Abstract:
A method and apparatus for runnerless injection molding of plastic materials utilizing a novel valve stem for controlling the opening and closing of at least two gates in a single injection nozzle The method and apparatus includes at least two separate melt streams whose flows are not obstructed by the valve stem. These melt streams may contain the same plastic material or different plastic materials and the injection nozzle may be either simultaneously or sequentially activated for filling the mold cavity.

Description:
This is a divisional of application Ser. No. 08/954,728, filed Oct. 20, 1997, now U.S. Pat. No. 5,972,258. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for runnerless injection molding provided with a novel valve gate for permitting at least two gates to be controlled in a single nozzle. In particular, the invention relates to an improved method and apparatus for molding hollow articles and preforms for blow molding which have a layered wall structure. 
     BACKGROUND OF THE INVENTION 
     This invention concerns injection molding nozzles used to inject plastic material into the cavity of a mold. Such nozzles receive molten plastic material from an injection molding machine and direct the same into a mold cavity through a passage called a gate. Two methods exist for this transfer: thermal, or open, gating; and valve gating. 
     In thermal gating, the gate is an open aperture through which plastic can pass during injection of plastic material. The gate is rapidly cooled at the end of the injection cycle to “freeze” the plastic material which remains in the gate to act as a plug to prevent drool of plastic material into the mold cavity when the mold is open for ejection of parts. In the next injection cycle, the cooling to the gate is removed and hot plastic material pushes the plug into the mold cavity, where it melts and mixes with the new melt stream. 
     In valve gating, gate opening and closing is independent of injection pressure and/or cooling, and is achieved mechanically, with a pin that travels back and forth, to open and close the gate. 
     Generally, valve gating is preferable to thermal gating because the gate mark left by valve gating on the finished molded part after injection is complete is much smaller than that which results from thermal gating. Larger gate sized can also be used in valve gate systems, leading to faster filling of the mold cavities and therefore shorter molding cycle times. 
     However, some disadvantages are frequently associate with valve gates. These disadvantages include “weld lines”, which are areas where multiple melt flow fronts meet, and valve stem wear. Weld lines tend to introduce weakness or loss of mechanical strength into the finished part and result from the fact that the valve stem is surrounded by the plastic material, splitting the melt stream, which is later rejoined at the end of the stem, and this re-combining of the stream leads to weld lines. Hence, there exists a need for a gate design which allows for the melt stream, or streams in the case of two or more plastic materials, to remain separate while still being controlled with a common valve stem. 
     The valve stem is also subject to wear from mechanical stress, due to stem deflection from the incoming pressurized melt, and thermal stress, from constant contact with the melt. This wear is exacerbated in cases where reinforced plastic materials, i.e., those containing glass or other fibers or materials, are injected. Hence, there exists a need for a design which mitigates the wear of the valve stem. 
     The injection of two or more separate melt streams into a mold cavity, whether simultaneously or sequentially, is referred to as co-injection, and leads to layered wall structures in hollow articles and blow molding preforms. The prior art includes a multitude of processes and apparatuses for forming molded articles from multiple plastic materials by co-injection. For example, U.S. Pat Nos. 5,028,226 and 4,717,324 show simultaneous and sequential co-injection apparatuses and methods, respectively. Both patents show one nozzle dedicated to each mold cavity wherein the cavity is filled by injecting two or more resins through a single gate. 
     In the systems shown in each patent, a valve stem is used to prevent resin flow through the gate after injection is complete. In these systems, the hot runner systems employed to receive the various resins from their source for conveyance to the mold cavities are very complicated. Consequently, such hot runner systems lead to mold designs which are not compact and thereby allow fewer cavities and fewer articles to be molded within a given space on a molding machine. 
     U.K. Patent No. 1,369,744 discloses a sequential co-injection system using separate channels, commonly referred to as sprue channels, for each melt stream, and sliding shuttles which function as valve stems to open and close the connection between the injection machine and the channels. However, these separate melt channels converge into a single common gate area prior to injection, so that some potential for contamination between streams exists. Furthermore, the shuttles are hydraulically actuated, increasing the complexity of the nozzle and allowing the risk of leaking hydraulic fluid to contaminate the streams. 
     U.S. Pat. No. 4,470,936 also discloses a sequential co-injection system using separate sprue channels for each melt stream, with each sprue channel being independently heated and converging to a common gate. In this system, a shuttle ball or swing gate switches the flow of material from one sprue channel to the other. This system also suffers from the potential for contamination between streams, such as described above for U.K Patent 1,369,744. This is a special concern as wear of the shuttle ball or swing gate is likely in normal use. 
     U.S. Pat. No. 5,651,998, assigned to the assignee of the present invention, discloses a method and apparatus for either sequential or simultaneous co-injection utilizing two opposing injection nozzles on the core and cavity sides respectively of the mold. Although effective, this arrangement requires an additional injection nozzle which must also receive resin from an injection unit on the opposite (movable) mold core half. This arrangement significantly increases the space requirements for the mold and may not be acceptable in some applications. 
     U.S. Pat. No. 5,125,816 is similar to U.S. Pat. No. 5,651,998 in that sequential co-injection is achieved by opposing gates on both the mold core and cavity respectively. However, in this arrangement the moveable mold half is fitted with slide cores containing tubular passages for feeding resin to one half of the molded part These slide cores move via hydraulic cylinders to define secondary mold cavities, which are in turn filled by gates on the opposing mold half This system suffers from disadvantages due to its complexity, the additional mold hardware requirements, including the aforementioned slide cores and additional injection nozzles, and the need for special manufacturing attention due to tight tolerances. 
     U.S. Pat. No. 3,873,656 shows a co-injection apparatus wherein at least two plastics are injected into a mold cavity through different gates, using a valve gating system. This design is only suitable for molding very large plastic articles. Also, the hot runner system taught does not have the capability for allowing separate temperature control of the different resin types, which inherently limits the variety of resins that can be used together in one system Furthermore, since the gates are far apart from one another, the flow of each resin will not be symmetrical throughout the part, but instead will be biased in the area of the gate. 
     U.S. Pat. No. 4,289,191 shows injection molding of molten wax into a precision metal die, wherein hollow parts are molded to extremely tight tolerances of ±0.012 mm. The wax stream flows from a nozzle having a central bore to a cavity or space formed between the nozzle tip, which has a relief channel, and the socket on the exterior of the die, and then into two or more separate sprue ports that feed into the mold cavity. Control of wax flow is accomplished by a retractable plunger in the nozzle which functions like a conventional valve stem. Although more than one sprue port is employed to supply material to the mold, these ports are downstream of the valve in the nozzle. Also, the valve stem obstructs the melt flow by being in the center of the melt stream, leading to weld lines. Finally, no provision is made for two or more separate resins to be injected through the two or more sprue ports, so this method cannot be used for co-injection purposes. 
     U.S. Pat. No. 5,645,874 shows a multiple gate noble in which each nozzle associated with a respective gate is equipped with an individual heater to allow independent thermal gating. In this arrangement a central flow passage feeds a plurality of radially extending branch passages leading to each respective gate, and as such, cannot accommodate multiple sources of resin or even sequential melt flow, and therefore cannot be used for co-injection purposes. 
     U.S. Pat. No. 4,702,686 shows a nozzle wherein a tapered plate divides a central flow channel into two partial channels prior to the nozzle tip and gate. This nozzle cannot accommodate the separate, different, resin sources require for coinjection purposes. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an novel apparatus and method for injecting at least two plastic materials into a mold cavity which obviates or mitigates at least one of the disadvantages of the prior art. 
     According to a first aspect of the present invention, there is provided a method of co-injecting at least two different plastic materials to form a multi-layer molded product using a hot runner injection molding machine with a separate channel for each material, each channel having an end in communication with a separate gate for feeding an injection mold, the method comprising: 
     (i) heating the plastic materials in their separate channels or storage areas; 
     (ii) injecting a selected amount of a first plastic material from a first channel through a first gate into the injection mold and preventing further flow of material from the first channel; 
     (iii) injecting a selected amount of a second plastic material from a second channel trough a second gate into the injection mold, said second gate being separated from said first gate by a gate separate means; 
     (iv) injecting a selected amount of a third material, said third material being selected from one of said first plastic material and any other plastic material, said third material being injected from its respective channel via its respective gate, said respective gate being separated from said second gate by a gate separating means; and 
     (v) moving a valve stem forward to close each said gate. 
     According to another aspect of the present invention, there is provided a method of co-injecting at least two different plastic materials to form an article having abutting portions of said different plastic materials using a hot runner injection molding machine with a separate channel for each different plastic material, each channel having an end in communication with a respective separate gate for feeding an injection mold, comprising the steps of: 
     (i) heating each different plastic material in its respective separate channel; 
     (ii) injecting a metered amount of a first plastic material from a first channel through a first gate into the injection mold and simultaneously injecting a metered amount of a second plastic material from a second channel through a second gate into the injection mold, said second gate being separated from said first gate by a gate separating 
     (iii) injecting a metered amount of a plastic material into tie injection mold from its respective individual channel through its respective gate; and 
     (iv) moving a valve stem to block all gates leading into the injection mold. 
     According to another aspect of the present invention, there is provided a method of co-injecting at least two different plastic materials to form a multi-layer molded product employing a hot runner injection molding machine with a separate channel for each material, each channel having an exit in communication with a respective separate gate for feeding an injection mold, the method comprising the steps of 
     (i) heating the plastic materials in their separate channels; 
     (ii) injecting a selected amount of a first plastic material from a first channel through a first gate into the injection mold and inhibiting further flow of material from said; 
     (iii) injecting a selected amount of a second material from a second channel through a second gate into the injection mold, said second gate being separated from said first gate by a gate separating means comprising a protrusion that engages a valve stem to support said valve stem; 
     (iv) injecting a selected amount of a third material, said third material comprising at least one of said first material and another material said third material being injected from its respective channel and its respective gate into said injection mold; and 
     (v) moving said valve stem to close at least one of said gates, each gate which is not closed by said valve stein being gated by thermal shut-off to inhibit the flow of plastic into the mold. 
     According to yet another aspect of the present invention, there is provided a hot runner injection molding apparatus for co-injecting at least two plastic materials into a forming mold, comprising: 
     a separate channel for each of said at least two plastic materials; 
     a separate heating means for each of said separate channels; 
     a separate gate for each of said at least two plastic materials, each said gate being in communication with a corresponding one of said separate channels 
     a valve stem movable between a first position wherein each said separate gate is open and a second position wherein each said separate gate is closed; and 
     a gate separating means comprising a protrusion separating each said separate gate from each other said separate gate, said protrusion co-operating with said valve stem to inhibit deflection thereof. 
     According to yet another aspect of the present invention, there is provided a hot runner injection molding apparatus for co-injecting at least two different plastic materials through separate channels to form a multi-layer molded product, each separate channel being independently heated and having an end in communication with a respective separate gate entrance into a forming mold, a gate separating means to prevent intermixing of the different plastic materials prior to exit at the gates, and a valve stem capable of longitudinal movement to permit and inhabit the flow of the different plastic materials through said gates, said gate separating means engaging a portion of said valve stem to inhibit lateral deflection thereof 
     The present invention provides a nose for plastic injection molding machines whereby flow disturbances and the resulting weld lines, which normally occur with known valve gate systems, are reduced Further, the present invention provides an injection system and method that employs relatively simple nozzle and hot runner designs. The present invention also provides a space-efficient, multi-material injection system for efficiently molding a plurality of articles in a multi-cavity mold. The present invention also provides an injection system and method wherein gates of different sizes can be accommodated in a single injection nozzle, each gate size being selected according to the viscosity of the particular plastic material flowing through it. 
     The present invention provides a novel method for runnerless injection molding, provided with a valve gate assembly, including at least two melt streams separated at the edge of the mold cavity by a gate separating means, a valve stem that is reciprocally movable and at least two gates that are opened and closed by the valve stem. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described; by way of example only, with reference to the attached Figures, wherein: 
     FIG. 1 is a sectional view of a hot runner-nozzle assembly for a mold cavity wherein two separate plastic materials fed to the nozzle tip and controlled by a single valve stem; 
     FIG. 2 is an expanded view of the nozzle assembly of FIG. 1; 
     FIG. 3 is a set of sectional views of a molded article detailing the layered wall structure after first, second, and third shots of plastic menial; 
     FIG. 4 a  is an end view of a nozzle assembly with three gates in accordance with the present invention; 
     FIG. 4 b  is a section of the nozzle assembly of FIG. 4 a , taken along line A—A of FIG. 4 a;    
     FIG. 4 c  is a section of the nozzle assembly of FIG. 4 a , taken along line B—B of FIG. 4 a;    
     FIG. 5 a  is a side view of a valve stem for the nozzle assembly of FIG. 4 a ; and 
     FIG. 5 b  is an end view of the valve stem of FIG. 5 a.   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 an embodiment of a valve gate assembly and injection nozzle in accordance with the present invention is indicated generally at  20  which is, in this embodiment of the present invention, a co-injection hot runner system which accommodates two plastic materials. One plastic material is provided from a source comprising extruder  24  and the other plastic material is-provided from a separate extruder (not shown) As used herein, different plastic materials are not intended to be limited to different material compositions, such as PET versus EVOH, but can also comprise, without limitation, materials with generally the same composition but different characteristics, such as PET in different colors or virgin PET versus recycled PET, foamed plastic materials versus non-foamed plastics, etc. 
     In this example, the portion of the hot runner system connected to extruder  24  is maintained at a temperature ranging from 500° to 550° F., the optimum processing temperature for a thermoplastic resin such as polyethylene teraphthalate, or PET, by suitable heaters in well-known fashion. Conversely, the portion of the system, illustrated in broken lines, which is connected to the second extruder is maintained at a different temperature, such as the range from 400° to 440° F., the optimum processing temperature for a thermoplastic resin such as EVOH. It is to be noted that the plastic materials selected and their optimum processing temperatures are merely examples of the present invention and their use in the present description is not intended as a limitation of the present invention. 
     A central manifold block  51  maintained at an operating temperature ranging from 500 to 550° F. by heating elements  52  and receives plasticized resin from extruder  24  through channels  53  and  54 . A spool or rotary, valve  56  is in circuit with channel  54  and operated by link mechanism  57 , and controls the charging of reservoir  58  of the shooting pot, or injection cylinder,  59  equipped with an injection piston or charging piston  61 . Valve  56  is formed with a transverse roughbore  62  and is shown in the closed position in FIG.  1 . 
     With reference now to FIGS. 1 and 2, reservoir  58  communicates with a nozzle assembly  64  via channel  63 . Heating elements  52  maintain the desired processing temperature of channel  63  as the PET or other plastic material progresses through to channel  90  of nozzle assembly  64  to a gate  76   a  As shown, gate  76   a  is separated from an adjacent gate  76   b  by a gate separating means. In a preferred aspect of the present invention, the gate separating means is in the form of a protrusion  86  that partially overlap central valve stem  83 , which is shown in the retracted position in these Figures. 
     This partial overlap of valve stem  83  and protrusion  86  inhibits any lateral alignment problems that might ordinarily occur where the stems moves longitudinally backwards and forwards over millions of injection cycles under very high injection pressures exceeding twenty thousand psi. While the overlap between protrusion  86  and stem  83  is preferred, it is not essential to the invention and, as will be understood by those of skill in the art, the gate separating means need not be a protrusion and can instead be any suitable barrier between the gates  76 . 
     As best seen in FIG. 1, a manifold segment  65  is secured to manifold block  51  and is heated by elements  66  to maintain optimum temperature (400° to 440° F.) in the hot runner connecting the second extruder (not shown) to channel  67  and to a reservoir  68  of a second shooting pot  69  which is equipped with an injection or charging piston  71 . Here again, a spool or rotary valve  72  (shown in the closed position relative to channel  67  in FIG. 1) controls charging of reservoir  68 . In the closed position of the spool valve  72 , reservoir  68  communicates with nozzle assembly  64  via a channel  70  through a cut-out  75 . When the spool valve  72  is open, channel  70  is closed and a link mechanism  85  operates to rotate valve  72 . 
     As shown in FIG. 2, nozzle assembly  64  includes a central spigot  73  in thermal contact with manifold block  51  immediately adjacent local heating elements  52  and spigot  73  is preferably fabricated from a good metallic thermal conductor such as beryllium copper. Spigot  73  is supported by minimal bearing surfaces  77 , 78 , best seen in FIG. 2, in a housing  79  and is spaced from spigot  73  along substantially its entire length to form an insulating air gap  81 . Air gap  81  inhibits conduction of heat from the spigot  73  to the housing  79  to maintain the desired process temperature, controlled by heating means  82 , as the plastic material, such as EVOH, progresses through channel  80  of housing  79  to gate  76   b.    
     The size of each of gates  76   a  and  76   b  can be selected as desired, largely independent of the other of gates  76   a  and  76   b , which is advantageous in situations where the viscosities of the different resin streams are significantly different or wherein a significantly larger amount of one material than the other is to be injected in an injection cycle. 
     Thus, it is apparent that the nozzle and valve gate and the hot runner system of the present invention is effective to maintain different optimum process temperatures appropriate to two different plastic materials from the source of the plastic materials to the nozzle gates. 
     As will be apparent to those of skill in the art because the plastic material is supplied to gates  76   a  and  76   b  via channels  80  and  90 , respectively, the plastic materials do not contact the majority of stem  83  and thus wear of stem  83  is reduced in comparison to conventional designs. 
     A preferred method of operation will now be described with reference to the PET and EVOH example described above. To prime the hot runner system initially, extruder  24  and the second extrude, including their respective co-operating shooting pots  59  and  69  are purged and the elders are moved into operative position relative to their respective manifolds. With valve stem  83  and spool valves  56  and  72  in the open position, shooting pot reservoirs  58  and  68  are charged with PET and EVOH material, respectively. Next, valve stem  83  is closed by a piston  84  and purged resin in the mold cavity is removed. 
     Thereafter the mold is closed and clamped, valve stem  83  is opened and the following sequence is performed. First, spool valve  56  is closed and injection piston  61  is advanced until it bottoms at the point indicated by the reference numeral  100 , discharging a measured amount of PET into the mold cavity through channel  63  and gate  76   a , which is separated from the adjacent gate  76   b  by a protrusion  86 . This constitutes the first shot of PET into the mold cavity, as shown schematically at F in FIG.  3 . 
     Piston  61  is held forward (in its bottomed position  100 ) blocking access to reservoir  58  to prevent backflow of PET compound from channel  63  into reservoir  58 . That is, the piston  61  is held bottomed to block access to reservoir  58  because upon subsequent operation of piston  71  to inject EVOH, the EVOH injection pressure would have a tendency to displace PET from channel  63  back into reservoir  58 . 
     Next, spool valve  72  is closed to the second extruder and opened to channel  70 . Injection piston  71  is moved until it bottoms at  101  and thus discharges a measured amount of EVOH into the cavity through channel  70  and gate  76   b . This constitutes the first shot of EVOH into the mold cavity (second shot of resin) to develop the three-layer wall as shown schematically at S in FIG.  3 . As will be apparent, the volume of the first and second shots of resin is less the total volume of the mold cavity. 
     Next channel  70  is closed by appropriate rotation of spool valve  72  and spool valve  56  is opened, allowing ever  24  to complete the filling of the mold cavity with PET and to pack the molded part while piston  61  remains bottomed, blocking access to reservoir  58 . This step constitutes the second shot of PET (third sot of resin) to develop a five-layer wall, as shown schematically at T in FIG.  3 . Thus, a five-layer wall structure is molded using two resins. 
     After packing is completed, valve stem  83  is moved forward to the closed position, where it blocks both gates  76   a  and  76   b  and piston  61  is now freed to move. Extruder  24  is operated to recharge reservoir  58  of shooting pot  59 , displacing piston  61  until it contacts an injection stop Sa, shown in FIG.  1 . The positioning of stop Sa controls and measures the amount of PET introduced intone reservoir  58 . 
     In similar fashion, the injection stop Sb controls and measures the amount of EVOH introduced into the reservoir  68 . During the course of packing the mold cavity, the reservoir  68  is recharged by opening spool valve  72  to allow the second extruder to displace piston  71  until the piston contacts its injection stop Sb, thus charging reservoir  68  with a measured amount of EVOH compound. After a suitable cooling interval, the mold is opened and the article is ejected by known means. The above cycle is can then be repeated, in continuous, automatic fashion, to generate additional layered articles. 
     It is also contemplated that articles comprising two or more layers of materials can be manufactured with the present invention, wherein one of the layers comprises a foamed material. For example, a first plastic material, such as a co-polymer of ethylene and vinyl acetate, can be injected into the mold to form the outer layer of the final article and a second plastic material, such as polypropylene, is then injected to form a foamed core. Another layer of the firs plastic material can then be injected to seal the foam material between the layers of the first material, much like a sandwich. It is also contemplated that the simultaneous injection of two or more different materials can also be performed with the present invention. This allows, for example, the manufacture of articles of PET-PEN resin blends. 
     As will be apparent to those of skill in the art the present invention need not be limited to nozzle and valve gate assemblies with only two gates and can instead include three or more gates, if desired. In another embodiment of the present invention, shown in FIGS. 4 a ,  4   b  and  4   c , a nozzle assembly is shown wherein three separate gates feed three different plastic materials into one mold cavity. In this embodiment, the gates  200 ,  204  and  208 , shown in FIG. 4 a , can be different sizes or the same size (not shown) and each gate is separated from the other two by a protrusion  212 , best seen in FIGS. 4 b  and  4   c.    
     FIG. 4 a  shows the pie-shaped arrangement of the three nozzle portions  216 ,  220  and  224  with insulating plates  228   a ,  228   b  and  228   c , made of a suitable material as will occur to those of skill in the art. Plates  228  separate each nozzle portion  216 ,  200  and  224  to maintain different thermal profiles for each plastic material type being carried to each gate  200 ,  204  and  208 , as dictated by the properties of particular materials. 
     FIGS. 5 a  and  5   b  show a valve stem  240  for the nozzle assembly of FIGS. 4 a ,  4   b  and  4   c  and the slot  244  which engages protrusion  212 , slot  244  being defined between pins  248 ,  252  and  256  which close respective ones of gates  200 ,  204  and  208  when stem  240  is advanced toward protrusion  212 . While the discussion above has only described a single stem in the nozzle assembly, it is contemplated that in some circumstances more than one valve stem can be employed in the assembly, each valve stem being individually actuated and gating one or more gates. 
     It is contemplated that in some circumstances both valve gating and thermal gating can be employed in a single nozzle assembly in accordance with the present invention. For example, as illustrated in FIG. 4 a  wherein gate  204  is much smaller than gates  200  and  208 , one or more gates can be much smaller, relative to the other gates, in the nozzle assembly and these smaller gates can be thermal gated in a conventional manner while larger gates, such as gates  200  and  208 , can be valve gated 
     It will be understood, of course, that modifications can be made to the embodiments of the invention illustrated and described herein without departing from the scope and purview of the invention as defined by the appended claims.