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This invention relates to blown plastic film in sheet form and an improved
device for producing same. More particularly, the invention relates to
films, made from polyethylene or other thermoplastic resins, prepared in
tubular form from a blowhead, but converted to sheet form as part of the
manufactured process prior to winding the film for shipment or storage.
The invention further relates to improved equipment for manufacturing
sheet film from tubular blown film, characterized by a slitting device,
dual takeoff rolls, and improved cooling means as part of the
manufacturing equipment. The invention further relates to improved methods
for cooling extruded blown tubular plastics film exiting from a
conventional blowhead.
In general, manufacture of tubular thermoplastic films, such as low or high
density polyethylene, polyvinyl chloride, or nylon films, or mixtures
thereof prepared by coextrusion utilizing a blowhead adjacent to an
extrusion die, is well known in the art. Utilization of internal and
external air cooling devices, such as cooling rings, to cool the molten
film, together with takeoff rolls which form a vertically supported
tubular bubble, is also generally known. Representative disclosures are
found in U.S. Pat. No. 3,088,167 issued May 7, 1963 to Corbett; U.S. Pat.
No. 3,170,011 issued Feb. 16, 1965 to Cheney et al; and U.S. Pat. No.
3,709,290 issued Jan. 9, 1973 to Upmeier.
In conventional processing using a blowhead, the film is prepared in
tubular form, flattened, and taken off onto winding rolls. The present
invention relates only to the more efficient manufacture of blown film
sheeting.
The use of slitting means in other types of film extrusion and tubular
blowing devices is disclosed in U.S. Pat. No. 3,291,876 issued Dec. 13,
1966 to Justus, British Pat. No. 1,429,064 (1966), British Pat. No.
1,188,817 (1970) and U.S. Pat. No. 4,003,973 issued Jan. 18, 1977 to
Kurokawa et al. These disclosures deal with different methods of film
manufacture. In the Justus patent, the tubing is extruded downwardly, the
bubble being formed between the die and the takeoff rolls; an air nozzle
projects into the tube, and, at the end of this nozzle are a pair of
cutting blades which continuously slit the tube. In British Pat. No.
1,429,064 heat softening tubular plastic is extruded over a gas bearing
and contacted with a mandrel whose temperature is maintained below the
softening temperature of the tube; a slitting means is provided in
conjunction with this device above a tube collapsing means. British Pat.
No. 1,188,817 employs a trapped bubble process for the production of
thermoplastic polymeric film wherein a coolant is circulated through a
liquid-gas heat exchanger which is located in the interior of the bubble
to effect cooling of the interior gas. This reference discloses slitting
devices adjacent the nip rolls for providing the film in sheet form. U.S.
Pat. No. 4,003,973 discloses the production of polypropylene film by
extruding resin downwardly from an annular die in contact with cooling
liquids on the outside and inside of the film, with the side ends of the
film being cut lengthwise before the film is withdrawn by the nip rolls
and the cooling liquid being withdrawn through the slit in the film. None
of these references disclose the particular method of the present
invention, whereby the slitting operation is used to facilitate internal
air cooling and exchange.
In accordance with the present invention, there is provided a process for
manufacturing thermoplastic blown film sheeting utilizing a tubular film
blowhead apparatus which comprises forming a blown tubular film bubble
from an extrusion blowhead, the blowhead having internal and external air
cooling means, conveying the tubular film upwardly within a collapsing
frame positioned above the freeze line of the film, collapsing the film to
a relatively flat cross-sectional tubular form, and immediately prior to
the takeoff rolls, slitting the edges of the tubular film with suitable
cutting devices, such as knives, the cutting devices being positioned
adjacent to the edges of the film and opposite the top of the collapsing
frame whereby separate sheets of film are formed, when withdrawing the
sheets utilizing a double set of takeoff rolls. The internal cooling air
present is conveyed upwardly and out of the interior of the slit tubular
film bubble through the aperture between the takeoff rolls; the rolls are
provided with means to adjust the size of the aperture by separating the
takeoff roll axes.
Internal cooling air in the present invention can be withdrawn either
through a central stack as well as upwardly through the gap created when
the sheets are formed. Alternatively, the exhaust stack in the blowhead
can be closed or replaced with air inlet supply conduits so that the air
flow through the blowhead is inward only, thereby further increasing the
cooling potential and overall process rate.
The process and apparatus of the present invention offer a number of
advantages, mainly a substantial rate improvement in the manufacture of
film sheeting. Moreover, conventional internal air exchange designs tend
to concentrate the high temperature air in the upper portion of the
tubular bubble above the reach of the customary central exhaust stack. In
the present invention, the dual takeoff rolls and slitting of the tubular
film prior to the takeoff rolls creates an aperture permitting the natural
convective flow to assist discharge of hot air, and in most cases,
eliminates the need for the customary central exhaust stack. As noted
previously, additional cooling air can be introduced by replacing the
exhaust stack with air inlet means, thus increasing the cooling efficiency
and throughput potential of the overall system. An air exhaust hood is
positioned above the aperture in the tube to remove internal cooling air.
The aperture is controllable to provide for further regulation of air flow
by adjusting the relative positioning of the takeoff roll axes which can
move in a horizontal plane.
It is well known that corona treatment of film is facilitated with warm
film; a further advantage of this invention is that it enables this
treatment step to be carried out more efficiently by covering each takeoff
roll with a dielectric material suitable for corona discharge. The energy
requirements of the process are reduced, since the film is still warm when
the corona treatment is carried out. The ozone generated may be removed
using a common fume hood. Further advantages involve improvements in sheet
film quality in that the blocking tendency, that is, the tendency of film
layers to adhere together when removed in tubular form, is effectively
eliminated.
The process and apparatus of the present invention are generally applicable
to all types of plastic film sheeting capable of being manufactured with a
blowhead, and is particularly suitable for use in connection with
polyethylene film.
The invention is specifically illustrated with reference to the
accompanying drawings and the following description thereof.
FIG. 1 represents the apparatus used in practicing the invention. Molten
plastic, such as polyethylene, is conveyed from an extruder (not shown)
through a die 2 which is part of a blowhead housing 1. The blowhead is
fitted with both external 3 and 3a and internal 4 air cooling rings which
have annular discharge passages for directing a circumferential air stream
upon the molten film tubular bubble 5 as it leaves the extrusion die.
Internal cooling air may be introduced and removed through the exhaust
stack 6 filled into the blowhead. Above the freeze line 7 of the tubular
film is positioned a collapsing frame 8 and 8a for flattening the tubular
film and at the upper terminal portion of the frame are positioned knife
edges 9 and 9a for slitting the tubular film along its edge to produce two
film sheets 10 and 10a which are taken off by the double set of
horizontally adjustable takeoff rolls 11 and 11a which are movable to
control the size of the aperture 12 formed as a result of slitting the
tubular film. Internal air is removed through exhaust hood 13 positioned
above the controlled aperture 12. A dielectric roll and nip is indicated
at 14 and 14a, a corona treatment bar adjacent thereto at 15 and 15a with
ozone exhaust hoods at 16 and 16a.
FIG. 2 is a top view of the flattened tubular film 5, the knife edges 9 and
9a and the collapsing frame 8 and 8a.
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