Note: Descriptions are shown in the official language in which they were submitted.
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`EXTRUDED, SURFACE-TEXTURED, SYNTHETIC RESIN
STRANDS OF HOLLOW CROSS-SECTION
The present invention relates to extruded strands
of thermoplastic resin, which strands have a hollow cross-
section.
Hollow strands of this type for the most part have
a smooth surface. Such strands can also be prepared with a
textured surface which is uniform in the direction of
extrusion if a correspondingly formed extrusion tool or a
shaping arrangement (calibration arrangement) having a
corresponding structure on its inner surface is employed.
However, it is often desired to prepare hollow strands of
this type with a surface structure which varies in the
extrusion direction, for example in order to effect a more
uniform distribution of light passing therethrough or to
impart a more pleasing appearance.
It is known to prepare hollow strands having a
granular surface structure by the extrusion of a partially
thermoplastic forming mass which contains finely-divided
synthetic resin particles which are difficultly fusible
and therefore not thermoplastic. After leaving the
extrusion nozzle or the calibrating arrangement, the
included particles produce a granular surface structure
as a result of their thermoelastic restoring forces. The
inhomogeneous forming masses suitable for such a purpose
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are only processed reluctantly because waste material,
particularly the useless extrudate which is produced
when an installation is started up, cannot be used again
together with waste material from a homogeneous forming
mass.
Extruded strands of a thermoplastic synthetic
resin which have a solid cross section can be readily
provided with a textured surface varying in the extrusion
direction by running them through an embossing calender.
However, this process cannot be used for strands having a
hollow cross section, since such strands would be pressed
together between the calender roll. The present invention
solves the problem of providing extruded strands of a
thermoplastic synthetic resin, which strands have a hollow
cross section and a textured surface varying in the extrusion
direction. By such a textured surface is to be understood
every structure not showing the same cross section on every
section of surface taken across the extrusion direction.
According to the invention, thermoplastic synthetic resins
are such resins which completely melt under the conditions
of extrusion or --as in the case of pigmented synthetic
resins-- permit an exact reproduction, by molding, of a
tool surface. Forming masses containing portions which
do not melt or which melt only with difficulty and which
have an influence on the surface structure of shaped
bodies produced therefrom are unsuitable.
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Summary of the Invention
The invention is a method for making an extruded
hollow strand of a thermoplastic synthetic resin having at
least one textured surface with a surfaced texture variable
in the extrusion direction. The method comprises extruding
a thermoplastic synthetic resin through an extrusion nozzle
having one or more core pieces to form a hollow stand,
introducing the extruded hollow strand into a vacuum calibra-
tion apparatus together with at least one air-impermeable
flexible band having a negative textured surface, said band
covering the surface of the strand which is to be textured
thereby. The band and strand adhere to each other with a
force of at least 0.1 N/cm , so that the band supports the
strand adhering thereto within the vacuum calibration apparatus.
The strand and band are passed together through the vacuum
calibration apparatus while maintaining the strand and band
in contact with the apparatus by the application of a vacuum
to the band and while cooling the resin of the strand to
a temperature below its softening point. The strand and band
are then separated as they leave the calibration apparatus.
Detailed Description of the Invention
For preparing the hollow strands according to the
invention, a hollow strand is prepared by the extrusion of
a thermoplastic synthetic resin through an extrusion nozzle
having one or more core pieces, which strand is then cooled
beneath the softening temperature of the synthetic resin in
a forming apparatus or calibration apparatus. According to
the invention, a flexible band provided with a "negative
textured surface" is introduced into the calibration arrange-
ment together with the synthetic resin strand and is separated
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from the strand after exit from the calibration arrangement.
The "negative textured surface" describes that
form, complementary to the texture to be imparted to the
synthetic resin strand, against which the textured surface
of the synthetic resin strand is molded. Since the textured
surface can have any desired form, the form of the "negative
textured surface" is also widely variable according to choice.
The depth of the structure, i.e. the difference between the
highest ridges and the deepest indentations within the
structure, can vary between fractions of a millimeter up to
several millimeters. In order tG be moldable from the surface
of the flexible band, the textured surface of the hollow
strand must be largely free of undercutting, which is not
however necessarily true of the negative structure of the
band. For example, a structured surface can be molded on a
sieve mesh without the forming mass penetrating through the
sieve openings and in this way
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inextricably anchoring itself therein by the formation
of undercuts. Within the aforementioned limits, there is
a great degree of freedom of design. Irregular or
geometrically regular or ornamental structures, letters and
characters, pictorial representations, or structures with
particular technical effects can be made use of, whereby
even broader limits pertain for the extent of the area
of the structural elements than apply for their depth.
A variety of textured surfaces can be formed directly from
planar textured materials, i.e. from rough papers, graded
or woven structures formed from metal threads or strips,
sandpaper, open-pored foams, crackled lacquered surfaces,
and the like. Other structures, for example surfaces
decorated with ornamental forms, can be imparted to the
hollow strand by molding the structured surface as a
negative structure on a band and then texturing the hollow
strand on such a band according to the present invention.
The material of the band provided with a negative
surface structure is so chosen that the band can be
removed from the hollow strand after the latter has been
cooled off. The band should at least be flexible enough
so that it can be applied to and removed from the surface
of the hollow strand with a short radius of curvature,
for example on a guide roll. An adhesive force of at
least 0.1 N/cm2 is required between the band and the hollow
strand when working with a band impermeable to air and
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vacuum calibration apparatus. In this case, the vacuum
acts then only on the band and supports the hollow strand
adhering thereto. If the adhesive force is too small,
the hollow strand separates from the band and collapses.
When using a air-permeable band, for example a woven band,
the vacuum can act through the band directly on the
surface of the hollow synthetic resin strand and maintain
its cross section. In this case, the adhesion value can
also be below the aforementioned value. This is also true
in all cases not employing a vacuum calibration apparatus,
but an apparatus employing an excess gas pressure in the
cavity of the strand.
Surprisingly, by a proper choice of the band
material, a sufficient adhesion arises immediately upon the
mutual entry of the textured band and the thermoplastic
hollow strand into the mouth of the calibration apparatus,
particularly if the hollow strand is minimally compressed
thereby. An adhesion of sufficient magnitude is present
between most of the synthetic resins which come into
consideration for use in the process and metals such as
steel, aluminum, and copper. The hollow strand adheres
to a textured band of synthetic resin if there is a
sufficient difference in polarity between the materials.
Thus, the adhesion of polymethyl methacrylate or polyvinyl
chloride on a corona-treated polyethylene, cross-linked
polyvinyl acetate, polyamides, and rubber is over 0.1 N/cm2,
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but is generally beneath this value on non-pretreated
polyethylene or polypropylene.
The adhesion between polymethyl methacrylate and
polyvinyl chloride or non-cross-linked polyvinyl acetate is
usually so strong that these materials cannot be separated
from one another after cooling. A certain adhesion of the
band is also desired, for example, if the band is to
remain as a protective coating on the surface of the
hollow strand until the latter is used, for example a
paper band or strip.
Bands or strips of paper, cardboard, and
synthetic resin can be provided in a known fashion with
the desired negative textured surface by means of embossing
rolls. If the structured band is itself made of a
thermoplastic synthetic resin, then it is sufficient that
its surface structure not soften plastically so long as
it is in contact with the hot hollow strand and until the
surface of the latter has been conformed to the structure.
Therefore, if the band is to be used several times, it is
advantageous to employ a material which will not soften
plastically under the operating conditions: in the case of
synthetic resins, then, such bands should be used which are
crossed-linked or hardened after impartation thereto of the
negative textured surface.
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On passage through the calibration apparatus,
the hollow strand is pressed against the inner surface of
the calibration apparatus by an excess gas pressure in the
hollow or cavity of the strand or by a reduced pressure
acting in the forming channel of the calibration apparatus.
An excess pressure in the cavity can be maintained in a
known fashion by means of a stopper attached to the
extrusion nozzle core and which slips through the hollow
strand, or by a periodic melt-sealing of the end of the
strand, if a pressurizing gas can find access to the cavity
by way of a gas conduit in the extrusion nozzle core.
Preferably, calibration apparatus employing a vacuum is
used since in this case a gas-tight seal of the cavity is
dispensable.
Forming masses suitable for the preparation of
strands having a hollow cross-section according to the
present invention are, for example, acrylic glass,
polyvinyl chloride, polycarbonate~ polystyrene, or impact-
resistant copolymers of styrene. Acrylic glass is
particularly preferred. By this term is to be understood
polymethyl methacrylate and copolymers predominantly
comprising methyl methacrylate and having molecular weights
between 100,000 and 250,000. As needed, the forming masses
can be optically clear, opacified, or colored as desired.
A preferred embodiment of the invention involves double-
walled planar structures having two parallel outer walls and
a plurality of supports, arranged between the outer walls
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and bonding the outer walls. These structures can be
textured on one or on both of the parallel exterior walls.
A better understanding of the present invention
will be had by referring to the accompanying drawings,
wherein Fig. 1 is a side view, in section, of an
- extrusion train and calibration chamber for texturing a
hollow synthetic resin strand according to the invention
and Fig. 2 is an end view of a preferred strand embodiment.
More in particular, Fig. 1 shows extrusion
nozzle 10 having nozzle core 11 from which issues hollow
strand 12 of a thermoplastic resin. Strand 12 enters
calibration apparatus 13 together with flexible band 14,
such as of wire mesh, having negative textured surface 15
which can vary in the extrusion direction. Within
apparatus 13, a vacuum is applied to strand 12 and/or
band 14 through passages 16. Cooling tubes 17 present
within apparatus 13 ultimately cool strand 12 to a
temperature below the softening temperature of the resin.
On leaving apparatus 13, band 14 is separated from
solidified strand 12 at guide roll 18, leaving strand 12
with textured surface 19 thereon. Band 14 may run through
apparatus 13 continuously or may be taken off a supply roll
and rewound after separation. These variations are not
shown.
Fig. 2 is an end view of a preferred embodiment
of a hollow extruded structure 20 comprising parallel outer
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walls 21, one or both of which can be provided with
textured surface 22 according to the invention. Walls 21
are separated by plurality of supports 23 running the
length of the structure and defining a plurality of
cavities 24 within the structure.
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