the heat shields 15 have perforations to direct an outflow flow of cooling air 16 to enter the combustor 7 the outflow of cooling air 16 mixes with the fuel air mixture in the combustor 7 and immediately on exit also forms a cooling air film to cool and protect the heat shields 15 from heat and combustion gases the intermediate chamber 14 has a higher internal pressure than the combustor 7 and the heat shields 15 have rails 17 along their external edges to seal the intermediate chamber 14 against the surface of the combustor shield 13 the combustor shell 13 is generally manufactured by machining of a metal alloy with a high heat resistance or more recently by direct metal laser sintering dmls additive manufacturing the heat shields 15 are generally cast of ceramic or metal alloy and can be produced by the metal injection molding mim process due to manufacturing dimensional tolerances and fitting of two parts with mating surfaces the assembly will always result in a gap 18 through which some compressed air leakage 19 occurs shown as dashed arrows some degree of air leakage 19 is beneficial since outward air flow purges hot combustion gases that would otherwise be trapped in eddies within the edge joint area of the heat shields 15 however leakage through the gap 18 that is uncontrolled or excessive is detrimental since leakage 19 can decrease engine efficiency through increased use of compressed cooling air and dilution of combustion gases leakage.