Head assembly for blow-extruding a tubular plastic foil

An apparatus for blow extruding a tubular plastic foil has a nozzle having an upwardly open annular mouth generally centered on a longitudinal axis for upwardly extruding a tubular plastic strand and an inner tube extending upward from the nozzle within the mouth thereof, having an upper end spaced axially from the mouth, and formed between the upper end and the mouth with at least one radially throughgoing hole. A stabilizer head carried on the inner-tube upper end is formed with passages having inner ends opening into the inner tube at the upper end thereof and outwardly directed outer ends. An outer tube surrounding the inner tube defines an inner longitudinal passage therewith, extends upward from the nozzle within the mouth thereof, and has an upper end above the hole and below the inner-tube upper end. A partition extends crosswise across the inner tube immediately below the hole and subdivides the inner tube into a lower tube section and an upper tube section and a blocking element engaged between the outer-tube upper end and the inner tube above the hole closes the upper-tube upper end. A guide sleeve between the head and the outer tube has a foraminous upper portion, an imperforate lower portion. One or more radially extending conduits below the partition between the inner and outer tubes form a flow path between an outer passage between the strand and the outer tube and an interior of the lower inner-tube section.

FIELD OF THE INVENTION 
The present invention relates to an apparatus for blow-extruding a tubular 
plastic foil or film. More particularly this invention concerns a head 
assembly for such an apparatus. 
BACKGROUND OF THE INVENTION 
A tubular plastic foil of the type used to make bags is formed by extruding 
a tubular plastic strand from an annular mouth of a nozzle. A stabilizer 
extends centrally from this nozzle and has an open outer end. Cooling air 
is directed radially inward at the strand as it exits from the nozzle and 
is also injected into the annular space between the strand and the 
stabilizer. The cooling air trapped in the strand serves both to expand 
and cool it, and is aspirated at some longitudinal spacing from the nozzle 
into the stabilizer. 
This arrangement is typically oriented vertically with the strand moving up 
and away from the nozzle. After being expanded by air pressure inside it, 
the tube is flattened and then is usually cross-welded and perforated or 
cut into bags. 
The stabilizer is typically of circular section and has a closed outer end 
and perforations at this outer end through which the cooling air is 
aspirated. Such an arrangement is satisfactory so long as the neck part of 
the strand, that is the strand before it is radially expanded to the 
desired size, is relatively short. When, however, high-pressure 
polyethylene is being extruded this neck is fairly long, so that it is 
fairly common for the resin to come into contact with the intake end of 
the stabilizer tube and become caught, forcing one to shut down the 
machine and clear the jam. 
It has been suggested to avoid this problem by providing along the stream 
of cooling air flowing along the outside of the strand and spaced from 
this strand a sleeve which is provided over its length with spaced 
circularly annular disks which extend into the flow of the outer stream of 
cooling air. Such a system is only a poor partial solution to the problem 
in that jams still occur. 
In U.S. Pat. No. 4,955,802 the apparatus has a nozzle having an annular 
mouth generally centered on a longitudinal axis for longitudinally 
outwardly extruding a tubular plastic strand, an inner stabilizing tube 
extending longitudinally outward from the nozzle within the mouth thereof 
and having an outer end spaced a predetermined longitudinal distance from 
the mouth, and an outer stabilizing tube surrounding the inner tube and 
defining a longitudinal passage therewith, extending longitudinally 
outward from the nozzle within the mouth thereof, and having an outer end 
lying between the outer end of the inner tube and the mouth. The passage 
opens outward between the outer ends. Cool air is fed to the passage at 
the mouth and to a space defined between the strand where it emerges from 
the mouth and the outer tube for flow of this cooling air longitudinally 
outward between the strand and the outer tube and from the outer end of 
the outer tube. Thus the strand is expanded transversely generally at the 
outer ends of the tubes. The cooling air is withdrawn from within the 
strand by drawing this cooling air into the outer end of the inner tube 
and longitudinally backward through the inner tube. 
Similarly in U.S. Pat. No. 5,322,431 this problem is at least partially 
solved by distributing over the length of the device formed by the inner 
tube and the outer jacket or tubular member, between the outlet at the 
foot side and the outlet at the head side, at least one blowing air intake 
device at which the blowing air is withdrawn from the flow passage formed 
by the clearance between the jacket of the device and the thermoplastic 
tube, into the inner tube, and toward the head of the device, but spaced 
from the outlet at the head side. At least one further blowing air outlet 
for feeding the blowing air into the flow passage is provided between the 
jacket and the thermoplastic tube. The intake as well as the additional 
blowing-air outlet are aerodynamically matched with respect to flows 
therethrough to stabilize the plastic tube and/or the film balloon. 
Both such arrangements are fairly efficient but could stand some 
improvement. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
blow-extruding system. 
Another object is the provision of such an improved blow-extruding system 
which overcomes the above-given disadvantages, that is which operates on 
the long-neck principle but which surely prevents the workpiece from being 
damaged by the stabilizer. 
SUMMARY OF THE INVENTION 
An apparatus for blow extruding a tubular plastic foil has according to the 
invention a nozzle having an upwardly open annular mouth generally 
centered on a longitudinal axis for longitudinally upwardly extruding a 
tubular plastic strand and an inner tube extending longitudinally upward 
from the nozzle within the mouth thereof, having an upper end spaced 
axially from the mouth, and formed between the upper end and the mouth 
with at least one radially throughgoing hole. A stabilizer head carried on 
the inner-tube upper end is formed with passages having inner ends opening 
into the inner tube at the upper end thereof and outwardly directed outer 
ends. An outer tube surrounding the inner tube defines an inner 
longitudinal passage therewith, extends longitudinally upward from the 
nozzle within the mouth thereof, and has an upper end above the hole and 
below the inner-tube upper end. A partition extends crosswise across the 
inner tube immediately below the hole and subdivides the inner tube into a 
lower tube section and an upper tube section and a blocking element 
engaged between the outer-tube upper end and the inner tube above the hole 
closes the upper-tube upper end. A guide sleeve has a foraminous upper 
portion, an imperforate lower portion, and an upper end secured to the 
head, and a lower end secured to the outer tube below the partition and 
forming an annular outer passage with the outer sleeve. One or more 
radially extending conduits below the partition between the inner and 
outer tubes form a flow path between the outer passage and an interior of 
the lower inner-tube section. Cooling air is fed to the inner passage at 
the mouth and to a space defined between the strand where it emerges from 
the mouth and the outer tube for flow of this cooling air longitudinally 
upward between the strand and the outer tube and along the inner passage 
and through the hole into the inner-tube upper section and thence through 
the passages of the head and outward so that the strand is expanded 
transversely into a tubular plastic foil. This air is withdrawn from 
within the strand by drawing it into the outer passage, through the 
conduit, and thence down along the inner-tube lower section. 
The invention is based on the surprising discovery that the substantial 
spacing between the stabilizer-head rings and the long neck of the 
workpiece, just where it transitions to the expanded foil, is not 
necessary. Instead it is more advantageous to actually let the workpiece 
contact the stabilizer at this location. The invention thus employs the 
inventive foraminous grill element constituting the upper portion of the 
guide sleeve. It allows the blow air to be guided into the inner tube so 
that a low pressure or suction is created in the region of this stabilizer 
grill element that pulls the workpiece in against the grill element 
immediately upstream of where it is expanded. The resultant stabilization 
is therefore excellent. 
According to this invention the upper foraminous portion is generally 
cylindrical and centered on the axis. It can also be generally 
frustoconical, upwardly flared, and centered on the axis. The latter 
system somewhat expands the plastic strand. 
The upper foraminous portion can be formed of a grill-work of bars or wires 
or of perforated sheet metal. In the latter case it is coated with a 
material having a very low coefficient of friction, e.g. 
polytetrafluoroethylene. The plastic being blow-extruded is normally 
high-density polyethylene.

SPECIFIC DESCRIPTION 
As seen in FIG. 1 an extruder head 1 has an annular gap 11 centered on a 
vertical axis A and from which issues a plastic tube 12 that is expended 
at a neck region 25 to form a tubular foil 5 of relatively large diameter. 
The extruder head 1 has a blow-air outlet 2 and a blow-air intake 3 and 
outside cool air is blown at 4 on the tube 12 issuing from it. 
An inner tube 6 having a lower end fixed at a connection 9 in the head 1 
extends upward along the axis A as does an outer tube 7. A head 8 mounted 
on the upper outer ends of the tubes 6 and 7 serves to stabilize the 
workpiece 5, 12, 25. An inner passage 10 formed between the tubes 6 and 7 
is connected to the blow-air output 2 to receive blow air therefrom while 
the outer tube 7 forms another passage 13 with the small-diameter extruded 
tube 12 that also receives blow air at 4. The interior of the lower end of 
the tube 6 is connected to the blow-air intake or return 3. 
As best shown in FIG. 2, the head 8 includes an upper end deflector 14 
comprising a pair of parts 18 and 19 mounted on a rod 27 fixed in the tube 
6 and forming an outwardly opening passage 28. The tube 6 is subdivided by 
a partition disk 16 in which the lower end of the rod 27 is fixed into a 
lower passage or compartment 6a and an upper passage or compartment 6b, 
and is formed immediately above the partition disk 16 with radially 
throughgoing holes 17. The upper end of the tube 6 is provided with a 
spacer ring 29 formed with axially throughgoing holes 30. A blocking ring 
15 is provided between the upper end of the outer tube 7, which is shorter 
than the core tube 6, and the tube 6. Thus air rising from the output 2 in 
the inner passage 10 between the tubes 6 and 7 will be forced in through 
the holes 17 into the upper part 6b of the tube 6 and then through the 
holes 30 to the passage 28 whence it will exit into the expanded tube part 
5. 
A collar 22 on the outer tube 7 well below the partition disk 16 carries 
the lower end of an outermost tube or sleeve 21 which defines an annular 
outer passage 23 with the outside of the tube 7 and whose upper end is 
fixed to an upwardly flaring foraminous sleeve or grill 20 whose upper end 
in turn is secured in the deflector part 19. The grill 20 can be formed of 
perforated sheet metal with holes 26 and is coated with a low-friction 
material such as polytetrafluoroethylene. Small radially extending tubes 
or conduits 24 have outer ends opening into the outer passage 23 and inner 
ends into the interior of the lower part 6a of the tube 6 and traverse the 
inner passage 10. Thus return air is sucked into the upper end of the 
outer passage 23 and thence passes through the conduits 24 into the lower 
tube part 6a and thence to the intake 3. 
Thus air rising in the inner passage 10 will be blown out of the top of the 
head assembly from the passage 28. Meanwhile air rising in the space 13 
between the outer tube 7 and the lower part 12 of the workpiece will bow 
it out at 31 at the lower end of the grill 20. Most of this latter flow of 
blow air will be sucked back down into the outer passage 23 along with the 
air blown in from the passages 28. This pulls the long neck of the 
workpiece back in at 25 against the foraminous grill 20.