Abstract: A method of manufacturing a molded article from a settable mixture includes the steps of in-taking a plurality of components into a plurality of separate material discharge containers sized relative to each other to provide the required mix ratio of the settable mixture. The components contained in the material discharge containers are discharged to a mold tool through a mixing head that receives each of the components separately and thoroughly mixes them into the settable mixture. The settable mixture flows from the mixing head to fill the mold tool. The mold tool rotates about an axis during the filling step to evacuate air from the mold tool. Pressure of the settable mixture is monitored during the filling step and rate of discharge of the settable component is varied in response to changes in pressure of the settable mixture.

Abstract: A method is disclosed for casting a concrete product, in which method the concrete product is cast in an essentially horizontal slipform casting process during which the top surface height and profile of the concrete product being cast is measured continually. Based on the measurement results, the relative proportion of the concrete mix flowing to the upper layer or lower layer of the concrete product is controlled by way of moving or tilting a feed trough (12) located below the concrete mix feed means (2).

Abstract: Thermoplastic polymers, rubbery polymeric components and reinforcing fillers are mixed and heated to a range of 380° F. to 440° F. The resulting mixture is then fed into a Banbury mixer, with the resulting mixture fed into a diverter feeding two molds. The velocity of the piston of each mold being filled is controlled to thereby control the density of the molded product along its length. As each mold is filled, it is deposited in a cool water bath (50° F.-60° F.) while the next mold is being filled. The molds are sequentially transferred to an air cooling rack to complete the process. A texture, generally comprising indentations perpendicular to the longitudinal axis on the molded object, provides a frictional surface between the railway crosstie and the ballast beneath the crosstie.

Abstract: A closed mould method of producing composite mouldings is provided. A sensor system (4, 5, 6, 7, 8) is provided for sensing distortion of one (1) of the mould elements while the resin mix is being fed into the mould cavity. A signal is generated if a predetermined degree of distortion is sensed.

Abstract: An apparatus for manufacturing semi-finished products and molded articles of metallic materials includes an extruder for producing a flow of the metals, with appliances being connected thereafter for shaping the semi-finished products and the molded articles. The extruder has a screw system consisting of two or more meshing screws. This design has an improved functionality and can produce components with reproducible quality.

Abstract: Method of making a proximity probe including providing a preform having an interior cavity accessible by an opened rearward end; coupling a coil to the preform proximate a forward most end of the preform for defining an assembly; locating a single support pin through the rearward end such that the support pin extends within the interior cavity while having an end emanating from the rearward end; cantilevering the emanating end between an upper and a lower mold plate defining a mold cavity for supporting the assembly; injecting moldable material into the mold cavity for molding an encapsulation of material over the assembly for defining an encapsulated probe tip, allowing the encapsulated probe tip to cure; removing the encapsulated probe tip from the mold cavity; removing the support pin from the assembly, and coupling a cable to the encapsulated probe tip for forming the proximity probe.

Abstract: An unstable weighing due to the plunger sliding resistance in a prepla type injection molding machine is prevented by dividing control of the plunger retraction movement for the weighing into speed control and pressure control. A plastic resin supply start position and a forced retraction release position are set along the way of the plunger retraction stroke by reference to a weighing start position. A plunger is moved from the weighing start position after injection and pressure-keeping to a weighing stop retraction position. Plastic resin is supplied to the forward portion of the injection cylinder when the plunger attains the supply start position from the weighing start position. The plunger is moved by forced retraction up to the forced retraction release position by speed control; the weighing is stabilized by changing the speed control to the pressure control once the plunger attains the forced retraction release position.

Abstract: For observing a flow of molten resin in an injection molding apparatus, a transparent cylinder, which is formed of transparent glass or transparent plastic material, is used in place of a mold of the apparatus. Because filling the transparent cylinder with the molten resin up to the cylinder's full capacity will burst the cylinder, the molten resin injection is terminated on the way when the injected molten resin has occupied a predetermined percentage, in a range of 50-90%, of the total volume of the cylinder, so that a burst of the cylinder can be avoided. The transparent cylinder provides for an increased resin-flow observation range because of its transparency, and can be less expensive because it is formed of inexpensive glass or plastic. As a consequence, there can be provided an efficient resin-flow observing experiment apparatus at lower cost.

Abstract: An injection molding method comprises the steps of melting molten resin within a heating cylinder, advancing an injection screw within the heating cylinder to inject the molten resin into a mold, retracting the injection screw to a predetermined position before completion of the injection, further advancing the injection screw under pressure control.

Abstract: Disclosed are a method and apparatus for matching the extrusion rate of a liquifier which extrudes a flow of material to a predetermined target output rate. The present invention adjusts the volumetric rate of material going in to the liquifier to account for a predicted melt flow component of material being extruded from the liquifier. Compensating for the predicted melt flow results in reduced error in an extrusion profile produced by material deposited along a toolpath followed by the liquifier.

Abstract: The invention relates to an electric injecting molder and an injection velocity and injection pressure controlling method for an electric injecting molder, and it is an object of the present invention to make it possible, in injection molding by an electric injection molder, to mold stable resin products having a minimized dispersion in the quality even if the operation environment varies. When an injection screw (23) disposed in an injection cylinder (22) is driven to move in forward and backward directions by an electric motor (29A) to inject resin material (3) in the injection cylinder (22) into a metal mold (1, 2) to fill the metal mold (1, 2), the electric motor (29A) is first velocity-controlled to fill the resin material (3) into the metal mold (1, 2), and, after the filling, the electric motor (29A) is pressure-controlled to replenish the resin material for shrinkage of the resin.

Abstract: In an injection molding machine having first and second nozzles for delivering melt material from a common manifold to one or more mold cavities, apparatus for controlling delivery of the melt material from the nozzles to the one or more mold cavities, each nozzle having an exit aperture communicating with a gate of a cavity of a mold and being associated with an actuator interconnected to a melt flow controller, the apparatus comprising a sensor for sensing a selected condition of the melt material through at least one of the nozzles; an actuator controller interconnected to each actuator, each actuator controller comprising a computer interconnected to a sensor for receiving a signal representative of the selected condition sensed by the sensor, the computer including an algorithm utilizing a value corresponding to a signal received from the sensor as a variable for controlling operation of an actuator for the at least one nozzle; wherein the melt flow controller controls the rate of flow of the melt materi

Abstract: Filler, processing fluid and silicone polymer are controllably compounded into homogeneous filled heat cured rubber compositions with requisite reinforcing properties and levels of volatiles. In the process, a continuous feed of a silicone polymer to a compounding apparatus is monitored and a signal is generated in accordance with the monitored rate of feed. Rate of feed of filler and additives to the compounding apparatus are then adjusted in accordance with the signal.

Abstract: A roll forming apparatus for rolling selected materials around a cylindrical core. The apparatus comprises a core support, a roll support device, and a material supply device. The core support is used to support the cylindrical core. The core support has an associated core rotation device for rotating the core about a core rotation axis at a controllable rotation rate. The roll support device supports a plurality of forming rolls positioned to surround the core about the core rotation axis. The roll support device has a roll control mechanism operable to both radially position the plurality of forming rolls relative to the core rotation axis and to constrain the plurality of forming rolls to be equally spaced from the core rotation axis in order to provide integrated adjustment of the plurality of forming rolls to control a radial dimension of a substantially symmetrical forming space defined by the plurality of forming rolls.

Abstract: A method and an apparatus capable of extrusion molding a comparatively wide, thin ceramic sheet using a screw extruder while suppressing the wrinkling thereof are disclosed. A molding apparatus (1) comprising a screw-type extruder (2) and a mold (11) at the forward end of the extruder (2) is used to extrusion mold a ceramic sheet from the ceramic material introduced into the extruder (2) by way of the mold (11). The extrusion molding is carried out while regulating the temperature of the portion of the ceramic material passing through the mold (11) corresponding to each of a plurality of areas (control zones) into which the mold (11) is divided.

Abstract: A system and method are disclosed for molding a part, such as a point-of-purchase display, is shown. The system comprises a plasticator and press. The molded part comprises an in-molded graphics sheet which becomes molded integral with the part.

Abstract: Molding of relatively large articles such as automobile body parts by gas-assisted injection molding with sequential gating of injected thermoplastic resin into the mold cavity. Elongated ribs in the parts form gas channels and strengthening ribs for the parts. Structural parts are made by adhesively joining two such parts made by gas-assisted injection molding through a combination of hollow ribs and joining flanges at sides of the parts.

Abstract: A process and device for the stable widthwise regulation of the bulb angle in connection with the extrusion of sheets of heat-sensitive plastic (2) in an extrusion device with a calender opening formed by two intake rollers (3, 2) and a traversing measuring device (7) provided according to the calender slot by altering positioning elements on the extrusion device. At least two traversing measurements of a parameter correlating with the bulb angle are started consecutively on the heat-sensitive plastic sheet synchronously with the positioning of one of the two calender rollers. Like measurement conditions therefore exist and the deviation of the measured values obtained on the basis of the measurements in relation to specified expected values is used to regulate the bulb angle over its width by altering a setting element on the extrusion device.

Abstract: A method for injecting a plastic material into a mold cavity to precisely fill the mold cavity under low pressure and to avoid packing the mold cavity with plastic material. The plastic material is injected into the mold cavity at a rate sufficient to maintain an unbroken melt front and to avoid any spraying or splashing of the plastic material within the mold cavity. After an initial unbroken melt front is established, the rate at which the material is injected is increased in proportion to the size of the melt front. After a predetermined amount of the material has entered the mold cavity, the material is injected based upon a rate-dominated control algorithm which maintains the unbroken melt front. Once the mold cavity has reached a pre-determined level of fill, the rate-dominated control algorithm is changed to a pressure-dominated control algorithm to prevent overrunning the end point of the mold process and to eliminate splashing and associated backflow problems.

Abstract: A method for injecting a plastic material into a mold cavity to precisely fill the mold cavity under low pressure and to avoid packing the mold cavity with plastic material. The plastic material is injected into the mold cavity at a rate sufficient to maintain an unbroken melt front and to avoid any spraying or splashing of the plastic material within the mold cavity. After an initial unbroken melt front is established, the rate at which the material is injected is increased in proportion to the size of the melt front. After a predetermined amount of the material has entered the mold cavity, the material is injected based upon a rate-dominated control algorithm which maintains the unbroken melt front.

Abstract: An apparatus for stabilizing the amount of belt-form edible dough produced per hour by a production system. The apparatus features a calculating device (67) to calculate the hourly throughput of the edible dough (W) by dividing the weight of the proper number of the divided edible dough bodies (W) by the time used to charge that proper number of the divided dough bodies (W). If by the calculation the hourly throughput of the edible dough is found to be below the predetermined goal, a controlling section (65) controls a motor (47) for gap-adjusting to expand the size of a gap for discharging between a pair of roller members (29L) and (29R). If the hourly throughput is found to be above the predetermined goal, the controlling section (65) controls the motor (47) to reduce the size of the gap for discharging.

Abstract: An electric motor type injection apparatus for a vertical molding machine which is compact in size and can be installed on a square measure limited is provided. According to this apparatus, an improper movement due to self-weight of injection mechanism members can be prevented, and deviation of a nozzle tip resulting in the cause of damage of a nozzle tip and a gate can be corrected.

Abstract: A method for injecting a plastic material into a mold cavity to precisely fill the mold cavity under low pressure and to avoid packing the mold cavity with plastic material. The plastic material is injected into the mold cavity at a rate sufficient to maintain an unbroken melt front and to avoid any spraying or splashing of the plastic material within the mold cavity. After an initial unbroken melt front is established, the rate at which the material is injected is increased in proportion to the size of the melt front. After a predetermined amount of the material has entered the mold cavity, the material is injected based upon a rate-dominated control algorithm which maintains the unbroken melt front.

Abstract: A dough sheeting system having a cutter assembly for pulling and cutting elongated chunks of dough from a hopper containing dough is disclosed. A receiving surface is disposed beneath the cutter assembly. A motor advances the receiving surface in a direction of dough feed. A calendar assembly receives dough output by the receiving surface and outputs a sheet of dough. A level sensor monitors the height of accumulated dough at the input of the calendar assembly and produces a signal when the input exceeds a predetermined level. The motor is responsive to the signal from the level sensor to stop advancing the receiving surface when the level exceeds the predetermined level. A movement sensor monitors the advancement of the receiving surface to actuate the cutter assembly to pull and cut a chunk of dough when the surface has advanced a predetermined distance from the position at which the surface was positioned when a prior chunk of dough was delivered to the surface.

Abstract: This plant for manufacturing fiber-reinforced thermoplastic structural components, especially for motor vehicles, is characterized in that it includes:means (1) of continuous production of a dough of a fiber-reinforced thermoplastic compound, receiving as input, on the one hand, the thermoplastic (4) and, on the other hand, the reinforcing fibers (6);means (14) of accumulation and of temporary storage of the dough produced; andmeans (26) of removing the dough from the accumulation and temporary-storage means (14) and of introducing this removed dough into a mould (29) for forming the component.

Abstract: Molten resin, plasticized in the plasticization chamber, is fed to the injection chamber through a connection passage where it is measured. Before it is injected by the plunger, the connection passage is blocked and the molten resin is preliminarily compressed. This invention provides a method using a preplasticization type injection molding machine that controls the volume of the filled molten resin with little variation utilizing blocking and preliminary compression. By means of blocking or preliminary compression, the position movement by which the plunger moves from the position at which the measurement is completed is calculated, and the position at which the next measurement will be complete is corrected. Furthermore, the molten resin fill operation is done on the basis of the position of the plunger detected at the time of the preliminary compression.

Abstract: A device for producing discrete rolls of chilled solutions or dispersions wherein the combination of a distribution bar, an auxiliary weir hopper and a puddle hopper allows for continuous coating of a rotating chiller drum in producing discrete rolls separated from each other by a time gap built into the deposition of the solutions or dispersions from the hopper to the rotating chilled drum.

Abstract: A dough portioning machine comprises a metering chamber with a metering piston which is displaceable therein. The metering piston is provided with a degassing channel which is formed in the piston wall and to which a cleaning pin in the cylindrical wall of the metering chamber can be allocated.

Abstract: A material loader includes a material hopper, an air filter, a hopper support and a control box. The filter is positioned on top of the hopper, and the hopper is positioned on top of the hopper support. The entire unit is mounted directly over the throat of a plastic injection molding machine and feeds a mixture of virgin and regrind plastic into the machine. The control box, mounted on the side of the plastic injection molding machine, contains timers and other electronic equipment which run the unit to control amount, ratio and timing of conveyance of feed material to the hopper, and ultimately to the press. Two inlet tubes feed plastic into the hopper, one feeding virgin plastic and the other feeding regrind plastic. The control box contains load timers, which may be adjusted manually to control the exact amount of each type of plastic which is fed into the hopper.

Abstract: A multi-stage vacuum kneading-extruder includes a degassing chamber for storing material to be processed and reducing pressure of space for storing the material, a kneading-conveyer device for kneading while conveying the material sent from the degassing chamber, and an extruding mechanism for extruding the kneaded material, at a conveyer terminal end of the kneading-conveyer device, into a predetermined shape. The kneading-conveyer includes a pair of conveyer screws disposed one after another in a conveying direction of the material and a grinding-kneading mechanism interposed between the front and rear screws. The grinding-kneading mechanism includes a plurality of unit kneading mechanisms disposed in opposition to each other relative to the material conveying direction. Each of the unit kneading mechanisms includes an un-rotatable stationary disc, a rotary disc rotatable relative to the stationary disc and a stationary filter interposed between the stationary and rotary discs.

Abstract: A dough sheeting system having a cutter assembly for pulling and cutting elongated chunks of dough from a hopper containing dough is disclosed. A receiving surface is disposed beneath the cutter assembly. A motor advances the receiving surface in a direction of dough feed. A calendar assembly receives dough output by the receiving surface and outputs a sheet of dough. A level sensor for monitoring the height of accumulated dough at the input of the calendar assembly and for producing a signal when the input exceeds a predetermined level. The motor is responsive to the signal from the level sensor to stop advancing the receiving surface when the level exceeds the predetermined level and is responsive to the signal when the height is reduced.

Abstract: Apparatus for and related methods of forming three-dimensional objects out of a building material, which is normally solid, but which is flowable when heated. In one embodiment, a support material is used to fill in portions of layers which are not to be solidified as part of the object, thus providing support to otherwise unsupported portions of other layers. Advantageously, the support material is also normally solid and flowable when heated, and has a lower melting point than the building material enabling the support material to later be removed without damaging the object. In an alternative embodiment this support material can be used to build a support such as a web support, or the like, for supporting an object surface from a second surface.

Abstract: An extruding apparatus for intermittently supplying a predetermined quantity of highly viscoelastic material, e.g. food material. The apparatus has a hopper having a supply pump, a supply pipe having one end connected to the pump and the other end to an extrusion valve, and an accumulator (constant pressure device) mounted to the extrusion valve for maintaining the pressure of the food material in the extrusion valve at a predetermined constant pressure.

Abstract: Apparatus for and related methods of forming three-dimensional objects out of a building material, which is normally solid, but which is flowable when heated. In one embodiment, a support material is used to fill in portions of layers which are not to be solidified as part of the object, thus providing support to otherwise unsupported portions of other layers. Advantageously, the support material is also normally solid and flowable when heated, and has a lower melting point than the building material enabling the support material to later be removed without damaging the object. In an alternative embodiment this support material can be used to build a support such as a web support, or the like, for supporting an object surface from a second surface.

Abstract: Apparatus for and related methods of forming three-dimensional objects out of a building material, which is normally solid, but which is flowable when heated. In one embodiment, a support material is used to fill in portions of layers which are not to be solidified as part of the object, thus providing support to otherwise unsupported portions of other layers. Advantageously, the support material is also normally solid and flowable when heated, and has a lower melting point than the building material enabling the support material to later be removed without damaging the object. In an alternative embodiment this support material can be used to build a support such as a web support, or the like, for supporting an object surface from a second surface.

Abstract: When molding is carried out by injecting and charging a measured resin into a mold by a pushing-out member, a measured value of a stroke length of the pushing-out member is calculated and set in advance when the pushing-out member is advanced up to the maximum forward position. The stroke of the pushing-out member is measured from the maximum forward position based on the measured value at a time of a measuring. The pushing-out member is advanced from a measuring completion position to the maximum forward position and then the injecting is carried out. Accordingly, a pressure retaining process becomes unnecessary for reserving a cushion amount and controlling a pressure, and also the resin is charged into the mold based on an always constant molten time. Therefore, a molding product can be made uniform.

Abstract: A method of making a plurality of optical record disc substrates includes the steps of injecting a resin material into a plurality of disc-shaped cavities which are formed by closing a fixed mold half and a movable mold half; and cooling the injected resin material in the cavities. The cavities are grouped into a plurality of groups so that each group includes not less than one cavity, and each group is provided with a common actuation device so that each actuation device is independently operated.

Abstract: Apparatus for dispensing predetermined quantities of particulate material is disclosed which includes a casing defining a dispensing chamber and an inlet tube for delivering material to the chamber. The inlet tube extends into and is surrounded by the chamber. The chamber has an outlet for discharging material from the chamber. A mechanism is provided for effecting relative movement between the tube and the casing whereby to vary the volume of material introduced into the chamber on opening of an inlet valve.

Abstract: A liquid filling method and in particular a lens material liquid filling method suitable for manufacturing a plastic lens. A thermal sensing portion (2a) of a temperature sensor (2) is set and kept to a temperature different from that of a liquid (3). The liquid (3) is poured into a vessel (8), a change of an output from the temperature sensor (2) is detected when the surface of the liquid (3) comes into contact with the thermal sensing portion (2a) by temperature detecting means (5), and control means (6) determines that the liquid surface has reached a desired level and sends a pouring stop command to a pouring mechanism (7) to complete the filling of the liquid.

Abstract: A vacuum kneading and extruding method includes the steps of separately and continuously measuring plural types of material and continuously feeding the material into a kneading and degassing tank; kneading the plural types of material fed, and degassing the material by decompressing a material accommodating space into a vacuum; kneading and transporting the material in a decompressed condition through a multi-stage kneading mechanism; and forming and extruding kneaded material in a predetermined shape. An apparatus for use in executing this method is included.

Abstract: An apparatus for forming compressible material into discrete solid blocks or briquettes in which the material is compressed in a compression chamber by a pre-press ram and a main ram. A control system is provided which includes sensors for generating signals that are a function of the density of the compressed and densified material within the compression chamber, and an arrangement which is responsive to the sensors is provided for varying the operation of the material feeding device to change the volumetric flow of material fed to the compression chamber in response to predetermined changes in the density of the material in the chamber.

Abstract: An efficient numerical procedure for the simulation of Newtonian fluids with non-negligible inertial effect which satisfies the Navier-Stokes equations and the energy equation. The method simulates the molding-filling process of incompressible viscous liquid, e.g. semi-solid metal, in a thin and irregular cavity. The moving free surfaces in an irregular domain is tracked using a fixed-mesh method. The material discontinuities across the interface between air and the liquid are removed by replacing the air with a pseudo-gas which has small density and dynamic viscosity but its kinematic viscosity is the same as that of the liquid. During the filling process, the (semi-solid) liquid may solidify on the mold surface. As a result, the effective cavity thickness reduces non-uniformly according to the result thermal analysis. The Navier-Stokes equations are integrated across the remaining gap by assuming that the velocity profile (not magnitude) is similar to that in a fully-developed flow.

Abstract: Apparatus for and related methods of forming three-dimensional objects out of a building material, which is normally solid, but which is flowable when heated. In one embodiment, a support material is used to fill in portions of layers which are not to be solidified as part of the object, thus providing support to otherwise unsupported portions of other layers. Advantageously, the support material is also normally solid and flowable when heated, and has a lower melting point than the building material enabling the support material to later be removed without damaging the object. In an alternative embodiment this support material can be used to build a support such as a web support, or the like, for supporting an object surface from a second surface.

Abstract: An injection pressure monitoring method in which the correlations between a screw position, injection pressure, and cavity fill for an injection molding machine are monitored and displayed on a screen. The pressure for the position of a screw is graphically displayed on the screen, and a picture of a cavity is also displayed on the screen. By inputting the screw position, the pressure for the screw position is monitored, and at the same time, the resin filling state in the cavity corresponding to this screw position is displayed as an image.

Abstract: An apparatus for hot-briquetting iron sponge includes a gravity feeder which has a feed shaft and a regulating tongue which supplies material in a dosed quantity to a moulding gap of a roller press, in such a way that the apparatus can also be used for pressing hot iron sponge at temperatures of up to 1000.degree. C. for obtaining high-density briquettes of good quality. The regulating tongue is provided with a heat-reducing insulating layer and cooling system. The feed shaft and a feed hopper also may have a heat-insulation layer and a cooling system. A flow-rate control is provided for gravity feeders where bulk material or flowable material is supplied by at least one regulator through a feed shaft to a moulding gap formed between at least two rolls and is processed by the rolls. At least one of the rolls is motor-driven. The material is supplied to the moulding gap by the regulating tongue in response to a driven roll torque sensed by a torque detector.

Abstract: Equipment, in particular pressure gelling equipment, is provided to fill one or more casting molds with castable, liquid materials of which the components are fed by pumps through closable conduits to at least one plunger-cylinder system from where the components are moved to a mixing chamber and from there to the particular casting mold. In order to process especially abrasive casting materials of high viscosity and short pot life, the invention provides that at least one plunger-cylinder system be operated at a pressure substantially higher than are the pumps and that the mixing-chamber volume be comparatively slight.

Abstract: In an apparatus for resin transfer molding, the resin is introduced into a heated resin pot, and a piston member is driven through the resin pot to displace the resin into a heated resin line. The temperature of the resin in the resin pot is substantially constant, and is above room temperature but below the catalyzing temperature of the resin. The preheated resin flows from the resin line into a preheater, which heats the resin to molding temperature and catalyzes the resin, and the catalyzed resin is then introduced into the mold cavity of a resin transfer mold containing a structural preform. The resin is maintained at a substantially constant pressure within the heated resin line, and the movement of the piston member is continuously monitored during the resin transfer process. The resin transfer process is terminated when the rate of movement of the piston member decreases below a predetermined minimum value approaching zero.

Abstract: Apparatus for and related methods of forming three-dimensional objects out of a building material, which is normally solid but which is flowable when heated. In one embodiment a support material is used to fill in portions of layers which are not to be solidified as part of the object to provide support to otherwise unsupported portions of other layers. Advantageously, the support material is also normally solid and flowable when heated, and has a lower melting point than the building material enabling the support material to later be removed without damaging the object. In an alternative embodiment this support material can be used to build a support such as a web support, or the like, for supporting an object surface from a second surface.

Abstract: A non-return valve (10) primarily for use with an injection molding machine utilizes a frame (12) which surrounds a first bore (34) and a second bore (32). The second bore (32) is accessed by inlets (30). Outlet passages (40) are located downstream of said inlet (30) and connect second bore (32) with an accumulation area (6). A piston (60) is dimensioned to slidably fit within the first bore (34) and extend into the second bore (32). In a downstream position, the piston (60) allows positive flow of material from the inlet (30) to the outlet (40). In an upstream position, the piston (60) positively blocks flow of material from the inlet (30) to the outlet (40). The piston (60) can be forced to its upstream position at the beginning of the injection step or can be preclosed by eliminating rearward movement prior to injection.