Preparation of oriented polymer films

Claim 1

IN A PROCESS OF STRETCHING A CONTINUOUS LENGTH OF AN ORIENTABLE, CRYSTALLIZABLE LINEAR POLYMER FILM IN TWO DIRECTIONS AT RIGHT ANGLES IN THE MAJOR PLANE OF THE FILM IN WHICH THE FILM IS STRETCHED A SUBSTANTIAL AMOUNT FIRST IN ONE SAID DIRECTION AND THEN IN THE OTHER SAID DIRECTION AS CONSECUTIVE STEPS, THE IMPROVEMENT WHICH COMPRISES MAINTAINING CONSTANT THE DESIRED LEVEL OF ORIENTATION IN SAID FIRST STRETCHING STEP BY MAINTAINING THE WORK OF STRETCHING PER UNIT VOLUME OF FILM CONSTANT DURING SAID STEP AND DETERMINING THE LEVEL OF WORK PER UNIT VOLUME IN THE SECOND STRETCHING STEP REQUIRED TO OBTAIN A FILM EXHIBITING MINIMUM BIREFRINGENCE BY TAKING SAMPLE OF SAID FILM WHICH HAS BEEN SUBSTANTIALLY STRETCHED IN ONE DIRECTION AND STRETCHING SAID SAMPLE IN SAID SECOND DIRECTION TO OBTAIN A FILM EXHIBITING MINIMUM BIREFRINGENCE, AND THEN STRETCHING THE CONTINUOUS LENGTH OF FILM WHICH HAS BEEN STRETCHED IN ONE DIRECTION ,AND ANY OTHER CONTINUOUS LENGTH OF FILM OF THE SAME TYPE WHICH HAS BEEN SO STRETCHED, IN SAID SECOND DIRECTION AT SAID DETERMINED WORK LEVEL, AND THEREAFTER MAINTAINING THE WORK PER UNIT VOLUME OF THE FILM CONSTANT AT SAID SECOND LEVEL DURING THE SECOND STRETCHING STEP.