Abstract:
In a process for welding plastic parts together, an IR shield redirects IR radiation emitted from a heater of a heating tool so that IR radiation which would otherwise be directly incident on certain portions of a plastic part and/or on other parts of the heating tool is avoided.

Description:
REFERENCE TO RELATED APPLICATION AND PRIORITY CLAIM 
       [0001]    This application claims the priority of Provisional Patent Application No. 62/366,178, filed 25 Jul. 2016, the entire content of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to welding of plastic parts. 
       BACKGROUND 
       [0003]    Plastic parts are commonly manufactured by a molding process, such as blow molding or injection molding, and then further processed by one or more operations such as boring, drilling, and/or welding. 
         [0004]    Welding is a process for joining plastic parts together by melting plastic at a location in each part where a weld joint is to be created, then placing the parts together so that their melt pools merge together, and then allowing the melt pools to solidify and create the finished weld joint. 
         [0005]    One type of plastic welding known as contact welding is performed by a heating tool which has a heater element, a hot plate for example, which is heated by a heat source. Thermally conductive parts of the tool which are in contact with the heater element are heated by thermal conduction of heat from the heater element. Other parts not in direct contact with the heater element are exposed to heat radiated by either the heater element and/or parts which have been heated by thermal conduction. A portion of the heater element has a feature which is placed in direct contact with a plastic part to melt plastic at a location where a weld joint is to be created. 
         [0006]    Another type of plastic welding known as non-contact welding is performed by a heating tool which has a part which is either a heater, or a heater element heated by a heater, and which is placed a short distance from a surface portion of a plastic part where a weld joint is to be created to radiate heat which melts the plastic at that surface portion. 
         [0007]    In both contact welding and non-contact welding, radiated heat lies in the infra-red (IR) band of the electromagnetic spectrum and may comprise wavelengths ranging from short to long. Various types of IR heater elements used in non-contact welding include IR bulbs, wire foil heaters, and ceramic heaters. Even a hot plate heater may be used for non-contact welding if not placed in direct contact with a plastic part. 
         [0008]    Because one or more parts of a plastic welding heating tool emit IR radiation, and because that radiation may disperse to some degree before arriving at a surface portion of a plastic part which is to be melted, some IR radiation may be incident on surface portions of a plastic part beyond a portion which is to be melted, and may have an undesired effect on the part. 
       SUMMARY OF THE DISCLOSURE 
       [0009]    This disclosure relates to heating tools for melting plastic of plastic parts by either contact welding or non-contact welding, characterized by mitigation of dispersion of IR radiation beyond an intended location at which a weld joint is to be created, and to a related method for mitigating dispersion of IR radiation beyond an intended location for a weld joint. Operating efficiency of a heating tool may be increased by recovery of dispersed IR radiation. 
         [0010]    A plastic welding heating tool comprises a heater, and/or a heater element heated by a heater, for melting plastic of a plastic part where a weld joint is to be created. An IR radiation shield is mounted on the heating tool and is shaped to selectively shield surfaces of the heating tool and/or of the plastic part from receiving incident IR radiation. The shield confronts one or more unshielded zones of the tool and/or plastic part, leaving one or more other zones unshielded. IR radiation is directly incident on an unshielded zone. The shield redirects incident IR radiation by reflecting a significant percentage of IR radiation which would otherwise be incident on a shielded zone. The direction of reflection is a function of the angle of incidence of IR on any particular area of the shield surface. IR radiation may be reflected fully or partially back toward its point of origin, allowing energy of the reflected radiation to be recovered by the heating tool. The energy recovery is additive to the thermal energy which is being added by the heater so that the sum of the energies maintains desired temperature of the heating tool for melting plastic. That capability enables the quantity of thermal energy being added by a heater to be significantly reduced from what it would be in the absence of the shield. The result is a more energy-efficient method for both non-contact welding and contact welding of a plastic part. 
         [0011]    If IR radiation is directly incident on portions of the surface of a plastic part beyond a surface portion where a weld joint is intended to be created, the plastic part may be affected in undesired ways, such as distortion, discoloration, etc. If the plastic part in which the weld joint is being created is one part of a multi-part assembly, dispersed IR radiation may be incident on other parts of the assembly, both plastic and non-plastic, and those other parts may be affected in undesired ways. Use of an IR shield can avoid such undesired consequences. 
         [0012]    Emissivity is a parameter which defines a material&#39;s ability to emit IR radiation. The emissivity of any particular material lies within a numerical range between 0.0 and 1.0. In physics, a material which absorbs 100% of incident IR radiation is called a “black body”. A “black body” has an emissivity value of 1.0. The opposite of a “black body” material is a material which reflects 100% of incident IR radiation, and such a material has an emissivity value of 0.0. Most materials have emissivity values between those two extremes. A shield which is used as described here for plastic welding reflects a significant percentage of IR radiation as recoverable energy and has an emissivity value closer to 0.0 than to 1.0. 
         [0013]    Some examples of materials suitable per se for use as a shield, or as a coating for a shield, and their emissivity values (e), are: polished titanium (e=0.1); polished aluminum (e=0.1); gold (e=0.05); titanium nitride (e=0.1); silver and silver coating polished (e=0.02); zinc and zinc coating polished (e=0.05); chromium and chromium coating polished (e=0.1); nickel, electroplated (e=0.03); and nickel, polished (e=0.072). 
         [0014]    A material having emissivity no greater than 0.3 is preferred for plastic welding applications. For non-contact plastic welding, aluminized steel (steel which has an aluminum-silicon alloy coating) provides an efficient and cost-effective shield. It can be easily shaped to simple- and/or complex-shaped heating tool geometries, it can withstand high temperatures required for non-contact plastic welding, and it has good corrosion resistance. Furthermore, it has emissivity significantly less than 0.3. 
         [0015]    In one general respect, the claimed subject matter relates to a heating tool for performing an operation on a plastic part to melt plastic at a location where a weld joint is to be created. The heating tool has: a) a thermally conductive body having a base, a top, and a side which joins the top and the base, the top having a shape conforming to that of plastic to be melted for creating a weld joint in a plastic part; b) a heater for heating the thermally conductive body to create a temperature at the top for enabling the top to melt plastic; c) an IR shield confronting at least a portion of the side of the thermally conductive body without obstructing the top of the body, the shield comprising an inner first layer having an inner face facing the side for reflecting incident heat radiated from the body back toward the body and an outer face disposed against an inner face of an outer second layer, material of the inner first layer having lower emissivity than material of the second outer layer. 
         [0016]    In another general respect, the claimed subject matter relates to a heating tool for melting a surface portion of a plastic part at a location where a weld joint is to be created during a plastic welding process. The heating tool comprises: a) a heater having a shape for melting a surface portion of a plastic part at a location where a weld joint is to be created and for emitting IR radiation in a direction beyond the surface portion where a weld joint is to be created; and b) an IR shield having a reflecting face having an emissivity no greater than 0.3 for redirecting IR radiation emitted by the heater in a direction beyond the surface portion where a weld joint is to be created in a different direction. 
         [0017]    In still another general respect, the claimed subject matter relates to a plastic welding process comprising: a) operating a heater to melt a surface portion of a plastic part at a location where a weld joint is to be created while the heater is also emitting IR radiation in a direction beyond the surface portion where a weld joint is to be created; and b) redirecting IR radiation being emitted beyond the surface portion where a weld joint is to be created by reflecting IR radiation which is incident on an IR shield off a reflecting surface of a first of two layers of the IR shield which are disposed against each other, material of the first layer having lower emissivity than material of a second of the two layers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a perspective view of a hot plate assembly mounted on a tooling plate of a welding machine. 
           [0019]      FIG. 2  is a view like  FIG. 1  showing a low emissivity shield assembled to the hot plate assembly. 
           [0020]      FIG. 3  is an enlarged fragmentary view of a portion of  FIG. 2  in the direction of arrows  3 - 3  in  FIG. 2 . 
           [0021]      FIG. 4  is an enlarged view of a portion of  FIG. 3 . 
           [0022]      FIG. 5  is a perspective view of a part of a low emissivity shield. 
           [0023]      FIG. 6  is a perspective view of the part of  FIG. 5  formed to a different shape. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  shows a heating tool  10  for performing an operation on a plastic part (not shown) to melt plastic at a location where a weld joint is to be created. Tool  10  comprises, as its heater element, a hot plate assembly  12  which is supported on a tooling plate  14  by a support structure  16 . 
         [0025]    Hot plate assembly  12  comprises a hot plate which has a thermally conductive body  18  whose shape has a feature which corresponds to that of a surface of a plastic part at a location where a weld joint is to be created.  FIGS. 3 and 4  show hot plate assembly  12  to further comprise an electric heater  20  (shown in phantom) which is disposed within a channel  22  in body  18  and captured within channel  22  by a retainer plate  24  fastened to body  18 . An example of heater  20  is a conventional flex heater pressed against body  18  by fastening of retainer plate  24  to body  18 .  FIG. 1  shows electrical leads  28 ,  30  of heater  20  extend outward from body  18  for connection to an electric power source (not shown). When heater  20  is energized by electric current, heat is transferred primarily from the heater into body  18  but also to some extent into retainer plate  24 . 
         [0026]    The hot plate shape shown by the example of the Figs. may be described as generally polygonal and having an endless length surrounding an open center.  FIGS. 3 and 4  show that body  18  has a base  32 , a top  34 , and opposite sides  36 ,  38 , side  36  being an inner perimeter side and side  38  being an outer perimeter side. Top  34  comprises a ridge  40 , a feature whose shape conforms to that of the weld joint to be created. Along the endless length of body  18  surrounding the open center, ridge  40  has an elevation which, as measured to tooling plate  14 , varies along that length for conforming to a three-dimensional shape of the surface of the plastic part which is to be melted to create the weld joint. 
         [0027]      FIG. 1  shows support structure  16  to comprise a platform  42  fastened to tooling plate  14  and a lower tier  44  of individual flat plates  46  supported by several vertical walls  48  which are fastened to platform  42  and have various portions of their upper edges shaped to support plates  46  of tier  44  for general conformance with the perimeter shape of the hot plate. An upper tier  50  of support structure  16  comprises individual plates  52  each parallel with and overlying a respective plate  46  of lower tier  44 . Tubular posts  54  separate each plate  52  of upper tier  50  from the respective plate  46  of lower tier  44 . A respective fastener  56  passes through each post  54  to secure plates  52  to plates  46 . Hot plate assembly  12  is fastened to plates  52  in the same manner by tubular posts  58  and fasteners  60 . 
         [0028]      FIG. 2  shows a multi-part shield  62  confronting sides  36 ,  38  of body  18  while leaving an opening  64  which leaves ridge  40  unobstructed by the shield. Shield parts  62   a ,  62   b ,  62   c ,  62   d ,  62   e ,  62   f ,  62   g ,  62   h ,  62   i ,  62   j , and  62   k  ( FIGS. 2-4 ) confront essentially the entire perimeter of outer perimeter side  38 . Other shield parts, some of which are parts  621 ,  62   m ,  62   n ,  62   o ,  62   p , and  62   q , confront essentially the entire perimeter of inner perimeter side  36 , it being understood that the direction from which  FIGS. 2-4  are viewed prevents some shield parts which confront inner perimeter side  38  from being seen. 
         [0029]    The parts forming shield  62  comprise multiple layers of materials which have different emissivities. An inner first layer of a shield part has an inner face facing body  18  for reflecting a majority of IR radiation incident on the inner face of the inner first layer back toward the body. The inner first layer has an outer face opposite its inner face. That outer face is disposed against an inner face of an outer second layer of the part. Material of the inner first layer has lower emissivity than material of the outer second layer. Aluminized steel is an example of a material having sheet steel as its second layer and an aluminum-silicon alloy coating on one face of the steel as its first layer. 
         [0030]    The shield parts of shield  62  comprise various slits  66  and bends for conforming each shield part to the length of ridge  40  and to varying elevation of ridge  40  along its length. Slits  66  extend in the individual shield parts from edges of the individual shield parts which confront portions of opening  64 . In the example of the Figs., ridge  40  protrudes through opening  64  a short distance beyond shield  62 . 
         [0031]    Shaping of the illustrated shield parts from flat aluminized steel sheet stock is explained with reference to  FIGS. 5 and 6 . The stock is cut to an appropriate size and shape for each part, including cutting which creates slits  66 . The cut stock is formed to a desired shape like the one shown in  FIG. 6 . Attachment holes  68  are also punched or drilled. A part can be formed to a desired shape, as exemplified by  FIG. 6  which provides a foot  70  containing holes  68  for fastening the part to a plate  52  using fasteners  72  as shown in  FIGS. 2-4 . While it is intended that the shield parts not be in contact with the heater element which melts plastic. occasional incidental contact may be tolerated without significant consequences, provided that such contact does not interfere with the plastic surface being melted. 
         [0032]    Heating tool  10  is used by mounting a plastic part on one platen of a welding machine, mounting heating tool  10  on an opposite platen, heating the heating tool, and closing the platens to bring ridge  40  into contact with, or close to, the plastic part. Leveling screws  74  are used to set the planarity of platform  42  to tooling plate  14  so that ridge  40  has the proper match to the portion of the plastic part which is to be melted.