Abstract:
An injection molding nozzle having a nozzle tip insulator assembly affixed thereon comprised of an inner sleeve and an outer sleeve is disclosed. The inner sleeve is preferably made from a metal with relatively low thermal conductivity and the outer sleeve is preferably made from a compressible non-metal material that can withstand typical injection pressures and temperatures.

Description:
RELATED APPLICATION DATA 
     This application is a continuation-in-part of application Ser. No. 09/272,251, filed Mar. 19, 1999, now U.S. Pat. No. 6,315,549, and is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of injection molding. More particularly, the invention relates to a tip insulator assembly that attaches to a distal end of an injection nozzle. The tip insulator assembly is held onto the nozzle preferably by a removable retainer means. 
     2. Summary of the Prior Art 
     Insulators are used to thermally insulate the heated tip or nozzle of a hot runner nozzle assembly from the surrounding cooled gate insert of a mold cavity. U.S. Pat. No. 4,662,837 to Anderson (incorporated herein by reference) shows such an insulator assembly. The insulative material is typically made of a high temperature resistant, resilient material such as VESPEL (polyimide resin). The insulators occupy space that would otherwise be filled with a resin “bubble”, well known in the art. If the resin being processed tends to degrade over time and it is allowed to form the insulating bubble around the tip, the degraded resin eventually is drawn into each part being molded causing unsatisfactory properties and appearance. It is therefore essential that the insulating function of the bubble be performed by some other material other than the resin being processed if it is thermally sensitive or if plastics of differing colors are to be processed and errant color streaks are undesirable. 
     U.S. Pat. No. 5,208,052 to Schmidt et al. (incorporated herein by reference) shows another insulator construction that surrounds a heated tip of a hot runner nozzle assembly. The insulator is made of titanium and separated from both the tip and cooled gate area by air gaps on either side. The insulator is threaded onto the nozzle housing and retains the tip in the housing by means of the contact area there between. The tip must seal against the bubble wall to prevent resin leaking behind it and occupying the air gap space. Sealing is effected by a seal in combination with deflection of the insulator leg pressing between the tip and the bubble wall. This design has the disadvantage of being relatively costly and titanium is not as effective as a thermal insulator as VESPEL (polyimide resin). 
     Co-pending U.S. application Ser. No. 09/272,251, filed Mar. 19, 1999, now U.S. Pat. No. 6,315,549, to Jenko et. al, shows a two piece insulator that surrounds the tip. An inner titanium sleeve is surrounded by a rather large outer VESPEL (polyimide resin) sleeve and the assembly is releasably fastened to an injection nozzle. In field testing has revealed that the structural design of the Jenko titanium inner sleeve is less than adequate if large enough preload forces are applied to the nozzle tip. Due to the location and orientation of the shoulder on the inner sleeve of the Jenko insulator assembly, a significant amount of the preload is applied to the unsupported shoulder which results in tensile failure of the inner sleeve. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a nozzle tip insulator assembly that may be releasably connected to an injection nozzle and withstand the pressure and temperature associated with injection molding plastics thereby overcoming the drawbacks of the prior art. 
     Another object of the present invention is to provide a nozzle tip insulator assembly that is comprised of a low profile “Vespel” outer insulator sleeve that is positively retained to an inner titanium sleeve when the mold plates are opened. 
     Another object of the present invention is to provide a nozzle tip insulator assembly that is comprised of an outer “Vespel” insulator sleeve that is retained in place on an inner titanium sleeve during an injection molding cycle or cycles. 
     Another object of the present invention is to provide a nozzle tip insulator assembly that uses a smaller amount of expensive “Vespel” to reduce cost and to further reduce the amount of heat transferred out of the nozzle. 
     Another object of the present invention is to provide a nozzle tip insulator assembly that provides improved structural performance over the prior art. 
     Another object of the invention is to provide a nozzle tip insulator that acts as the only nozzle seal for an injection nozzle. 
     A nozzle tip insulator assembly comprises an inner sleeve formed to fit around a nozzle tip, an outer sleeve on the inner sleeve, and a joining means located between the inner and outer sleeves. A shoulder protrudes from the inner sleeve to restrain the movement of the outer sleeve. The shoulder is located to direct a portion of loads applied to the outer sleeve into the nozzle tip, thereby reducing tensile loads in the inner sleeve. The inner sleeve is preferably releasably affixed to an injection nozzle. The outer sleeve is preferably made of a material with a low thermal conductivity, such as VESPEL (polyimide resin). 
     Further objects and advantages of the present invention will appear hereinbelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified cross-sectional view of an injection nozzle in accordance with one exemplicative embodiment of the present invention; 
     FIG. 2 is an enlarged cross-sectional view of one exemplicative embodiment in accordance with the present invention; 
     FIG. 3 is an exploded isometric view of an exemplicative embodiment in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, which shows a simplified cross-section of an injection nozzle assembly  10  in accordance with an exemplicative embodiment of the present invention. The nozzle assembly  10  is inserted in a cavity  40  in a mold plate  34  as well known in the art. Nozzle assembly  10  is comprised of an elongated nozzle bushing  12  having an internal first melt channel  30  therein for receipt of a fluid from a hot runner manifold (not shown) or the like. A heater  24  is placed in thermal communication with the nozzle bushing  12  for maintaining the elevated temperature of the nozzle  10 . Affixed coaxially to nozzle bushing  12  is an elongated nozzle tip  14  with a second melt channel  32  formed therein and placed in alignment with first melt channel  30 . Fluid entering first melt channel  30  is further communicated to second melt channel  32  and is then communicated to one or more third melt channels  36  at the lower distal end of the nozzle tip  14 . 
     In this embodiment, and not by limitation, the nozzle tip  14  is affixed to nozzle bushing  12  by an attachment means  42 , ie. threads. Alternative attachment methods could easily be employed by someone with ordinary skill in the art, and such alternatives are fully contemplated herein. 
     A tip insulator assembly  16  is affixed to the lower distal end of the nozzle tip  14  adjacent a mold gate  26 . In one preferred embodiment, the assembly  16  is releasably attached by a retainer means  18 , such as a spring clip. The tip insulator assembly  16  is comprised of an inner sleeve  20  made from a material with a relatively low thermal conductivity, such as titanium. The inner sleeve  16  is generally a hollow cylindrically shaped body adapted to sealingly fit around the lower distal end of the nozzle tip  14  as shown in the figure. 
     An outer sleeve  22 , also made from a material with a relatively low coefficient of thermal conductivity, is affixed to an outer surface of the inner sleeve  20  such that an outer surface of the outer sleeve  22  sealingly abuts against a gate wall  38  during an injection molding cycle. In one embodiment, the outer sleeve is made from VESPEL (polyimide resin). With this sealing arrangement, molten fluid is communicated from the space denoted by FIG. 28 to the gate  26 , thereby not allowing material to become stagnate for an extended period of time and degrade. 
     Referring now to FIGS. 2 and 3, where like features have like numerals, enlarged views of the tip insulator assembly  16  is shown. The outer sleeve  22  is affixed to and protrudes from the inner sleeve  20 . A shoulder  48  located on the inner sleeve  20  is provided to prevent the sealing pressure (denoted P) from forcing the outer sleeve  22  off the inner sleeve  20  during an injection cycle. 
     To prevent the outer sleeve  22  from falling off the inner sleeve  22  when the gate wall  36  is removed, a joining means  44  is provided between the inner sleeve  20  and the outer sleeve  22 . The joining means  44  would preferably be formed on the outer surface of the inner piece  20  and could be a series of circumferential ridges, an external thread, an array of through holes or protrusions or the like. In the embodiment shown in FIGS. 2 and 3, a series of circumferential ridges are used. The exact configuration of the joining means  44  could be easily modified as long as it allows the outer sleeve  22  to remain affixed to the inner sleeve  20 . 
     In the preferred embodiment, the outer sleeve  22  is allowed to deform under the pressure (denoted P) created during the initial heat up the nozzle assembly into the joining means  44  such that when the injection cycle operating temperature is reached, the outer sleeve  22  is mechanically retained on the inner sleeve  20 . As the nozzle assembly  10  is heated up from room temperature to the molding process temperature, the overall length of the nozzle assembly  10  will grow and bring the outer sleeve  22  into sealing contact with the gate wall  38 . The amount of thermal growth of a given nozzle assembly  10  is well known in the art and provides the sealing force between the gate wall  38  and the outer sleeve  22  and causes deformation of the outer sleeve  22  and joins the outer sleeve  20  to the inner sleeve  20 . Alternatively, the nozzle assembly could already be in contact with the gate wall at room temperature, but as the nozzle assembly is heated up, the preload on the outer sleeve increases to provide a more positive seal. 
     Located at the distal end of the inner sleeve  20  adjacent space  28  is an angled protrusion  50 . This wedge like protrusion  50  interfaces with the outer sleeve  22  to form a highly efficient sealing interface to prevent the high-pressure molten fluid from migrating between the inner and outer sleeves. 
     In one preferred embodiment, at least one slot  19  is provided through a wall of the inner piece  20  for insertion of the retainer means  18 . The slot  19  aligns with a complimentary slot in the nozzle tip  14  for insertion of the retainer means  18 . In this manner, tip insulator assembly  16  is releasably attached to the nozzle tip  14 . Alternative arrangements of the retainer means  18  could easily be provided, for example, threads, snap in detents, a pin and hole arrangement to name just a few. Although it is preferably to have the tip insulator assembly  16  releasably attached to the nozzle tip  14 , it could also be permanently affixed by brazing, press fitting on the nozzle tip, welding or the like. 
     In one preferred embodiment, the size of outer sleeve  22  has been substantially reduced due to the use of the shoulder  48  and the joining means  44  on the inner sleeve  20 . Since a preferred embodiment of the outer sleeve  22  uses the expensive “Vespel” material due to its compressibility, low coefficient of thermal conductivity and ability to withstand high temperatures, reducing the size of the outer sleeve  22  substantially reduces the cost to produce the tip insulator assembly. Further still, by reducing the size of the outer sleeve  22 , the amount of heat conducted out of the nozzle tip  14  has been reduced which results in better performance of the injection molding process. 
     Placement of the shoulder  48  is such that the preload force P is supported by both the protruding shoulder  48  and the nozzle tip  14 . This arrangement reduces the tensile forces in the outer sleeve  22  and eliminates the possibility of structural failure of the outer sleeve  22 . 
     As a result of the improved structural performance of the nozzle insulator assembly  16 , back up sealing interfaces  52  (FIG. 1) that were provided between the nozzle bushing  12  and the mold plate  34  in the prior art have been eliminated. As a result, machining tolerances and heat loss between these two components have been improved. 
     It is to be understood that in the context of the present invention, the term nozzle or nozzle tip may be used interchangeably, and may refer to either of a nozzle tip for a hot runner application, or a nozzle tip on the end of an injection molding machine&#39;s injection unit that is coupled to a mold sprue bushing. Insulator assemblies that can be attached to either type of injection molding nozzle tip are considered useful and within the scope of the present invention, which should not be limited to one application or the other. 
     It is to be further understood that the present invention should not be limited only to the use of hot runner nozzles with molds. The present invention includes the use of hot runner nozzles that are installed as extensions between machine injection units and inlets to mold hot runners, which are outside the mold structure. The hot runner nozzle tip may form a connection between hot runner structures, or between an injection machine nozzle and a heating channel for conveying melted materials. Insulators that can be removably attached to hot runner nozzle tips used in any setting are considered useful and within the scope of the present invention, which should not be limited to the use of hot runner nozzles with molds. 
     It is to be understood that the invention is not limited to the illustrations described herein, which are deemed to illustrate the best modes of carrying out the invention, and which are susceptible to modification of form, size, arrangement of parts and details of operation. For example, the releasable retainer means  18  has many easily identifiable equivalents. The invention is intended to encompass all such modifications, which are within its spirit and scope as defined by the claims.