Abstract:
A pressure molding apparatus comprises a furnace defining a heating space for accommodating a mold assembly including a mold body charged with a raw material to be molded, and a pressure piston for pressurizing the raw material; heating means for heating the heating space; and load imposing means for imposing a load on the pressure piston to mold the raw material. The load imposing means includes at least one weight means and weight supporting means. The weight supporting means is selectively set in a load-free state in which the weight supporting means supports the weight means so that the weight of the weight means is not imposed on the pressure piston, and a load imposed state in which the weight supporting means releases the support of the weight means so that the weight of the weight means is imposed on the pressure piston.

Description:
FIELD OF THE INVENTION 
     This invention relates to, but is not restricted to, a pressure molding apparatus suitable for forming a dental crown by heating and pressurizing a ceramic raw material, such as glass-ceramics. 
     DESCRIPTION OF THE PRIOR ART 
     As is well known among people skilled in the art, a mold assembly composed of a mold body and a pressure piston is used to mold a ceramic raw material, such as glass-ceramics, into a dental crown. In the mold body, a molding cavity, a raw material charging cavity, and a sprue are formed. The raw material charging cavity is open upwards, and the sprue extends downward from the raw material charging cavity to the molding cavity. The raw material charging cavity of the mold body is charged with a raw material to be molded, i.e., a ceramic material such as glass-ceramics. Then, a lower portion of the pressure piston is inserted into the raw material charging cavity. An upper portion of the pressure piston protrudes upward from the mold body. Then, the mold assembly is accommodated into a heating space of a furnace in a pressure molding apparatus to heat the raw material to a suitable temperature. At the same time, a load is imposed on the pressure piston to pressurize the raw material. As a result, the raw material charged into the raw material charging cavity is flowed into the molding cavity through the sprue to mold it into a required form. Usually, the amount of the raw material charged is larger than the capacity of the molding cavity. Thus, the resulting dental crown is accompanied by an additional portion located in the sprue and the raw material charging cavity of the mold body. Such an additional portion is later removed from the dental crown by machining. Imposition of the load on the pressure piston is performed by a pneumatic cylinder mechanism. 
     The ceramic softened by heating has a relatively high viscosity of, say, about 10 4  to 10 6  poises. Such a raw material needs to be flowed, as required, against resistance due to the escape of air from the molding cavity (the mold body is usually formed of a porous material, and air in the molding cavity escapes through the mold body itself). For this purpose, it is necessary to apply a pressure of, say, about 5 kg/cm 2  to the pressure piston. To impose a load on the pressure piston fully stably, a considerably large, high performance pneumatic cylinder mechanism is needed. Hence, a molding apparatus has to be considerably bulky and expensive. The use of a relatively small, low performance pneumatic piston mechanism may often result in a stick-slip phenomenon (a phenomenon in which when the load is imposed on the pressure piston to flow the raw material, descent of the pressure piston is stopped accidentally, thereby terminating the flow of the raw material). The stick-slip phenomenon would make the raw material flow into the molding cavity insufficient and form flow mark on the molded dental crown. 
     SUMMARY OF THE INVENTION 
     A main object of the present invention is to provide a novel and improved pressure molding apparatus which does not require a large, expensive pneumatic cylinder mechanism, can thus be produced in a small size and at a low cost, and nonetheless can impose a completely stable load on a pressure piston. 
     Another object of the invention is to provide a novel and improved pressure molding apparatus which can bring a mold assembly to a required position fully easily, when accommodating the mold assembly into a heating space of a furnace. 
     The inventors of the present invention have conducted in-depth studies, and found that the main object can be attained by selectively applying the weight of weight means to a pressure piston, instead of using a pneumatic cylinder mechanism. 
     That is, the present invention provides, as a pressure molding apparatus for attaining the main object, a pressure molding apparatus comprising: 
     a furnace defining a heating space for accommodating a mold assembly including a mold body charged with a raw material to be molded, and a pressure piston for pressurizing the raw material; 
     heating means for heating the heating space; and 
     load imposing means for imposing a load on the pressure piston to mold the raw material, wherein 
     the load imposing means includes at least one weight means and weight supporting means, and the weight supporting means is selectively set in a load-free state in which the weight supporting means supports the weight means so that the weight of the weight means is not imposed on the pressure piston, and a load imposed state in which the weight supporting means releases the support of the weight means so that the weight of the weight means is imposed on the pressure piston. 
     Preferably, the load imposing means includes first weight means and second weight means, and the weight supporting means is selectively set in the load-free state in which the weight supporting means supports both the first weight means and the second weight means so that none of the weights of the first weight means and the second weight means are imposed on the pressure piston, a low load imposed state in which the weight supporting means releases the support of the first weight means so that the weight of the first weight means is imposed on the pressure piston, and a high load imposed state in which the weight supporting means releases the support of both the first weight means and the second weight means so that the weights of the first and second weight means are imposed on the pressure piston. Preferred embodiments are as follows: The furnace has a floor wall defining a lower surface of the heating space, and a ceiling wall defining an upper surface of the heating space; the mold assembly is laid on the floor wall; the pressure piston is stretched outward upwardly from the mold body; the first weight means is placed above the pressure piston and stretched through the ceiling wall; the second weight means is placed above the first weight means; the weight supporting means includes a supporting member, and hoisting and lowering means for hoisting and lowering the supporting member; when the supporting member is brought to a hoisted position, the second weight means supports the first weight means, and the supporting member supports the second weight means, and supports the first weight means via the second weight means; when the supporting member is lowered beyond a low load imposed position, the first weight means contacts the pressure piston, so that the support of the first weight means by the second weight means is released, whereby the load of the first weight means is imposed on the pressure piston; when the supporting member is further lowered beyond a high load imposed position, the second weight means contacts the first weight means, so that the support of the second weight means by the supporting member is released, whereby the load of the first weight means is imposed on the pressure piston, and the load of the second weight means is also imposed on the pressure piston via the first weight means. The second weight means includes a bracket member having an opening formed in a bottom wall; the first weight means includes a rod extending in an up-and-down direction through the opening; an engagingly stopping flange positioned above the bottom wall of the bracket member, and a contact flange positioned below the bottom wall of the bracket member are disposed on the rod; the bottom wall of the bracket member supports the engagingly stopping flange, whereby the second weight means supports the first weight means; and the bottom wall of the bracket member contacts the contact flange, whereby the load of the second weight means is imposed on the pressure piston via the first weight means. A contact member is disposed on the bracket member, and the supporting member supports the second weight means by contacting a lower surface of the contact member. In an embodiment for attaining the other object, the furnace includes a floor wall defining a lower surface of the heating space, and a furnace floor plate removably mounted on the floor wall, and position regulating means for regulating the position of the mold assembly to be placed on an upper surface of the furnace floor plate is disposed on the upper surface of the furnace floor plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view, partly in cross section, of a preferred embodiment of a pressure molding apparatus constructed in accordance with the present invention; 
     FIGS.  2 ( a ) and  2 ( b ) are sectional views of a mold assembly for use in the pressure molding apparatus of FIG. 1, shown in a state before pressure molding and a state after pressure molding, respectively; 
     FIG. 3 is a perspective view showing a furnace floor plate for use in the pressure molding apparatus of FIG. 1; 
     FIG. 4 is a perspective view showing a modification of the furnace floor plate; 
     FIG. 5 is a perspective view showing another modification of the furnace floor plate; 
     FIG. 6 is an exploded perspective view showing first weight means and second weight means in the pressure molding apparatus of FIG. 1; 
     FIG. 7 is an exploded perspective view showing the second weight means and weight supporting means in the pressure molding apparatus of FIG. 1; 
     FIG. 8 is a schematic plan view showing the weight means (the first weight means and the second weight means) and the weight supporting means in the pressure molding apparatus of FIG. 1; 
     FIG. 9 is a partial sectional view showing the second weight means and part of the weight supporting means in the pressure molding apparatus of FIG. 1; and 
     FIG. 10 is a schematic sectional view showing a modification of load imposing means (the weight means and the weight supporting means). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in more detail by reference to the accompanying drawings which show preferred embodiments of a pressure molding apparatus constructed in accordance with the present invention. 
     With reference to FIG. 1, a pressure molding apparatus shown entirely by the numeral  2  has a heating furnace  4 . This heating furnace  4  has a heating space  6  which may be in a nearly rectangular parallelopipedal shape. The heating space  6  is demarcated by a floor wall  8 , a ceiling wall  10 , and four side walls, i.e., a rear wall  12 , a front wall  14 , and both lateral walls  16  (FIG. 1 shows only one of the lateral walls  16 ). The floor wall  8 , ceiling wall  10 , rear wall  12 , front wall  14 , and lateral walls  16  that define the heating space  6  are formed of a refractory material such as firebrick. The floor wall  8 , ceiling wall  10 , rear wall  12 , and lateral walls  16  are held by suitable holding means (not shown) at required positions inside a housing  18  which is in the shape of a box with an open front surface. On the front surface of the housing  18 , a door  20  is mounted which is turned about a turn axis extending along an edge thereof, whereby the door  20  is brought to a closing position for closing the front surface of the housing  18  and an opening position for opening the front surface of the housing  18 . The front wall  14  is held at an inner surface of the door  20 . Thus, when the door  20  is brought to the closing position, the heating space  6  is closed. When the door  20  is brought to the opening position, the front surface of the heating space  6  is opened, so that the interior of the heating space  6  can be accessed. On the ceiling wall  10 , heating means  22 , which may be an electric resistance heater, is disposed. By this heating means  22 , the interior of the heating space  6  is heated to a suitable temperature. Through the rear wall  12 , a temperature sensor  24  is disposed. By this temperature sensor  24 , the temperature inside the heating space  6  is detected. Based on the temperature detected, energization and deenergization of the heating means  22  are controlled. 
     On the floor wall  8 , a furnace floor plate  26  which may be formed of a suitable refractory material is laid. The contour of the furnace floor plate  26  may be substantially the same as the cross sectional shape of the heating space  6 . The furnace floor plate  26  is laid on the floor wall  8 , with its rear edge and lateral edges being contiguous to or close to the rear wall  12  and the lateral walls  16 , respectively. On the furnace floor plate  26 , a mold assembly entirely indicated at  28  is laid. With reference to FIG.  2 ( a ), the mold assembly  28  is constituted of a mold body  30  and a pressure piston  32 . The mold body  30 , advantageously formed of a refractory porous material, is in a generally cylindrical shape, and has a molding cavity  34 , a raw material charging cavity  36 , and a sprue  38 . The molding cavity  34  corresponds to the shape of a product to be molded, e.g., a dental crown. The raw material charging cavity  36  has substantially the same sectional shape, e.g., a circular sectional shape, except its lower end portion, and its upper end is open. The sprue  38  extends from the raw material charging cavity  36  to the molding cavity  34 . Such mold body  30  can be formed, for example, by forming wax pattern of which shapes correspond to the molding cavity  34 , the raw material charging cavity  36  and the sprue  38 , fixing the wax pattern in the casting ring using a crucible former, placing an investment (e.g. gypsum) around the wax pattern, setting the investment, and then burning the wax pattern away. The raw material charging cavity  36  of the mold body  30  is charged with a raw material  40  to be molded, which may be a ceramic material such as glass-ceramics. The pressure piston  32  is in the shape of a cylinder having a sectional shape corresponding to the sectional shape of the raw material charging cavity  36 . A lower part of the pressure piston  32  is inserted into the raw material charging cavity  36 , while an upper part thereof is protruded upward from the mold body  30 . As will be further mentioned later, the mold assembly  28  is laid on the furnace floor plate  26 , and accommodated in the heating space  6 . There, the mold assembly  28  is heated to a suitable temperature, and a load is imposed on the pressure piston  32 . Thus, as shown in FIG.  2 ( b ), the raw material  40  to be molded is softened and pressurized, and then flowed through the sprue  38  into the molding cavity  34  to form a dental crown  42 . The amount of the raw material  40  charged into the raw material charging cavity  36  for molding is larger than the capacity of the molding cavity  34 . Hence, the dental crown  42  involves an additional portion  44 . Such an additional portion  44  can be removed mechanically from the dental crown  42  after removing the mold body  30  and withdrawing the dental crown  42  together with the additional portion  44 . 
     Further referring to FIG. 3 along with FIGS.  1  and  2 ( a ), a relatively large diameter circular concave portion  46 , and a relatively small diameter circular concave portion  48  placed concentrically at a bottom surface of the circular concave portion  46  are formed at an upper surface of the furnace floor plate  26 . The circular concave portions  46  and  48  constitute position regulating means for regulating the position of the mold assembly  28 . As shown by a two-dot chain line in FIG. 3, when there is used the mold assembly  28  having a relatively large mold body  30  having an outer diameter corresponding to an inner diameter of the circular concave portion  46 , a lower end portion of the mold body  30  is inserted into the circular concave portion  46 . Thus, the mold assembly  28  is located at a required position in the heating space  6 . When there is used the mold assembly  28  having a relatively small mold body  30  having an outer diameter corresponding to an inner diameter of the circular concave portion  48 , a lower end portion of the mold body  30  is inserted into the circular concave portion  48 . Thus, the mold assembly  28  is located at a required position in the heating space  6 . Instead of forming a concave portion at the upper surface of the furnace floor plate  26 , it is possible to form a protrusion of a suitable shape, thereby regulating the position of the mold assembly  28 . As illustrated in FIG. 4, for example, an arcuate protrusion  50  having an inner diameter corresponding to the outer diameter of the mold body  30  may be formed at the upper surface of the furnace floor plate  26 , and an outer peripheral surface of the lower end portion of the mold body  30  can be brought into contact with the protrusion  50 . When the mold body  30  is in a prismatic shape, a channel-like protrusion  52  corresponding to the contour of the mold body  30  may be formed on the furnace floor plate  26 , and the lower end portion of the mold body  30  can be positioned inwardly of the protrusion  52 . 
     As shown in FIG. 1, load imposing means indicated generally at the numeral  54  is disposed on an upper surface of the housing  18 . This load imposing means  54  is composed of weight means  56  and weight supporting means  58 . In the illustrated embodiment, the weight means  56  is composed of first weight means  60  and second weight means  62 . 
     With reference to FIG. 6 along with FIG. 1, the first weight means  60  is formed from a slender rod  64  extending through the ceiling wall  10  of the aforementioned heating furnace  4  and the housing  18 . The rod  64  which may be formed of a suitable refractory material has a circular sectional shape, and is disposed so as to be movable upward and downward relative to the heating furnace  4 . As will be easily understood by reference to FIG. 1, the rod  64  is matched to the pressure piston  32  of the mold assembly  28  placed in the heating space  6  of the heating furnace  4 . To a nearly middle portion, and an upper end portion of the rod  64 , circular flanges  68  and  70 , respectively, are fixed by suitable means such as welding. 
     Further referring to FIGS. 6 and 7 together with FIG. 1, the second weight means  62  includes a nearly rectangular parallelopipedal casing  72  with an open upper surface and an open front surface, and a cover member  74  to be detachably combined with the casing  72 . The casing  72 , which may be formed from a suitable metal, has a bottom wall  76 , a rear wall  78 , and lateral walls  80 . The cover member  74 , which may similarly be formed from a suitable metal, has an upper wall  82  closing the upper surface of the casing  72 , a front wall  86  closing the front surface of the casing  72 , an engagingly stopping piece  88  protruding from a rear edge of the upper wall  82 , and engagingly stopping pieces  90  protruding from both lateral edges of the upper wall  82  and both lateral edges of the front wall  86 . The engagingly stopping piece  88  is engaged with an outer surface of the rear wall  78 , and the engagingly stopping pieces  90  are engaged with outer surfaces of both lateral walls  80 , whereby the cover member  74  is combined with the casing  72 . In a central part of the bottom wall  76  of the casing  72 , a circular opening  92  is formed. The rod  64  constituting the first weight means  60  is passed through the opening  92 , and entered into the casing  72 . The flange  70  fixed to the upper end portion of the rod  64  is positioned in the casing  72 . The outer diameter of the flange  70  is larger than the inner diameter of the opening  92 , so that the flange  70  cannot move through the opening  92 . On the bottom wall  76  of the casing  72 , a plurality of weight plates  94  which may be formed from a suitable metal are stacked. The number of the weight plates  94  can be determined by the required weight of the second weight means  62 . In a central part of each of the weight plates  94 , a circular opening  96  is formed. The outer diameter of the opening  96  is substantially the same as, or somewhat larger than, the outer diameter of the flange  70  fixed to the upper end portion of the rod  64 . Thus, the flange  70  can freely move up and down through the openings  96 . 
     With reference to FIG. 7 together with FIG. 1, a metallic channel member  98  extending in an up-and-down direction is fixed to an outer surface of the rear wall  78  of the casing  72  by suitable means such as welding. As shown in FIGS. 7 and 8, a short shaft  102  is fixed to an outside surface of each of side walls  100  of the channel member  98 . On each of these short shafts  102 , a guided roller  104  is mounted rotatably. As will be easily understood by referring to FIG. 8, a pair of channel-like guide members  106  extending substantially vertically and spaced in a transverse direction (a direction perpendicular to the sheet face in FIG. 1, and a right-and-left direction in FIG. 8) are fixed on the housing  18  of the heating furnace  4 . The width between the side walls of the guide member  106  which may be metallic corresponds with the outer diameter of the guided roller  104 . Each of the guided rollers  104  is accommodated into each of the pair of guide members  106 , whereby the channel member  98 , accordingly, the second weight means  62 , is mounted so as to be free to move up and down. To the channel member  98 , a bracket  108  which may be metallic is fixed by suitable means such as welding. As will be understood by reference to FIG. 9 along with FIG. 7, a supported member  112  is fixed to a bottom wall  110  of the bracket  108 . In more detail, a circular opening  114  is formed in the bottom wall  110  of the bracket  108 . The supported member  112 , which may be metallic, has a cylindrical upper portion  116  of relatively small diameter, and a cylindrical lower portion  118  of relatively large diameter. The outer diameter of the upper portion  116  is substantially the same as the inner diameter of the opening  114 , and an annular groove  120  is formed at an outer peripheral surface of the upper portion  116 . As clearly illustrated in FIG. 9, the upper portion  116  of the supported member  112  is inserted through the opening  114 , and a snap ring  122  is engaged with the annular groove  120  of the upper portion  116 , whereby the supported member  112  is fixed to the bottom wall  110  of the bracket  108 . 
     The weight supporting means  58  will be described by reference to FIGS. 7 to  9  along with FIG. 1. A support block  124  is fixed onto the housing  18  of the heating furnace  4 . On the support block  124 , a threaded shaft  126  extending substantially vertically upward is mounted rotatably. An external thread  128  is formed at an outer peripheral surface of the threaded shaft  126 . As illustrated in FIG. 1, a gear  130  is fixed to a lower end portion of the threaded shaft  126 . On the housing  18 , an electric motor  132  is mounted. On an output shaft of the motor  132 , a rack (not shown) is formed, and engaged with the gear  130 . Thus, when the motor  132  is energized, the threaded shaft  126  is rotationally driven via the rack and the gear  130 . On the threaded shaft  126 , a support member  134  is mounted. The support member  134  is in a cylindrical shape, and has a flange  136  formed at its upper end. In the support member  134 , an internally threaded hole  137  (FIG. 9) extending therethrough is formed. This internally threaded hole  137  is screwed on the external thread  128  of the threaded shaft  126 . Thus, when the threaded shaft  126  is rotated, the support member  134  is lowered or raised. The supported member  112  fixed to the bracket  108  of the aforementioned second weight means  62  is matched to the support member  134 . As will be further mentioned later on, an upper surface of the support member  134  contacts a lower surface of the supported member  112 , whereby the weight supporting means  58  supports the second weight means  62  (and also supports the first weight means  60  via the second weight means  62 ). An inner diameter of the supported member  112 , which is in a cylindrical shape, is larger than an outer diameter of the threaded shaft  126 , so that the supported member  112  can freely move up and down around the threaded shaft  126 . 
     A detected piece  138  is fixed onto the flange  136  of the support member  134 . This detected piece  138  has a connecting portion  140 , an upright portion  142  extending upward from a rear edge of the connecting portion  140 , and a detected portion  144  extending rearward from the upright portion  142 . The connecting portion  140  is fixed to the flange  136  by four bolts  146 . At a center of the connecting portion  140 , a circular opening  148  is formed. An inner diameter of this opening  148  is somewhat larger than an outer diameter of a lower portion of the supported member  112 . Thus, the upper surface of the support member  134  is directly contacted with the lower surface of the supported member  112 . As will be understood by referring to FIG. 9 along with FIG. 7, a frame member  150  extending substantially vertically upward is disposed on the housing  18  of the heating furnace  4 . In the frame member  150 , an elongated slit  152  extending in a vertical direction is formed. The detected portion  144  of the detected piece  138  is protruded rearward through the slit  152 . On the frame member  150 , three detectors  154   a,    154   b  and  154   c  are mounted with spacing in an up-and-down direction. These detectors  154   a,    154   b  and  154   c  can each be composed of a microswitch. As will be further mentioned later on, when the support member  134  is raised to an uppermost position indicated by a solid line in FIG. 1, the detector  154   a  detects the detected piece  138 . When the support member  134  is lowered to a position slightly beyond a low load imposed position indicated by a two-dot chain line  134   b  in FIG. 1, the detector  154   b  detects the detected piece  138 . When the support member  134  is lowered to a position slightly beyond a high load imposed position indicated by a two-dot chain line  134   c  in FIG. 1, the detector  154   c  detects the detected piece  138 . 
     Mainly referring to FIG. 1, a typical example of the manner of molding by the above-described pressure molding apparatus will be explained in summary. Prior to the initiation of molding, the heating means  22  of the heating furnace  4  is energized to preheat the interior of the heating furnace  4  to, say, 900° C. The door  20  of the heating furnace  4  is opened, and the mold assembly  28 , in which the raw material  40  to be molded has been charged into the mold body  30 , and the pressure piston  32  has been partly inserted, as shown in FIG.  2 ( a ), is laid on the furnace floor plate  26  of the heating space  6 . Then, the door  20  is closed, and a molding start switch (not shown) is operated to start molding. During molding, the heating space  6  inside the heating furnace  4  is maintained at, say, 900° C. 
     At the start of molding, the support member  134  of the weight supporting means  58  is located at the uppermost position indicated by the solid line in FIG. 1, and the detector  154   a  detects the detected piece  138 . In this state, the support member  134  of the weight supporting means  58  contacts the supported member  112  of the second weight means  62  to support the second weight means  62 . Furthermore, the flange  70  fixed to the upper end portion of the rod  64  in the first weight means  60  is supported on the bottom wall  76  of the casing  72  of the second weight means  62 . Thus, the first weight means  60  is also supported by the weight supporting means  58  via the second weight means  62 . As shown by a solid line in FIG. 1, the lower end of the rod  64  in the first weight means  60  is positioned apart in an upward direction from the pressure piston  32  of the mold assembly  28 , so that no load is imposed on the pressure piston  32 . 
     When the molding start switch is operated in the foregoing manner, normal rotation of the motor  132  of the weight supporting means  58  is started, whereby the support member  134  is gradually lowered. Accordingly, the second weight means  62  and the first weight means  60  are also gradually lowered. When the support member  134  is lowered to the position indicated by the two-dot chain line  134   b  in FIG. 1, i.e., the position of starting low load imposition, the lower end of the rod  64  in the first weight means  60  contacts an upper end of the pressure piston  32  in the mold assembly  28 , as shown by a two-dot chain line  64   b  in FIG. 1, so that the lowering of the first weight means  60  is inhibited. When the support member  134  is further lowered, slightly, beyond the position of starting low load imposition, the flange  70  fixed to the upper end portion of the rod  64  in the first weight means  60  is separated upward from the bottom wall  76  of the casing  72  in the second weight means  62 . As a result, suspension of the first weight means  60  by the second weight means  62  is released. Thus, the load of the first weight means  60  is imposed on the pressure piston  32  of the mold assembly  28  to put a low load on the raw material  40  to be molded, thereby initiating its molding. At the same time, the detector  154   b  detects the detected piece  138 , thereby stopping the motor  132 . Hence, a low load imposed state is continued. 
     When a predetermined time, e.g., about 10 to 15 minutes, elapses after detection of the detected piece  138  by the detector  154   b,  normal rotation of the motor  132  is resumed. Thus, the support member  134  and the second weight means  62  supported thereby begin to descend again. When the support member  134  is lowered to a position of high load imposition indicated by a two-dot chain line  134   c  in FIG. 1, the bottom wall  76  of the casing  72  in the second weight means  62  contacts the flange  68  fixed to a nearly middle part of the rod  64  in the first weight means  60 , as shown by two-dot chain lines  68   c  and  76   c  in FIG.  1 . Thus, the lowering of the second weight means  62  is inhibited. When the support member  134  is further lowered, slightly, beyond the position of starting high load imposition, the supported member  112  in the second weight means  62  is separated upward from the support member  134 . As a result, suspension of the second weight means  62  by the weight supporting means  58  is released. Thus, the load of the second weight means  62  is imposed on the pressure piston  32  of the mold assembly  28  via the first weight means  60  to increase the load on the raw material  40 , which is to be molded, from a low load to a high load. At the same time, the detector  154   c  detects the detected piece  138 , thereby stopping the motor  132 . Hence, a high load imposed state is continued. 
     When a predetermined time, e.g., about 30 to 40 minutes, elapses after detection of the detected piece  138  by the detector  154   c,  reverse rotation of the motor  132  is started to raise the support member  134  gradually. When the support member  134  is raised past the high load imposition start position indicated by the two-dot chain line  134   c  in FIG. 1, the support member  134  contacts the supported member  112  to support it. Thus, the load of the second weight means  62  is removed from the pressure piston  32  of the mold assembly  28 . When the support member  134  is raised past the low load imposition start position indicated by the two-dot chain line  134   b  in FIG. 1, the bottom wall  76  of the casing  72  in the second weight means  62  contacts the flange  70  fixed to the upper end portion of the rod  64  in the first weight means  60  to support the flange  70 . Thus, the load of the first weight means  60  is also removed from the pressure piston  32  of the mold assembly  28 . When the support member  134  is raised to the highest position indicated by the solid line in FIG. 1, the detector  154   a  detects the detected piece  138 , whereupon the motor  132  is stopped. 
     When a predetermined time, e.g., about 45 to 60 minutes, elapses after initiation of reverse rotation of the motor  132  in the foregoing manner, the heating means  22  in the heating furnace  4  is deenergized. Moreover, a suitable alarm (not shown) is energized to inform an operator of the completion of the molding process. The operator opens the door  20 , and withdraws from the heating furnace  4  the mold assembly  28  containing a molded dental crown  42 . During the period from the removal of load from the pressure piston  32  of the mold assembly  28  until the door  20  is opened and the mold assembly  28  withdrawn from the heating furnace  4 , the temperature inside the heating furnace  4  can be lowered to a predetermined temperature, if desired. 
     In the above-described pressure molding apparatus  2 , a low load is imposed on the pressure piston  32  of the mold assembly  28 , and then, after a lapse of a predetermined time, a high load is imposed thereon. If desired, a required high load can be imposed from the start of load imposition. According to the inventors&#39; experience, however, imposing the required high load from the start of load imposition tends to form flow mark on the molded dental crown  42 . 
     If desired, the load to be imposed may be gradually increased in 3 or 4 or more steps. FIG. 10 illustrates a modification of the load imposing means constituted so as to increase the load in 3 steps. In load imposing means  254  shown in FIG. 10, weight means  256  is composed of first weight means  260 , second weight means  262 , and third weight means  263 . Weight supporting means  258  is provided on and along the third weight means  263  to support the third weight means  263  directly, support the second weight means  262  via the third weight means  263 , and support the first weight means  260  via the third weight means  263  and the second weight means  262 . With reference to FIG. 10, the first weight means  260  is substantially the same as the first weight means  60  in the aforementioned pressure molding apparatus  2 , except that a relatively large, circular flange  265  is added to an intermediate portion of a rod  264 . The second weight means  262  is substantially the same as the second weight means  62  in the aforementioned pressure molding apparatus  2 , except that there is no member provided on and along an outer surface of a rear wall of a casing  272 , and that the casing  272  is housed in a casing  273  of the third weight means  263 . The third weight means  263  has the casing  273  of a relatively large size housing the casing  272  of the second weight means  262 . In a bottom wall of this casing  273 , a somewhat smaller opening than a cross sectional shape of the casing  272  of the second weight means  262  is formed. Inside the casing  273 , a plurality of weight plates  295  are housed. In a central part of each of the weight plates  295 , a rectangular opening  297  slightly larger than the cross sectional shape of the casing  272  in the second weight means  262  is formed. The casing  272  of the second weight means  262  can freely move up and down through the openings  297 . On and along a rear wall of the casing  273  in the third weight means  263 , there is provided substantially the same constituent element as the constituent element provided on and along the rear wall  78  of the casing  72  of the second weight means  62  in the aforementioned pressure molding apparatus  2 . This constituent element can be supported by the weight supporting means  258 . The constitution of the weight supporting means  258  may be substantially the same as the constitution of the weight supporting means  58  in the aforementioned pressure molding apparatus  2 , except that the support member  234  is detected at four positions, i.e., an uppermost position, a low load imposition start position, a medium load imposition start position, and a high load imposition start position. 
     In the load imposing means  254  shown in FIG. 10, at the start of molding, the support member  234  of the weight supporting means  258  is located at the uppermost position indicated by a solid line in FIG.  10 . In this state, the support member  234  of the weight supporting means  258  contacts a supported member  212  of the third weight means  263  to support the third weight means  263 . Also, the casing  272  of the second weight means  262  is supported on a bottom wall of the casing  273  of the third weight means  263 . Thus, the second weight means  262  is supported by the weight supporting means  258  via the third weight means  263 . Furthermore, a flange  270  fixed to an upper end portion of a rod  264  in the first weight means  260  is supported on the bottom wall of the casing  272  of the second weight means  262 . Thus, the first weight means  260  is also supported by the weight supporting means  258  via the third weight means  263  and the second weight means  262 . As shown by a solid line in FIG. 10, a lower end of the rod  264  in the first weight means  260  is positioned apart in an upward direction from a pressure piston  232  of a mold assembly  228 , so that no load is imposed on the pressure piston  232 . 
     When a molding start switch is operated, normal rotation of a motor  132  of the weight supporting means  258  is started, whereby the support member  234  is gradually lowered. Accordingly, the third weight means  263 , the second weight means  262  and the first weight means  260  are also gradually lowered. When the support member  234  is lowered to a position indicated by a two-dot chain line  234   b  in FIG. 10, i.e., the position of starting low load imposition, the lower end of the rod  264  in the first weight means  260  contacts an upper end of the pressure piston  232  in the mold assembly  228 , as shown by a two-dot chain line  264   b  in FIG.  10 . Thus, the lowering of the first weight means  260  is inhibited. When the support member  234  is further lowered, slightly, beyond the position of starting low load imposition, the flange  270  fixed to the upper end portion of the rod  264  in the first weight means  260  is separated upward from the bottom wall of the casing  272  in the second weight means  262 . As a result, suspension of the first weight means  260  by the second weight means  262  is released. Thus, the load of the first weight means  260  is imposed on the pressure piston  232  of the mold assembly  228 . 
     When the support member  234 , and the third weight means  263  and second weight means  262  supported thereby are lowered further, whereby the support member  234  is lowered to the position of medium load imposition indicated by a two-dot chain line  234   c  in FIG. 10, the bottom wall of the casing  272  in the second weight means  262  contacts a flange  268  fixed to a nearly middle part of the rod  264  in the first weight means  260 , as shown by two-dot chain lines  268   c  and  276   c  in FIG.  10 . Thus, the lowering of the second weight means  262  is inhibited. When the support member  234  is further lowered, slightly, beyond the position of starting medium load imposition, the casing  272  in the second weight means  262  is separated upward from the bottom wall of the casing  273  in the third weight means  263 . As a result, support of the second weight means  262  by the third weight means  263  is released. Thus, the load of the second weight means  262  is imposed on the pressure piston  232  of the mold assembly  228  via the first weight means  260  to increase the imposed load from a low load to a medium load. 
     When the support member  234 , and the third weight means  263  supported thereby are lowered further, whereby the support member  234  is lowered to a position of high load imposition indicated by a two-dot chain line  234   d  in FIG. 10, the bottom wall of the casing  273  in the third weight means  263  contacts a flange  265  fixed to a nearly middle part of the rod  264  in the first weight means  260 , as shown by two-dot chain lines  265   d  and  276   d  in FIG.  10 . Thus, the lowering of the third weight means  263  is inhibited. When the support member  234  is further lowered, slightly, beyond the position of starting high load imposition, the supported member  212  in the third weight means  263  is separated upward from the support member  234 . As a result, support of the third weight means  263  by the weight supporting means  258  is released. Thus, the load of the third weight means  263  is also imposed on the pressure piston  232  of the mold assembly  228  via the first weight means  260  to increase the imposed load from the medium load to a high load. 
     The preferred embodiments of the pressure molding apparatus constituted in accordance with the present invention have been described in detail above by reference to the accompanying drawings. However, it should be understood that the present invention is not limited to these embodiments, and various changes and modifications may be made without departing from the spirit and scope of the invention.