A selective laser sintering apparatus is adapted contain or receive a build frame and comprises a cap assembly which includes a removable plate assembly having at least one gas inlet formed therein. The removable plate assembly sealingly covers the build frame. The cap assembly further comprises at least one gas line fluidly connectable to the gas inlet via a gas port to deliver inert gas to the interior of the build frame. The plate assembly comprises upper and lower plates defining a spacing therebetween. A seal extends around a periphery of the upper and lower plates and is sandwiched therebetween to enclose the spacing. The lower plate has a plurality of gas inlets formed therein which are in fluid communication with the spacing such that inert gas may be delivered to the build volume while the cap assembly is installed thereupon.