Abstract:
A method and an apparatus to form a sheet of polymeric material have first and second mold halves which clamp together on a polymeric sheet. A vacuum mechanism draws the sheet into an interior space. The vacuum mechanism has a plurality of controllable vacuum ports positioned about the interior space to direct the vacuum at specific portions of the sheet. Also, the sensing mechanism determines the draw depth of the sheet.

Description:
FIELD  
       [0001]     The present invention relates generally to forming polymeric material and, more particularly, to a vacuum system in a mold to form the polymeric materials.  
       BACKGROUND  
       [0002]     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.  
         [0003]     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 retain as high an impact strength as do other polymeric materials and thus, are less preferred for applications where impact strength is of importance.  
         [0004]     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, 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 by the protective coating which is applied regardless of the material used.  
         [0005]     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 to form 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.  
         [0006]     Applicant&#39;s prior application, now U.S. Pat. No. 6,733,714 issued May 11, 2004 and entitled Method for Forming High-Impact Transparent, Distortion-Free Polymeric Material, the specification and drawings of which are expressly incorporated by reference, illustrates an apparatus and method for forming distortion free polymeric materials. While this apparatus and method functions well for its intended purpose, designers strive to improve the art.  
       SUMMARY  
       [0007]     Accordingly, the present invention provides an apparatus to form a sheet of polymeric material. The apparatus includes a first mold half with a bottom wall and a first side wall defining a first interior space. The first sidewall has a first edge. A second mold half has a top wall and a second sidewall which defines the second interior space. The second wall has a second edge. The first and second halves clamp the sheet between the first and second edges. The vacuum mechanism draws the sheet into one of the first and second interior spaces. The vacuum mechanism includes a plurality of controllable vacuum ports. The plurality of controllable vacuum ports is positioned about the mold halves to direct the vacuum at specific portions of the sheet. Also, a mechanism is included to sense a draw depth of a sheet within one of the first or second interior spaces.  
         [0008]     The plurality of ports may include a mechanism to vary the vacuum at the plurality of ports to direct the vacuum at a specific portion of the sheet. Each port may include a valve. Ordinarily, the ports are positioned at the periphery of one of the first or second interior spaces. Also, a number of the plurality of ports may be positioned near the center of one of the first or second interior spaces.  
         [0009]     In accordance with a second aspect of the teaching, an apparatus to form a sheet of polymeric material comprises a first mold half having a bottom wall and a first sidewall defining a first interior space. The first side wall has a first edge. A second mold half has a top wall and a second sidewall defining a second interior space. The second sidewall has a second edge. The first and second halves clamp the sheet between the first and second edges. A vacuum mechanism draws the sheet into one of the first or second interior spaces. The vacuum mechanism includes one or more manifolds. The one or more manifolds include a plurality of ports. Each port has an independently controlled vacuum source to direct a vacuum at a specific portion of the sheet to draw the specific portion of the sheet into the first or second interior space. A mechanism to sense the draw depth of the sheet is within one of the first or second interior spaces.  
         [0010]     Further, the vacuum source may include two manifolds. One of the manifolds is positioned adjacent the periphery of one of the first or second interior spaces. One of the manifolds is positioned adjacent to the center of one of the first or second interior spaces. Each port on the manifolds includes a valve to independently control its vacuum source to direct the vacuum source at a specific portion of the sheet.  
         [0011]     Accordingly to an additional aspect of the teaching, a method to form a sheet of polymeric material comprises the steps of heating the sheet to a first temperature. The sheet is retained between the first and second mold halves with the sheet supported along the peripheral edge. A vacuum is generated on one side of the sheet thereby drawing the sheet into an interior space of one of the first or second mold halves. The vacuum is controlled at specific locations along the sheet in the interior space of one of the first or second halves so that a different vacuum pressure is created at the different locations to enhance drawing of the sheet into the interior space at the specific location. The sheet is cooled from the first temperature to a second temperature upon achieving a specific draw depth of the sheet within one of the first or second mold halves. A first vacuum is drawn adjacent at least a portion of the peripheral edge of one of the mold halves. A second vacuum is drawn adjacent a center of one of the interior spaces. The first vacuum is greater than the second vacuum. Further, a plurality of independent vacuums may be generated at specific locations along the sheet. The vacuums are controlled at the specific locations.  
         [0012]     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     DRAWINGS  
       [0013]     The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.  
         [0014]      FIG. 1  is a perspective view of a forming mold.  
         [0015]      FIG. 2  is a top plan view of a mold half with a vacuum system.  
         [0016]      FIG. 3  is a cross-section view through the mold halves along line  3 - 3  of  FIG. 2 .  
         [0017]      FIG. 4  is a cross-section view along line  4 - 4  of  FIG. 2 .  
         [0018]      FIG. 5  is a cross-section view along line  5 - 5  of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0019]     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.  
         [0020]     Turning to the figures, particularly  FIG. 1 , a forming mold  10  is illustrated with an upper and lower half  12 ,  14  which come together to form a heated sheet of polymeric material  16 . The sheet  16  is preferably an optical quality polycarbonate material and may be retained within a rigid frame  18 . The frame has a length and width somewhat greater than that of upper and lower halves. The frame clamps the sheet  16  about its periphery. Edges  22 ,  24  of the upper and lower halves  12  and  14  are contoured to define a desired end form for the peripheral edge of the sheet  16 . A vacuum is created by a vacuum system  30  within an interior space  26  of a lower mold half  14  to draw the sheet  1  downward to form the sheet as defined by the contoured edges  22  and  24  of upper and lower halves  12  and  14 . The drawing process ensues until the sheet is registered by a sensor eye  32  which, in turn, ceases the vacuum. A cooling mechanism  25  is disposed within an interior space  34  of the upper half  12 . The cooling mechanism  25  cools the sheet into its desired form.  
         [0021]     In an exemplary embodiment, the forming mold  10  is configured to form an aircraft windshield. As seen in  FIGS. 1 and 2 , the perimeter of the mold corresponds to the shape for particular application. It will be appreciated, however, that the 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  42  and four sidewalls  44 ,  46 ,  48 , and  50  defining the interior space  26 . The sidewalls  44 ,  46 ,  48 , and  50  have upper edges  52 ,  54 ,  56 , and  58 , respectively, and are selectively contoured along their lengths for defining the end form of the peripheral edge of the sheet  16 . The upper edges  52 ,  54 ,  56 , and  58  are preferably beveled, sloping downward towards the interior of the lower half  14 . The upper half  12  includes a top wall  62  and four sidewalls  64 ,  66 ,  68 , and  70  defining the interior space  34 . The sidewalls  64 ,  66 ,  68 , and  70  have lower edges  72 ,  74 ,  76  and  78 , respectively, and are correspondingly contoured along their lengths to engage the upper edges  52 ,  54 ,  56 , and  58 . The lower edges  72 ,  74 ,  76  and  78  are preferably beveled slipping downwards toward the interior of the lower half  14  for corresponding alignment with the beveled upper edges  52 ,  54 ,  56 , and  58 .  
         [0022]     The sheet  16  is retained in the molds by mechanism like those disclosed in Applicant&#39;s U.S. Pat. No. 6,733,714 issued May 11, 2004 and entitled. Method for Forming High-Impact Transparent, Distortion-Free Polymeric Material, the specification and drawings of which are expressly incorporated by reference. Also, a mechanism for trimming the polycarbonate sheet is like those disclosed in the patent.  
         [0023]     The vacuum system  30  is coupled with a vacuum source through the bottom wall  34  of the lower half  14 . The vacuum system  30  includes one or more manifolds. Here, the lower mold half includes three manifolds  92 ,  94 , and  96 . Each manifold is connected with a vacuum source which extends through the bottom of the mold half. The manifolds  92 ,  94 , and  96  include a plurality of ports  98 . The ports  98  include adjustable valves  100 . The valves  100 , as shown, are manually operated. The valves  100  are at a desired angle with respect to the manifolds. This enables “aiming” to the vacuum source at the sheet  16 . Alternatively, the valves could be solenoid valves and be operated by a computer. The valves  100  enable independent control of the vacuum source at each valve at a specific location or position along the sheet  16 . Thus, by adjusting the vacuum of each valve, it in turn enables the sheet to be drawn into the mold at different positions along the sheet  16 . This enables control of the direction of the vacuum to areas where the sheet must be drawn to deeper depths or where the surface shape of the sheet needs to be altered.  
         [0024]     Preferably, the manifolds  92  and  94  are positioned about the periphery of the lower mold half  14 . The manifold  96  is positioned near the center of the mold half  14 . The angle and direction of the valves  100  can be altered to fit a specific design of the to be molded sheet. It has been found that to draw the sheet into the mold in a preferred position, that the valves along the periphery are substantially wide open enabling full vacuum directed at the sheet. The valves on the center manifold  96  may be opened to less than halfway to draw the sheet into the desired shape of the specific position. Ordinarily, the valves in the center manifold may be opened about 40%. Thus, each port enables independent control of the vacuum source at that position along the sheet. Thus, the depth of draw can be controlled at a specific position along the sheet  16 .  
         [0025]     The sensor eye  32  may be positioned adjacent the manifolds  92 ,  94  and  96 . Ordinarily, the sensor is of the ultrasonic type which includes an eye to control the draw of depth. Also, a sensing system like that disclosed in Applicant&#39;s U.S. Pat. No. 6,733,714 issued May 11, 2004 and entitled Method for Forming High-impact Transparent, Distortion-Free Polymeric Material, the drawings and specification of which are herein incorporated by reference, could be used.  
         [0026]     A cooling mechanism may be disposed in the interior surface of the upper half  14  fixedly attached to the top wall. The cooling mechanism may be like those disclosed in Applicant&#39;s previous U.S. patent.  
         [0027]     A controller is provided and is in electrical communication with various components of the forming mold  10 , including the sensing mechanism  32  and the cooling mechanism. Depending upon the particular embodiment, the controller  200  may also be in electrical communication with laser or water jet trimming mechanisms for controlling their activity. The controller  200  controls the forming process as discussed in detail below.  
         [0028]     The present invention provides a method of forming the sheet  16  of polymeric material, preferably utilizing the above-detailed forming mold  10 . The method of the present invention will be described in detail. Initially, the sheet  16  is loaded into the frame  18 . The sheet  16  is heated in stages 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 the thickness of material used. Heating the sheet  16  in stages avoids possible blistering or other deformation of the surface of the sheet  16  that could otherwise occur.  
         [0029]     The sheet  16  is subsequently brought into the forming stage and placed on top of the lower half  14 , with a bottom surface  102  resting on the upper edges  52 ,  54 ,  56 ,  58  of the sidewalls  44 ,  46 ,  48 ,  50 . The upper half  12  travels downward in alignment with the lower half  14 , whereby the lower edges  72 ,  74 ,  76 ,  78  of the side walls  64 ,  66 ,  68 ,  70  engage an upper surface  104  of the sheet  16  thereby retaining the sheet  16  therebetween. The frame holds the perimeter of the sheet  16  in rigid form, whereby the sheet  16  is pulled and stretched as it is enclosed within the forming mold  10 . Concurrently, a retention mechanism provides a downward force, biasing the bottom surface  102  of the sheet  16  into tight engagement with the upper edges  52 ,  54 ,  56 ,  58  of the sidewalls  44 ,  46 ,  48 ,  50  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 . In an alternative embodiment, however, trimming may occur subsequent to the upper and lower halves  12 ,  14  closing. Alternatively, the trimming operation may be performed once sheet  16  has been formed.  
         [0030]     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 manifold ports  98  via valve  100 . The vacuum is achievable due to the airtight seal between the bottom surface  102  of the sheet  16  and the upper edges  52 ,  54 ,  56   58  of the sidewalls  44 ,  46 ,  48 ,  50 . The vacuum ports  98  can be directed at the sheet to ensure proper drawing of the sheet. Also, each port vacuum source can be controlled via its valve  100 . As a result, the sheet  16  is drawn downward by the controlled directed vacuum force into the interior space  26 , thus forming the desired shape. The sensing mechanism  32  senses when the sheet achieves a particular draw depth within the interior space  26 . Upon sensing the sheet  16  achieving the draw depth, the cooling mechanisms 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  200 , 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  18 , with excess sheet material, is also withdrawn, thereby leaving the formed sheet  16  accessible for removal from the forming mold  10 . A secondary clamping mechanism is used to grasp a perimeter edge of the sheet  16  and carry it through the remaining processes.  
         [0031]     Subsequent to the forming process, the frame and excess material are carried away 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, primer and coating stages at steps, respectively, and a second quality check. The first and second quality checks 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 preferably include priming via dip or spray, a primer drying sub-step, a hard coat application sub-step and a hard coat drying sub-step. It will be appreciated, however, that the herein described 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, the finished sheet  16  is packaged for customer delivery.  
         [0032]     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 .  
         [0033]     Although the supporting description herein describes 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.  
         [0034]     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.