Abstract:
A base sheet of transparent polycarbonate material has a formed shape including a center portion and a peripheral portion. The center portion is formed by a non-contact forming operation occurring at a temperature above the glass transition temperature and below the melt temperature of the transparent polycarbonate material, thereby resulting in the center portion of the base sheet being free of visual distortion. A top coat layer is disposed over the base sheet. The top coat layer has a greater resistance to abrasion than the polycarbonate material and provides UV protection to the base sheet. A primer coating is disposed between the base sheet and the top coat layer to facilitate bonding therebetween. The peripheral portion can have a shape created by a contact forming operation occurring at a temperature above the glass transition temperature and below the melt temperature of the polycarbonate material.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This is a divisional of an application filed Aug. 13, 2001 and assigned U.S. Ser. No. 09/928,827. 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   The present invention relates generally to forming polymeric materials and more particularly to a method and apparatus for forming distortion-free polymeric materials. 
   Polymeric materials are used in a wide variety of applications. Typically, polymeric materials are used to manufacture transparent panels such as windows or windshields for various applications including aircraft, automobiles, motorcycles, boats and the like. Such applications, especially those for aircraft, require an extremely clear, undistorted, transparent panel, which is resistive to scratching and impact in order to afford the pilot a clear view of the surroundings. 
   Traditionally, acrylic plastic is used to form such transparent panels. Acrylic plastic is noted for its excellent optical properties and weatherability, having outstanding resistance to the effects of sunlight and exposure to the elements over long periods of time. Subjected to long term exposure to the elements, acrylic plastic does not experience significant yellowing or any other significant changes in its physical properties. Acrylic plastic, however, does not have as high an impact strength as do other polymeric materials and thus, are less preferred for applications where impact strength is of importance. 
   Polycarbonate is a high-performance thermoplastic with the characteristics of high impact strength, optical clarity, heat resistance and dimensional stability. Polycarbonate, on the other hand, does not include the same weatherability characteristics of acrylic plastic. However, the transparent panels, whether produced using acrylic plastic or polycarbonate, include a hard protective coating to prevent scratching, abrasions or other markings that would reduce the service life of the transparent panel. Further, the hard protective coating protects the base sheet, whether acrylic plastic or polycarbonate, from UV degradation. As a result, the transparent panel is protected from any degradation, such as yellowing, abrasion distorting, and the like, even though the base sheet (e.g. polycarbonate) would otherwise degrade from such exposure. Therefore, it is desirable in the industry to use polycarbonate for producing transparent panels because of its high impact strength, while it remains protected from UV degradation and abrasion by the protective coating which is applied regardless of the material used. 
   Traditionally, polymeric sheets of acrylic plastic are formed using molds that include contoured upper and lower surfaces. The contoured surfaces define the desired shape of the polymeric sheet, directly contacting the entire upper and lower surfaces of the polymeric sheet. Because of the hardness of the upper and lower surfaces of an acrylic plastic sheet, it may be formed in this manner without distorting the upper and lower surfaces. However, the upper and lower surfaces of a polycarbonate sheet are not as hard and therefore, when heated, may be distorted upon contact during the forming process. For this reason, the use of traditional molds, which directly contact the upper and lower surfaces of the polymeric sheet, are not desirable for forming polycarbonate sheets. Traditional molds have increased potential for distorting the surfaces of the polycarbonate sheet, thus producing an increased number of rejected panels and driving up production costs. 
   Accordingly, the present invention provides an apparatus for forming a polymeric material, such as polycarbonate. The present invention enables forming of a polycarbonate sheet without distorting the key visibility areas of the sheet. The apparatus of the present invention provides a forming mold including a first half having a bottom wall and a first side wall defining a first interior space and a first edge and a second half having a top wall and a second side wall defining a second interior space and a second edge. The first and second halves come together to clamp the peripheral edge portions of a sheet of polymeric material therebetween for forming the sheet whereby the sheet is vacuum drawn into one of the first and second interior spaces. A cooling mechanism is disposed within one of the first and second interior spaces and a sensing mechanism is attached to one of the first and second halves for sensing a draw depth of the sheet within one of the first and second interior spaces. The first edge is preferably contoured for defining a final edge contour of the sheet and the second edge correspondingly contoured for facilitating engagement of the first and second halves. Further, the first edge is preferably beveled and the second edge correspondingly beveled for facilitating engagement of the first and second halves. 
   In a preferred embodiment, a trimming mechanism is provided for trimming a perimeter of the sheet to a desired shape. A retention mechanisms is also provided and operatively supported by one of the first and second halves for biasing the sheet into contact with one of the first and second edges of the first and second halves. 
   The present invention further provides an improved method for forming a sheet of polymeric material. The method of the present invention includes the steps of: heating the sheet to a first temperature, retaining a sheet between first and second mold halves of a forming mold, generating a vacuum on one side of the sheet thereby drawing the sheet into an interior space of one of the first and second mold halves, and cooling the sheet from the first temperature to a second temperature upon achieving a specified draw depth of the sheet within one of the first and second mold halves. The method preferably includes the step of detecting a draw depth of the sheet within one of the first and second mold halves for initiating the cooling. Alternatively, the heated sheet may be formed by use of blow air to exert a pressure on the other side of the sheet in lieu of the vacuum forming process or perhaps by use of a combination of both blow air and vacuum on opposite sides of the sheet. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of a forming mold in accordance with the present invention; 
       FIG. 2  is a top view of the forming mold of  FIG. 1 ; 
       FIG. 3  is a side view of a lower half of the forming mold; 
       FIG. 4  is a sectional view of the forming mold taken along line  4 - 4  of  FIG. 2 ; 
       FIG. 5A  is a detailed view of a retention mechanism of the forming mold; 
       FIG. 5B  is a view of an alternative embodiment of a trimming means; 
       FIG. 6  is a perspective view of the lower half of the forming mold having a finished polymeric sheet resting thereon. 
       FIG. 7  is a perspective view of the forming mold including an alternative trimming means; and 
       FIG. 8  is a schematic view of an exemplary processing line for forming the polymeric material. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   With reference to the Figures, there is shown a forming mold  10  including upper and lower halves  12 , 14  that come together to form a heated sheet of polymeric material  16  therebetween. The sheet  16  is preferably an optical quality polycarbonate material and is retained within a rigid frame  17  having a length and width somewhat greater than that of the upper and lower halves  12 , 14 , and that clamps about the complete peripheral edge of the sheet  16 . Edges of the upper and lower halves  12 , 14  are contoured to define a desired end form for peripheral edges of the sheet  16 . A vacuum is created within an interior space  18  of the lower half  14  for drawing the sheet  16  downward, thereby forming the sheet  16  as defined by the contoured edges of the upper and lower halves  12 , 14 . The drawing process ensues until the sheet  16  crosses a trigger point whereby the vacuum draw ceases and cooling mechanisms  20 , disposed within an interior space  22  of the upper half  12 , act to cool the sheet  16  in its desired end form. Alternatively, rather than drawing a vacuum in interior space  18 , the gas pressure in interior space  18  may be increased by supplying pressurized gas thereto to exert a forming force on sheet  16  or both a vacuum within space  18  and increased gas pressure within space  22  may be utilized to accomplish the forming operation. 
   In an exemplary embodiment, the forming mold  10  is configured for forming an aircraft windshield. As best seen in  FIG. 2 , the perimeter of the forming mold  10  is correspondingly shaped for the particular application. It will be appreciated, however, that the forming mold  10  can be configured to form sheets  16  into various shapes and contours in accordance with the requirements of a variety of applications. The lower half  14  includes a bottom wall  24  and four sidewalls  26 , 28 , 30 , 32  defining the interior space  18 . The sidewalls  26 , 28 , 30 , 32  have upper edges  34 , 36 , 38 , 40 , respectively, and are selectively contoured along their lengths for defining the end form of the peripheral edge of sheet  16 . The upper edges  34 , 36 , 38 , 40  are preferably beveled, sloping downward toward the interior of the lower half  14 . The upper half  12  includes a top wall  42  and four sidewalls  44 , 46 , 48 , 50  defining the interior space  22 . The sidewalls  44 , 46 , 48 , 50  have lower edges  52 , 54 , 56 , 58 , respectively, and are correspondingly contoured along their lengths to engage the upper edges  34 , 36 , 38 , 40 . The lower edges  52 , 54 , 56 , 58  are preferably beveled sloping downward toward the interior of the lower half  14  for corresponding alignment with the beveled upper edges  34 , 36 , 38 , 40 . The lower half  14  further includes an opening  60  for drawing air from the interior space  22 . In this manner, a vacuum may be created within the interior space  18  for forming the sheet  16 , as will be described in further detail below. 
   As seen in  FIG. 4  a series of retention mechanisms  62  are preferably included around the perimeter of the upper half  12  and are operatively disposed within the sidewalls  44 , 46 , 48 , 50  of the upper half  12 . As best shown in  FIG. 5A , the sidewalls  44 , 46 , 48 , 50  include a series of cavities  64  therein having openings  66  through the beveled lower edges  52 , 54 , 56 , 58 . The retention mechanisms  62  each include a retention pin  68  that is partially disposed within the cavity  64 . The retention pin  68  includes a pin portion  70  slidably disposed in and extending outwardly through the opening  66  and an enlarged diameter head  72  slidably disposed within the cavity  64 . The pin portion  70  includes a rounded end face  71 . The retention mechanism  62  further includes a spring  74  disposed between an upper face  76  of the cavity  64  and a top face  78  of the retention pin  68 . The spring  74  biases the retention pin  68  downward through the opening  66 . Also included is an access cover  77  for providing access to the cavity  64 . The access cover  77  runs the length of the cavity  64  and is held in position by a series of screws  79 . In this manner, the retention mechanisms  62  can be assembled into and accessed within the sidewalls  44 ,  46 ,  48 ,  50 . 
   The retention mechanism  62  retains the sheet  16  in position between the upper and lower halves  12 , 14  throughout the hereindescribed forming process, whereby the rounded end face  71  of the pin portion  70  is biased into contact with the sheet  16 . It will be appreciated, however, that the retention mechanisms  62  may be alternatively housed within the sidewalls  26 , 28 , 30 , 32  of the lower half  14 , whereby the spring  74  biases the retention pin  68  upward through the opening  66 . 
   A sensing mechanism  80  is provided and is mounted to the sidewall  28  of the lower half  14 . In accordance with a first preferred embodiment, the sensing mechanism  80  includes a laser  82 . The laser  82  selectively generates a beam of laser light  84  that travels across the interior space  18  of the lower half  14  and is reflected by a reflector  86 , fixedly attached to the side wall  32 . The laser  82  includes a sensor for sensing the reflected beam  84 . In accordance with a second preferred embodiment, the sensing mechanism  80  includes an optical sensor such as a video camera or the like. The beam emitted by the sensing mechanism  80  or the line of sight is positioned so as to be intersected and/or interrupted by the lowest most point of sheet  16  as it is formed to its finished shape. When this point is detected by sensor  80 , sensor  80  generates a control signal to discontinue the vacuum as well as to trigger a cooling stage, as will be described in further detail hereinbelow. 
   The cooling mechanisms  20  are disposed within the interior space  22  of the upper half  14 , fixedly attached to the top wall  42 . In a first preferred embodiment, the cooling mechanisms  20  include fans for circulating air through the interior space  22  of the upper half  14 . Alternatively, it is anticipated that the cooling mechanisms  20  may also include other air blowing or circulating means known in the art, such as blowing ducts and the like which may draw air from outside mold  10  or may include apparatus for cooling the air being circulated thereby. The cooling mechanisms  20  circulate cooling air for cooling the sheet  16  after forming, as described in further detail hereinbelow. 
   The forming mold  10  further includes trimming means  88  for trimming edges of the sheet  16  as defined by the external shape of the forming mold  10 . In a first preferred embodiment, the trimming means  88  includes a series of blades  90  fixedly attached about the perimeter of the upper half  12  by bolts  91 . The blades  90  extend downward past the lower edges  52 , 54 , 56 , 58  of the sidewalls  44 , 46 , 48 , 50  and include a sharpened leading edge  92 . As the upper and lower halves  12 , 14  come together to retain the sheet  16  therebetween, the blades  90  simultaneously cut through the sheet  16 , cutting away excess material and forming a perimeter of the sheet  16  as defined by the perimeter of the forming mold  10 . As shown the pin portion  70  preferably extends past the leading edge  92  so as to contact the sheet  16  prior to the engagement of the sharpened leading edge  92  therewith so as to insure it is securely retained in position during the trimming operation. Additionally, it should be noted that in a preferred embodiment, as shown in  FIG. 5B , there are a plurality of blades  90  along each edge with each leading edge  92  being angled relative to the surface of the sheet  16  so as to provide a series of progressive trimming sections along each side of sheet  16  and thus reduce the force required to accomplish same. 
   It is also anticipated that alternative trimming means  88  may be implemented for trimming the perimeter of the sheet  16 . Such means include a laser, a high-speed water jet, and the like. In such an arrangement, a laser trimming or water jet trimming mechanism may be provided to orbit the perimeter of the forming mold  10 , trimming away excess material as it travels. After the mold has been moved to a closed position. An exemplary embodiment of the alternative trimming means in detailed in  FIG. 7 . 
   A controller  100  is provided and is in electrical communication with various components of the forming mold  10 , including the sensing mechanism  80  and the cooling mechanism  20 . Depending upon the particular embodiment, the controller  100  may also be in electrical communication with laser or water jet trimming mechanisms for controlling their activity. The controller  100  controls the forming process as discussed in detail below. 
   The present invention provides a method of forming the sheet  16  of polymeric material, preferably utilizing the above-detailed forming mold  10 . With particular reference to  FIG. 8 , the method of the present invention will be described in detail. Initially, at step  200 , the sheet  16  is loaded into the frame  17 . The sheet  16  is heated in one or more stages, represented as steps  210 ,  220 ,  230 , until it is heated past a glass transition temperature, achieving a glass-transition stage, thereby becoming viscous or rubbery. It should be noted, however, that the sheet should not be heated to the point that it reaches a melting temperature, whereby the sheet would melt and become, scrap. The number of heating stages, their respective lengths and temperatures, may vary in accordance with the type of material and thickness of material used. Heating the sheet  16  in stages is believed preferable to avoid possible blistering or other deformation of the surface of the sheet  16  that could otherwise occur. 
   The sheet  16  is subsequently brought into the forming stage, at step  240 , and placed on top of the lower half  14 , with a bottom surface  102  resting on the upper edges  34 , 36 , 38 , 40  of the sidewalls  26 , 28 , 30 , 32 . The upper half  12  travels downward in alignment with the lower half  14 , whereby the lower edges  52 , 54 , 56 , 58  of the side walls  44 , 46 , 48 , 50  engage an upper surface  104  of the sheet  16  thereby forming the area around the periphery of the sheet  16  to the contour of edges  52 , 54 , 56 , 58  and retaining the sheet  16  between the upper and lower edges. The frame holds the perimeter of the sheet  16  in rigid form, and thus the sheet  16  is pulled and stretched as it is enclosed within the forming mold  10 . Concurrently, the retention mechanisms  62  provide a downward force, biasing the bottom surface  102  of the sheet  16  into tight engagement with the upper edges  34 , 36 , 38 , 40  of the sidewalls  26 , 28 , 30 , 32 , creating an airtight seal therebetween. Additionally, the edges of the sheet  16  are trimmed in accordance with the perimeter shape of the forming mold  10 . In accordance with the preferred embodiment, trimming of the sheet  16  occurs concurrently with the closing of the upper and lower halves  12 , 14 , whereby the blades  90  cut through the sheet  16  as the upper half  14  engages the upper surface  104  of the sheet  16 . In an alternative embodiment, however, trimming may occur subsequent to the upper and lower halves  12 , 14  closing, whereby a laser or water-jet trimming mechanism travels about the perimeter of the forming mold  10  or the knives may be movable relative to upper half  12  and employ a separate activating mechanism to perform the trimming operation. Alternatively, the trimming operation may be performed once sheet  16  has been formed by any one of a laser, water-jet or separately actuated knives. 
   Once the sheet  16  is retained between the upper and lower halves  12 , 14 , a vacuum is created within the interior space  18  of the lower half  12  by drawing air from the interior space  18 , through the opening  60 . The vacuum is achievable due to the airtight seal between the bottom surface  102  of the sheet  16  and the upper edges  34 , 36 , 38 , 40  of the sidewalls  26 , 28 , 30 , 32 . As a result, the sheet  16  is drawn downward by the vacuum force into the interior space  18 , thus forming the desired shape. The sensing mechanism  80  senses when the sheet achieves a particular draw depth within the interior space  18 . Upon sensing the sheet  16  achieving the draw depth, the cooling mechanisms  20  are activated for cooling the sheet  16  below its glass-transition temperature, thereby again achieving a rigid state. The vacuum is held at steady state during the cooling process and is not relieved until the sheet  16  is sufficiently cooled. The cooling time of the sheet may be monitored by the controller  100 , which controls each of the above-described activities. Once the sheet  16  is sufficiently cooled, the vacuum is relieved from the lower half  14  and the upper half  12  withdraws. The frame  17 , with excess sheet material, are also withdrawn, thereby leaving the formed sheet  16  accessible for removal from the forming mold  10 . This is best shown in  FIG. 6 . A secondary clamping mechanism  110  is used to grasp a perimeter edge of the sheet  16  and carry it through the remaining processes. 
   Subsequent to the forming process, the frame and excess material are carried away at step  250  for reprocessing of the excess material and the formed sheet  16  undergoes several finishing processes for producing an end product. These stages preferably include a first quality check, at step  260 , primer and coating stages at steps  270 ,  280 , respectively, and a second quality check at step  290 . The first and second quality checks  260 ,  290  are preferably achieved using optical means, such as a camera, for checking the polymeric sheet  16  for any distortion, scratches and/or abrasions. The primer and coating stages  270 ,  280  preferably include a wash substep, preferably with water, to remove any dust or other particles from the surfaces of sheet  16  followed by a drying stage and then priming via dip, flow coating or spray process, a primer drying sub-step, a hard coat application by dip, flow coating or spraying process sub-step and a hard coat drying sub-step. It will be appreciated, however, that the hereindescribed finishing processes are merely exemplary in nature and may be substituted for or further include any one of a number of other finishing processes commonly known in the art. Finally, at step  300 , the finished sheet  16  is packaged for customer delivery. 
   It should be noted that at least the primer and coating stages will be performed under strict temperature humidity and dust controlled conditions to ensure proper flow free coating of sheet  16 . The primer coat may be of any suitable material capable of providing a clear distortion free bond with sheet  16  and the top coat. At present, the preferred primer and top coating materials are experimental materials supplied by General Electric Co. applied by a flow coating process that are believed to offer an improved life span of 8-10 years which is significantly longer than currently available materials which may be utilized for this purpose. Preferably the primer and hard coat will be applied to both surfaces of sheet  16 . 
   Although  FIG. 8  and the supporting description herein, describe a generally linear processing line for forming polymeric material, it will be appreciated by those skilled in the art that the processing line may vary in layout. For example, it is anticipated that the processing line may be a rotary line, whereby the processing steps are generally organized as a circle. In this manner, the sheet  16  rotates about the circular layout through each of the processing stages for forming the finished product. 
   While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to provide the above-stated advantages, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.