Abstract:
A vent for a mold having a mold cavity comprises a valve member housed within a main body sleeve and reciprocally moveable therein in an axial direction between open and closed positions. The sleeve is mounted within a mold wall and contains an expansion agent such as silicon. An increase in mold temperature causes the expansion agent to expand and move the valve member from the closed position into the open position. An external annular groove is provided within the sleeve to receive displaced mold wall material as the sleeve is press inserted into the mold wall. A tight fit of the sleeve within the mold wall results. The sleeve and valve are configured to allow air to escape in the open position. The valve member is moved to the closed position by material within the mold cavity pressing against the valve member.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to vents for molds used in forming molded products and particularly but not exclusively to vents for tire molds. 
     BACKGROUND OF THE INVENTION 
     In molding rubber articles such as tires, the initial heating of the tire surface in contact with the hot mold rubber so that under the influence of internal molding pressure local rubber flow occurs. Venting is required to allow pockets of air which may become trapped between the green tire carcass and the hot mold to escape so that every part of the curing tire surface contacts the mold and the tire is thus vulcanized with a perfect impression of the mold detail. 
     Variously configured mold vents have thus been proposed and used to accomplish the above-stated objective. One approach is in the use of small diameter holes drilled through a mold wall normal to the interior surface. Another common approach is the use of “insert vents” that comprise small bore tubes introduced through the mold wall. Once trapped air has vented through the hole rubber begins to flow through the vent. However, the small diameter of the hole ensures that the rubber cures rapidly thus plugging the vent hole and sealing the mold. After completion of the tire curing process these plugs of rubber that are still attached to the tire surface are pulled out of the vent holes when the tire is extracted from the mold. 
     Such plugs of rubber detract from the visual appeal of the molded tire and are usually removed by trimming. A trimming operation, however, is time consuming and undesirably adds to the cost of the finished tire. 
     Another problem frequently encountered is that such plugs of rubber may break off when the tire is being extracted from the mold, thus blocking the vent hole. A blocked vent hole may not be immediately apparent and can cause subsequent poor quality moldings. 
     To solve the aforementioned problems, “spueless” vents have been proposed that allow trapped air to escape but which close to prevent rubber flow. U.S. Pat. Nos. 4,492,554 and 4,347,212 disclose examples of such “sprueless” vents. These known vents comprise a valve held normally open by a coil spring to allow the passage of air. The valve is closed by flowing rubber that moves a valve head portion against the spring tension into seated engagement with a valve seat. 
     While working well, certain problems remain unsolved by such existing vent configurations. One problem that reoccurs with existing vent configurations is that the vent sleeve is prone to becoming dislodged over time from the mold wall to which it attaches. In other instances, a problem may exist when molds containing such vents are cleaned. Cleaning is commonly effected by blasting the mold surface with a mildly abrasive material such as a plastic grit. However, because conventional vents are open during the cleaning operation it is possible for the cleaning medium to enter and block the vent. 
     United Kingdom published application GB 2,339,163 A discloses a further embodiment of a known vent that seeks to overcome the problem that occurs when a cleaning medium penetrates into the vent. The application discloses a vent configuration in which a first closure means (bias spring) is used in combination with a second closure means to close the vent when it is cold. This mechanical closure system is an improvement over the prior art but is relative expensive and complicated to fabricate. Moreover, the vent construction does not prevent the vent sleeve from becoming dislodged over time from the mold wall. 
     Accordingly, the industry remains in need of a vent plug system that can function effectively in venting air when open yet avoid vent obstruction when subjected to a cleaning procedure. Still further, a suitable vent system will be relatively simple and inexpensive to manufacture, incorporate and utilize within a mold, and replace if necessary. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a vent for a mold having a mold cavity is provided comprising a vent main body, a vent valve member disposed within the vent main body and moved by an expansion of a viscous material within the main body to cause the vent valve member to move with respect to the vent main body from a closed to an open position. A further aspect of the invention composes the expanding viscous material of silicon. 
     According to a further aspect of the invention, a mold with improved venting is provided having a mold cavity defined by a mold wall; a vent main body received within the mold wall; a viscous material contained within a portion of the vent main body; a vent valve member disposed within the vent main body to cause the vent valve member to move with respect to the vent main body from a closed to an open position. Expansion of the viscous material tightens an interference fit between the vent main body and the mold wall to which it attaches. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described by way of example and with reference to the accompanying drawings in which: 
         FIG. 1  is an exploded perspective view of the subject vent assembly; 
         FIG. 2  is a longitudinal section view through the assembled vent shown in the closed position; 
         FIG. 3  is a longitudinal section view through the assembled vent rotated ninety degrees from the position shown in  FIG. 4 ; 
         FIG. 4  is a side elevation view of the assembled vent partially inserted into a sidewall region of a mold. 
         FIG. 5  is a side elevation view shown partially in section of the assembled vent fully inserted into a mold sidewall region; and 
         FIG. 6  is a side elevation view show partially in section of the assembled vent fully inserted into a mold sidewall region and with the vent valve in a fully open position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to  FIGS. 1 and 2 , one embodiment of the subject invention is shown to comprise a mold vent  10  for disposition within a mold wall region. The vent assembly  10  includes a vent main body  12  and a vent closure member  14  (interchangeably referred to herein as “valve member”). The vent main body  12  is generally a tubular structure or sleeve open at both ends to provide an air escape passage through the interior of the main body. At the upper end  16  of the vent main body  12 , representing the end disposed at the molding surface, there is an internal conical seat  18 . A complementary external conical-shaped vent closure face  20  is disposed on the valve member  14 . Closure of the valve occurs when the complementary conical-shaped surfaces of the closure face  20  and the vent seat  18  are brought together by downward movement of the valve member  14  within the main body  12 . While a conical seat is preferred, other alternative seat variations known in the art are intended to be within the scope of the invention. 
     An axial through-bore  21  extends through the sleeve  12 . The external profile of the sleeve  12  is configured to provide an annular shoulder  22  at end  16  separated by a second annular shoulder  24  by annular recess or groove  26 . As will be appreciated, the groove  26  functions to receive a surplus of segment material forced back by tightening the shoulders  22 ,  24  of the sleeve in a hole. A series of three orifices  28 ,  30 , and  32  are spaced along the main body  12  and extend through the body sides to communicate with the internal through-bore  21 . A transverse counter-bore  34  extends from orifice  32  through the main body  12 . The through-bore  21  forms a reservoir  36  rearward from the counter-bore  34  and a lower end opening  38  to bore  21  extends from reservoir  36  through a lower end  40  of the main body  12 . The reservoir  36  and counter-bore  34  are filled with an expansion agent  42  pursuant to the invention having the characteristic of volumetric expansion when subjected to a thermal energy source. One such agent is silicon; however alternative expansion agents known in the art may be employed pursuant to the invention. 
     The valve member  14  is generally a tubular structure having a valve head  44  of relatively large diameter. The conical valve seat  20  represents an underside, rearward facing surface of head  44 . Extending rearward from head  44  is a cylindrical valve body  46  that steps radially inward to a valve stem portion  48  along a conical annular shoulder  50 . Stem  48  is generally cylindrical and steps outward at a rearward end of the valve  14  to a disc-flange  52 . The construction of the members  12 ,  14  are from any suitably hard material such as, but not limited to, steel. 
     The vent valve member  14  is received axially within the axial bore  21  of the vent main body  12 . When fully inserted, as best seen from  FIGS. 2 and 3 , there exists a gap  54  between internal sidewalls defining bore  21  and the main body  12 . The valve member  14  has an axial length sufficient to extend the end flange  52  into the reservoir  36  when the head  44  is fully seated against upper end  16  of the body  12 . The diameter of the assembled vent plug is preferably in the range of two to three millimeters but may be varied according to the application. It will be seen that the rearward portion of the axial bore  21  is filled with the expansion agent such as silicon. The silicon fill extends from the opening  38  forward and fills the reservoir  36  and counter-bore  34 . Silicon fills a rearward portion of gap  54  as shown and excess silicon material may be evacuated through the middle orifice  30 . 
     Implementation and operation of the vent plug  10  is illustrated in  FIGS. 4 ,  5 , and  6 . The vent plug  10  is inserted into a socket bore  58  within a region  56  of a mold. Insertion may be effected through the use of a suitable tool such as a mallet. The vent plug  10  is driven into the socket bore  58  until end  16  is flush with the internal surface  59  of the mold. The groove  26  receives any surplus material forced back by a tightening of shoulders  22 ,  24  of the sleeve in the hole  58 . The groove improves the maintenance of the position of the vent plug within the hole  58  by reducing the force required to install the vent plug. Reduction in force effected by the groove  26  also avoids damage to the head of the vent plug  16  and  44 . A snug fit between the sleeve and the hole with minimal insertion force is thereby achieved. The presence of sleeve  12  simplifies the mounting and makes it easier for the vent plug to be brought into a flush relationship with the curing surface. Moreover, the sleeve better protects the conical seating surfaces during mounting. The bore  58  is sized generally to accept the vent plug with minimal interference. Full insertion of the vent plug is shown in FIG.  5 . In the position shown, annular flanges  22 ,  24  abut against internal walls of bore  58  in a friction fit. The diameter of the main body  12  is slightly undersized with respect to the diameter of the bore  58  such that a gap is defined between the body  12  and the bore sidewalls. In the condition shown, the head  44  of the valve  14  is seated against the main body  12  and blocks air from flowing therebetween. 
     It will be appreciated that the dimensions of the vent plug can be smaller than a conventional vent plug employing a coil spring to allow the passage of air. A vent plug with a diameter of 2 mm for shoulders  22 ,  24  is possible. The smaller dimensions possible through the use of the invention allows for a better evacuation of the air in a small area. 
     When temperature increases within the mold up to the curing temperature, the valve opens to the position shown in FIG.  6 . Heating of the mold causes the silicon  42  to expand against the valve member  14 . Expansion of the silicon against conical annular shoulder  50  forces the valve member  14  to move away in an axial direction, opening an air path  60  between the valve member  14  and the main body  12 , beginning at the seated surfaces  18 ,  20 . The axial movement of the valve  14  during the work corresponds to approximately 0.05 mm. The air path  60  proceeds along the outside of the valve member  14  until reaching orifice  28 . The air then escapes through orifice  28  and progresses rearward along the gap  54  between the valve member  14  and the sleeve  12 . Upon reaching the end  40  of the vent plug, the air flows into the mold bore  58  and is evacuated. 
     Expansion of the silicon within the sleeve  12  further brings a complementary radially outward tightening of the sleeve against the mold bore sidewalls. As a result, the mold plug fit is tightened and unintended extraction of the mold plug from the mold wall during the heating and cooling cycle is avoided. In operation of the vent in the molding of a rubber tire, trapped air escapes through the open vent until the rubber impinges onto the end head  44  and pushes the valve member  14  into the main body  12  to bring the conical faces  18 ,  20  into seated engagement so to close the air escape passage  60  therebetween. When the vent-plug is closed, the conical surfaces  20  of the valve and  18  of the sleeve have a perfect fit because the elasticity of the silicon permits the valve  14  to have a limited movement, approximately 0.05 mm due to silicon expansion. This limited movement is beneficial and favors the adjustment of conical surfaces  20  and  18  as the valve closes. The silicon material  42  then cools to room temperature, the mold internal surfaces are cleaned by conventional means such as abrasive cleansing, and the cycle is repeated. 
     It will be noted that the subject vent plug accomplishes the stated objectives by providing an assembly comprising relatively few component parts. Intricate and expensive means for biasing the valve member open are avoided. The use of silicon or other suitable expansion agent functions predictably through repeated thermal cycles. In addition, the expansion of the silicon not only acts to open the valve but also tightens the sleeve within the mold sidewall. It will further be appreciated that the valve closes as when rubber flowing within the mold contacts head  44  and forces the valve member  14  into seated engagement with the main body  12 . At room temperature, the valve is in the closed position represented in  FIGS. 2 and 3  and allows for a sand blasting of the mold internal surfaces with the vent plugs flush within the mold sidewalls. Moreover, the subject vent valve is mounted flush with the interior wall of the mold and the valve seats flush with the molding surface. Thus, there is little material migration into the valve resulting in a sprueless end product such as a tire. The elimination of sprues avoids the cost of post-mold removal and enhances the appearance of the finished product. 
     Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described which will be within the full intended scope of the invention as defined by the following appended claims.