Abstract:
A thermal expansion compensation support and method are used in a hot runner injection mold for maintaining liquid-tight relationships between parts that define a flow passageway. The support includes an annular housing having an annular recess that receives a spring, such as a Belleville washer, and a cover that overlies the annular recess in the housing. The housing and the cover are spring biased from each other to maintain a pressure force on the surfaces that are in contact with the outer faces of each of the cover and of the annular housing. The support imparts a holding force on the several parts of the machine that define the molding material flow passageway to prevent leakage. The method involves holding the parts together at a first holding force level for an initial portion of a warm-up temperature range and holding the parts together at a second holding force level for the remainder of the temperature range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hot runner injection molding systems, and more particularly to a thermal expansion compensation support for a manifold used in a hot runner injection molding system. 
     2. Description of the Related Art 
     In hot runner injection molding systems a heated manifold is provided to convey molten plastic molding material from a source, such as a plastication barrel, to a plurality of injection nozzles. The nozzles are in fluid communication with respective mold cavities that define the shapes of parts to be molded. The manifold is heated to maintain the molten plastic material at a desired elevated temperature so that the material does not cool excessively as it flows from the plastication barrel to the mold cavities. The heat is typically provided by electrical heating elements within the manifold, or by circulating heated fluids through passageways within the manifold. The added heat maintains the molding material in a fluid state so it can readily be conveyed and completely fill the respective mold cavities to provide fully formed molded parts. Although the manifold is heated, the adjacent structural elements of the injection molding machine, which generally include a clamp plate and an injection nozzle retainer plate, are not heated and may actually be cooled by adjacent mold elements. 
     The hot runner manifold is generally spaced from the adjacent structural elements of the machine by spacers or supports, which are often disc-shaped or annular metallic members that serve to support the hot runner manifold within the mold assembly and space the manifold from the adjacent mold elements. The molding material is conveyed to the hot runner manifold, and then conveyed through the manifold to the respective injection nozzles. When starting such an injection molding machine from a “cold” (start-up) condition, the hot runner manifold is initially spaced from the respective adjacent machine structural elements at a predetermined distance. This spacing distance diminishes when the manifold expands as its temperature increases during the course of the operation of the injection molding machine. However, the adjacent structural elements, which are not directly heated, are at a lower temperature and therefore expand to a lesser degree. 
     As the molten molding material is conveyed from the plastication barrel to the hot runner manifold and from the manifold to the respective mold cavities, it passes through flow passageways that must remain aligned with each other to prevent leakage of the fluent molding material. Thus, it is essential that the molding material flow passageways within the respective adjoining elements of the machine be properly aligned throughout the machine warm-up process and subsequent operation, even though the parts expand at different rates and may ultimately have different operating temperatures. 
     In the past, various structural arrangements have been proposed in an effort to ensure that the respective parts of a hot runner injection molding machine are properly aligned and are arranged in leak-tight relationship. For example, in U.S. Pat. No. 4,588,367, entitled “Hot Runner Manifold For Injection Molding Machine”, which issued on May 13, 1986, to Schad, the injection nozzle is retained in sealing engagement with a hot runner manifold block by means of a pair of Belleville washers. These springs are positioned to maintain engagement between the injection nozzles and the manifold block from initial start-up, through warm-up, to normal operating temperature. However, if the springs were to fail either before or during the time the machine is at normal operating temperature, the failure of the springs would allow the flow passageway between the injection nozzle and the manifold block to open. The parts would separate as a result of removing the spring force, thereby allowing the molten molding material to leak from the open flow passageway into the space between the manifold block and the mold plate, possibly interfering with the molding process by not supplying sufficient material to form the part properly. 
     Another arrangement for maintaining contact between a manifold block and a nozzle to avoid molding material leakage involves the use of a somewhat flexible spacer. Such an arrangement is disclosed in U.S. Pat. No. 5,125,827, entitled “Injection Molding Apparatus Having An Insulative And Resilient Spacer Member”, which issued on Jun. 30, 1992, to Gellert. That patent discloses the use of an annular metallic spacer that is positioned between a hot runner manifold and the clamp plate that contacts the plastication barrel. The spacer is defined by a plurality of peripherally interconnected, V-shaped concentric rings that allow the spacer to deflect during the expansion of the manifold block relative to the clamp plate during warm-up to maintain the parts that define the material flow passageway in contact with each other. However, the structure of the spacer and the elasticity of the metallic material from which it is formed limits the degree of deflection that the spacer can undergo, and therefore full sealing contact of the injection nozzle and the manifold block throughout the range from cold start-up to full operating temperature cannot be achieved. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the deficiencies of the spacer configurations in the prior art arrangements. It is another object of the present invention to provide a thermal expansion compensation support that is effective to maintain the parts that define the material flow passageway in continuous contact throughout the range of operation of a hot runner injection molding machine, from cold start-up through normal operating temperature, without allowing leakage of molding material from between the adjoining mold elements within which the molten material flow channel is provided. 
     Briefly stated, in accordance with one aspect of the present invention, a thermal expansion compensation support is provided in a hot runner mold assembly. The support includes a housing having a recess that defines an opening in the housing. A spring is positioned within the housing recess and extends outwardly of the housing opening. A cover overlies the opening and is in surface contact with the spring. A connector extends between the housing and the cover for holding the cover against the spring, thereby compressing the spring to apply a “preload” to the support, while leaving a defined gap between the housing opening and the cover. 
     In accordance with another aspect of the present invention, a method is provided for liquid-tight interconnections between the several members of the injection mold construction in which the molding material flow passageway is contained. The method includes holding the members together at a first holding force level for a first portion of the predetermined temperature range, and holding the members together at a second force level for a second portion of the predetermined temperature range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary cross-sectional view showing a hot runner manifold block and adjacent mold elements for use in an injection molding machine. 
     FIG. 2 is an enlarged, longitudinal cross-sectional view of a manifold support in accordance with the present invention. 
     FIG. 3 is an enlarged, fragmentary longitudinal cross-sectional view of an alternate embodiment of a manifold support in accordance with the present invention and shown in its operative position while the parts of the mold are in a cold, start-up condition. 
     FIG. 4 is an enlarged, fragmentary longitudinal cross-sectional view similar to that of FIG. 3, but showing the respective mold parts at a point shortly before the mold reaches its normal operating temperature. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and particularly to FIG. 1 thereof, there is shown a portion of the molding material flow passageway of a hot runner injection mold  10 . The mold  10  includes a top clamp plate  12  and a nozzle retainer plate  14  positioned adjacent the clamp plate  12 . The nozzle retainer plate  14  has a U-shaped cross section that defines a recess  16  adjacent a face of the clamp plate  12  for receiving a hot runner manifold block  18  that includes heating elements  19 , preferably in the form of electrical resistance heaters. 
     Positioned between the manifold block  18  and nozzle retainer plate  14  is an injection nozzle body  20  that is received in an appropriately sized bore  22  formed in the retainer plate  14 . The nozzle body  20  includes a central passageway  24  that terminates in a flow outlet  26  that is adapted to be in fluid communication with a gate to a mold cavity (not shown), as is generally known in the art. The nozzle passageway  24  communicates with a material flow channel  28  provided in the manifold block  18 . A manifold extension  30  passes through the top clamp plate  12  and is retained in position by a locating ring  32  that is connected with the clamp plate  12  by means of cap screws  34 . The manifold extension  30  includes a central passageway  36  that communicates with the material flow channel  28  in the manifold block  18 . At its outermost end, the central passageway  36  terminates in a seat  38  that is adapted to engage with an outlet of an injection unit barrel (not shown) that provides a source of molten molding material. 
     As shown in FIG. 1, the hot runner manifold block  18  is spaced from each of the top clamp plate  12  and nozzle retainer plate  14  by means of supports  40 ,  42 , and  44  that are positioned between and are in contact with the opposed surfaces of the respective parts. The supports  42  and  44  shown beneath the manifold block  18  in FIG. 1 are conventional, known support structures, generally cylindrical or disk-like in configuration. The support  42  is retained in position by means of a dowel pin  46  that extends into the nozzle retainer plate  14  and that also serves to orient properly the nozzle retainer plate  14  and manifold block  18  by engaging with a mating hole  47  provided in the manifold block  18 . The support  44  is attached to the nozzle retainer plate  14  by screws  48 . 
     The injection nozzle body  20  is in contact with the manifold block  18  and includes an injection nozzle support collar  50  that extends between an outwardly-extending flange  52  on the nozzle body  20  and a counterbore  54  provided in the nozzle retainer plate  14 . 
     On the face of the manifold block  18  opposite from the conventional supports  42  and  44  is a manifold support  40  to compensate for thermal expansion in accordance with the present invention. Support  40  extends between and is in contact with the opposed faces of each of the top clamp plate  12  and hot runner manifold block  18 . 
     As will be appreciated by those skilled in the art, the molten molding material passes into the central passageway  36  provided in the manifold extension  30 , on into the material flow channel  28  in the manifold block  18 , through the respective injection nozzle bodies  20  (only one of which is shown in FIG.  1 ), and finally into the respective mold cavities (not shown). To maintain contact between the manifold block  18  and injection nozzle body  20 , thereby preventing leakage of the fluent molding material, the support  40  preferably includes a spring arrangement whereby a continuous force is maintained on the manifold block  18  so that the manifold block  18  and injection nozzle body  20  do not separate. 
     Referring now to FIG. 2, there is shown in enlarged form, and in longitudinal cross section, a manifold support  40  in accordance with the present invention. The support  40  includes an annular housing  56  that is defined by an annular base wall  58  that transitions to a longitudinally-extending outer wall  60  and a longitudinally-extending inner wall  62  spaced inwardly from the outer wall  60 . The base wall  58 , inner wall  62  and outer wall  60  thus define an annular, U-shaped channel  64 . An annular washer  66  is positioned within the channel  64 , and rests against the inner surface of the base wall  58  to provide a wear surface for a spring  68  that overlies the washer  66 , as will be more fully explained later. 
     The spring  68  can be a Belleville washer, as shown, a coil spring, or any other elastic, annular ring that can maintain its resiliency when subjected to the higher than ambient temperatures encountered by the manifold support  40 . The ensuing discussion will be based upon the use of springs in the form of Belleville washers, but it will be understood by those skilled in the art that other types of springs can also be utilized. A cover  70  in the form of an annular disk is provided to overlie annular channel  64  and to engage and retain the Belleville washer  68  carried within the channel  64 . 
     The inner wall  62  includes an inwardly-extending flange  72  adjacent the open end of the annular channel  64 . The flange  72  defines a through-bore  74  that slidably receives a tubular sleeve  76  having an outwardly extending flange  78  at one end. The flange  78  of the sleeve  76  and the flange  72  of the housing  56  are adapted to engage each other and thereby limit movement of the sleeve  76  relative to the housing  56 . The end of the sleeve  76  opposite from the flange  78  contacts the cover  70  to space it from the flange  72  of the housing  56  when the flanges  72  and  76  are in contact with each other. A screw  79  extends through the interior of the sleeve  76  so that the screw head  80  engages an end surface  82  of the sleeve  76 . A jam nut  84  is threaded on the screw  79  to hold together the several parts of the support  40  in the relative positions as shown in FIG.  2 . More particularly, the nut  84  is hand-tightened so that the cover  70 , sleeve  76  and screw head  80  into “metal-to metal” contact; the elements of the support  40  are sized such that the cover  70  will just come into contact with the spring  68  (no compression) when assembled in this manner. 
     As can also be seen in FIG. 2, when in its assembled form and without any axial load imposed, the support  40  includes a gap  86  between the cover  70  and the annular housing  56 . Thus, when the support  40  is used in a mold assembly as shown in FIG.  1  and an axial compressive load is applied to the support  40 , the housing  56  will move toward the cover  70 , compressing the spring  68 . When the compressive load is sufficient to force the opposed surfaces of the cover  70  and housing  56  to move into contact with each other, no further compression of the spring  68  can occur and the support  40  consequently acts as a solid, non-resilient spacer. The size of the initial gap  86  when not under load can be varied by changing the axial dimensions of the several parts of support  40 , particularly the length of the sleeve  76 . The resistance of the support  40  to compressive loads can be varied by changing the spring constant for the spring  68 . Further, although shown in FIG. 2 as having a single spring  68 , two or more such springs can be utilized, if desired. FIGS. 3 and 4 show a configuration for the support  40  in which the spring force is provided by two back-to-back Belleville washers. 
     Additionally, although other forms of annular springs can be employed in the support  40 , Belleville washers are preferred because they provide the desired spring constant and have a low axial height, thereby occupying less space in the mold assembly. However, when a Belleville washer flexes or deflects with changes in the magnitude of the compressive force, the inner and outer edges of the Belleville washer move slightly in a radial direction, thereby tending to scuff the supporting surfaces; in this case, the washer  66  and cover  70  are in contact with the spring  68 . Accordingly, both the washer  66  and cover  70  are preferably surface hardened to prevent wear that can occur from movement of the Belleville washer over the contacting surfaces. In that regard, the washer  66  and cover  70  can have a surface hardness that exceeds the hardness of the interposed Belleville washer(s). Further, the annular housing  56  is preferably formed from a material that can withstand relatively high temperatures, of the order from about 400° C. to about 500° C. and that can also withstand the compressive loads to which it can be subjected when in use. A suitable material for the annular housing  56  is titanium alloy, such as Ti-6AI-4V, which is widely available. 
     The manner of use for the support  40  can best be seen in FIGS. 3 and 4. In each of FIGS. 3 and 4 the support  40  is positioned between the top clamp plate  12  and manifold block  18  and includes two back-to-back Belleville washers  68  and  69 . FIG. 3 shows the respective parts of the system when they are in their “cold” condition, at start-up of the molding machine before the mold elements have reached their operating temperature. When in this condition, the support  40  is at least partially compressed to transmit a “preload” force to the manifold  18 , thereby maintaining a sealed relationship between the flow nozzle  20  and the manifold  18 , as shown in FIG.  1 . The gap  86  between the annular housing  56  and the cover  70  allows limited relative movement between those parts and a corresponding increase in force. In the unloaded state (before assembly in the mold), the gap  86  can be of the order of about 0.015 in. (FIG.  2 ); the gap  86  is preferably reduced to about 0.007 in. when the mold is fully assembled in the “cold” state with the preload applied (FIG.  3 ), as described above. When the manifold block  18  is heated it expands, causing the annular housing  56  and cover  70  of the support  40  to move together, thereby reducing the size of gap  86  and increasing the compressive force as the manifold expands with increasing temperature. 
     At a time shortly before the respective parts of the injection mold reach normal operating temperatures, the size of gap  86  is reduced to zero, so that there is direct metal-to-metal contact between the annular housing  56  and cover  70 , as shown in FIG.  4 . The narrowing of the space between the top clamp plate  12  and manifold block  18  with increasing temperature has at least partially compressed springs  68  and  69 , and has caused the annular housing  56  of the support  40  to contact the cover  70 . Further thermal expansion of the manifold block  18  serves to compress all of the elements of the support  40  and increase the force tending to hold injection nozzle  20  (see FIG. 1) in tight contact with manifold block  18 . Thus, leakage of molten molding material through gaps between the respective parts that define the flow passageway is avoided. 
     Because direct, metal-to-metal contact occurs in the support  40  at a time shortly before the mold reaches its normal operating temperature, should spring failure occur, or should excessive spring relaxation occur due to temperature, the direct, metal-to-metal contact between the manifold block  18 , support  40 , and clamp plate  12  will prevent any such spring failure from allowing leakage of molding material. In the prior art arrangements, on the other hand, spring tension has to be maintained throughout the operation of the machine, from cold start-up through and including normal operating temperature; consequently, reliance was placed upon the springs to maintain the parts in liquid-tight relationship at all times. As will be appreciated, at normal operating conditions the present invention provides a more positive seal between the respective contacting parts defining the material flow passageway, one that is independent of the spring force and even of the spring condition. 
     As is apparent from FIGS. 2 and 3, the amount of preload that is applied to the spring  68  is determined by the spring constant and the amount the spring  68  is compressed when the support  40  is initially assembled into the hot runner system of an injection mold (“cold” condition). The support  40  is secured to the top clamp plate  12  by the screw  79 , as shown in FIG. 3, and compressed slightly to apply the preload when the top clamp plate  12  is attached to the nozzle retainer plate  14 . Depending on the size and shape of the hot runner manifold block  18 , as well as the total number of nozzles  20 , several supports  40  are used to apply a uniform force and maintain contact between the manifold block  18  and nozzles  20 . 
     Although particular embodiments of the present invention have been illustrated and described, it would be apparent to those skilled in the art that various changes and modifications can be made without departing from the concepts of the present invention. Accordingly, it is intended to encompass within the appended claims all such changes and modifications that fall within the scope of the invention as described herein.