Injection molding method

Abstract

To provide an injection molding method capable of preventing rupture of a surface layer while suppressing ejection of a molten resin for an inner layer from a molten resin for a surface layer.SOLUTION: An injection molding method of a sandwich molding (100) according to the present invention includes: a first step of injecting a first molten resin (A), for forming a surface layer (OL), into a molding cavity (17) provided in a pair of molds, in an amount less than a volume of the molding cavity; a second step of forming an additional cavity (17S) inside the first molten resin (A) by conducting a core back operation that retracts one of the pair of molds from the other during or after completion of the injection of the first molten resin in the first step; and a third step of injecting a second molten resin (B), for forming an inner layer (IL), into the additional cavity (17S) during or after completion of the core back in the second step.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to an injection molding method suitable for obtaining a sandwich molded body including an inner layer and a surface layer covering the inner layer.

Background Art

[0002] Conventionally, a sandwich molded body including an inner layer and a surface layer covering the inner layer has been known. As an injection molding method for molding a sandwich molded body, a multi-stage molding method is known. The inner layer is sometimes referred to as a core layer, and the surface layer is sometimes referred to as a skin layer. The multi-stage molding method is a method of injecting a molten resin for the surface layer into a molding cavity, and then injecting a molten resin for the inner layer into the inside of the molten resin for the surface layer to fill the molding cavity with these two molten resins.

[0003] However, in the conventional injection molding method, there is a risk that the molten resin for the inner layer injected later may spout from the molten resin for the surface layer injected earlier. Therefore, Patent Document 1 proposes a so-called core back for expanding the volume of the molding cavity before injecting the molten resin for the inner layer. By this core back, a void is formed inside the molten resin for the surface layer injected earlier, into which the molten resin for the inner layer is injected. Patent Document 1 injects the molten resin for the inner layer into the void formed by this core back, so that it is possible to suppress the molten resin for the inner layer from spouting from the molten resin for the surface layer.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The injection molding method disclosed in Patent Document 1 fills the molding cavity provided before core-back with the surface layer molten resin and then performs core-back. In the sandwich molded body obtained by performing injection molding according to the disclosure of this Patent Document 1, it has been found that the surface layer may break in the direction of core-back. This phenomenon is particularly prominent in the surface layer molten resin with high viscosity. From the above, an object of the present invention is to provide an injection molding method capable of preventing breakage of the surface layer in a sandwich molded body while suppressing the ejection of the inner layer molten resin from the surface layer molten resin.

Means for Solving the Problems

[0006] The molding method of the present invention is an injection molding method of a sandwich molded body including an inner layer and a surface layer covering the inner layer, a first step of injecting a first molten resin for forming a surface layer in an amount less than the volume of the molding cavity into the molding cavity provided in a pair of molds; a second step of performing core-back to retract one of the pair of molds from the other during or after the injection of the first molten resin in the first step to form an additional cavity inside the first molten resin; a third step of injecting a second molten resin for forming an inner layer into the additional cavity during or after the core-back in the second step; and comprises.

[0007] In the first step, preferably, in a second direction orthogonal to the first direction which is the direction of core-back, a gap is provided between the first molten resin and the other mold, and the injection of the first molten resin is terminated.

[0008] In the second step, preferably, the gap is maintained during the core-back.

[0009] In the third step, preferably, The injection of the second molten resin is performed until the gap is filled with the first molten resin.

[0010] In the injection molding method of the present invention, preferably, after performing core-back in the second step, the first molten resin, the additional cavity, and the first molten resin are arranged in this order in the first direction.

[0011] In the third step, preferably, by injecting the second molten resin into the additional cavity, the molding cavity is filled with the first molten resin.

[0012] In the injection molding method of the present invention, preferably, in the core-back of the second step, the mold is retracted until the thickness of the molding cavity becomes larger than the thickness of the final sandwich molded body, after the third step, the mold is advanced until the thickness of the molding cavity becomes the thickness of the sandwich molded body.

Advantages of the Invention

[0013] According to the injection molding method of the present invention, by finishing the injection of the first molten resin before the molding cavity is filled, it is possible to prevent the breakage of the surface layer in the sandwich molded body while suppressing the ejection of the inner layer molten resin from the surface layer molten resin.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0015] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the embodiment, after starting the injection of the first molten resin A for forming a surface layer in an amount less than the volume of the molding cavity, core - back is performed, and then the second molten resin B is further injected and filled. In this way, in the embodiment, by finishing the injection of the first molten resin A before the molding cavity is filled, it is possible to suppress the ejection of the inner - layer molten resin from the surface - layer molten resin and prevent the breakage of the surface layer in the sandwich molded body. Finishing the injection before the molding cavity is filled is called a short - shot. Hereinafter, the common matters of the embodiment and the second embodiment will be described first, and then the embodiment and the second embodiment will be described in order.

[0016] [Main part of the injection molding apparatus 1: Refer to FIG. 1] FIG. 1 shows the main part of the injection molding apparatus 1 applied to the embodiment. The injection molding apparatus 1 includes a mold part 10 for molding an injection - molded product, and an injection part 20 for injecting and filling the first molten resin A and the second molten resin B toward the mold part 10. The operation of the mold part 10 and the injection part 20 of the injection molding apparatus 1 is controlled according to an instruction from a controller composed of a computer apparatus (not shown). In the injection molding apparatus 1, the thickness direction (first direction) T, the width direction (second direction) W orthogonal to the thickness direction T, the front (F) and the rear (R) are defined as shown in FIG. 1 and the like.

[0017] [Mold part 10] The mold part 10 includes a fixed mold 11 whose position is fixed and a movable mold 15 that can move forward and backward with respect to the fixed mold 11. In a mold clamping state where the fixed mold 11 and the movable mold 15 are abutted against each other, a molding cavity 17 is formed between the fixed mold 11 and the movable mold 15. A first resin gate 12 and a second resin gate 13 are formed in the fixed mold 11, and the molten resin plasticized by the injection part 20 is injected and filled into the molding cavity 17 through the first resin gate 12 and the second resin gate 13. A first molten resin A for the surface layer is supplied to the first resin gate 12, and a second molten resin B for the inner layer is supplied to the second resin gate 13. A first gate valve 18 for controlling the flow of the first molten resin A is provided at the first resin gate 12, and a second gate valve 19 for controlling the flow of the second molten resin B is provided at the second resin gate 13. Controlling the flow means allowing the flow of the molten resin toward the molding cavity 17 and stopping the flow of the molten resin toward the molding cavity 17. The control of the first gate valve 18 and the second gate valve 19 is performed by a controller. Note that in the mold part 10 shown in FIG. 1(a), a convex part that is a part of the movable mold 15 enters the cavity of the fixed mold 11, but a mold part having another structure including a molding cavity 17 capable of molding the sandwich molded body 100 can be adopted. For example, the movable mold 15 may have a cavity by itself in the same manner as the fixed mold 11, or the movable mold 15 may have a flat shape without a convex part. Further, the fixed mold 11 and the movable mold 15 can be made into an insert mold structure.

[0018] In the mold part 10, the operation in which the movable mold 15 moves forward (F) away from the fixed mold 11 is called mold opening or retraction, and conversely, the operation in which the movable mold 15 moves backward (R) closer to the fixed mold 11 is called mold closing or advancement.

[0019] Although not shown in the drawings, the fixed mold 11 is attached to the fixed mold plate, and the movable mold 15 is attached to the movable mold plate. A plurality of tie bars connecting the two are provided between the fixed mold plate and the movable mold plate. By applying the pressure of, for example, a hydraulic cylinder between the fixed mold plate and the movable mold plate via the tie bars, mold clamping or mold opening is performed.

[0020] [Injection unit 20] The injection unit 20 includes a first injection unit 21 capable of injecting and filling the first molten resin A for the surface layer into the molding cavity 17, and a second injection unit 22 capable of injecting and filling the second molten resin B for the inner layer into the molding cavity 17. The first molten resin A from the first injection unit 21 is injected into the molding cavity 17 through the first resin gate 12, and the second molten resin B from the second injection unit 22 is injected into the molding cavity 17 through the second resin gate 13.

[0021] Each of the first injection unit 21 and the second injection unit 22, although not shown in the drawings, includes, for example, a heating cylinder, an injection nozzle provided at the front end of the heating cylinder, a screw rotatably provided inside the heating cylinder, and a heater wound around the heating cylinder. The pellet-shaped raw material resin is supplied into the heating cylinder through a raw material inlet provided in the heating cylinder. The heater heats the resin inside the heating cylinder. The screw can move forward or backward and rotate forward or backward inside the heating cylinder. The injection nozzle has a resin gate penetrating in the front-rear direction inside, and the plasticized molten resin is supplied to the first resin gate 12 and the second resin gate 13 through the resin gate. Each of the first injection unit 21 and the second injection unit 22 includes, although not shown in the drawings, a driving device for moving the screw forward or backward and a driving device for rotating the screw forward or backward. These driving devices include, for example, an electric motor, and this electric motor performs necessary operations according to the instructions of the controller.

[0022] [Sandwich molded body 100: See Figure 6] The sandwich molded body 100 is one of the multi-layer molded products in resin molded products, which consists of various combinations of different materials, the same materials, different colors, and the same colors. The sandwich molded body 100 is a multi-layer molded product in which the entire circumference of the inner layer IL is covered by the surface layer OL, and the inner layer IL is not exposed to the outside from the surface layer OL. Therefore, the sandwich molded body 100 ensures the appearance and design of the resin molded product with the surface layer OL made of non-foaming resin, while the inner layer IL employs, for example, low-cost recycled resin. In this case, it is preferable to increase the volume of the low-cost inner layer IL as much as possible while reducing the volume of the high-cost surface layer OL. Further, the inner layer IL uses a foaming resin for the purpose of weight reduction, vibration damping, heat insulation and sound insulation, flexibility imparting, or a functional resin such as a high-strength resin, a resin for blocking and absorbing electromagnetic waves and ionizing rays, a water-absorbing resin, a non-permeable resin, etc., and bears the functionality according to the application.

[0023] As shown in FIG. 6(a), the sandwich molded body 100 may have a constant thickness, or as shown in FIG. 6(b), it may have a plurality of different thicknesses. The sandwich molded body 100 includes a thick portion 101, an intermediate thick portion 102, and a thin portion 103. This embodiment can mold any form of the sandwich molded body 100, and can also mold sandwich molded bodies in other forms.

[0024] Although it has been described that the surface layer OL in which the first molten resin A is solidified breaks, FIG. 6(a) shows an example of the location of the break Br. As shown in the figure, the break Br is in the width direction W, and the break Br occurs at the end portion where the first molten resin A flows from the first resin gate 12, the resin flow end portion.

[0025] 〔Injection molding method: FIGS. 2, 3, 4〕 With reference to FIGS. 2, 3 and 4, the injection molding method of the sandwich molded body 100 according to the embodiment will be described. The injection molding method according to the embodiment injects the first molten resin A for forming the surface layer OL before core-back, and then injects the second molten resin B for forming the inner layer IL to fill the molding cavity 17 with the first molten resin A and the second molten resin B.

[0026] [First Step: Injection of First Melted Resin A; see FIGS. 2 and 4] The first process starts from the first step of injecting the first melted resin A into the molding cavity 17 (FIG. 4 S101). At this time, the first gate valve 18 provided at the first resin gate 12 is open, while the second gate valve 19 provided at the second resin gate 13 is closed. When the first gate valve 18 and the second gate valve 19 are open, the illustration of the first gate valve 18 and the second gate valve 19 is omitted in FIGS. 2 and the like. Before the injection of the first melted resin A, the molding cavity 17 is empty. The first melted resin A injected in the first step is in an amount less than the volume VC1 of the molding cavity 17, and when the first step is completed, the molding cavity 17 has a gap 17T that is not filled with the first melted resin A (S103). This gap 17T is at both ends in the width direction W of the molding cavity 17 and exists across the fixed mold 11 and the movable mold 15 in the thickness direction T. Therefore, the end portion of the flow of the first melted resin A is not in contact with the fixed mold 11. That is, the gap 17T is formed between the fixed mold 11 and the movable mold 15 in the thickness direction (the first direction), and is formed between the first melted resin A and the fixed mold 11 in the width direction W (the second direction). Although not shown in the figure, gaps 17T are also formed at both ends on the back side and the front side of the paper surface. The injection in which the above molding cavity 17 is not filled is called a short shot. This gap 17T and its vicinity correspond to the resin flow end portion FT.

[0027] In FIG. 2, an example is shown in which both edges 11E on both sides of the fixed mold 11 in the width direction W are parallel to the thickness direction T, but this is only an example in the present invention. For example, both edges 11E may be inclined with respect to the thickness direction T. Even in this case, the gap 17T is formed between the first melted resin A and the fixed mold 11 in the width direction (the second direction) W that is orthogonal to the thickness direction T which is the core-back direction (the dimension component of the width direction W of the gap 17T is not zero). Also, although an example of a rectangular shape is shown here as the shape of the longitudinal section of the gap 17T, it is not limited to a rectangle. Corresponding to the external shape of the sandwich molded body 100, for example, the shape of the longitudinal section such as a parallelogram or a trapezoid other than a rectangle is adopted as the shape of the gap 17T. Also, the surfaces surrounding the gap 17T are not limited to flat surfaces and may be curved surfaces. This is also adopted according to the external shape of the sandwich molded body 100.

[0028] [Second Step: Core Back; refer to FIGS. 3 and 4] Next, a core back is performed to retract the movable mold 15. At this time, both the first gate valve 18 and the second gate valve 19 are closed. The core back in the second step can start during the injection of the first molten resin A in the first step, or can also start after the completion of the injection of a predetermined amount of the first molten resin A. When the core back is performed, an additional cavity 17S is formed inside the resin portion formed by the first molten resin A (S105). However, since the first step ends with a short shot, the state where the gap 17T remains empty also continues during the core back. During the core back, tensile forces Tf are generated in the thickness direction T at both ends in the width direction W of the first molten resin A. The tensile force Tf is the main factor for the breakage of the first molten resin A, and the short shot functions effectively against this tensile force Tf, as will be described later.

[0029] Note that, with respect to the injection rate (injection amount per unit time) of the first molten resin A injected into the molding cavity 17 in the first step, when the expansion rate of the molding cavity 17 due to the core back in the second step is large, the core back in the second step may start before the injection of a predetermined amount of the first molten resin A in the first step is completed. When the volume expansion rate of the molding cavity 17 is larger than the injection rate of the first molten resin A, the injection amount of the first molten resin A into the molding cavity 17 cannot catch up with the expansion of the molding cavity 17, so the density of the first molten resin A decreases. As a result, even if the sufficient size of the additional cavity 17S cannot be obtained inside the first molten resin A, the resistance to the inflow of the second molten resin B is reduced, so that it is possible to easily fill the second molten resin B to the end portion.

[0030] In the second step, the inside of the first molten resin A is divided in the thickness direction T by the additional cavity 17S. As a result, except for the gaps 17T at both ends in the width direction W, the first molten resin A, the additional cavity 17S, and the first molten resin A are arranged in order in the thickness direction T.

[0031] The additional cavity 17S is a void into which the second molten resin B is injected in the next third step. Assuming that the volume of the additional cavity 17S is VC2 and the volume in the gap 17T portion is VC3, the volume VC2 + the volume VC3 coincides with the volume VB of the second molten resin B injected in the third step. When the additional cavity 17S with the required dimensions is formed, the second step ends.

[0032] [Third Step: Injection of the Second Molten Resin B; FIGS. 3 and 4] Next, a third step of injecting the second molten resin B toward the additional cavity 17S is performed (FIG. 4 S107). The injection of the second molten resin B is performed until the additional cavity 17S is filled with the second molten resin B (FIG. 4 S109). The molding cavity 17 is filled with the first molten resin A and the second molten resin B. The injection of the second molten resin B can start during the core back or can start after the completion of a predetermined amount of the core back. The first step ends with a short shot, and gap 17T created by the short shot continues in the second step. Therefore, in the third step, second molten resin B is injected until gap 17T corresponding to the short shot is filled with first molten resin A (FIG. 7, S109). The first molten resin A filling gap 17T is pressed against the wall surfaces on both sides in the width direction W of fixed mold 11 by the injection pressure of second molten resin B. This completes the third step.

[0033] In addition, the injection of the second molten resin B in the third step is not limited to start after the core-backing in the second step as described above, but may start during the core-backing. In this case, the first molten resin A injected into the molding cavity 17 in the first step has not been cooled down since not much time has passed since the injection, and the first molten resin A has a low viscosity and is in a state where it flows easily. Therefore, when the second molten resin B is injected into the first molten resin A during the core-backing in the second step, a part of the first molten resin A at the part in contact with the injected second molten resin B is dragged or caught up in the second molten resin B and carried to the flow end side. As a result, the dimension of the first molten resin A constituting the surface layer OL in the thickness direction T is reduced, and the dimension of the first molten resin A in the width direction W at the flow end is increased by the first molten resin A carried to the flow end. This makes it easy to prevent the second molten resin B from breaking through the first molten resin A, which is the outer layer OL, at the flow end portion and being exposed to the outside of the molded product. When injection of a predetermined amount of second molten resin B is completed, the first molten resin A, the second molten resin B, and the first molten resin A are arranged in the thickness direction T in this order in molding cavity 17.

[0034] [Actions associated with steps 1 to 3] Prior to the first step, a mold clamping process is carried out in which the fixed mold 11 and the movable mold 15 are brought into close contact with each other, and a plasticization and measuring process is carried out in which the raw material resin pellets P of the first molten resin A and the second molten resin B are heated, melted, and plasticized inside the heating barrel. Also, after completing the third step, a holding and cooling process of cooling until the filled first molten resin A and second molten resin B solidify, a mold opening process of opening the fixed mold 11 and the movable mold 15, and a take-out process of taking out the sandwich molded body 100 cooled and solidified in the cavity 17 are sequentially performed.

[0035] [Effects achieved by the embodiment] According to the injection molding method according to the embodiment, the following first effect and second effect are achieved. [First effect] According to the present embodiment, since the gap 17T is provided between the first molten resin A and the molding cavity 17, it is possible to suppress the first molten resin A from contacting the wall surfaces at both ends in the width direction W of the fixed mold 11 and decreasing in temperature. Therefore, even if tensile forces Tf are generated in the thickness direction T at both ends in the width direction W during core back, since solidification or an increase in viscosity of the first molten resin A is suppressed at both ends in the width direction W, breakage of the said part can be prevented. In particular, when the first molten resin A is a resin material with a high viscosity, the present embodiment in which a short shot is intentionally made before core back is effective.

[0036] [Second effect] In the present embodiment, the first step of injecting the first molten resin A that constitutes the surface layer OL ends before the molding cavity 17 is filled with the first molten resin A. That is, the second process ends with an intentional short shot. Further, core back is performed as the second step after the first step. The additional cavity 17S formed by this core back is formed over the entire area of the molding cavity 17 and is a region with a constant thickness void or an extremely low resin density. Therefore, the second molten resin B injected in the third step can be filled into the additional cavity 17S without being resisted except for the friction with the first molten resin A injected earlier.

Example

[0037] Hereinafter, the experimental results of actual injection molding will be described as examples. The experiment produced sample molded products (circular basins) according to three steps corresponding to the embodiments, and evaluated the number of breakages at the end portions of the resin flow in the process for two types of resin materials. Also, the core-back rate in the second step related to core-back was changed. Regarding the injection of the first molten resin A in the first step, a comparative example under the same conditions as this embodiment was evaluated, except that it was performed with a full shot that fills the molding cavity 17.

[0038] The results are shown in FIG. 5. The present embodiment that performs a short shot can obtain a remarkable effect with a core-back magnification of 1.2 times or more.

[0039] Without departing from the gist of the present invention, it is possible to select the configurations exemplified in the embodiments or to appropriately change them to other configurations.

[0040] The injection compression process can be adopted in the present invention. This injection compression process is related to the backward and forward movement of the movable mold 15 after core-back (second step) and the third step as follows. Core-back (second step): The movable mold 15 is moved backward until the thickness of the molding cavity (17) becomes larger than the wall thickness of the sandwich molded body (100) as the final molded body. After the third step: After the third step in which the filling of the second molten resin B is completed, the movable mold 15 is moved forward until the thickness of the molding cavity 17 becomes the wall thickness of the sandwich molded body 100 as the final molded body.

[0041] According to the present invention adopting the above injection compression process, the following effects are achieved. Since the second molten resin B inside the first molten resin A can be crushed and spread in the width direction W, even when the thickness of the additional cavity 17S is thin and it is difficult for the second molten resin B to be filled, in addition to or instead of the filling residual pressure, it becomes easy to wash away the second molten resin B and reach the end of the second molten resin B. Particularly in the second process, in addition to the effect of allowing the second molten resin B to reach the end even when the dimension in the thickness direction T of the additional cavity 17S is small and thin, by crushing the second molten resin B, even at the flow end portion generated in the second molten resin B, due to the high resin pressure, it becomes easy to press the first molten resin A against the wall surface of the fixed mold 11.

[0042] In the injection molding apparatus 1 shown in FIG. 1, the first molten resin A or the second molten resin B is injected into the molding cavity 17 from each of the independent first resin gate 12 and second resin gate 13. However, the present invention is not limited to this, and the first molten resin A or the second molten resin B can also be sequentially injected into the molding cavity 17 from a common resin gate.

Explanation of Signs

[0043] 1 Injection molding apparatus 10 Mold part 11 Fixed mold 11E Edge 12 First resin gate 13 Second resin gate 15 Movable mold 17 Molding cavity 17S Additional cavity 17T Gap 18 First gate valve 19 Second gate valve 20 Injection part 21 First injection unit 22 Second injection unit 100 Sandwich molded body IL Inner layer OL Outer layer A First molten resin B Second molten resin

Claims

1. An injection molding method for a sandwich molded body including an inner layer and a surface layer covering the inner layer, comprising: a first step of injecting a first molten resin for forming the surface layer in an amount less than the volume of the molding cavity into a molding cavity provided in a pair of molds; a second step of performing a core back to retract one of the pair of molds from the other during or after the injection of the first molten resin in the first step to form an additional cavity inside the first molten resin; a third step of injecting a second molten resin for forming the inner layer into the additional cavity during or after the core back in the second step; An injection molding method comprising the above steps.

2. In the first step, in a second direction orthogonal to a first direction which is the direction of the core back, a gap is provided between the first molten resin and the other mold, and the injection of the first molten resin is terminated. The injection molding method according to Claim 1.

3. In the second step, the gap is maintained during the core back. The injection molding method according to Claim 2.

4. In the third step, the injection of the second molten resin is performed until the gap is filled with the first molten resin. The injection molding method according to Claim 3.

5. The molding cavity has a first direction in which the inner layer and the surface layer are laminated, and after performing the core back in the second step, the first molten resin, the additional cavity, and the first molten resin are arranged in this order in the first direction. The injection molding method according to Claim 1.

6. In the third step, by injecting the second molten resin into the additional cavity, the molding cavity is filled with the first molten resin. The injection molding method according to Claim 1.

7. In the core back of the second step, the mold is retracted until the thickness of the molding cavity becomes larger than the wall thickness of the final sandwich molded body, and after the third step, the mold is advanced until the thickness of the molding cavity becomes the wall thickness of the sandwich molded body. The injection molding method according to any one of Claims 1 to 6.

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