Injection molding method and injection mold

An injection molding method and an injection mold used therefor are provided, in which a molded product free from burrs, whitening and gate marks can be obtained with a simple structure mold, and it can adequately serve the needs for multicavity molding as well. In the injection molding method, molten resin material is introduced and charged into a resin reservoir and a molding cavity of an injection mold, where a depth of the resin reservoir is larger than a thickness of a communicating portion. A cut punch is moved, when a portion of the resin material in the resin reservoir is still molten, to push the molten resin in the resin reservoir back from a gate into a runner so that the cut punch not only closes the communicating portion but also cuts a resin solidified portion in the resin reservoir away from a resin molded product in the molding cavity at the communicating portion.

TECHNICAL FIELD

The present invention relates to an injection molding method and an injection mold.

BACKGROUND ART

Injection molding methods have heretofore been used as a molding method for producing resin molded products. It is well known that an injection molding method is a method involving the steps of: forming a cavity between a fixed die and a movable die; charging a molten thermoplastic resin material from a gate provided on part of the cavity by injection; and solidifying the thermoplastic resin material into a predetermined shape corresponding to a space in the cavity to obtain a resin molded product; and taking the resin molded product out.

Further, injection molds used in injection molding methods come in two types: one having means for heating part of a runner that introduces a molten resin material into a cavity from an injection molding apparatus, and the other not having such means. It is also well known that a method using the former type of mold having the heating means is called a hot runner system and that a method using the latter type of mold not having the heating means is called a cold runner system.

By the way, immediately after the end of an injection molding process based on the cold runner system, a resin molded product formed in the cavity and a resin portion solidified in the runner are in one piece. Therefore, the resin molded product and the solidified resin portion in the runner must be cut into separate pieces at a gate portions and thus some kind of mechanical cutting means is usually provided in the mold.

On the other hand, the injection molding method based on the hot runner system has a mold structure so that the resin in the runner can be kept molten. Therefore, the cutting means is not generally required. However, to fabricate a molded product having an opening in its part, it is preferred not to form weld lines (lines formed in the cavity by the confluence of molten resin portions flowing in different directions, and it is desirable to minimize the formation of such lines from the viewpoints of external appearance and mechanical strength). To prevent the formation of weld lines, it is desired that molding be done using a mold in which the gate is previously provided so as to correspond to an opening forming portion and that the portion be thereafter cut away to form the opening. In this case, even the hot runner system must have mechanical cutting means similarly to the cold runner system.

Thus, irrespective of the systems, whether it is the cold runner system or the hot runner system, various types of methods and molds that involve the cutting operation are proposed in injection molding methods. For example, Japanese Patent Application Laid-open No. Sho 55-15834 proposes a molding method for obtaining a ring-like molded product. In this molding method, a molten resin is charged into a cavity through a sprue bush, and then the sprue bush is retracted as a cut pin for cutting a solidified resin portion near a gate portion moves forward, so that the gate portion is sheared to separate a subrunner from a resin molded product. Further, Japanese Patent Application Laid-open No. Hei 6-278177 proposes a method that provides a second bush. In this method, a cut bush for cutting a solidified resin portion near a gate portion is moved forward, and in synchronism with such forward movement of the cut bush, the second bush retracts from a cavity in such a manner as to mechanically absorb an amount of resin pushed out by the cut bush. Still further, Japanese Patent Application Laid-open No. Hei 2-67115 proposes a mold that cuts a number of disk gates. That is, not only a sprue bush is operated while interlocked with the movement of a fixed die away from a fixed table, but also a punch cutter provided on a movable die is projected, so that a number of disk gates are cut.

Furthermore, Japanese Patent Application Laid-open No. Sho 58-158231 proposes a method for separating a molded product from a disk gate formed in a cavity. That is, in a gate portion within the cavity, a gate cutting blade provided on a movable die so as to face the inner surface of a fixed die is operated to bring a front end of the blade into contact with the fixed die, so that the gate disk is cut away.

Furthermore, Japanese Patent Application Laid-open No. Hei 8-294944 proposes a method for obtaining a resin molded product having an opening at a central portion. In this method, a specially shaped core pin having an external shape corresponding to the opening of the molded product is caused to shuttle between a first position and a second position. The first position stops at a gate a flow of molten resin injected into a hot runner. The second position opens the gate to introduce the molten resin into a cavity, and is located at the opening portion that is the center of axis corresponding to a disk gate portion. Moreover, Japanese Patent Application Laid-open No. Hei 7-276437 proposes a method for obtaining a resin molded product having an opening at the center of axis. In this method, a fixed pin is slidably provided in a hot runner provided in a fixed die and serves also as a hot gate, and a movable pin is provided on a movable die so as to be slidable in correspondence with the fixed pin and has a distal end formed so that the distal end engages with and abuts against a distal end of the fixed pin so as to be connected to and disconnected from the distal end of the fixed pin. The distal end of the fixed pin and that of the movable pin are engaged with and abutted against each other in the cavity so as to correspond to the opening portion of the molded product, so that the resin molded product having the opening at the center of axis is obtained.

Still further, Japanese Patent Application Laid-open No. Hei 1-99821 proposes a method for cutting a gate portion using a sleeve-like cutter that is driven toward a fixed die from a movable die after a resin material charged into a cavity has substantially solidified and before a resin portion at the gate portion completely solidifies.

However, the methods and mold disclosed in Japanese Patent Application Laid-opens Nos. Sho 55-15834, Hei 6-278177 and Hei 2-67115 are based on the concept that complicates the mold structure in order to achieve the above object, and thus elevates the cost and is highly likely to impair mechanical reliability. Further, if such methods and mold are applied to multicavity molds, the structure becomes further complicated. Thus, one can easily guess that the cost increases and the mechanical reliability impairs significantly.

Still further, the method disclosed in Japanese Patent Application Laid-open No. 58-158231 proposes the use of a simply structured mold, but imposes not only a problem that the gate cutting blade comes in contact with the mold when cutting the gate portion, and thus has difficulty stably cutting a solidified film layer of the resin material, but also a problem that the fixed die could be damaged by the gate cutting blade abutting against the mold strongly.

The methods disclosed in Japanese Patent Application Laid-open Nos. Hei 8-294944 and Hei 7-276437 impose a problem that the molds are expensive since both methods require an exclusive valve gate system having specially shaped or structured pins. The methods also impose, e.g., a problem that burrs are likely to be produced at a punched portion since an opening is punched before the gate portion solidifies.

Still further, the method disclosed in Japanese Patent Application Laid-open No. 1-99821 proposes a simply structured mold, but imposes a problem that the method is effective when there is an amount of resin plenty enough to absorb the driving stroke of the sleeve-like cutter at the disk gate portion, but if the volume of the disk gate portion is relatively small, the cutter is hard to drive and thus the method is not applicable.

DISCLOSURE OF THE INVENTION

Therefore, an object of the present invention is to provide an injection molding method that can obtain a molded product free from burrs, whitening and gate marks using a simply structured mold, and that can adequately serve the needs for multicavity molding.

Further, another object of the present invention is to provide an injection mold beneficially applicable to the aforementioned molding method.

To overcome the aforementioned problems, one aspect of the present invention provides an injection molding method wherein a molten resin material, which is injected into a cavity defined by a fixed die and a movable die via a runner provided in the fixed die, is introduced from a gate provided in the fixed die and is molded, and the method is characterized in that:

when the molten resin material is being charged into the cavity through the resin reservoir from the gate, a cut punch, which is provided on the side of the movable die that confronts the gate through a resin reservoir formed by recessing the fixed die toward the gate and which is movably provided so that the cut punch is inserted into the resin reservoir so as to be in slidable contact with the resin reservoir, has a distal end thereof extending in a moving direction thereof that is toward the resin reservoir, and the distal end is located between the resin reservoir and the cavity at such a position as to open a communicating portion that allows the resin reservoir and the cavity to communicate with each other so that the molten resin material is introduced into the cavity via the resin reservoir; and

when the resin material that is still molten is present in the resin reservoir after the molten resin material has been charged into the cavity and the resin reservoir, the cut punch moves toward the gate so that the cut punch is inserted into the resin reservoir, whereby the cut punch not only closes the communicating portion while forcibly pushing the still molten resin material present in the resin reservoir back into the gate, but also cuts the resin material at the communicating portion so that a resin molded product formed in the cavity is separated from a resin solidified portion formed in the resin reservoir.

Further, another aspect of the present invention provides an injection mold wherein a fixed die and a movable die form a cavity into which a molten resin material injected via a runner provided in the fixed die is introduced from a gate provided in the fixed die, and the mold is characterized in that:

the gate is connected to the cavity through a resin reservoir formed by recessing the fixed die toward the gate;

a cut punch is provided on the side of the movable die that confronts the gate through the resin reservoir, the cut punch being movable so that the cut punch can be inserted into the resin reservoir so as to be in slidable contact with the resin reservoir;

when the molten resin material is being charged into the cavity, a distal end of the cut punch extending in a moving direction of the cut punch that is toward the resin reservoir is located between the resin reservoir and the cavity at such a position as to open a communicating portion that allows the resin reservoir and the cavity to communicate with each other so that the molten resin material is introduced into the cavity via the resin reservoir; and when the resin material that is still molten is present in the resin reservoir after the molten resin material has been charged into the cavity and the resin reservoir, the cut punch moves toward the gate so that the cut punch is inserted into the resin reservoir, whereby the cut punch not only closes the communicating portion while forcibly pushing the still molten resin material present in the resin reservoir back into the gate, but also cuts the resin material at the communicating portion so that a resin molded product formed in the cavity is separated from a resin solidified portion formed in the resin reservoir.

BEST MODES FOR CARRYING OUT THE INVENTION

An injection molding method of the present invention (hereinafter referred to as the “method of the present invention”) and an injection mold of the present invention will be described below in details.

In the method of the present invention, a molten resin material injected by an injection molding machine is charged into a cavity defined by a fixed die and a movable die from a gate provided in the fixed die via a runner provided in the fixed die. At this time, the molten resin material introduced from the gate is charged into the cavity while flowing into a resin reservoir that is formed on the side of the gate in the fixed die. When the molten resin material is introduced into the cavity via the resin reservoir, a communicating portion that allows the molten resin material to pass through a space between the resin reservoir and the cavity is formed. A cut punch is provided on the side of the movable die that confronts the gate through the resin reservoir and is movable so that the cut punch is inserted into the resin reservoir so as to be in slidable contact with the resin reservoir in order to form the communicating portion, and the distal end of such cut punch is arranged on the side of the movable die. As shown in, e.g.,FIG. 2to be referred to later, when a molten resin material is charged into a cavity10via a resin reservoir12with a cut punch15not yet in operation, a distal end23of the cut punch15is held on the side of the movable die in such a position as to open a communicating portion11through which the resin passes from the resin reservoir12to the cavity10. Then, as shown in, e.g.,FIG. 4to be referred to later, after the molten resin material has been charged into the cavity and the resin reservoir, the resin material gradually solidifies from a portion that is in direct contact with the cooled dies toward the inner portion. At this time, when the resin material that is still molten is present in the resin reservoir, the cut punch is operated so that the punch moves toward the gate in such a manner as to be inserted into the resin reservoir. Then, as shown in, e.g.,FIG. 5, the cut punch closes the communicating portion while forcibly pushing the still molten resin material present in the resin reservoir back into the gate. At the same time, the cut punch cuts the resin material at the communicating portion, so that a resin molded product formed in the cavity is separated from a resin solidified portion formed in the resin reservoir.

In an injection mold used in the method of the present invention, the depth as viewed in a moving direction of the cut punch in the resin reservoir (i.e., a distance between the distal end of the cut punch that is not yet in operation and held on the side of the movable die and a surface on the side of the gate of the fixed die forming the resin reservoir), e.g., a distance L1shown inFIG. 2and an opening distance of the communicating portion in the moving direction of the cut punch, e.g., a distance L2shown inFIG. 2are arranged so that L1>L2. As a result of such arrangement, the solidifying speed of the resin material in the resin reservoir becomes completely lower than that of the resin material in the communicating portion. Hence, even if the resin material in the communicating portion has solidified almost completely or half-solidified, the resin material that is still molten is present in the resin reservoir. If the cut punch is operated to move toward the fixed die under such condition, the still molten resin material is forcibly pushed back into the runner via the gate, and this gives the cut punch a moving stroke. Therefore, the larger the thickness of the resin reservoir is when compared with the opening distance of the communicating portion, the larger the difference between the solidifying time of the resin material in the resin reservoir and that of the resin material in the communicating portion becomes. As a result, such arrangement is advantageous in increasing the range of timings at which the cut punch is stably operated. On the other hand, the resin reservoir is scrapped every time the molding process ends, and thus it is not economically desirable to make it thicker than necessary. Thus, as a range in which the molding process can be performed stably and economically and in which burring and whitening do not occur at such a part of a resin molded product as to correspond to the communicating portion, the depth as viewed in the moving direction of the cut punch in the resin reservoir is preferably set from 1.5 to 10 times the opening distance of the communicating portion, or more preferably from 2 to 6 times.

Further, in the injection mold used in the method of the present invention, as shown inFIG. 14a, such a cross section of the resin reservoir that the depth as viewed in the moving direction of the cut punch15in the resin reservoir12is gradually increased toward the gate14is advantageous in increasing the operating area of the cut punch. Further, as shown inFIG. 14b, such a cross section that the depth of the outermost peripheral portions12aat both ends of the resin reservoir12is increased is also advantageous in ensuring the operating stroke of the cut punch. Still further, as shown inFIG. 14c, such a cross section that the depth as viewed in the moving direction of the cut punch15in the resin reservoir12is gradually increased toward the gate14and the depth of the outermost peripheral portions12aat both ends of the resin reservoir12is increased may also be acceptable and is advantageous in ensuring the operating stroke of the cut punch.

Still further, it is so arranged that the diameter of the shaft of a pusher pin for pushing the resin solidified portion in the resin reservoir is increased so that the area of projection of the distal end of the moving cut punch as viewed in the moving direction of the cut punch is decreased, and that the cut punch is operated independently of the pusher pin so that when the cut punch operates, the pusher pin does not operate. Such an arrangement is advantageous in ensuring the operating area and stroke of the cut punch.

In the method of the present invention, when the resin material in the communicating portion between the resin reservoir and the cavity semi-solidifies and the resin material that is still molten is present in the resin reservoir after the charging of the molten resin material into the cavity has been completed, the cut punch is operated to move toward the gate of the fixed die, so that the cut punch closes the communicating portion. The operating timing of the cut punch is selected appropriately in accordance with the shape of the molding cavity, i.e., the shape of a resin molded product, the solidifying speed of the resin material, the shape of the resin reservoir, and the like. Generally, if the cut punch is operated when the resin material in the communicating portion is still molten, i.e., if the cut punch is operated at an early timing, the resin material creeps into an operating clearance provided between a side surface of the resin reservoir and a side surface of the cut punch since the mold is designed to have a slight clearance from manufacturing and structural considerations. As a result, there is a danger that burrs are formed at the cut end of the resin molded product.

Further, if the clearance is decreased to overcome the burring problem, the side surface of the resin reservoir abuts against the cut punch to cause scorings, and in the extreme case, there may be an operation problem, e.g., that the communicating portion of the fixed die is damaged. In addition, if the cut punch is operated after the resin material in the communicating portion has completely solidified, i.e., if the cut punch is operated at a later timing, tiny cracks are present at the cut end of a resin molded product obtained and whitening appears over the exterior of the product. Therefore, it is important to determine while confirming the operating timing of the cut punch for the method of the present invention through tests and the like so that the timing comes later than when the cut end burrs and earlier than when the cut end whitens in the resin molded products obtained.

In the method of the present invention, the larger the diameter of the opening of the gate through which the molten resin material passes when the resin material that is still molten in the resin reservoir is pushed back into the runner by the operation of the cut punch, the easier the molten resin material is caused to flow back into the runner, and thus the larger diameter of the opening of the gate is advantageous in operating the cut punch as desired. On the other hand, in the case where a cold runner type injection mold is used, a larger diameter of the opening of the gate is likely to make it hard to separate the resin solidified portion formed in the resin reservoir from the runner. To overcome this problem, in the case where a cold runner type injection mold is used, it is preferred to previously fabricate the mold having an ordinary pin gate diameter, examine the operating conditions of the cut punch by checking the burring or whitening conditions of an obtained molded product, and determine an appropriate opening diameter by increasing the diameter as necessary. This applies similarly to the diameter of the opening of the gate of the injection mold of the hot runner structure. In the case where a hot runner type injection mold having a valve gate is used, there is no need to consider the problem of separating the resin solidified portion in the resin reservoir from the runner at the gate since the gate is mechanically closed by a needle pin. Therefore, it is preferred to increase the diameter of the opening of the gate as much as possible. For example, the diameter of the opening of the gate preferably ranges from 1.0 to 2.5 mm.

EMBODIMENTS OF THE INVENTION

An injection molding method and an injection mold, which are preferred embodiments 1 and 2 of the present invention will now be described based onFIGS. 1 to 13.FIGS. 1 to 7show a first embodiment for fabricating a bent plate-like resin molded product using a cold runner type mold that is a preferred embodiment of a mold of the present invention.FIGS. 8 to 13show a second embodiment for fabricating a thin annular part using a hot runner type mold.

Cold runner type injection mold1used in the first embodiment whose main portion is shown inFIG. 1in the form of a cutaway section includes a fixed die2and a movable die3. The fixed die2includes a fixed mounting plate2aand a fixed die plate2cmounted on the fixed mounting plate2athrough a fixed back plate2b. A sprue bush6in which an introducing hole5is formed is provided on the fixed mounting plate2a. The introducing hole5introduces a molten resin material injected from an injection nozzle4of an injection molding machine into the mold. The introducing hole5of the sprue bush6is formed inside the fixed back plate2band the fixed die plate2c, and communicates with a runner7through which the molten resin material flows.

The movable die3has a movable mounting plate3aand a movable die plate3cthat is mounted on the movable mounting plate3athrough a spacer block3b.

A resin reservoir12is formed by recessing the fixed die plate2cof the fixed die. That is, at the time the fixed die2and the movable die3of the injection mold1are matched, a stepped molding surface8formed on the fixed die plate2cof the fixed die2and a stepped molding surface9formed on the movable die plate3cof the movable die3so as to correspond to the molding surface8forms the resin reservoir12that communicates to a cavity10that forms the resin molded products through the communicating portion11as shown by a plan view ofFIG. 3. InFIG. 3, reference numeral14denotes a gate for introducing the molten resin material into the cavity.

Further, as shown inFIG. 2, the resin reservoir12is connected to the runner7via the gate14formed in a resin reservoir surface13on the molding surface8of the fixed die plate2cof the fixed die. The runner7introduces the molten resin material from the nozzle4of the injection molding machine.

Further, a cut punch15is provided on the side of the movable die plate3cof the movable die3that confronts the gate14through the resin reservoir12. The cut punch15is so arranged that when the molten resin material is being charged into the cavity, a distal end23of the cut punch is located between the resin reservoir12and the cavity10at such a position as to open a communicating portion11that allows the resin reservoir12and the cavity10to communicate with each other so that the molten resin material is introduced into the cavity via the resin reservoir12, and that when the resin material that is still molten is present in the resin reservoir12after the molten resin material has been charged into the cavity, the cut punch is moved from the movable die3to the fixed die2so that the resin reservoir12is separated from the cavity10at the communicating portion11. The cut punch15is secured to a cut punch driving hydraulic cylinder17together with cut punch pusher plates16aand16b, and is urged by a spring18in a direction opposite to a driving direction.

Further, a resin reservoir pusher pin19for pushing out a resin solidified portion formed in the resin reservoir12is inserted into the cut punch15so as to be slidable independently of the cut punch15. Also, molded product pusher pins20aand20bfor pushing out a resin molded product formed in the cavity10are slidably inserted into the movable die plate3c. The resin reservoir pusher pin19and the molded product pusher pins20aand20bpass through the cut punch pusher plates16aand16band are mounted on pusher plates22aand22bthat are urged by a spring21in a direction opposite to a driving direction of the pusher pins. The resin reservoir pusher pin19and the molded product pusher pins20aand20bare driven forward (in the elevating direction as viewed inFIG. 1) when a pushing rod (not shown) of the molding machine pushes first a pusher rod receiver22cand then the pusher plate22bthat is integrally mounted on the pusher rod receiver22c.

As shown inFIGS. 1 and 2, undercut portions25aand25bare formed in the distal end23of the cut punch15. The portions25aand25ballow the cut punch to hold the resin solidified portion formed in the resin reservoir12so that the resin solidified portion does not remain on the side of the fixed die at the time of mold opening.

In the injection mold1, the communicating portion11provided between the resin reservoir12and the cavity is arranged so that the depth as viewed in the moving direction of the cut punch15in the resin reservoir becomes larger than the opening distance of the communicating portion in the moving direction of the cut punch as shown inFIG. 2. That is, a distance L1between a molding surface8aof the fixed die plate2con a side surface26of the resin reservoir12and the distal end23of the cut punch15is arranged to be larger than the opening distance of the communicating portion11extending in the moving direction of the cut punch, i.e., a distance L2between a molding surface8bof the fixed die plate2cand the distal end23of the cut punch15(L1>L2). As a result of such arrangement, when the cut punch15is driven in a direction indicated by an arrow A, not only a side surface15aof the cut punch15closes the communicating portion11, but also cuts the resin molded product formed in the cavity10away from the resin solidified portion formed in the resin reservoir12at the communicating portion11.

In the injection molding method using the injection mold1that has the cold runner structure shown inFIG. 1, first, the movable die3is driven to close the fixed die2and the movable die3, which in turn forms the cavity10and the resin reservoir12communicating with the cavity10. Then, the molten resin material is injected from the injection nozzle4of the injection molding machine, the nozzle4being brought into contact with the introducing hole5that is formed in the sprue bush6provided in the fixed mounting plate2a. The injected molten resin material is introduced into the resin reservoir12from the gate14while flowing through the runner7, and is further charged into the cavity10via the communicating portion11. At this time, the cut punch15does not operate and is held on the side of the movable die3.

The resin material charged into the resin reservoir12, the communicating portion11and the cavity10is cooled by cooling water circulating through cooling means, e.g., cooling water passages appropriately provided within the fixed die2and the movable die3, and thus solidified. After the charging of the resin material has been completed, the charged resin material is cooled to solidify, as shown inFIG. 4, from the outside portion that is in contact with the molding surface8of the fixed die plate2cand the molding surface9of the movable die plate3c, and the resin material at the communicating portion11where the molding surface8neighbors the molding surface9solidifies or semi-solidifies first. On the other hand, in the resin reservoir12that is thicker than the communicating portion11, i.e., in the resin reservoir12in which the distance between the molding surface8of the fixed die plate2cand the molding surface9of the movable die plate3cis large, the outside portion of the resin material that is in contact with the molding surfaces8and9solidifies or semi-solidifies, but a resin material portion27that is still molten is present in the inside.

At this time, the cut punch15that is mounted on the cut punch pusher plates16aand16bis operated by the cut punch driving hydraulic cylinder17against the urging force of the spring18, so that the cut punch moves from the movable die3to the fixed die2. The still molten resin material27in the resin reservoir is pushed back from the gate14to the runner7by the pushing force of the moving cut punch15. As a result, the volume of the resin in the resin reservoir is reduced, which in turn provides a stroke for allowing the cut punch15to move. Hence, as shown inFIG. 5, the cut punch15advances toward the fixed die plate2c, so that the side surface15aof the cut punch15closes the communicating portion11and a resin molded product28in the cavity10is cut away from a resin solidified portion29formed in the resin reservoir12at the communicating portion11. At this time, the cut punch15is mounted with its base30held between the cut punch pusher plates16aand16bshown inFIG. 1, so that the moving distance of the cut punch15is regulated by the distance between an outside surface31of the cut punch pusher plate16aand a rear surface32of the movable die plate3c. The resin reservoir pusher pin19inserted into the cut punch15so as to be slidable independently of the cut punch15does not move together with the cut punch15when the cut punch moves, and thus a top face24of the pin19forms a recess33.

The resin material in the cavity10and the resin reservoir12is cooled and solidifies, so that the resin molded product28is formed in the cavity10, and the resin solidified portion29including a resin material portion solidified at the recess33and the undercut portions25aand25bis formed in the resin reservoir12. Then, as shown inFIG. 6, the movable die3is driven to open the injection mold1. At this time, the resin solidified portion29formed in the resin reservoir is separated from the resin molded product, and held on the distal end of the cut punch15. Further, the solidified portion formed in the undercut portions25aand25bfunctions as a holding portion for holding the resin solidified portion29on the distal end of the cut punch15.

Next, as shown inFIG. 7, the pusher rod (not shown) of the molding machine drives the resin reservoir pusher pin19and the molded product pusher pins20aand20bmounted on the pusher plates22aand22bagainst the urging force of the spring21, and the resin molded product28and the resin solidified portion29are pushed out independently of each other and obtained in the form of cut pieces.

Further,FIGS. 8 to 13show the second embodiment for fabricating thin annular parts using a hot runner type four-cavity mold based on a valve gate system.FIG. 8shows only a half of the four-cavity injection mold, omitting the other half. The following description is based on the half of the mold with two cavities shown inFIG. 8, and a description of the other half is omitted.

A hot runner type injection mold41used in the second embodiment whose main portion is shown inFIG. 8in a cutaway form includes a fixed die42and a movable die43. The fixed die42has a fixed mounting plate42aand a fixed die plate42cmounted on the fixed mounting plate42athrough a spacer block42b. A sprue bush46in which an introducing hole45is formed is provided on the fixed mounting plate42a. The introducing hole45introduces a molten resin material injected from an injection nozzle44of an injection molding machine into the mold. The introducing hole45of the sprue bush46is connected to a runner48that is provided in a manifold block47interposed between the fixed mounting plate42aand the fixed die plate42c, and communicates with hot runners50aand50bof hot runner nozzles49aand49bprovided in the fixed die plate42c. Needle pins53aand53bare inserted into the hot runners50aand50bof the hot runner nozzles49aand49b, respectively. The pins53aand53bare driven by needle pin driving cylinders51aand51bprovided in the fixed mounting plate42aand control the introduction of the molten resin material into resin reservoirs58aand58b, and cavities56aand56bwhich will be described later.

The movable die43includes a movable mounting plate43aand a movable die plate43esecured to movable back plates43cand43dthat are mounted on the movable mounting plate43athrough a spacer block43b.

The resin reservoirs58aand58bare formed by molding surfaces54aand54band molding surfaces55aand55b, respectively. That is, as shown inFIGS. 8 and 9, when the fixed die42and the movable die43of the injection mold41are closed, the stepped molding surfaces54aand54bformed on the fixed die plate42cof the fixed die42and the stepped molding surfaces55aand55bformed on the movable die plate43eof the movable die43so as to correspond to the molding surfaces54aand54bform the resin reservoirs58aand58bthat communicate with annular cavities56aand56bfor forming resin molded products through communicating portions57aand57b. Further, gates52aand52bare formed in resin reservoir surfaces59aand59bof the molding surfaces54aand54bof the resin reservoirs58aand58bon the side of the fixed die plate42c, respectively.

The resin reservoirs58aand58bare connected to the hot runners50aand50bvia the gates52aand52bformed in the resin reservoir surfaces59aand59bof the molding surfaces54aand54bon the fixed die plate42cof the fixed die42. The hot runners introduce the molten resin material.

Cut punches60aand60bare provided on the side of the movable die plate43eof the movable die43that confronts the gates52aand52bthrough the resin reservoirs58aand58b. The cut punches60aand60bare so arranged that when the molten resin material is being charged into the cavities, distal ends67aand67bof the punches are located between the resin reservoirs and cavities at such positions as to open the communicating portions that allow the resin reservoirs58aand58band the cavities56aand56bto communicate with each other so that the molten resin material is introduced into the cavities via the resin reservoirs58aand58b, and that when the resin material that is still molten is present in the resin reservoirs58aand58b=after the molten resin material has been charged into the cavities, the cut punches are moved from the movable die43to the fixed die42so that the resin reservoirs58aand58bare separated from the cavities56aand56bat the communicating portions57aand57b. The cut punches60aand60bare connected to cut punch driving hydraulic cylinders61aand61bprovided in the movable back plate43ctogether with the movable back plates43cand43d, and are urged by springs62aand62bin a direction opposite to a driving direction.

Further, resin reservoir pusher pins63aand63bfor pushing out resin solidified portions formed in the resin reservoirs58aand58bare inserted into the cut punches60aand60bso as to be slidable independently of the cut punches60aand60b. Also, molded product pusher sleeves64aand64bfor pushing out resin molded products formed in the cavities56aand56bare slidably inserted into the movable die plate43e.

The resin reservoir pusher pins63aand63band the molded product pusher sleeves64aand64bpass through the movable back plates43cand43dand are mounted on pusher plates66aand66burged by a spring65in a direction opposite to a moving direction of the pusher pins and sleeves.

The resin reservoir pusher pins63aand63band the molded product pusher sleeves64aand64bare driven forward (in the elevating direction as viewed inFIG. 8) when a pushing rod (not shown) of the molding machine pushes first a pusher rod receiver66cand then the pusher plates66aand66bthat are integrally mounted on the pusher rod receiver66c.

As shown inFIGS. 8 and 9, undercut portions69aand69bare formed in the distal ends67aand67bof the cut punches60aand60b. The portions69aand69ballow the cut punches60aand60bto hold the resin solidified portions formed in the resin reservoirs58aand58bso that the resin solidified portions do not remain on the side of the fixed die at the time of mold opening.

Further, in the injection mold41, the communicating portion57aprovided between the resin reservoir58aand the cavity56aand the communicating portion57bprovided between the resin reservoir58band the cavity56bare arranged so that the depth of the resin reservoirs become larger than the opening distance of the communicating portions in the moving direction of the cut punches60aand60b. That is, when the cut punches60aand60bare not operated and held on the side of the movable die plate43e, a distance L3between the molding surface54aor54bof the fixed die plate42con a side surface101of the resin reservoir58aor58band the distal end67aor67bof the cut punch60aor60bis arranged to be larger than the opening distance of the communicating portion57aor57bextending in the moving direction of the cut punch, i.e., a distance L4between the molding surface54aor54bof the fixed die plate43eand the distal end67aor67bof the cut punch60aor60b(L3>L4). As a result of such arrangement, the cut punch60ais operated to move in a direction indicated by an arrow A, so that a side surface102aof the cut punch60anot only closes the communicating portion57a, but also cuts the resin molded product formed in the cavity56aaway from the resin solidified portion formed in the resin reservoir58aat the communicating portion57a. Further, a similar operation is performed at the resin reservoir58band the cavity56b.

In the injection molding method using the injection mold41that has the hot runner structure shown inFIG. 8, first, the movable die43is driven to close the fixed die42and the movable die43, which in turn forms the cavities56aand56band the resin reservoirs58aand58bthat communicate with the cavities56aand56bthrough the communicating portions57aand57b. Then, the injection nozzle44of the injection molding machine is brought into contact with the introducing hole45formed in the sprue bush46provided in the fixed mounting plate42a, and the molten resin material is injected. The injected molten resin material is introduced into the hot runners50aand50bwhile flowing through the runner48. At this time, the needle pins53aand53bare driven upward as viewed in the drawing by the needle driving cylinders51aand51bto open the gates52aand52b, so that the molten resin material is introduced and charged into the cavities56aand56bvia the resin reservoirs58aand58band the communicating portions57aand57b. At this time, the cut punches60aand60bare not operated and held on the side of the movable die43.

The resin material charged into the resin reservoirs58aand58b, the communicating portions57aand57band the cavities56aand56bis cooled by cooling water flowing through cooling means, e.g., cooling water passages appropriately provided within the fixed die42and the movable die43, and thus solidified.

Next, an operation at the resin reservoir58a, the communicating portion57aand the cavity56awill be described as an example. An operation at the resin reservoir58b, the communicating portion57band the cavity56bis also similar.

After the charging of the molten resin material has been completed, the charged resin material is cooled and solidifies, as shown inFIG. 10, from the outside portion that is in contact with the molding surface54aof the fixed die plate42cand the molding surface55aof the movable die plate43e, and the resin material at the communicating portion57awhere the molding surface54aneighbors the molding surface55asolidifies or semi-solidifies first. On the other hand, in the resin reservoir58athat is deeper than the communicating portion57a, i.e., in the resin reservoir in which the distance between the molding surface54aof the fixed die plate42cand the molding surface55aof the movable die plate43eis larger than the communicating portion57a, the outside portion of the resin material that is in contact with the molding surfaces54aand55asolidifies or semi-solidifies, but a resin material portion103athat is still molten is present in the inside.

At this time, the cut punch60amounted on the movable back plates43cand43dis moved from the movable die43to the fixed die42by the cut punch driving hydraulic cylinder61aagainst the urging force of the spring62a. The still molten resin material portion103ain the resin reservoir is pushed back into the hot runner50afrom the gate52aby the pushing force of the driven cut punch60a. As a result, the volume of the resin in the resin reservoir is reduced, which in turn provides a stroke for allowing the cut punch60ato move. Hence, as shown inFIG. 11, the cut punch60amoves toward the fixed die plate42c, so that the side surface102aof the cut punch60anot only closes the communicating portion57a, but also cuts a resin molded product105ain the cavity56aaway from a resin solidified portion106aformed in the resin reservoir58aat the communicating portion57a. At this time, the resin reservoir pusher pin63athat is inserted into the cut punch60aso as to be slidable independently of the cut punch60ais not driven together with the cut punch60awhen the cut punch60ais driven, and thus a top face68aof the pin63aforms a recess107a.

Then, the needle pin53ais driven to close the gate52a, and the resin material in the cavity56aand the resin reservoir58ais cooled and solidifies. Then, the resin molded product105ais formed in the cavity56a, and the resin solidified portion106aincluding a resin material portion solidified at the recess107aand the undercut portions69aand70ais formed in the resin reservoir58a. As shown inFIG. 12, the movable die43is thereafter driven to open the injection mold. At this time, the resin solidified portion106aformed in the resin reservoir is separated from the resin molded product105a, and held on the distal end of the cut punch60a. Further, the solidified portion formed in the undercut portions69aand70afunctions as a holding portion for holding the resin solidified portion106aon the distal end of the cut punch60a.

Then, as shown inFIG. 13, the pusher rod (not shown) of the molding machine drives the resin reservoir pusher pin63aand the molded product pusher sleeve64amounted on the pusher plates66aand66bagainst the urging force of the spring62a, so that the thin annular resin molded product105aand the disk-like resin solidified portion106aare pushed out independently of each other in cut pieces.

EXAMPLES

Test Example 1

Tests were carried out to fabricate bent plate-like resin molded product using the cold runner type injection mold of the present invention shown inFIG. 1by changing the used resin, mold temperature, clearance of the sliding portion between the inner surface of the resin reservoir and the outer periphery of the cut punch.

The thicknesses of the product forming portion, the communicating portion and the resin reservoir (at the time of injection) of the injection mold used for the tests were as follows.

Thickness of product forming portion1.5mm(cavity 10)Thickness of communicating portion (11)0.5mmThickness of resin reservoir (12)1.0mmat the time of injection

In the tests, the cutting quality given by the cut punch and associated problems are evaluated based on the following criteria. The results are shown in Table 1.

Cutting quality evaluation criteria:

◯ Satisfactory without cracks and burrs at cut end

Δ Acceptable in terms of external appearance although with some cracks and burrs at cut end

X Defective with cracks and burrs at cut end

K Unsatisfactory in terms of mold durability because of scorings at sliding portion

B Burring at cut end of molded product

H Whitening at cut end of molded product or cutting of molded product impossible

It can be said from the results shown in Table 1 that narrower clearances of the sliding portion between the inner surface of the resin reservoir and the outer periphery of the cut punch provide a better cutting quality, but too narrow a clearance makes it easy to bring the cut punch into contact with the resin reservoir, and this is not desirable in terms of mold durability. Particularly, if the clearance of the sliding portion is less than 5 μm, a mold of high accuracy must be used to avoid the mold durability problem, and this increases the cost for the mold and hence is not desirable economically. Therefore, when economic advantages are considered, it is desirable to optimize the clearance in accordance with the resin properties used. It has been verified from the results shown in Table 1 that clearances of 10 to 20 μm are preferred for GPPS and PC, whereas clearances of 5 to 10 μm are preferred for PA.

Further, tests were carried out to fabricate bent plate-like resin molded products using the same mold by changing the used resin, mold temperature and cut punch drive timing to evaluate the cutting quality given by the cut punch and associated problems under the same criteria. The results are summarized in Table 2. “Drive timing” means a time interval (in seconds) from injection completion (pressure holding start) to cut punch drive start.

It can be said from the results shown in Table 2 that the cut punch drive timing differs depending on the resin properties. What is common to the above three kinds of resins is that an early cut punch drive timing produces burrs at the cut ends and a late cut punch drive timing produces whitening and a further late timing makes it impossible to cut the products.

Test Example 2

Tests were carried out to fabricate thin annular parts by using the hot runner type injection mold of the present invention shown inFIG. 8and using PA as a resin at a mold temperature of 70° C. and by changing the clearance of the sliding portion between the inner surface of the resin reservoir and the outer periphery of the cut punch to obtain cut punch driving timings that can give satisfactory cutting quality. The results are shown in Table 3.

The thicknesses of the product forming portion, the communicating portion and the resin reservoir (at the time of injection) of the injection mold used for the tests were as follows.

Thickness of product forming portion (cavity56a) 0.17 mm

Thickness of communicating portion (57a) 0.17 mm

Thickness of resin reservoir (58a) at the time of injection 0.5 mm

It can be said from the results shown in table 3 that narrower clearances of the sliding portion between the inner surface of the resin reservoir and the outer periphery of the cut punch provide a wider drive timing range for giving satisfactory cutting quality, whereas wider clearances provide a narrower drive timing range for giving satisfactory cutting quality. Further, too narrow a clearance makes it easy to bring the cut punch into contact with the resin reservoir similarly to Test Example 1, which is not desirable in terms of mold durability. When PA is used as a resin, a clearance of 15 μm makes the cutting quality-optimizing drive timing as short as 0.1 sec, and this is not desirable in terms of stable production. Further, a clearance of 20 μm provides no drive timing range that can optimize the cutting quality. Therefore, the optimal cut punch drive timing that can give satisfactory clearance and cutting quality differs depending on the resin properties.

Further, tests were carried out to fabricate thin annular parts by using the same mold and using PA as a resin at mold temperatures of 60 and 80° C. by changing the thickness of the resin reservoir to obtain cut punch drive timings for giving satisfactory cutting quality. The results are shown in Tables 4 and 5.

It is understood from the results shown in Tables 4 and 5 that thicker resin reservoirs provide a wider range of cut punch drive timings for giving satisfactory cutting quality. Further, thickness ratios between the resin reservoir and the communicating portion of 1.5 or less cannot provide an adequately wide drive timing range, which imposes problems in terms of stable production. Still further, the resin in the resin reservoir is scrapped every molding cycle, and thus is a waste. Therefore, there is no need to have a drive timing range wider than necessary, and thus from the viewpoint of cut punch driving controllability and effective use of resources, it is understood that the thickness of the resin reservoir should preferably range from 1.5 to 10 times the thickness of the communicating portion, or more preferably from about 2 to 6 times.

INDUSTRIAL APPLICABILITY

As described in the foregoing, the injection molding method of the present invention can provide the following advantages.

(1) When the resin is injected and charged, the resin material in the resin reservoir solidifies from its outside surface, and at the time the resin material in the communicating portion that communicates with the cavity has solidified, the molten resin material remains in the resin reservoir since the resin reservoir is deeper than the communicating portion. When the cut punch is driven under such condition, the molten resin material in the resin reservoir can flow backward into the runner via the gate. As a result, the volume of the resin in the resin reservoir can be reduced, so that the moving stroke of the cut punch can be provided. Hence, the method of the present invention can eliminate the use of a complicated structure of the conventional mold, e.g., a moving sprue bush employed in a mold for fabricating optical disk substrates. That is, the method of the present invention can simplify the mold structure.
(2) If the resin reservoir is considered as part of a product, the gate in the mold of the present invention functions as a gate in an ordinary mold. Therefore, by adding only a cut punch drive system, the mold of the present invention can be designed to have a structure as simple as that of the conventional mold. As a result, the design of a multicavity mold can be similar to that of the conventional example, and thus becomes easy. Still further, even if a hot runner type is employed, the structure of the mold of such type can be designed as simple as that of a conventional example. Therefore, a commercially available hot runner system can be directly used, and thus an inexpensive mold can be provided.
(3) The solidified (or semi-solidified) resin material in the communicating portion is cut away. Therefore, the cut end has no burrs into which the molten resin can creep, and thus a resin molded product having no gate marks can be obtained. Further, since the molten resin does not creep into the cut end, an adequate clearance between the cut punch and the resin reservoir into which the cut punch is inserted can be provided, and thus the user does not have to worry about scorings at this portion. Hence, a durable mold can be provided and mold parts can be fabricated inexpensively.