Abstract: A plastic resin kneading/extruding method using one auxiliary equipment including a screw drive source and a resin feed hopper, and a plurality of extruding units each including a cylinder with a feed opening and a screw and having their operations composed of an extruding step of kneading and extruding the plastic resin and a cleaning step of cleaning the screw and the cylinder, characterized in that the extruding step and the cleaning step are sequentially alternated for each of the extruding units. Thus, it is possible to realize a kneading/extruding method and an apparatus therefor, which can accomplish synthetically continuous extruding operations without the extruding step zone being interrupted by the cleaning step zone and which can eliminate the defects of the prior art thereby to have a high extruding efficiency.

Abstract: A process for continuously producing a chewing gum base comprises the steps of continuously adding an elastomer, a filler and a plasticizer into a continuous mixer, subjecting the elastomer, filler and plasticizer to a highly distributive mixing operation followed by a restriction element and continuously discharging the resulting chewing gum base from the mixer while the adding and mixing steps are in progress.

Abstract: An apparatus and method for extruding an elastomeric polymer which is subject to periodic pressure surges. The apparatus includes a barrel having an upstream portion and a downstream portion, a hopper positioned for delivering polymer to the upstream portion of the barrel, a shaft rotationally mounted within the barrel and a drive means for rotating the shaft. The shaft has a conveying screw flight for conveying polymer downstream from the hopper through the barrel. The barrel has a discharge port located downstream of the hopper. A surge suppressor is provided on the shaft for reducing the pressure and flow rate surges in the polymer. The surge suppressor includes a screw flight downstream of the discharge port for urging the polymer upstream toward the discharge while permitting a portion of the polymer to flow downstream into the surge suppressor. A polymer seal is provided downstream of the surge suppressor.

Abstract: A rotary plug 1 has recesses 5 debouching on its side face 12 which are in communication with the inlet 6 and recesses 5 debouching on its side face 13 which are in communication with the outlet 7. These recesses 5 pass across a port 4 which permits passage to and from a unit consisting of cylinder 2 and piston 3.

Abstract: A screw of a screw-in-line type plasticizing device of an injection molding machine has a cylindrical main part and a spiral screw flight on the main part. A portion of the screw flight of the screw facing a resin supply port has a parallelogrammic axial sectional shape which is inclined towards the discharge end of the screw.

Abstract: A process and an apparatus for the direct, continuous and flexible modification of polymer melts by branching off a branch melt stream from the main melt stream to be modified, dispersing the initially introduced additive in the melt in a specially designed twin-screw extruder, and subsequent recycling of the additive melt concentrate to the melt stream to be modified.

Abstract: For the production of extrudates such as profiles or granules, pulverulent or finely divided ultra-high molecular weight polyethylene is processed on single screw extruders. By the use of a screw with a specific geometry, thermal degradation of the polymer into low molecular weight products is avoided. Furthermore, profiles are obtained which have perfect surfaces, are free of voids and pores, and have no internal stresses.

Abstract: An apparatus and method for controlling thermoplastic mixing in a plastic extrusion machines used for injection molding, blow molding or other reciprocating molding process. The plastic extruder having a rotating helical screw within a cylindrical shell with an automatic shut-off and mixing valve secured to the helical screw at the downstream end of the screw. The automatic shut-off and mixing valve includes a unitary valve body with a poppet. A plurality of mixing groove pairs are arranged extending substantially longitudinally on the surface of the valve body. Select angular bores extend from the groove pairs to the poppet. The poppet is axially movable within and rotatable relative the valve body. The poppet has an axial stem extending into a longitudinal axial bore within the valve body. An elastic means is positioned on the rearward end of the poppet maintaining the valve in a normally closed position against a seating surface of the valve body when there is no rotation of the helical screw.

Abstract: Side by side extruders provide an extrusion apparatus which has an imposed order of longitudinal displacement of the process material in relation to itself so that cross-shearing is directed to newly aligned material in a manner which achieves volumetric blending with little unproductive shearing. The extrusion apparatus mixes and extrudes polymer like materials with as little waste shearing as possible, so as to preserve the viscosity of the polymer materials and to produce a uniformity processed material of the highest quality possible from the starting materials.

Abstract: A method of thermoplastifying a synthetic resin which is displaced by a worm in a worm housing. High-frequency energy is applied to the plastic in one or more regions along the path of the plastic in which there is no or little frictional heat generated by the interaction of the screw with the housing.

Abstract: An apparatus for extruding material is provided which includes a barrel and a screw. The barrel has an axis and an internal operating surface, the internal operating surface defining a barrel interior and a barrel helical thread portion which includes a plurality of grooves reducing in cross-sectional area from a maximum value to a minimum value in a direction axially opposite to the axial direction of flow of material through the barrel. The screw has an external operating surface and extending into the barrel interior, the external operating surface of the screw forming a screw helical thread portion having at least a first and second groove. Each of the first and second grooves has an upstream end, a downstream end, and a body portion extending between the upstream and downstream ends thereof, the first and second grooves being axially co-incident at at least a transition location and being at least partially axially co-extensive.

Abstract: A method of continuously making chewing gum base employs low pressure, adequate residence time, and the flexibility of selectively feeding different ingredients at different locations on the mixer. The method involves the use of a paddle mixer, characterized by a plurality of paddles mounted on a longitudinal rotating shaft, preferably mounted on two parallel co-rotating shafts. The paddles cover at least about 40 percent of the active length of the shafts. This configuration provides for significant residence time in the mixer and low pressure gum base manufacture.

Abstract: In a serial two-stage extruder, the first-stage extruder has a screw including a feed screw section having a length within a range of 5 to 9 the major diameter thereof, a mixing screw section having a hexagonal cross section and a pitch of 1 to 3 times the major diameter thereof, and a metering screw section having a screw groove of an appropriate depth. The second-stage extruder includes an extruding screw and a barrel. The extruding screw has a screw groove of an appropriate depth and a dulmage formed at the tip end of the screw. The dulmage has a plurality of fins cooperating with fins of the barrel. Alteratively, in the second-stage extruder, the dulmage may have a plurality of axially spaced stages of fins. Configurations of the fins in each respective stage are identical but various characteristics of the fins vary from stage to stage. In this case, the barrel does not have fins.

Abstract: An apparatus is disclosed for processing extrudable materials in such a way as to apply a controlled and predetermined degree of shear force to the materials. Typically, an extruder apparatus includes a housing having first and second screws mounted therein for rotation on a common axis, the arrangement being such that longitudinal thrust forces exerted by the first screw are at least partially compensated by corresponding forces generated by the second screw, inlet means for introducing material to be processed into the housing and a shear generating device located between said screws, said device including a restriction through which material is forced from said housing to exit from said housing. The restriction is preferably an annular gap formed by a sleeve fixed relatively to the housing and a shaft portion which extends into the sleeve.

Abstract: A continuous method of making chewing gum base is provided wherein two continuous mixers are arranged in series. A higher viscosity portion including elastomer, filler, and elastomer plasticizer is continuously added and mixed in the first continuous mixer to form a blend. The blend is ventilated after leaving the first continuous mixer. The blend from the first continuous mixer is continuously added to the second continuous mixer. A lower viscosity portion including softener/emulsifier and, optionally, wax and additional elastomer plasticizer, is also continuously fed to the second continuous mixer and mixed therein with the blend from the first continuous mixer to form chewing gum base.

Abstract: A plasticating apparatus having a barrel and a screw rotatably disposed therein. The screw has a mixing section including a helical conveying flight to form a helical passage for transporting resinous material along the screw. The mixing section further includes two helical divider flights, each spaced from the conveying flight to divide the helical passage into three channels of varying cyclic depth in the helical direction of the channels. The portions of minimum depth of the channels define wave crests which are helically displaced from each other.

Abstract: A method and apparatus in the context of a rotating conveying screw situated within a cylindrical housing which has an inlet port on one end of the cylinder and a discharge opening on the other end of the cylinder. A resistance plate is mounted transversely to the conveying screw near the discharge opening for corotation with the screw. The plate is mounted to define a discharge gap between the opening and the plate. A sleeve on the housing is employed to adjust the size of the gap between the plate and the opening. The resistance plate can be a flat disc, conical, or other shape, so long as it is effectively attached to and rotates with the screw. The helical screw is preferably supported at both ends by a bearing means. Breaker bars are mounted on the transverse plate and extend over the gap between the discharge opening and the plate. Breaker bars may also be mounted to the cylindrical sleeve and extend over the gap between the sleeve and the plate.

Abstract: A plastifying extruder of the double-screw type, to avoid dead zones in the flow, has at its throttle location throttle disks on the screws formed with outwardly open recesses angularly equispaced on the disks. Pins distributed around the screws in the throttle region project radially into gaps between the disks and can penetrate to a greater or lesser extent into these gaps to provide a variable flow cross section.

Abstract: An extruder having a reduced nip ratio and a process employing the same.

Abstract: A screw-type resin injection apparatus for injection molding which includes a cylinder element having a cylinder chamber for receiving resin therein, and having, at an end portion, a nozzle for injecting the resin. A screw element is arranged within the cylinder chamber in a manner allowing rotation and axial movement. A rotary drive shaft is connected to the screw element telescopically so as to be capable of transmitting a torque. The rotary drive shaft is rotatively driven by a motor. A fluid pressure cylinder unit is arranged laterally of the cylinder element in parallel relation thereto. The fluid pressure cylinder unit is rotatably engaged with the screw element to drive the screw axially. Preferably, the screw element is provided with a mixing screw portion which has a screw groove small in pitch and large in lead angle and an axial groove extending axially.

Abstract: An extruder housing is formed for a double-worm or double-screw extruder by providing a steel body with a seamless one-piece structure and hot-isostatically pressing the body and the powder-metallurgical wear-resistant material lining the preferably conical bores of the housing to form a wear-resistant layer which is bonded to the steel body by a diffusion layer.

Abstract: An extrusion apparatus or extruder for mixing and extruding of thermo-plastic and rubber like process materials has a rotor member rotatable within the bore of a barrel member. Interposed between the feed end and the discharge end, the rotor and barrel members are provided with one or more mixing stages, consisting of process material transfers, rotor to barrel, barrel to enlarged rotor, enlarged rotor to enlarged barrel, enlarged barrel to enlarged rotor, enlarged rotor to barrel and barrel to rotor. Each process material transfer has the rotor or barrel member provided with multiple grooves which communicate with co-acting receiving rotor or barrel member multiple grooves. Each revolution of the rotor transfers process material from each sending groove to all of the co-acting receiving grooves, an action which reduces the thickness of the transferred material in the ratio of one to ten.

Abstract: In the preparation and production of plastic materials by a continuously operating double shaft screw-type extruder, the efficiency of production often depends on good delivery of material to the extruder. In accordance with the invention for processing various types of plastic materials, a primary stage is mounted in a pressure-tight sealed manner on the housing of the extruder, the primary stage being formed by a vertical double conical container in which a pair of screw-type helices are rotatably supported and driven from above. The helices are connected to two replaceable feed elements at their lower ends so that delivery of material to the extruder is achieved without any intermediate dead spaces.

Abstract: A process and apparatus for the production of plastic material containing an additive in which the plastic material is supplied into a mixing tank to undergo vigorous mixing therein and the mixed plastic material is discharged by an extrusion screw at the bottom of the tank into a conveying cylinder in which the extrusion screw extends. The mixed plastic material is conveyed by the screw in the conveying cylinder for discharge of the plastic material at a discharge outlet. The plastic material is heated in the conveying cylinder to achieve melting of the plastic material and the additive is supplied to the melted plastic material within the conveying cylinder so that the additive is thoroughly mixed with the melted plastic material before discharge thereof at the discharge outlet.

Abstract: A screw extruder having a cylinder, an extrusion head, a screw in the cylinder and a motor for driving the screw in rotation has insert members which extend into the cylinder to effect plastification and homogenization of the material being extruded. The web of the screw is provided with annular recesses into which the insert members project. Some of the insert members are stationary, in the form of pins projecting radially into the cylinder, while others are movable so as to vary the extent of their projection into the cylinder. The movable insert members may be pins extending radially into the cylinder and movable radially, rotatable threaded pins extending tangentially into the cylinder, rotatable pins extending tangentially into the cylinder and having non-circular portions in the cylinder, or sector-shaped discs which are movable pivotally.

Abstract: A screw extruder having a barrel with a longitudinal bore for receiving a rotatable screw, a transverse bore disposed in the barrel tangential to the longitudinal bore for receiving and supporting a pin wherein the pin extends partially into the longitudinal bore. The tangential arrangement of the pin substantially decreases the susceptability of the pin to bending or breaking. The pin may also be adjustably positioned in the transverse bore and further may be subject to thermal heating and cooling.

Abstract: A method of manufacture of a extruded vinyl product to provide a textured stake construction includes an extruder screw and a die having specific structural, and dimensional characteristics and temperature parameters. The extruder screw has a flight depth in the meter zone which is about one half the flight depth in the feed zone and deeper than extruder screws of conventional design. The die includes a frusto-conical section connected to a cylindrical passage. The extruded vinyl stake will have a knobby or textured surface.

Abstract: A screw kneader for a plastic material in which a pair of longitudinal screws are rotatably mounted in respective bores for conveying plastic material through the kneader. The screws have spaced portions for advancing the plastic material from an inlet region to an outlet region and between the spaced portions is a section with reversed threads opposing plastic flow to effect mixing of the plastic material. The threads of the intermediate section are provided with openings to permit passage of up to 20% of said plastic material in the direction of advance. Transfer elements with helical threads connect opposite ends of the threads in the intermediate section with the threads of the inlet and outlet regions. The transfer elements of the two screws mutually engage one another to strip plastic material from one another and from the walls of the bores.

Abstract: A screw for use in an plasticating injection apparatus includes a plurality of interrupted primary flights. An outer surface of a primary valley is formed by the primary flights and features a plurality of plateaus. The plateaus span adjacent sections of the primary flights and define shallow levels between deeper levels of the primary valleys. This depth differential contributes to a turbulent mixing action which, along with the primary and secondary valleys, contributes to improve mixing action within a metering section of the screw. The primary flights are interrupted by secondary valleys which are of opposite hand to the primary valleys.

Abstract: In the manufacture of a battery cell, a shear thinning cathode material that has been mixed to a homogeneous state is transported to an extrusion opening while being subjected to a non-decreasing shear stress. The shear thinning cathode material is extruded onto a conductive substrate material at a constant rate. An electrolyte material is applied over the shear thinning cathode material, and an anode material is applied over the electrolyte material.

Abstract: A plasticizing unit is provided for palsticizing material which is subsequently supplied to an injection molding press. The unit includes a screw disposed in a barrel which has an inlet, an outlet, a reservoir zone portion, and a plasticizing zone portion. The plasticizing zone portion of the barrel and the screw each have a continuous helical groove of opposite hand as the other groove and the grooves transfer material therebetween to effect plasticizing of the material. The reservoir zone portion of the barrel receives a shot volume of material which has been plasticized during passage through the helical grooves of the barrel and the screw. A volume displacement member which is axially movably disposed in a bore of the screw is movable within the reservoir zone portion to selectively control the shot volume of plasticized material received in the reservoir zone portion.

Abstract: A Transfermix cold feed extruder having a plasticizing section that includes a plurality of screw flights therein in a transfer zone that is formed over a common length of the screw and barrel of the extruder. In the transfer zone, a helical groove in the screw varies in cross section from a full area to a zero area, and a helical groove of opposite hand in the barrel correspondingly varies in cross section from a zero area to a full area. The extruder is provided with a set of flow deflectors in a cut in the screw positioned immediately upstream of the plasticizing section. By the coaction of the flow deflectors with the plurality of screw flights, an increase of throughput for difficult-to-extrude compounds, with an improvement in the quality of the extrudate, is obtained in a surprising manner.

Abstract: Apparatus for extruding plastic and/or rubber material has a screw extruder, which plasticizes the material and delivers it to a gear pump which develops sufficient pressure for extrusion of the material through an extrusion head. The apparatus is characterized by the fact that the axes of gears of the gear pump and the axis of the screw of the extruder are parallel to one another. The housing of the gear pump has a side opening into which a cylindrical bushing and the screw of the extruder extend so that the material is delivered by the extruder directly to the gear pump as a non-oriented plasticized mass. The extrusion head may be a transverse extrusion head with an axis parallel to the axes of the gear wheels of the gear pump and the axis of the screw of the extruder. A second like extrusion apparatus including a screw extruder and a gear pump may be connected to an opposite side of the extrusion head.

Abstract: Equipment for briqueting plant stalk-produce, includes a conical-screw compactor, a tubular press-cavity, an annular degassing device and a control device including a molding pressure sensor. The annular degassing device adjoins a discharge aperture of a compactor space with a plurality of narrow-axial degassing slits open to the atmosphere, which issue into the discharge aperture of the compactor space.

Abstract: A double worm extruder for kneading units along the worms which are rotatable in the same sense. Each kneading unit has three or more circumferentially offset polygonal disks which have sealing surfaces at diametrically opposite locations closely juxtaposed with the inner wall of the housing. These sealing disks are flanked by edges which are inclined to the axes of the worms so as to insure greater flow cross sections past the disks and thereby avoid shear peaks in the extruded material and thermal decomposition thereof.

Abstract: In the production of foamed plastic articles using a screw extruder, it is known to introduce a gaseous foaming agent into the plastic while the latter is passing through the extruder. It has been found that a foamed thermoplastic with pores or cells of uniform size and distribution can be produced by passing a gaseous foaming agent, e.g. nitrogen gas through a central passage extending substantially the entire length of the extruder screw, discharging the agent into the thermoplastic proximate the discharge end of the extruder, and mixing the foaming agent with the plastic immediately prior to discharge of the mixture into a mold.

Abstract: A polymer-extruder screw has a mixing section with relatively deep channels between flight lands at a proximal end, progressively shallower channels with a progressively larger screw core of a central transition section, and finally a metering section having a plurality of at least three pressure channels juxtaposed helically between flight lands having channel interruptions comprised of helical inclines and constant-radii tops in helical progression to a distal end of the extruder screw where the pressure channels end with equal depth at an interface with a metering gate of an extruder. The pressure channels can be symmetrical or asymmetrical in angles of incline and decline at opposite helical sides of the channel interruptions for optimizing channel depths, for positioning of maximum channel depths in relation to tops of channel interruptions, for equalizing channel depths at the distal end of the extruder screw and for optimizing incline and decline advantages for polymers with different characteristics.

Abstract: Baker's dough moves from feed hopper (21) through dough inlet opening (25) into auger housing (19). Reduced pressure is maintained in the auger housing by gas exhaust port (28) and vacuum pump (27). The depth and amplitude of the helical cavity (31) in the pumping section (34) of the auger (20) is greater than the depth and amplitude of the helical cavity (33) in the gas exhaust section (35) of the auger, so that more resistance is applied by the interior cylindrical surface (29) of the auger housing to the dough in the gas exhaust section (35) than in the pumping section (34). Therefore, the dough does not clog the gas exhaust port, allowing increased vacuum to be applied to the housing, thereby firmly drawing dough to the auger. Additionally, transverse dough feed and transverse dough exhaust with respect to length of the auger tends to reduce the tendency of the dough to move circumferentially with the rotation of the auger, thereby enhancing the pumping efficiency of the auger.

Abstract: The invention relates to an apparatus for producing monodisperse powdered rubber and includes an extruder and individual conveying, grinding and discharge sections with respective cooling units corresponding thereto. The cooling units are arranged outside of the extruder and receive the grinding stock after each grinding process in order to cool and transport the stock to the feed opening of the following conveying, grinding and discharge section of the extruder.

Abstract: Multi-screw, continuous mixing and kneading machine for plasticizable compounds with screws, which rotate in a housing and which penetrate the cylindrical housing section, enclosing the screw concerned, and exhibit feed and discharge zones and intermediate melting, mixing and kneading zones, where the feed and discharge zones are formed by intermeshing screws that rotate in the same direction, and kneading elements are disposed in the melting, mixing and kneading zones. The kneading elements comprise axially extended polygons, whose surfaces are approximately flat and whose edges travel by one another with small clearance relative to the envelopes of the respective other polygon so as not to intermesh while rotating, where the edges of the housing section enveloping them maintain a distance that is significantly greater than the clearance.

Abstract: A compact and precision extrusion system for extruding strips of rubber compounds with profiles of accurate dimensions. Strips of rubber compound are uniformly heated and mixed in an extruder and then passed through a low pressure drop strainer to remove foreign matter. The strained compound then is fed directly into a gear pump which delivers precise volumes of the rubber compound immediately into the spread chamber of an extrusion head after which it passes through a die to form a precise strip of the rubber compound. The strainer and extrusion head are movably mounted with respect to the gear pump for axial separation from the gear pump for easy removal of the rubber compound from the various components of the system to facilitate compound changes. A control system regulates the speed of the extruder to provide a near constant pressure at the entrance port of the gear pump to precisely regulate the rate of extrusion of the compound from the extrusion head.

Abstract: A process is provided for modifying the residence time distribution of materials in an extruder which results in a residence time distribution comparable to that obtained in a continuous stirred tank reactor. An extruder is provided which has a section which modifies the residence time distribution of materials loaded into the extruder.

Abstract: A one-piece barrel for a hot-fed extruder extends from drive means at its upstream end to its discharge mouth near the downstream end of the feed screw. This one-piece extruder barrel has a split-level, in-feed opening adapted for connecting a strong, reinforcing converging hopper in close tolerance therewith for feeding molten plastic into the extruder barrel through this in-feed opening. A feed screw propels along the barrel molten plastic fed into the barrel through the in-feed opening. The barrel is a one-piece, integral, strong, cylindrical member with an axial bore whose entire length may be finish machined at one time. Such one-time finishing of the entire bore of a barrel is not possible in prior hot-fed extruder barrels, because they were built up from multiple sections assembled end-to-end to form the whole barrel.

Abstract: A highly viscous or dough-like material, especially glass fiber-filled polyester, polyamide or polypropylene is fed from a rotating funnel via a feed worm lying along a generatrix of the funnel directly into an extruder or an injection molding cylinder and without passing through an elbow or other jacket. The system is thereby able to operate at reduced pressure with least damage to the fibers and reduced drive energy for the feed worm or screw.

Abstract: An extruder for poorly miscible extrudates which includes a pin-lined barrel section and a transfer mixing section used together. Restricting pins are arranged in the transfer mixing section in the region of the greatest barrel channel volume and permit universal use of the extruder with respect to the extrudate. The combination of the two mixing systems allows in a surprising way an increase in the material throughput of 60% to 100% with the mixing quality remaining the same and a halving of the drive torque. Due to a constant number of channels of the barrel channels and screw channels, the number of barrel flights and screw flights remains independent of the channel cross-sectional area of barrel and screw. A helical barrel groove in the feed zone of the extruder barrel additionally improves the mixing quality of the extrudate and the output of the extruder.

Abstract: In a process for the manufacture of thick, preferably sheet-shaped, thermoplastic molded parts or molded parts having areas of variable cross sections, at least one part of the plastic material is conveyed as a melt into a heatable and optionally coolable compression mold and is subsequently compressed. Plastic granules are mixed in with the melt before the melt is conveyed into a compression mold. A device to prepare such mixture of melt and plastic granules includes a first section designed as a melt extruder and a subsequent section designed as a mixing extruder. The granules can be fed into the melt between both extruder sections or into the second section.

Abstract: The present invention provides an extruder for highly foamed plastic material for coating a central conductor of a coaxial insulated cable with an insulator made of a mixture of a plastic material, preferably polyethylene, and an inert gas, such as nitrogen gas, to serve as a foaming agent. An inert gas is introduced approximately halfway down the extruder through a lead-in portion. At the lead-in portion of the extruder is a vent portion wherein the screw is provided with a plurality of stirring blades extending radially from the screw toward the inner surface of the extruder. These stirring blades provide a mechanism by which the inert gas is mixed thoroughly into the plastic material. The above invention provides an apparatus for producing an insulating material in which an inert gas (such as nitrogen), rather than hydrocarbon chloride fluoride, is used as a foaming agent, thus avoiding potential harm to the ozone layer.

Abstract: The specification is concerned with a process and apparatus for continuous manufacture of moisture curable silicone compositions.

Abstract: A process and an apparatus for optimal operation of a high-speed extruder in which a pin-lined barrel section and a Transfermix section, both known per se but separately used, are utilized together in an extruder. The combination of these mixing systems allows in a surprising way an increase in the material throughput of 60% to 100%, with the mixing quality remaining the same and the drive torque being halved. The static extrudate pressure can be further increased by arranging an adjustable restriction device downstream of the extruder screw, thereby permitting a further increase in the output rate.

Abstract: An extruder for elastomeric material has an axial screw in an extruder chamber defined by a cylindrical housing. Two radial planes of vented kneading pins are provided upstream of the extruder housing succeeded by spaced-apart planes of non-vented kneading pins. Each vented kneading pin includes small apertures at the inlet end of the pin which communicate with a stepped bore of gradually increasing crosssectional area from the inlet to the outlet thereof whereby gases within the elastomeric material can be vented through the apertures communicating with the stepped bore to the exterior of the extruder housing.