Patent Publication Number: US-6221292-B1

Title: Apparatus and method for molding polymeric fibers into products

Description:
This is a divisional of application Ser. No. 09/053,106 filed on Apr. 1, 1998 and now U.S. Pat. No. 6,063,317. 
    
    
     TECHNICAL FIELD 
     This invention relates to an apparatus and method for molding polymeric fibers into products, such as pads or cushions for furniture, car seats, mattresses, and the like, and more particularly to the process conditions and equipment necessary to make products from polymeric fibers at a cost, quality, and performance level acceptable for commercial production. 
     BACKGROUND ART 
     The use of certain polymeric fibers, as opposed to primarily polyurethane foam, to make filled articles such as pads or cushions has been shown to result in improved performance characteristics. Compared with foam products, fiber-filled products may be more durable, have lighter weight, have greater permeability, be less costly, and be more readily recyclable. 
     A further improvement in the quality of fiber-filled articles was described in U.S. Pat. No. 4,940,502, wherein clusters of fibers, sometimes termed fiberballs, were used to create fiber-filled products. Fiberballs have a three dimensional structure which provides resilience upon deformation. From experiments reported in U.S. Pat. No. 5,169,580, fiberball cushions showed firmer support and higher resistance to repetitive compressions than batt cushions, even when the fiberball cushions were of a lower density. When molded, the forces which bond the fiberballs to each other are generally much weaker than the forces which resist compression of the individual fiberballs. This structure allows for resilience while also providing adequate local stiffness. 
     Although several patents, for example U.S. Pat. Nos. 5,169,580; 5,454,992; and 5,492,662, disclose processes and machinery for molding fiber-filled products, little attempt has been made to relate particular aspects of the molding method and apparatus to the performance and utility of the articles produced. 
     SUMMARY OF THE INVENTION 
     In order to be molded, fiber clusters must contain at least some thermoplastic fibers, defined as fibers capable of being repeatedly softened by heating, and hardened by cooling through a characteristic temperature range. Fiber clusters are often made from a mixture of high melting point matrix fibers and low melting point, thermoplastic binder fibers. The clusters are fused into a desired shape by placing the clusters in a mold formed with holes in its surfaces, then passing a heating fluid such as air through the mold. By using a heating fluid temperature which is above the binder fiber melting point, but below the matrix fiber melting point, substantially only the binder fibers soften during heating and the matrix fibers remain firm. The resulting product is cooled with a fluid which is cooler than the binder fiber melting point, thereby bonding the binder fibers such that the mixture retains the shape of the mold. 
     It is an object of the present invention to relate molding process conditions and machinery design to the performance of the fiber-filled products created therefrom. 
     Accordingly, an apparatus and method are provided for molding clusters of fibers, including at least some thermoplastic fibers, into products at a cost, quality, and performance level acceptable for commercial production. The clusters, which are typically shipped in a compressed state, are dedensified and then placed into an empty mold. The mold has apertures to facilitate the transfer of thermal energy into the mold. A conveyor is used to transport the mold through a plurality of process locations on an indexed, continuous, or semi-continuous basis. Included are one or more heating locations wherein the mold is heated, providing an influx of thermal energy sufficient to fuse the thermoplastic fibers to form a heated product. One or more cooling locations follow the heating location, wherein the mold and the heated product are cooled to produce a cooled product. The cooled product is then ejected from the mold. 
     In a preferred embodiment, parameters such as the heating fluid temperature, direction of heating fluid application, cooling fluid temperature, mold thickness, and mold aperture size have been selected which provide improved performance characteristics of the molded product. 
     The specific features and advantages of the present invention are more readily understood from a review of the attached drawings and the accompanying specification and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a molding method in accordance with the present invention; 
     FIG. 2 is a perspective view of a mold constructed in accordance with the present invention; 
     FIG. 3 is a top, schematic view of a linear conveyor and process locations utilized in the molding method of the present invention; 
     FIG. 4 is a end, cross-sectional view of a heating station provided in the present invention; and 
     FIGS. 5 a  and  5   b  depicts relationships of temperature vs. time for different sections of the mold during heating cycles illustrative of prior art methods (FIG. 5 a ) and a preferred method of the present invention (FIG. 5 b ). 
    
    
     BEST MODE(S) FOR PRACTICING THE INVENTION 
     The automotive seating and furniture industries, among others, test the performance of their molded products. Common tests include load vs. deflection plots to determine stiffness or softness, resistance to cyclic compressions which indicates fatigue resistance, resistance to constant compression at elevated temperature, known as heat set resistance, and surface texture. Through properly designed experiments, Applicant has determined certain aspects of the molding apparatus and method which improve these performance characteristics, disclosed as follows. 
     The present invention is an apparatus and method for molding products from a blend of at least two types of polymeric fibers: binder fibers and matrix fibers. The binder fibers, which are thermoplastic, possess a melting point separated from and lower than the matrix fibers. Preferably, the binder fibers have a melting point at least 55° C. lower than the melting point of the matrix fibers. In the present invention, binder fibers comprise ˜20% by volume of the fiber blend, but more or less may be used. 
     Binder fibers and matrix fibers may be made separately from relatively homogeneous polymers, or they may be combined within bicomponent fibers which have regions of different melting points. An example of the latter is a fiber with a matrix core having a high melting point, and a full or partial binder sheath having a lower melting point. Binder fibers and matrix fibers may be composed of any of a variety of natural or synthetic polymeric materials, including polyethylene, polypropylene, and polyester. 
     In a preferred embodiment of the present invention, the fibers used are Dacron® polyester fiberfill from E. I. du Pont De Nemours &amp; Co., Inc. Specifically, these are clusters of 0.75 in. long, hollow, bicomponent fibers which are 6 denier per filament. The matrix fiber core has a melting point of 260° C., and the binder fiber sheath has a melting point of 110 to 160° C., or some other temperature less than the matrix fiber melting point. In an alternative embodiment, a third type of polymeric fiber may be combined with the fiber clusters. In this embodiment, longer fibers are intertwined with the clusters prior to heating to increase the integrity of the molded product. 
     Clusters are typically transported in densely packed containers. As a result, the use of fiber clusters directly from the container might result an overly dense product. Consequently, the fiber clusters are typically dedensified, usually to less than 2 lb m /ft 3 . Such dedensification can be accomplished by a variety of lofting techniques including paddles, stirrers, blowers, agitators, or mixers. A preferred method is to pass the clusters through a centrifugal fan. 
     The method of the present invention is outlined in FIG.  1 . As described above, the fiber clusters are first dedensified manually or automatically, as shown in block  10 . Next, in block  12 , a mold is at least partially filled with the fiber clusters. The mold is then heated, as shown in block  14 , whereby an influx of thermal energy fuses the thermoplastic fibers to form a heated product. In block  16 , the mold and the heated product are cooled, thereby producing a cooled product molded to shape. Lastly, in block  18 , the cooled product is ejected from the mold. 
     The method of the present invention applies equally well to batch or continuous molding processes, and further details of the method will be given in the description that follows. 
     Referring now to FIG. 2, a mold  20  constructed in accordance with the present invention is shown. Mold  20 , which may be of any shape, comprises a plurality of walls  22 , a base  24 , and a mold cover  26 . Preferably, base  24  and cover  26  each have apertures  28  of any shape that allow the passage of fluids therethrough. In an alternative embodiment, walls  22  may also be provided with apertures  28 . Apertures  28  preferably provide an open area of over 25 to 35%, and may be arranged in a straight row or staggered pattern. Apertures  28  may be made by a mechanical process, such as perforating, expanding, or etching, by casting, by explosive bonding, or by any other process that creates holes. 
     Prior art molds have typically been constructed of sheet metal having a thickness of 1.5 mm (16 gage) or larger, and formed with circular apertures with a diameter of 3 mm or larger. In the present invention, sheets of metal thinner than 1.5 mm are preferred to construct mold  20 , with apertures  28  of a diameter less than 3 mm formed therein. A series of experiments has confirmed the durability of thinner sheets of metal to repeated molding cycles. In addition, Reynolds numbers were calculated which indicated that flow through the smaller apertures was still laminar and sufficient for molding. When implemented, the mold construction of the present invention improved performance characteristics such as the heat set resistance and fatigue resistance. By providing a smaller heat sink, the thinner metal also decreases the energy used in the molding process as well as decreasing the necessary molding time. Furthermore, the smaller apertures improve the surface texture of the molded product as less fiber tends to stick through the apertures to form dimples in the finished product. 
     Mold  20  can alternatively be constructed from materials with less heat capacity than steel, aluminum, or other metals. Lower heat capacities result in less energy being absorbed by mold  20 , resulting in lower molding costs. Mold  20  can be made of non-metallic materials, including ceramics and polymers, particularly those that can be welded. 
     Referring again to FIG. 2, a frame  30  holds mold  20  and allows the shape of mold  20  to be altered within the dimensions of frame  30 . Alternatively, frame  30  may contain several molds  20 , such that more than one product may be molded at a time. A plurality of frames  30  may be used simultaneously so that a continuous or semi-continuous molding process can be achieved. Prior art molding systems, such as that disclosed in U.S. Pat. No. 5,454,992, utilize a carousel arrangement for moving molds  20  through the various process locations. In the present invention, molds  20  are moved through process locations using a linear conveyor  32  to which a plurality of frames  30  may be affixed. Conveyor  32  provides a more economical system than belt or carousel systems due to quicker, higher volume throughput. 
     Conveyor  32  may be of the indexed type, wherein molds  20  are moved from one process location to the next with a predetermined time interval. In an alternative embodiment, a continuous conveyor may be used. In this embodiment, molds  20  are passed through continuous heating and cooling zones, as opposed to keeping molds  20  fixed relative to the fluid flow mechanisms. 
     Shown in FIG. 3 is a molding apparatus, including conveyor  32  and the plurality of process locations, or stations, constructed in accordance with a preferred embodiment of the present invention. In this embodiment, linear conveyor  32  is of the indexed type, comprising two transfer conveyors  34  and  36  and two shuttle conveyors  40  and  42  to allow operation of the molding apparatus within a relatively compact space. If necessary, a third shuttle conveyor  38  may be used to move mold  20  to transfer conveyor  34  as shown. As stated above, conveyor  32  may carry a plurality of frames  30  such that many molds  20  can be circulated through the various process stations on a semi-continuous basis. It will be appreciated that the disclosed linear conveyor  32  may be of any length necessary to suit the manufacturing requirements. 
     First, mold  20  is at least partially filled with fiber clusters at filling station  44 . Mold  20  is then transported via shuttle conveyor  38  to a mold closing station  46  on transfer conveyor  34 , where mold cover  26  is manually or automatically secured to mold  20 . Next, mold  20  is moved via shuttle conveyor  40  to a heating station  48 . Transfer conveyor  36  then moves mold  20  to a cooling station  50 . As shown, an additional cooling station  52  is preferably incorporated prior to transporting mold  20  to an ejection station  54  via shuttle conveyor  42 . Two mold ready stations  56  and  58  allow for time delays between closing station  46  and heating station  48  and between cooling station  52  and ejection station  54 , respectively. 
     Placement of fiber clusters into mold  20  can be accomplished in several ways. Clusters can be manually loaded into mold  20  at filling station  44 , but preferably clusters are introduced into mold  20  automatically by a transport device  60 . Tests determined that a standard textile feed apron can transport fiber clusters without significant degradation, such that acceptable performance properties are retained in the subsequently molded material. Alternatively, a screw feeder of increasing pitch in the direction of transport may be used to transport fiber clusters into mold  20 , or clusters may be fed by gravity into mold  20  from a hopper. A DIAMONDBACK® hopper provides gravity flow without bridging over at the discharge like a standard conical hopper would at the same discharge size. Transport device  60  preferably has an automatic weighing apparatus  62  attached thereto for weighing the clusters prior to filling mold  20 . Alternatively, a mass flow measurement of fiber clusters may be obtained using X-rays, active acoustic, or passive vibration monitoring. 
     Clusters are placed in mold  20 , typically to a height in excess of twice the height of the finished molded product. Preferably, the fibers are interlaced manually or mechanically by means for interlacing, such as means for mixing, moving, shaking, imparting or vibration before mold  20  is closed at closing station  46 . Alternatively, an electrostatic charge can be used to create a more uniform distribution of clusters prior to closing mold  20 . 
     The clusters may be compressed beyond the pressure necessary to close mold  20  in order to improve performance of the molded product. Experiments were conducted to determine the effect of multiple compressions of the fiber clusters, both before heating and while the mold material was at least partially above the melt point of the binder fiber. The performance of the molded product was relatively little affected by dwell time and the speed of compressions. However, the heat set resistance, fatigue resistance, and stiffness of the products were significantly affected by the number of compression cycles and the pressure of the compressions. In the present invention, the fiber clusters may be compressed more than once during the molding process under a pressure greater than 25 lb/ft 2 . 
     At heating station  48 , which is illustrated in FIG. 4, a heating fluid, typically air, is passed through mold  20  for a time long enough to allow fusing of the binder fibers. Heating station  48  includes a heater  64  for heating the air to a temperature sufficient to soften the binder fibers. Heater  64  is in communication with a fan  66  which is rotatably driven by a motor  68 . Fan  66  directs air toward mold  20  through a duct  70 , and receives air from mold  20  through a duct  72 . 
     Heating station  48  includes a plurality of dampers  74 ,  76 ,  78 ,  80 , and  82  which are selectively opened and closed in order to direct air flow through or around mold  20 . To force air through the top  84  of mold  20 , dampers  76  and  80  are closed, and dampers  74 ,  78 , and  82  are opened. Air moves from fan  66  into duct  70 , through damper  74  into a duct  86 , through damper  78  into a duct  88 , through mold  20  into a duct  90 , through damper  82 , and back to fan  66  through duct  72 . To force air through the bottom  92  of mold  20 , dampers  74  and  82  are closed, and dampers  76 ,  78 , and  80  are opened. Air moves from fan  66  into duct  70 , through damper  80  into duct  90 , through mold  20  into duct  88 , through damper  78  into duct  86 , through damper  76 , and back to fan  66  through duct  72 . 
     U.S. Pat. No. 5,169,580 describes a molding machine equipped to provide heating fluid flow which is initially up through the mold and is then reversed to be blown downward through the mold halfway through the heating cycle. This method was used to attempt to ensure uniform bonding of the fibers. In the present invention, experiments were performed in which thermocouples were placed in the center, near the top  84 , and near the bottom  92  of mold  20 . A graph of time vs. temperature from each thermocouple is shown for a reversal in the direction of air flow halfway through the heating time in FIG. 5 a , and one-third of the way through the heating time in FIG. 5 b . Concentrating on the portion of the graphs above the binder fiber melting point, the area under each curve was calculated. The results indicated that the greatest uniformity in heating between the three areas of the mold was observed when the direction of flow was reversed at a point less than halfway through the molding cycle, as in FIG. 5 b , and this is the method preferred in the present invention. 
     In a preferred embodiment of the present invention, the heating fluid has a temperature less than 55° C. over the melting point of the binder fiber. This constraint on the heating fluid temperature was shown through experimentation to improve both the fatigue resistance and heat set resistance of the molded products. Preferably, the heating fluid flow rate is between 70 and 110 ft/min. 
     Referring again to FIG. 4, ducts  88  and  90  are preferably centered symmetrically over the top  84  and bottom  92  of mold  20 , respectively, such that air flow is directed perpendicularly to top  84  or bottom  92  of mold  20  and is uniformly distributed across the entire mold  20 . An alternative embodiment allows air to be directed through walls  22  of mold  20  as well. In a preferred embodiment, less than 6% of the air flow is allowed to bypass mold  20  and flow between mold walls  22  and frame  30 . To save energy, the hot air can be recovered, reheated to the working temperature, and recycled. While conveyor  32  is in the process of moving a new mold  20  into heating station  48 , dampers  78 ,  80 , and  82  are closed, and dampers  74  and  76  are opened. Air flows from fan  66  into duct  70 , through damper  74  into duct  86 , through damper  76 , and back to fan  66  through duct  72 . 
     After heating, mold  20  is transported to cooling station  50 , which is arranged similarly to heating station  48  shown in FIG. 4. A cooling fluid having a temperature below the melting point of the binder fiber is passed through mold  20 , either by force or by natural convection. With forced convection, the air flow can be through the top  84  of mold  20 , the bottom  92  of mold  20 , the walls  22  of mold  20 , or a combination of these directions. 
     A series of experiments was performed in which it was determined that the slower the product is cooled, the better are certain performance properties. The molded product can be cooled more slowly by slowing the flow rate of cooling fluid, increasing cooling fluid temperature, or both. In the method of the present invention, the flow rate of cooling fluid is between 20 and 40 ft/min, much lower than the preferred heating fluid flow rate. Alternatively, the molded product can be cooled more slowly using natural convection. In order to lengthen the cooling process, additional cooling station  52  is incorporated along transfer conveyor  36 . In addition, the cooling fluid temperature is preferably at or above a typical ambient temperature of 15° C., but below the melting point of the binder fiber. 
     As with heating station  48 , a substantial part of the energy can be recovered from cooling stations  50  and  52  and channeled to oven  64  using an external air duct (not shown). Once the product has cooled enough so that the polymeric bonds have solidified, mold  20  can be opened manually or automatically at ejection station  54  and the cooled product will retain its shape. Densities of the molded structures will generally be on the order of 1 to 5 or more lb m /ft 3 . 
     In addition to cushions and pads, it is understood that molded fiber-filled products produced by the apparatus and method of this invention may include acoustic attenuators, spacers, filters, and other end-use applications. 
     It is understood, of course, that while the form of the invention herein shown and described constitutes a preferred embodiment of the invention, it is not intended to illustrate all possible forms thereof. It will also be understood that the words used are words of description rather than limitation, and that various changes may be made without departing from the spirit and scope of the invention disclosed.