CA1039911A - Apparatus for extruding a tubular film of thermoplastic material including a die and a cooled rotary mandrel - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The invention provides an apparatus for producing a predominantly mon-axially molecularly oriented film of thermo-plastic polymeric material, comprising a die for extruding a tube of thermoplastic polymeric material along the axis thereof, means within said die for the introduction of gas into said tube and maintaining sufficient gas pressure within said tube to prevent the collapse thereof, roll means spaced from said extruding die for hauling off said tube, at least one mandrel positioned within said tube at least one said mandrel being rotatable about said axis means positioned within at least one said mandrel for cooling said mandrel, means for blowing gas onto the outer surface of the film between said die and the mandrel nearest to said die, and means within said tube for heating at least the inner surface of said tube, and at least one of said roll means and said die being also rotatable about the die axis so that the film is spirally stretched upstream of said roll means.

Description

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The present invention relates to the production of a predominantly monaxially oriented film of a thermoplastic material extrudable through an annular die. In particular, the present invention relates to an apparatus for use in the process.
This application is a divisional application of - copending application No. 172,798 filed May 31, 1973.
;- According to known methods thermoplastic films are ` prepared by the blown film or trapped bubble process, in which a thermoplastic film tube is extruded from an annular die, inflated by air to the required size, cooled suitably with externally blown air, collapsed to a flat tube whilst being drawn down by the haul-off rollers and wound into rolls of flattened tubular film. In addition the thermoplastic tubular films can be produced using internal and/or external mandrels to supplement or replace air cooling. Subsequently said tubular ~-films may be further stretched in order to obtain higher ;
orientation.
,; Methods of preparing predominantly monaxially oriented `
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; 20 films of a thermoplastic material have been previously proposed `;-`
~, ~ in U.S. patent No. 3,'4 ~ issued Sept. 19, 1967 to Dyer, describes a process of preparing a monaxially oriented continuous :, .:, . :
strip of thermoplastic film in which the direction~of orientation is disposed at an angle to the longitudinal direction of said ` film, which process comprises thermoplastic material in its , ~ ~
formative state from extruding a tube of thermoplastic material -~

from a rotating annular die, cooling the rotating tube to below ,: :: .
the temperature at which said material is in its formative state ;
by passing it over a cooled internal mandrel, advancing the rotating tube and heating the tube to the orientation temperature range by passing it over a heated mandrel, drawing said rotating tube, cooling said rotating tube to below the temperature at , . . .. .
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which said material is in its formative state. Said tube is slit along helical lines, in order to obtain a monaxially oriented continuous strip of thermoplastic film, in which the direction `
of orientation is at an angle to the longitudinal direction of said film. The tube obtained before the slitting step is carried out is a tube which has been oriented longitudinally. -British patent No. 1,072,174 issued June 14, 1967 to Reifenhauser Kommandit Gesellschaft describes a process of prepar-ing a tube of thermoplastic material, in which the tube just after having left the annular die is pulled over a slowly rotating cali-brating mandrel, so that the tube is subjected to a limited rotary movement and a movement in the direction of squeezing rolls. The -helix angles formed in the tube obt-ained do not exceed 10, as there is no relative rotational motion between the die and the nip of the squeezing rolls.
U.S. patent No. 3,576,935, issued April 27, 1971 to Dyer vaguely indicates that it is known to prepare a tube of thermo-.:
plastic material by means of an annular die, a mandrel and a nip-roll in which one of said three parts rotates slowly, the remaining two parts being kept stationary. The slow rotation ~ -may be also a reciprocating motion.
When the stretching operation is carried out below a certain temperature, dependent on the polymer, an opaque film results owing to the formation of microvoids. When higher temperatures are used then the microvoids will not be present.
Reference will ~ be made to the accompanying drawings in which, Fig. 1 is a top view of a spirally cut cold stretched predominantly oriented opaque film and Fig. 2 is a side elevation of an apparatus according ;~
to one embodiment of the present invention.
Thus if the tubular layflat film is cold stretched and then cut spirally a predominantly monaxially oriented opaque : --``\` :
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film having the structure of Fig. 1 is obtained. Referring to Fig. 1, reference numeral 1 indicates the direction or orienta- ;~
tion and reference numeral 2 designates a side fold. The side ~ folds lie parallel to the now oblique orientation direction.
-i This situation gives rise to periodic variations in the final -product caused by (a) non uniform stretching at the edges of the layflat film and (b) the folded crease formed as the tubing passed through the nip rollers. Heretofore it has not been possible to preven-t the structure of the film adjacent the side folds from being different from the structure of the remainder of the film.
This means that the film thickness and impact strength in the side fold region is different from the thickness and irnpact strength of the remainder of the film.
It has now been found possible to manufacture improved l -and highly oriented films which have a helical orientation and are naturally transparent or translucent instead of opaque when prepared from a non pigmented resin. The improvement is firstly ~i ;~~, :.
due to the elimination of the aforernentioned stretching operation '!, on the layflat film after the nip rolls which causes non-uniform ,. -stretching at the edges of the layflat film, and secondly to ~;
the fact that the final crease formed by the nip rollers is at an angle to the orientation direction.
Copendiny application No. 172,798 discloses and claims ~ `~
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a predominantly monaxially oriented film tube or composite film tube of a thermoplastic material extrudable through an annular die haviny a direction of orientation within the tube that follows .' 1' ' a helical path, at an angle with the axis of the tube of at least 10C.
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The copending application also discloses and claims .~ ;, a method of manufacturing a predominantly monaxially oriented film of a thermoplastic material extrudable through an annular ~ ~
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film die, ~7hich method comprises extruding a -tube of thermoplastic material above its meltiny temperature, inflating said tube by ~ 3 ~
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`` lV39~1 a gas, partially cooling the tube with an external blown gas, maintaining the required diameter by means of at least one mandrel, collapsing the tube to form a flat tube and hauling it off by means of a haul-off assembly, the film being spirally stretched by contact with at least one mandrel between the die and the hauling-off assembly, one oE the die assembly and the haul-off assembly being rotated continuously in one direction at a constant angular velocity, the remaining elements in con-i tact with the film being rotated at an angular velocity of at ;~ 10 least 0.
The present invention provides an apparatus ;for pro-ducing a predominantly mon-axially molecularly oriented film of ` thermoplastic polymeric material, comprising a die for extruding a tube of thermoplastic polymeric material along the axis --~ thereof, means wîthin said die for the introduction of gas into said tube and maintaining sufficient gas pressure within said ~ -tube to prevent the collapse thereof, roll means spaced from said extruding die for hauling off~said tube, at least one man-1 drel positioned within said tube at least one said mandrel being rotatable about said axis means positioned within at least one , said mandrel for cooling said mandrel, means for blowing gasb onto the outer surface of the film between said die and the mandrel nearest to said die, and means within said tube for heating at least the inner surface of said tube, and at least one of said roll means and said die being also rotatable about , the die axis so that the film is spirally stretched upstream of said roll means.
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The films are so-called spirally stretched films in ' ~' which the resulting direction of orientation within the tube -~ 3q follows a helical path which direction of orientation and`the -j direction of scratch lines formed by contact of the polymer ~ with one or more mandrels are mutually disposed at an angle.
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The predominantly monaxially oriented film of a thermoplastic material extrudable through an annular die, may be produced by extruding a tube of said thermoplastic material above its melting temperature, inflating said tube by a gas to prevent collapse, partially cooling the tube with an externally blow gas, further cooling it with internal and/or external mandrels, collapsing it to a flat tube and hauling it off. The ;
film is spirally stretched by contacting it with at least two -~
mandrels between the die and the haul-off device, either the ' die or the nip roll assembly of the haul-off device rotating - continuously in one direction at a constant angular velocity, ;
and the remaining elements in contact with the film rotate at ;- the same or a different angular velocity as the die or the nip roll assembly, alternatively are kept stationary.
Consequently, spirally stretched films of the parent ; invention may be manufactured according to several embodiments of the above method. Some of the embodiments are: ~
Mark I II III -Nip rolls Rotating Stationary Rotating*
Upper mandrel Rotating Stationary Stationary Lower mandrel Stationary Rotating Rotating Die Stationary Rotatin~ Stationary !` ' ' '" : ' " _ ',~'''' ' .
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- Mark IV V VI
Nip rolls Stationary Rotating)' Rotating Upper mandrel Rotating Rotating)' Rotating Lower mandrel Rotating Rotating in opposite direction)' Die Rotating idem)' Stationary B In the above Marks~a rotating die means a die rotating separate-ly or in combination with a rotating extruder.
In the examples given above where the die rotates ~-additional variations include a die fixed to the extruder, both -~
the die and the extruder rotating as a single unit. Further- ;
, . .. more, it should be understood that the upper and/or lower mandrels can be supplemented by additional mandrels acting as ~`
~` upper or lower mandrels. ~ `
In the above table the apostraphe and asterisk have `~
" the following significance: ' The die and lower mandrels -together with a framework housing the nip rolls and ~ upper mandrel are, for example, rotating in a clock-; wise direction whereas the nip rolls and upper mandrel -.
are rotating in an anticlockwise direction within said framework.
* Nip roll assembly rotational direction is counter to that of the lower mandrel. ~ '`
Thermoplastic materials that can be extruded through an , annular die are disclosed in "Plastic film technology" by W.R.R.
: Park, pages 21 - 22 (1969). Examples of said materials are `
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polyethylene, polypropylene, vinylpolymers such as polyvinyl-chloride, polystyrene, polyvinylidene chloride, vinylchloride/
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vinylidenechloride copolymers, superpolyamides, e.g. adipic acid-30 hexamethylene diamine condensation polymers and polyesters such as polyethylene terephthalate.
O~viously thermoplastic materials that can be extruded -~
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., through an annular die may contain customary additives such as plasticizers, colouring materials, pigments, fillers, etc. ~ -It is also possible to use the above and o-ther polymexs so that the film according to the invention is a combination of more than one polymer. - -The preferred thermoplastic materials for the present ".,:. ~
process are polyolefins. g -Irrespective of what embodiment of the novel method . ':'... .
according to the invention is useu for the manufacture of the film, the tube may be extruded from an annular die, inflated , by air to prevent collapse, partially cooled with externally blown air, then further cooled by a mandrel system collapsed to a flat tube having been drawn down by the action of the haul off wound into rolls of tubular film.
~ ~o special requirements in addition to those needed by Conventional mandrel systems exist with respect to the materials of which the mandrels are constructed be their function internal r ;. .
or external contact witn the film. '`;~
The diameter of the mandrels is not critical bu-t from ~ ;
a practical point of view their size is related to the diameter f of the die. Under certain conditions it is preferable to have both mandrels of the same diameter but this need not be the case.
' Owing to shrinkage of the polymer as it cools and the i~
resulting frict!ional forces between the film and the mandrel a slight taper can be used to advantage to reduce these forces.
~' The surface roughness and individual contac-t lengths on the mandrels are arranyed to give continuous production at the desired level of friction and cooling. The use of a knife edge at the bottom side of any mandrel may also be of advantage. Preferably said mandrel has a surface roughness of at least 5 micro inches CLA.
Cooling of the mandrels is normally achieved by passing water of the required temperature and flowrate through the mandrel system.
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Slight creasing during the deflation step may occur.
Introducing a difEerential speed relationship between the elements ~ --can help to overcome this defect. In addition techniques such as ~ ~
vacuum between mandrels or pressurizing of the film onto a mandrel ~ -can be used advantageously in combatting this problem, as can gas pressure between the mandrel farthest from the die and the nip roll assembly. ¦-rrhe distance between the die and tile lower mandrel is ,~
largely dependent on their relative diameters for a given die size and the desired physical properties of the product.
An air ring through which air is directed onto the outer surface of the extrudate at a position between the die and the ~-lowest mandrel is beneficial in order to partially cool the outer F
surface of the film, thus giving additional stability to the -process. The exact position of the air cooling will depend on several factors such as melt temperature, type of material used, output rate of material, distance of the lower mandrel to the die, !-the direction, velocity and volume of air coming from the air ring.
Preferably, in additlon to said air ring, controlled gas, normally air under controlled conditions should be present at all air spaces formed by the film and any two adjacent elements.
Means for reheating the film may be present where ~;
appropriate. The presence of such means may be advantageous in view of the fact that the stretching step may be performed in two stages and that additional heat may be required for causing the tubular film to be bonded by an appropriate technique during J
production.
The angle of orientation in the tubular material may be modified by changing the relative speeds of the haul-off and ~` 30 rotating members of the system. Preferably the method is carried -out in such a way that the angle of orientation is about 45 to the layflat fold line.

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' Obviously the method may be carried out so that the - final product hauled off is a ready-for-use cross-laminate by using known bonding techniques prior to wind-up but not excluding ; slitting followed by a conventional lamination step.
The freeze line, where crystallization is visually ,: , i~ taking place should preferably be wholly on a mandrel or in a -region between two adjacent mandrels.
It appears that the ultimate tensile strength (UTS) of . .
the film increases, coupled with a reduction in the elongation, as the final film becomes thinner.
Increasing the volume of air cooling appears to slightly increase the UTS.
An apparatus, using a two mandrel system, for producing ~F`:
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~ a predominantly ~onaxially oriented film of a plastic polymeric . ~ "" ,, .
7 material, provided according to the prevent invention comprises ;~
(vide fig. 2).
1. an annular film die

2. a mandrel support system ~

3. a lower mandrel ~ -

4. an upper mandrel :, f~ :.

5. water and air services passing through the die ~ -

6. mandrel water pipes ~ ~
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7. an internal heater ~-3. nip or hauling-off rollers 9. polymeric film 10. an air-ring f l~ote that the deflation system is not included.
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In tlle table of examples results obtained according to Mark I, III, IV and VI are stated~
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The monaxially oriented film tubes may be bound internally in face to face engagement without slitting, by means of methods known from U.S. patents 2,3~2,657 and 3,576,935 and British patent 1,072,074 indicated as welding, puncturing, lub-welding, crystal- ~ -welding and co-extrusion, so that the direction of the orientation of one face makes an angle with the direction of the orientation --of the other face. Preferably the lines of orientation of the layers make an angle to each other and the lines of orientation ~ ' and the scratch lines in each layer make an angle of 1-89. It is most preferred that the lines of orientation of at least 2 layers are at right angles to each other.
A, spirally stretched tubing produced using the ~lark I
system was laminated by the following technique. ~ polyurethane 2 component adhesive system was introduced onto the inner surface of the tubing (adhesive conveyed through mandrel system and sprayed onto surface), whilst manufacturing said tubing. During ' the deflation and nipping stages the tubing was collapsed into layflat form thus bringing the now opposite sides of the tube into contact one with the other. l`he final bond characteristics were achieved during the storage of the lay-flat tubing in roll form.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for producing a predominantly mon-axially molecularly oriented film of thermoplastic polymeric material, comprising a die for extruding a tube of thermoplastic polymeric material along the axis thereof, means within said die for the introduction of gas into said tube and maintaining sufficient gas pressure within said tube to prevent the collapse thereof, roll means spaced from said extruding die for hauling off said tube, at least one mandrel positioned within said tube at least one said mandrel being rotatable about said axis means positioned within at least one said mandrel for cooling said mandrel, means for blowing gas onto the outer surface of the film between said die and the mandrel nearest to said die, and means within said tube for heating at least the inner surface of said tube, and at least one of said roll means and said die being also rotatable about the die axis so that the film is spirally stretched upstream of said roll means.