Method for manufacturing injection molded product

Abstract

To provide a method of manufacturing an injection molded article capable of expressing a shading pattern in a simple configuration using resin containing a fiber.SOLUTION: A method of manufacturing an injection molded article comprises an injection step of injecting a composite material in a cylinder into a mold. In the composite material, fibers are dispersed in resin. In the injection step, the composite material is injected in multiple stages at a relatively high speed and a low speed.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an injection molded product.

Background Art

[0002] Patent Document 1 discloses a method for manufacturing an injection molded product of a cellulose fiber composite resin. In this method, a resin insert piece containing a colorant or cellulose fibers is preliminarily placed in the runner portion, and the cellulose fiber composite resin is injected into the cavity of the mold. The cellulose fiber composite resin injected from the molding machine flows into the cavity through the runner portion. At this time, a part of the resin insert piece placed in the runner portion melts due to the heat of the flowing cellulose fiber composite resin or shear heat generated by the shear force with the resin. When a part of the resin insert piece melts, the colorant contained in the resin insert piece flows out, or the cellulose fibers contained in the resin insert piece turn brown when melting. In the technique of Patent Document 1, a dark color different from the cellulose fiber composite resin injected from the molding machine randomly flows into the injection molded product due to the outflow of the colorant or the browning of the cellulose fibers, thereby expressing a wood grain pattern.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the technique of Patent Document 1, it is necessary to prepare a resin insert piece containing a colorant or cellulose fibers, which increases the number of parts. In the technique of Patent Document 1, a step of preliminarily placing the resin insert piece in the runner portion is required, which increases the number of steps.

[0005] One object of the present invention is to provide a method for manufacturing an injection molded article that can exhibit a shading pattern with a simple configuration using a resin containing fibers.

Means for Solving the Problems

[0006] (1) The method for manufacturing an injection molded article according to one aspect of the present invention includes an injection step of injecting a composite material in a cylinder into a mold. The composite material has a plurality of fibers dispersed in a resin. In the injection step, the composite material is injected in multiple stages at a relatively fast speed and a slow speed.

[0007] (2) In the method for manufacturing an injection molded article, in the injection step, the composite material may be alternately and repeatedly injected at a relatively fast speed and a slow speed.

[0008] (3) In the method for manufacturing an injection molded article, in the injection step, the composite material may be injected so as to gradually increase in speed or gradually decrease in speed.

Advantages of the Invention

[0009] The method for manufacturing an injection molded article of the present invention can exhibit a shading pattern using a resin containing fibers due to the difference in injection speed. When the composite material is injected at a fast speed, the shear heat generation becomes relatively high, and the fibers in the cylinder are likely to be charred. The shear heat generation is caused by the friction between the fibers or the shear stress associated with the friction between the fibers and the resin. When the composite material is injected at a slow speed, the shear heat generation becomes relatively low, and the fibers in the cylinder are less likely to be charred. The greater the degree of charring of the fibers, the darker the color can be exhibited, and the less the degree of charring of the fibers, the lighter the color can be exhibited. Due to this degree of charring of the fibers, a shading pattern can be exhibited on the injection molded article.

[0010] As described in (2) above, when the composite material is repeatedly injected alternately at a relatively fast speed and a slow speed, a wood grain pattern or stripe pattern in which dark and light colors alternate can be expressed. As described in (3) above, when the composite material is injected gradually at an increasing speed, a gradation pattern that changes in order from a light color to a dark color can be expressed. When the composite material is injected gradually at a decreasing speed, a gradation pattern that changes in order from a dark color to a light color can be expressed.

[0011] Since the method for manufacturing an injection molded product of the present invention expresses a shading pattern due to the difference in injection speed, it can be used by making simple modifications to an existing molding machine. Therefore, compared with the case where a resin insert piece containing a colorant or cellulose-based fiber is arranged in advance between the molding machine and the mold, the method for manufacturing an injection molded product of the present invention is suppressed from increasing costs or decreasing productivity.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0013] A specific example of the method for manufacturing an injection molded product of the present invention will be described with reference to the drawings. The same reference numerals in the drawings indicate the same objects. In each drawing, for convenience of explanation, a part of the configuration may be shown in an exaggerated or simplified manner. The dimensional ratios of each part in the drawings may also be different from the actual ones.

[0014] <Overall Configuration> The manufacturing method of the injection molded product according to the embodiment includes an injection step of injecting the composite material 8 in the cylinder 2 into the mold 7 using the molding machine 1 shown in FIG. 1. One of the features of the manufacturing method of the injection molded product according to the embodiment is that in the injection step, the composite material 8 is injected in multiple stages at a relatively fast speed and a slow speed. Due to the difference in injection speed, as shown in FIGS. 3 and 5, a shading pattern 90 is developed on the surface 9f of the injection molded product 9. Hereinafter, first, the molding machine 1 and the composite material 8 used in the manufacturing method of the injection molded product will be described, and then the manufacturing method of the injection molded product will be described.

[0015] <Molding Machine> As shown in FIG. 1, the molding machine 1 includes a cylinder 2, a hopper 3, a heater 4, and a screw 5. The cylinder 2 is filled with the composite material 8. In front of the cylinder 2, a nozzle 20 for injecting the composite material 8 into the mold 7 is provided. The composite material 8 is fed into the cylinder 2 from the hopper 3. A heater 4 is disposed on the outer periphery of the cylinder 2. By this heater 4, the composite material 8 in the cylinder 2 is heated. The heating temperature depends on the melting point of the resin 80 contained in the composite material 8. The screw 5 performs a rotational motion and a reciprocating motion in the cylinder 2. By the rotation of the screw 5, the composite material 8 is sent forward in the cylinder 2 along the groove of the screw 5. At this time, the composite material 8 melts by the heat from the heater 4. Simultaneously with the melted composite material 8 being sent forward in the cylinder 2, the screw 5 moves backward. When a predetermined amount of the melted composite material 8 accumulates in front of the cylinder 2, by pushing the screw 5 forward toward the front of the cylinder 2, the above-mentioned composite material 8 is injected from the nozzle 20 into the cavity 70 of the mold 7. The screw 5 does not rotate when it is pushed forward toward the front of the cylinder 2. The rotation and reciprocation of the screw 5 are operated by a motor 6. The motor 6 is, for example, a servo motor. If the motor 6 is a servo motor, the reciprocating motion of the screw 5 can be controlled microscopically as compared with the case where the screw 5 reciprocates by a hydraulic cylinder.

[0016] In the molding machine 1, a series of operations are repeatedly performed, including feeding the composite material 8 from the hopper 3 into the cylinder 2, melting the resin 80 by rotating the screw 5, filling the composite material 8 in front of the cylinder 2 by retracting the screw 5, and pushing the screw 5 forward to inject. In the molding machine 1 of this example, the speed of pushing the screw 5 forward, that is, the moving speed of the screw 5, is controlled in multiple steps. In other words, the moving speed of the screw 5 is variably controlled for each operation from the start to the completion of injection.

[0017] The mold 7 has a cavity 70 corresponding to the shape of the injection molded product 9 to be produced. The composite material 8 injected into the cavity 70 is cooled within the mold 7. By this cooling, the composite material 8 is solidified into the injection molded product 9. When the composite material 8 flows into the cavity 70, a so-called skin layer is formed in a film-like state immediately solidified by cooling from the portion in contact with the inner surface of the mold 7. The skin layer is formed in order from the region near the gate 71 of the mold 7. This skin layer constitutes the surface 9f of the injection molded product 9 shown in FIGS. 3 and 5.

[0018] <Composite material> In the composite material 8, a plurality of fibers 81 are dispersed in the resin 80. The resin 80 is, for example, polypropylene (PP). Each fiber 81 is a plant fiber. If each fiber 81 is a plant fiber, each fiber 81 is likely to burn at a high injection speed. The injection speed and the burning of the fiber 81 will be described later. Each fiber 81 is, for example, a cellulose-based fiber. Each fiber 81 is, for example, white ash, palm, or rice husk. The plurality of fibers 81 may be composed of a plurality of fibers 81 of the same kind, or may be composed of a combination of a plurality of fibers 81 of different kinds.

[0019] The content of the plurality of fibers 81 in the composite material 8 is, for example, 40% by mass or more and 70% by mass or less. The above content is the ratio when the entire composite material 8 is 100% by mass. When the above content is 40% by mass or more, it is easy to develop the shading pattern 90 described later on the surface 9f of the injection molded product 9. When the above content is 70% by mass or less, the content of the resin 80 relatively increases, and it is easy to perform injection molding. The above content may be 45% by mass or more and 65% by mass or less, or 50% by mass or more and 65% by mass or less.

[0020] <Method for manufacturing an injection molded product> In the method for manufacturing an injection molded product, in the injection step of injecting the composite material 8 in the cylinder 2 into the mold 7, the composite material 8 is injected in multiple stages at a relatively fast speed and a slow speed. This multi-stage injection is performed by controlling the speed at which the screw 5 of the molding machine 1 is pushed forward, that is, the moving speed of the screw 5, in multiple stages. The above speed is called the injection speed.

[0021] When the screw 5 reciprocates in the cylinder 2, shear heat generation occurs due to the frictional force between the fibers 81 or the shear stress associated with the friction between the fibers 81 and the resin 80 in the cylinder 2. In addition to the heat from the heater 4, when the above shear heat generation occurs, the fibers 81 may burn in the cylinder 2 depending on the temperature. At a fast injection speed, the shear heat generation becomes relatively high, and the fibers 81 are likely to burn in the cylinder 2. At a slow injection speed, the shear heat generation becomes relatively low, and the fibers 81 are less likely to burn in the cylinder 2. Depending on the degree of burning of the fibers 81 in the cylinder 2, the color of the composite material 8 injected from the nozzle 20 of the cylinder 2 changes. This color is almost determined near the nozzle 20 in the cylinder 2.

[0022] The number of times of multi-stage injection, that is, the number of times the injection speed is changed, is, for example, 5 times or more. If the above number of times is 5 times or more, the shading pattern 90 described later is developed. To increase the number of shading changes, the more the number of times of multi-stage injection, the better. The above number of times can be appropriately selected according to the desired shading pattern 90. The above number of times may be 10 times or more, or 15 times or more.

[0023] In the injection process, as shown in FIG. 2, relatively fast injection speeds and slow injection speeds may be alternately repeated. That is, in the injection process, the injection speed may be changed in a pulsed manner. In the graph shown in FIG. 2, the horizontal axis represents time and the vertical axis represents the injection speed. The unit of the injection speed is mm / sec. For example, the fast injection speed is 80 mm / sec or more, and the slow injection speed is 40 mm / sec or less. In this case, the difference between the fast injection speed and the slow injection speed is 40 mm / sec or more. The fast injection speed may be 100 mm / sec or more, and the slow injection speed may be 40 mm / sec or less. In this case, the difference between the fast injection speed and the slow injection speed is 60 mm / sec or more. If the above difference is 60 mm / sec or more, the difference between the dark region 91 and the light region 92 of the shading pattern 90 described later is easily distinguishable by the naked eye. The fast injection speed may be 100 mm / sec or more, and the slow injection speed may be 20 mm / sec or less. In this case, the difference between the fast injection speed and the slow injection speed is 80 mm / sec or more. The difference between the fast injection speed and the slow injection speed is, for example, 100 mm / sec or less. If the above difference is 100 mm / sec or less, the responsiveness of the screw 5 is likely to be improved. The ratio of the fast injection speed to the slow injection speed is, for example, 3 times or more.

[0024] In this example, a constant high injection speed A and a constant low injection speed B are repeated. In this case, on the surface 9f of the injection molded product 9, as shown in FIG. 3, a shading pattern 90 in which dark regions 91 and light regions 92 are alternately repeated is formed. When injection molding is performed, a gate mark (not shown) corresponding to the gate 71 of the mold 7 is formed on the injection molded product 9. Centered on this gate mark, a shading pattern 90 corresponding to the injection speed is formed. In this example, as shown in FIG. 2, starting from the high injection speed A, thereafter, the low injection speed B and the high injection speed A are alternately repeated. Therefore, in this example, on the surface 9f of the injection molded product 9, as shown in FIG. 3, a dark region 91 is formed centered on the gate mark, and thereafter, a thin region 92 and a dark region 91 are alternately formed. Starting from the low injection speed B, thereafter, the high injection speed A and the low injection speed B may be alternately repeated. In that case, a light region 92 is formed centered on the gate mark, and thereafter, on the surface 9f of the injection molded product 9, a dark region 91 and a light region 92 are alternately formed. Thus, when the relatively high injection speed A and the low injection speed B are alternately repeated, a wood grain pattern is formed as the shading pattern 90.

[0025] The change in the injection speed may combine different high injection speeds or different low injection speeds. For example, a set in which a high injection speed A1, a high injection speed A2, a low injection speed B1, and a low injection speed B2 are performed in order may be repeated.

[0026] In the injection process, as shown in FIG. 4, the injection speed may be gradually decreased. That is, in the injection process, the injection speed may be changed stepwise. The horizontal axis and the vertical axis of the graph shown in FIG. 4 are the same as those of the graph shown in FIG. 2. When the injection speed is gradually decreased, as shown in FIG. 5, on the surface 9f of the injection molded product 9, a shading pattern 90 with a gradation from a dark region 91 to a light region 92 is formed centering on a gate mark (not shown). The injection speed may be gradually increased. In that case, on the surface 9f of the injection molded product 9, a shading pattern 90 with a gradation from a light region 92 to a dark region 91 is formed centering on a gate mark (not shown). To increase the gradation levels of this gradation, it is better to increase the number of multi-stage injections. The number of multi-stage injections is 5 or more, 10 or more, or 15 or more.

[0027] The change amount of the injection speed is, for example, 5 mm / sec or more and 40 mm / sec or less. The change amount of the injection speed is the change amount of the injection speed at each stage in the multi-stage. If the change amount is 5 mm / sec or more, the gradation of the shading pattern 90 is easily visible to the naked eye. If the change amount is 40 mm / sec or less, the responsiveness of the screw 5 is likely to be improved. The change amount may be 10 mm / sec or more and 20 mm / sec or less.

[0028] The injection time at each injection speed is, for example, 0.2 sec or more and 1.0 sec or less. If the injection time at each injection speed is 0.2 sec or more, it is easy to control the injection speed. If the injection time at each injection speed is 1.0 sec or less, it is easy to form a relatively fine shading pattern 90. The injection time at each injection speed may be 0.5 sec or less.

[0029] The present invention is not limited to these examples, but is shown by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims.

Explanation of Signs

[0030] 1 Molding machine 2 Cylinder 20 Nozzle 3 Hopper 4 Heater 5 Screw 6 Motor 7 Mold 70 Cavity 71 Gate 8 Composite Material 80 Resin 81 Fiber 9 Injection Molded Product 9f Surface 90 Shading Pattern 91 Dark Region 92 Light Region

Claims

1. An injection process for injecting a composite material in a cylinder into a mold, wherein the composite material has a plurality of fibers dispersed in a resin, and in the injection process, the composite material is injected in multiple stages at a relatively fast speed and a relatively slow speed so as to change the degree of burning of the plurality of fibers due to shear heating in the cylinder. A method for manufacturing an injection molded product.

2. The method for manufacturing an injection molded product according to claim 1, wherein in the injection process, the composite material is repeatedly injected alternately at a relatively fast speed and a relatively slow speed.

3. The method for manufacturing an injection molded product according to claim 1, wherein in the injection process, the composite material is injected so as to gradually become faster or gradually become slower.

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