Abstract:
A mold tool includes upper and lower sections defining a cavity within which a plastic film preform is placed before injection of plastic forming a molded article. The plastic film extends between an interface defined between the sections of the mold. The interface secures a position of the plastic film within the mold tool preventing movement during injection of plastic. The interface is specifically configured to secure the plastic film without damage. The specific configuration of the interface is determined according to a relationship between molding process parameters and physical characteristics of the plastic film.

Description:
This application claims priority to provisional application Ser. No. 60/281,813 filed on Apr. 5, 2001. 

   BACKGROUND OF THE INVENTION 
   The invention relates to an assembly and method of holding plastic film within a mold. 
   In-mold decorating is a term of art describing the process of applying a decorative coating to a plastic part during the molding process. Conventional methods of applying a decorative coating include the use of a paint film placed within the mold that adheres to a plastic substrate injected into the mold during the molding process. A currently known alternative to the use of paint film is the use of a plastic film. The use of plastic film provides several advantages over the use of paint film. The plastic film can stretch to a greater extent than plastic film, allowing application to complex and intricate shapes. Further, plastic film provides the same color matching capabilities as do paint films. Finally, plastic film is formed of substantially the same base material as the substrate so removal of the plastic film is not required prior to recycling, as is required for paint based films. 
   Although plastic film includes many advantages over the use of paint films, the use of plastic film presents several design and processing challenges. Typically, plastic film includes at least three layers, a clear layer, a color layer and a substrate layer. The plastic film may include additional layers depending on the composition of the molded article to which the plastic film is adhered. The plastic film must be secured within the mold with sufficient force to prevent movement during molding at pressure, while not crushing the plastic film and the color layer. Crushing of the color layer may cause discoloration and tearing of the plastic film. 
   Accordingly, it is desirable to develop an apparatus and method of holding the plastic film within the mold with sufficient force to secure the plastic film in place against high molding pressure without causing damage to the plastic film. 
   SUMMARY OF THE INVENTION 
   An embodiment of this invention is a mold tool including first and second sections defining an interface for holding a plastic film during injection without damaging the plastic film. 
   The mold tool assembly includes upper and lower sections cooperating to define a mold cavity. A surface on each of the upper and lower sections about the perimeter of the cavity defines an interface when clamped together to secure the plastic film within the mold. The plastic film is preformed into a desired shape and inserted into the mold. A portion of the plastic film is trapped between the upper and lower sections of the mold within an interface. The interface is configured to apply sufficient clamping force to the plastic film to prevent the high pressure of plastic injected into the mold from blowing out the plastic film, without damaging the plastic film. 
   The interface is defined by upper and lower surfaces on the upper and lower mold sections. The plastic film is held within a gap between the upper and lower surfaces. Each of the surfaces also defines a width. The gap and width of the interface determine the amount of clamping force exerted on the plastic film when the mold sections are closed. The clamping force prevents plastic injected at high pressure into the cavity from shifting the plastic film during molding. The level of clamping force is balanced to prevent excessive application of force that can damage or crush the plastic film. 
   The balance of clamping force securing the plastic film within the mold includes properly dimensioning the gap and width relative to physical properties of the plastic film. The gap and width extend about the perimeter of the cavity and form the interface. The width and gap thickness are specifically adapted according to physical properties of the specific plastic film material used in each application. Physical properties of the plastic film material include compressive strength, flexure strength, compressive modulus, and elongation of the plastic film material. Each specific composition of plastic film includes unique physical properties used to determine the proper dimensions for the interface. 
   Further, specific molding process variables are considered for the determination of the specific dimension of the interface. A maximum clamping force is used along with other characteristics to determine the dimension of the interface. In addition, the specific molded part configuration along with the pressure of plastic injected into the mold dictates a clamping force required for a particular part that is also related to the physical properties of the plastic film to determine the specific dimensions of the interface. 
   The configuration of the mold tool of this invention provides proper application of clamping force on the plastic film to secure the plastic film, while not causing damage to the plastic film. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
       FIG. 1  is cross sectional view of a mold assembly according to this invention; and 
       FIG. 2  is an enlarged cross-sectional view of an interface between an upper section and a lower section of a mold for holding a plastic film according to this invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIGS. 1 ,  2  a mold tool  10  is shown and includes upper and lower sections  12 ,  14 . The upper and lower sections  12 , 14  of the mold  10  cooperate to form a cavity  20 . Surfaces  16 ,  18  disposed about a perimeter  32  of the cavity  20  form an interface  30  to secure the plastic film  26  within the mold  10 . Preferably, the plastic film  26  is preformed into a desired shape and inserted into the mold  10 . A portion  34  of the plastic film  26  is trapped between the upper and lower sections  12 , 14  of the mold  10  at the interface  30 . 
   Referring to  FIG. 2 , the interface  30  is defined by the upper surface  16  on the upper mold portion  12  and the lower surface  18  disposed on the lower mold portion  14 . The plastic film  26  is held within a gap  22  between the upper and lower surfaces  16 ,  18 . Each of the surfaces  16 ,  18  also defines a width  24 . The gap  22  and width  24  of the interface  30  determine the amount of clamping force exerted on the plastic film  26  when the mold sections  12 ,  14  are closed. The clamping force prevents plastic injected at high pressure into the cavity  20  from shifting the plastic film  26  during molding of a molded article  28 . The level of clamping force is balanced, preventing excessive application of force that can damage or crush the plastic film  26 . 
   The balance of clamping force securing the plastic film  27  within the mold  10  includes the steps of properly dimensioning the gap  22  and width  24  relative to physical properties of the plastic film  26 . The gap  22  and width  24  extend about the perimeter  32  of the cavity  20  and form the interface  30 , that provides the clamping force secming the plastic film  26 . The width  24  combined with the length of the perimeter  32  is determined to provide an area over which to apply the required clamping force. The gap  22  is sized in relation to the thickness of the plastic film  26 . The width  24  and gap  22  are specifically adapted according to physical properties of the specific plastic film material used in each application. A worker skilled in the art will recognize the physical properties of the plastic film and understand that such information is of common knowledge. Further, the specific units of each characteristic are not limited to those disclosed within this application. 
   Physical properties of the plastic film  26  include compressive strength, flexure strength, compressive modulus, and elongation of the plastic film  26 . Each specific composition of plastic film  26  includes unique physical properties used to determine the proper dimensions for the interface  30 . 
   Further, specific molding process variables are considered for the determination of the specific dimension of the interface  30 . The molding process includes specific parameters common to all molding configurations. The mold  10  is supported within a molding machine (not shown) that exerts specific amounts of clamping force. A maximum clamping force of the machine is used along with other characteristics to determine the dimension of the interface  30 . In addition, the specific molded part configuration along with a pressure of plastic injected into the mold  10  dictates a clamping force required for a particular part that is also related to the physical properties of the plastic film  26  to determine the specific dimensions of the interface  30 . 
   The specific relationship between characteristics of the plastic film  26  along with molding process parameters determine the specific width  24  and gap  22  of the interface  30  portion of the mold  10 . Minimum gap thickness is determined by a relationship between the minimum material thickness and the elongation properties of the plastic material. Preferably, the minimum gap thickness is determined by the following relationship: 
               Minimum   ⁢           ⁢   gap   ⁢           ⁢   thickness     =     m2     (     E   3     )               Equation   ⁢           ⁢   1             
 
wherein:
         m2=maximum thickness of the plastic film in inches; and   E=Elongation as a percentage of plastic film thickness.       

   Further, the maximum gap thickness is determined by a relationship between the film thickness, the compressive strength of the film material and the maximum injection pressure utilized in the process. This relationship is preferably expressed in the flowing equation: 
               Maximum   ⁢           ⁢   gap   ⁢           ⁢   thickness     =     m1     [     2   ⁢     (     i   L     )       ]               Equation   ⁢           ⁢   2             
 
wherein:
         m1=minimum thickness of plastic film in inches;   i=compressive strength of the plastic film (rupture or yield) (psi)   L=injection pressure of plastic into cavity of mold (psi)       

   The width  24  of the interface  30  is determined by a relationship between the clamping force of the press, the clamping force used in the process, the total length of the perimeter  32 , injection pressure of plastic into the mold cavity  20 , flexure strength of the film material, elongation properties of the plastic film, a compressive modulus of the film  26 , along with minimum and maximum thickness of the plastic materials. Preferably, the optimum, minimum and maximum width  24  of the interface  30  is determined by the below listed equations. 
               Optimum   ⁢           ⁢   width     =       [       (     H   *   j     )       (   f3   )       ]     *     (     E   100     )               Equation   ⁢           ⁢   3                 Maximum   ⁢           ⁢   width     =     (       H   *   i     f1     )             Equation   ⁢           ⁢   4                 Minimum   ⁢           ⁢   width     =         L   *     (     maximum   ⁢           ⁢   gap     )     *   H     n               Equation   ⁢           ⁢   5             
 
where
         f1=available clamping force of molding machine (lbs)   f2=clamping force required for the part (lbs)   f3=clamping force used in process (lbs)   H=total length of mold perimeter (inches)   i=compressive strength of film material (rupture or yield) (psi)   j=flexure strength (rupture or yield) (psi)   L=maximum injection pressure require to mold part (psi)   m1=minimum film thickness (inches)   m2=maximum film thickness (inches)   n=compressive modulus of film material (psi)   E=elongation as a percentage of thickness.       

   The resulting interface  30 , facilitates a desired amount of clamp force applied to the plastic film  26 . The specific gap  22  and width  24  of the interface  30  provides proper clamping force to the plastic film  26 . 
   The invention also includes a method of securing the plastic film  26  within a mold  10 . The method includes the steps of placing the preformed plastic film  26  into the mold cavity  20  and placing a portion of the plastic film  26  within the interface  30  between the mold sections  12 ,  14 . The interface  30  includes the gap  22  sized according a relationship between physical properties of the plastic film  26  and process parameters of the molding operation. The relationship used to determine the specific configuration of the gap  22  and width  24  provides proper clamping force application to the plastic film  26  within the mold cavity  20 . 
   The foregoing description is exemplary and not just a material specification. The invention has been described in an illustrative manner, and should be understood that the terminology used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications are within the scope of this invention. It is understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.