APPARATUS AND A PROCESS FOR PROCESSING INHOMOGENEOUS RUBBER MIXTURES

DISPOSITIF ET METHODE DE TRAITEMENT DES MELANGES HETEROGENES DE CAOUTCHOUC

English Abstract

AN APPARATUS AND A PROCESS FOR PROCESSING
INHOMOGENEOUS RUBBER MIXTURES
Abstract of the Disclosure
A process for the production of homogeneous
compact mouldings from vulcanisable inhomogeneous rubber
mixture by extrusion or injection moulding, characterised
in that the inhomogeneous rubber mixture is forced to
form a progressing, continuously rotating bead within a
screw channel of the extrusion screw by all the constituents
of the mixture being compressed and forcibly separated
from one another again, and is subsequently converted
into a homogeneous plastic mass on passing through a
shearing gap in which the individual particles lose their
structure and an extrusion screw construction for carrying
out the process.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A screw extruder for processing inhomogeneous rubber mixtures, com-
prising at least one mixing and shearing section in which the depth of the
screw channels, measured in a direction transverse to the screw, increases
gradually and continuously from almost zero and suddenly decreases again to
almost zero, resulting in the formation of a "chamber" in which the material
being processed is set rotating.
2. A process for the production of homogeneous compact mouldings from
vulcanisable inhomogeneous rubber mixtures by extrusion or injection moulding,
which comprises forcing the inhomogeneous rubber mixture to form a progress-
ing, continuously rotating bead within a screw channel of an extrusion screw
by all the constituents of the mixture being compressed and forcibly separated
from one another again, and subsequently converting the bead into a homo-
geneous plastic mass on passing through a shearing gap in which individual
particles lose their structure.

Description

~8Z7~
Quality requirements in -technical rubber articles
become more stringent and, simultaneously, the cycle times
in processing rubber mixtures by injection moulding have -to
be shortened to increase output and to improve economics.
Attempts have been made to produce technical rubber
articles simpler, less expensive and with less energy consump-
tion by using powder-form "rubber mixtures" in strip or granular
form instead of completely homogeneous rubber mixtures.
"Rubber mixtures" contain the unvulcanised rubber
and all the additives such as fillers (carbon black or
silicates), plasticiser oils, vulcanisation agents (cross-
linkers), additives (antiagers, antiozonants or stabilisers)
and processing aids. They are processed simply by shaping and
vulcanising. To produce rubber mixtures the generally very tough
constituents have to be intensively mixed in internal mixers or
on mixing rolls with a high consumption of energy in either case,
Powder-form rubber mixtures are, however, easy to produce because
only powder-form cons-tituents starting with the powder-form
rubber have to be mixed with plasticiser oils, provided, the
necessary specially developed mixing units are avilable.
Powder-form rubber mixtures are normally compacted
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and homogenised before processing by extrusion or injection moulding. This
can be done in apparatus specially developed for this purpose.
The present invention relates to screw extruders for processing
rubber which, in addition to their normal delivering and compacting efect,
also mix, exert a shearing ef~ect and homogenise. These extruders are suit-
able for processing powder-form rubber mixtures directly, i.e. without inter-
mediate compacting.
The present invention provides a screw extruder for processing
inhomogeneous rubber mixtures, comprising at least one mixing and shearing
section in which the depth of the screw channels, measured in a direction
transverse to the screw, increases gradually and continuously from almost
zero and suddenly decreases again to almost zero, resulting in the formation
of a "chamber" in which the material being processed is set rotating.
The present invention also relates to a process for the production
of homogeneous, compact mouldings from vulcanisable, inhomogeneous rubber
mixtures by extrusion or injection moulding, in which the inhomogeneous rub-
ber mixture is forced to form a progressing, continuously rotating bead
within a screw channel of an extrusion screw in which the constituents of the
mixture are compressed and forcibly separated from one another again, and
wherein the rubber mixture is subsequently converted into a homogeneous
plastic mass on passing through a shearing gap in which ~h~ individual parti-
cles lose their structure.
The principle of the extrusion screw used in the extruder according
to the invention is shown in Figures 1 ~diagrammatic view) and 2 (cross-
section). The housing of the screw machine is denoted by the reference 4.
The reference 3 denotes the screw land which extends as near as possible to
the housing wall. The reference 1 denotes the body of the screw. When it
rota~es in the direction of the arrow (Figure 2), this screw delivers partly
a powder form material and partly a plastic viscous material.

`" 9.~LZ~
The depth of the screw chanrlel is variable in the lntermediate
space bctween -two screw lands. At Z th~ depth of the screw
charlllcl is very smal~ so -tha-t u natrow s~lea-ring gup is
; for~c~a. Thereafter, -the channel (!ept}1 increases g-raduaLly
and continuously llntil~ just before -the next s~learing gap
2' it suddenly clecrcases again so that a sharp edge is
formed in flont of the gap. In Fig. 2 , three such
"chambers" are provided per screw channel. Although this
has proved -to be best, it is also possible to provide one,
two or more chambers.
A partly powder-form, partly plastic material,
which is situated in the rotating screw, is initially
offered a space in which it is set rotating, as shown at
5, by the rotation of the screw. The conditions are
similar to those prevailing in the bead which is formed in
front of the gap of mixing rolls. Thereafter, the material
has to pass through the gap 2' where it is e~posed to intense
shear forces. In the example of Figures l and 2, this
process is repeated $wice for each revolution of the screw.
The screw does not have to be and, in general, is
not constructed in the manner described above over i$s
entire length. In general, a feed zone is present at its
beginning; in other words~ this feed section and the
surrounding screw housing are designed in such a way that
powder-form or inhomogeneous s-trip material can be received.
The feed zone is usually followed by a delivery zone and
then by a mixing zoneS as described above. This mixing
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zone may in turn be followed by ano-ther delivery ~one,
anothcr mix:ing zone such as de.scribe(l above and, fLn~lly,
by a discharge zone desigrled to build llp pressure.
The pitch of the screw lands ol th~ mixin~
section may by made variable according to -the diame-ter of
the screw. The shearing gap between the barrel sleeve and
-the screw channel may be from O.Ol to lO mm wide and the
width of the shearillg flight land may be from l.0 to 20 mm.
The channel depth between two shearing lands extends in an
arbitrary convex curve 6 from the shearing gap ~ to the
point of reversal and terminates in an arbi-trary concave
curve 7 which runs into the next shearing flight edge.
Practical tests were carried out in the mixing
section described above using a typical extrusion mixture
which is intended to be representa-tive o-f the whole range
of mixtures used in the rubber industry. The screw used,
complete with mixing section, had a diameter of 90 mm and
a length of preferably from 16 to 24 D.
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