Patent Publication Number: US-2003224080-A1

Title: Automated pin for gas assisted injection molding

Description:
[0001] This Application is a Continuation-in-Part of U.S. patent application Ser. No. 09/553,807, Filed Apr. 21, 2000 
    
    
     
       TECHNICAL FIELD  
       [0002] The present invention relates generally to pressure assisted injection molding apparatuses and processes, and more particularly to a nozzle for injection of fluid in such an apparatus or process.  
       BACKGROUND OF THE INVENTION  
       [0003] Gas assisted injection molding of plastic has long been known in the industry. During gas assisted injection molding, molten plastic is forced into an enclosed mold, and gas is injected into the mold within the plastic material. The gas will raise the internal mold pressure and create an expanding gas pocket which will force the cooling plastic to the extreme recesses of the mold, maximizing the fill-out of the mold surface and reducing the sag of the plastic from the mold surface as the plastic shrinks during cooling, thus producing a better finished surface. In gas injection systems, there are two main methods of delivering gas into the mold cavity. The first is directly injecting the gas into the mold cavity, known as in article, while the second is injecting the gas into a channel leading into the mold, which is known as in-runner. The injection of the gas remotely into the cavity is generally preferred over the channel method.  
       [0004] Some more recent designs incorporate the use of both compressible and incompressible fluids in injection molding processes. Apparatuses and processes are known utilizing a multi-step process in which incompressible fluid is injected prior to injecting a compressible fluid. The incompressible fluid, for instance, water, substantially cools the plastic, lessening the time between molding cycles. In some systems, the injected fluid is actually used to drive molten plastic from the mold to a remote reservoir. In one system in particular, pressurized compressible fluid is injected through a nozzle following the injection of water. The mold is then fluidly connected to a low pressure space such as a reservoir or drain, and the water flows from the mold.  
       [0005] In many designs, the fluid/gas supply is positioned remotely from the mold, and is connected to the mold cavity with a supply passage. During injection of pressurized gas into the mold, a delay can occur while the supply system and mold cavity are pressurized to the desired level. The delay increases cycle time. Because injection molding tends to be a relatively high volume production process, designers are continually searching for ways to increase the number of molded parts that can be manufactured in a certain time.  
       [0006] Fluid is typically injected into the mold through a nozzle with a reciprocable rod for valving the fluid injection. In many such designs, the nozzle has an internal passage connectable to the mold cavity within which the rod reciprocates, and the rod is journaled by a portion of the nozzle housing. A significant challenge to designers has been overcoming the tendency of the nozzle to leak fluid through the housing around the area journaling the pin.  
       [0007] In addition to the foregoing concerns, further design challenges relate to the problems of plastic intrusion into fluid injection nozzles/pins during system operation. Gas or fluid injection nozzles are typically located near the plastic injection nozzle so that the fluid injected can best assist the flow of the plastic material through the mold. This, however, typically subjects the fluid injection nozzles to the flow of molten plastic at its most liquid state and highest pressure, which tends to clog or pack fluid injection nozzles. Further, fluid injection nozzles are often used as gas exhaust outlets, so that any molten material will tend to flow toward and into the outlet during the venting process. Cycle time of the molding process is critical to production cost, so venting before the interior of the part has completely cooled may be desirable, creating the potential for un-cooled material flow toward and into the fluid nozzle. Two approaches have typically been used to inhibit the flow of molten resin into the fluid nozzle: a valved fluid nozzle (e.g. U.S. Pat. No. 5,232,711), or an injection pin with very small orifices, which tend to resist the flow of the molten resin (e.g. U.S. Pat. No. 5,820,889). Another method employed to avoid the clogging of the gas supply passages with molten resin is to delay gas injection until the plastic injection is completed, as described in U.S. Pat. No. 5,295,800. However, this allows the plastic to cool somewhat, which reduces the flowability of the material, and reduces the efficacy and efficiency of the fluid-injection process.  
       [0008] The use of valved gas nozzles adds complexity and expense to the entire system. Because injection molding is a relatively high volume production process, such nozzles are subjected to repeated exposure to molten resin under pressure. A valved nozzle requires a reciprocating motion opposing the intrusion of plastic or overcoming the fluid injection pressure, a motion that requires a relatively large force which may lead to wear and failure of the valve and nozzle components. Since repairing or replacing such reciprocating nozzles or valves is time consuming and expensive in material cost and system down time, it is necessary to have a heavy duty but simple device. Exemplary reciprocating nozzles or pins are shown in U.S. Pat. Nos. 4,740,150; 4,905,901; 5,151,278; 5,164,200; 5,198,238 and 5,464,342.  
       SUMMARY OF THE INVENTION  
       [0009] In one aspect, the present invention includes an injection molding apparatus including a molding tool having a molding cavity. A source of pressurized gas is provided and is selectively connected to the mold cavity with a gas valve proximate the mold cavity. A reciprocable rod is provided and valve gas injection into the molding cavity.  
       [0010] In another aspect, the present invention provides a nozzle for an injection molding apparatus that includes a nozzle housing defining a fluid passage and an outlet. A rod extends through the fluid passage and is reciprocable therein to valve the outlet. The invention further provides an annular seal in the housing, the seal comprising a compressible elastomeric member and a resilient sleeve positioned about the rod. Compression of the elastomeric member deforms the sleeve, creating a fluid seal with the rod. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is a schematic illustration of a gas assisted injection molding apparatus in accordance with a preferred embodiment of the present invention;  
     [0012]FIG. 2 is a side view of an injection molding apparatus according to a second preferred embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION  
     [0013] In one aspect, the present invention is a gas injection nozzle pin assembly for a gas assisted plastic injection molding system as shown in the attached FIG. 1. The assembly  10  generally comprises a nozzle having a gas inlet port which communicates with a stored gas which is used to control the metering and flow of the gas into the nozzle. The nozzle includes a generally cylindrical body member  12 . Extending from the body member on one end thereof is a pin member  14  containing an elongated cylindrical bore  16 . The cylindrical bore has a conical nozzle end  18  which is used to mate with and accept an automatically controlled rod  20 . The rod extends along the entire length of the cylindrical bore and into the body member of the nozzle assembly. The rod includes a frustoconical shape  22  on its end such that it mates with the conical nozzle end  18  of the cylindrical bore  16  to create a specific outlet size for gas escaping into the interior of the mold assembly.  
     [0014] The rod is controlled by the use of an electromagnetic solenoid  24  or other type of electronic actuator which will use electrical power to control movement of the rod in and out of the conical nozzle end, based on a sequence of events occurring in the molding operation. The electronic actuator is located within the body of the nozzle assembly and is securely connected to one end of the rod. The electronic actuator is controlled via an electronic control assembly which is attached to the control unit for the gas assisted injection molding assembly. The electronic actuator is activated by introducing current through an electromagnetic coil which is attracted to magnetically conductive metal at an end of the body  12 . Additional electromagnetic inserts are preferably located at each end of the body  12  to increase the magnetic attractive forces, and to increase the return force when deactivated. It is preferred to have the device magnetically biased toward the closed position. A coil spring can also be utilized to bias the mechanism toward the closed position.  
     [0015] An alternate embodiment includes a pneumatically controlled actuator for reciprocating the rod. Still another embodiment includes a ball screw drive for driving the rod, which is provided with a threaded end.  
     [0016] The use of the electronically controlled rod will allow the operation of the valve at precise intervals during the plastic injection in the mold such that flow-back does not occur within the cylindrical nozzle bore member. The ease of operation of the electronic pin will also allow for quicker reaction times to an overflow condition that might occur in the nozzle of the cylindrical bore member. Furthermore, the use of the electronically controlled actuating rod will allow for a closed pin while injecting resin and an open large end to pass fluid when cleaning of the nozzle is necessary.  
     [0017] It should be noted that the embodiment disclosed above uses an electronic actuator to control the movement of the rod thus releasing gas during various stages of the gas assisted plastic injection molding operation. It will allow for various amounts of gas to be released depending on the size of the outlet opening created at the nozzle end by actuated movement of the rod in the chamber. It should be noted that any other type of electronic or mechanical switch that can be electronically controlled by the operator or a computer system may be used in controlling the movement of the rod within the nozzle assembly.  
     [0018] Referring to FIG. 2, there is shown an injection molding apparatus  110  according to a second preferred embodiment of the present invention. Apparatus  110  includes a fluid injection nozzle  112 , positioned adjacent and extending into a mold cavity  114 . Apparatus  110  further includes a pressurized gas supply  116 , which may be two discrete supplies, but is preferably a single supply for the entire system, and a source of incompressible fluid  118 . Both source  118  and  116  are fluidly connectable to mold cavity  114  via a fluid passage  120  in nozzle  112 . A reciprocable rod  122  is located in nozzle  112 , and reciprocates in passage  120 , valving fluid communications between passage  120  and mold cavity  114  with an enlarged distal portion  124 . Rod  122  preferably includes an enlarged proximal portion  126  that includes a pressure surface  127  exposed to fluid pressure in a chamber  130 . A fluid supply line  132  supplies fluid, preferably a compressible fluid, to chamber  130 , allowing the axial position of rod  122  to be adjusted by varying the fluid pressure therein. It should be appreciated that any known reciprocation means might be incorporated into apparatus  110  without departing from the scope of the present invention. For instance, rather than using compressible fluid in chamber  130  to reciprocate rod  122 , an incompressible fluid such as conventional hydraulic oil might be used. Similarly, an electrical actuator (not shown) could be used to reciprocate rod  122 , employing a solenoid and stator apparatus. A biasing spring (not shown) can also be disposed within nozzle  112  to bias rod  122  toward a retracted position, wherein distal portion  124  blocks fluid communications between passage  120  and mold cavity  114 .  
     [0019] In a preferred embodiment, nozzle  112  is operable to valve the injection of both compressible and incompressible fluids into cavity  114 . Source  118  is connectable to fluid passage  120  via a fluid supply line  134 . A first valve  136 , for example an electrically actuated valve, is preferably positioned adjacent nozzle  112 , and is actuatable to supply fluid, for instance water, to fluid passage  120 . Supply  116  is also connectable to fluid passage  120  via a second fluid supply line  138 , fluid communications being controlled by a second valve  139 . Because the position of rod  122  (controlled with pressure in chamber  130 ) controls fluid communications between passage  120  and cavity  114 , the present invention actually provides two means for controlling delivery of each fluid to cavity  114 . As with source  118 , the position of rod  122  controls fluid communications between passage  120  and cavity  114 , thus controlling at least in part fluid communications between supply  116  and cavity  114 . Those skilled in the art will appreciate that fluid pressures are preferably controllable at both supply  116  and source  118 , and rod  122  can be extended to open fluid communications with cavity  114  merely by raising the fluid pressure in passage  120  sufficiently, either with fluid from source  118 , supply  116 , or a combination of both. Increased fluid pressure on enlarged distal portion  124  can force rod  122  to its extended position, allowing fluid to flow into cavity  114 . Still further injection styles and sequences are possible with system  110 . For instance, a vacuum can be pulled on chamber  130 , biasing rod  122  toward a retracted position, while fluid pressure builds in passage  120 . When the fluid pressure has increased to the desired level, the vacuum can be relaxed, allowing pressure in passage  120  to drive rod  122  toward an extended position. Because passage  120  is pressurized prior to extending rod  122 , in such a process the initial burst of fluid into cavity  114  can take place at maximum pressure, reducing cycle time in many instances. Valve  139 , controlling delivery of compressible fluid, is preferably located proximate the mold cavity  114 . In a preferred embodiment, valve  139  is positioned directly adjacent the exterior of nozzle  112 , however, the valve could be positioned more remotely to nozzle  112  without departing from the scope of the present invention. The positioning of valve  139  adjacent nozzle  112  allows fluid in supply line to be delivered into nozzle  112  in a relatively highly pressurized state. By maintaining valve  139  in a closed state, fluid from source  116  can pressurize the entire supply line  138  upstream of valve  139 , reducing the time delay upon opening valve  139  before high pressure fluid is delivered to the mold.  
     [0020] A seal  140  is preferably positioned in nozzle  112 , and prevents fluid from leaking along rod  122  past the point where rod  122  extends into fluid passage  120 . In a preferred embodiment, seal  140  comprises a deformable sleeve  142  and a compressible elastomeric member  154 . Sleeve  142  is preferably formed from polytetrafluoroethylene (TEFLON®) or some other suitable low friction material, and is circumferential of rod  122 , having a match clearance therewith, although the interface might be looser without departing from the scope of the present invention so long as the sleeve is sufficiently deformable to make an essentially fluid-tight seal with rod  122 . Sleeve  142  preferably has an angular exterior surface  143 , against which compressible elastomeric member  154  abuts. Axial compression of member  154 , which is preferably an O-ring, causes member  154  to flatten slightly, squeezing inwardly/circumferentially against surface  143 . Consequently, sleeve  142  is radially inwardly deformed about rod  122 , forming a fluid-tight seal therewith. In the embodiment pictured in FIG. 1, member  154  bears against a plate  155 , integral with nozzle  112 , allowing axial compression of member  154  by axially urging plate  155  against member  154 , or alternatively, urging the components of seal  140  against plate  155 . An additional plate or some other type of stop is preferably placed to abut sleeve  142  opposite plate  155 , and assists in holding the components of seal  140  in place. Many different means for axially compressing member  154  may be employed. It is merely necessary that sleeve  142  may be securely positioned and member  154  axially compressed, thereby flattening and inwardly deforming sleeve  142 .  
     [0021] A typical injection molding cycle according to the present invention begins by injecting a quantity of fluent plastic into mold cavity  114 , preferably packing the cavity to as full a state and as great a pressure as possible. At this point, gas pressure in chamber  130  is relatively low and rod  122  is retracted, blocking fluid communications between passage  120  and cavity  114 . Valves  136  and  139  are preferably closed. Once the plastic has been introduced into cavity  114 , valve  136  is preferably actuated to fluidly connect supply line  134  with fluid passage  120 . Close to this time, gas pressure is preferably increased in chamber  130 , causing rod  122  to move toward an extended position and fluid, preferably water from source  118 , begins to flow past valve  136 , and through passage  120  into the mold cavity. The injected water forces still-fluid plastic to the furthest recesses of the mold and forces plastic against the mold surfaces, and cools the plastic in the mold relatively rapidly. In one embodiment, water drives some of the fluent plastic into one or more overspill reservoirs as it is injected. Once a suitable quantity of water has been injected, valve  136  is preferably closed. It should be appreciated that embodiments are contemplated in which rod  122  is not actuated apart from the injected fluid, which acts on enlarged distal portion  124  to extend rod  122  and initiate fluid communications with the mold cavity  114 . In a preferred embodiment, a quantity of gas is injected into the mold cavity  114  following injection of water. The mold cavity is packed relatively tightly with water and plastic, and the gas is therefore injected under pressure. The mold cavity is preferably substantially sealed, such that the aforementioned injection of gas creates a relatively small pocket or bubble of pressurized gas, increasing the overall internal mold pressure. After a desired dwell time has elapsed (if any), the mold cavity  114  is preferably fluidly connected to a low pressure space such as a fluid reservoir or drain. Fluid connection of cavity  114  to lower pressure allows the pocket of pressurized to begin to expand, expelling the water from the mold cavity and yielding a hollow molded plastic part.  
     [0022] By locating the gas injection port and its respective valve adjacent the nozzle, the present invention allows a short burst of relatively high-pressure gas to be injected into the mold cavity. In a preferred embodiment, the gas supply line connecting gas supply  116  to valve  139  is pressurized prior to a molding cycle. Thus, when valve  139  is actuated, there is already gas at a pressure suitable for injection proximate the mold. This design contrasts with earlier systems in which a gas valve is actuated remote from the mold, requiring the entire gas supply system from the source to the mold to be pressurized in order to inject create a sufficiently pressurized gas bubble in the water and plastic filled mold cavity. Moreover, the use of separate gas and water ports into nozzle  112  allows pressure to be maintained in the gas line during water injection, in contrast to designs wherein the gas and water are injected via a single nozzle. Thus, rather than a relatively long period of lower pressure gas injection, a relatively short period of higher pressure gas injection can occur. Allowing a relatively small quantity of pressurized gas to be “burped” into the mold in this fashion decreases injection molding cycle time.  
     [0023] The present description is for illustrative purposes only, and should not be construed to narrow the breadth of the present invention in any way. While various preferred embodiments have been disclosed herein, those skilled in the art will appreciate that alterations might be made to many aspects of the presently disclosed embodiments without departing from the scope and spirit of the invention, defined in terms of the claims set forth below. Other aspects, features and advantages will be apparent upon an examination of the attached drawing figures and appended claims.