Multi-layer heating element

Abstract

A heating element and, in particular, a ceramic heating element, such as ceramic heating elements used in high temperature glow plugs for diesel engines and gas igniters. The heating element includes an electrical insulator and an electrically conductive layer. The conductive layer is formed from a single material and single composition. The method of manufacture includes the steps of forming the insulative layer and molding a conductive layer around the insulative layer.

1 . A heating element comprising: 
 an electrically conductive layer; and    an electrically insulative layer, and wherein said electrically conductive layer includes a portion that is completely formed within said insulative layer, and said insulative layer includes a portion that is completely formed within said conductive layer.    
     
     
         2 . The heating element of  claim 1  wherein said heating element has a first outer surface defined by said electrically conductive layer and wherein said electrically insulative layer has an outer insulative surface, said conductive layer having a thickness between said outer insulative surface and said outer surface.  
     
     
         3 . The heating element of  claim 2  wherein said thickness is variable with areas of less thickness having greater electrical resistance.  
     
     
         4 . The heating element of  claim 3  wherein said thickness varies along the length of the heating element.  
     
     
         5 . The heating element of  claim 3  wherein said thickness varies about the circumference of the heating element.  
     
     
         6 . The heating element of  claim 1  wherein said heating element has a length and said electrically insulative layer includes an outer insulative surface and wherein said outer insulative layer has a diameter that varies along said length.  
     
     
         7 . A heating element comprising: 
 a base portion having a first diameter; and    a heating portion formed at an end of the base portion, said heating portion having a second diameter and wherein said first diameter is greater than said second diameter, and wherein said heating portion includes an electrically insulative layer and an electrically conductive layer, said conductive layer having a first thickness and a second thickness, said second thickness being greater than said first thickness.    
     
     
         8 . The heating element of  claim 7  wherein said base portion includes said second thickness and said heating portion includes said first thickness.  
     
     
         9 . A heating element comprising: 
 a first layer having a first geometric profile; and    a second layer molded over said first layer, said second layer having an outer surface forming a second geometric profile and wherein said first and second geometric profiles create a variable thickness in the second layer.    
     
     
         10 . The heating element of  claim 9  wherein said variable thickness creates areas of higher electrical resistance and areas of lower electrical resistance.  
     
     
         11 . The heating element of  claim 10  wherein said first and second geometric profiles are designed to optimize a heating profile.  
     
     
         12 . The heating element of  claim 9  wherein said first layer is an electrical insulator and said second layer is an electrically conducting material.  
     
     
         13 . The heating element of  claim 12  wherein said electrical insulative layer and said electrically conductive layer have approximately the same thermal expansion characteristics.  
     
     
         14 . The heating element of  claim 13  wherein said electrically insulative layer is formed from polystyrene.  
     
     
         15 . A heating element having a length comprising: 
 a first layer including at least two projections; and    a second layer substantially surrounding said first layer for at least a portion of the length of the heating element, wherein said second layer and said at least two projections form an outer surface.    
     
     
         16 . The heating element of  claim 15  wherein said first layer includes at least three projections.  
     
     
         17 . A method of forming a heating element comprising: 
 forming a first layer having at least three projections;    inserting said first layer into a die so that said projections engage the die; and    forming an electrically conductive layer at least partially around said first layer.    
     
     
         18 . The method of  claim 17  wherein said step of forming an electrically conductive layer includes the step of injecting a molten conductive material into said die.  
     
     
         19 . The heating element of  claim 17  wherein said first layer has a center passage and an outer surface and wherein said second layer is disposed within said center passage and surrounds at least a portion of said outer surface.  
     
     
         20 . The heating element of  claim 19  wherein said second layer is formed without joints or interfaces.  
     
     
         21 . The heating element of  claim 19  wherein said second material is formed as a single piece.  
     
     
         22 . A heating element having: 
 a first layer having a first end, a second end and a center portion between said first and second ends, and wherein said first and second ends have a diameter that is greater than the diameter of the center portion; and    a second layer.    
     
     
         23 . The heating element of  claim 22  wherein said second layer is electrically conductive to provide resistance heating.  
     
     
         24 . The heating element of  claim 22  wherein said first layer is an electrical insulator.  
     
     
         25 . The heating element of  claim 22  wherein said second layer is molded around said electrically insulative layer.  
     
     
         26 . The heating element of  claim 22  wherein said second layer has an outer surface and wherein the diameter of said outer surface proximate to said first end of said first layer is less than the diameter of the outer surface proximate to the center portion of said first layer.  
     
     
         27 . A method of forming a heating element comprising: 
 molding a core from a first material;    overmolding an electrically conductive material on said core; and    removing said core from said conductive material.    
     
     
         28 . The method of  claim 27  further including the step of adding an electrically insulative material to said conductive material after said step of removing said core.