JPH0866948A - Injection molding method of synthetic resin molded piece - Google Patents

Abstract

PURPOSE: To provide an injection molding method of synthetic resin molded piece which can prevent generation of sink marks in a molded piece having ribs and can prevent degradation of strength by restraining thinning of ribs and bosses. CONSTITUTION: When a cavity 8 of molds 4, 5 is charged with a melting resin so as to form thickened parts at a base of ribs 91 and the melting resin is cooling and solidifying, a compressed liquid is injected in a rear face side of a molded piece 90 in the cavity 8. The molded piece 90 is cooled and solidified in a state that its top face is being pressed to the inner face of the cavity 8 by the injection of the compressed liquid. Even when the molded piece 90 has the ribs 91, sink marks do not generate on the top face of the molded piece 90. Although thinned parts generate at the base of the ribs 91, they are replenished by the thickened parts, so that the thinning is restrained and degradation of the strength of the ribs 91 can be prevented.

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 a synthetic resin molded product, and more particularly to a synthetic resin molded product having bosses and ribs, which easily causes sink marks on the surface of the molded product, such as automobile dashboards and doors. It can be used for molding interior and exterior products such as handle covers and casings for home electric appliances.

[0002]

BACKGROUND ART Synthetic resin molded products produced by injection molding have inherent molding shrinkage. Therefore, the shrinkage of parts (cooling delay parts) where cooling is delayed, such as the center of bosses and ribs and thick parts, causes There were sink marks on the surface. Therefore, in conventional injection molding, the resin injected into the cavity of the mold is
It has been attempted to prevent sink marks by applying excessive holding pressure, but it is difficult to completely eliminate sink marks, and rather warps due to excessive holding pressure applied to surfaces other than bosses, ribs and thick parts. There was a problem that.

On the other hand, as a method for preventing sink marks without applying such an excessive holding pressure, Japanese Patent Application Laid-Open No. 50-75247.
As disclosed in JP-A-59-220337 and JP-A-59-220337, there is known an injection molding method in which a compressed fluid such as compressed air is pressed into a cavity to press a resin against the inner surface of the cavity to prevent sink marks.

[0004]

However, when such a pressure fluid is press-fitted into the cavity, the sink mark due to the contraction of the cooling delay portion is not the front surface side of the resin but the back surface side of the resin into which the pressure fluid is injected. And a large flesh portion was formed especially at the base of the boss or rib. For example,
As shown in FIG. 7A, the fixed mold 4 and the movable mold 5
The surface of the cavity 8 formed by
When a concave groove (recess) 19 for forming the rib 91 of 0 is formed and a molten resin is injected into the cavity 8 and a pressure fluid is injected between the resin and the mold 5, as shown in FIG. As shown, a large cutout portion 94 was formed in the base portion 91A of the rib 91.

Similarly, as shown in FIG. 8A, when a ring-shaped concave groove (recess) 19 for forming the boss 93 of the molded product 90 is formed on the surface of the cavity 8 on the mold 5 side. Also, as shown in FIG. 8 (B), a large cutout portion 94 was formed in the base portion 93A of the boss 93. Such a thin portion 94
Since it occurs on the back surface side of the molded product 90, the appearance of the molded product 90 is not deteriorated. However, when the amount of lacking thickness is large, the strength of the rib 91 or the boss 93 is deteriorated.

An object of the present invention is to provide a synthetic resin molded article which can prevent sink marks on the surface side of a synthetic resin injection molded article having ribs, bosses, and the like, and which can prevent the ribs and bosses from having a lack of wall thickness to prevent a decrease in strength. It is to provide an injection molding method.

[0007]

The injection molding method for a synthetic resin molded product of the present invention is an injection molding method for molding a synthetic resin molded product having bosses, ribs, etc., in which a molten resin is filled in a cavity of a mold. After the thickened portion is provided on the base portion of the boss, rib, etc., and when the filled molten resin is in a state of being cooled and solidified, the back surface side of the molded article in the cavity (molding with the cavity surface and injection resin It is characterized in that a compressed fluid is injected into the space (between the products). At this time, the volume of the added portion is
It is desirable to set the volume to 20 to 70% of the volume of the cooling delay portion generated in the boss, rib and the like of the synthetic resin molded product.

[0008]

In the present invention as described above, when the molten resin injected into the cavity is in the state of being cooled and solidified, the compressed fluid is injected into the back surface side of the molded article, so that the molten resin inside the cavity is injected. The (molded product) is cooled and solidified in a state where the surface side thereof is pressed against the inner surface of the cavity, so that even if the molded product has ribs or bosses, sink marks on the surface side of the molded product are prevented. In addition, since the ribs and bosses on the back surface of the molded product are prone to lack of wall thickness, a thickened part is provided during injection filling, so even if the cooling delay part of the ribs or bosses contracts The added amount is replenished by the added portion, a required thickness is secured, and strength reduction is prevented.

Further, since the sink mark is prevented by injecting the compressed fluid, it is not necessary to apply an excessive holding pressure to the molten resin injected into the cavity, and low pressure injection molding can be performed, so that the warp of the molded product Distortion is prevented and productivity is improved.

[0010]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic configuration diagram of an injection molding apparatus 1 used in the injection molding method of the present invention. The injection molding device 1 includes an injection device 3 that has a screw 2 to melt and knead a resin, and a mold clamping device 6 to which a fixed mold 4 and a movable mold 5 are attached. The movable die 5 is provided with an ejection pin 9 that is pushed through the ejection plate 7 and has its tip protruding into the cavity 8 to take out a molded product through the through hole 10 of the movable die 5.

The movable mold 5 is formed with a gas supply passage 11 which communicates with each of the through holes 10, and the gas supply passage 11 is connected to a gas injection controller 12. The gas injection control device 12 is driven by the drive air from the compressor to increase the pressure of the nitrogen gas for injection into a compressed fluid, and a signal from the injection device 3, that is, the injection timing, to supply the gas supply path. An on-off valve 14 that controls the supply of nitrogen gas to the valve 11 and a control device 15 that controls the operation of the booster 13 are provided. Therefore, the compressed fluid supply means is constituted by the gas supply path 11 and the gas injection control device 12.

The on-off valve 14 includes a supply electromagnetic valve 16 for controlling the supply of nitrogen gas, an exhaust electromagnetic valve 17 for exhausting the injected nitrogen gas, and an electromagnetic valve 18 for controlling the pressure of the supplied nitrogen gas. The valves 16, 17, and 18 are individually controlled to be opened and closed by the control device 15.

As shown in FIGS. 2 and 3, the cavity 8 of this embodiment forms a plate-shaped molded article 90 having two ribs 91. The movable die 5 is provided with ribs for forming ribs. Two recessed grooves 19 are formed. Cavity 8 of groove 19
A cutout portion 19A having a shape in which a corner portion is obliquely cut out is formed at the side edge portion. Further, as shown in FIG. 3, the concave groove 19 on the inner surface of the cavity 8 of the movable mold 5 is used.
A groove 20 having a triangular cross-section for forming a barrier 92 on the molded product 90 is formed along the concave groove 19 on the outer side of.
The shape of the groove 20 is not limited to the triangular shape in cross section, and may be, for example, a rectangular shape in cross section similar to the concave groove 19. The depth dimension of the groove 20 (height dimension of the barrier 92) is set to about 2 to 10 mm in consideration of the nitrogen gas holding capacity and the increase amount of the resin used, which will be described later.

The through hole 10 of the movable mold 5 is formed in the concave groove 19
Between the concave groove 19 and the groove 20, and as shown in FIGS. 2 and 3, the tip of the cavity 8 has a small diameter, and the clearance A with the protruding pin 9 is 1/100 to 8/100 mm ( Ten
˜80 μm). A seal member 21 such as an O-ring that seals a gap with the protrusion pin 9 is provided on the protrusion plate 7 side of the through hole 10 (on the side opposite to the cavity 8).

Next, the procedure of injection molding in this embodiment will be described. First, using the mold clamping device 6, the mold 4,
5 is closed, and a predetermined amount of molten resin is injected into the cavity 8 by the injection device 3. At this time, the resin pressure applied to the protrusion pin 9 increases as the resin is filled, but the clearance A at the tip of the through hole 10 on the cavity 8 side is 1/100 to 8/100.
Since the width is as narrow as mm, the resin is prevented from flowing into the through hole 10, and the resin is filled without clogging the clearance.

As shown in FIG. 4A, the cutout portion 19A is also filled with the resin, so that the base portion 91A of the rib 91 is provided with a thickened portion 96 when the resin is filled. . The thickened portions 96 are formed along the ribs 91 on both side portions of the ribs 91, and the combined volume of the respective thickened portions 96 has a rib 91 and is thickened.
20-70% of the volume of the cooling delay part 97 occurring in the central part of the
The size of the cutout portion 19A is set so that

When a predetermined amount of molten resin is filled and a signal indicating the end of filling is sent from the injection device 3 to the gas injection control device 12, the pressure booster 13 is operated and the supply valve 16 is opened to supply the gas. Through hole 10 through passage 11
Nitrogen gas is injected into. At this time, the molten resin is being cooled and solidified, and a gap is formed between the molten resin and the cavity 8. At the side of the protruding plate 7 of the through hole 10, the sealing material 21 is formed.
Since it is sealed with, the nitrogen gas injected into the through hole 10 is injected into the cavity 8 through the clearance of the protrusion pin 9.

At this time, at the initial stage of injecting the nitrogen gas (compressed fluid), the pressure is reduced by opening the pressure control valve 18 and exhausting a part of the nitrogen gas to lower the pressure (for example, the booster 13).
Nitrogen gas at a pressure of 0.5 to 3 MPa is injected for a predetermined time (for example, 0.2 to 3 seconds), and then the valve 18 is closed to a high pressure (for example, 3.5 to 20 MPa). The opening / closing valve 14 is controlled by the control device 15 so as to inject the gas for a predetermined time (for example, 2 seconds or more).

When low-pressure nitrogen gas is injected into the cavity 8, the nitrogen gas is injected between the inner surface of the cavity 8 of the movable mold 5 and the back surface side of the molded article 90 in contact with the cavity 8 to move the molded article 90. A space is formed between the mold 5. Further, when high-pressure nitrogen gas is injected, this space is filled with high-pressure nitrogen gas and a sufficient holding pressure is applied to the molded product 90, the surface side of the molded product 90 is pressed against the inner surface of the cavity 8 of the fixed mold 4, and Occurrence is prevented.

On the other hand, on the back surface side of the molded product 90, the surface of the molded product 90 shrinks due to the contraction of the cooling delay portion 97, and a thinned portion 94 is produced in the conventional case, but in this embodiment, it is shown in FIG. 4 (B). As described above, since the resin of the portion filled as the additional portion 96 is replenished to the lacking portion, the amount of sinking (sink) is reduced. Therefore, the added portion 96 is formed only at the initial stage of the filling of the resin into the cavity 8, and if the molded product 90 shrinks due to the progress of the cooling and solidification of the resin, the shrinkage is supplemented and disappears. is there.

The nitrogen gas is, as shown in FIGS. 2 and 3, between the concave grooves 19 (between the ribs 91 of the molded product 90) and the concave grooves 1.
9 and the groove 20 (the rib 91 and the barrier 9 of the molded product 90)
2), nitrogen gas does not leak from the rib 91 portion where sink marks are likely to occur, and a sufficient holding pressure on the rib 91 portion is maintained.

After the molten resin has cooled and solidified, the exhaust valve 17 is opened to release the gas in the cavity 8, the molds 4 and 5 are released, and the ejection pin 9 is ejected to take out the molded product 90. , End one cycle of injection molding. Molded product 9
0 is manufactured sequentially.

According to this embodiment, the following effects are obtained. Groove 19 that forms rib 91 of molded product 90
The cutout portion 19A is formed in the rib 91 and the thickened portion 96 is formed in the base portion 91A of the rib 91. It is possible to reduce or eliminate flesh (sink) of the base portion 91A. For this reason, the thickness of the base portion 91A of the rib 91 can be ensured to prevent a decrease in strength, and the defective product occurrence rate due to the decrease in strength can also be reduced.

Further, the volume of the thickening portion 96 is set to the cooling delay portion 9
Since it is set to 20 to 70% of the volume of No. 7, it is possible to replenish the deficient portion and prevent the strength from decreasing, without significantly increasing the amount of resin. That is, if the volume of the added portion 96 is 20% or less of that of the cooling delay portion 97, depending on the shape of the molded product 90 and the type of resin, the supplement amount of the deficient portion is small and it is not possible to suppress the strength reduction. On the other hand, if it is 70% or more, the resin amount increases more than necessary. However, if it is in the range of 20 to 70% as in the above-mentioned embodiment, the balance between the supplement of the wall thickness and the injection resin amount is improved. can do.

Further, since nitrogen gas is injected into the cavity 8 to maintain the gas pressure and press the molten resin against the inner surface of the cavity 8, the resin surface side and the inner surface of the cavity 8 are kept until the molten resin is cooled and solidified. The close contact state can be maintained, and even when the molded product 90 having the ribs 91 and the like is molded, sink marks on the front surface side can be reliably prevented.

Since sinking of the molded article 90 can be prevented by injecting nitrogen gas into the cavity 8, it is not necessary to perform high pressure injection molding for sinking prevention as in the conventional case, and sinking occurs. No low pressure injection molding can be realized. Therefore, low-pressure molding is possible and the molding cycle can be shortened, so that the productivity can be improved without degrading the quality of the molded product 90. In addition, since low-pressure injection molding can be performed, an excessive holding pressure is not applied to the molded product 90, the distribution of molding strain such as warpage and distortion can be reduced, and the accuracy of the molded product 90 can be improved. The molded product 90 can be molded.

Since the groove 20 is formed in the movable mold 5 in addition to the concave groove 19 and the barrier wall 92 is formed in the molded product 90 in addition to the rib 91, the injected gas is liable to sink due to the rib 91 or the like. The molded product 90 can be maintained at a predetermined holding pressure until the resin is cooled and solidified.
It is possible to surely prevent sink marks.

The cavity 8 is passed through the clearance between the protrusion pin 9 usually provided in the mold 5 and the through hole 10.
Since nitrogen gas is injected into the inside, the structure of the mold 5 can be simplified as compared with the case where a valve hole is provided by forming a gas injection hole or a porous member is arranged. It can be provided at low cost. At this time, since the clearance A around the protrusion pin 9 is set to 1/100 to 8/100 mm, it is possible to prevent the molten resin from flowing into the through hole 10 and prevent the nitrogen gas from flowing into the cavity 8. Therefore, sufficient gas can be smoothly supplied, and the holding pressure of the nitrogen gas can be increased to reliably prevent sinking of the molded product 90.

At the initial stage of cooling and solidification immediately after injecting the molten resin, that is, in the initial stage of injecting nitrogen gas, the pressure control valve 18 is opened to make the pressure of the nitrogen gas low, so that the resin surface is solidified in the initial stage of cooling. When the layer is thin, the solidified layer is not broken by the pressure of nitrogen gas and the gas does not dive inside the resin, and there is no reduction in strength due to the gas entering the resin 90. Can be manufactured.

Since noncombustible nitrogen gas is used as the compressed fluid, there is no danger of explosion even if expanded or heated by the injection into the cavity 8, and the safety of injection molding can be ensured.

Nitrogen gas is injected between the back side of the molded product 90 and the movable mold 5 to form a gap, so that the molded product 90 can be easily released from the mold, and troubles due to defective release can be prevented. Therefore, efficient injection molding can be performed.

Next, an example of an experiment conducted to confirm the effect of the present invention will be described. In this experimental example, a molded article 90 having a rib 91 as shown in FIG.
As shown in FIG. 7, the case where the cutout portion 19A is formed in the concave groove 19 of the mold 5 and the thickening portion 96 is provided, and the cutout portion 19A
Various conditions are changed in the case where the thickened portion 96 is not formed without forming the mold, and the molding state (the state of occurrence of sink mark or lack of thickness of the molded product 90, whether injection molding can be performed without trouble) is evaluated.

When molding the molded article 90, IS-200 manufactured by Toshiba Machine is used as an injection molding machine, and MFR is used as a resin.
(Melt flow ratio) [230 ℃, 2.16kgf] ＝ 10g / 10min
Block PP (polypropylene) of was used. FIG.
As shown in the portion (B), the general thickness d of the molded product 90 other than the rib 91 was set to 2.5 mm, and the base thickness (width) b of the rib 91 was set to 2 mm. Further, the height dimension of the barrier 92 was set to 2 mm. The molded product 90 is 250 mm
A 250 mm plate having four ribs 91 formed at 40 mm intervals was used.

The mold clamping pressure was set to 200 t, the mold temperature was set to 30 ° C. in each example, and the resin temperature was set as shown in Table 1. Then, Experimental Examples 1 to 6 were carried out by changing the size of the pin clearance A and the amount of additional thickness (volume percentage of the additional thickness portion 96 with respect to the cooling delay portion 97), and the sink state of the molded product 90 was measured by a surface roughness meter ( It was detected using a surf coder SE-30D manufactured by Kosaka Laboratory. Furthermore, the lacking part 9
The amount of lacking wall of No. 4 was divided into the amount of missing portion of base portion d1 and the amount of lacking rib portion b1 for measurement. The base thickness deficiency d1 is obtained by measuring the general thickness d2 of the base 91A portion of the rib 91 of the molded product 90 and calculating the shortage (d1 = d2-d) with respect to the design thickness d of the mold. It is a thing. Similarly, the rib lacking amount b1 is also the rib 9
The base thickness b2 of No. 1 was measured, and the shortage (b1 = b2-b) with respect to the design wall thickness b was calculated.

On the other hand, as Reference Examples 1 to 6, experiments were performed in the case where the thickened portion 96 was not formed by changing the pin clearance A and the gas pressure (Reference Examples 1 to 6).

[0036]

[Table 1]

As shown in Table 1, by forming the thickness-increasing portion 96, the amount of deficiency was reduced and the decrease in strength could be prevented. In particular, when the amount of the thickness of the thickness-added portion 96 was increased (the percentage was increased), the amount of thickness loss on the rib side could be reduced. Since the cutout portion 19A is formed in the concave groove 19, the base thickness b2 on the rib side may be larger than the designed thickness b. In addition, the amount of wall deficiency increased as the holding gas pressure increased.

On the other hand, in the case where the thickened portion 96 is not provided as in the reference example, if the conditions such as the holding pressure and the size of the pin clearance are the same, there will be more defects than in the case where the thickened portion 96 is provided. The amount of meat has increased. From the above, it can be seen that the provision of the thickened portion 96 reduces the amount of wall thinning and can prevent the decrease in strength of the molded product 90, thus confirming the usefulness of the present invention.

Although the present invention has been described with reference to the preferred embodiment, the present invention is not limited to this embodiment, and various improvements and design changes can be made without departing from the scope of the present invention. Is possible.

For example, the present invention is not limited to the injection molding of the molded product 90 having the rib 91, but can be used when the molded product 90 having the boss 93 is injection molded as shown in FIGS. That is, the groove 19 that forms the boss 93
A notch portion 19A is formed at the edge of the cavity 8 side of
As shown in FIG. 6 (A), the base portion 93A of the boss 93 may be provided with the thickening portion 96. It is preferable to set the added portion 96 to have a volume of 20 to 70% of the cooling delay portion 97 formed in the boss 93, as in the above embodiment. Also in this case, as shown in FIG. 6 (B), by providing the thickening portion 96, the thickness reduction portion 94 of the boss 93 portion is reduced, and the reduction in strength is prevented.

Further, when molding the molded article 90 having the boss 93, the groove 20 is formed in the mold 5 as shown in FIG. 5 so that the gas having a predetermined pressure is retained in the boss 93. It is desirable to provide a barrier wall 92 having a circumferential shape or a rectangular shape around the boss 93. Further, according to the present invention, the rib 91 and the boss 9 are provided.
It can also be used for manufacturing various molded products such as a molded product in which both No. 3 and 3 are formed, a molded product without the rib 91 and the boss 93.

In the above embodiment, the compressed fluid (nitrogen gas) was injected from the clearance of the protrusion pin 9, but as shown in FIG. 5, the through hole 22 communicated with the gas supply passage 11 was formed in the mold 5. Formed and exit 22A on the side of the cavity 8
A porous member 40 formed of a heat resistant material such as metal or ceramics may be arranged in the portion. The porous member 40 is immovably fixed at the stepped portions of the fixing pin 30 and the through hole 22. Even if this porous member 40 is arranged, the compressed fluid supplied to the through hole 22 can be injected into the cavity 8 through the porous member 40, and the molten resin in the cavity 8 can be injected into the through hole 22. Can be prevented from flowing into. The diameter of a large number of through-holes formed in the porous member 40 may be appropriately set according to the type of resin used, and may be set to, for example, 1/100 to 8/100 mm, which is the same as in the above embodiment. .

Further, a sleeve pin consisting of an inner pin and an outer pin is arranged in the die 5, and a compressed fluid is injected into the cavity 8 through a gap between the pins, or a pouring formed on the die 5 is performed. A valve rod for opening and closing the inlet may be provided and the valve may be opened to inject the compressed fluid into the cavity 8. In short, the configuration for injecting the compressed fluid into the cavity 8 may be set appropriately for implementation.

Although the barrier 92 is not necessarily provided,
The provision thereof has an advantage that the injected compressed fluid does not leak, and the pressure holding effect can be maintained for a long period of time even in a portion where the rib 91 is not present. Further, the compressed fluid is not limited to nitrogen gas, and other gas such as compressed air may be used. However, since the temperature of the compressed fluid increases in contact with the molten resin, the use of a nonflammable gas such as nitrogen gas has the advantage of higher safety.

The pressure of the compressed fluid to be injected is not limited to the case where the pressure is controlled in two steps as in the above-mentioned embodiment, but the pressure may be controlled in three steps or more or the pressure value is continuously changed.
Further, the pressure value may be constant. The pressure value at this time may be appropriately set according to the type of resin used and the like. Further, the pressure control method is not limited to the method of providing the pressure control valve 18 as in the above embodiment,
A known appropriate pressure control method using a pressure reducing valve or the like may be used.

The present invention also provides the molded article 90 of the above embodiment.
Not only in the case of manufacturing a sheet, but also, for example, a plate member having a large number of grid-like ribs used for a paper supply component of a copying machine, various resins having ribs or bosses for an automobile door handle cover, etc. It can be used for molded products.

[0047]

According to the present invention as described above, it is possible to prevent sink marks on the surface side of a synthetic resin injection-molded article having ribs, bosses, etc., and to prevent the ribs and bosses from having a lack of thickness to prevent strength reduction. effective.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view showing a central portion of the mold of the embodiment.

FIG. 3 is an enlarged sectional view showing an upper end portion of the mold of the embodiment.

FIG. 4 is an enlarged sectional view showing a rib portion of the embodiment.

FIG. 5 is an enlarged sectional view showing a main part of a modified example of the present invention.

FIG. 6 is an enlarged cross-sectional view showing a boss portion of the modified example.

FIG. 7 is an enlarged sectional view showing a rib portion of a conventional example of the present invention.

FIG. 8 is an enlarged sectional view showing a boss portion of a conventional example of the present invention.

[Explanation of symbols]

 1 Injection Molding Device 4 Fixed Die 5 Movable Die 8 Cavity 9 Ejection Pin 10 Through Hole 11 Gas Supply Channel 12 Gas Injection Control Device 13 Booster 14 Open / Close Valve 15 Control Device 19 Recessed Groove 19A Notch 20 Groove 22 Through Hole 40 Porous member 90 Molded product 91 Rib 91A Base 92 Wall 93 Boss 93A Base 94 Missing part 96 Added part 97 Cooling delay part

Claims (2)

[Claims] 1. An injection molding method for molding a synthetic resin molded article having bosses, ribs, etc., after filling a molten resin in a cavity of a mold to form a thickened portion on a base of the bosses, ribs, etc. An injection molding method for a synthetic resin molded article, which comprises injecting a compressed fluid into the back surface side of the molded article in the cavity when the filled molten resin is in a state of being cooled and solidified. 2. The injection molding method for a synthetic resin molded article according to claim 1, wherein the volume of the added portion is 20 times the volume of the cooling delay portion generated in a portion such as a boss or rib of the synthetic resin molded article.
An injection molding method for a synthetic resin molded article, characterized by being set to ˜70%.