Abstract:
In a method and apparatus for injection molding a multi-component mixture, in particular liquid silicone rubber; a starting material, containing at least two components and, optionally, at least one additive, is conveyed from a storage and metering assembly through a mixer for mixing the starting material, to a reservoir at an amount which is a multiple of an amount required for an injection molding cycle. The mixed starting material is transferred in increments from the reservoir to an injection unit of an injection molding machine. When the reservoir is emptied to a predetermined level a multiple of the amount of starting material required for the injection molding cycle is fed to the reservoir.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims the priority of German Patent Application, Ser. No. 101 45 560.7, filed Sep. 14, 2001, pursuant to 35 U.S.C. 119(a)–(d), the subject matter of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a method and apparatus for injection molding multi-component mixtures, in particular liquid silicone rubber. 
   Silicone rubber is a generic term for a particular elastomer based on siloxane and has a chemical structure which is characterized by the siloxane chain (—Si—O—Si—O—), whereby organic methyl groups, phenyl groups, hydroxyl groups or the like are bounded as ligands. A distinction is made between solid silicone rubbers and liquid silicone rubbers (LSR) whereby the difference is characterized by their chain length. While liquid silicones are composed of a union of up to 1000 molecules, solid silicones contain a union of up to 6000–10000 molecules. As far as silicone cross-linking is concerned, two groups are distinguished: the RTV type (cross-linked at room temperature), and the HTV type (cross-linked at high temperature), i.e. cross-linked under hot-vulcanization condition. 
   The present invention relates primarily to the area of liquid silicones of the HTV type which become increasingly more relevant in the field of injection molding. This type of liquid silicones are two-component mixtures made of low viscous vinyl—or hydrogen-functional polysiloxanes with fillers and small quantities of a platinum catalyst which react according to the principle of the addition cross-linkage. The term “addition cross-linkage” denotes hereby a reaction of educts with one another under the influence of a catalyst to produce the end product, without formation of a cleavage product. These mixtures contain further an inhibitor, also called pot life regulator, to influence the period until commencement of cross-linkage. The inhibitor is added to one component whereas the catalytic cross-linkage agent is added to the other component of the two-component mixtures. This means in the context of liquid silicones that hydrogen-functional polysiloxane bonds with vinyl-functional polysiloxane under the influence of the catalyst which impacts only the reaction speed and is still in original state after the reaction. 
   Another method for making a two-component mixture involves peroxide cross-linkage where organic peroxides dissociate into free radicals as a consequence of the instability of the O—O bond under the influence of heat. The radicals react with the vinyl groups or methyl groups of the polysiloxane structure while forming hydroxyl compounds and enable in this way cross-linkage that leads to the formation of an atomic union between the individual monomers. 
   Turning now to  FIG. 1 , there is shown a conventional injection molding machine for injection molding formed parts made of liquid silicone rubber. Reference is also made in this context to German publication no. DE 34 003 09 A1. The injection molding machine includes a metering assembly  1  which has a storage tank  2  for containing component A, a storage tank  3  for containing component B as well as a storage tank  4  for containing an additive or a coloring agent F. A motor  5  drives barrel pumps  6 ,  7 ,  8  to transport the starting components A, B and the additive (or coloring agent) F through tubular conduits  9 ,  10 ,  11  via respective check valves  12 ,  13 ,  14  and a mixing head  15  to a static mixer  16 . The actual mixing of the components is implemented in the static mixer  16  by means of flow dividing elements by which the material stream is layered many times. Subsequently, the mixture is fed via a shut-off valve  17  and a fitting  18  into the feed zone  19  of a plastification screw  20  of a plastification and injection unit  21 . The rotary motion of the plastification screw  20  moves the material to an antechamber in front of the screw  20  in order to improve a mixing of the components through dynamic mixture. The screw cylinder is equipped with a water-based temperature-moderating structure to prevent excessive heating of the mixture. During subsequent injection operation, the screw  20  operates as plunger which forces material via a, not shown, cold runner system into a molding tool  22 . Backflow of the very low-viscous mixture from the antechamber into the leading screw space is prevented by a particular backflow valve (LSR design)  23 . Addition cross-linkage is realized in cavity  24  as soon as the required activation energy is exceeded as a result of a heating of the mixture. Vulcanization is effected at a temperature of about 170–200° C. and at cross-linking speeds of about 5 s/mm wall thickness of the formed part. 
   A main field of application for liquid silicone rubbers are small items such as, e.g., baby bottle nipples, mouth pieces of anesthetic equipments, and seals for various purposes, e.g., seal for a plunger of a syringe in the medical field or O rings in the automobile industry and engineering. 
   Conventional metering assemblies convey both components A and B at a particular, adjustable pressure from the storage tanks  2 ,  3 , through the static mixer  16  directly into the plastification and injection unit  21  of the injection molding machine. As the distance traveled by the material during transport between the storage tanks  2 ,  3  and the plastification and injection unit  21  is oftentimes very long, the pressure may fluctuate in the area of the feed zone of the plastification and injection unit so that the material intake is uneven, resulting in metering variations or variations of the shot weight. In particular, when smallest amounts of liquid silicone are processed, problems can be encountered that adversely affect the overall quality of the molding result because injection of material may be insufficient or may even be excessive. Another problem involves the very small throughput of the components through the static mixer as a consequence of the small shot weights because the components are not thoroughly mixed enough. 
   It would therefore be desirable and advantageous to provide an improved method and apparatus for injection molding liquid silicone rubber or other mixtures comprised of several components, to obviate prior art shortcomings and to allow a thorough mixture of the components while avoiding pressure fluctuations in the area of the feed zone of the plastification and injection unit. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a method for injection molding a multi-component mixture, in particular liquid silicone rubber; includes the steps of conveying a starting material, containing at least two components and, optionally, at least one additive, from a storage and metering assembly through a mixer for mixing the starting material, to a reservoir at an amount which is a multiple of an amount required for an injection molding cycle; transferring the mixed starting material in increments from the reservoir to an injection unit of an injection molding machine; and supplying a multiple of the amount of starting material required for the injection molding cycle to the reservoir, when the reservoir is emptied to a predetermined level. 
   The present invention resolves prior art problems by conveying starting material through the mixer at an amount which is a multiple of the amount needed for an injection molding cycle. As a result the mixing result is much improved. 
   According to another aspect of the present invention, a method for injection molding a multi-component mixture, in particular liquid silicone rubber, includes the steps of conveying a starting material, containing at least two components and, optionally, at least one additive, from a storage and metering assembly through a mixer for mixing the starting material, to two reservoirs in alternating fashion at an amount which is a multiple of an amount required for an injection molding cycle; transferring the mixture in increments from the reservoirs to an injection unit of an injection molding machine; and supplying a multiple of the amount of starting material required for the injection molding cycle to the reservoirs, when the reservoirs are emptied to a predetermined level, whereby one of the reservoirs is intended to deliver mixed starting material to the injection unit, while the other reservoir is being filled with mixed starting material. 
   According to still another aspect of the present invention, an apparatus for injection molding a multi-component mixture, in particular liquid silicone rubber, includes a storage and metering assembly for conveying a starting material containing at least two components and, optionally, at least one additive; a mixer receiving the starting material from the storage and metering assembly to provide a mixture of the starting material; an injection unit receiving the mixture from the mixer for injection into a molding tool; and a reservoir positioned between the injection unit and the mixer and dimensioned to receive a multiple of an amount of starting material required for an injection molding cycle. 
   According to still another aspect of the present invention, an apparatus for injection molding a multi-component mixture, in particular liquid silicone rubber, includes a storage and metering assembly for conveying a starting material containing at least two components and, optionally, at least one additive; a mixer receiving the starting material from the storage and metering assembly to provide a mixture of the starting material; an injection unit receiving the mixture from the mixer for injection into a molding tool; and a pair of reservoirs positioned between the injection unit and the mixer and dimensioned to receive a multiple of an amount of starting material required for an injection molding cycle. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
       FIG. 1  is a schematic illustration of a conventional injection molding machine; 
       FIG. 2  is a schematic illustration of a first embodiment of an injection molding machine according to the present invention; 
       FIG. 3  is a schematic illustration of a second embodiment of an injection molding machine according to the present invention; 
       FIG. 4  is a schematic illustration of a third embodiment of an injection molding machine according to the present invention; 
       FIG. 5  is a schematic illustration of a modified connection between mixer and reservoir for an injection molding machine according to the present invention; and 
       FIG. 6  is a schematic illustration of a variation of the injection molding machine of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Turning now to the drawing, and in particular to  FIG. 2 , there is shown a schematic illustration of a first embodiment of an injection molding machine according to the present invention for injection molding liquid silicone rubber. Parts corresponding with those in  FIG. 1 . are denoted by identical reference numerals and not explained again. In accordance with the present invention, provision is made for a buffer in the form of a metering cylinder, generally designated by reference numeral  25  and positioned between the static mixer  15  and the shut-off valve  17  of the plastification and injection unit  21 . Accommodated in the metering cylinder  25  is a metering plunger  26  which has formed therein a channel  29  with a leading end  28  connecting into an antechamber  29  in front of the metering plunger  26  to serve as reservoir  29 . The trailing end  30  of the channel  27  is fluidly connected via a tubular conduit  31  with an exit port  32  of the static mixer  16 . As an alternative, the fluid connection may also be realized via a telescopic pipe connection, as shown in  FIG. 5 . The reservoir  29  is fluidly connected via shut-off valve  17  to the entry zone  18  of the plastification and injection unit  21 , whereby a pressure sensor  33  provides constant pressure conditions in the reservoir  29 . 
   Although not shown in  FIG. 2 , there may be provided in addition to the check valves  12 ,  13 ,  14 , disposed upstream of the mixing head  15 , a further check valve in the channel  27 . Also not shown is a cooling structure for the metering cylinder  25  and the metering plunger  26  to maintain proper temperature of the mixture. 
   The injection molding machine of  FIG. 2  operates as follows: By setting a suitable pressure in the storage and metering assembly  1  and a suitable output of the motor  5  and the pumps  6 ,  7 ,  8 , the components A, B as well as additive (or coloring agent) F in the storage tank  4  are conveyed to the static mixer  16  and mixed there. The resultant mixture then advances through tubular conduit  31  and via channel  27  of the metering plunger  26  into the reservoir  29 . As a consequence, the mixture fills the reservoir  29  as the metering plunger  26  moves back until reaching an end position. When the metering plunger  26  reached the end position, the filling operation of the reservoir  29  is over. As the plastification screw  20  rotates, a desired amount of mixture is actively drawn into the feed zone  19  of the plastification and injection unit  21  while a commensurate advance of the metering plunger  26  maintains a constant pressure in the reservoir  29 . The movement of the metering plunger  26  may hereby be implemented by any suitable hydraulic, electric or pneumatic drives. 
   Each injection molding cycle results in an increment advance of the metering plunger  26  by a distance which corresponds to the required quantity of mixture for executing an injection molding cycle. Thus, the reservoir  29  is emptied in increments by the respective shot weight. Once the metering plunger  26  reaches an adjustable forward end position, the pumps  6 ,  7 ,  8  are activated to force new material into the metering cylinder  25  at a pressure which is higher than the pressure in the reservoir  29 . As the pressure sensor  33  maintains a constant pressure in the reservoir  29 , the metering plunger  26  moves back until reaching the rear end position. During filling procedure of the reservoir  29 , there is no need to interrupt the injection molding process. 
   Referring now to  FIG. 3 , there is shown a schematic illustration of a second embodiment of an injection molding machine according to the present invention. Parts corresponding with those in  FIG. 2  are denoted by identical reference numerals and not explained again. In this embodiment, provision is made for an injection unit in the form of a plunger-type injection unit, generally designated by reference numeral  34 . The plunger-type injection unit  34  has an injection plunger  36  which moves within a cylinder  56  and defines with the cylinder  56  an antechamber  35  in front of the injection plunger  36 . Filling of the antechamber  35  is realized by lateral fill openings, indicated at reference numeral  57 , in a forward area of the antechamber  35 . Of course, filling of the antechamber  35  may also be implemented in a manner analogous to the metering cylinder  25  of the embodiment of  FIG. 2 , i.e. via a channel extending through the injection plunger  36  and having a trailing end fluidly connected with the exit opening of the reservoir  29  via a tubular conduit. As an alternative, the fluid connection may also be realized via a telescopic pipe connection, as shown by way of example in  FIG. 5 . Normally, the antechamber  35  is filled with the respective shot weight plus the amount of mixture required for applying the after-pressure. As a consequence of the low viscosity of the liquid silicone rubber mixture, the injection plunger  36  is suitably provided with elastic members  37 , e.g., O rings, about its outer wall that glides along the cylinder  56 . 
   As the dynamic mixture through a plastification screw downstream of the reservoir  29  is omitted in the configuration of the injection unit in the form of a plunger-type injection unit according to  FIG. 3 , it may be necessary to replace the static mixer with a dynamic mixer, even though the higher throughput amount by the static mixer attains more thorough mixing results, when compared with prior art devices. 
   The mode of operation of the injection molding machine of  FIG. 3  is similar to the injection molding machine of  FIG. 2 , i.e. the reservoir  29  of the metering cylinder  25  receives an amount of mixture which is a multiple of the amount required for an injection molding cycle, i.e. the amount drawn from the reservoir  29  by the plunger-type injection unit  34  and forced into the molding tool  22 . 
     FIG. 4  shows a schematic illustration of a further variation of an injection molding machine according to the present invention. In this embodiment two metering cylinders  38 ,  39  are provided downstream of the mixer  16  so that two reservoirs  40 ,  41  can alternatingly be filled. In the non-limiting example of  FIG. 4 , the metering plungers  42 ,  43  are actuated by hydraulic piston and cylinder units  44 ,  45  and include piston rods  54 ,  55 , whereby the piston rods  54 ,  55  are each extended outwards via another cylinder  58  with accommodated piston  59  to interact with position transmitters  46 ,  47  and  48 ,  49 , respectively, so as to establish and ascertain the rear and front end positions of the metering plungers  42 ,  43 . Control valves  60  are provided to assist a movement of the metering plungers  42 ,  43  through respective fluid regulation into the cylinders  59 . 
   At operation of the injection molding machine of  FIG. 4 , the mixture of components A, B and additive (or coloring agent) F is alternately conveyed downstream of the static mixer  16  via tubular conduits  50 ,  51  and control valves  52 ,  53  into the reservoirs  40 ,  41  of the metering cylinders  38 ,  39 . Pressure sensors  33  monitor the pressure in the respective reservoirs  40 ,  41  to keep the pressure constant. Suitably, the cylinders  38 ,  39  are each embraced by a cooling system  61 . 
   The injection molding machine of  FIG. 4  has the benefit that a re-charging of the one metering cylinder that has been emptied will not impact a metering operation of the plastification and injection unit  21  because the other metering cylinder, that has been filled previously, is then available for the injection molding process. 
   Referring now to  FIG. 6 , there is shown a schematic illustration of a variation of the injection molding machine of  FIG. 3 . Parts corresponding with those in  FIG. 3  are denoted by identical reference numerals and not explained again. In this embodiment, provision is made, in addition to the fluid connection between the trailing end  30  of the channel  27  via tubular conduit  31  to exit port  32  of the static mixer  16 , for the arrangement of at least one other fill opening at the forward end of the antechamber  29  which is fluidly connected via a further tubular conduit (or pipe connection)  31  with a further exit port of the static mixer  16 . Of course, the provision of at least one other fill opening at the forward end of the antechamber  29 , as shown in  FIG. 6  by way of example, is equally applicable for the injection molding machine of  FIG. 2 . 
   It will be understood by persons skilled in the art that the principles described in the preceding description with respect to the processing of a liquid silicone rubber mixture are generally applicable to any mixtures of different components. 
   While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.