JP3469643B2 - Injection molding method for synthetic resin molded products - Google Patents

Description

Description: 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 having bosses and ribs, which tends to cause sink marks on the surface of the molded product. It can be used for molding of molded products, for example, interior and exterior products such as dashboards and door handle covers of automobiles, and casings of home electric appliances. 2. Description of the Related Art Since a synthetic resin molded product obtained by injection molding has an inherent molding shrinkage, the shrinkage of a portion where cooling is delayed (cooling delay portion) such as a center portion of a boss or a rib or a thick portion is particularly caused. The surface of the molded article had sink marks. For this reason, in the conventional injection molding, the resin injected into the mold cavity is
Although excessive sink pressure was applied to prevent sink marks, it was difficult to completely eliminate sink marks, but rather warped by applying excessive holding pressure to surfaces other than bosses, ribs, and thick parts. There was a problem that occurs. On the other hand, Japanese Patent Application Laid-Open No. 50-75247 discloses a method for preventing sink marks without applying such an excessive holding pressure.
As described in Japanese Patent Application Laid-Open No. HEI 59-220337, an injection molding method is known in which a pressurized fluid such as compressed air is press-fitted into a cavity and a resin is pressed against the inner surface of the cavity to prevent sink marks. However, when such a pressure fluid is press-fitted into the cavity, the shrinkage due to the contraction of the cooling delay portion is caused not by the surface of the resin but by the resin into which the pressure fluid is injected. On the back side of the boss, and particularly a large underfill portion at the base of the boss or rib. For example,
As shown in FIG. 7A, the fixed mold 4 and the movable mold 5
A molded product 9 is provided on the surface of the cavity 8 formed by
After forming a concave groove (recess) 19 for forming a rib 91 of zero, injecting a molten resin into the cavity 8 and then injecting a pressure fluid between the resin and the mold 5, FIG. As shown, a large underfill 94 was formed at the base 91A of the rib 91. Similarly, as shown in FIG. 8A, a ring-shaped groove (recess) 19 for forming a boss 93 of a molded product 90 is formed on the surface of the cavity 8 on the mold 5 side. Also, as shown in FIG. 8B, a large underfill 94 was formed at the base 93A of the boss 93. Such a missing portion 94
Since it occurs on the back side of the molded article 90, there is no problem such as a decrease in the appearance of the molded article 90, but there is a problem that the strength of the rib 91 and the boss 93 decreases when the thickness of the underfill increases. SUMMARY OF THE INVENTION An object of the present invention is to provide a synthetic resin molded article which can prevent sink on the surface side of a synthetic resin injection molded article having ribs, bosses, etc., and can prevent a decrease in strength by suppressing underfill of the rib or boss portion. An object of the present invention is to provide an injection molding method. According to the injection molding method of the present invention, a synthetic resin molded article having a boss, a rib or the like is melted in a cavity of a mold. after the resin is filled is provided a pressurized meat portions at the base of such a boss rib, when the molten resin filled is filled by the end of the molten resin is being cooled and solidified, the back surface of the molded product in the cavity Inject the compressed fluid into the side (between the cavity surface and the molded product made of injection resin), and when the molten resin has cooled and solidified,
It is characterized in that the mold is released and the molded product is taken out. According to the present invention, when the molten resin injected into the cavity is being cooled and solidified, the compressed fluid is injected into the back surface of the molded product. The molten resin (molded product) is cooled and solidified in a state where the surface side is pressed against the inner surface of the cavity, so that sinking on the surface side of the molded product is prevented even if it has ribs or bosses. In addition, the base of the ribs or bosses on the backside of the molded product, which is likely to be underfilled, is provided with a thickened portion at the time of injection filling. The portion is replenished by the thickened portion, the required thickness is secured, and the strength is prevented from lowering. Furthermore, since sinkage 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, so that low-pressure injection molding can be performed, and warpage of the molded article and the like can be prevented. Distortion is prevented and productivity is improved. An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is 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 having a screw 2 for melting and kneading 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 a push-out pin 9 which is pushed through the protrusion plate 7 and whose tip projects into the cavity 8 to take out a molded product through the through hole 10 of the movable die 5. A gas supply path 11 is formed in the movable mold 5 so as to communicate with the through hole 10. The gas supply path 11 is connected to a gas injection controller 12. The gas injection control device 12 is driven by driving air from the compressor to increase the pressure of the nitrogen gas for injection to increase the pressure of the nitrogen gas into a compressed fluid. An opening / closing valve 14 for controlling the supply of nitrogen gas to 11 and a control device 15 for controlling the operation of the pressure intensifier 13 are provided. Therefore, the gas supply path 11 and the gas injection control device 12 constitute a compressed fluid supply unit. The opening / closing 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. These valves 16, 17, and 18 are individually controlled to open and close by the control device 15. As shown in FIGS. 2 and 3, the cavity 8 of this embodiment forms a plate-like molded product 90 having two ribs 91, and the movable mold 5 has a rib forming rib. Two concave grooves 19 are formed. Cavity 8 of concave groove 19
A notch 19A having a shape in which a corner is cut obliquely is formed at the side edge. As shown in FIG. 3, a concave groove 19 on the inner surface of the cavity 8 of the movable mold 5 is provided.
A groove 20 having a triangular cross section for forming a barrier 92 in the molded product 90 is formed along the concave groove 19 on the outside of the molded product 90.
The shape of the groove 20 is not limited to a triangular cross section, but may be, for example, a rectangular cross section similar to the concave groove 19. The depth dimension of the groove 20 (the height dimension of the barrier wall 92) is set to about 2 to 10 mm in consideration of a nitrogen gas holding capacity and an increase in the amount of resin used, which will be described later. The through hole 10 of the movable mold 5 is
As shown in FIGS. 2 and 3, the front end of the cavity 8 has a small diameter and the clearance A with the protruding pin 9 is 1/100 to 8/100 mm (see FIGS. 2 and 3). Ten
8080 μm). Further, a sealing material 21 such as an O-ring for sealing a gap with the protruding pin 9 is provided on the protruding plate 7 side (the side opposite to the cavity 8) of the through hole 10. 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 the injection device 3 injects a predetermined amount of the molten resin into the cavity 8. At this time, the resin pressure applied to the protruding pin 9 increases with the resin filling, but the clearance A at the tip of the through hole 10 on the cavity 8 side becomes 1/100 to 8/100.
Since the width is reduced to 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, since the resin is also filled in the notch 19A, a thickened portion 96 is formed in the base 91A of the rib 91 when the resin is filled. . The thickened portion 96 is formed along the rib 91 on both side surfaces of the rib 91, and the combined volume of the thickened portions 96 is a thick molded product 90 provided with the rib 91.
20 to 70% of the volume of the cooling delay part 97 generated at the center of the
The size of the notch 19A is set such that When a predetermined amount of the molten resin is charged and a signal indicating the end of the charging is sent from the injection device 3 to the gas injection control device 12, the pressure intensifier 13 is operated and the supply valve 16 is opened to supply the gas. Through hole 10 through road 11
Is injected with nitrogen gas. At this time, the molten resin is being cooled and solidified, so that a gap is formed between the molten resin and the cavity 8.
Therefore, the nitrogen gas injected into the through-hole 10 is injected into the cavity 8 through the clearance of the protruding pin 9. At this time, in the initial stage of the injection of the nitrogen gas (compressed fluid), the pressure is reduced by opening the pressure control valve 18 and partially evacuating the nitrogen gas to reduce the pressure (for example, the pressure intensifier 13
Is injected for a predetermined time (for example, 0.2 to 3 seconds), and thereafter, the valve 18 is closed and the nitrogen gas is set to a high pressure (for example, 3.5 to 20 MPa). The opening and closing valve 14 is controlled by the control device 15 so as to inject for a predetermined time (for example, 2 seconds or more). When a 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 of the molded product 90 in contact with the cavity 8. A space is formed between the mold 5. Further, when a high-pressure nitrogen gas is injected, the space is filled with the high-pressure nitrogen gas, and a sufficient holding pressure is applied to the molded product 90, and the surface side of the molded product 90 is pressed against the inner surface of the cavity 8 of the fixed mold 4, and the sink The occurrence is prevented. On the other hand, on the back side of the molded product 90, the surface thereof sinks due to the shrinkage of the cooling delay portion 97, and a thinned portion 94 occurs in the conventional case, but in this embodiment, it is shown in FIG. As described above, since the resin in the portion filled as the fill portion 96 is replenished to the missing portion, the amount of missing portion (shrinkage) decreases. Therefore, the thickened portion 96 is formed only at the initial stage of filling the cavity 8 with the resin, and if the molded product 90 shrinks due to the progress of cooling and solidification of the resin, it is replenished by the shrinkage and disappears. is there. As shown in FIGS. 2 and 3, nitrogen gas is supplied between the grooves 19 (between the ribs 91 of the molded article 90) and the grooves 1
9 and the groove 20 (the rib 91 of the molded product 90 and the barrier 9
2), the nitrogen gas does not leak from the rib 91 where the sink is likely to occur, and a sufficient holding pressure on the rib 91 is maintained. When 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 protruding pins 9 are protruded to take out the molded product 90. One cycle of injection molding is completed. By repeating the above molding cycle, molded article 9
0 are sequentially manufactured. According to this embodiment, the following effects can be obtained. The concave groove 19 forming the rib 91 of the molded product 90
A notch 19A is formed on the base 91A of the rib 91 so that a thickened portion 96 is formed on the base 91A. Of the base portion 91A can be reduced or completely eliminated. For this reason, the thickness of the base portion 91A of the rib 91 is ensured, so that a decrease in strength can be prevented, and the occurrence rate of defective products due to the decrease in strength can be reduced. Further, the volume of the thickened portion 96 is reduced by the cooling delay portion 9.
Since the volume is set to 20 to 70% of the volume of No. 7, it is possible to replenish the underfill without significantly increasing the amount of resin, thereby preventing a decrease in strength. That is, if the fill portion 96 has a volume of 20% or less of the cooling delay portion 97, depending on the shape of the molded article 90 and the type of resin, the replenishment amount of the underfill is reduced, and the reduction in strength cannot be suppressed. On the other hand, if it is 70% or more, the amount of resin increases more than necessary. However, if it is in the range of 20 to 70% as in the above embodiment, the balance between the replenishment of the underfill and the amount of injected resin is improved. can do. Further, since nitrogen gas is injected into the cavity 8 to hold the gas and pressurize the molten resin against the inner surface of the cavity 8, the resin surface side and the inner surface of the cavity 8 are not cooled until the molten resin is cooled and solidified. The close contact state can be maintained, and even when the molded article 90 having the ribs 91 and the like is molded, sink on the surface side can be reliably prevented. Since injection of nitrogen gas into the cavity 8 can prevent sinking of the molded article 90, there is no need to perform high-pressure injection molding to prevent sinking as in the prior art. No low pressure injection molding can be realized. For this reason, low-pressure molding is possible and the molding cycle can be shortened, so that the productivity can be improved without lowering the quality of the molded article 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 distortion such as warpage and distortion can be reduced, and the accuracy of the molded product 90 can be improved. Can be formed. A groove 20 is formed in the movable mold 5 in addition to the concave groove 19, and a barrier wall 92 is formed in the molded product 90 in addition to the rib 91. , And a predetermined holding pressure can be maintained until the resin is cooled and solidified.
Sink can be reliably prevented. The cavity 8 is formed through a clearance between the protrusion pin 9 and the through hole 10 which are usually provided in the mold 5.
Since the nitrogen gas is injected into the inside, the structure of the mold 5 can be simplified as compared with the case where a gas injection hole is formed to provide a valve stem or a porous member is arranged. It can be provided at low cost. At this time, since the clearance A around the protruding pin 9 is 1/100 to 8/100 mm, it is possible to prevent the molten resin from flowing into the through hole 10 and to prevent the nitrogen gas from flowing into the cavity 8. Therefore, a sufficient gas can be supplied smoothly, and the holding pressure by the nitrogen gas can be increased to reliably prevent the molded product 90 from sinking. In the initial stage of cooling and solidification immediately after the injection of the molten resin, that is, in the initial stage of nitrogen gas injection, the pressure control valve 18 is opened to reduce the pressure of the nitrogen gas to a low pressure. When the layer is in a thin state, the solidified layer is not broken by the pressure of the nitrogen gas and the gas does not dive into the resin, and the high quality molded article 90 does not have a decrease in strength due to the gas entering the resin. Can be manufactured. Since non-combustible nitrogen gas is used as the compressed fluid, there is no danger of explosion even if it is expanded or heated by injection into the cavity 8, and the safety of injection molding can be ensured. Since a gap is formed by injecting nitrogen gas between the back side of the molded product 90 and the movable mold 5, the molded product 90 can be easily released, and the occurrence of trouble due to poor release is prevented. Thus, efficient injection molding can be performed. Next, a description will be given of an experimental example conducted to confirm the effects of the present invention. In this experimental example, a molded article 90 having a rib 91 as shown in FIG.
As shown in the figure, the notch 19A is formed in the concave groove 19 of the mold 5 to provide the thickened portion 96, and the notch 19A
This is an evaluation of the molding state (whether sinking or underfilling of the molded article 90 occurs or whether injection molding can be performed without any trouble) by changing various conditions in the case where the thickened part 96 is not provided without forming the solid part. In molding the molded product 90, Toshiba Machine's IS-200 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) was used. FIG.
As shown in (B), the general thickness d of the molded product 90 other than the ribs 91 was set to 2.5 mm, and the base thickness (width) b of the ribs 91 was set to 2 mm. The height 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 case, and the resin temperature was set as shown in Table 1. Then, Experimental Examples 1 to 6 were performed by changing the dimensions of the pin clearance A and the thickness of the thickened portion (volume percent of the thickened portion 96 with respect to the cooling delay portion 97), and the sinking state of the molded product 90 was measured with a surface roughness meter ( It was detected by using a surf coder SE-30D manufactured by Kosaka Laboratory. Furthermore, the missing portion 9
The underfill amount of No. 4 was measured separately for the base underfill amount d1 and the rib underfill amount b1. In addition, the base part thickness d1 is obtained by measuring the general thickness d2 of the base part 91A 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 underfill amount b1 is
1 is obtained by measuring the base thickness b2 and calculating the shortage (b1 = b2-b) with respect to the design thickness b. On the other hand, as Reference Examples 1 to 6, experiments were conducted with the pin clearance A and the gas pressure changed in the case where the thickened portion 96 was not formed (Reference Examples 1 to 6). [Table 1] As shown in Table 1, when the thickened portion 96 was formed, the amount of underfill was reduced, and a decrease in strength could be prevented. In particular, when the thickness of the thickened portion 96 was increased (when the percentage was increased), the thickness of the ribs was reduced. In addition, since the notch 19A was formed in the concave groove 19, the base thickness b2 on the rib side sometimes became larger than the design thickness b. In addition, the underfill amount increased as the holding pressure 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, the gap is smaller than 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 thickness of the underfill, and also prevents the strength of the molded article 90 from being reduced, confirming the usefulness of the present invention. Although the present invention has been described with reference to preferred embodiments, the present invention is not limited to these embodiments, and various modifications and changes in design can be made without departing from the spirit of the present invention. Is possible. For example, the present invention is not limited to the injection molding of the molded article 90 having the ribs 91, but can also be used for the injection molding of the molded article 90 having the boss 93 as shown in FIGS. That is, the concave groove 19 forming the boss 93
A notch 19A is formed at the edge of the cavity 8 side of
As shown in FIG. 6A, a configuration may be such that a thickened portion 96 is provided on the base 93 </ b> A of the boss 93. It is preferable that the fill portion 96 is set to have a volume of 20 to 70% of the cooling delay portion 97 formed in the boss 93 as in the above-described embodiment. Also in this case, as shown in FIG. 6B, by providing the thickened portion 96, the thinned portion 94 of the boss 93 is reduced, and a reduction in strength is prevented. When a molded product 90 having a boss 93 is formed, a groove 20 is formed in the mold 5 as shown in FIG. It is desirable to provide a circumferential or square circumferential barrier wall 92 around the boss 93. Further, the present invention provides the rib 91, the boss 9
It can also be used for the production of various molded products such as a molded product in which both of them are formed, a molded product without the rib 91 and the boss 93, and the like. In the above-described embodiment, the compressed fluid (nitrogen gas) is injected from the clearance of the protruding pin 9, but as shown in FIG. And the outlet 22A on the cavity 8 side
A porous member 40 formed of a heat-resistant material such as metal or ceramic may be disposed in the portion. The porous member 40 is immovably fixed at the steps of the fixing pin 30 and the through hole 22. Even if the 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 prevented. 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, for example, to 1/100 to 8/100 mm, which is the same as in the above embodiment. . Further, a sleeve pin composed of a middle pin and an outer pin is arranged in the mold 5 so that a compressed fluid is injected into the cavity 8 from a gap between the pins, or a note formed in the mold 5 is formed. 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 as appropriate for implementation. Although it is not always necessary to provide the barrier 92,
The arrangement is advantageous in that the injected compressed fluid does not leak, and the pressure-holding effect can be maintained for a long time even in a portion where the rib 91 is not provided. Further, the compressed fluid is not limited to nitrogen gas, and other gases such as compressed air may be used. However, since the temperature of the compressed fluid in contact with the molten resin increases, there is an advantage that the use of a nonflammable gas such as nitrogen gas provides higher safety. The pressure of the compressed fluid to be injected is not limited to two-stage control as in the above embodiment, but may be controlled to three or more stages or a pressure value to be changed continuously.
Further, the pressure value may be constant. The pressure value at this time may be appropriately set according to the type of the resin used and the like. Further, the pressure control method is not limited to the method in which the pressure control valve 18 is provided as in the above-described embodiment.
A known appropriate pressure control method using a pressure reducing valve or the like may be used. The present invention also relates to the molded article 90 of the above embodiment.
Not only in the case of manufacturing, for example, a plate member formed with a large number of grid-like ribs used for paper supply parts of a copying apparatus, and various resins having ribs and bosses of a door handle cover of an automobile and the like. It can be used for molded products. According to the present invention, sink of the surface side of a synthetic resin injection molded article having ribs and bosses can be prevented, and the ribs and bosses can be reduced in thickness to reduce the strength. There is an effect that it can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic configuration diagram showing one embodiment of the present invention. FIG. 2 is an enlarged sectional view showing a central portion of a mold according to the embodiment. FIG. 3 is an enlarged sectional view showing an upper end portion of a mold according to 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 modification of the present invention. FIG. 6 is an enlarged 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. [Description of Signs] 1 Injection molding apparatus 4 Fixed mold 5 Movable mold 8 Cavity 9 Protruding pin 10 Through hole 11 Gas supply path 12 Gas injection control device 13 Pressure intensifier 14 Open / close valve 15 Control device 19 Depressed 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 Underfill 96 Filling 97 Cooling delay

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B29C 45/00-45/84

Claims (1)

(57) [Claim 1] In an injection molding method for molding a synthetic resin molded article having bosses, ribs, etc., a molten resin is filled in a cavity of a mold, and the bosses, ribs, etc. after forming the pressure-walled portion to the base, when the molten resin filled is filled by the end of the molten resin is being cooled and solidified, it injected molded article of the back side to a compressed fluid in the cavity, the molten resin An injection molding method for a synthetic resin molded product, which comprises releasing the mold after cooling and solidifying, and taking out the molded product.