JP2954898B2 - Injection mold - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to shrinkage caused by shrinkage,
The present invention relates to an injection molding die suitable for injection molding thermoplastic resin molded products having different thicknesses without forming defects such as warpage on the surface.

[0002]

2. Description of the Related Art When a thermoplastic resin molded article having a thick part and a thin part is manufactured by an injection molding method, the thick part is finally solidified in a process of cooling and solidifying a resin filled in a mold. . Therefore, the volumetric shrinkage of the resin is concentrated on the thick portion, and the surface of the thick portion tends to have sink marks or the like. The commercial value of a molded article is significantly reduced due to the occurrence of sink marks, warpage, and the like (hereinafter, this is represented by sink marks). In the conventional injection molding method, a pressure-holding step is provided immediately after the completion of the injection filling of the resin to compensate for the volume shrinkage, and a pressure for preventing sink is applied to the molten resin in the cavity. However, when the product shape becomes complicated, a gate seal precedes and sufficient pressure is not applied, so that a product having a satisfactory appearance cannot be obtained.

As means for suppressing the occurrence of sink marks, for example, Japanese Patent Publication No. 61-53208 and Japanese Patent Application Laid-Open No. 63-268
No. 611, JP-A-64-63122, etc., introducing high-pressure gas into a cavity through a resin flow path and applying a dwell pressure through a hollow gas flow path to suppress the occurrence of surface defects. Have been. However, when manufacturing a molded product with a complicated shape, necessary hollowing is not performed sufficiently and sinks may occur, or the gas flow path may become a flow leader for the molten resin, causing an air trap or severe flow marks. Surface defects are likely to occur. In extreme cases, the high-pressure gas may break through the surface resin layer, making molding difficult. For this reason, applicable shapes are restricted, which is not a practical measure. Further, handling of the high-pressure gas itself is not easy. It is also known to press the thick part from the back side of the molded article after filling with resin. As a pressurizing means, there is a method in which a part of a mold is made movable and pressurized by hydraulic pressure or pneumatic pressure, or as in Japanese Patent Publication No. 61-9126, a back surface of a thick portion is pressurized by a gas body. . However, in the former case, the versatility is poor, such as a complicated mold structure and a special molding machine. In the latter case, irregular shaped sink marks occur on the back surface of the molded product, and other defects such as significantly impairing the commercial value occur.

[0004]

By the way, with resins such as polycarbonate and PMMA having high set-up properties,
When performing injection molding under conditions where the cooling and solidification of the surface layer are fast,
The strength developed in the surface layer exceeds the volume shrinkage force of the resin. Therefore, a cavity called a void is formed inside the thick portion without deforming the surface layer. In this case, no defect such as sink occurs on the surface of the thick portion. This void phenomenon is generally regarded as a defect that lowers the strength of a molded product. However, the use of the void phenomenon for the prevention of sink marks is disclosed in Japanese Patent Publication No. Hei 2-13886.
That is, by providing a sink control member that reaches the cavity from the inner surface of the mold corresponding to the back surface of the part where the sink is likely to occur on the molded product, and by generating a void around the tip of the sink control member, the occurrence of sink is prevented. ing. This method is effective because sink can be prevented by simple means.

[0005] However, Plastic Year Bo
ok (1974) p. 115, as reported in documents such as “Plastic Guide / Molding Process”, in order to generate voids, the mold has a portion that is difficult to cool, that is, the sink control member is made of a material having a large heat capacity. It is required to be made and kept at a higher temperature. For this reason, it is difficult to reduce the size of the sink control member, and there are restrictions on the locations where the sink control member can be incorporated, causing problems such as the inability to stably maintain a high temperature. As a result, it is difficult to surely prevent the thick portion of the entire molded product from sink. Also, the locations where sinks are likely to occur are greatly affected by the molding conditions. Even in the same molded product, the locations where sinks occur are not constant and vary from one molding cycle to another, making it difficult to identify the location. In addition, it is easily affected by the shape and size of the thick portion. In addition, in the case of a resin such as polyacetal, voids do not develop sufficiently to compensate for molding shrinkage, and it is not possible to sufficiently prevent sinks on the surface of a molded product.

[0006] Also, Japanese Patent Publication No. 48-41264 discloses:
The nozzle is projected into the mold cavity by the reciprocating mechanism,
An injection molding method is disclosed in which a gas is injected into a resin in a cavity at a high pressure from a tip of the nozzle through a passage formed in the axis of the nozzle before the resin is solidified. However, in this method, it is difficult to control the pressure so as to form a preferable void because a high-pressure gas is used.
Furthermore, the nozzle is gripped by the contraction force of the solidified resin. Therefore, the nozzle cannot smoothly remove the resin molded product from the mold cavity unless a mechanical reciprocating mechanism is used. Therefore, the installation position and quantity are also restricted. The present invention has been devised to solve such a problem, and a void insertion pin having a function of reliably and forcibly inducing a void in a molten resin near a tip end of the void insertion pin is provided. It is an object of the present invention to obtain a thermoplastic resin molded article having excellent surface properties even when the thickness is significantly different by providing the mold at an appropriate position in the mold.

[0007]

According to the present invention, there is provided an injection mold having a cavity having a thick portion and a thin portion corresponding to the shape of a resin molded product to be produced. A mold, and a void insertion pin fixedly disposed with a tip protruding from a raised portion formed on the inner surface of the thick portion of the mold,
Formed on the peripheral surface along the longitudinal direction of the void insertion pin,
A pressurized gas introduction path for feeding a pressurized gas at a pressure that breaks through the resin skin layer generated near the tip of the pin; and the tip of the void insertion pin is pointed. One or more void insertion pins are arranged according to the size of the thick portion. The pressurized gas introduction path formed around the void insertion pin is connected to the pressurized gas supply source via the pressurized gas supply path, and a switching valve for communicating the void to the atmosphere via the pressurized gas introduction path is provided. It is provided in the pressurized gas supply path.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Injection molding die 10 to which the present invention is applied.
As shown in FIG. 1, for example, a fixed mold 11 and a movable mold 12 are supported by a mold base 13. Fixed mold 11 and movable mold 1
2, a cavity 14 corresponding to the shape of the injection molded product to be manufactured is defined. The movable die 12 incorporates one or more void insertion pins 20, 20,.... The installation position of the void insertion pin 20 is set to a thick portion or a peripheral portion where defects such as sink marks and warpage easily occur. Further, it is preferable to form a raised portion 16 on the inner surface of the movable die 12 at a position where the void insertion pins 20, 20,. The void insertion pin 20 has a sharp tip 21 having a conical shape, and the sharp tip 21 projects from the movable mold 12 into the cavity 14. Void insertion pin 20
The insertion hole 30 is formed in the movable die 12 into which the is inserted as shown in FIG. The fitting hole 30 has a large diameter portion 31 and a small diameter portion 32 located on the cavity 14 side. The large diameter portion 31 has a diameter larger than the cross section of the void insertion pin 20, and forms an annular space 22 between the large diameter portion 31 and the side surface of the void insertion pin 20. The small-diameter portion 32 has a diameter slightly larger than the cross section of the void insertion pin 20, and forms an annular space 23 with a narrow gap between the small-diameter portion 32 and the side surface of the void insertion pin 20.

The space 22 functions as a pressurized gas introduction passage, and is connected to a pressurized gas supply source 40 via a pressurized gas supply passage 41 (see FIG. 1). A switching valve 42 is provided in the middle of the pressurized gas supply path 41 to shut off the gas pressure from the pressurized gas supply source 40 and open the pressurized gas supply path 41 to the atmosphere. The annular space 23 communicates with the pressurized gas supply source 40 or the atmosphere via the space 22 and the pressurized gas supply path 41, and communicates with the cavity 14 via the inclined peripheral surface 24 of the void insertion pin 20. The molten resin 50 is injected and filled into the mold cavity 14, and after the pressure-holding step is completed, or when the pressure-holding step is omitted, immediately after the injection and filling, the pressurized gas is injected through the void insertion pin 20 projecting into the cavity 14. Let it work. The pressurized gas breaks through the resin skin layer 51 being formed at the tip of the void insertion pin 20 and inserts microbubbles into the molten resin 50 to form void nuclei 5 serving as nuclei for void growth.
Form 2

The sharp tip 21 of the void insertion pin 20
It protrudes from the inner surface 15 of the movable mold 12 and is located at a position where the heat removal action of the mold 10 is not easily received. Therefore, the molten resin 50 injected into the cavity 14 is
Even when the solidification is started from a portion in contact with the resin, the resin 50 near the sharp tip portion 21 is in a state where the fluidity is relatively high. That is, the resin skin layer 51 that is being formed at the tip of the void insertion pin 20 is sufficiently softer than the resin solidified layer 55 that grows at a location that contacts the inner surface 15 of the mold, and easily breaks at a relatively low gas pressure. Can be In particular, when the void insertion pin 20 protrudes into the cavity 14 from the raised portion 16, the influence of the heat removal action by the mold 10 is further reduced, and the sharp tip portion 21 is located far from the mold inner surface 15. Therefore, the resin skin layer 51 is thin and can be easily pierced. Further, since the annular space 23 is sealed by the contraction force of the resin, air leakage is also prevented. After the void nuclei 52 are once formed, even if the action of the pressurized gas is stopped, the volume shrinkage force accompanying the cooling and solidification of the surrounding resin 50 concentrates on the void nuclei 52. Therefore, voids 53 are generated using the void nuclei 52 as seeds as shown in FIG. On the other hand, the resin 55 in contact with the inner surface 15 of the mold is discharged through the fixed mold 11 and the movable mold 12 to be cooled and solidified. The resin 50 shrinks due to the cooling and solidification, and a shrinking force acts from the inside toward the mold inner surface 15, so that the inside of the resin 50 is in a negative pressure state.

The void 53 communicates with the pressurized gas supply source 40 or the atmosphere via the annular space 23, the space 22, and the pressurized gas supply path 41 via the skin layer breakage 54. Therefore, an amount of gas or air corresponding to the negative pressure inside the resin 50 is drawn into the void 53. That is, the void 53
While absorbing the volume shrinkage of the resin 50 one after another, it continues to grow to a size sufficient to prevent sink marks. Thereby, not only the periphery of the void insertion pin 20 but also the entire thick portion has a sink suppressing effect. At this time, if the action of the pressurized gas is continued, the volume contraction force of the resin and the pressure of the pressurized gas are synergistic, and the growth of the void 53 is promoted, and not only the thick portion but also the peripheral thin portion and the adjacent thick portion It is also possible to suppress sink marks in the part. As a result, even if the resin molded product 60 has a thick portion such as the rib 61 as shown in FIG.
A product having no defects such as sink marks on the surface portion 63 corresponding to the ribs 61 of the flat plate portion 62 can be obtained. Since the void insertion pin 20 has the sharp distal end portion 21, the action of the pressurized gas is concentrated on the distal end, and the formation of the void nucleus 52 is promoted, and the reproducibility of generating the void 53 at an arbitrary position is also improved.

The skin layer breakage 54 generated when the pressurized gas penetrates the resin skin layer 51 being formed at the tip of the void insertion pin 20 has a diameter of about 0.5 mm in many cases, and is almost indistinguishable in appearance. Therefore, product 60
Appearance is not impaired. Although the diameter of the void insertion pin 20 is not particularly limited, it is required to be 1 mm or more in consideration of strength, and preferably 1 to 10 m.
m, more preferably 1 to 5 mm. The position of the tip of the void insertion pin 20 only needs to enter the thick portion of the cavity 14 and does not necessarily have to be at the center. Furthermore, the installation position of the void insertion pin 20 is not particularly limited, and can be provided at a convenient location in terms of product design, and a sufficient effect can be obtained even if it is installed at the end of the thick portion. In this regard, since it is not necessary to increase the heat capacity or limit the locations where sinks are likely to occur as in the sink control member described in Japanese Patent Publication No. Hei 2-13886, the degree of freedom in product design can be compared. It becomes so large that it does not. In addition, the void insertion pin 20 does not need to be a particularly cylindrical column as long as the action of the pressurized gas is effectively concentrated on the distal end portion, and the degree of freedom regarding the shape is relatively large.

[0013] The pressure of the pressurized gas used is as follows:
No. 53208, etc., which has a pressure much lower than the pressing force of the high-pressure gas used for hollowing the thick portion, 5 to 15 kg / c.
m ² and relatively requires low pressure. In most cases, 10kg /
Air pressure of not more than cm ² is sufficient, and a pressurized gas source can be easily obtained. That is, Japanese Patent Publication No. 57-14968
No need to use high-pressure nitrogen gas of up to 150 kg / cm ² required for hollowing a thick part described in JP-B-61-53208, etc. Is not required, and a molded article having excellent surface properties without easily sinking under general operating conditions can be easily obtained, and its superiority is incalculable. In addition, the pressurized gas has the effect of promoting the growth of the void 53 even at a low pressure, and is particularly effective in preventing sinking of a resin having a strong molding shrinkage force. Further, when the action of the pressurized gas is continued during cooling of the resin to promote the growth of the voids 53 and sufficiently absorb the shrinkage of the resin 50, the occurrence of sink marks is more reliably prevented. The gas is sent into the resin along the outer periphery of the void insertion pin, and a slight gap is formed between the void insertion pin 20 and the resin 50 by the gas injection.
It is also easy to extract the molded product 60 from zero.

The resin to be injection-molded according to the present invention is not particularly limited in its kind, and may be polystyrene,
Most thermoplastic resins such as polypropylene, ABS, and polycarbonate resins, and resins mixed with a filler reinforcing material are available. In particular, when the present invention is applied to a resin or the like having a large molding shrinkage such as polypropylene or ABS, the effect is remarkably exhibited. As the pressurized gas to be used, for example, a gas such as nitrogen or air is used at normal temperature.
Since the pressurized gas acts at a low pressure, the gas temperature does not adversely affect the resin in combination with the normal temperature of the gas. From this point, it is possible to use compressed air which is advantageous in terms of cost and handling. Void insertion pin 2
0 is set at an arbitrary position of the target thick portion (rib 61). In the case where the thickness of the thick portion is so large that only one void cannot absorb the shrinkage of the entire thick portion, a plurality of void insertion pins 20 are provided at appropriate distances. The installation distance between the void insertion pins 20, 20,... Is determined by the correlation with the volume of the thick portion where one void insertion pin 20 compensates for the shrinkage force of the resin, but is also affected by the molding conditions, especially the resin temperature. Therefore, it is preferable to install them with a margin.

The thermoplastic resin 50 is injected into the cavity 14 of the mold 10 in a molten state from an injection molding machine via a gate, and fills the cavity 14. When the filling of the resin 50 is completed, the pressurized gas is immediately applied to the void insertion pin 20 provided in the thick portion of the cavity 14. The pressurized gas flows from a supply source outside the mold 10 toward the sharp tip portion 21 of the void insertion pin 20 via the pressurized gas introduction path 41, and the peripheral surface of the void insertion pin 20 and the surface skin layer 51 of the molten resin 50. While concentrating on the tip of the void insertion pin 20. When the molten resin 50 is cooled and the volume contraction starts, the resin skin layer 51 at the tip of the void insertion pin 20 is pierced by the pressure of the pressurized gas. At this time, the formation of the resin skin layer 51 is delayed and becomes thinner as it is closer to the sharp tip portion 21 of the void insertion pin 20, so that the thin resin skin layer 51 at the pin tip portion is easily pierced by the pressurized gas, and the skin layer is thinned. A break 54 is formed. Then, a pressurized gas is sent as fine bubbles into the molten resin 50, and void nuclei 52 serving as seeds for void growth are formed.

The pressurization by the pressurized gas can be stopped immediately at this point. At this time, the remaining gas pressure is
The switching valve 42 is switched to open the pressurized gas supply passage 41 by communicating with the atmosphere. However, it is not necessary to open the pressurized gas. Rather, void 53
This has the effect of promoting the growth of and improving the releasability of the void insertion pin 20. Further, when the action of the pressurized gas is continued positively, the volume contraction force of the resin 50 and the pressure of the pressurized gas are synergistically promoted to promote the growth of the void 53, and the resin molded product
It is possible to suppress not only the thick part 61 of 0, but also the thin part around the thick part 61 and the sink of the adjacent thick part. In this regard, it is effective to continue pressurizing until the cooling and solidification of the resin is completed.

As the cooling process proceeds, the void nuclei 52 are sucked by the volume shrinkage caused by cooling and solidification of the surrounding molten resin 50, and grow into voids 53 as shown in FIG. At this time, the voids 53 continue to grow from the shape I to the shape II in the direction of the molten resin 50 whose cooling and solidification is delayed. When the cooling step is completed, the mold 10 is opened and the molded product is taken out. The obtained molded article 60 has no surface sink marks at all, and has excellent surface properties with an appearance in which the cavity shape is completely transferred. Further, since the skeleton of the molded product itself can be formed by the freely designed thick portion 61, a molded product having strength and rigidity that cannot be realized by a conventional general injection molded product can be obtained.

[0018]

EXAMPLE In this example, as shown in FIG. 4, an injection molded body 60 having a thick rib 61 on the back surface of a flat plate portion 62 is manufactured. A gate 64 for injecting a molten thermoplastic resin is formed on a side surface of the flat plate portion 62. The gate 64 is cut off and becomes a product. Rib 61
Has a length of 230 mm, a height of 17 mm, and a thickness of 10 mm as compared with a thickness of 3 mm of the flat plate portion 62. Due to this thickness, when a normal injection molding method is used, sink marks are significantly generated on the surface 63 of the flat plate portion 62 corresponding to the rib 61. Thus, as shown in FIG. 3, the injection molding die 1 for forming the cavity 14 corresponding to the shape of the injection molded body 60.
The three movable insertion pins 20 having a diameter of 2 mm were attached to the movable mold 12 having a pitch of 100 mm. The void insertion pin 20 was incorporated into the movable mold 12 such that the sharp tip 21 protruded into the cavity 14.

Movable mold 1 into which void insertion pin 20 is inserted
2, a fitting insertion hole 30 is formed as shown in FIG. The fitting hole 30 has a large diameter portion 31 and a small diameter portion 32 located on the cavity 14 side. The large diameter portion 31 has a diameter larger than the cross section of the void insertion pin 20, and an annular space 23 is formed between the large diameter portion 31 and the side surface of the void insertion pin 20. The space 23 functions as a pressurized gas introduction path,
It is connected to a pressurized gas supply source 40 via a pressurized gas supply path 41 (see FIG. 1). The small-diameter portion 32 is
It has a diameter slightly larger than the cross section of 0, and forms an annular space portion 23 with a narrow gap between the void insertion pin 20 and the void insertion pin 20.

A molten polystyrene resin (Estyrene H-65 manufactured by Nippon Steel Chemical Co., Ltd.) is injected into the cavity 14,
Immediately after completing the pressure holding process for 9.5 kg / cm
^The room temperature compressed air pressurized to ² is operated from the pressurized gas supply source 40 through the pressurized gas supply path 41 and the annular spaces 22, 23 of the void insertion pins 20, 20, 20 by operating the switching valve 42. It was fed along the peripheral surface of the void insertion pin 20 and acted on the sharp tip 21. After 8 seconds, the supply of the pressurized gas was stopped, and the pressurized gas supply passage 41 was connected to the atmosphere via the switching valve 42. In this state, the polystyrene resin was cooled and solidified. After a cooling process for 60 seconds, the obtained injection molded body 60 was taken out of the mold 10. At this time, a skin layer break portion 54 having a diameter of about 0.5 mm is formed at the tip of the void insertion pin 20, 20, 20, and the skin layer break portion 54 is formed.
It was confirmed that the void 53 had grown at the end of the step. When the surface condition of the obtained injection molded body 60 was examined, no surface defects such as sink marks were detected.

[0021]

As described above, the injection molding die of the present invention is suitable for injection molding of resin molded products having different wall thicknesses, resin molded products having complicated shapes, etc. A void insertion pin is installed at an arbitrary position corresponding to the portion, and a space portion connected to a pressurizing source or the atmosphere is formed around the void insertion pin. After filling the cavity of the mold with the injected resin, pressurized gas is applied to the tip of the pin along the peripheral surface of the void insertion pin, and the resin skin layer that is being created near the tip of the pin is compressed by the pressurized gas. Air bubbles that are broken through and serve as void nuclei are introduced into the molten resin. Since the void nucleus is in communication with the pressurized gas supply source or the atmosphere through the skin layer breakage, the gas from the pressurized gas supply source or the atmosphere is generated due to the negative pressure generated inside the resin due to volume contraction accompanying cooling and solidification. Aspirate,
It grows into a void whose size matches the volume shrinkage.

Therefore, the injection-molded article obtained by using this mold has no surface defects such as sink marks and is excellent even in a complicated shape having both extremely thick and thin portions. It has high surface value and high commercial value. In addition, the pressure-holding step can be omitted compared to the conventional injection molding method in which the filling and holding pressure of the molten resin is kept high to prevent sink marks, so that a molding machine having a smaller mold closing force can be used. This has the advantage that molding distortion can be reduced. Also, compared to the molding method in which the thick part is hollowed out, the molding is extremely easy since the uncooled resin in the center of the thick part is not accompanied by excessive resin flow such as hollowing out. ,
Also, special high pressure gas which is difficult to handle is not required.

[Brief description of the drawings]

FIG. 1 is an internal sectional view of a mold incorporating a void insertion pin.

FIG. 2 is an explanatory diagram showing that void nuclei are introduced into a molten resin.

FIG. 3 is a diagram showing a process in which void nuclei grow in voids in accordance with volume contraction of a resin.

FIG. 4 is a perspective view of a resin molded product having a thick portion.

[Explanation of symbols]

10: Mold for injection molding 11: Fixed mold 12: Movable mold 13: Mold base 14: Cavity 15: Mold inner surface 16: Raised portion 20: Void insertion pin 21: Sharp tip 22:
Space part (pressurized gas introduction path) 23: Narrow gap annular space part 24: Slant peripheral surface 130: Fitting hole 31: Large diameter part 32: Small diameter part 40: Pressurized gas supply source 41: Pressurized gas supply path 4
2: Switching valve 50: Molten resin 51: Resin skin layer 52: Void nucleus 53: Void 54: Breaking portion of skin layer 55: Solidified layer that grows in contact with the inner surface of the mold 60: Resin molded product 61: Rib (thick portion) ) 62:
Flat plate part 63: Surface part corresponding to rib 64: Gate

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-281818 (JP, A) JP-A-4-47517 (JP, U) JP-A-53-88574 (JP, U) 9126 (JP, B2) JP 2-13886 (JP, B2) JP 48-41264 (JP, B1) (58) Fields investigated (Int. Cl. ⁶ , DB name) B29C 45/00-45 / 84

Claims (3)

(57) [Claims] 1. A mold provided with a cavity having a thick part and a thin part corresponding to the shape of a resin molded product to be manufactured;
A void insertion pin whose tip protrudes and is fixedly arranged at a raised portion formed on the inner surface of the thick-walled mold; formed on the peripheral surface along the longitudinal direction of the void insertion pin, and formed near the tip of the pin; A pressurized gas introduction path for feeding a pressurized gas at a pressure that penetrates the resin skin layer, wherein a tip of the void insertion pin is pointed. 2. The injection mold according to claim 1, wherein one or more void insertion pins are arranged according to the size of the thick portion. 3. A pressurized gas supply path is connected to a pressurized gas supply source via a pressurized gas supply path, and a switching valve for communicating a void to the atmosphere via the pressurized gas supply path is provided with a pressurized gas supply path. The mold for injection molding according to claim 1, which is provided in a road.