An annular co-extrusion die for extruding multi-layer tubular plastic film
has inner die members stacked one upon another and having radially
outwardly extending surfaces forming radially outwardly extending helical
passages between respective pairs of inner die members. An annular outer
die member surrounds the stacked inner die members and forms a
longitudinally extending annular passage therebetween for conveying
plastic material from the radially outwardly extending helical passages
to an annular extrusion orifice. A lower inner die member has a first
feed passage extending from a lower surface thereof to a substantially
horizontal upper surface thereof at a position spaced from a central
longitudinal axis of the longitudinally extending annular passage, the
substantially horizontal upper surface having a first groove extending
from the upper end of the first feed passage to the longitudinal axis. An
upper inner die member immediately above the lower inner die member has a
substantially horizontal lower surface engaged with the substantially
horizontal upper surface of the lower inner die member, the substantially
horizontal lower surface having a second groove extending from the upper
end of the first feed passage to the longitudinal axis and forming a
second fed passage with the first groove. The upper inner die member has
a third feed passage extending substantially vertically upwardly from the
lower substantially horizontal surface thereof at the longitudinal axis
and in communication with the second feed passage. At least one fourth
feed passage in the upper inner die member extends from an upper end of
the third feed passage to the helical passages between the upper and
lower inner die members.