Injection molding apparatus

Abstract: An injection assembly (10) for the injection molding of plastics and rubbers (M) is disclosed which is comprised of a cylinder (20), a nozzle (30) mounted at one end portion of the cylinder (20), a torpedo (40) fitted at its one end to the nozzle (30) and a plunger (50) arranged to reciprocate in the cylinder (20). The torpedo (40) has a hollow extension (42) extending opposite to the nozzle (30) and protruding in the cylinder (20), and the plunger (50) is provided therein with an elongate aperture (51) held in slidably movable relation to the extension (42) of the torpedo (40). Uniform heating of the molding material (M) is attainable at high speed.

Description: This invention relates to apparatus for the injection molding of plastic and rubbery materials. More particularly, the invention is directed to an injection assembly adapted to be used on such apparatus to thereby convert a molding material from solid particles or pellets into a continuous semi-fluid or fluid mass and to subsequently inject the same into one or more cavities defined in two mating molds. Various apparatus of the character mentioned are known, a typical example of which is built with an upright heating cylinder, a nozzle unit attached to a bottom end of the cylinder, an injection plunger formed to be solid and supported centrally upwardly of the cylinder for up-and-down movement and a feed hopper located to communicate with the cylinder. Fitted centrally of the nozzle unit is a torpedo provided at its exit end with a needle valve. Heaters are arranged both on a peripheral surface of the nozzle unit and within a surrounding wall of the cylinder. A pair of molds are superimposed one on the other, thereby defining a cavity therebetween, and held in cooperative releasable relation to the nozzle unit. The foregoing apparatus is wholly unsatisfactory as it is feasible only with large heat consumption and even with long molding cycle and often with objectionable material degradation. From US-A-2 476 550, it is known an injector for molding machines, comprising an heating cylinder, a torpedo fitted in the cylinder and an apertured injection plunger reciprocable in said cylinder; However, in that apparatus, the plunger is stopped in its downward stroke by the ribs which maintain the torpedo in the cylinder so that a great quantity of plastics material is liable to remain i n the channel between the cylinder and the torpedo and may harden and plug subsequently this channel and cause a failure of the apparatus or may be overheated and spoil the quality of the final holdings. Furthermore, in such an apparatus, the needle valve is located outside the nozzle unit. This is an inconvenient because the needle valve is at a lower temperature than the other parts of the apparatus, and the required uniformity of the temperature of the melt plastics material may be disturbed with the result that the moldings will not have a quality as good as the moldings obtained with the present apparatus wherein the plastics material is always in contact with hot parts until it penetrates into the mold. Moreover, the needle valve cannot funtion properly since it is not actuated by a spring and cannot be perfectly closed. In addition, the location of the needle valve outside of the nozzle unit does not allow to obtain moldings having no hole therein, which is a limitation in the possibility of molding. Generally, thermoplastic materials for instance of a pellet shape are poor in thermal conductivity and hence requisite of substantial preheat in a spacious region present in the cylinder. The preheated plastics flows through a constricted channel in the torpedo where it gets softened or plasticized with heat for injection into the mold cavity. In such instance the molding material is difficult to preheat with uniformity due primarily to that region in the cylinder being relatively large in cross-sectional area. To be more specific the molding material is subject to heat in a shorter duration at a location contiguous to an inner wall of the cylinder than at that remote from the wall, i.e. in a less uniform manner, and much time is consumptive in preheating the material on the whole. To cope with those problems it has been proposed that preheating be effected at elevated temperature. However, this will render the plastic material susceptible to overheat on or nearby the inner cylinder wall, eventually leading to inadequate discoloration and even to serious decomposition. The foregoing situation of the prior art has lent an impetus for the provision of injection assemblies that can work with great reliability in injection-molding plastics and rubbers. The present invention seeks to provide a novel injection assembly for use in injection molding apparatus which will enable uniform heating of molding materials at high speed and with utmost simplicity without involving undesirable decomposition, thus ensuring ultimate moldings of superior quality. The above and other objects and features of the invention will become better understood from the following description upon reading in conjunction with the accompanying drawings. As claimed, the injection assembly for use in injection molding apparatus, of the invention comprises a heating cylinder, a spreader or torpedo having a tapered end snugly fitted in a nozzle unit, a region defined between said cylinder and said torpedo for heating the molding material, a needle valve, and an apertured injection plunger reciprocable in said cylinder. Said torpedo has an extension extending from said tapered end and protruding into said cylinder. The cylinder, the nozzle unit and the torpedo are maintained hot, According to the invention, the needle valve is mounted in said tapered end of the torpedo and fitted inside a hollow part of said extension adapted for receiving said needle valve, the torpedo has spring means positioned in said extension for urging said valve normally toward a delivery direction. Furthermore, the apertured plunger is mounted in sliding interengagement with and over the torpedo extension. In such a way, the molding material is exposed to preheat in the cylinder with the aid of three sources of heat from the cylinder , the nozzle unit and torpedo elements and softened while passing through the nozzle unit. Figure 1 is a vertical cross-sectional view showing an injection assembly embodying the present invention. Figure 2 is a view similar to Figure 1 but explanatory of an injection assembly of the prior art. Referring now to the drawings and Figure 1 in particular, there is shown an injection assembly designated at 10 and constructed in accordance with the present invention. This assembly is designed to suit application to injection molding apparatus in actual use. No explanation will be required with respect to clamping mechanisms and other structural details as commonly accepted in the art. The injection assembly 10 contemplated under the invention is structured in vertical posture, though not restricted, and comprised essentially of a heating cylinder 20, a nozzle unit 30, a spreader or torpedo 40 and an injection plunger 50. Each such element is configured in a manner to be described. The cylinder 20 has a body surrounded by a wall 21 of increased thickness and adapted to accommodate a molding material M, i.e. a solid plastics or rubber of a powder or pellet shape. To this end the cylinder body diverges and opens at its one side for entry of the material M. The cylinder 20 acts to preheat or preplasticize the material M. The nozzle unit 30 is mounted integrally on a bottom end portion of the cylinder 20 and arranged to receive part of the torpedo 40 as will be mentioned. The torpedo 40 has at its bottom end a tapered portion 41 snugly fitted to the nozzle unit 30 and a hollow extension 42 extending from the tapered portion 41 and protruding into the cylinder 20. A plurality of fins 43, two seen from Figure 1, are disposed on the tapered portion 41 which are shaped to tightly fit against an inside surface of the nozzle unit 30. A needle valve 44 is attached to a tip end of the tapered portion 41. The torpedo includes a compression spring 45 positioned in the extension 42 and a screw 46 mounted threadedly on a top end opposed to the tapered end. The spring 45 and the screw 46 cooperate with each other in normally urging the valve 44 downwardly toward a delivery direction. The plunger 50 has an elongate aperture 51 of an inside diameter corresponding to an outside diameter of the extension 42 of the torpedo 40. Thus this plunger reciprocates in sliding interengagement with and over the torpedo. Heating means 60 is conveniently mounted around the nozzle unit 30. Where it is found desirable, an additional heater may be disposed in conventional fashion within the wall 21 of the cylinder 20. Designated at 70 is a feed hopper for supplying the molding material M into the cylinder 20. Two separate molds 80 are mated to define therebetween a single cavity 90 into which a soft plastic or rubbery mass is injected by means of the nozzle unit 30. With the structure specified above, the molding material fed from the hopper is exposed to preheat in the cylinder with the aid of three sources of heat from the cylinder, nozzle and torpedo elements that can all be maintained hot upon switching on of the heater. This constitutes one important feature of the invention from the structure and performance points of view. The plastics or rubber is then softened or plasticized while in passage through the nozzle unit. Downward movement of the plunger allows the soft mass to successively inject through the needle valve into the mold cavity after which a final molding is obtained. It has now been found that a plastics or rubber can be heated with greater uniformity and at higher speed. This is interpreted to mean that moldings of improved quality are provided with shortened molding cycle. To facilitate comparison a prior injection assembly referred to hereinabove is represented in Figure 2 in which like and corresponding parts are denoted by like reference numerals primed. This assembly is uncomparable to and different from the assembly of the invention particularly in regard to the torpedo and plunger arrangements.

Claims: An injection assembly for use in injection molding apparatus, which comprises a heating cylinder (20), a spreader or torpedo (40) having a tapered end (41) snugly fitted in a nozzle unit (30), a region defined between said cylinder and said torpedo for heating the molding material, a needle valve (44), and an apertured injection plunger (50) reciprocable in said cylinder (20), said torpedo (40) having an extension (42) extending from said tapered end (41) and protruding in to said cylinder (20), the cylinder, the nozzle unit and the torpedo being maintained hot, characterized in that : said needle valve (44) is mounted in said tapered end of the torpedo and fitted inside a hollow part of said extension adapted for receiving said needle valve; said torpedo (40) has spring means (45) positioned in said extension (42) for urging said valve (44) normally toward a delivery direction; and said apertured plunger (50) is mounted in sliding interengagement with and over the torpedo extension; so that the molding material is exposed to preheat in the cylinder with the aid of three sources of heat from the cylinder , the nozzle unit and torpedo elements and softened while passing through the nozzle unit. An injection assembly according to claim 1, wherein screw means (46) are mounted on another end of the torpedo opposed to said tapered end (41), said spring and screw means cooperating for urging said valve toward the delivery direction.

Inventors: ITO MICHIO, YOSHIDA HIROSHI, ITO, MICHIO, YOSHIDA, HIROSHI
Applicants: YKK CORP, YKK CORPORATION
Application year: 1990
Main classification: B29C45
IPC codes: B29C45, B29K21