JP2000108185A - Method for injection molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a sink mark of a crystal resin by slightly opening a die after the die is opened and a molten resin is first injected, the die is clamped and shaped while molten resin is then injected in a die cavity and shaped, providing a gap between the molding and the cavity, and clamping the die while surface resin is capable of being flowed after a given time. SOLUTION: A polypropylene resin of a molten state in a die cavity clamped by a mold clamping unit 300 and in an injection unit 200 is metered by a first injection timer 533. At the time of elapsing a given time after completion of filling, a movable die 404 is retracted so that a gap between the molding surface and the cavity surface becomes a given value. A pressure holding state is maintained for a given time after the first injection, and then a second injection is executed. At the time of elapsing given time, the mold 404 is advanced, the die is again clamped, a thick part generated by the secondary injection is corrected, shaped, and held for a given time. Then after a given time is elapsed, a gap is provided as a heat insulation layer between the surface of the molding and the surface of the cavity, held for a given time, the die is clamped and then opened. Thus, a sink mark can be prevented at the thick part of a crystalline resin.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an injection molding method for eliminating surface defects called sink marks which occur in a molded article having a partially thick portion such as a rib or a boss.

[0002]

2. Description of the Related Art In injection molding, if a product has a locally thick part such as a rib or a boss, the cooling and solidification of the thick part is delayed as compared with other parts. Had a surface defect called sink mark on the surface. FIG.
Since the high temperature portion shown in FIG. 2A solidifies and contracts in volume, sink marks occur on the surface on the side opposite to the ribs as shown in FIG. 2B.

[0003] In order to eliminate this, in the pressure-holding step, compensation filling is performed to compensate for the volume shrinkage due to cooling and solidification of the molten resin. ) Had no holding pressure effect, and it was practically impossible to completely eliminate sink marks. In particular, a crystalline resin such as a polypropylene resin has a large volume shrinkage, and the occurrence of sink marks cannot be avoided.

[0004]

In order to make this sink mark less noticeable, various efforts have been made in terms of product design and molding conditions, but they have not been satisfactory. In particular, since shrinkage becomes large with respect to a crystalline resin such as a polypropylene resin having a large volume shrinkage, it is a great limitation from the viewpoint of product design, and it takes time to determine molding conditions.

[0005]

In order to solve the above-mentioned problems, in the present invention, a molten resin is injected and filled into a mold cavity (primary injection), and after completion of shaping, the molten resin is filled. During the flowable time, once the mold is slightly opened or after the mold is opened, the molten resin is further injected (secondary injection), and then the mold is closed and shaped again. The mold is slightly opened again, a gap is provided between the surface of the molded product and the cavity surface of the mold, the mold is held for a certain time, and the mold is closed again within a time in which the surface-side molten resin can flow. And

[0006]

[Function] Even if the product shape has a partially thick portion such as a rib or a boss, a gap as a heat insulating layer is provided on the surface side (anti-rib / boss side) of the product to prevent the rib / boss from being formed. Cooling and solidification proceeds only from the boss side). For this reason, the injected resin is significantly delayed in cooling and solidification on the surface side of the product as compared with the opposite surface side, so that it is still in a molten state and can flow. By performing re-clamping within a time when the product surface side is still in a molten state and after a lapse of a predetermined time from the start of injection, it is possible to cause resin to flow and prevent sink marks.

[0007] Since the opposite surface side (rib / boss side) of the thick portion including the rib / boss portion is in close contact with the mold, the cooling rate is high and the cooling / solidification in this portion takes place on the surface side of the product. It is significantly ahead. Therefore, even if the front side is in a molten state, the cooling / solidification on the opposite side is almost completed. The resin in the high-temperature portion in the molten state on the front surface has a substantially uniform thickness with a thin skin layer formed on the surface as shown in FIG. There is a high-temperature molten resin inside the skin layer, and the molten resin can be made to flow by pressing. The rib side (non-surface side) is cooled and solidified,
Volume contraction is almost complete. That is, the resin in the molten state exists with a uniform thickness inside the skin layer.

In this state, when re-clamping is performed, the solidified resin on the opposite side acts as a heat insulating layer, so that cooling and solidification of the molten resin on the front side mainly proceeds from the front side of the product. Therefore, the cooling of the resin in the portion having the ribs proceeds substantially uniformly similarly to the other portions. In this way, it is possible to prevent the occurrence of sink marks even in a molded product having a large thickness due to a crystalline resin having a large volume shrinkage.

However, in the state where the secondary injection is performed after the mold is opened, the thickness of the molded article near the gate is thick, and when the mold opening is small, the surface of the thick portion generated by the secondary injection is gold. May contact mold cavity surface. When the molten resin comes into contact with the surface of the mold, the cooling capacity of the mold is large, so that the molten resin is rapidly cooled, and the resin cannot flow even if re-clamping is performed. In order to avoid such a state, in the present invention, the mold is closed and shaped again by applying a mold clamping force after the second injection, and then the mold is opened again to form the surface of the molded product and the surface of the mold cavity. And a gap as a heat insulating layer is provided between them. By doing so, it is possible to promote cooling / solidification on the opposite side (anti-heat insulating layer side) and to delay cooling / solidification on the front side (insulating layer side).

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIGS. 1 to 4 relate to an embodiment of the present invention, FIG. 1 is an overall configuration diagram of an injection molding apparatus according to the present invention, FIG. FIG. 3 is a flowchart of the injection molding method according to the present invention, and FIG. 4 is a plan view and a longitudinal sectional view of a molded product used in an experiment for confirming the effects of the present invention.

As shown in FIG. 1, an injection molding apparatus 100 according to the present invention comprises an injection apparatus 200, a mold clamping apparatus 300, a mold apparatus 400, and a control apparatus 500.

The mold apparatus 400 includes a fixed mold 403 attached to a fixed board 401 and a movable mold 404 attached to a movable board 402. The movable board 402 and the movable mold 404 are molds of a mold clamping device. The tightening cylinder 301 is configured to be able to move forward and backward. The mold clamping device 300 includes a mold clamping cylinder 301 for opening and closing the mold of the mold device. The movable mold moves forward and backward by being guided by a tie bar (not shown) with respect to the fixed mold. .

The injection device 200 comprises an injection unit 201, an injection cylinder 202 and a hydraulic motor 203. On the other hand, as shown in FIG. 1, the control device 500 includes a molding device control unit 501, an injection control unit 510 that controls plasticization of the molding material resin and injection of the molten resin into the mold cavity, opening and closing of the mold, Mold clamping controller 5 for controlling mold clamping force
20 and a comparison control unit 530 for controlling the mold opening timing.

The comparison control section 530 is connected to the switching timing condition setting section 534, the timer 533, the temperature monitoring section 532, and the mold clamping control section 520. Further, the mold clamping control section 520 includes a molding apparatus control section 501, a mold clamping position monitoring section 52.
2. It is connected to the mold clamping condition setting unit 521, the comparison control unit 530, and the like. In the present embodiment, an injection molding machine having a direct pressure type mold clamping device is used, but an injection molding machine of a toggle type mold clamping device, or a vertical injection molding machine or an electric type mold clamping device is used. An injection molding machine may be used.

Next, the outline and operation method of the injection molding method of the present invention will be described with reference to the flowchart of FIG. A molding method using a PP (polypropylene) resin having a large volume shrinkage due to cooling and solidification as a molding material will be described in detail as an example. The molded product is a disk with a rib and a window having a diameter of 340 mm and a shape as shown in FIG. The gate shape is a pinpoint gate having a diameter of 2 mm.

As the injection molding machine, a machine having a mold clamping force of 350 tons was used. The injection device 200 primarily injects a preset amount of the PP resin in a molten state into the mold cavity clamped by the mold clamping device 300 with a predetermined clamping force, and starts the timer 533 (t1). When ta seconds have elapsed after the completion of the injection filling, the movable mold 404 is retracted by the mold clamping device 300 so that the gap between the surface of the molded product and the mold cavity surface becomes a predetermined value. After the completion of the primary injection, the pressure-holding state is entered. In this pressure-holding state, the secondary injection is performed continuously from the completion of the primary injection (the details will be described later). When the timer 533 (t1) elapses tb seconds, the holding pressure (secondary injection) ends.

Next, When the timer 533 (t1) elapses tc seconds, the movable mold 404 is advanced by the mold clamping device 300 to perform re-clamping with a predetermined mold clamping force, and the thick part generated by the secondary injection is corrected and applied. Shape. This state is maintained until the timer 533 (t1) reaches td seconds, and after the elapse of td seconds, a gap as a heat insulating layer is provided between the surface of the molded product and the surface of the mold cavity. In this state, the timer 533 (t
1) is maintained until te seconds are reached, and after te seconds elapses, mold clamping is performed again with a predetermined mold clamping force. After that, the timer 533
When (t1) reaches tf seconds, the mold is opened and the molded product 6 is opened.
00 is taken out to complete one molding cycle and be ready for the next cycle.

The distinction between the primary injection and the secondary injection is as follows. That is, the operation of injecting the molten resin into the cavity before starting the operation of removing the mold clamping force to open the mold is set as the primary injection, and the operation of removing the mold clamping force to open the mold is started. The operation of injecting after the above is referred to as secondary injection.
Therefore, the primary injection and the secondary injection may be performed separately (discontinuously), or the secondary injection may be performed by a pressure-holding operation (step) performed continuously after the primary injection.

Next, a confirmation experiment was performed to compare the injection molding method according to the present invention with the conventionally used injection molding method. Table 1 shows the results. The sink was measured at the position of the rib 150 mm away from the pinpoint gate at the center as shown in FIG.

[Table 1] Notes: (1) The molding material is PP resin. (2) The timer setting values are as follows. ta: 3 seconds, tb: 8 seconds, tc: 13 seconds, td:
19 seconds te: 30 seconds, tf: 70 seconds (3) The mold opening amount is 0.3 mm both times. (4) Set the injection pressure to the resin pressure converted value of 1400 kgf.
/ Cm 2, but the actual injection pressure was considerably small. (5) The sink is measured at the center of the surface of the rib 150 mm from the center of the gate.

As is apparent from Table 1, the degree of sink marks on the surface side of the thick portion is significantly reduced in the injection molding method according to the present invention as compared with the conventional molding method. Thus, the injection molding method of the present invention is a revolutionary measure against sink marks.

In the present invention, there are concerns about the surface state (transferability of the mold) and the shaping property of the molded article, which are in good condition without any difference from the conventional injection molding method. Is added.

It should be noted that the present invention is not limited to the above-described embodiments, and it goes without saying that various modifications can be made without departing from the spirit of the present invention.

As described above, according to the injection molding method of the present invention, even if the molded article has a partially thick portion, it cannot be avoided by the conventional injection molding method. It became possible to eliminate sink marks generated in the part. According to the present invention, a molded product having a partially thick portion can be efficiently designed and produced with high quality.
That is, not only the amorphous resin but also the crystalline resin such as the polypropylene resin having a large volume shrinkage, the molding conditions can be easily determined without restriction from the product design, and efficient production can be performed. . In addition, when using crystalline resin, the required amount of resin to be filled during initial injection filling is small, and re-injection to compensate for volume shrinkage due to cooling and solidification is performed in the mold open state to reduce the required mold clamping force. The size can be greatly reduced, and the compactness of the mold clamping device can be achieved.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an injection molding apparatus according to the present invention.

FIG. 2 is an explanatory diagram for illustrating the occurrence of sink marks on a thick-walled portion and a preventive measure.

FIG. 3 is a flowchart of an injection molding method according to the present invention.

FIG. 4 is a plan view and a longitudinal sectional view of a molded product used in an experiment for confirming the effect of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 injection molding device 200 injection device 201 injection unit 202 injection cylinder 203 hydraulic motor 300 mold clamping device 301 mold clamping cylinder 400 mold device 401 fixed platen 402 movable platen 403 fixed mold 404 movable mold 500 control device 501 molding device control unit 510 Injection control unit 520 Mold clamping control unit 521 Mold clamping condition setting unit 522 Mold clamping position monitoring unit 523 Position detection sensor 530 Comparison control unit 531 Temperature sensor 532 Temperature monitoring unit 533 Timer 534 Switching timing condition setting unit 600 Molded product

Claims (1)

[Claims] After the injection of a molten resin into a mold cavity (primary injection) and the completion of shaping, the mold is once slightly opened or the mold is temporarily opened within a time in which the molten resin can flow. After the mold is opened, the molten resin is further injected (secondary injection), and then the mold is closed and shaped again. Then, the mold is slightly opened again to remove the gap between the surface of the molded product and the surface of the mold cavity. Wherein the mold is closed again within a time period during which the surface side molten resin can flow, and a gap is provided in the mold.