as sodium bicarbonate and citric acid as is well known in the art for use in extruding a polystyrene foam sheet which has functional properties suitable for use in application by shrinkage from a formed sleeve to conform to the contour shape of a container to perform as a label the method comprises utilizing other atmospheric gases such as carbon dioxide nitrogen or air as the sole and exclusive blowing agent for producing said foam sheet the method preferably incorporates the injection of the high molecular weight non volatile blending agent having very low volatility at extrudate temperatures e g 300 u00b0 f 325 u00b0 f into the polymer melt such material provides a solvating action weakening intermolecular bonding of the polymer and reducing melt viscosity preferably directly downstream following the injection of the organic fluid air carbon dioxide nitrogen or air or blends thereof after having been compressed in excess of extruder injection point pressure typically in excess of 3000 psi is injected through a second injection port into the polymer as indicated in fig1 the extrusion equipment used may be a conventional tandem system employing two extruders working in conjunction with each other whereby the first extruder melts the polymer mixed with nucleating agent which accepts by injection and mixes in a blowing agent and transfers said mix to a second extruder which cools the melt and conveys it into an annular die which transforms the melted mass into a continuous sheet foam no extrusion equipment modifications such as screw designs die designs or special mixing devices are necessary standard type pumps as suitable for organic fluids and as suitable for boostering industrial type gas pressure with appropriate metering devices which are known to those skilled in the art.