PURPOSE: To enable lapping a strip of an extensible material like rubber from an extrusion molding machine over a complex configuration by controlling the thickness correctly and thereby to dispense with machining of the outer surface.

CONSTITUTION: An insulation casing for an object like a rocket-motor casing is produced by lapping an insulation-material strip 18 over the surface of a rotating mandrel 10. During the lapping process, the said strip 18 conforms most correctly to the geometric configuration of the mandrel 10 through a molding machine with a unique structure to form the configuration of the strip 18. The molding machine automatically varies both taper and width-size of the strip 18. The lapping-processed final product has a smooth surface which requires only minimum finish-processing.