JP7587227B2 - Two-tone molding - Google Patents

Description

This invention relates to two-color molded products.

Conventionally, there is a so-called two-color molding method in which two types of molten molding resin with different colors, materials, etc. are sequentially injection molded to produce a two-color molded product in which two different types of molded resin parts are joined (Patent Document 1). In addition, there is a method for obtaining a decorated two-color molded product, in which a decorated sheet with a printed pattern or the like is sandwiched between a base sheet by closing the mold, and the molten molding resin is injection molded while the decorated sheet is bonded to the injection molded product at the same time, known as a two-color molding simultaneous decorating method.

Conventionally, a second cavity mold is used in which a decorative sheet is not supplied during secondary molding. For this reason, a decorative sheet can be attached to the surface of the primary molded resin part of a two-color molded product, but not to the surface of the secondary molded resin part. In order to form a pattern on the surface of the secondary molded resin part, the only option is to apply screen printing or the like to the surface of the secondary molded resin part of the two-color molded product in a separate process after the two-color molding process. A screen-printed pattern is far inferior to a pattern on a decorative sheet in terms of the number of colors and the precision of the pattern. In addition, it is extremely difficult to accurately align and screen print the pattern on the decorative sheet that is already attached to the surface of the molded resin part. Therefore, production efficiency is poor because a separate process is required in addition to the two-color molding process. Furthermore, the surface of the two-color molded product will have both the pattern on the decorative sheet and the pattern on the screen printing, and only two-color molded products with an uneven appearance due to the uneven number of colors and precision of the pattern can be produced. Furthermore, the pattern on the surface of the primary molded resin part and the pattern on the surface of the secondary molded resin part are not accurately aligned, so the only products that can be produced are those that are painted in two colors and look bad.

In order to provide a two-color molded product with an attractive pattern, the inventor proposed a two-color molded product consisting of an inner part made of a crystalline plastic material and an outer part made of an amorphous plastic material (Patent No. 6608690). According to the two-color molded product invention previously proposed by the inventor, the inner part is made of a high-shrinkage crystalline plastic material and the outer part is made of a low-shrinkage amorphous plastic material, so that when cooled, the crystalline plastic of the inner part shrinks more than the amorphous plastic of the outer part. As a result, a small gap is formed between the material of the inner part and the material of the outer part, so that if a decorative pattern is applied to the material of the inner part, the decorative pattern of the material of the inner part can be clearly seen due to the difference in refractive index between the material of the inner part and the material of the outer part.

JP 2012-126079 A

The two-color molded product proposed by the present inventor can be used for various purposes, for example, as a container for storing medicinal liquids (liquid medicines such as eye drops, liquid quasi-drugs, liquid cosmetics, etc.). It is preferable that two-color molded products storing such medicinal liquids have excellent durability. In this respect, the two-color molded product previously proposed by the present inventor can form an attractive pattern, but when used as a container for eye drops, for example, cracks may occasionally occur on the outer part when too much force is continuously applied to the fastening when the container is opened and closed, or the outer part may occasionally be damaged when a large impact is temporarily applied, such as when the container is dropped while being carried, so there is room for improvement in terms of durability.

The present invention was made in consideration of the improvements in the conventional technology, and its purpose is to provide a two-color molded product with excellent durability.

The present invention relates to a two-color molded product consisting of an inner part of the primary injection molded product obtained by injection molding a first synthetic resin, and an outer part of a secondary injection molded product obtained by injection molding a second synthetic resin so as to cover the primary injection molded product, the inner part being the primary injection molded product and the outer part being the secondary injection molded product, in which the first synthetic resin is a crystalline synthetic resin, and the second synthetic resin is a polyester having a spiral flow length of 200 mm or more at 200°C.

The spiral flow length is the flow length in Archimedes spiral flow when injection molding is performed using the following mold under the following measurement conditions.
The mold used had a resin injection port at the center and an Archimedes spiral curved groove with constant groove spacing starting from the resin injection port. The spiral shape was 10 mm wide, 2 mm thick, and had a maximum flow length of 1,300 mm.
The measurement conditions were an injection pressure of 100 MPa, an injection time of 10 seconds, an injection speed of 50 mm/second, a mold temperature of 50°C, and a molding temperature of 200°C.
The Archimedes spiral is a type of algebraic spiral, and is a curve expressed by the polar equation r=aθ (a is a constant) in polar coordinates (r, θ). The Archimedes spiral is a spiral in which the lines are spaced equally apart.

Generally, thermoplastic resins are in a solid state at temperatures below their melting point, but when heated above their melting point, they melt and become fluid. Representative methods for expressing the fluidity (viscosity) of thermoplastic resins when molten include melt flow rate, methods using measuring instruments such as capillary rheometers, and flow length evaluations using actual injection molding machines. In general, melt viscosity characteristics show a tendency to depend on the molecular weight of the polymer, but in reinforced grades and elastomer-modified grades, the fluidity does not necessarily correlate with the molecular weight because it is affected by factors such as the content of reinforcing materials.

<Melt flow rate>
Melt flow rate is an index of fluidity expressed by converting the amount of sample (between the marked lines) extruded from an orifice by applying a certain weight to a molten sample in a cylinder (heated barrel) into weight per 10 minutes (unit: g/10 minutes). Under the same cylinder temperature and load conditions, a material with a higher melt flow rate has better fluidity. However, since thermoplastic resins are generally non-Newtonian fluids in which the shear stress of the flow and the flow velocity gradient are not proportional, when the apparent shear rate changes significantly due to molding conditions or molded product shape, such as in injection molding, the relationship between the fluidity during molding and the melt flow rate may not match.

<Capillary rheometer>
The capillary rheometer is a test method in which a sample molten in a cylinder is extruded through a capillary, similar to the melt flow rate. The difference from the melt flow rate is that it can be calculated as melt viscosity (unit: Pa sec) instead of resin weight per unit time. However, melt viscosity depends on shear rate and temperature, so when using melt viscosity characteristics to judge fluidity, it is cumbersome to confirm the appropriate shear rate range according to the molding method and shape.

<Flow length>
The flow length is a method for evaluating the flowability using an actual injection molding machine and mold. Since the flow length depends on the molding conditions and shape, the flowability can be relatively judged by keeping the molding temperature, mold temperature, injection pressure, injection speed, and injection time constant. In addition, the flow length is an index of flowability that takes into account not only the melt viscosity but also the solidification characteristics of the material, and is more practical than the melt flow rate and capillary rheometer mentioned above. A long spiral flow length at 200°C indicates that the resin has high flowability at 200°C, which is a relatively low molding temperature, and that the resin can be used under a wide range of molding conditions and can be adapted to various shapes.

The present invention provides a two-color molded product with excellent durability, as described below, by using a crystalline synthetic resin as the first synthetic resin in the inner portion and a polyester as the second synthetic resin in the outer portion, the spiral flow length of which is 200 mm or more at 200°C.

Even though they are called crystalline synthetic resins, not all of them are crystalline; they contain a mixture of crystalline and non-crystalline parts. The proportion of the crystalline parts is called the degree of crystallinity, and those with a relatively high degree of crystallinity are crystalline synthetic resins. Examples of crystalline synthetic resins include polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyamide, polyacetal, polyphenylene sulfide, polyether ether ketone, and polytetrafluoroethylene.

Crystalline synthetic resins have the following characteristics: "They have a glass transition temperature and melting point. They are difficult to make transparent. Volume changes occur due to crystallization (large shrinkage during molding). They are difficult to achieve dimensional precision. They have excellent fatigue resistance. They have excellent mechanical strength. They have excellent chemical resistance. They have excellent sliding properties. They are highly reinforcing due to additives. They have excellent mechanical properties. They are suitable for functional parts. They have excellent rigidity and springiness." By constructing the inner part of the present invention from a crystalline synthetic resin, the following effects can be expected.

Crystalline synthetic resins shrink, reducing in volume, when they change from liquid to solid. The ratio of this shrinkage is called the shrinkage rate. In other words, it is a numerical value defined as: shrinkage rate = [(length before shrinkage) - (length after shrinkage)] / (length before shrinkage). Table 1 below shows the numerical values for the shrinkage rates of representative crystalline plastics.

By using a high shrinkage material of crystalline synthetic resin for the inner part, the crystalline synthetic resin of the inner part shrinks more than the second synthetic resin of the outer part when cooled. As a result, a small gap is formed between the material of the inner part and the material of the outer part, so that if a decorative pattern is applied to the material of the inner part, the decorative pattern of the material of the inner part can be clearly seen from the outside due to the difference in refractive index between the material of the inner part and the material of the outer part. Therefore, polypropylene, high density polyethylene, polyacetal, and polybutylene terephthalate, which have a large shrinkage rate, can be particularly preferably used as the crystalline synthetic resin of the inner part in Table 1. The difference in shrinkage rate between the crystalline synthetic resin and the second synthetic resin (the value obtained by subtracting the shrinkage rate of the second synthetic resin from the shrinkage rate of the crystalline synthetic resin) is preferably 3/1000 or more. If this shrinkage rate difference is less than 3/1000, the difference in refractive index between the material of the inner part and the material of the outer part is small, so that the decorative pattern of the material of the inner part cannot be clearly seen. It is preferable for the difference in shrinkage rate between the crystalline synthetic resin and the second synthetic resin to be large, as this allows the decorative pattern of the material on the inside to be more clearly visible, but judging from the materials that are actually available, the upper limit for the difference in shrinkage rate between the crystalline synthetic resin and the second synthetic resin is approximately 50/1000.

The second synthetic resin can be injection molded to cover the primary injection molded product at a temperature that does not melt the primary injection molded product, and a polyester with a spiral flow length of 200 mm or more at 200°C can be used. The spiral flow length is more preferably 250 mm or more at 200°C, and even more preferably 300 mm or more at 200°C. The polyester is not particularly limited, but examples include polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc., and blends of these can also be used. Among these, polyethylene terephthalate is preferable. By adopting a polyester with a spiral flow length of 200 mm or more at 200°C as the second synthetic resin for the outer part of the present invention, a two-color molded product with high fluidity, a wide range of molding conditions, adaptability to various shapes, and excellent durability can be provided.

The present invention makes it possible to provide two-color molded products with excellent durability.

FIG. 1(a) is a vertical cross-sectional view of the inner part of one embodiment of a two-color molded product of the present invention, FIG. 1(b) is a vertical cross-sectional view of the outer part of one embodiment of a two-color molded product of the present invention, and FIG. 1(c) is a vertical cross-sectional view of one embodiment of a two-color molded product of the present invention. FIG. 2 is a photograph showing the results of a compression test on a two-color molded product of an embodiment in which polyethylene terephthalate (spiral flow length at 200° C.: 320 mm) is used for the outer portion. FIG. 3 is a photograph showing the results of a compression test on a two-color molded product of a comparative example in which acrylonitrile styrene was used for the outer portion.

The present invention will be described in detail below based on the embodiments. Note that the embodiments described below are merely examples, and the present invention is not limited to the embodiments. Various changes and modifications are possible without departing from the technical scope of the present invention.

The two-color molded product of the present invention can be obtained by injection molding using first and second fixed molds and a movable mold. Figure 1(a) is a diagram showing a vertical cross section of an inner part 1 obtained by performing a primary injection molding using a crystalline synthetic resin in a first cavity defined by a first fixed mold and a movable mold. Then, the primary injection molding part is moved together with the movable mold, and a secondary injection molding is performed in a second cavity defined by a second fixed mold and a movable mold using a polyester with a spiral flow length of 200 mm or more at 200°C, and a two-color molded product 3 in which the outer part 2 and the inner part 1 are integrated can be obtained as shown in Figure 1(c).

As shown in FIG. 1(a), an undercut portion 4 that protrudes in the radial direction is formed at the lower end of the inner portion 1, so that the outer portion 2 is held in the undercut portion 4 of the inner portion 1 as shown in FIG. 1(c), and a good fit can be maintained.

<Compression test>
In order to evaluate the strength of the two-color molded article of the present invention, a compression test was carried out.
As an example, polypropylene was used for the inner part 1 and easily moldable polyethylene terephthalate (spiral flow length at 200°C: 320 mm) was used for the outer part 2, and injection molding was performed as described in paragraph 0024 to prepare a two-color molded product with a predetermined decorative pattern on the outside of the inner part 1.

As a comparative example, polypropylene was used for the inner part 1 and acrylonitrile styrene for the outer part 2, and by performing injection molding as described in paragraph 0024, a two-color molded product was prepared in which a predetermined decorative pattern was applied to the outside of the inner part 1.

The two-color molded products of the examples and comparative examples were examined for the state of damage to the caps when a load of 180 N was applied at a speed of 100 mm/min using a tension and compression tester ("TRAPEZIUM LITE X" manufactured by Shimadzu Corporation). The results are shown in the photographs in Figures 2 and 3.

As shown in Figure 3, in the two-color molded product of the comparative example in which acrylonitrile styrene was used for the outer part, numerous cracks in the form of white stripes running diagonally and horizontally were observed in the outer part which appears gray. However, as shown in Figure 2, in the two-color molded product of the example in which polyethylene terephthalate (spiral flow length at 200°C of 320 mm) was used for the outer part, no cracks like those seen in Figure 3 were observed. In this way, it was confirmed that the two-color molded product of the present invention has remarkably high durability.

As explained above, the two-color molded product of the present invention is useful as a container or a part of a container (e.g., a lid) for storing cosmetics, medicines, food, and various other contents.

1 Inner part 2 Outer part 3 Two-color molding 4 Undercut part

Claims (5)

The two-color molded product is composed of an inner part of the primary injection molded product obtained by injection molding a first synthetic resin, and a secondary injection molded product obtained by injection molding a second synthetic resin so as to cover the primary injection molded product, and is composed of an outer part of the secondary injection molded product, wherein the first synthetic resin is a crystalline synthetic resin, and the second synthetic resin is a polyester having a spiral flow length of 200 mm or more at 200°C during injection molding using the following mold and measurement conditions, and the difference in shrinkage rate between the crystalline synthetic resin and the second synthetic resin is 3/1000 or more and 50/1000 or less .
The mold used had a resin injection port at the center and an Archimedes spiral curved groove with constant groove spacing starting from the resin injection port. The spiral shape was 10 mm wide, 2 mm thick, and had a maximum flow length of 1,300 mm.
The measurement conditions were an injection pressure of 100 MPa, an injection time of 10 seconds, an injection speed of 50 mm/second, a mold temperature of 50°C, and a molding temperature of 200°C. The two-color molded product according to claim 1, wherein the crystalline synthetic resin is any one of polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyamide, polyacetal, polyphenylene sulfide, polyether ether ketone, and polytetrafluoroethylene. The two-color molded product according to claim 1 or 2, wherein the polyester contains polyethylene terephthalate. The two-color molded product according to any one of claims 1 to 3, wherein the secondary injection molded product is transparent. The two-color molded product according to any one of claims 1 to 4, in which a decorative pattern is applied to the outside of the primary injection molded product.