown in FIGS. 3A and 3B, powder box  19  may then be brought into contact with mold  17  and engaged with it in order to prevent leakage of plastic powder  21 . Next, as shown in FIG. 3B, air present in the enclosed area defined by an outer surface of mold  17  and an inner surface of vacuum element  22 , and by mold surface  18  and the inner surface of powder box  19 , may be removed through vacuum connections  23  and  24  to simultaneously form a seal between mold  17  and powder box  19  via vacuum rated seal  25 . The vacuum level may range between atmospheric and absolute vacuum. As shown in FIG. 3C, mold  17  may now be rotated, at which state plastic powder  21  strikes mold surface  18 . After a predetermined time period, rotation of mold  17  is stopped as shown in FIG.  3 D and vacuum within the enclosed area discussed above is released. At this stage in FIG. 3D, when mold  17  stops rotating, plastic powder  21  on mold surface  18  fuses to form the shape of mold surface  18 . Any remaining plastic powder  21  drains back into powder box  19  for subsequent casting, or may be discarded. As shown next in FIG. 3E, powder box  19  may now be disengaged from mold  17  and returned to its original location (shown also in FIG.  3 A). Finally, as shown in FIG. 3F, mold  17  may be rotated to a predetermined orientation to allow an operator to remove the fused layer (or skin) on mold surface  18 . 
     It should be evident from the above discussion that for the second embodiment of MEMVC assembly  16 , instead of vacuum element  22  being mounted on mold  17  and powder box  19  being capable of vacuum loading, a vacuum element may likewise be mounted on powder box  19  (which may not capable of vacuum loading) and mold  17  may instead be designed with vacuum loading capabilities, as would be evident to a skilled artisan. 
     Third Preferred Embodiment 
     As shown in FIG. 4A, a third preferred embodiment of the Multi Element Moving Vacuum Chamber (MEMVC) assembly generally designated  26  may include mold  27  having mold surface  28 . MEMVC assembly  26  may also include powder box  29  having disposed therein plastic powder  31 . In MEMVC assembly  26 , mold  27  and powder box  29  may each be formed to hold vacuum pressure when engaged with each other (discussed below). In other words, mold  27  and power box  29  may be capable of vacuum loading. Powder box  29  may include vacuum connection  32  for drawing and releasing vacuum by means of a conventional pump (not shown). Additionally, powder box  29  may include vacuum rated seal  33  for maintaining a seal between itself and mold  27 . 
     Mold  27  may be made of a metal such as nickel, a metallic material, or any equivalent material known in the art. Plastic powder  31  may be a powder such as Thermo Plastic Urethane (T.P.U.), Thermo Plastic Olefinic (T.P.O.), Poly Vinyl Chloride (P.V.C.), or any equivalent material that may be used with mold  27  to form a cast product. Additionally, plastic powder  31  may have a fine or granulated composition, or may be a liquid. Vacuum connection  32  and vacuum rated seal  33  may be placed on powder box  29  (as shown in FIG.  4 A), or may likewise be placed on mold  27 . 
     As shown next in FIG. 4A, in order to cast plastic powder  31  into the shape of mold surface  28 , mold  27  may first be heated to a predetermined optimum molding temperature. As shown in FIGS. 4A and 4B, powder box  29  may then be brought into contact with mold  27  and engaged with it in order to prevent leakage of plastic powder  31 . Next, as shown in FIG. 4B, air present in the enclosed area defined by mold surface  28  and the inner surface of powder box  29  may be removed through vacuum connection  32  to simultaneously form a seal between mold  27  and powder box  29  via vacuum rated seal  33 . The vacuum level may range between atmospheric and absolute vacuum. As shown in FIG. 4C, mold  27  may now be rotated, at which state plastic powder  31  strikes heated mold surface  28 . After a predetermined time period, the rotation of mold  27  is stopped as shown in FIG.  4 D and the vacuum within the enclosed area discussed above is released. At this stage in FIG. 4D, when mold  27  stops rotating, plastic powder  31  on mold surface  28  fuses to form the shape of mold surface  28 . Any remaining plastic powder  31  drains back into powder box  29  for subsequent casting, or may be discarded. As shown next in FIG. 4E, powder box  29  may now be disengaged from mold  27  and returned to its original location (shown also in FIG.  4 A). Finally, as shown in FIG. 4F, mold  27  may be rotated to a predetermined orientation to allow an operator to remove the fused layer (or skin) on mold surface  28 . 
     In the first, second and third preferred embodiments of the MEMVC assembly discussed above, it should be noted that mounting separate vacuum elements on mold  6  and powder box  8  (first embodiment), and on mold  17  (second embodiment), may virtually eliminate pressure on these members ( 6 ,  8  and  17 ) related to vacuum loading. Therefore, for the first and second embodiments, it may only be necessary to modify mold  6  and powder box  8 , and mold  17 , respectively, in order to attach vacuum elements  11  and  12 , and  22 , respectively. On the contrary, for the second and third embodiments discussed above, it may be necessary to reinforce powder box  19 , and mold  27  and powder box  29 , respectively, in order for these members ( 19 ,  27  and  29 ) to withstand the pressure associated with vacuum loading. Moreover, in the first through third embodiments discussed above, it may be necessary to attach one or more latches onto the respective molds, powder boxes, and/or vacuum elements, in order to permit access to the mold surface for removal of the cast skin, and/or introduction of additional plastic powder between cycles. Additionally, in the first through third embodiments discussed above, molds  6 ,  17  and  27 , respectively, may be heated in a hot air furnace (not shown) to a predetermined optimum molding temperature, or may instead be heated to the optimum molding temperature by any equivalent method known in the art. 
     Although particular embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those particular embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.