JPS6258287B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an injection molded product made of rubber-reinforced polystyrene resin and having a good appearance. Polystyrene resin (hereinafter abbreviated as PS) is an amorphous resin with excellent moldability, rigidity of the molded product,
It has many characteristics such as excellent strength, toughness, dimensional accuracy, and dimensional stability, and can be easily colored.
For this reason, they are widely used as materials related to our daily lives, such as housings for electrical equipment such as televisions, radios, and lighting equipment, and daily miscellaneous goods. Among them, rubber-reinforced PS, commonly known as HIPS (high-impact polystyrene) and MIPS (medium-impact polystyrene), etc., have the effective reinforcement effect of rubber and have many of the characteristics of PS mentioned above.
Widely used. However, because of the presence of reinforcing rubber, when rubber-reinforced PS is molded, especially injection molded, when the PS flows inside the mold, the rubber in the resin deforms, and the deformation of the rubber causes When the rubber contacts the mold and is cooled, the deformed rubber will be exposed on the resin mold contact surface, that is, the surface of the injection molded product, or the deformed rubber will cause rough skin, flow marks, silver streaks, etc. on the surface of the injection molded product. Poor appearance and L/t (resin flow distance in injection molded products, ratio of L to average wall thickness t of molded products) =
In molded products of 10 to 20 or more, a difference occurs in the gloss of the molded product surface between the start of flow of the resin (gate part) and the end of flow of the resin (dead end part).
A so-called gloss gradient occurs. These drawbacks have been commercialized even though they are unsatisfactory, based on the recognition that they cannot be prevented in principle considering the principles of injection molding described below. Furthermore, due to these drawbacks, the use of plastic molded products that require extremely high gloss has been abandoned. Reconsidering the principles of injection molding of thermoplastic resins, mainly PS, in order to explain in more detail, in general, injection molding of thermoplastic resin molded products utilizes the plasticity of the thermoplastic resin, in other words, screws, etc. The basic principle is to obtain a molded product by heating, fluidizing, and shaping thermoplastic resin, and then cooling and solidifying it in a mold. That is, in order to solidify and release the molded product from the mold, the thermoplastic resin is cooled below the heating deformation temperature and taken out of the mold. Therefore, the mold is generally maintained at a temperature lower than the heating deformation temperature. Furthermore, in order to increase productivity, a refrigerant is used to cool the mold to a temperature on the verge of condensation.
In other words, in current injection molding, the mold is cooled, and even when heating and storing heat at the temperature of the molten resin, the mold temperature is in principle controlled so as not to exceed the heating deformation temperature of the thermoplastic resin. and mold. In other words, when the mold surface and thermoplastic resin come into contact, the thermoplastic resin is rapidly cooled at the contact surface, and the fluidity of the thermoplastic resin becomes extremely poor, resulting in poor adhesion of the thermoplastic resin to the mold surface, resulting in poor molding. The surface of the product is extremely uneven. In addition, in the case of HIPS, as mentioned above, when HIPS flows in the mold, the rubber in the resin is deformed, and the deformation of the rubber comes into contact with the mold and is cooled, so that the mold contact surface of the resin, That is, the deformed rubber is exposed on the surface of the resin injection molded product, or the surface of the injection molded product becomes uneven due to the influence of the deformed rubber, resulting in the aforementioned poor appearance. When using these resins for molded products that require a beautiful appearance, methods such as painting, pasting a film, or attaching a thin film material to the surface of the thermoplastic resin molded product are used. This method has drawbacks such as not being able to obtain a molded product with an appearance that has the original soft luster, and also requiring time and effort to manufacture, making it expensive. Furthermore, in the case of painting, the paint component (thinner) used to form a paint film dissolves the thermoplastic resin, reducing mechanical strength such as impact strength. Furthermore, when attaching a film, if the shape of the molded product is complex, it is almost impossible in terms of industrial production to cover the entire surface of the molded product. Buffing is also sometimes used as a method of partially modifying the appearance of a molded product and imparting gloss to it. However, only minor appearance defects can be corrected by buffing, and it requires the skill of an expert to obtain a uniform appearance. In addition, even if a person is an expert, the molded product will still have scratches caused by buffing (bubble scratches). Furthermore, if a molded product has unevenness or grooves, even if it is buffed, it will not be possible to obtain a molded product that shines uniformly. If buffing is applied forcefully, inconveniences will occur, such as the corners of the molded product being rounded. Many studies have been conducted to improve the appearance of reinforced PS injection molded products, especially the gloss of the molded products mentioned above. In principle, the gloss decreases. That is, HIPS has not yet been able to obtain a resin composition with excellent gloss and impact resistance. In order to solve this problem, the present inventors formed a layer in which the rubber is less deformed near the surface of the rubber-reinforced PS molded product. We succeeded in obtaining a rubber-reinforced PS injection molded product with a good appearance and no appearance defects such as silver streaks. Rubber-reinforced PS injection molded products with this good appearance can be applied not only to molded products with simple shapes, but also to injection molded products with complex grid-like shapes. , it is preferable to form a skin layer with little orientation. That is, the present invention provides an injection molded product made of rubber-reinforced polystyrene resin containing a rubber component of 4% by weight or more, in which a layer of at least 1 to 100 microns of rubber with little deformation is provided near the surface of the molded product substantially at the bonding interface. It is integrally formed during injection molding without having any
80% or more, preferably 90% or more, more preferably
We provide rubber-reinforced polystyrene resin injection molded products with a gloss of 95% or more. In addition, the injection molded product of the present invention not only has a good gloss on the surface of the molded product, but also eliminates the appearance of rubber on the surface, which is a drawback of rubber-reinforced PS molded products, and weld lines such as silver streaks and flow marks. This is a rubber-reinforced PS injection molded product with a good appearance and no appearance defects caused by the flow or uneven flow of the reinforced PS during injection molding. In other words, the surface of the injection molded product is smooth and has the soft luster characteristic of thermoplastic resins, and there are no appearance defects such as silver streaks or flow marks, and weld lines are virtually invisible. It is a rubber reinforced PS injection molded product. Furthermore, in the case of conventional rubber-reinforced PS injection molded products, based on the above-mentioned principle, for molded products with L/t = 10 to 20 or more, the gloss of the gate part and the gloss of the gate end part (flowing end part) There is a difference, the so-called gloss gradient is 1-5%/cm
There are many cases. However, in the case of the injection molded article of the present invention, the gloss gradient is 0 to 0.5% cm, preferably 0.
It is an injection molded product having a uniform and high gloss without a gloss gradient, which is extremely small at ~0.2%/cm, more preferably within 0 to 0.1%/cm, which is unimaginable for conventional injection molded products. Further, in normal injection molding, a weld line is a confluence line that occurs at a confluence point of resin, and when a vertical cross section of this line is observed under a microscope, it forms a recess with a width of 10 microns or more and a depth of 3 to 5 microns or more. In the present invention, "the weld line is substantially invisible" means that the line-like appearance is not visible in a recess with a width of 5 μm or less and a depth of 1 μm or less. There are various possible causes of flow marks. For example, when there is a variation in the wall thickness of a molded product, the flow of resin is disturbed or pressure is transmitted unevenly in that area, and the uneven resin is cooled and solidified on the mold surface, resulting in uneven appearance. However, the molded product of the present invention does not have such flow marks. Silver streaks are caused by volatile substances in the resin being volatilized during molding, and silver streaks are created on the surface of the molded product when the resin is cooled and solidified during the volatilization.
The molded article according to the present invention does not have the silver streak. Jetting is a phenomenon that is often seen in the gate area of molded products. During injection molding, the flow rate of resin increases at areas where the resin flow path is narrowed, such as at the gate area, so the resin partially protrudes inside the mold. However, the product of the present invention does not have such jetting. In the product of the present invention, the appearance defects caused by the non-uniformity of the resin flow state in the mold are not cooled and solidified while the resin flow state is non-uniform. This makes it possible to produce molded products with uniform gloss. The method for obtaining the injection molded article of the present invention is as follows. In order to improve the adhesion between the resin and the mold, the surface of the mold is maintained at a temperature higher than the heating deformation temperature of the rubber-reinforced PS, thereby molding while maintaining its plasticity. On the other hand, it is impossible to release the mold from the mold while keeping the surface of the mold above the heating deformation temperature of the rubber-reinforced PS, and in order to obtain the desired molded product without deformation, the mold must be cooled. The molded article is cooled to a temperature lower than the heating deformation temperature of the thermoplastic resin and released from the mold in a solidified state. For this heating and cooling, the principle of high-frequency induction heating is utilized, and by selectively heating the surface layer of the mold, the injection molded product of the present invention can be obtained by rapidly heating and rapidly cooling the mold surface. Next, the molded product according to the present invention and its molding method will be explained with reference to the drawings. In injection molding of rubber-reinforced PS, a high-frequency induction heating inductor is installed between the stationary mold and the movable mold, as shown in Figure 1. When an inductor is inserted between the movable mold and the stationary mold and a high frequency is oscillated in the sandwiched state, the mold surface (point A and point B) appears as shown in Figure 2.
The temperature rises rapidly only inside the mold (point C and D).
It can be confirmed that there is almost no temperature rise due to high-frequency induction heating. In the case of the example shown in FIG. 2, the mold is not cooled with cooling water, but simply shows an example of the change over time in the temperature distribution of the mold due to high-frequency induction heating. After that, the mold was opened once, the inductor was extracted from between the stationary side and movable side molds, the mold was closed again, and the rubber reinforced PS was injection molded in the same manner as normal injection molding, resulting in the desired rubber reinforcement with a beautiful appearance. A PS injection molded product was obtained. An injection molded HIPS product of the present invention was obtained, and an electron micrograph of a cross section in the thickness direction of the molded product is shown in FIG. The magnification is 7000x. For reference, the same material was injection molded using the same mold at a mold temperature of 60°C, and a photograph of the same cross section was taken in the same manner as shown in Figure 4. The magnification is the same
The island pattern in each photo is 7000x HIPS
It is a reinforcing rubber, and the sea pattern part is GPPS.
(polystyrene homopolymer). The horizontal line is the surface of the molded product. As is clear from comparing Figures 3 and 4, in the case of the molded product of the present invention, the reinforcing rubber hardly deforms near the surface of the molded product, and the reinforcing rubber does not appear on the surface of the molded product. do not have. On the other hand, in the case of Figure 4 obtained by the normal injection molding method, the reinforcing rubber is deformed due to distortion due to insufficient flow during injection molding, and the reinforcing rubber is exposed on the surface of the molded product. Recognize. For this reason, it is only possible to obtain a molded product with a poor appearance. In order to quantify the good appearance and gloss of the above-mentioned HIPS molded product of the present invention, the glossiness Gs (60°) of the molded product was measured according to ASTM D523, and the result was 103%.
It was hot. On the other hand, a molded product with a mold temperature of 60℃ has a gloss level of 50.
%, indicating the smoothness and gloss of the molded product appearance of the present invention. Also, perhaps because the molded product of the present invention has low flow resistance during injection molding and is less likely to cause orientation distortion, when the heat distortion temperature specified in JIS K6871 was measured, the heat distortion temperature was 22% higher than that of a normal molded product. It was confirmed that the so-called practical heat resistance temperature was improved by ~3°C, and as a result of comparing the drop strength of molded products, the practical toughness was also improved. The rubber-reinforced PS referred to in the present invention refers to HIPS, MIPS
etc. The appropriate frequency of the high frequency used for high frequency induction heating in the present invention is 50 Hz to 10 MHz, preferably 1 KHz to 1000 KHz. In this application, when evaluating the gloss of molded products,
Gs (60°)% in ASTM D523 is used, which has traditionally been used to evaluate the appearance and gloss of plastic molded products.
It follows the custom of using (60°)%.
Gs (60°) according to ASTM D523
% is 70% or more, gloss Gs (20
゜)% is to be applied. Therefore, Gs (60°)% and Gs (20°)% were measured for molded products of the present invention and molded products including comparative examples, and the correlation between the two was determined, as shown in FIG. 5. Therefore, if you follow ASTM D523 exactly, Gs (60
For molded products with a gloss level of 70% or more, the corresponding Gs(20°)% value should be displayed as shown in Figure 5, but the relationship between conventional industry practice and comparative examples Please note that I purposely used Gs (60°)% to make the difference more clear. Example 1 A conventional commercially available HIPS was molded using a conventional in-line injection molding machine. The mold is a normal S-
The mold is made of 45C steel and can form plate-shaped products with a diameter of 10 cm, depth of 2 cm, and average wall thickness of 3.5 mm, and the gate is a center direct gate. The inductor is made by spirally forming a 6mm diameter copper tube into a dish shape at 5mm intervals, then casting it with epoxy resin to a thickness of 3cm and solidifying it into a flat plate. As for the injection molding conditions, the cylinder temperature was set so that the temperature of the HIPS was 220°C. Before injecting the HIPS into the mold, the above-mentioned inductor was inserted between the molds and a 400KC, 6KW high frequency oscillator was used to inject the HIPS into the mold.
It oscillated for 15 seconds, then the mold was opened, the inductor was pulled out from between the molds, and the mold was closed again. During this time, the mold cooling water is not allowed to flow inside the mold.Then, the HIPS is injected into the mold for 10 seconds at an injection pressure of 60 kg/cm ² as in normal injection molding. After cooling for 20 seconds through cooling water, the molded product was taken out. The total cycle time was 60 seconds. The surface of the molded product is so glossy that it is unimaginable for conventional HIPS molded products, and the Gs (60°)% is 103%.
It was hot. Moreover, there was no flow mark at the gate part, and the gloss of the gate part and the edge of the plate (the flow end part of the molded product) was the same at Gs (60°)% of 103%, so a molded product without a so-called gloss gradient was obtained. We obtained a molded product that is so tight that it overturns the concept of conventional HIPS molded products. Example 2 Normal HIPS with a normal injection molding machine at a resin temperature of 220
Molded at ℃. The mold is a mold that can produce dumbbells and strips of shape specified in JIS K6871, and the material is S-
Made with 55C. The inductor used was one in which 3 mm diameter copper tubes were spirally arranged at 5 mm intervals, which were cast into a 2 cm thick flat plate with epoxy resin and solidified. The molding method was the same as in Example 1, but
4KHz, 6KW, high frequency oscillation time 10 seconds, injection 10 seconds,
Cooling 15 seconds, whole molding cycle 50 seconds, injection pressure 50Kg/cm ²
It is. In this way, a molded HIPS product with a beautiful and glossy surface appearance was obtained. The physical properties of the molded products were evaluated according to JIS K6871, and the results are shown in Table 1. As is clear from the results in Table 1, the appearance, gloss,
A molded product with excellent physical properties could be obtained. Comparative Example 1 Using the same molding machine and mold as in Example 2, using the same molding material, resin temperature 220℃, mold temperature 60℃
℃, injection 10 seconds, cooling 15 seconds, total molding cycle 40 seconds,
The molded product was molded at an injection pressure of 50 kg/cm ² and the physical properties of the molded product were measured. The results are shown in Table 1, Comparative Example 1.

[Table] Example 3 Using a mold capable of molding a pair of audio cassette housings (referred to as cassette halves in the industry), HIPS was molded using an ordinary injection molding machine. The inductor used was one in which copper tubes with a diameter of 5 mm were arranged in a flat spiral shape at 5 mm intervals, which was then cast into a 3 cm thick flat plate with epoxy resin, and the copper tubes were fixed and solidified. The inductor was inserted between molds and a high frequency of 7 KHz and 20 KW was oscillated for 15 seconds, and then the inductor was extracted from between the molds and injection molding was performed in the same manner as in Example 1. The molded product has a complex shape such as ribs, bosses, perforations, and embossed parts, and in normal molding, flow marks and weld lines are noticeable, but the molded product in this example has a complicated shape. The appearance of the product is beautiful, there are no flow marks, and no weld lines are visible.
In addition, the glossiness of the grained areas was improved, resulting in a so-called tight grained texture, which significantly increased the product value. The dimensional accuracy is also the same as the conventional product,
There are no sleds, etc. When the glossiness of the flat part of the molded product was measured, the Gs (60°) of the molded product of this example was 99 to 98%.
However, the Gs (60°) of molded products made by conventional injection molding methods is 30 to 50%, and only molded products with large flow marks, welds, and gloss gradients can be obtained. . Incidentally, the mold temperature for obtaining molded products using this conventional method is 70°C, which is the upper limit temperature for industrial production. Example 4 In this example, a series of experiments were conducted using various rubber-reinforced PSs at different mold surface temperatures, and the resulting gloss was compared and examined. The planar shape is shown in Figure 6, and the wall thickness is 3 mm.
In a mold for obtaining an injection molded product with a 1.5 cm hole chip, the gate is a 4 x 2 mm restriction gate as shown by the arrow in Figure 6. The mold material is carbide mold material (NAK
The mold surface was given a mirror finish. The inductor (coil) is a 10mm copper pipe with a diameter of 5mm.
They were arranged in a planar spiral shape at intervals and hardened with epoxy resin to a thickness of 3 cm. The high frequency oscillator used was a 7KHz, 10KW, steplessly variable output. The molding machine used was Toshiba IS80 (5 oz injection molding machine). Using these devices, various resins were molded under normal injection molding conditions and the injection molding method using the inductor insertion method of the present invention, and the results of comparison are shown in Table 2. As shown in Table 2, for any resin obtained by the method of the present invention,
It can be seen that a molded product can be obtained that has a high gloss level on the surface of the molded product and has almost no gloss gradient called gloss unevenness. In other words, when comparing the gloss of part A and the gloss of part B, there is a difference in the case of ordinary injection molded products, but in the molded product of the present application, the molded product gloss of each part is almost the same and high gloss. I understand that there is something.

[Table] Example 5 A pocket radio housing having external dimensions of 12 cm in length, 8 cm in width, and 2 cm in depth was molded from HIPS resin. The inductor was created in substantially the same manner as in Example 3, and molded in the same manner. This molded product is commercialized by covering the appearance defects by applying acrylic paint to prevent weld lines and flow marks from appearing on the speaker grille of pocket radios and parts with holes for various knobs. However, the molded product of this example had no flow marks or weld lines visible, and the gloss of each part was so high that it did not require painting at all.
゜) was 98 to 99%, and a uniform and high gloss molded product was obtained. As mentioned above, the rubber-reinforced PS molded product of the present invention has a molded product appearance that is so good that it overturns the concept of conventional rubber-reinforced PS injection molded products.In other words, the gloss level of the molded product is very good, and the gloss gradient , no unevenness, no flow marks, and no visible weld lines. Moreover, the advantages of conventional rubber-reinforced PS, such as ease of molding, rigidity, toughness, and dimensional stability, remain the same, and productivity is also slightly higher than that of conventional injection molding, with a total molding cycle of 2 to 5 It will only be a little longer. The appearance of molded products has improved, eliminating the cheap image traditionally associated with plastics, and not only significantly increasing product value, but also eliminating the need for painting on parts that previously had to be painted to prevent appearance defects. The usefulness of inventions is immeasurable.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram for manufacturing a molded article according to the present invention. FIG. 2 shows an example of the temperature distribution of the mold in the apparatus shown in FIG. FIG. 3 is a cross-sectional micrograph (x7000x) of a HIPS molded product in the thickness direction, which is one embodiment of the present invention. FIG. 4 is a comparison photograph, which is a cross-sectional micrograph (x7000x) of a HIPS molded product in the thickness direction obtained by a normal molding method. FIG. 5 is a curve showing the correlation between glossiness Gs (20°)% and glossiness Gs (60°)%. FIG. 6 shows the dimensions and shape of the sample piece used in Example 4. FIG. 7 is an explanatory diagram of the weld line. 1: Fixed side mold in the mold, 2: Moving side mold, 3: Inductor in high frequency oscillator,
Point A, point B: surface of the mold, point C, point D: inside of the mold, a: weld line width, b: weld line depth.

Claims (1)

[Scope of Claims] 1. In an injection molded product made of rubber reinforced polystyrene resin containing 4% by weight or more of a rubber component, at least 1 to 100 μm is added near the surface of the molded product.
The layer with little rubber deformation is formed integrally during injection molding with virtually no bonding interface, and the surface of the molded product has a gloss level of Gs (60°)% at an incident angle of 60° as specified in ASTM D523. A rubber-reinforced polystyrene resin injection molded product characterized by having a gloss of 80% or more. 2 Claim 1 in which the gloss level is 90% or more
Injection molded products as described in section. 3 Claim 1 in which the gloss level is 95% or more
Injection molded products as described in section. 4. The injection molded product according to any one of claims 1 to 3, wherein the surface of the injection molded product is free from defects such as flow marks, jetting, and silver streaks, and has substantially inconspicuous weld lines. 5. The injection molded article according to any one of claims 1 to 4, having a gloss gradient of 0 to 0.5%/cm.