JP7087778B2 - Injection molding method and injection molding equipment - Google Patents

Description

The present invention relates to an injection molding method and an injection molding apparatus.

In recent years, in the production process of resin molded products for automobiles, efforts aimed at reducing the environmental load and cost have been active by abolishing the painting process, which emits a large amount of CO ₂ and is expensive. As one of the painting alternative techniques, a molding method in which injection molding is performed using colored pellets in which a resin material is pre-colored is widely known.

Injection molding is a molding method for molding a resin molded product by filling a product portion having a product shape formed in a molding mold with a resin and solidifying the product. The resin is conveyed to the product part in the mold through a flow path called a runner in the mold. Examples of the runner include a hot runner that heats the inside of the flow path and causes the resin to flow in a molten state, and a cold runner that does not heat the inside of the flow path. When a hot runner is used, the structure of the molding die becomes complicated, so that the cost of the molding die and the maintenance cost are significantly higher than those of the cold runner. Therefore, from the viewpoint of cost reduction, it is preferable to use a cold runner rather than a hot runner. For example, Patent Document 1 below discloses an injection molding method using a cold runner.

Japanese Unexamined Patent Publication No. 2013-129121

By the way, in general, an amorphous resin, which is widely used as a resin material for a raw coating method for automobiles, is a very expensive material as compared with a crystalline resin. Therefore, the present inventor has proceeded with a study on using a crystalline resin in the original deposition method for the purpose of reducing the material cost.

However, it has been found that when injection molding as described in Patent Document 1 is carried out using a crystalline resin, color unevenness occurs in the resin molded product. As a result of diligent studies by the present inventor, it was found that the cause is the fountain flow phenomenon occurring in the cold runner. That is, due to the fountain flow phenomenon, a skin layer that is solidified in contact with the molding die, a core layer in which the resin flows without solidifying, and a shear layer formed between the skin layer and the core layer are generated. In the shear layer, the temperature rises due to frictional heat, so that the curing time progresses slowly and crystallization is promoted. As a result, in the shear layer, a highly crystallized region having a higher crystallinity than the skin layer and the core layer is formed. It was found that the distance (depth) of the highly crystallization region from the surface of the resin molded product varies, so that light is diffusely reflected in the highly crystallization region and color unevenness occurs.

Therefore, the present invention has been made to solve the above problems, and an object of the present invention is to provide an injection molding method and an injection molding apparatus capable of suppressing color unevenness of a resin molded product.

The injection molding method according to the present invention that achieves the above object flows through the main flow path of the cold runner in the process of filling the product portion formed in the molding mold with the thermoplastic resin melted via the cold runner. The flow of the thermoplastic resin is divided into a plurality of branch paths and then merged , and the flow path cross-sectional area of each of the branch paths has a size larger than the flow path cross-sectional area of the main flow path .

The injection molding apparatus according to the present invention that achieves the above object has a product part formed in a molding die and a cold runner that forms a flow path for filling the product part with the molten thermoplastic resin. The cold runner is a confluence that merges the main flow path, a plurality of branch paths that divide the flow of the thermoplastic resin flowing through the main flow path, and the flow of the thermoplastic resin flowing through the plurality of branch paths. And, the flow path cross-sectional area of each of the branch paths has a size larger than the flow path cross-sectional area of the main flow path .

According to the above-mentioned injection molding method and injection molding apparatus, the core layer and the shear layer generated in the cold runner are separated by the fountain flow phenomenon and then agitated by rejoining the core layer and the shear layer. The temperature can be made uniform. As a result, the distance (depth) from the surface of the resin molded product in the highly crystallized region formed by solidifying the shear layer in the product portion can be made uniform. As a result, it is possible to suppress color unevenness caused by diffused reflection of light in the highly crystallization region.

It is the schematic sectional drawing of the resin molded article painted by the painting method. It is a schematic cross-sectional view of the resin molded product which was original-bonded by the original-bonding method. It is a schematic diagram which shows the crystal structure of the crystalline resin which concerns on a comparative example. It is a schematic diagram which shows the crystal structure of the crystalline resin which concerns on one Embodiment of this invention. It is a schematic diagram for demonstrating the fountain flow phenomenon of the crystalline resin flowing in a flow path. It is sectional drawing which shows the crystallization distribution of the resin molded article which concerns on a comparative example. It is sectional drawing which shows the crystallization distribution of the resin molded article which concerns on one Embodiment of this invention. It is sectional drawing which shows the crystallization distribution of the resin molded article which concerns on one Embodiment of this invention. It is a graph which shows the relationship between the transmittance of a resin molded article, and the appropriate depth of a high crystallization region. It is a perspective view which shows the injection molding apparatus which concerns on one Embodiment of this invention. It is a perspective view which shows the sprue, the cold runner, and the product part of the injection molding apparatus shown in FIG. It is a top view for demonstrating the flow of the resin in the branch path and the junction flow path of the cold runner shown in FIG. 7. It is a figure which shows the temperature distribution of the resin flowing in the vicinity of a cold runner and a fan gate which concerns on a comparative example. It is a figure which shows the temperature distribution of the resin flowing around the cold runner and the pin gate of the injection molding apparatus shown in FIG. 7. FIG. 7 is a cross-sectional view of a flow path showing the temperature distribution of the resin flowing in the branch path and the junction flow path of the cold runner of the injection molding apparatus shown in FIG. 7. It is a figure which shows the distribution of the crystallinity of the cross section of the resin molded article which concerns on a comparative example. It is a figure which shows the distribution of the crystallinity of the cross section of the resin molded article which concerns on one Embodiment of this invention. It is a figure which shows the temperature distribution of the resin flowing in the molding die of the injection molding apparatus which concerns on modification 1. FIG. It is a figure which shows the branch path and the junction flow path of the cold runner which concerns on modification 2. FIG. It is a figure which shows the branch path and the junction flow path of the cold runner which concerns on modification 3. FIG. It is a figure which shows the cold runner and product part which concerns on modification 4.

FIG. 1A is a schematic cross-sectional view of a resin molded product painted by a painting method. As shown in FIG. 1A, in the coating method, the coating layer 1 is formed on the surface of the resin layer 2. The coating layer 1 is generally composed of a plurality of layers including a primer coating layer 1a for facilitating the adhesion of paint, a base layer 1b for imparting color, and a clear layer 1c for imparting gloss. According to the painting method, the surface of the resin molded product has high designability with good color development, as well as scratch resistance and impact resistance (hereinafter, scratch resistance and impact resistance are collectively referred to as "durability"). Can be given. On the other hand, since the painting method has a large amount of CO ₂ emissions and a high cost, the original coating method is attracting attention as a painting alternative technique from the viewpoint of reducing the environmental load and cost.

FIG. 1B is a schematic cross-sectional view of a resin molded product deposited by the original deposition method. In the original molding method, injection molding is performed using colored pellets 3 in which a resin material is colored with a high-luminance agent or a coloring pigment. Since there is no need to paint, the man-hours can be reduced, the manufacturing cost can be reduced, and the environmental load can be reduced.

However, there are many problems in the original landing method, and there are two major problems especially when it is applied to parts for automobiles where high quality is required. The first issue is durability (scratch resistance and impact resistance). Since the surface hardness of the raw material is lower than that of the painted molded product, it is easily scratched. Therefore, it is necessary to increase the hardness of the material, but as a trade-off, the impact resistance is lowered and it is difficult to achieve both. The second issue is the cost of the resin material for the original deposition method. The resin material for the original deposition method is required to have high surface hardness in order to improve scratch resistance, and at the same time, high transparency is required to improve color development. For this reason, amorphous materials such as polycarbonate (PC) resin and acrylic (PMMA) resin have been used in the conventional landslide method. However, the above-mentioned amorphous resin, which is widely used as a raw material for automobiles, has good color development (transparency) and scratch resistance, but is compared with crystalline resins such as PP resin used in the coating method. The price of the material is very high.

Here, the "amorphous resin" means a resin that is difficult to crystallize, and the "crystalline resin" means a resin that is easy to crystallize. The ease of crystallization can be determined using the degree of crystallization as an index. Specifically, a resin having a crystallinity lower than a predetermined reference value is defined as an amorphous resin, and a resin having a crystallinity higher than a predetermined reference value is defined as a crystalline resin. The reference value of the crystallinity can be set in the range of, for example, 20 to 50%. The crystallinity is not only due to the type of material, but can also be adjusted by molding conditions and the like.

Crystalline resin, which is widely used for painting methods for automobiles, is difficult to use as a raw material because the material cost is low, but the color development (transparency) is poor and durability is also a problem. Has been said.

Therefore, the present inventor has made extensive studies on improving transparency and durability in order to use a crystalline resin as a raw material. FIG. 2A is a schematic view showing the crystal structure of a general crystalline resin. As shown in FIG. 2A, there were variations in the size of the crystals 5 of the crystalline resin. On the other hand, FIG. 2B is a schematic view showing the crystal structure of the crystalline resin according to the embodiment of the present invention. The present inventor has succeeded in improving the transparency of a crystalline resin having a high degree of crystallinity, which has been difficult to make transparent, by making the crystal 6 finer and more uniform as shown in FIG. 2B. did. Furthermore, surprisingly, the present inventor has discovered that by refining the crystals of the crystalline resin, a crystalline resin having extremely high hardness and strength as compared with the conventional crystalline resin can be obtained. However, it has been found that the durability, which has been a problem in crystalline resins, can be improved.

However, it has been found that the crystalline resin developed by the present inventor has improved transparency and durability, but has high transparency, so that the problem of color unevenness becomes remarkable. In general, crystallinity and transparency are inversely proportional. The higher the crystallinity, the lower the transparency. Therefore, when the crystallinity of the crystalline resin is non-uniform, the refractive index changes depending on the difference in the crystallinity. Therefore, as shown in FIG. 1B, the incident light 7 incident on the crystalline resin is recognized as color unevenness by the reflected light 8 diffusely reflected due to the difference in the refractive index.

The reason why the crystallinity of the crystalline resin becomes non-uniform will be described with reference to FIG. FIG. 3 is a schematic diagram for explaining the fountain flow phenomenon of the crystalline resin flowing in the flow path. The crystalline resin flows in the flow path in the flow direction F while causing a resin flow phenomenon called fountain flow (spout flow) 16 at the flow front. The crystalline resin flowing in the flow path is a low-temperature (about 40 ° C. for PP resin) skin layer 13 (solidified layer) that is cooled and solidified in contact with the molding mold that forms the flow path, and the resin does not solidify. A high temperature (about 200 ° C. for PP resin) core layer 18 (fluid layer) is formed. Here, since the resin has started to solidify in the skin layer 13, the viscosity rapidly increases. On the other hand, in the core layer 18, the resin flows, the temperature of the resin is high, and the viscosity is low. Therefore, a shear layer 17 in which a shear force is generated due to a difference in viscosity is formed between the skin layer 13 and the core layer 18.

In the shear layer 17, since a shearing force is constantly generated during the flow of the resin, heat is generated by frictional heat and the temperature becomes high (about 205 to 210 ° C. for PP resin). Here, the degree of crystallization increases as the temperature of the crystallization temperature range is slowly lowered (the cooling rate is slowed down) in the process of cooling from the molten state and solidifying. Therefore, in the shear layer 17, crystallization progresses as the curing time slowly progresses from a high temperature state, and the high crystallinity region 14 (FIG. 14) has a higher degree of crystallization than the layer in which the skin layer 13 and the core layer 18 are solidified. 4A) is formed.

On the other hand, the skin layer 13 has a lower crystallinity than the core layer 18 because it comes into contact with the molding die and is rapidly cooled. Therefore, the degree of crystallinity after solidification increases in the order of the skin layer 13, the core layer 18, and the shear layer 17 (highly crystallization region 14). As mentioned above, crystallinity and transparency are inversely proportional. According to the study of the present inventor, even in the crystalline resin in which the crystal is made finer and the transparency is increased, the transparency is lowered in the highly crystallized region 14 where the crystallinity is increased, and it looks like color unevenness. found. In particular, in the case of the highly transparent crystalline resin developed by the present inventors, this problem becomes remarkable because the sensitivity of the color unevenness in the highly crystallized region 14 becomes high.

FIG. 4A is a cross-sectional view showing the crystallization distribution of the resin molded product according to the comparative example. 4B and 4C are cross-sectional views showing the crystallization distribution of the resin molded product according to the embodiment of the present invention. As a result of diligent studies on the cause of color unevenness, the present inventor has experimented with the occurrence of color unevenness due to variations in the depth D1 from the surface of the resin molded product in the highly crystallization region 14 as shown in FIG. 4A. Was found by analysis. As shown in FIG. 4A, when the depth D1 of the high crystallization region 14 varies, the incident light 7 is diffusely reflected to become the reflected light 8, which is recognized as color unevenness. According to the research conducted by the present inventor so far, by making the molding temperature of the crystalline resin uniform, the depths D2 and D3 of the high crystallization region 14 can be uniformly molded as shown in FIGS. 4B and 4C. I found that I could do it. This makes it possible to suppress the occurrence of color unevenness.

Further, according to the study of the present inventor, as shown in FIG. 4C, the highly crystallization region 14 is arranged in the central portion 11 away from the skin layer 13, and the depth D3 from the surface of the resin molded product is made deeper. It was found that the sensitivity of color unevenness can be reduced by doing so. This is because, as shown in FIG. 4B, when the depth D2 from the surface of the resin molded product in the high crystallization region 14 is shallow, the incident light 7 is reflected in the high crystallization region 14 close to the skin layer 13, so that the reflected light is reflected. The sensitivity at which 8 is recognized as color unevenness increases. On the other hand, as shown in FIG. 4C, by making the depth D3 from the skin layer 13 of the highly crystallization region 14 deeper, the incident light 7 incident on the region where the distance from the skin layer 13 is shallow is high. Since the reflected light 8 is reflected before reaching the crystallization region 14, the sensitivity recognized as color unevenness is lowered. As described above, by increasing the depth D3 from the surface of the resin molded product in the high crystallization region 14, the sensitivity to color unevenness can be reduced.

FIG. 5 is a graph showing the relationship between the transmittance (transparency) of the resin molded product obtained by the sensory evaluation and the appropriate depth of the high crystallization region 14. If the depth from the surface of the resin molded product in the high crystallization region 14 is shallower than the appropriate depth, color unevenness will occur, and if the depth is deeper than the appropriate depth, color unevenness within the allowable range for automobile parts will be suppressed. be able to. As shown in FIG. 5, the higher the transmittance (transparency) of the resin molded product, the deeper the appropriate depth of the high crystallization region 14. This is because a resin molded product having a higher transmittance (transparency) transmits light deeper and has a higher sensitivity to color unevenness caused by the high crystallization region 14. Therefore, the resin molded product having a high transmittance needs to have a deeper depth from the surface of the resin molded product in the high crystallization region 14. On the other hand, the lower the transmittance (transparency) of the resin molded product, the lower the sensitivity of color unevenness caused by the high crystallization region 14 because the light transmission is reduced. Therefore, even if the depth of the high crystallization region 14 is shallow, it is possible to suppress the sensitivity to color unevenness. From the graph of FIG. 5, the optimum depth of the high crystallization region 14 can be obtained from the transmittance of the resin molded product.

Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the technical scope of the present invention should be determined based on the description of the scope of claims, and is not limited to the following embodiments.
The dimensional ratios in the drawings are exaggerated for convenience of explanation and may differ from the actual ratios.

[resin]
One embodiment of the present invention is an injection molding method and an injection molding apparatus using a crystalline resin among thermoplastic resins. As the resin applied to the present invention, it is preferable to use a crystalline resin in which crystals are refined and homogenized as shown in FIG. 2B. As a result, as described above, transparency and durability can be improved as compared with the conventional crystalline resin, and the material cost can be reduced as compared with the amorphous resin. The transparency of the crystalline resin is not particularly limited, but is preferably 50% or more, more preferably 70% or more, and particularly preferably 80% or more. Even when a crystalline resin having a non-uniform crystal size and low transparency as shown in FIG. 2A is applied to the present invention, the effect of the present invention of suppressing color unevenness is exhibited. In the following description, the thermoplastic crystalline resin is also simply referred to as "resin".

The thermoplastic crystalline resin applicable to the present invention is not particularly limited, and examples thereof include polypropylene (PP) resin, polyamide (PA) resin, polystyrene (PS) resin, polyphenylene sulfide (PPS) resin, and the like. Among the above, PP resin, which is widely used as a resin component for automobiles, can be preferably used from the viewpoint of reducing material cost. Depending on the required performance, additives such as rubber, talc, and oil may be added to the PP resin, or a neat PP resin to which no additive is added may be used.

[Injection molding equipment]
FIG. 6 is a perspective view showing an injection molding apparatus 100 according to an embodiment of the present invention. The injection molding apparatus 100 is an apparatus for molding a resin molded product by filling the product portion 50 formed in the molding die 110 with the molten resin, cooling the product portion 50, and solidifying the product portion 50.

The injection molding apparatus 100 includes a molding die 110 and a resin injection section 120 that injects the resin into the molding die 110 in a pressurized state. The operation of the molding die 110 and the resin injection unit 120 may be controlled by a known controller. Further, a sensor for measuring the pressure and temperature of the resin in the molding die 110 may be appropriately provided.

The molding die 110 has a fixed die 111 and a movable die 112 that can move in a direction that approaches and separates from the fixed die 111. The mold opening direction of the molding mold 110 is not particularly limited, and a horizontal opening mold or a vertical opening mold can be used. From the viewpoint of easy molding of a larger molded product, it is preferable to use a side-opening mold as shown in FIG. Further, in the present embodiment shown in FIG. 6, a two-divided split type of a fixed type 111 and a movable type 112 is used, but the present invention is not limited to this, and a three-divided or more split type may be used.

The movable mold 112 is a cavity-side mold having a groove having a shape corresponding to the shape of the cold runner 30 and the product portion 50 on the surface facing the fixed mold 111. The fixed mold 111 is a core-side mold having a molding surface corresponding to the groove of the movable mold 112. The movable type 112 further has a sprue 20 that forms a flow path for transporting the resin injected from the resin injection unit 120 to the cold runner 30. With the molding die 110 closed, the cold runner 30 and the product section 50 are formed between the fixed die 111 and the movable die 112. The movable mold 112 may be used as the core-side mold, and the fixed mold 111 may be used as the cavity-side mold.

FIG. 7 is a perspective view showing a sprue 20, a cold runner 30, and a product unit 50 of the injection molding apparatus 100 shown in FIG. As shown in FIG. 7, a pin gate 40 is formed between the cold runner 30 and the product unit 50.

The sprue 20 is a flow path for transporting the molten resin to the cold runner 30, and is formed by a through hole penetrating the movable type 112. The sprue 20 shown in FIG. 7 is formed so that the cross-sectional area of the flow path increases from the upstream side (outside of the molding die 110) to the downstream side (cold runner 30 side). As a result, the pressure loss of the sprue 20 can be reduced and the resin can flow smoothly. In addition, in this specification, "the cross-sectional area of a flow path" means the area of the cross section orthogonal to the flow direction of a resin.

The cold runner 30 includes a main flow path 31, a plurality of branch paths 32 that diverge the resin flow flowing through the main flow path 31, a merging flow path 33 that merges the resin flows flowing through the plurality of branch paths 32, and a merging flow path 33. It has a connecting portion 35 that connects the pin gate 40 and the pin gate 40. A slag reservoir 34 is provided on the extension line of the combined flow path 33. The slag pool 34 can prevent the cooled and solidified resin from flowing into the product section 50.

In the injection molding method using the injection molding apparatus 100, the flow of the resin flowing through the main flow path 31 of the cold runner 30 is diverted into a plurality of branch paths 32 and then merged with the merging flow path 33 (hereinafter, also referred to as “split merging”). .). As a result, referring to FIG. 3, the core layer 18 and the shear layer 17 generated in the cold runner 30 are separated by the fountain flow phenomenon and then agitated by rejoining the core layer 17 and the core layer. The temperature of 18 can be made uniform. As a result, the temperature of the resin in the product section 50 becomes uniform, so that the distances (depths) D2 and D3 of the high crystallization region 14 from the surface of the resin molded product become uniform as shown in FIGS. 4B and 4C. Can be. As a result, it is possible to suppress color unevenness caused by diffused reflection of light in the high crystallization region 14.

Further, the main flow path 31 and the combined flow path 33 are formed so as to extend substantially linearly. Since the resin flows linearly before and after the splitting in the branch path 32, the stirring action due to the splitting and merging can be improved as compared with the case of meandering.

FIG. 8 is a top view for explaining the flow of the resin in the branch path 32 and the junction flow path 33 of the cold runner 30 shown in FIG. 7. As shown in FIG. 8, the shape of the branch path 32 is a triangle. The resin flow splits into two at the branch path 32 and joins the merging flow path 33. The angle θ1 formed by the flow direction F of the resin split into two is an acute angle (about 60 degrees in FIG. 8). The flow of the separated resin merges in the directions facing each other, and then flows into the confluence 33. As described above, by merging the resins in the opposite directions after relatively gently splitting the resin, the stirring action due to the split merging is further improved.

In this embodiment, an example of branching into two branch paths 32 is shown, but the number of branch paths 32 is not limited to this, and may be, for example, three or more. As the number of branch paths 32 increases, the effect of stirring the resin by merging and merging becomes higher, but the structure of the molding die 110 becomes complicated, so that the manufacturing cost increases. From the above viewpoint, it is preferable to set the number of branch paths 32 to an optimum value.

Since the resin in the cold runner 30 is discarded after molding, it is preferable to make the cross-sectional area of the flow path as small as possible from the viewpoint of reducing the amount of the discarded resin and improving the yield. On the other hand, if the cross-sectional area of the flow path of the cold runner 30 is too small, the flow resistance becomes large and the resin cannot flow smoothly. From the above viewpoint, it is preferable to appropriately select the optimum flow path cross-sectional area of the cold runner 30.

Further, the cross-sectional area of the flow path of the branch path 32 has a size larger than the cross-sectional area of the flow path of the main flow path 31. As a result, the pressure loss in the branch path 32 can be reduced and a smooth resin flow can be formed.

The shape of the cross section of the flow path of the cold runner 30 is not particularly limited, and examples thereof include a semicircle, a perfect circle, a semi-ellipse, a trapezoid, and a square. In the present embodiment, the cross-sectional shape of the flow path of the cold runner 30 is a semicircle. The cross section of the flow path of the branch path 32 may have the same shape as the main flow path 31 or the combined flow path 33, or may have a different shape. From the viewpoint of reducing the pressure loss in the branch path 32, it is preferable that the cross section of the flow path of the branch path 32 has the same shape as the main flow path 31 and the junction flow path 33.

The flow path cross-sectional area S1 of the pin gate 40 is formed to be smaller than the flow path cross-sectional area of the cold runner 30, more specifically, the flow path cross-sectional area S2 of the connecting portion 35. As a result, when passing through the pin gate 40, the cross-sectional area of the flow path becomes small, so that a shearing force is generated in the resin to generate heat, and the temperature of the resin flowing into the product section 50 rises. In this way, by using the merging and merging together with the pin gate 40, the temperature rise of the resin and the uniform temperature distribution can be obtained after passing through the pin gate 40. Due to the uniform temperature rise of the resin, the temperature difference between the skin layer 13 and the core layer 18 becomes smaller, so that the formation of the skin layer 13 is delayed, and the shear layer generated by the shearing force between the skin layer 13 and the core layer 18 is generated. The generation of 17 is delayed. By the time the shear layer 17 is formed, heat is taken from the molding die 110 and the skin layer 13 gradually begins to solidify. Therefore, the highly crystallization region 14 formed by solidifying the shear layer 17 in the product section 50. The distance (depth) D3 from the surface of the resin molded product becomes longer. As a result, as shown in FIG. 4C, since the highly crystallization region 14 can be formed in the central portion 11 away from the surface layer, the sensitivity of color unevenness caused by diffused reflection of light in the highly crystallization region 14 is reduced. be able to.

While the above effect can be obtained by making the flow path of the resin smaller by the pin gate 40, if the flow path is too small, there is a concern that the moldability may be deteriorated due to an increase in flow resistance or the like. From this point of view, the throttle ratio (S1 / S2) of the flow path cross-sectional area S1 of the pin gate 40 to the flow path cross-section S2 of the connecting portion 35 is preferably 10 to 40%, preferably 20 to 30%. More preferred. It is preferable to appropriately select an optimum drawing ratio of the pin gate 40 according to the size of the resin molded product (product) to be molded and the viscosity of the resin to be used.

The product part 50 is formed by a cavity in the mold 110 having the product shape of the resin molded product.

Next, with reference to FIGS. 9 to 11B, the analysis results performed for confirming the effect of the injection molding method and the injection molding apparatus 100 according to the present embodiment will be described.

FIG. 9 is a diagram showing the temperature distribution of the resin flowing in the vicinity of the cold runner 30a and the fan gate 40a according to the comparative example. In the comparative example shown in FIG. 9, the cold runner 30a is different from the above-described embodiment in that the cold runner 30a does not have a branch path and a fan gate 40a is used instead of the pin gate. The fan gate 40a is formed so that the cross section of the flow path increases from the connecting portion 35 of the cold runner 30a toward the product portion 50.

As shown in FIG. 9, when the cold runner 30a does not have a branch path, the resin is not agitated, so that the temperature unevenness of the resin generated in the cold runner 30a is transmitted to the product unit 50 as it is. Therefore, in the analysis result shown in FIG. 9, the temperature of the resin after passing through the fan gate 40a is not symmetrical, and a region where the temperature is extremely low is generated in the center. When the temperature unevenness of the resin occurs in the product unit 50, light is diffusely reflected due to the difference in the crystallinity of the resin after solidification, and color unevenness occurs on the surface of the resin molded product.

FIG. 10A is a diagram showing the temperature distribution of the resin flowing in the vicinity of the cold runner 30 and the pin gate 40 of the injection molding apparatus 100 according to the present embodiment shown in FIG. 7. FIG. 10B is a cross-sectional view showing the temperature distribution of the resin flowing in the branch path 32 and the junction channel 33 of the cold runner 30 of the injection molding apparatus 100 shown in FIG. 7. In the examples shown in FIGS. 10A and 10B, the cross section of the flow path of the cold runner 30 has a semicircular shape with a diameter of 5 mm, and the main flow path 31, the branch path 32, and the combined flow path 33 have substantially the same shape and size. Further, the cross section of the pin gate 40 is a square of 1.5 mm × 1.5 mm. The aperture ratio of the pin gate 40 was 22.9%.

As shown in FIGS. 10A and 10B, the temperature of the resin that was non-uniform due to the fountain flow phenomenon in the main flow path 31 becomes uniform after passing through the branch path 32 (the temperature between the high temperature portion and the low temperature portion of the resin). The difference is reduced). Further, as shown in FIG. 10A, it was found that the temperature of the resin after passing through the pin gate 40 increased, became more uniform, and showed a symmetrical temperature distribution.

FIG. 11A is a diagram showing the distribution of the crystallinity of the cross section of the resin molded product according to the comparative example. FIG. 11B is a diagram showing the distribution of crystallinity in the cross section of the resin molded product according to the embodiment of the present invention.

In the distribution of the crystallinity of the cross section of the resin molded product obtained by solidifying the resin passed through the fan gate 40a as shown in FIG. 9, the high crystallinity region 14 is located near the surface as shown in FIG. 11A. Is formed in. As described above, when the depth of the high crystallization region 14 is shallow, the incident light 7 is reflected in the high crystallization region 14 close to the skin layer 13, so that the sensitivity recognized as color unevenness when the reflected light 8 is diffusely reflected is high. It gets higher.

On the other hand, as shown in FIGS. 10A and 10B, in the distribution of the crystallinity of the cross section of the resin molded product obtained by solidifying the resin passed through the pin gate 40, the high crystallinity region is shown in FIG. 11B. 14 is formed in the central portion 11 away from the surface layer. This makes it possible to reduce the sensitivity of color unevenness caused by diffused reflection of light in the high crystallization region 14.

As described above, in the injection molding method according to the present embodiment, in the process of filling the product portion 50 formed in the molding die 110 with the resin (thermoplastic resin) melted via the cold runner 30. The flow of the resin flowing through the main flow path 31 of the cold runner 30 is divided into a plurality of branch paths 32 and then merged.

Further, the injection molding apparatus 100 according to the present embodiment includes a product part 50 formed in the molding die 110 and a cold runner 30 forming a flow path for filling the product part 50 with the molten resin. .. The cold runner 30 has a main flow path 31, a plurality of branch paths 32 for dividing the resin flow flowing through the main flow path 31, and a confluence flow path 33 for merging the resin flows flowing through the plurality of branch paths 32.

According to the injection molding method and the injection molding apparatus 100 having the above configuration, the core layer 18 and the shear layer 17 generated in the cold runner 30 are separated by the fountain flow phenomenon and then rejoined to be agitated. The temperatures of the shear layer 17 and the core layer 18 can be made uniform. As a result, the distance (depth) of the highly crystallization region 14 formed by solidifying the shear layer 17 in the product portion 50 from the surface of the resin molded product can be made uniform. As a result, it is possible to suppress color unevenness caused by diffused reflection of light in the high crystallization region 14.

Further, since the fountain flow phenomenon occurs in the flow path by using the cold runner 30, the low temperature skin layer 13 and the low temperature skin layer 13 as shown in FIG. 3 also in the product part 50 in which the resin flows through the cold runner 30. A high-temperature shear layer 17 and a core layer 18 are formed. Therefore, a skin layer 13 having a low crystallinity and a high hardness is formed on the outermost layer of the resin molded product, and a layer having a high crystallinity and a high toughness (toughness) is formed on the central portion 11 side of the outermost layer. Will be done. As a result, the outermost layer has high scratch resistance, and the central portion 11 side of the outermost layer can have higher impact resistance to absorb energy due to impact, so that the durability can be further improved.

Further, the resin that has passed through the cold runner 30 is passed through the pin gate 40 having a cross-sectional area smaller than that of the cold runner 30 and filled in the product section 50. As a result, when passing through the pin gate 40, the cross-sectional area of the flow path is rapidly reduced, so that a shearing force is generated in the resin to generate heat, and the temperature of the resin rises. By using the diversion and merging together with the pin gate 40, a temperature rise and a uniform temperature distribution can be obtained after passing through the pin gate 40. Therefore, due to the uniform temperature rise of the resin, the temperature difference between the skin layer 13 and the core layer 18 becomes smaller, so that the formation of the skin layer 13 is delayed, and the shear layer 17 generated by the shearing between the skin layer 13 and the core layer 18 is delayed. Generation is delayed. By the time the shear layer 17 is formed, heat is taken from the molding die 110 and the skin layer 13 gradually begins to solidify. Therefore, the highly crystallization region 14 formed by solidifying the shear layer 17 in the product section 50. The distance (depth) from the surface of the resin molded product becomes longer. As a result, since the highly crystallization region 14 can be arranged in the central portion 11 away from the surface layer, the sensitivity of color unevenness caused by diffused reflection of light in the highly crystallization region 14 can be reduced.

Further, when the cold runner 30 is made to flow in the resin, the resin is made to flow linearly before and after the distribution into the branch path 32. As a result, the resin flows linearly before and after the splitting, so that the stirring action due to the splitting and merging can be improved as compared with the case of meandering. As a result, the temperatures of the shear layer 17 and the core layer 18 are further made uniform, and the distance (depth) from the surface of the resin molded product of the highly crystallization region 14 formed by solidifying the shear layer 17 in the product section 50. ) Can be made uniform. As a result, it is possible to suppress color unevenness caused by diffused reflection of light in the high crystallization region 14.

Further, the cross-sectional area of the flow path of the branch path 32 has a size larger than the cross-sectional area of the flow path of the main flow path 31. As a result, the pressure loss in the branch path 32 can be reduced and a smooth resin flow can be formed. As a result, the stirring action due to the splitting and merging can be further improved, so that the color unevenness caused by the diffused reflection of light in the high crystallization region 14 can be suppressed.

The resin (thermoplastic resin) is a crystalline resin. As a result, the highly crystallization region 14 is likely to occur, so that the effect of suppressing color unevenness due to divergence and merging becomes more remarkable.

Next, an injection molding method and an injection molding apparatus according to a modification will be described. The same components as those in the above-described embodiment are designated by the same reference numerals and the description thereof will be omitted.

<Modification 1>
FIG. 12 is a diagram showing the temperature distribution of the resin flowing in the molding mold of the injection molding apparatus according to the first modification. The molding mold according to the first modification is provided with a fan gate 240 formed so that the cross section of the flow path increases from the connecting portion 35 of the cold runner 30 toward the product portion 50 instead of the pin gate. Is different from. As shown in FIG. 12, in the injection molding apparatus according to the first modification, the core layer 18 and the shear layer 17 can be stirred and made uniform by the diversion and merging in the branch path 32, as in the above-described embodiment. As a result, the distance (depth) of the highly crystallization region 14 formed by solidifying the shear layer 17 in the product portion 50 from the surface of the resin molded product can be made uniform. As a result, it is possible to suppress color unevenness caused by diffused reflection of light in the high crystallization region 14, as in the above-described embodiment.

<Modification 2 and Modification 3>
13 and 14 are diagrams showing a modified example of the branch path of the cold runner. In the above-described embodiment, the shape of the branch path of the cold runner is assumed to be triangular (see FIG. 8), but the present invention is not limited to this, for example, as in the branch path 232 of the modification 2 shown in FIG. It may be circular, rectangular (diamond) as in the branch path 332 of Modification 3 shown in FIG. 14, or the branch path may be arranged three-dimensionally. By making it circular as in the branch path 232 of the modification 2 shown in FIG. 13, the flow direction F of the resin follows a curve, so that the generation of eddy currents and the like can be suppressed. Further, by forming a rhombus like the branch path 332 of the modification 3 shown in FIG. 14, the angle θ2 formed by the flow direction F of the resin split into two and the angle θ3 formed by the flow direction F of the merging resin. Can be 90 degrees or a sharp angle. As a result, the flow of splitting and merging of the resin can be made smoother, so that the stirring action can be further improved.

<Modification example 4>
FIG. 15 is a diagram showing a cold runner and a product unit 50 according to the modified example 4. In the above-described embodiment, only one pin gate is arranged in the product unit 50, but the number and positions of the pin gates are not particularly limited, and for example, a plurality of pin gates (for example, three places) as in the modified example 4 shown in FIG. A pin gate 440 may be provided. Further, the main flow path 431 may be branched into a plurality of (for example, three), and a branch path 432 may be provided in each of the branches. It is preferable that the number and arrangement of the pin gates 440 are appropriately selected according to the size and shape of the product unit 50 and the like.

Although the injection molding method and the injection molding apparatus according to the present invention have been described above through the embodiments and modifications, the present invention is not limited to the contents described in the embodiments and modifications, and is within the scope of claims. It can be changed as appropriate based on the description.

For example, a hot runner may be provided between the sprue and the cold runner. According to such a configuration, the resin can be transported over a long distance in a state of being melted by the hot runner. As a result, even when the distance between the sprue and the product part is long, the resin can be smoothly flowed without solidifying the resin and blocking the flow path. Further, since the amount of the resin solidified in the runner as the skin layer can be reduced, the yield can be improved.

Further, the main flow path and the combined flow path of the cold runner are said to be formed in a straight line, but the present invention is not limited to this, and the main flow path and the combined flow path may be formed in a curved line.

In addition, each configuration described in the embodiment and the modified example may be combined as appropriate.

13 skin layer,
14 High crystallization region,
17 Shear layer,
18 core layer,
20 sprue,
30 cold runners,
31, 431 Main flow path 32, 232, 332, 432 branch road,
33 combined flow path,
35 connecting part,
40, 440 pin gate,
50 Product Department,
100 injection molding equipment,
110 molding mold,
111 fixed type,
112 Movable type.

Claims (8)

In the process of filling the product part formed in the mold with the thermoplastic resin melted via the cold runner.
The flow of the thermoplastic resin flowing through the main flow path of the cold runner is divided into a plurality of branch paths and then merged .
An injection molding method in which the cross-sectional area of the flow path of each of the branch paths is larger than the cross-sectional area of the flow path of the main flow path . The injection molding method according to claim 1, wherein the thermoplastic resin that has passed through the cold runner is filled in the product portion by passing through a pin gate having a cross-sectional area smaller than that of the cold runner. The injection molding method according to claim 1 or 2, wherein when the cold runner is made to flow into the thermoplastic resin, the thermoplastic resin is made to flow linearly before and after splitting into the branch path. The injection molding method according to any one of claims 1 to 3 , wherein the thermoplastic resin is a crystalline resin. The product part formed in the molding mold and
It has a cold runner that forms a flow path for filling the product part with the molten thermoplastic resin.
The cold runner includes a main flow path, a plurality of branch paths for branching the flow of the thermoplastic resin flowing through the main flow path, and a confluence flow path for merging the flow of the thermoplastic resin flowing through the plurality of the branch paths. Have,
An injection molding apparatus having a cross-sectional area of the flow path of each branch path larger than the cross-sectional area of the flow path of the main flow path . It has a pin gate between the cold runner and the product section.
The injection molding apparatus according to claim 5 , wherein the flow path cross section of the pin gate is smaller than the flow path cross section of the cold runner. The injection molding apparatus according to claim 5 , wherein the main flow path and the combined flow path of the cold runner are formed in a straight line. The injection molding apparatus according to any one of claims 5 to 7 , wherein the thermoplastic resin is a crystalline resin.

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