Patent Application
20060138689
1. Field of the Invention
The present invention relates to an apparatus and method for molding a chair mat. More specifically, the present invention relates to an apparatus and method for compression molding a chair mat.
2. Background Information
The development of relatively low cost synthetic polymers such as nylon and polyester made the production of office carpet economically viable. Prior to such development, only a few executive offices had carpeting. However, with the development of such synthetic polymers, the need to have a hard surface for chair casters to roll on became imperative. Consequently, there was a need for a chair mat that was aesthetically pleasing and that would allow the carpet to be seen. Acrylic was the only viable material during the early stages of the development of chair mats. Initially, such chair mats were cell cast and later were manufactured by a continuous cast process. Acrylic sheets for chair mats were first produced by the cell cast method and later by a continuous cast process, whereby methyl methacrylate liquid monomer was polymerized directly into a sheet. These processes require very large capital investment. The cast sheets were sold to fabricators who would “router” the shape and impart a beveled edge to the resultant chair mat to reduce sharpness.
Subsequently, the production of Lexan polycarbonate sheets for glazing, signs and many other applications were being produced for a market that had historically been dominated by acrylics. Although the production of chair mats using polycarbonate was a logical extension, such polycarbonate was much more expensive on a pound basis, so there was a need to make the chair mat thinner. Also there was a need to fabricate the finished product and to market directly to large consumers and office supply companies. Although a lifetime warranty was offered against breakage, such polycarbonate chair mats did not perform as anticipated.
Additionally, experimentation with the extrusion of polycarbonate sheets was made but did not pose a serious threat to the production of acrylic chair mats. More specifically, polycarbonate resin was used to extrude sheet through a die into a roll forming station where the embossed or polished finished surface was formed. This station also functioned as a cooling point.
Flexible plasticized polyvinyl chloride (PVC) had been used to make carpet runners for high traffic areas. These carpet runners typically had “grippers” to keep the “runners” from sliding on the carpet, a problem that had plagued acrylic and polycarbonate chair mats. Another problem with such prior art chair mats was the tendency for the edges to warp upwardly, presenting a potential “tripping” hazard. Therefore, the manufactures of such chair mats would instruct users to rotate the chair mat periodically, turning the top over. With carpet runners, the PVC was a softer product and would lay flat and combined with the grippers, would stay in place. Consequently, manufactures of carpet runners developed the technique of extruding finished chair mats from PVC. The use of the softer PVC permitted the production of a chair mat having half the thickness of the corresponding acrylic chair mats. More particularly, PVC resin is mixed with stabilizers, plasticizers and other additives to achieve the desired softness. This mix (called a compound) in a powder form or pellet form, is extruded in much the same manner as polycarbonate (single screw extruder) or with multiple screw extruders for powder compounds. Two significant improvements in the process are: (1) the coextrusion of a softer strip in the center allowing the mat to be folded and (2) the design of the mat is engraved onto one of the rolls in the forming station, which gives a polished beveled edge versus one that is mechanically (router) produced.
All the aforementioned prior art plastic chair mats have have been produced via the cast or extrusion process.
U.S. Pat. No. 5,073,428 to Lancelot et al teaches the production of a foldable floor mat. Also, U.S. Pat. No. 5,048,182 describes the production of a chair mat using counter rotating rolls.
In the apparatus and method according to the present invention, a single or multiple screw extruder is used similar to the extruders used for extruding polycarbonate and PVC for the prior art chair mats, however no sheet forming die or roll forming station is used. Rather, “lumps” of accurately weighed melted thermoplastic polymer (starting with, but not limited to PVC) is extruded onto polymer transfer trays which are conveyed to forming presses where the top finish and bottom finish with grippers are produced. The ability to form a finished shape without the need to trim large amounts of excess results in significant labor, re-work, scrap and raw material reduction.
Therefore, it is a primary feature of the present invention to provide an apparatus for compression molding a chair mat that overcomes the problems associated with the prior art arrangements.
Another feature of the present invention is the provision of an apparatus for compression molding a chair mat that greatly reduces the production cost of chair mats.
A further feature of the present invention is the provision of an apparatus for compression molding a chair mat that reduces the amount of waste material generated during the production of chair mats.
Other features and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description of a preferred embodiment of the present invention contained herein.
The present invention relates to an apparatus for compression molding a chair mat. The apparatus includes an extruder for extruding a quantity of accurately weighed melted polymer. A polymer transfer tray is disposed adjacent to the extruder for receiving the quantity of accurately weighed melted polymer thereon. A forming press is disposed downstream from the extruder for compression molding the quantity of accurately weighed melted polymer. A conveyor is provided for moving the polymer transfer tray with the quantity of melted polymer thereon from the extruder to the press. The forming press includes a first mold for molding an upper impression of the chair mat. A second mold is provided for molding a lower impression of the chair mat. The first and second molds are movable relative to each other from an open disposition thereof for permitting loading of the quantity of accurately weighed melted polymer between the molds to a closed disposition of the molds in which the molds are moved towards each other for compression molding the weighed melted polymer between the molds.
In a more specific embodiment of the present invention the extruder includes a housing which has an upstream and a downstream end. The housing defines a chamber having a first and a second end. The first end of the chamber is disposed adjacent to the upstream end of the housing. An electric motor or hydraulic source drives a screw device that is rotatably disposed within the chamber and a hopper is disposed adjacent to the first end of the chamber. The arrangement is such that when the polymer is deposited within the hopper, the polymer is fed from the hopper through the first end of the chamber towards the second end of the chamber due to rotation of the screw device within the chamber so that the polymer flows from the chamber towards the downstream end of the housing.
Those skilled in the art will appreciate that a variety of extruder screw configurations may be employed as the reduced back pressure allows for more intensive mixing without the need for the pressure building required of a sheet die. Further, the extruder horsepower requirement is lessened by this reduced melt pressure requirement. Additionally, the reduction in pressure building reduces the degradation of thermally sensitive polymers such as PVC.
The apparatus further includes a weighing device which is disposed adjacent to the downstream end of the housing for accurately weighing the quantity of extruded polymer so that the weighing device meters the quantity of extruded polymer deposited on the polymer transfer tray.
Additionally, the polymer transfer tray includes a planar portion of generally rectangular configuration, the planar portion defining a peripheral edge. A ridge extends upwardly from the peripheral edge of the planar portion for containing the quantity of accurately weighed melted polymer when deposited on the polymer transfer tray.
Furthermore, the forming press is a hydraulic forming press. Alternatively, the press can be a pneumatically, electro/mechanically or other energy transfer forming press.
Also, the conveyor includes a robotic transfer arm. The arrangement is such that the conveyor moves the polymer transfer tray with the polymer thereon from the extruder to the forming press.
Moreover, the apparatus further includes a device for removing the polymer from the polymer transfer tray when the polymer transfer tray is disposed adjacent to or at the forming press such that the polymer is placed within the forming press between the two molds within the forming press.
Additionally, the first mold includes a movable platen. The first mold defines the upper impression of the chair mat. A plurality of guides extend from the movable platen to the second mold for guiding the movable platen and the first mold between the open and closed dispositions of the molds. A driving device is provided for moving the movable platen between the open and closed dispositions of the molds. The arrangement is such that when the quantity of accurately weighed polymer is disposed between the molds, the movable platen is urged by the driving device towards the second mold, the guides guiding the first mold into register with the second mold so that the polymer disposed between the first mold and the second mold is compression molded between the molds.
More specifically, the driving device includes a hydraulic, pneumatic, electro/mechanical or other energy device ram for urging the movable platen from the open towards the closed disposition.
Also, the driving device further includes a process control for controlling the ram such that the ram is selectively controlled to move the movable platen from the closed to the open disposition for permitting removal of the molded chair mat from between the molds.
The present invention also includes a method for compression molding a chair mat. The method includes the steps of extruding a quantity of accurately weighed melted polymer through an extruder and receiving the quantity of accurately weighed melted polymer onto a polymer transfer device such as a polymer transfer tray disposed adjacent to the extruder for transfer to the forming press.
The method also includes conveying the polymer transfer tray with the quantity of melted polymer thereon from the extruder to a forming press and moving a first and a second mold of the press relative to each other from an open disposition of the molds for permitting loading of the quantity of accurately weighed melted polymer between the molds. The molds are moved to a closed disposition for compression molding the quantity of weighed melted polymer between the molds.
Throughout the disclosure of the present invention, the term “conveyor” is to be construed as including preferably, a robot transfer arm but also in the alternative, a conventional conveyor or any other suitable material handling device. Also, throughout the disclosure of the present invention, it will be understood by those skilled in the art that with regard to the extruder, the term “a screw device” includes not only a single screw device but also a multiple screw device.
Furthermore, it is to be understood by those skilled in the art that, in the disclosure of the present invention, with regard to the press, the power source thereof in an alternative arrangement would be pneumatic, electro/mechanical or any other suitable energy device.
Accordingly, many modifications and variations of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description contained hereinafter taken in conjunction with the annexed drawings which show a preferred embodiment of the present invention. However, such modifications and variations fall within the spirit and scope of the present invention as defined by the appended claims.
Similar reference characters refer to similar parts throughout the various views of the drawings.
The extruder 12 includes an extruder screw or screws housed in a barrel or housing. The configuration of the screws may be varied to give the optimum level of mixing and temperature. The screws are driven by an electric or hydraulic source through a gear reduction unit to provide the necessary torque to rotate the screw or screws. More particularly, the screw or multiple screws are secured to a feed section that is affixed to the gear reducer that also houses a thrust bearing. The discharge end of the barrel has an adapter 48 to give a specific shape/volume output. Prior to the adapter, a screen changer to screen out impurities may be employed.
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Moreover, the apparatus 10 includes the push mechanism device generally designated 64 for removing the quantity 14 of polymer from the polymer transfer tray 16 when the polymer transfer tray 16 is disposed adjacent to the forming press 18 such that the quantity 14 of polymer is placed within the forming press 18.
Additionally, the first mold 22 includes a movable platen 66. The first mold 22 defines an upper impression 68 of the chair mat. A plurality of guides 70 and 72 extend from the movable platen 66 to the second mold 24 for guiding the movable platen 66 and the first mold 22 between the open and closed dispositions of the molds 22 and 24 as shown in
Additionally, the second mold 24, as shown in
More specifically, the driving device 74 includes a hydraulic ram 76 for urging the movable platen 66 from the open disposition as shown in
Also, the driving device 74 further includes a control 78 for controlling the ram 76 such that the ram 76 is selectively controlled to move the movable platen 66 from the closed disposition shown in
In operation of the apparatus 10 according to the present invention a chair mat is compression molded by extruding a quantity 14 of accurately weighed melted polymer through an extruder 12. The accurately weighed quantity 14 is received onto the polymer transfer tray 16 disposed adjacent the extruder 12. The quantity 14 of polymer is compression molded in the forming press 18 which is disposed downstream from the extruder 12. The polymer transfer tray is conveyed with the quantity of melted polymer thereon from the extruder 12 to the press 18. The molds 22 and 24 are moved relative to each other from the open disposition of the molds as shown in
The present invention provides a unique apparatus for compression molding chair mats which greatly reduces the amount of material needed to fabricate a chair mat.
