wherein {HEAD ~ each} of the two pieces of the mold insert comprises at least one portion partly defining said mold cavity which is made of a material or a combination of {TAIL ~ materials}, other than glass the thermal diffusivity α of which fulfills the condition 
wherein { HEAD ~ each } of the two pieces of the mold insert comprises at least one portion partly defining said mold cavity which is made of a material or a combination of { TAIL ~ materials } , other than glass the thermal diffusivity α of which fulfills the condition
OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH