Abstract:
An imitation candle having an exterior wax shell and an interior illumination source is structured to reduce cracking of the exterior wax shell by incorporating a bonding layer into a central cavity in the shell between the interior illumination source and the wax shell. The bonding layer terminates well spaced from terminal edges of the shell to transfer stress between the illumination source and the shell caused by differing coefficients of thermal expansion to points removed from edges of the shell to retard crack genesis.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The invention relates to wax imitation candles and more particularly to an imitation candle resistant to cracking at low temperatures.  
         [0003]     2. Description of the Problem  
         [0004]     Many people find candle light pleasant. The flickering of light and movement of shadows across a floor or on a nearby wall can be almost hypnotically soothing. As a result, candles have remained popular for generations since the invention of more practical electrical lighting, especially for decorative and mood setting purposes.  
         [0005]     Consequently, numerous manufacturers have attempted to meet a demand for a candle like luminary using electrical illumination. A now popular imitation candle is taught in International Publication Number WO 03/016783 A1. This imitation candle uses an internal LED as a light source within a solid appearing body. While a classical image of a candle is of a long, thin, tapering rod, which stands upright in a candle stick and which leaves its flame exposed as it burns down, this imitation candle comes as a relatively short to circumference block or cylinder which is self supporting. Such candles commonly leave the outer wall of the candle intact as the candlewick burns down. When this happens, the candle flame is no longer directly visible when viewed from the side. This results in a diffuse, flickering glow visible through the paraffin wall of the candle, which is imitated by the external shape of the imitation candle.  
         [0006]     While the imitation candle of WO 03/016783 appears to be a solid body to users it is in fact hollow. This provides space for the installation of batteries, the LED, LED excitation circuitry and possibly light directing internal components. In addition, the contour of the void&#39;s internal surface may be chosen for light transmission issues. While the imitation candle can readily be made in plastic, fabricating it in more realistic wax has presented particular problems.  
         [0007]     Wax is highly susceptible to compressive and tensile stress. Waxes also tend to have high coefficients of thermal expansion. Differential heating and cooling of sections of a cast wax body introduces stress. Stress tends to be focused along sharp corners and edges of a wax body. Stress can occur during manufacturing and shipping of the wax shell imitation candles when the imitation candles are subjected to rapid cooling or great temperature extremes, respectively. The cavity adds the problem of internal edges, as well as reducing the strength of the body compared to a solid wax body. In addition, the insert on which battery, excitation circuitry and the LED are mounted will typically be constructed by plastic with the wax body being formed in part on the insert body. Wax will typically have a higher coefficient of expansion than the plastic does, which results in additional stress as temperature of the body decreases and contributes further to the problems of the inherent weakness of wax.  
         [0008]     Wax bodies, such as candles, are formed by a process of casting. Where it is desired to incorporate a plastic module in the wax body the plastic module may be fixed in position in a mold and hot wax poured around the module, adding wax as earlier poured wax cools and shrinks, until all voids around the module are filled. Alternatively, a wax shell can be formed that produces the outer visible surfaces of the candle while leaving a space for the module. After the shell is produced a second pour is done to secure the module in position. The amount of wax in the second pour is less than in the first, with the attendant advantages of quicker cooling and faster production speeds. While true, solid wax candles have reasonable durability to withstand cold temperature induced stress, wax bodies made by either of the foregoing casting techniques have proven highly susceptible to cracking. Thin sections of the casting adjacent the module cool more rapidly than thicker sections. Leading edges of the imitation candle also cool rapidly. These sections of rapid cooling result in differential rates of contraction, which can easily result in formation of a crack to relieve stress. Once such a crack propagates into a thicker section of the body it can become a focal point for other stresses and can extend to encircle the imitation candle body.  
       SUMMARY OF THE INVENTION  
       [0009]     According to the invention there is provided an imitation candle. The imitation candle has a wax shell having a central cavity defined by an interior surface. A artificial lighting module, which tends to exhibit a different thermal coefficient of expansion than the wax, is positioned in the central cavity. A bonding layer between a portion of the module and the interior surface of the wax shell retains the module in the shell. The bonding layer leaves a gap between the insert and the interior surface near any exterior edges of the wax shell. The gap is preferably filled with air.  
         [0010]     Additional effects, features and advantages will be apparent in the written description that follows. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0012]      FIGS. 1 and 2  are perspective views from different angles of a wax shell and artificial illumination source for insertion thereto.  
         [0013]      FIGS. 3A and 3B  are cross sectional views of wax shell imitation candles constructed in accordance with each of two preferred embodiments of the invention.  
         [0014]      FIG. 4-7  depict steps in a process for fabricating the wax shell imitation candle of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Referring now to  FIGS. 1 and 2  a shell  10  and an insert or illumination module  12  which includes circuitry, batteries and a light emitting diode for insertion into the shell are shown from above and below. Shell  10  is a generally squat, cylindrical body, with dimensions common to free standing, thick walled candles. An upper surface  22  of shell  10  is depressed into the interior of the shell to simulate a previously burned candle the center of which is partially melted and consumed. Insert  12  fits into and is retained within cavity  14  defined by an interior surface  15  of shell  10 . Cavity  14  is open along a bottom surface of shell  10  and is slightly oversized, as described below, to admit insert  12 . Shell  10  is preferably a cast wax body. Insert  12  has an exterior casing  18  made enclosing the battery, circuitry and an LED enclosed in an upper surface  16  of the insert  12 . Insert  12  is introduced to cavity  14  lead by upper surface  16 . The wax material of shell  10  and the plastic material of casing  18  exhibit substantially different coefficients of thermal expansion. The present invention concerns mating of the interior surface of shell  10  and casing  18  of insert  12  to inhibit cracking of the wax of the shell.  
         [0016]     Referring to  FIGS. 3A and 3B  some of the features of the invention as incorporated into each of two preferred embodiment of the invention may be seen to advantage. The central depression in upper surface  22  begins spaced inwardly from a rounded circumferential exterior edge  27  with a shallow downwardly slanted ledge  26 , which terminates moving toward the vertical center axis of shell  10  in a rounded shoulder  24  where the upper surface drops to a central depression defined by a second shoulder  25 . Insert  12  is illustrated fitted into cavity  14  from the bottom of shell  10 . Cavity  14  is defined by an interior surface  15  which, in a fashion similar to the central depression in the upper surface  22 , has rounded transitions between portions of the surface which exhibit substantial intersecting angles vis-a-vis one another. Rounded transition  23  is characteristic forming a boundary between a cylindrically shaped, vertically oriented section of interior surface  15  and a horizontally oriented disk like section at the top of cavity  14 .  
         [0017]     Insert  12  is undersized compared to the cavity  14  in which it is to be retained. Bonding between a plastic insert casing  18  is provided by bonding layer  20  which lines the upper portion of cavity  14  between casing  18  and interior surface  15 . As described below, bonding layer  20  is formed by a second pouring of a small quantity of molten wax into an inverted, but already cooled and hardened shell  10 . Bonding layer  20  is shaped by fitting insert  12  into cavity  14  while the second poured wax is still molten. Bonding layer  20  does not line all of interior surface  15  in the preferred embodiment, but only enough to cover casing  18  around LED  16  and about the top half of the main body of insert  12 . An air gap  30  surrounds the bottom half of insert  14  spacing the insert from interior surface  15 . The top  34  of illumination module  14  abuts an upper horizontal face  34  of interior surface  15 , displacing molten wax and positioning the illumination module vertically. Horizontal positioning of illumination module may be achieved by careful reference to the spacing between casing  18  and interior surface  15  and by the careful, mutually parallel orientation of the elements. The bottom surface of insert  12  is slightly recessed (2.5 mm) from the surrounding bottom surface of shell  10  allowing accurate determination disposition of the insert in cavity  14 .  
         [0018]     While use of a bonding layer  20  is preferred due to the assurance of a good fit between the bonding layer and insert  12 , it is possible to substitute a molded or shaped shoulder  60  which is formed as part of interior surface  15  defining cavity  14 . As seen in  FIG. 3B  shoulder  60  is part of shell  12  and slants inwardly into cavity  14  partway into the cavity from the bottom surface of shell  10 . Construction of shell  10  to incorporate such a circumferential shoulder is easily done by modification of the bit used to shape cavity  14  or form  42 . It is important that a gap be left between the body of insert  12  and interior surface  15  in the lower part of cavity  14 . This saves processing steps. However, the difficulty in this technique is that extremely close tolerances in dimensional matching between the insert  12  and the shell  10  are required to avoid introducing stress on introducing the insert to cavity  14 . It may be possible to time the introduction to a point while the wax of shell  10  is still slightly soft.  
         [0019]      FIGS. 4 through 7  help illustrate a process for fabricating the imitation candle of the present invention. The first step of the process is to pour molten wax  11  into a mold  40  giving the body of wax which cools to form shell  10  its exterior shape. Mold  40  should be slightly taller then the desired eventual size of shell  10  to allow trimming of the cooled body to the desired size. Cavity  14  may be formed in one of two ways. In one process, a form  42  is held in the mold  40  to leave cavity  14  upon withdrawal from the hardened shell  10 . Alternatively, no form is used and the mold  40  is substantially filled with wax on the first pouring. In a preferred embodiment mold  40  is 111 mm deep allowing trimming of shell  10  to a desired height of 105 mm.  
         [0020]     After pouring of the wax for shell  12  the wax is allowed to cool. Where no form is used the wax is allowed to cool until the wall thickness is at least 10 mm. Where a form  42  is used the wax is allowed to cool until the entire shell  10  has hardened. A water bath may be used to expedite the cooling process. If no form was used a hole is formed into the cooling body from what will be become the bottom surface of the shell to the interior, still molten wax. The mold is partially inverted to allow the molten wax to be poured out and reclaimed. Removal of the central, molten wax speeds the cooling process and relieves stress on the walls of shell  10 . The shell continues cooling, again potentially placed in a water bath to quicken the process. Mold  40  is advantageously shaped to impress an upper surface central depression into shell  10 . Where, however, the mold did not incorporate such a shape, a bit contoured with the cross section of the upper surface may be used to shape the upper surface after withdrawal of the shell  10  from mold  40 .  
         [0021]     The position of insert  12  is controlled by the depth of cavity  14 . An inner bit may be used trim the bottom of shell  10  and to machine cavity  40  where no interior form  42  is used, or where adjustment of the shape of a cavity left by a form is required. Shell  10  should be properly fixtured during shaping with a bit to insure a uniform core depth and candle height.  
         [0022]     With the shell  10  fully hardened and the shape of cavity  14  finalized, shell  10  is reinverted and a second pour  46  of a small quantity of molten wax is made into the top of cavity  14 . By the term “small” it is meant that the amount of wax in the second pour is a small percentage of the quantity of wax in the first pour. Where the depth of cavity  14  is 86 mm, the pour will leave the upper 58 mm empty before insertion of the insert  12 . The formulation of the wax may be the same for both pours. With the second pour  46  still molten, insert  12  is lowered into cavity  14  of the inverted shell  10 , displacing molten wax of the second pour  46  upwardly around the insert along the interior surface  15  of the cavity to form a bonding layer  20 . Insert  12  is pressed as far as possible into shell  10 , until the casing around upper surface  16  hits the top surface of the interior surface  15 . An air gap of about 30 mm extends upwardly from the bottom of shell  10  into cavity  14  around insert  12 . This helps prevent cracking.  
         [0023]     The invention impedes the genesis and spread of cracks in the wax shell of a two component imitation candle. The assembly method for embedding insert  12  moves the point of maximum stress to a position where the stress is more readily tolerated. This is achieved by forming a gap between the insert and thin walled sections of the wax starting from a leading edge of the wax (e.g. the bottom edges of the shell). The gap can be air, or it can be filled with substances which offer insubstantial resistance to contraction of the wax as it cools. Leaving a gap between the bottom edge of the shell moves the point of maximum stress to an area of the shell where the gap ends and the bonding layer begins. This places the point of maximum stress away from any corners or edges. Cooling of the shell is also retarded here due to the greater local thermal mass, allowing more time for internal stress relief. The invention also achieves reduced concentration of stress by maintaining a maximum degree of uniformity in wax wall thickness and eliminating sharp corners.  
         [0024]     While the invention is shown in only two of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.