JP7711566B2 - Resin molded product and method for manufacturing said resin molded product - Google Patents

Description

The present invention relates to a resin molded product and a method for manufacturing the resin molded product, and more specifically, to an injection-molded resin molded product and a method for manufacturing the resin molded product.

Injection molding is used in a wide range of fields because it is possible to produce large molded products and molded products with complex shapes, and because it requires a relatively small number of steps, it allows for low costs.

For example, large parts such as automobile bumpers and instrument panels are often made using injection molding, and for these large parts, injection materials containing added fillers are used to improve their rigidity and strength.

When injection molding large parts, the injection material is often injected from multiple gates to shorten the length that the injection material flows through the mold. Also, if the molded product has an opening, the injection material meets from two directions, forming an interface where the injection material meets within the resin molded product.

The injection material injected into the cavity flows like a fountain flow at its tip, spurting out from the fluidized layer in the center of the cavity toward the mold. If the filler is needle- or plate-shaped, the filler will be oriented upright in the thickness direction of the resin molded product at the joining interface.

Even if the molten thermoplastic resin shrinks in volume as the injected material cools and solidifies, the filler in the injected material is originally solid, so there is almost no volume shrinkage.

Therefore, if filler is oriented vertically in the thickness direction of a part, shrinkage in the thickness direction is prevented only in that area, resulting in a visual defect (secondary weld line) in which the area near the joining interface is raised.

Patent Document 1 discloses that at the resin confluence in the cavity, a part of the mold surface of the mold element that comes into contact with the injected material is rotated in situ to apply an external force to the injected material, mechanically changing the orientation of the filler and suppressing secondary weld lines.

JP 2014-172256 A

However, the method described in Patent Document 1 involves rotating the surface of the mold to apply an external force to the injected material, so the rotating part of the mold must be flat. This makes it difficult to prevent secondary weld lines from appearing on the curved surfaces of the resin molded product, and also increases the complexity of the mold, resulting in increased costs.

The present invention was made in consideration of the problems associated with the conventional technology, and its purpose is to provide a resin molded product that can prevent the occurrence of secondary weld lines without being restricted by the external shape, and a method for manufacturing the resin molded product.

As a result of extensive research into achieving the above object, the inventors discovered that by thinning a portion of a resin molded product to provide a gradually changing portion where the plate thickness gradually changes, a difference occurs in the cooling rate of the injected material on either side of the gradually changing portion, and the filler is oriented in the in-plane direction, which led to the completion of the present invention.

That is, the resin molded article of the present invention contains a thermoplastic resin and needle-like and/or plate-like filler.
The molded article has a gradual-change portion where the thickness gradually changes, a primary weld line on the surface extends near the gradual-change portion, a joining interface of thermoplastic resin extending from the primary weld line penetrates from the thick side to the thin side at the center in the thickness direction, and the filler at the joining interface is oriented such that its longitudinal direction intersects with the thickness direction of the resin molded article.

The method for producing a resin molded article of the present invention is a method for producing the above-mentioned resin molded article, and includes a step of preparing a mold and a molding step of injecting an injection material into the mold.
The step of producing the mold comprises:
A step of analyzing the flow of the injection material in the mold from a mesh model of the resin molded product and confirming the occurrence position of the primary weld line and the occurrence direction of the secondary weld line relative to the primary weld line;
A step of changing the mesh model of the resin molded product into a mesh model in which a gradually changing part whose thickness gradually changes and a thin-walled part connected to the gradually changing part are formed based on the occurrence position of the primary weld line;
A step of analyzing the flow of the injection material in the mold from the changed mesh model and confirming whether or not a primary weld line occurs in the gradually changing portion;
The method includes a process of repeatedly performing a step of changing the mesh model and a step of checking whether a primary weld line occurs in the gradual change portion, and creating a mold based on a mesh model in which a primary weld line occurs in the gradual change portion.

According to the present invention, a part of a resin molded product is thinned to provide a gradually changing part whose thickness gradually changes, and a difference in the cooling rate of the injected material is created on either side of the gradually changing part, thereby orienting the filler in the in-plane direction. This makes it possible to provide a resin molded product and a method for manufacturing a resin molded product that suppresses the occurrence of secondary weld lines without being restricted by the external shape.

1 is a cross-sectional view showing a main part of an example of a resin molded product of the present invention. FIG. 2 is a cross-sectional perspective view of a main portion illustrating a joining interface of a resin molded product having a constant thickness. FIG. 2 is a cross-sectional view illustrating the state of pressure applied to a joining interface of the resin molded product of the present invention. FIG. 2 is a cross-sectional view of a main part illustrating the state of a solidified portion and a fluidized layer in a gradually changing portion of the resin molded product of the present invention. FIG. 2 is a cross-sectional view of a main portion illustrating the penetration state at the joining interface of the resin molded product of the present invention and the orientation state of the filler. 1 is a flowchart of a mold manufacturing process. 1 is a cross-sectional view of a main portion showing an example of a penetration state at a joining interface and an orientation state of a filler. FIG. 1 is a graph showing the relationship between the thickness of the thick-walled portion and the height of the secondary weld line in an example and a comparative example. FIG. 2 is a schematic diagram showing the orientation state of fillers present in the vicinity of the joining interface in an example and a comparative example.

The resin molded article of the present invention will now be described in detail.
The resin molded article of the present invention contains a thermoplastic resin and needle-like and/or plate-like fillers, and has a gradually changing portion whose thickness gradually changes, and has a thick portion and a thin portion sandwiching the gradually changing portion, as shown in FIG.

The gradual change portion is provided near the location where the injected material joins during injection molding. Therefore, since the injected material joins near the gradual change portion, a primary weld line extends along the gradual change portion on the surface of the resin molded product near the gradual change portion.

If necessary, the resin molded product may further have a gradually changing portion on the opposite side of the thin-walled portion to the gradually changing portion, and a thick-walled portion connected to this may be formed.

These optional gradual change sections are provided from the perspective of the strength balance of the resin molded product resulting from the formation of the thin-walled sections, and are not provided to prevent the occurrence of secondary weld lines, so there is no need for primary weld lines to extend near all of the gradual change sections of the resin molded product.

In the present invention, "the vicinity of the gradually changing portion" refers to the combined range of the entire gradually changing portion, the range from the boundary between the thick-walled portion and the gradually changing portion to 10 mm toward the thick-walled portion, and the range from the boundary between the thin-walled portion and the gradually changing portion to 10 mm toward the thin-walled portion.

In a typical resin molded product with a constant thickness, the cooling rates of the flow fronts of the merged injected material are the same, so the merged injected material stops flowing at almost the same time.
If the pressure at the merging interface is uneven, a bow-shaped merging interface is formed, as shown by the dotted line in Figure 2, and the filler near the merging interface is oriented upright in the thickness direction of the resin molded product, forming a secondary weld line at a position shifted from the primary weld line.

The resin molded product of the present invention has different thicknesses on either side of the gradual change section, so when the injection material joins the gradual change section from the in-plane direction of the resin molded product, the thinner injection material cools first and stops flowing, while the thicker injection material still retains fluidity.

Therefore, at the joining interface during the pressure holding process, a pressure difference occurs, as shown by the size of the arrow in Figure 3, and pressure is applied from the thicker injection material to the thinner injection material.

In addition, when viewed in the out-of-plane direction of the resin molded product, the injected material is cooled by contacting the mold, so even if the injected material on the mold side solidifies, the injected material in the center retains fluidity, as shown in Figure 4.

And because pressure is applied from the thicker side to the thinner side at the merging interface as described above, as shown in Figure 5, the injected material penetrates from the thicker side to the thinner side at the center of the plate thickness, forming a merging interface with the tip bent at an acute angle.

The filler near this merging interface is oriented along the merging interface that is bent at an acute angle, so its longitudinal direction is oriented in a direction that intersects with the plate thickness direction of the resin molded product.

As a result, the proportion of filler in the thickness direction of the joining interface is reduced, and the difference in shrinkage between the vicinity of the joining interface and other areas due to the volumetric shrinkage of the thermoplastic resin is reduced, so the entire injected material shrinks uniformly, preventing the occurrence of secondary weld lines.

It is preferable that the filler near the above-mentioned merging interface has few fillers oriented in a vertical direction where the angle between its longitudinal direction and the thickness direction of the resin molded product is less than 30°, and the thickness of the region where the above-mentioned vertical fillers are present is preferably 0.7 mm or less.

The filler present near the above-mentioned joining interface can be confirmed by observing a cross-sectional image of the resin molded product. In the present invention, the filler present near the joining interface was observed, and the thickness of the region where the above-mentioned standing filler was present was measured.

The penetration length of the above-mentioned merging interface, i.e., the length from the root where the curvature of the merging interface changes suddenly in the center of the plate thickness direction to the tip of the penetration, is preferably 2 mm or more.

The surface side of the resin molded product is made smooth and only the back side is inclined to form the gradual change portion, so that no change in appearance occurs due to the formation of the gradual change portion, and appearance defects due to secondary weld lines can be prevented.

Furthermore, since the resin molded product of the present invention has a curved merging interface, even if stress is applied near the merging interface, this stress is dispersed at the curved merging interface, preventing a decrease in strength near the merging interface compared to when the merging interface is flat.

In the present invention, "the longitudinal direction of the filler is oriented in a direction intersecting the plate thickness direction of the resin molded product" means that the filler at the joining interface is inclined relative to the plate thickness direction to an extent that the occurrence of secondary weld lines can be prevented, and does not mean that there is no filler at all facing the plate thickness direction at the penetration tip. Also, the cross-sectional views in this specification only show the filler near the surface of the resin molded product and near the joining interface.

The above-mentioned gradual change portion depends on the thickness of the resin molded product, but for example, if the thickness of the resin molded product is 1.5 to 5 mm, it is preferable that the amount of change in thickness is 0.5 mm to 1.5 mm, and the length in the direction in which the thickness changes is preferably 30 mm or more.

By gradually changing the thickness of the resin molded product rather than suddenly, the fluid part in the center of the thickness direction remains wedge-shaped as the injected material solidifies, so the joining interface penetrates deeply toward the thinner side and its tip forms a sharp angle.

Therefore, the filler near the joining interface is oriented so that its longitudinal direction lies flat in a direction approximately perpendicular to the thickness direction, preventing appearance defects due to secondary weld lines.

The above-mentioned thermoplastic resin may be any conventional resin used in injection molding, such as polypropylene resin, polyamide resin, thermoplastic urethane resin, thermoplastic polyester resin, acrylic resin, polyolefin resin, etc.

The filler may be needle-like or plate-like fillers such as talc, mica, graphite, wollastonite, glass flakes, barium sulfate, or calcium carbonate, as well as PAN- or pitch-based carbon fibers and glass fibers. The filler content in the resin molded product is preferably 10 to 30% by mass.

Next, a method for manufacturing a resin molded product capable of preventing the occurrence of secondary weld lines will be described.
As described above, the formation of a joining interface near the gradual change portion can prevent the occurrence of secondary weld lines, so in the present invention, a mold is used in which a gradual change portion is formed near the position where the joining interface is formed during injection molding. The manufacturing procedure for the mold is shown in Figure 6.

Specifically, a mesh model is created from CAD shape data of a resin molded product of the desired shape, allowing for flow analysis of the injected material using the finite element method, finite volume method, etc.

The flow of the injected material within the mold is analyzed using this mesh model, injection molding conditions such as injection pressure and gate position, and physical property data of the injected material, and the analysis data is used to confirm the location of the primary weld line and the direction of the secondary weld line relative to the primary weld line.

Then, based on the location of the primary weld line and the direction of the secondary weld line, the mesh model is changed to a mesh model that has a gradual change section near the location of the primary weld line.

At this time, by setting the position of the gradual change portion so that a thin-walled portion connected to the gradual change portion is formed on the side where the secondary weld line occurs, the injected material on the thin-walled portion side will lose fluidity first, making it easier for a joining interface to form near the gradual change portion.

Depending on the gate position and the shape of the resin product, the pressure from the thin-walled section may be higher than that from the thick-walled section, and the position of the gradual change section can be set so that a thin-walled section connected to the gradual change section is formed on the opposite side to the side where the secondary weld line occurs.

Then, based on the modified mesh model having the above-mentioned gradual change portion, the flow of the injected material in the mold is analyzed in the same manner, and it is confirmed whether or not a primary weld line occurs in the above-mentioned gradual change portion.

If the primary weld line does not appear in the set gradual change area, the mesh model is changed again to one that changes the position and length of the gradual change area and the thickness of the thin-walled area, and the flow of the injected material is analyzed. The mesh model is repeatedly changed until the primary weld line appears in the gradual change area.

Then, if a primary weld line occurs in the gradual change portion, a mold is created based on a mesh model in which this primary weld line occurs in the gradual change portion.

By using this prepared mold and performing injection molding under the injection molding conditions for which the above-mentioned flow analysis was performed, a primary weld line is formed near the gradual change section, and a resin molded product of the present invention can be obtained in which the filler at the joining interface is oriented in a direction that intersects the thickness direction.
FIG. 7 shows an example of the shape of the tip of a joining interface formed in the vicinity of the joining interface of a resin molded product.

In addition, the flow analysis of the injection material uses a mesh model to simulate the flow of the injection material, and does not completely match the flow state of the injection material when actually producing a resin molded product.
Therefore, the occurrence of a primary weld line in a gradual change portion in a mesh model does not mean that a resin molded product in which a primary weld line is formed within the gradual change portion is obtained; in the resin molded product, a primary weld line is formed in the vicinity of the gradual change portion.

This method of manufacturing resin molded products eliminates the need to provide a mover or valve gate inside the mold, which reduces the cost of the mold, and eliminates the need to control these moving mechanisms during molding, simplifying the molding conditions and improving robustness.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples.

[Example]
The cavity was set up so that a 1.5 mm-thick thin-walled section was provided in the center of a cavity 320 mm long and 80 mm wide, with a 60 mm-long gradual-change section on one side of this thin-walled section and a 30 mm-long gradual-change section on the other side, and thick-walled sections 2.5 mm thick were formed on both ends so that the change in thickness of these gradual-change sections was 1.0 mm.

Plate-shaped polypropylene resin containing 20% by weight of talc was injected from both ends of the cavity and merged in the center of the gradual change section, which was 60 mm in length, to obtain a plate-shaped resin molded product.

The amount of change in thickness of the gradually changing portion was fixed at 1.0 mm, and the thickness of the thin-walled portion and the thick-walled portion were varied to obtain a plate-shaped resin molded product.

[Comparative Example]
A plate-shaped resin molded product was obtained in the same manner as in Example 1, except that no gradually changing portion was provided and a cavity having a constant thickness of 1.5 mm was set.
Similarly, a plate-shaped resin molded product was obtained by changing the thickness without providing a gradually changing portion.

The cross sections of the resin molded articles of the example and the comparative example were observed.
In the resin molded product of the Example, the filler particles near the joining interface were oriented in a direction intersecting the plate thickness direction, whereas in the resin molded product of the Comparative Example, the filler particles near the joining interface were oriented upright in the plate thickness direction.

Figure 8 shows the relationship between plate thickness and the amount of rise of the secondary weld line for the example and comparative example, and Figure 9 shows a schematic diagram of the orientation state of the filler near the gradual change part for the example and comparative example.

When the plate thickness was 2.5 mm, the thickness (L) of the region where the filler existing near the merging interface is present and the angle between the longitudinal direction of the filler and the thickness direction of the resin molded product is less than 30° was 0.2 mm for the resin molded product of the example and 1.6 mm for the resin molded product of the comparative example.
In addition, all of the examples in which the plate thickness was varied were 0.7 mm or less, but the comparative examples were over 0.7 mm.

From Figure 8, it can be seen that the amount of rise of the secondary weld line in the resin molded product of the present invention is suppressed to about one-third of that in the resin molded product of the comparative example, and the occurrence of appearance defects caused by the secondary weld line can be prevented.

REFERENCE SIGNS LIST 1 resin molded product 2 gradual change portion 3 thin wall portion 4 thick wall portion 5 joining interface 51 penetration portion 6 primary weld line 7 secondary weld line 8 filler 9 solidified portion 10 fluidized layer

Claims (7)

A resin molded product containing a thermoplastic resin and a needle-like and/or plate-like filler,
The thickness of the plate is gradually changed.
A primary weld line on the surface extends near the gradually changing portion,
The joining interface of the thermoplastic resin extending from the primary weld line penetrates from the thick side to the thin side at the center in the thickness direction,
A resin molded article, wherein the filler at the joining interface is oriented such that its longitudinal direction intersects with the thickness direction of the resin molded article. The resin molded product according to claim 1, characterized in that the filler present near the joining interface has an angle of less than 30° between its longitudinal direction and the thickness direction of the resin molded product, and the thickness of the region where the filler is present is 0.7 mm or less. The resin molded product according to claim 1 or 2, characterized in that the amount of change in thickness in the gradual change portion is 0.5 mm to 1.5 mm. The resin molded product according to any one of claims 1 to 3, characterized in that the length of the gradually changing portion in the direction in which the thickness changes is 30 mm or more. The resin molded product according to any one of claims 1 to 4, characterized in that the thickness of the thick-walled portion connected to the gradually changing portion is 1.5 to 5 mm. A method for producing a resin molded product according to any one of claims 1 to 5, comprising the steps of:
The method includes a step of preparing a mold and a molding step of injecting an injection material into the mold,
The step of producing the mold comprises:
A step of analyzing the flow of the injection material in the mold from a mesh model of the resin molded product and confirming the occurrence position of the primary weld line and the occurrence direction of the secondary weld line relative to the primary weld line;
A step of changing the mesh model of the resin molded product into a mesh model in which a gradually changing part whose thickness gradually changes and a thin-walled part connected to the gradually changing part are formed based on the occurrence position of the primary weld line;
A step of analyzing the flow of the injection material in the mold from the changed mesh model and confirming whether or not a primary weld line occurs in the gradually changing portion;
A manufacturing method for a resin molded product, comprising a process of repeatedly performing a step of changing the mesh model and a step of checking whether a primary weld line occurs in the gradual change portion, and creating a mold based on a mesh model in which a primary weld line occurs in the gradual change portion. The method for manufacturing a resin molded product according to claim 6, characterized in that the step of modifying the mesh model includes a process for forming a thin-walled portion on the side where the secondary weld line occurs.