Abstract: Tubular bodies are extruded from thermoplastic material by forming a tubular preform at an exit of an extruder head die after which the tubular preform is then blown to the configuration of a cavity of a blow mold. The position of the preform relative to the blow mold is regulated and/or controlled, and waste portions produced in the blow mold are separated from the blown hollow body. The weight of the separated lower waste portions and/or the net weight of the hollow body when in accumulation head operation is measured in order to control and/or regulate the position of the preform and to change the die slit in the event of a weight deviation from a reference value.

Abstract: A parison thickness control system includes an extruder die and a mandrel which are movable relative to each other to change wall thickness of a parison extruded between the extruder die and the mandrel during an extrusion or forming cycle of the parison. A parison control pattern is derived by manually inputting master points each defined in terms of a target parison wall thickness and an elapsed time during the parison extrusion cycle. A further parison control pattern is derived by performing the spline interpolation to interpolate between the master points. A still further parison control pattern is derived by correcting the above-noted further parison control pattern to allow the maximum or minimum value of the master points to indicate a limit value for the target parison wall thickness during the parison extrusion cycle. The parison extrusion cycle is divided into a given number of equal time intervals to define cycle points.

Abstract: Extrusion of a tubular parison for blow molding is controlled according to a program of set point data defining parison wall thicknesses and extrusion rates. Progress of purge of plasticized material from an accumulator through a variable annular gap is measured. Parison profile set point signals defining wall section thickness at points correlated with progress of purge of the accumulator are produced in response to the measured progress. Purge velocity set point signals defining purge velocities correlated to selected profile points are produced in response to progress of purge of the accumulator. Variation of the annular gap is controlled in response to the parison profile set point signals and velocity of purge of the accumulator is controlled in response to the purge velocity set point signals.

Abstract: An extruder for extruding a multilayer parison and a method for doing the same. The extruder includes a cylinder having a cylindrical hole and a die slit which extrudes a parison, a ring piston movably fitted in the cylindrical hole, a multilayer head provided between the die slit and the ring piston, and a plurality of annular nozzles concentrically provided in the multilayer head, the annular nozzles facing and being opened to the side of the die slit.

Abstract: In a process and apparatus for the production of hollow bodies from thermoplastic material, the walls of the hollow bodies are of a laminate structure having at least first and second layers, by means of extrusion blow molding, the pressure within the extrusion head and more particularly at the location at which the laminate is formed by the discharge of flows of material from a storage chamber in the head is maintained within limits such as to avoid disturbing the laminate to such an extent as to have an adverse effect on its function. The pressure at the time of a transition between filling and emptying of the storage chamber and between emptying and filling of the storage chamber is maintained substantially at a pressure level which approximately corresponds to the storage chamber emptying pressure.

Abstract: A method of making tubular bodies from thermoplastic material by extruding a parison through an extruder head die slit which is then expanded in a blow mold. The position of the parison and particularly a thickened portion thereof is regulated and/or controlled relative to critical cross-sectional regions of the hollow body blown therefrom. The hollow body of the separated lower waste portions and/or the net weight of the hollow body is measured in order to control and/or regulate the position of the thickened portion of the parison and to change the die slit as need be upon deviation from a reference value. The latter is particularly beneficial in accumulation head operation, while in continuous operation, either the hollow body net weight and the weight of the lower waste-portion is measured and compared with a reference characteristic or the hollow body net weight is compared to a reference characteristic and markings reflective of the critical cross-sectional regions of the blown article are sensed.

Abstract: A plastic container having base and sidewall portions is provided with corrugations as a unitary part of the sidewall portion. The sidewall portion is of a thickness that would not be capable of supporting itself without the unitary corrugations. The corrugations serve to rigidify, and this impart self-supporting capabilities to, the sidewall portion. The upper edge of the sidewall preferably includes a unitarily formed rim which assists in maintaining the sidewall in its generally tubular condition (and thus more easily facilitates the placement of articles in the container). The container may be closed by a flexible and gatherable terminal portion which is an integral extension of the rim or the upper edge of the sidewall. The containers of the invention are formed by an extrusion blow-molding technique using an appropriately configured blow mold.

Abstract: A container having upper and lower portions, the lower portion having self-supporting side walls and a bottom wall. The upper portion is flexible and non-self-supporting. Preferably, the upper and lower portions are formed of plastic material with the upper portion forming a unitary continuation of the lower portion. The entire container may be formed using blow molding techniques or other conventional means such as co-extrusion.

Abstract: A parison forming device is disclosed for forming hollow polymer extrudes of variable wall thickness in either the circumferential or longitudinal directions, or in any combination thereof. The parison forming device consists of a die-head assembly and mandrel assembly, concentrically arranged, creating an annual opening. The inner or outer circumferential perimeter of the annular opening may be selectively modified at specific radial locations through radially displaceable slides positioned about the annular opening. The slides, attached to either the die-head assembly or mandrel assembly, may be actuated during parison formation via hydraulic or other methods so as to provide suitable wall thickness profiles for later finished polymer products.

Abstract: In the discontinuous manufacture of multilayer, co-extruded, hose-like thermoplastics material preforms used in the production of large volume multilayer hollow members in a divided blow mold, at least two different annular shaped material melts are brought together in series within an axially extending annular piston to form a multilayer material melt. An annular flow channel in the piston receives the layers of material melt. The flow channel widens in a funnel-like manner toward an annular storage space. The preform is subsequently ejected from the storage space through an annular nozzle gap by the annular piston moving in its axial direction. The annular piston acts as a co-extrusion head and is pressed upward by the multilayer material melts. In supplying the material melts from extruders into the flow channel of the piston, the material melts flow first in the extrusion direction and then are redirected for flow radially inwardly into the flow channel of the piston.

Abstract: Blow molded container and related process are provided wherein a container having at least an upper, non-self-supporting, flexible portion is blow molded to include an integral press-lock type closure about its upper open end. The blow mold cavity is formed to provide a rib or bead about one-half the periphery of the container open end, and a hollow recess about the other one-half of the periphery of the container open end. the upper end of the finished container may be flattened along its upper edge and closed by pressing the rib into the recess. It one embodiment of the invention, the lower portion of the container may be of greater thickness, but nevertheless integral with the upper portion, so as to be stable or self-supporting. In another embodiment, the container may be non-self-supporting throughout while, in still another embodiment, the container may be self-supporting throughout its length.

Abstract: The invention concerns a process and an apparatus for the production of hollow bodies from thermoplastic material, the wall of which is in the form of a laminate, by extrusion blow molding. Tubular preforms for making the hollow bodies are first produced in an intermittent mode of operation, the wall thereof also being in the form of a laminate which includes a suitable number of layers. Within an extrusion head having an annular storage chamber for receiving the laminate-formation preform and an annular piston for emptying the storage chamber, the flows of material for forming the layers of the laminate of the preform are passed through the annular piston into the storage chamber in which they combine to form the laminate. The width of the outlets at which the flows of material issue from the annular piston can be so selected that it is proportional to the thickness of the respective layers to be formed therefrom in the laminate produced in the storage chamber.

Abstract: The invention relates to a blow molding machine for the blow molding of hollow articles, with an accumulator head (1) for producing a preform and with a blow mold for forming a hollow article from the preform. After completion of a blow molding sequence, a piston positioning signal is derived by a control system (15), to regulate the position of a discharge piston (4) in the accumulator head (1), from a comparative signal of the actual position and set position of the discharge piston (4). The set position is determined by integration from a previously specified speed set point profile. The control system (15) further superimposes an influx signal on the comparative signal, which corresponds to the change in volume of plastic flowing from an extruder (2) to replace the discharged contents of the accumulator head (1).

Abstract: A blow molding machine for the blow molding of hollow articles includes an accumulator head (1) for producing a preform and a blow mold for forming a hollow article from the preform. After completion of a blow molding sequence and the filling of the accumulator head (1), a piston positioning signal is sent by a control system (15) to regulate the position of a discharge piston (4) in the accumulator head (1) derived from a comparative signal of the actual position and set position of the discharge piston (4). The set position is determined by integration from a previously specified speed set point profile. The control system (15) further superimposes an influx signal on the comparative signal, which corresponds to the change in volume of plastic flowing from an extruder (2) to replace the discharged contents of the accumulator head (1).

Abstract: A mold-core rod combination for producing a plastic preform for forming blow molded plastic bottles which comprises: a neck portion defining an opening; a tubular sidewall portion depending therefrom; and an integral base structure depending from the tubular sidewall portion to a closed end; the preform having an outside wall face and an inside wall face with one of these in the base structure having integrally formed thereon a plurality of filets, extending longitudinally of the preform and defining a continuous reinforcing ring of varying thickness spaced from the closed end and circumscribing the base structure, wherein the filets decrease progressively in width and radial thickness at least from the reinforcing ring toward the closed end. The preform is capable of forming a blow molded plastic bottle with a bottom portion having a continuous reinforcing ring of circumferentially continuous radially extending alterations in wall thickness with a regularly undulating cross-section along the circumference.

Abstract: A method for manufacturing large-volume hollow bodies of plastics material with multiple-layer walls. The apparatus defines an accumulation chamber and has a nozzle outlet. A plurality of material layers are joined into a multiple-layer plastic material melt within a tubular plunger which is movable in axial direction of the apparatus. The multiple-layer plastics material melt is conducted in a flow duct which widens in the shape of a funnel toward the accumulation chamber. The tubular plunger is forced by the melt to move in axial direction away from the nozzle outlet. The material melt is ejected as a multiple-layer extruded material by moving the tubular plunger in axial direction toward the nozzle outlet.

Abstract: An apparatus for manufacturing a hollow plastic product is provided. In accordance with one aspect, into a mold half having a tubular groove portion and a projecting groove portion which projects outwardly from the tubular groove section is supplied a parison into the tubular groove portion and a clump of molding material into the projecting groove portion. When blow molding is carried out by introducing a pressurized gas into the parison, the clump becomes integrated with the parison thereby providing a hollow plastic product of unitary structure. In accordance with another aspect of the present invention, a parison extrusion nozzle is provided with at least two passages each of which is connected, preferably through a valve, to a corresponding dispensing unit for dispensing a desired molding material. A control unit is provided as connected to each of the dispensing units to control the supply of molding material so that there is obtained a parison having regions of different molding materials.

Abstract: From the mold cross section and the position of the parison in the mold that parison wall thickness distribution (function of the wall thickness of the parison around the parison) is computed, which would result in the desired wall thickness distribution of the blow molded article; the die gap is then shaped to at least approximate the so computed parison wall thickness distribution.

Abstract: Plastic dust-proof boots which are made of thermoplastic resin by means of blow molding and provided at least on one end of a bellows with a stationary ring to be fitted to an engaging groove formed on an attaching shaft and methods for manufacturing the plastic boots. In the boots, an inner circumference base upright wall of the stationary ring is formed at a nearly vertical state, and the inner circumference base upright wall of the stationary ring is connected through a sleeve having an inner diameter so that the attaching shaft can be loosely fitted. In manufacturing the boots, the stationary ring and the sleeve are molded during the parison molding previously or during mold tightening at the blow molding stage, and the blow molding is performed while the stationary ring and the sleeve are grasped between molds.

Abstract: A method of forming a container having upper and lower portions, the lower portion having self-supporting side walls and a bottom wall. The upper portion is flexible and non-self-supporting. Preferably, the upper and lower portions are formed of plastic material with the upper portion forming a integral continuation of the lower portion. The entire container may be formed using blow molding techniques or other conventional means such as co-extrusion.

Abstract: Plastic cartridge for packaging sealants is made by blow molding an extruded parison into a metal mold with a cavity whose interior diameter gradually increases toward the bottom in correspondence with the variation in thickness between the top and bottom of the parison, which variation in thickness results from longitudinal stretching of the extruded parison under its own weight. A plastic cartridge having equal inside top and bottom diameters is produced.

Abstract: An accumulation head for a blow molding machine which produces a parison of a first melt material having a stripe of a second extruded plastic material formed by directing a second extruded plastic material into the stream of a first plastic material as the material in the accumulator is extruded to form the parison.

Abstract: A heat-softening plastic material is extruded such that the plastic material forms a pine-shaped outer member and at least one rod-shaped core member is positioned in the inside space of the pipe-shaped outer member. Divided forming molds are disposed about the extruded plastic material except for an entrance opening for introducing gas such that only the pipe-shaped outer member is expanded by the pressurized gas and the thus formed container is subsequently cooled and hardened. The thus formed hollow container has at least one rod-shaped support extending between the upper and lower ends on the inside of the hollow container. The container will not expand and remains stable even if a high pressure fluid is introduced into the inside of the container.

Abstract: A central body portion has first, second and third concentric cylinders of successively increasing diameter encircling the central body portion and each other in spaced relationship to form three concentric annular spaces for receiving molten material. Only one end of each of the first, second and third cylinders and the body portion are rigidly attached to each other so as to maintain the rigid spaced relationship. A mandrel is attached to the outer end of the central body portion and a die is attached to the outer end of the outer most concentric cylinder in spaced relationship to the mandrel to shape the molten material into a pipe having a multi-layered wall.

Abstract: An enclosure having a self-supporting side wall formed of a plurality of laterally spaced ribs with non-self-supporting thin webs therebetween whereby the ribs and webs cooperate one with the other to provide an enclosure having shape-retaining characteristics. A bottom wall is provided constituting an integral extension of the side wall with the ribs being scored at the juncture of the side and bottom walls to permit the material to be folded such that the side wall and bottom wall lie generally normal one to the other. The enclosure has a closure portion including either a substantially rigid portion having a neck with an opening or a self-supporting rib and web closure terminating in the neck and opening. A cap is provided for sealing the enclosure. Blow molding and blown film techniques are used to form the enclosure.

Abstract: A die ring for use in a blow molding process is provided with depressions at the downstream end of the land to provide the parison with a longitudinally thickened rib and depressions at the upstream end of the land to counteract parison deformation caused by the downstream depressions.

Abstract: The invention relates to a method for the manufacture of tubular preforms. Its purpose is to improve the flow pattern in the plastics chamber and simultaneously to shorten the dwell time of the plastics. This is achieved, in the course of the dip blow method, by the fact that the geometric form of a plastics chamber (that is distinct from a separate screw chamber) is changed and the volume is decreased so that the tubular preform acquires a certain thickness or thickness distribution.

Abstract: A machine has a nozzle formed with an annular passage centered on an axis and having at an outer axial end an annular mouth. A plastic mass is introduced under pressure into the inner axial end of the passage at two diametrally opposite locations and mixes and passes outwardly along the passage to emerge from the mouth as a tube. The axial outward flow of the mass along the passage is impeded upstream of the mouth to pressurize the mass with a pressure substantially greater than atmospheric. Such flow is impeded either by restricting the passage or by allowing the volume of the passage to increase against a predetermined counterforce. The mass will form when it leaves the nozzle a tube of almost perfectly uniform wall thickness.

Abstract: A method and apparatus for producing hollow plastic articles are disclosed. The articles have a blow-molded portion and a compression-molded portion, and are formed by downwardly extruding plastic at at least one extrusion rate to progressively form a parison having a downwardly facing open end from a first location adjacent an extrusion head toward a second location axially spaced therefrom. A male mold member is provided at the second location. Portions of the open end of the parison are engaged by a plurality of fingers, while the open end travels toward the second location and the engaged portions are moved downwardly at a rate at least corresponding to the extrusion rate. During the travel toward the second location, portions of the open end are radially expanded and guided over the male mold member.

Abstract: A die ring for use in a blow molding apparatus is provided with depressions at the downstream end of the land to provide the parison with a longitudinally thickened rib and depressions at the upstream end of the land to counteract parison deformation caused by the downstream depressions.

Abstract: In extrusion blowing a hollow member, a preform is extruded and then blown in a mold to produce the final member. The weight of the hollow member is determined after its removal from the mold and compared to a reference value, thereby to adjust the size of the extrusion discharge opening to control the amount of material in the preform. The length of the preform is detected to control the movement of the mold thereby to keep a substantially constant length of extruded preform, while the time required to produce a preform is also maintained at a substantially constant value.

Abstract: Improved extrusion head for use in forming plastic containers or tubular films (optionally with multi-ply walls) by the extrusion-blowing process. The typical extrusion head used in this process has flat compensating rings bounding a portion of the frustoconical flow passages which ultimately feed the extrusion orifice. These are replaced by spherical segmental rings having a beveled surface in the form of a spherical, conical, toric or ogival shape which bounds said flow passage and thus eliminates stagnating recesses (of the type encountered through use of the prior art flat rings).

Abstract: In the production of a tubular extruded section of thermoplastic material, the actual length of an extruded section at the end of the extrusion operation may be smaller or greater than the length of the extruded section which passed through the discharge orifice of the extrusion head during extrusion. If the wall thickness of the extruded section is varied during extrusion in accordance with a given program, inaccuracies in regard to distribution of the wall thickness over the length of the extruded section can be reduced by detecting the length of the extruded section which passes through the discharge orifice, and controlling the means for producing the programmed variation in wall thickness, in dependence on that discharge length. The same effect can be achieved by controlling the means for producing the programmed variation in wall thickness, in dependence on discharge speed and time.

Abstract: An extrusion head with an annular extrusion die gap or orifice for fabricating a hollow extrudate of thermoplastic material. The extrusion head has a central die core and outer die casing ring cooperating to define the die gap. The inner boundary of the die gap is formed by a wall of the die core, while the outer boundary of the die gap is formed by a wall of the casing ring. At least one boundary wall of the die gap comprises independently movable sections that move with respect to the other boundary wall to change the width of the die gap.

Abstract: A pipe of oriented thermoplastic polymeric material having an integral socket is manufactured by expanding a tubular blank having a portion of greater wall thickness which is positioned in a portion of a female mould defining the socket. The tubular blank is heated by circulation of hot water to a temperature at which deformation will induce oriention of the polymer molecules, and is then expanded radially outwards against the mould by application of internal pressure. In this manner the wall thickness and rigidity of the socket are increased.

Abstract: During the production of a preform of thermoplastic synthetic material by means of extrusion from an extruder head provided with a slot-shaped exit opening, the width of which is changed according to a specific program during the extrusion of the preform in order to change the wall thickness thereof, the wall thickness of at least one section of the preform should be controlled by at least one additional program, which is superposed on at least one portion of the first program, which is longer than the additional program. In this manner, one achieves greater flexibility in respect to fulfilling the requirements of each situation on the programming, namely both in respect to the resolution and in respect to precise determination of the location of the individual switch points, even when a very great number of changes of the slot width must be undertaken over a very short section of the preform or during a very short time period during the extrusion process.

Abstract: An extrusion head for use in blow molding machines for molding parison having a single layer or wall or a plurality of layers or walls, in which a plunger is slidably interposed between a head body and a core and the plastic material is successively accumulated in an accumulation chamber defined between the extruding end of the plunger, the head body and the core so that the faster the plastic material is introduced into the extrusion head, the faster it is extruded.

Abstract: An annular exit die for an extrusion head of the type having a die casing ring and a tapered die core cooperating to form an annular die gap or orifice for fabricating a hollow extrudate of thermoplastic material. The tapered die core is adjustable in the axial direction and changes the radial cross-sectional dimension of the die gap as a function of its position. The casing ring has a lower end and an inner wall defining the outer boundary of the die gap. The wall has a step at the lower end of the ring. The step has an inner radially undulating circumferential profile and a radially undulating edge defining the boundary between the step and the lower end of the casing ring. The inner wall of the ring also has another step disposed radially inwardly of the first-mentioned step. This step has an inner radially symmetrical profile and a radially symmetrical edge defining the boundary between the two steps.

Abstract: A plastic resin blow molding machine to form bottles from a thermoplastic resin forms a tubular parison of plastic resin by extruding the molded plastic from the circular orifice formed between a vertically aligned mandrel and a forming die. The mandrel is vertically adjusted relative to the forming die by an adjustment nut having screw threads which mate with the screw threads of the mandrel. The adjustment nut is rotatably mounted on a plate and is fixed to a ratchet ring having evenly spaced indentations, the ratchet ring being turnable by a ratchet wrench having a spring-loaded pawl.

Abstract: In extrusion blowing a hollow member, a preform is extruded and then blown in a mold to produce the final member. The weight of the hollow member is determined after its removal from the mold and compared to a reference value, thereby to adjust the size of the extrusion discharge opening to control the amount of material in the preform. The length of the preform is detected to control the movement of the mold thereby to keep a substantially constant length of extruded preform, while the time required to produce a preform is also maintained at a substantially constant value.

Abstract: A mold has a core surrounded by an outer sleeve which defines with the core an annular axially extending passage. An adjustment element in the form of a ring or sleeve is displaceable in this passage so as to vary the radial dimension of a tubular blank that is extruded from the outlet end of the passage. The adjustment ring or sleeve may be elastically deformable by means of hydraulic cylinders or screws so as to create the desired thickness at the desired location, and may even be displaced during operation of the apparatus so as to vary the thickness of the blank from one region to another in the direction in which it is extruded.

Abstract: Hollow fibers, particularly useful as semipermeable membranes in separatory devices such as blood dialyzers, having a non-uniform cross-sectional area along the length thereof. The fibers have recurring points of maximum outside diameter and minimum outside diameter and such points may recur on a regular basis or at random. Fiber wall thicknesses may be substantially uniform or non-uniform. Methods and apparatus for making the fibers are disclosed.

Abstract: Improved paint brush bristles formed of synthetic materials are hollow and tapered with a central axial hollow. The tapered hollow bristles are characterized by their consistent wall thickness, i.e., ratio of cross-sectional hollow area to cross-sectional wall area remains consistent from one end of the bristle to the other; and by their consistency in neck-down location.

Abstract: Hollow fibers, particularly useful as semipermeable membranes in separatory devices such as blood dialyzers, having a non-uniform cross-sectional area along the length thereof. The fibers have recurring points of maximum outside diameter and minimum outside diameter and such points may recur on a regular basis or at random. Fiber wall thicknesses may be substantially uniform or non-uniform. Methods and apparatus for making the fibers are disclosed.

Abstract: A motive power battery casing is blow molded in an oblong configuration of rectangular cross-section. The die is notched so as to create longitudinal ribs of relatively greater thickness of the molding material in a generally cylindrical parison. The casing mold is vented in the bottom at its corners to assure full development of the casing corners. The casing is formed with relatively thin side walls and a relatively thicker collar of the molding material near its top in order to provide sufficient strength for heat sealing at the top.

Abstract: An accumulator head is described as having an annular accumulation chamber, an annular ring-piston movable axially through the chamber, an inlet through which plasticized resinous material is forced into the chamber between opposing ends thereof, and a special feature built into the ring-piston which causes material, flowing into the chamber, to wipe the face or tip of the piston and keep it clean and free of any material. A discharge passageway leads from the accumulation chamber to a discharge orifice adjacent which a parison is formed by resinous material from the chamber. Another inlet is provided in the discharge passageways as a means of providing a final, outer layer or laminate of material to that flowing out the discharge passageway from the chamber.

Abstract: A mold has a core surrounded by an outer sleeve which defines with the core an annular axially extending passage. An adjustment element in the form of a ring or sleeve is displaceable in this passage so as to vary the radial dimension of a tubular blank that is extruded from the outlet end of the passage. The adjustment ring or sleeve may be elastically deformable by means of hydraulic cylinders or screws so as to create the desired thickness at the desired location, and may even be displaced during operation of the apparatus so as to vary the thickness of the blank from one region to another in the direction in which it is extruded.

Abstract: Disclosed are an integral plastic bowl, waterway and trap for a toilet and the method of making the product. The bowl is concave and leads into a trap which may be interconnected with a standard closet flange. The waterway is vertically aligned with the trap and slopes downwardly from an inlet to an outlet, which is preferably at a position near the bottom interior of the bowl and which also preferably includes a wall surface in overlying alignment with at least a portion of the downwardly sloping waterway portion. During flushing, water from a reservoir flows down the waterway, against the outlet wall surface causing a vortex in the bowl, and then out of the bowl into the trap. In the method of making the product, a plastic parison is positioned between the sections of a blow mold. Next, the mold sections are closed, pinching portions of the parison shut to partially form the trap and waterway and enclosing another parison portion in a bulbous mold cavity.

Abstract: A lower mold half is horizontally placed. The top surface of the lower mold half is provided with a first groove of any desired shape, one end of which is closed with the other end having a notch for allowing air to be blown in. An upper mold half has a bottom surface which matches with the top surface of the lower mold half and which is provided with a second groove corresponding to said first groove. Thus, when the upper and lower mold halves are in contact, there is formed a mold cavity defined by the first and second grooves. A nozzle for discharging a parison is disposed above the lower mold half. The relative positional relation between the nozzle and the lower mold half is changed in such a manner that the nozzle follows the first groove without changing the distance therefrom so that a parison can be properly placed in the first groove. If at least two lower mold halves are provided, it is possible to carry out molding operation continuously.

Abstract: .Iadd.An improved extrusion method is provided to produce plastic tubes having at least one end thereof an increased wall thickness. The method includes the steps of continuously advancing plastic material through an extruder to form a cylinder, cooling the extruded cylinder as it passes from the extruder, periodically increasing the speed at which the plastic material is fed to the extruder relative to the speed at which the cylinder is drawn from the extruder to produce a section having an increased wall thickness and dividing the cylinder thus formed. .Iaddend.