Injection mold capable of shearing film gate within mold

Abstract: An injection mold of the type in which a film gate of a resin is formed and in which shearing of the film gate is conducted by a movement of a gate cutter (1) alone. The gate cutter (1) has a stepped cylindrical form with a main cylindrical portion of a greater diameter (D) and a coaxial end cylindrical portion of a smaller diameter (d). When the mold is closed, the end cylindrical portion being received in the bore of a confronting die (4) leaving therebetween an annular gap (t) of a radial size which determines a gate thickness. The outer peripheral surface of the portion of the gate cutter received in the bore of the die presenting a gate land.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to an injection mold having a mechanism for shearing a film gate within the die. Such an injection mold is known from US-A-2,471,148. More particularly, the present invention is concerned with a configuration of the gate which is capable of optimizing the flow rate and the pressure of the resin supplied into the cavity. DESCRIPTION OF THE RELATED ART A die for molding an optical disk will be described as an example of a die having a mechanism for shearing a film gate within an injection mold. Referring to Fig. 2, a known injection mold has a gate which is defined by the end surface a of a gate cutter 1 and the end surface b of a sprue bush 2. In this mold the gap between the end surfaces a and b and the length ℓ of the lands presented by the above-mentioned end surfaces are determined to stabilize the flow of the flowing resin and optimize the application of pressure. The cutting of the gate is effected by a forward movement of the gate cutter 1 as indicated by an arrow in Fig. 3, while allowing the sprue bush 2 to be retracted. This in turn requires that the touch pressure of the nozzle 3 of the injection molding apparatus is reduced or the nozzle 3 is forcibly retracted. Anyway, a delicate and intricate control is necessary. If the shearing has to be conducted by the forward movement of the cutter 1 alone, it is necessary that the sprue bush 2 is set at the retracted position as shown in Fig. 4. In this case, the gate is defined by the clearance between the inner peripheral corner c of the dies 4 and the outer peripheral end d of the gate cutter 1. Namely, the land ℓ shown in Fig. 2 is substantially zero so that the effect for stabilizing the flow of the resin and the application of the pressure to the cavity 5 depends solely on the thickness of the gate. With this type of injection mold, therefore, it has been extremely difficult to produce a product having an essential characteristic which is worsened in accordance with a rise in the molding pressure, e.g., birefringence in case of an optical disk, since the production of such product requires a highly delicate control of the molding pressure. An injection mold in which a film gate of a resin is formed and in which shearing of the film gate is conducted by movement of a gate cutter pin alone which cooperates with a confronting die is known from US-A-2,471,148. In this known injection mold the gate cutter pin has a main cylindrical portion performing the cutting when fitted into a bore of corresponding inner diameter formed in the confronting die. The gate cutter pin further has a coaxial tip-rounded tapered upper end portion which facilitates entrance of the pin into the bore when the mold is being closed. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an injection mold of the type in which shearing of gate is effected by a forward movement of the gate cutter alone, wherein the configuration of the gate is improved to enable the flow rate and the pressure of the resin supplied to the cavity to be easily controlled with a simple construction. According to the present invention there is provided an injection mold in which a film gate of a resin is formed and in which shearing of the film gate is conducted by movement of a gate cutter alone which cooperates with a confronting die, said gate cutter having a main cylindrical portion and a coaxial end portion, said end portion being received in a bore of said confronting die when said mold is closed, the bore of said die having an inside diameter D' corresponding to an outside diameter D of said main cylindrical portion of said cutter so that it fits into said bore of said die when the gate is cut, characterized in that said gate cutter has a stepped cylindrical form with said main cylindrical portion being of the greater diameter D and said coaxial end portion being cylindrical and being of a smaller diameter d, said cylindrical end portion, when received in said bore of said confronting die leaving therebetween an annular gap t of a radial size which determines a gate thickness, the outer peripheral surface of the portion of said gate cutter received in said bore of said die presenting a gate land. The above and other objects, features and advantages of the present invention will become clear from the following description of a preferred embodiment when the same is read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of an injection mold in accordance with the present invention; Fig. 2 is a sectional view of a known injection mold in a state before the shearing of gate; Fig. 3 is a sectional view of the injection mold of Fig. 2 in a state after the shearing of the gate; and Fig. 4 is a sectional view of a modification of the known injection mold of Fig. 2 in which shearing is effected by a movement of the gate cutter alone. DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described with reference to Fig. 1. The injection mold of the present invention, which is of the type in which the shearing of the gate is effected by a movement of the gate cutter 1 alone, is characterized in that a gate land is formed on the gate cutter 1 so as to stabilize the flow of the resin into the cavity while optimizing the pressure applied to the resin in the cavity. Referring to Fig. 1, the injection mold has a gate cutter 1 and a die 4. The end of the gate cutter 1 adjacent the die 4 is stepped so as to provide a small-diameter end portion which is coaxial with the main portion of the gate cutter 1 and which has a diameter "d" smaller than the diameter D of the main portion. The arrangement is such that, when the injection mold is closed for injection, the small-diameter end portion of the gate cutter 1 is received in the bore of the die 4 leaving an annular gap t therebetween. The bore of the die 4 has an inside diameter D′ corresponding to the outside diameter D of the main portion of the gate cutter 1 so that, when the gate is cut, the main portion of the gate cutter fits in the bore of the die 4. With this arrangement, it is possible to preserve the gate thickness t or t′ and the gate length ℓ′, thus making it possible to stabilize the flow of the resin flowing into the cavity 5 and to optimize the transmission of pressure to the resin in the cavity 5. Although the sprue bush 2 and the die 4 are illustrated as independent members in Fig. 4, it is to be understood that these members may be constructed integrally with each other. The invention is generally applicable to various molds of the type having a film gate in which gate cutting operation is conducted while the material is still in the die and in which the quality of the product is largely affected by the pressure of the resin and the state of the resin flowing into the die cavity, although an injection mold for optical disks has been specifically described. As has been described, according to the present invention, it is possible to optimize the state of flow of the resin and the pressure of the resin in the die cavity by virtue of the gate thickness and the gate land length which are presented by the claimed structure of the mold. Although the invention has been described through its specific terms, it is to be understood that the described embodiments are only illustrative and various changes and modifications may be imparted thereto without departing from the scope of the appended claim.

Claims: An injection mold in which a film gate of a resin is formed and in which shearing of the film gate is conducted by movement of a gate cutter (1) alone which cooperates with a confronting die (4), said gate cutter (1) having a main cylindrical portion and a coaxial end portion, said end portion being received in a bore of said confronting die (4) when said mold is closed, the bore of said die (4) having an inside diameter (D') corresponding to an outside diameter (D) of said main cylindrical portion of said cutter (1) so that it fits into said bore of said die (4) when the gate is cut, characterized in that said gate cutter (1) has a stepped cylindrical form with said main cylindrical portion being of the greater diameter (D) and said coaxial end portion being cylindrical and being of a smaller diameter (d), said cylindrical end portion, when received in said bore of said confronting die (4) leaving therebetween an annular gap (t) of a radial size which determines a gate thickness, the outer peripheral surface (1a) of the portion of said gate cutter (1) received in said bore of said die (4) presenting a gate land.

Inventors: KITAMURA TAKEHIKO, KITAMURA, TAKEHIKO
Applicants: SUMITOMO HEAVY INDUSTRIES, SUMITOMO HEAVY INDUSTRIES, LTD
Application year: 1990
Main classification: B29C45
IPC codes: B29C45