This application relates to a method and apparatus for heating a surface marking, such as a thermoplastic pattern in an asphalt substrate. The marking may be selected for functional or decorative purposes. The method involves gradually applying heat to the marking to avoid scorching and to ensure a consistent bond with the underlying substrate, even in the case of markings having a very large surface area. In one embodiment the method a portable heating apparatus is provided having infrared heaters mounted for reciprocal movement in a travel path periodically passing over the marking and the underlying substrate. The heating method permits direct visual monitoring of the work site to achieve optimum adhesion of the marking to the asphalt or other substrate.

RELATED APPLICATIONS 
     This application claims priority on pending international application No. PCT/CA02/01864 filed 3 Dec. 2002 which is incorporated herein by reference. 
     TECHNICAL FIELD 
     This application relates to a method and apparatus for heating settable surface markings, such as thermoplastic markings applied to roadway or walkway surfaces. The markings may be selected for functional and/or decorative purposes. 
     BACKGROUND 
     Various methods for applying markings to roadway and walkway surfaces are known in the prior art. For example, it is well-known in the prior art to apply colored thermoplastic markings to asphalt roadways, such as traffic arrows or lane markings. The markings are fixed in place using heat which causes the settable material to bind to the underlying substrate. 
     It is also becoming increasingly common to apply surface markings to outdoor substrates for decorative or marketing purposes. For example, corporate logos and advertising designs may be applied to asphalt, concrete or other substrates, such as in parking lots, drive-throughs, store fronts and the like. 
     Thermoplastic surface markings are typically heated in situ by using hand-held open-flame torches. However, it is very difficult to consistently apply heat to surface markings using such hand-held heaters, particular if the markings are large in size. As a result, two primary problems have arisen, namely overheating and underheating. Often the thermoplastic material is either scorched due to the application of excessive heat or fails to bond consistently to the underlying substrate due to the application of insufficient heat. The failure to establish a consistent bond may result in delamination of the marking from the substrate over time, especially in high traffic areas. 
     The need has therefore arisen for an improved method and apparatus for gradually and consistently applying heat to surface markings to ensure a consistent bond with the underlying substrate, even in the case of markings having a very large surface area. 
     SUMMARY OF INVENTION 
     In accordance with the invention, a method of binding a thermally settable marking to a substrate is described comprising:
         (a) positioning said marking on said substrate;   (b) gradually heating said marking and said substrate in situ by periodically passing a heater in proximity to said substrate; and   (c) allowing said marking to bind to said substrate when said marking is heated to a sufficiently pliable state.       

     In one embodiment of the invention the marking may be partially or entirely in-laid within the substrate. The substrate may, for example, comprise an asphalt surface and the marking may be partially or entirely in-laid within an upper portion of the asphalt surface. The marking may be formed from a thermoplastic material. 
     The Applicant's method may include the step of providing a heating apparatus having a support frame extending over the marking. The method may include moving the heater on the support frame in a path which periodically passes over the marking to gradually increase the temperature thereof. For example, the heater may comprise an infrared heater which moves in a reciprocating motion on the support frame. In some embodiments of the invention multiple infrared heaters movable on the support frame may be provided. 
     The heater is preferably capable of heating a relatively large surface area, such as greater than 10 square feet, while permitting visual monitoring of the work site. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In drawings which illustrate embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way, 
         FIG. 1  is perspective view of an apparatus comprising reciprocating infrared heaters for gradually heating a marking applied to a substrate. 
         FIG. 2  is an end elevational view of the reciprocating heaters of  FIG. 1 . 
         FIG. 3  is a graph showing the gradual increase in the substrate surface temperature with successive passes of the reciprocating heaters of  FIG. 1 . 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     This application relates to methods and apparatus for heating settable surface markings  10 , such as markings comprised of a thermoplastic material. With reference to  FIG. 1 , a marking  10  may be applied to a substrate  12  and then gradually heated in situ until a consistent bond is achieved between marking  10  and substrate  12 . As used in this patent application the term heating “in situ” refers to heating the marking  10  and substrate  12  at the work site rather than using hot materials heated off-site. 
     Substrate  12  may comprise, for example, an asphalt surface. As used in this patent application “asphalt” means a paving compound for constructing roads, driveways, walkways and the like which consists of a combination of bituminous binder, such as tar, and an aggregate, such as sand or gravel. As will be appreciated by a person skilled in the art, substrate  12  could also alternatively comprise concrete or other materials capable of binding to thermoplastic markings  10 . 
     Settable markings  10  are well-known and are available from various suppliers. A suitable thermoplastic material is available, for example, from Lafarge Road Markings and is sold under the trademark THERMALINE™. Other suppliers of thermoplastic markings include Flint Trading, Inc. and Avery Dennison Corporation. Marking  10  may selected for a functional purpose, such as a traffic marking or corporate logo, or may be purely decorative. 
     As shown  FIG. 1 , a portable surface heating apparatus  14  is provided for heating marker,  10 . In the illustrated embodiment apparatus  14  includes a support frame  16  and a plurality of infrared heaters  18  supported for movement on support frame  16 . For example, support frame  16  may include elongated rails  20  which are supported above substrate  12  by support legs  22  and housing  24 . A heater truck  26  is provided for reciprocating movement on rails  20 . Truck  26  supports a bank of infrared heaters  18  at positions close to substrate  12  (e.g. approximately 2 inches above the ground). 
     As shown in  FIGS. 2 and 3 , in operation infrared heaters  18  travel back and forth over marking  10  to gradually heat marking  10  and substrate  12 . For example, in one embodiment heaters  18  move through three cycles per minute (each cycle being a traversal of truck  26  from housing  24  to the distal end of rails  20  and back again). An important advantage of the heating method of  FIG. 1  is that a relatively large marking  10  and underlying substrate  12  can be heated gradually and evenly. This approach avoids the disadvantages of hand-held torch heaters which cannot easily be used to evenly heat large areas and have a tendency to scorch the thermoplastic material and/or the substrate. For example, depending upon its composition, some thermoplastic markings  10  and/or substrates  12  can scorch when subjected to sustained temperatures above approximately 325° F. 
       FIG. 3  is a graph showing the changing temperature profile of an asphalt substrate  12  with successive passes of heaters  18 . Substrate  12  is allowed to cool after each heating cycle. The temperature of substrate  12  (and marking  10  applied thereto) gradually increases with successive heating cycles until the desired temperature suitable for thermoplastic/asphalt adhesion is achieved. The asphalt surface is subjected to a relatively slow heat soak to permit heat to gradually penetrate through and around marking  10  below the uppermost surface layer of the asphalt. 
     Heating apparatus  14  allows the operator to visually monitor the work site during the heating operation. For example, marking  10  could cover a large surface area. The reciprocating nature of Applicant's heating apparatus  14  enables the operator to visually monitor the heating process while it is ongoing to gauge the degree of adhesion and to avoid underheating or overheating. For example, the operator can determine when marking  10  becomes sufficiently pliable to flow into any interstices or impressions formed in substrate  10 , thereby enhancing adhesion to substrate  12 . Heater  14  is then removed and marking  10  is allowed to set in place. Depending upon the material used, marking  10  heated in situ to a temperature within the range of 100°–400° F., or more particularly 150°–350° F. Optionally marking  10  and/or substrate  12  may be pre-heated prior to placement of marking  10  at the work site. 
     Markings  10  may be applied directly on an upper or other exposed surface of substrate  12  or may be in-laid within substrate  12 , either partially or entirely. In some applications in-laid markings  10  may be preferred since they have less tendency to wear than exposed markings. Marking  10  may be compressed into substrate  12 , for example, with a mechanical compactor, such as a vibrating plate compactor  16  or a drum roller. In other applications it may be beneficial for markings  10  to project above substrate  12 . This may be useful, for example, in regulating the speed of vehicles traversing a paved roadway or the like. In one embodiment of the invention, the applicant's method could be employed to form an inlaid pattern in a substrate  12  where only the edge portion(s) of the marking  10  are inlaid. For example, a thermoplastic inlaid traffic marking  10  having a gently curved upper surface could be provided. An impression could be formed in substrate  12  conforming to the contour of the periphery of the marking  10 . The impression could be formed so that only edge portions of the marking  10  are inlaid to ensure that the edges will not be caught by snow plows in regions having winter snowfalls. Further, the curvature of the marking  10  could enhance the reflectivity of the thermoplastic material to improve traffic safety. 
     As will be appreciated by a person skilled in the art, the gradual heating method shown in  FIGS. 1–3  could be used to facilitate adherence of thermoplastic or other settable markings  10  which are relatively large in size, such as in-laid or projecting traffic markings. One advantage of this approach in comparison to conventional painted-on traffic markings is that the installation process is not weather dependent. Also, marking  10  would not become obliterated by wear of the surface layer (i.e. since the marking color would extend consistently throughout the thickness of the marking). 
     In still further alternative embodiments of the invention heating apparatus  14  may be modified to include one or more heat sensors  28  for sensing the temperature of substrate  12  ( FIG. 1 ). The heat sensors  28  could be mounted on truck  26  to travel over substrate  12  and scan the temperature thereof. Apparatus  14  may also include a controller for switching off one or more of the heaters  18  in the heater bank depending upon the measured surface temperature. For example, once the surface temperature achieves a target value, some of the heaters  18  could be switched off to prevent further heating and possible scorching of marking  10  or substrate  12  while other heaters  18  could remain on to maintain the surface temperature at or near the target value. Alternatively, the height, speed or heating intensity of some or all of the heaters  18  could be adjusted depending upon the sensed temperature. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.