a die stamper assembly comprises a first die part for holding a first stamper and a second die part for mounting a second stamper , the die parts cooperating to define a mould cavity . the first stamper is mounted at a boundary of the first die part and the second stamper is mounted at a boundary of the second die part . the die stamper assembly further comprises a venting ring configured around and moveable with respect to the first die part and defining an outer boundary of the mould cavity . a venting holder arranged for holding the circumference of the first stamper and positioned between the first die part and the venting ring to attach the first stamper on the first die part allows venting of air escaping from the mould cavity and prevents leaking of plastic material out of the mould cavity .