Injection molding method

Abstract

To provide an injection molding method capable of molding a sandwich molded article having surface and inner layers with different thicknesses while suppressing an ejection of an inner layer molten resin from a surface layer molten resin.SOLUTION: An injection molding method for a sandwich molded article of the present invention includes: a first step of injecting a first molten resin (A) for forming a surface layer into a molding cavity (17) provided in a pair of molds (11, 15) in the amount less than a volume of the molding cavity; a second step of injecting a second molten resin (B1) for forming an inner layer into the first molten resin (A); a third step of forming an additional cavity (17S) inside the first molten resin (A) and the second molten resin (B1) by performing a core-back by retracting one of the pair of molds from the other; and a fourth step of injecting a second molten resin (B2) into the additional cavity (17S) while one of the molds is retracted.SELECTED DRAWING: Figure 3

Description

[Technical field]

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

[0002] Conventionally, a sandwich molded body having 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 called a core layer, and the surface layer is sometimes called a skin layer. The multi-stage molding method is a method in which a molten resin for a surface layer is injected into a molding cavity, and then a molten resin for an inner layer is injected into the molten resin for the surface layer, filling the molding cavity with these two molten resins.

[0003] However, in conventional injection molding methods, there is a risk that the molten resin for the inner layer injected later will spurt out from the molten resin for the surface layer injected earlier. Therefore, Patent Document 1 proposes performing a so-called core-back to expand the volume of the molding cavity before injecting the molten resin for the inner layer. This core-back creates a gap into which the molten resin for the inner layer is injected inside the molten resin for the surface layer injected earlier. In Patent Document 1, the molten resin for the inner layer is injected into the gap created by the core-back, so that it is possible to prevent the molten resin for the inner layer from spurting out from the molten resin for the surface layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2012 / 160952 Summary of the Invention [Problem to be solved by the invention]

[0005] However, there is a demand for a sandwich molded body having a plurality of different wall thicknesses, and in addition to different wall thicknesses in the surface layers, the sandwich molded body may be required to have different wall thicknesses in the inner layers. However, the gap formed by the core back is determined by the amount of movement of the movable die by the core back, so the interval in the direction of the core back (first direction) is almost constant. Therefore, in Patent Document 1, the interval in the first direction formed by the core back is almost constant, and the thickness of the molten resin for the inner layer injected and filled therein becomes uniform. It is difficult to obtain a sandwich molded body with different thicknesses for both the surface layer and the inner layer by Patent Document 1. In view of the above, an object of the present invention is to provide an injection molding method capable of molding a sandwich molded body in which the surface and inner layers have different thicknesses while preventing the molten resin for the inner layer from ejecting from the molten resin for the surface layer. [Means for solving the problem]

[0006] The molding method of the present invention comprises the steps of: A method for injection molding a sandwich molded body having an inner layer and a surface layer covering the inner layer, comprising: A first step of injecting a first molten resin into a molding cavity provided in a pair of molds, the first molten resin being for forming a surface layer in an amount less than the volume of the molding cavity; a second step of injecting a second molten resin into the first molten resin to form an inner layer; a third step of performing a core-back by retracting one of the pair of dies from the other to form an additional cavity inside the first molten resin and the second molten resin; and a fourth step of injecting a second molten resin into the additional cavity while one of the dies is retracted.

[0007] In the injection molding method of the present invention, the core back in the third step is started during or after the injection of the second molten resin in the second step, or The second molten resin in the fourth step is started during or after the core-back operation in the third step.

[0008] In the injection molding method of the present invention, a preferred molding cavity is A first direction in which the inner layer and the surface layer are laminated; a second direction perpendicular to the first direction; and The molding cavity is The first dimension has a plurality of different dimensions.

[0009] In the injection molding method of the present invention, a preferred molding cavity is A first molding portion; The second molded portion is connected to one side of the first molded portion in the second direction and has a smaller dimension in the first direction than the first molded portion.

[0010] In the injection molding method of the present invention, a preferred molding cavity is The third molded portion is connected to the other side in the second direction of the first molded portion and has a smaller dimension in the first direction than the second molded portion.

[0011] In the injection molding method of the present invention, a preferred additional cavity is The first molded portion is formed to extend through the second molded portion in the second direction.

[0012] In the injection molding method of the present invention, preferably, After the core-back in the third step, The first molten resin, the second molten resin, the additional cavity, the second molten resin and the first molten resin are arranged in this order in the first direction.

[0013] In the injection molding method of the present invention, preferably, in the second step, The second molten resin is injected until the mold cavity is filled with the first and second molten resins.

[0014] In the injection molding method of the present invention, preferably, in the second step, The second molten resin is injected until the molding cavity is filled with the first molten resin and the second molten resin.

[0015] In the injection molding method of the present invention, preferably, in the fourth step, By injecting a second molten resin into the additional cavity, the molding cavity is filled with the first molten resin and the second molten resin.

[0016] In the injection molding method of the present invention, preferably, In the third step, the die is moved backward until the thickness of the molded cavity is greater than the wall thickness of the final sandwich molded body. After the fourth step, the mold is advanced until the thickness of the molding cavity becomes the wall thickness of the sandwich molded body. Effect of the Invention

[0017] According to the injection molding method of the present invention, by dividing the injection of the second molten resin into two injections with a core back sandwiched therebetween, it is possible to form a sandwich molded body in which the surface layer and the inner layer have different thicknesses while preventing the inner layer molten resin from ejecting from the surface layer molten resin. [Brief description of the drawings]

[0018] [Figure 1] 1 shows a schematic configuration of an injection molding apparatus according to an embodiment, with the upper diagram showing a clamped state and the lower diagram showing an unclamped state. [Diagram 2] 1A to 1C are diagrams illustrating a procedure of an injection molding method according to a first embodiment. [Diagram 3] 2, a diagram showing the procedure of the injection molding method according to the first embodiment. [Figure 4] FIG. 2 is a flow chart showing the procedure of the injection molding method according to the first embodiment. [Diagram 5] 6A to 6C are diagrams illustrating a procedure of an injection molding method according to a second embodiment. [Figure 6] 5A to 5C are diagrams showing the procedure of the injection molding method according to the second embodiment. [Figure 7] FIG. 11 is a flow chart showing the procedure of an injection molding method according to a second embodiment. [Figure 8] FIG. 2 is a diagram showing a longitudinal section of a sandwich molded body obtained in the embodiment. [Figure 9] 1 is a graph showing experimental results in an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Two embodiments of the present invention will be described below with reference to the accompanying drawings. In both of the embodiments, a core-back is performed in which one mold is retracted from the other mold between the start of molding and the completion of injection and filling. In addition, in both of the embodiments, a core-back is performed at a stage where a predetermined amount of the second molten resin (B1) for forming the inner layer is injected, and then the second molten resin (B2) is further injected and filled. In this way, in the embodiments, by dividing the injection of the second molten resin into two injections with the core-back sandwiched between them, it is possible to mold a sandwich molded body in which the thickness of both the surface layer and the inner layer are different while preventing the molten resin for the inner layer from spouting out of the molten resin for the surface layer. Hereinafter, common points between the first and second embodiments, and then the first and second embodiments will be described.

[0020] [Common points: see Figures 1, 5, and 8] FIG. 1 shows a main part of an injection molding apparatus 1 applied to the embodiment. The injection molding apparatus 1 includes a mold section 10 for molding an injection molded product, and an injection section 20 for injecting and filling a first molten resin A and a second molten resin B (B1, B2) into the mold section 10. The operations of the mold section 10 and the injection section 20 of the injection molding apparatus 1 are controlled according to instructions from a controller consisting of a computer device (not shown). In addition, regarding the second molten resin B, in order to distinguish between the portion injected before the core back and the portion injected after the core back, the former is referred to as the second molten resin B1 and the latter is referred to as the second molten resin B2. However, when there is no need to distinguish between the two, they are collectively referred to as the second molten resin B. In addition, in the injection molding device 1, the thickness direction (first direction) T, the width direction (second direction) W perpendicular to the thickness direction T, the front (F), and the rear (R) are defined as shown in FIG. 1 etc.

[0021] [Molding section 10] The mold section 10 includes a fixed mold 11 whose position is fixed, and a movable mold 15 that can move forward and backward relative to the fixed mold 11. In a mold clamping state in which the fixed mold 11 and the movable mold 15 are butted 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 molten resin plasticized in the injection section 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 a surface layer is supplied to the first resin gate 12, and a second molten resin B for an 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 in the first resin gate 12, and a second gate valve 19 for controlling the flow of the second molten resin B is provided in the second resin gate 13. Controlling the flow means allowing the molten resin to flow toward molding cavity 17 and stopping the flow of the molten resin toward molding cavity 17. The first gate valve 18 and the second gate valve 19 are controlled by a controller.

[0022] In the mold section 10, the action of the movable mold 15 moving forward (F) away from the fixed mold 11 is called mold opening or retreating, and conversely, the action of the movable mold 15 moving backward (R) towards the fixed mold 11 is called mold closing or advancing.

[0023] Although not shown in the figure, the fixed mold 11 is attached to the fixed platen, and the movable mold 15 is attached to the movable platen. A plurality of tie bars are provided between the fixed platen and the movable platen to connect them, and the mold is closed or opened by applying pressure from, for example, a hydraulic cylinder between the fixed platen and the movable platen via the tie bars.

[0024] In the injection molding apparatus 1, the molding cavity 17 of this embodiment has three different dimensions d1, d2, and d3 in the first direction (T) corresponding to the sandwich molded body 100 molded in the molding cavity 17. These dimensions are the distances between the fixed mold 11 and the movable mold 15 in the molding cavity 17 in a clamped state. The dimensions d1, d2, and d3 have the following relationship. The molding cavity 17 is divided into a thick molding section 171, an intermediate thickness molding section 172, and a thin molding section 173 corresponding to the respective dimensions. The thick molding section 171 is an example of the first molding section of the present invention, the intermediate thickness molding section 172 is an example of the second molding section of the present invention, and the thin molding section 173 is an example of the third molding section of the present invention. Dimensions: d1>d2>d3

[0025] [Injection part 20] The injection section 20 includes a first injection unit 21 capable of injecting and filling a first molten resin A for a surface layer into the molding cavity 17, and a second injection unit 22 capable of injecting and filling a second molten resin B for an 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.

[0026] Although not shown, each of the first injection unit 21 and the second injection unit 22 includes, for example, a heating barrel, an injection nozzle provided at the front end of the heating barrel, a screw rotatably provided inside the heating barrel, and a heater wound around the heating barrel. Pellet-shaped raw material resin is supplied to the inside of the heating barrel through a raw material inlet provided in the heating barrel. The heater heats the resin inside the heating barrel. The screw can move forward or backward inside the heating barrel and can rotate forward or backward. The injection nozzle has a resin gate penetrating therethrough in the front-rear direction, 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 a drive device for advancing or retreating the screw, and a drive device for rotating the screw forward or backward, both of which are omitted from the drawing. These drive devices include, for example, an electric motor, and the electric motor performs necessary operations according to instructions from a controller.

[0027] [Molded body 100 (sandwich molded body): Figs. 8(a) and (b)] As shown in FIG. 8(a), the sandwich molded body 100 molded in the molding cavity 17 has a thick part 101 having the largest thickness (T1), an intermediate thick part 102 connected to one side of the thick part 101 in the width direction W and having a smaller thickness (T2) than the thick part 101, and a thin part 103 connected to the other side of the width direction W of the thick part 101 and having a smaller thickness (T3) than the intermediate thick part 102. The thicknesses T1, T2, and T3 are thicknesses in the surface layer OL and have the following relationship. In addition, the thicknesses t1, t2, and t3 of the thick part 101, the intermediate thick part 102, and the thin part 103 in the inner layer IL are as follows. In this way, the sandwich molded body 100 has three different thicknesses, and the thicknesses are different in both the surface layer OL and the inner layer IL.

[0028] Thickness at surface OL: T1>T2>T3 Inner layer IL thickness: t1>t2>t3

[0029] The sandwich molded body 100 is one of the multi-layer molded products consisting of various combinations of different materials, the same material, different colors, and the same color in resin molded products. The sandwich molded body 100 is a multi-layer molded product in which the entire periphery of the inner layer IL is covered with 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 secures the appearance and design of the resin molded product with the surface layer OL made of a non-foaming resin, while the inner layer IL is made of, 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. In addition, the inner layer IL uses a foaming resin for the purpose of weight reduction, vibration damping, heat insulation, sound insulation, and flexibility, or a functional resin such as a high-strength resin, an electromagnetic wave / ionizing radiation blocking / absorbing resin, a water-absorbent resin, or a non-permeable resin, and is responsible for the functionality according to the application.

[0030] The sandwich molded body 100 shown in Fig. 8(a) shows an example in which the thickness t1 of the inner layer IL is uniform, but is not limited to this. For example, as shown in Fig. 8(b), this embodiment encompasses a sandwich molded body 100 in which the thickness of the inner layer IL is uneven, such as the thickness of the thick portion 101 being large at the center in the width direction W.

[0031] [First embodiment: Figs. 2, 3, and 4] An injection molding method (hereinafter, the first process) for the sandwich molded body 100 according to the first embodiment will be described with reference to Figs. 2, 3 and 4. In the injection molding method according to the first embodiment, before core-backing, the molding cavity 17 is filled with a first molten resin A injected to form a surface layer OL, and then filled with a second molten resin B1 injected to form an inner layer IL. In the molding cavity 17 at this time, the fixed mold 11 and the movable mold 15 are in close contact with each other by clamping. Note that the volume of the molding cavity 17 in this empty state is VC1, and the volumes of the thick molded portion 171, the intermediate thick molded portion 172 and the thin molded portion 173 are VC11, VC12 and VC13, respectively. Note that VC1 = VC11 + VC12 + VC13

[0032] The first process includes a 1A step, a 2A step, a 3A step, and a 4A step. Here, the 1st, 2nd, 3rd, etc., mean an order, but are not limited to the fact that the subsequent step starts after the preceding step is completed. In this embodiment, the operation of the subsequent step is allowed to start before the operation of the preceding step is completed.

[0033] [1st A step: Injection of 1st molten resin A; see Figures 2 and 4] The first process starts with a 1A step in which a first molten resin A is injected into the molding cavity 17 (FIG. 4, S101). At this time, a first gate valve 18 provided at the first resin gate 12 is open, whereas a 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, they are not illustrated in FIG. 2 and the like. Before the injection of the first molten resin A, the molding cavity 17 is empty. When a specified amount of the first molten resin A has been injected (FIG. 4, S103, Y), the injection of the first molten resin A is stopped. The amount of the first molten resin A injected in the 1A step is less than the volume VC1 of the molding cavity 17, and is particularly preferably less than the volume VC11 of the thick molding portion 171, as an example for obtaining a significant effect of the present invention. That is, in this case, the first molten resin A is not injected into the intermediate thick molding portion 172 and the thin molding portion 173 in the preferred 1A step.

[0034] [2nd A step: Injection of second molten resin B1: see Figures 2 and 4] After the 1A step is completed, the process proceeds to the 2A step in which the second molten resin B1 is injected (FIG. 4, S105). In the 2A step, the first gate valve 18 of the first resin gate 12 is closed, and the second gate valve 19 of the second resin gate 13 is opened. In other words, the resin to be injected into the molding cavity 17 is switched from the first molten resin A to the second molten resin B1. The second molten resin B1 that has passed through the second resin gate 13 is injected into the interior of the first molten resin A by passing through the first molten resin A that has been previously injected. When the molding cavity 17 is filled (packed) without gaps by injecting the second molten resin B1 and combining it with the first molten resin A injected previously, the injection of the second molten resin B1 is stopped (Fig. 4 S107). At this time, the second molten resin B1 is injected to such an extent that it fills the width direction W of the molding cavity 17 corresponding to the thick portion 101.

[0035] Assuming that the amount (volume) of the first molten resin A injected in the first A step is volume VA and the amount (volume) of the second molten resin B1 injected in the second A step is volume VB1, the relationship with the volume VC1 of the molding cavity 17 is as shown in the following formula (1). The volume VA and the volume VB1 are determined in consideration of the respective volumes of the surface layer OL and the inner layer IL in the finally desired sandwich molded body 100, and can have the relationship shown in the following formula (2). VA + VB1 = VC1 …(1) VA > VB1, VA = VB1, VA < VB1 … Formula (2)

[0036] [Third A step: Core back; refer to Figs. 3 and 4] After finishing the second A step, a core back is then performed to retract the movable mold 15. At this time, both the first gate valve 18 and the second gate valve 19 are closed. When the core back is performed, an additional cavity 17S is formed inside the resin portion formed by the first molten resin A and the second molten resin B1. At the start point of the third A step, the second molten resin B1 is covered by the first molten resin A over the entire area around it, but the interiors of the first molten resin A and the second molten resin B1 are divided in the thickness direction T by the additional cavity 17S. As a result, at the site of the molding cavity 17 corresponding to the thick portion 101, the first molten resin A, the second molten resin B1, the additional cavity 17S, the second molten resin B1, and the first molten resin A are arranged in order in the thickness direction T. In the molding cavity 172 corresponding to the intermediate thickness portion 102 and the molding cavity 173 corresponding to the thin portion 103, the first molten resin A, the additional cavity 17S, and the first molten resin A are arranged in order.

[0037] The additional cavity 17S penetrates the thick molded portion 171 in the width direction W, and is formed over both the intermediate thick molded portion 172 and the thin molded portion 173. The additional cavity 17S thus formed over the entire area of ​​the molding cavities 171, 172, 173 has a dimension in the thickness direction T that is approximately equal over the entire area of ​​the molding cavities 171, 172, 173. The additional cavity 17S is a gap into which the second molten resin B2 is injected in the next fourth step. This gap is formed in a part or all of the periphery of the molding cavity 17. If the volume of the additional cavity 17S is VC2, the volume VC2 is equal to the volume VB2 of the second molten resin B1 injected in the fourth step.

[0038] From the above, if the volume of molding cavity 17 after core-back is VC3, the relationship of the following formula (3) is satisfied: The above-mentioned formula (1) is also listed below. VC3 = VC1 + VC2 … (3) VC1 = VA + VB1 … (1)

[0039] [Step 4A: Injection of molten resin B2; Figures 3 and 4] After the 3A step is completed, the process proceeds to the 4th step of injecting the second molten resin B2 into the additional cavity 17S (FIG. 4, S111). The injection of the second molten resin B2 is continued until the additional cavity 17S is filled with the second molten resin B2 (FIG. 4, S113). When injection of second molten resin B2 is completed, in molding cavity 171 corresponding to thick portion 101, first molten resin A, second molten resin B1, molten resin B2, second molten resin B1 and first molten resin A are arranged in that order in thickness direction T. In molding cavity 172 corresponding to intermediate thick portion 102 and molding cavity 173 corresponding to thin portion 103, first molten resin A, second molten resin B2 and first molten resin A are arranged in that order.

[0040] [Actions associated with steps 1A to 4A] Prior to step 1A in the first process, 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. Furthermore, after completing step 4A, a holding and cooling process is carried out in which the filled first molten resin A and second molten resin B are cooled until solidified, a mold opening process is carried out in which the fixed mold 11 and the movable mold 15 are opened, and a removal process is carried out in which the sandwich molded body 100 that has been cooled and solidified in the cavity 17 is removed.

[0041] [Relationship between preceding and succeeding steps] In the above, an example has been described in which the 3A step is started when the 2A step is completed, and the 4A step is started when the 3A step is completed, but this embodiment is not limited to this. For example, the core-back in the 3A step may be started during the injection of the second molten resin in the 2A step. Also, the injection of the second molten resin in the 4A step may be started during the operation of the core-back in the 3rd step. This is the same in the second embodiment.

[0042] An example in which step 4A is started during the operation of step 3A will be specifically described below. In this case, the first molten resin A filled in the molding cavity 17 in the 1A step and the second molten resin B1 filled in the 2A step have not yet been cooled because they were filled not long ago. Therefore, they have low viscosity and flow easily. Therefore, when the second molten resin B2 is filled inside the first molten resin A and the second molten resin B1 during the operation of the 3A step, a part of the first molten resin A and the second molten resin B1 at the part in contact with the filled second molten resin B2 is dragged or caught by the second molten resin B2 and carried to the flow end side. As a result, the skin layer thickness of the first molten resin A, which is the surface layer material, becomes thinner, and the thickness of the first molten resin A at the flow end part increases due to the first molten resin A carried to the flow end part. This makes it easy to prevent the second molten resin B2 from breaking the first molten resin A, which is the surface layer OL, at the flow end part and being exposed to the outside of the molded product.

[0043] [Advantages of the First Embodiment] The first process according to the first embodiment provides the following first and second effects. [First effect] In the first embodiment, after the 2A step in which the second molten resin B1 constituting the inner layer IL is injected, a core-back is performed as the 3A step. The additional cavity 17S formed by this core-back is formed over the entire area of ​​the molding cavities 171, 172, 173, and is a gap with a constant thickness or a region with an extremely small resin density. Therefore, the molten resin B2 injected in the 4th step fills the additional cavity 17S without resistance other than friction with the first molten resin A and the second molten resin B1 injected earlier.

[0044] Suppose that the final required amount (VB) of the second molten resin B continues to be injected in the 2A step. In this case, the injected molten resin B flows preferentially into the molding cavity 172, which is thicker and has a smaller flow resistance than the molding cavity 173, which has a smaller thickness, and penetrates the first molten resin A, which was injected earlier, in the width direction W. The molded product obtained in this way has the inner layer IL exposed to the outside from a part of the surface layer OL, and cannot be said to be a sound sandwich molded body. If the amount of the second molten resin B injected is reduced, it is possible to prevent the second molten resin B from penetrating through the first molten resin A, but this would result in an excessive amount of the first molten resin A in the thick-walled portion 101.

[0045] [Second effect] According to the injection molding method of the first embodiment, in addition to the second molten resin B1 injected in the 2A step, molten resin B2 is injected into the additional cavity 17S. The second molten resin B1 is mainly injected corresponding to the thick portion 101 of the sandwich molded body 100, and the molten resin B2 is also injected into the thick portion 101. Therefore, while keeping the first molten resin A, which is the surface layer resin in the thick portion 101, thin, a large amount of the second molten resin B (B1, B2) can be filled inside the first molten resin A, and the amount of the first molten resin A can be minimized.

[0046] [Second embodiment: Figs. 5, 6, and 7] Next, a method for injection molding a sandwich molded body according to a second embodiment (hereinafter, referred to as the second process) will be described with reference to Figures 5, 6, and 7. In the second process, in step 2A of the first process, injection of the second molten resin B1 is stopped before the molding cavity 17 is filled. In other words, in the second process, step 2A of the first process is replaced with step 2B, in which the amount of the second molten resin B1 injected is intentionally reduced so as to end the injection with a short shot.

[0047] [1st A step: Injection of 1st molten resin A; see Figures 5 and 7] Step 1B in the second process is executed in the same manner as step 1A in the first process, as shown in FIG. 5 (FIG. 7, S101 and S103).

[0048] [2nd B step: Injection of second molten resin B1; Figures 5 and 7] After the 1A step is completed, the process proceeds to the 2B step in which the second molten resin B1 is injected (FIG. 7, S105). The 2B step is common to the 2A step of the first process in that the second molten resin B1 is injected into the molding cavity 17. Therefore, the matters that apply to the 2A step are also applied to the 2B step.

[0049] Step 2B differs from step 2A in that step 2B ends with a short shot. When the injection of second molten resin B1 in step 2B ends with a short shot, as shown in Fig. 5, first molten resin A does not reach both ends in the width direction W of molding cavity 17, and gaps 17T are left between first molten resin A and both sides in the width direction W of molding cavity 17 (Fig. 7, S108). The amount of the second molten resin B1 injected into the molding cavity 17 in the 2B step is reduced by an amount corresponding to the gap 17T. The amount of the first molten resin A to be injected may be reduced.

[0050] [Step 3B: Core back; Figures 6 and 7] After the 2B step is completed, the 3B step is performed to move the movable mold 15 back (S109 in FIG. 7). The 3B step is basically the same as the 3A step of the first process. However, the preceding 2B step is completed with a short shot, and the gap 17T is maintained during the core back. During the core back, a tensile force is generated in the first molten resin A in the thickness direction T at both ends in the width direction W. The short shot works effectively against this tensile force, as will be described later.

[0051] [4th B step: Injection of molten resin B2; Figures 6 and 7] After the 3B step is completed, the process moves to the 4B step in which the molten resin B2 is injected toward the additional cavity 17S (FIG. 7, S111). In the 4B step, the same operations as those in the 4B step of the first step are basically performed. However, since the 3B step is completed with a short shot, in the 4B step, the molten resin B2 is injected until the gap corresponding to the short shot is filled with the first molten resin A (FIG. 7, S113). The first molten resin A filling the gap is pressed against the wall surface of the fixed mold 11 by the injection pressure of the molten resin B2. This completes the 4B step.

[0052] [Relationship between preceding and succeeding steps] In the second embodiment as well, the operation of the subsequent step is permitted to start before the operation of the preceding step is completed. An example of this will be described. The example described here is a case where the expansion rate of the molding cavity 17 due to the core back in the 3B step is large compared to the injection rate (injection amount per unit time) of the second molten resin B1 filled into the molding cavity 17 in the 2B step. In this case, it is effective to start the 3B step before the 2B step is completed. When the volume expansion rate of molding cavity 17 is greater than the injection rate of second molten resin B1, the density of first molten resin A and second molten resin B1 decreases because the amount of second molten resin B1 being filled inside molding cavity 17 cannot keep up with the expansion of molding cavity 17. As a result, even if a sufficient size of additional cavity 17S cannot be obtained inside first molten resin A and second molten resin B1, the resistance to the inflow of second molten resin B2 is low, making it easier for second molten resin B2 to fill up to the ends.

[0053] [Effects of the second embodiment] The second process according to the second embodiment provides the following third effect in addition to the first and second effects of the first process. [Third effect] According to the second process, the gap 17T is provided until the filling of the second molten resin B2 is completed, so that the first molten resin A can contact the wall surface of the fixed die 11 and suppress the temperature drop. Therefore, even if a tensile force is generated in the thickness direction T at both ends in the width direction W during core back, the solidification or increase in viscosity of the first molten resin A is suppressed at both ends in the width direction W, so that the breakage of the corresponding parts can be prevented. In particular, when the first molten resin A is a resin material with high viscosity, the second process in which a short shot is intentionally performed before the core back is effective. On the other hand, even in the first process, when the first molten resin A is made of a resin material with low viscosity, the breakage of the first molten resin A at both sides in the width direction W can be prevented. EXAMPLES

[0054] The results of actual injection molding experiments will be described below as examples. In the experiment, a sample molded product (circular tray) was produced according to four steps corresponding to each of the first embodiment (first process) and the second embodiment (second process), and the number of breaks occurring at the end of the resin flow during that process was evaluated. The core-back ratio was changed in the 3A step and the 3B step related to the core-back. In addition, the two differ among the four steps in that the 2A process is a full shot, while the 2B process is a short shot. The injection molding machine used was a 450HH manufactured by UBE Machinery Co., Ltd. The following two injection molding conditions were adopted. The temperature of the mold during injection molding differs between the two conditions. Condition 2, with a low mold temperature, is intended to shorten the cycle of injection molding shots and can be considered a special condition. Here, an example is shown in which the mold temperature is 10°C lower than the normal temperature (T°C), but the low temperature is set to more than 10°C lower than the normal temperature (T°C), and may be, for example, 15°C or 20°C lower than the normal temperature T°C. Condition 1: Mold temperature Normal temperature (T℃) Condition 2: Mold temperature low temperature (T-10℃)

[0055] The results are shown in Figure 9. Under condition 1, there is no difference between the first and second embodiments, but under condition 2, the second embodiment in which a short shot is performed produces a more pronounced effect at a core-back ratio of 1.2 or more.

[0056] As long as it does not deviate from the gist of the present invention, it is possible to select and discard the configurations given in the first embodiment (first process) and the second embodiment (second process), or to change them to other configurations as appropriate.

[0057] The present invention can adopt an injection compression process, which involves the core back (step 3A of the first process, step 3B of the second process) and the retreat and advance of the movable mold 15 after the fourth step, as follows: Core back (step 3A or step 3B): The movable mold 15 is moved backward until the thickness (distances d1, d2, d3) of the molding cavity (17) becomes greater than the wall thickness (T1, T2, T3) of the sandwich molded body (100) as the final molded body. After the fourth step: After the fourth step in which filling of the second molten resin B2 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.

[0058] According to the present invention, which employs the above-mentioned injection compression process, the following effects are achieved. Since the second molten resin B2 inside the first molten resin A and the second molten resin B1 can be crushed and spread in the width direction W, even if the additional cavity 17S is thin and it is difficult to fill it with the second molten resin B2, it is easy to push the second molten resin B2 to reach the end in addition to or instead of the residual filling pressure. In particular, in the second process, in addition to the effect of allowing the second molten resin B2 to reach the end even though the dimension in the thickness direction W of the additional cavity 17S is small and thin, by crushing the second molten resin B2, the high resin pressure generated in the flow end portion of the second molten resin B2 makes it easy to press the first molten resin A against the wall surface of the fixed mold 11.

[0059] 1, a first molten resin A or a second molten resin B is injected into a molding cavity 17 from a first resin gate 12 and a second resin gate 13, which are independent of each other. However, the present invention is not limited to this, and the first molten resin A or the second molten resin B can also be injected into the molding cavity 17 in sequence from a common resin gate.

[0060] The injection molding method of the present invention is not limited to the sandwich molded articles having three different thicknesses described in the embodiment, but is also applicable to sandwich molded articles according to the following first to third aspects. First aspect: Sandwich molded body having a constant dimension (wall thickness) in the first direction (T) For example, as in the mold part 10 shown in Fig. 1, the distance from the second gate valve 19 to both end faces of the molding cavity 17 in the width direction W (the upper end face and the lower end face in the figure) may differ. In this case, even if the dimension (wall thickness) in the first direction (T) is constant for a sandwich molded body, there may be a bias in the ease with which the molten resin flowing in from the second gate valve 19 flows in the width direction W. In such cases, the present invention is effective.

[0061] Second embodiment: Sandwich molded body having two different dimensions (wall thicknesses) in the first direction (T) The present invention is effective for a sandwich molded body (100) whose dimensions (wall thickness) in the first direction (T) differ so much that differences in resin flow (pressure flow) occur. The second embodiment corresponds to, for example, a sandwich molded body made of a first molded body and a second molded body. Third aspect: Sandwich molded body having four or more different dimensions (wall thicknesses) in the first direction (T) [Explanation of symbols]

[0062] 1 Injection molding equipment 10. Mold Department 11 Fixed mold 12 First resin gate 13 Second resin gate 15 Movable mold 17 Molding Cavity 17S additional cavity 17T gap 171 Thick wall molding part 172 Intermediate thickness molding section 173 Thin wall forming part 18 First gate valve 19 Second gate valve 20 Injection part 21 First Launch Unit 22 Second Launch Unit 100 Molded body 101 Thick wall part 102 Medium thickness section 103 Thin section IL Inner layer OL surface layer A First molten resin B, B1, B2 Second molten resin d1,d2,d3 interval T1, T2, T3 thickness t1, t2, t3 wall thickness

Claims

1. A method for injection molding a sandwich molded body having 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 into a molding cavity provided in a pair of molds, the first molten resin having an amount less than the volume of the molding cavity; a second step of injecting a second molten resin into the first molten resin to form the inner layer; a third step of performing a core-back by retracting one of the pair of dies from the other to form an additional cavity inside the first molten resin and the second molten resin; and a fourth step of injecting the second molten resin into the additional cavity while one of the molds is retracted.

2. The core back in the third step is started during or after the injection of the second molten resin in the second step, or The injection of the second molten resin in the fourth step is started during or after the core-back operation in the third step is completed. The injection molding method of claim 1.

3. The molding cavity is A first direction in which the inner layer and the surface layer are laminated; a second direction perpendicular to the first direction; and The molding cavity is having a plurality of different dimensions in the first direction; The injection molding method according to claim 1 or 2.

4. The molding cavity is A first molding portion; a second molded portion connected to one side of the first molded portion in the second direction and having a smaller dimension in the first direction than the first molded portion, The injection molding method according to claim 3.

5. The molding cavity is a third molded portion connected to the other side of the first molded portion in the second direction and having a dimension in the first direction smaller than that of the second molded portion; The injection molding method according to claim 4.

6. The additional cavity is The first molded portion is formed in the second direction and is formed across the second molded portion. The injection molding method according to claim 4.

7. After the core-back is performed in the third step, the first molten resin, the second molten resin, the additional cavity, the second molten resin, and the first molten resin are arranged in this order in the first direction; The injection molding method according to claim 5.

8. In the second step, The second molten resin is injected until the molding cavity is filled with the first molten resin and the second molten resin. The injection molding method according to any one of claims 1 to 7.

9. In the second step, The second molten resin is injected until the molding cavity is filled with the first molten resin and the second molten resin. The injection molding method according to any one of claims 1 to 7.

10. In the fourth step, By injecting the second molten resin into the additional cavity, the molding cavity is filled with the first molten resin and the second molten resin. The injection molding method according to claim 9.

11. In the third step, the die is retracted until the thickness of the molding cavity becomes greater than the wall thickness of the final sandwich molded body. After the fourth step, the mold is advanced until the thickness of the molding cavity becomes equal to the wall thickness of the sandwich molded body. The injection molding method according to any one of claims 1 to 10.

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