POLYSTYRENE-BASED RESIN COMPOSITION FOR INJECTION MOLDING

Abstract: Disclosed is a polystyrene-based resin composition for an injection molding. The composition comprises a mixture of a first and a second styrene compositions having rubbers of different particle sizes, wherein the first styrene composition includes 6-8% of the rubber having the particle diameter of 3-5 mu m, and the second styrene composition includes 7-9% of the rubber having the particle diameter of 1-3 mu m. The large-sized rubber particles and the small-sized rubber particles contact closely after the injection molding and then the exterior view of the mould product can be further improved. And the forming of a weld line is restrained.

Description: POLYSTYRENE-BASED RESIN COMPOSITION FOR INJECTION MOLDING DESCRIPTION TECHNICAL FIELD OF THE INVENTION The present invention relates to a polystyrene-based resin composition for an injection molding and more particularly, to a polystyrene-based resin composition for an injection molding for reducing exterior defects such as a weld line formed during the injection molding and for eliminating a post-processing treatment such as a painting treatment. BACKGROUND ART Generally, polystyrene is produced b> the radical polymerization of styrene using an initiator such as an organic peroxide. Thus obtained polystyrene has a straight chain structure and has a good transparenc) . The specific gravitv of the poh styrene is low and an insulating characteristic on electricity and high frequency is good. However, the poh styrene has some defects of having a low softening temperature and being hard and so liable to break. Polystyrene-based resin has a low melting point viscosity in relatively low temperature range. And the decreasing degree of the viscosity according to the decrease of the temperature, is slow and the range of the molding temperature is wide. Moreover, the transparency and a molding characteristic of the polystyrene-based resin are good. Accordingly, high impact polystyrene (HIPS) and ABS resin are widely used as a case for household electric and electronic products. The HIPS obtained by mixing general purpose polystyrene (GPPS) obtained b polymerizing styrene. with 5-20% of synthetic rubber, has an improvement in liability in breaking of polystyrene, and the ABS resin is an impact-resistant thermoplastic resin composed of the three components of acrylonitrile. butadiene and styrene. etc. The ratio (L/t) of the fluid length L of the resin and the building thickness t of the mould product, is 230-250 for the GPPS resin. 220-230 for the HIPS resin and 190-200 for the ABS resin. Accordingly, the fluidity of the resins are good in order of the ABS resin, the HIPS resin and the GPPS resin. However, since the ABS resin is more expensive than the GPPS resin or the HIPS resin, the HIPS resin is widely used for the common electronic products such as television cases. However, since the HIPS resin is not good in fluidity, a weld line is generated in the manufactured mould product through the injection molding. Therefore, post-processing treatment such as painting treatment should be implemented after the injection molding. The weld line is formed as a thin solid line at the position where two melted and separated resins flow together. The main reason of the generation of the weld line is the incomplete mixing of two melted resins at the meeting portion. The weld line also can be generated b\ moisture or a volatile matter contained in the resin, or by a mold release used during the injection molding. For carrying out the injection molding, a gate for supplying the resin into the mold is needed. And when the mould product is large or complicated, two or more gates are needed. In these cases, the weld line is liable to be formed. Even though the melted resin is supplied through one gate, the weld line may be generated when an inserting component or a hole is present in the mould product or when the building thickness of the mould product is different each other. FIGs. 1A and I B are typical views for explaining the generating process of the weld line. In FIG. 1A. the melted resin is supplied into the cavity through two gates 12 and 14 and the weld line is generated where the two separated portions of the resin flow together. At this time, the weld line is not formed even though the separated and melted resins flow together near gates 12 and 14 but the weld line is formed far from gates 12 and 14. The melted resin gets cool through the mold and the two resin portions far from gates 12 and 14 could not mix together completely because of the low temperature of the resin, thereby generating the weld line. Meanwhile, in FIG. I B. the weld line 16 is generated even though the melted resin is supplied through one gate 12 when a hole 18 is provided in the mould product. Because the melted resin is separated around the hole 18 and then How together, the weld line is generated. The weld line is inevitably generated when butadiene is included in the HIPS resin for increasing the high intensity and the fluidity of the resin, because the fluidity is lowered. The generated weld line lowers the strength of the mould product and the degree of the exterior quality for the mould product. Accordingly, the weld line should be eliminated after the injection molding process through a painting process. DISCLOSURE OF THE INVENTION Accordingly, it is an object of the present invention considering the above- mentioned problem, to provide a polystyrene-based resin composition for an injection molding for minimizing the generation of the weld line during the injection molding through increasing the fluid characteristic when the resin composition is melted. To accomplish the object, there is provided in the present invention a polystyrene-based resin composition for an injection molding comprising a mixture of a first and a second styrene compositions having rubbers of different particle sizes. At this time, the first styrene composition includes 6-8% of the rubber ha\ ing the particle diameter of 3-5μm. And the second styrene composition includes 7-9% of the rubber having the particle diameter of l -3μm. Further, the polystyrene-based resin composition comprises 70-90% of the first styrene composition, 9-29% of the second styrene composition, a thermo- stabilizer and a silicon oil. According to the present invention, the fluid characteristic of the resin is improved by mixing polystyrene resin compositions including rubbers having different particle sizes. Therefore, the generation of the weld line is restrained and the exterior view of the mould product is improved. BRIEF DESCRIPTION OF THE DRAWINGS The above object and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: FIG. 1A is a typical view for illustrating the forming process of a weld line; and FIG. IB is a typical view for illustrating another forming process of a weld line. MODES OF CARRYING OUT THE INVENTION Hereinafter, the polystyrene-based resin composition for the injection molding according to preferred embodiments of the present invention will be explained in more detail with reference to the accompanying drawings. Generally, when the polystyrene resin includes the particles of a synthetic rubber, only the large-sized rubber particles alone or only the small-sized rubber particles alone have been used. The rubber particles are distributed in the fiber structure of the butadiene, which is present in the base of the polystyrene. When an impact is applied to the mould product, the stress coincides on the fiber structure and minute crazing is generated. Through this, the impact can be absorbed. However, when a large amount of the large-sized rubber particles or the small-sized rubber particles is used, the fluidity during the injection molding is proportional to the amount of the used particles. Therefore, it is desired that the amount of the using rubber particles is as little as possible, while reinforcing the impact-resistance. When the size of the rubber particles included in the polystyrene is only one kind of particle size among the large one or the small one. a homogeneous distribution of the rubber particles in the polystyrene becomes difficult. When a composition containing both the large-sized rubber particles and the small-sized rubber particles is used, the rubber particles can be mixed homogeneously and both of the impact-resistance and the fluidity can be satisfied for the polystyrene. Accordingly, the molding characteristic during the injection molding can be improved and the exterior view of the mould product also can be improved. The present invention will be explained in detail with the following preferred embodiments. 1 ) examples 1 -5 The first polystyrene composition was obtained by mixing styrene with 7% of a rubber having the mean particle diameter of 3-5 μm. The second polystyrene composition was obtained by mixing styrene with 8% of a rubber having the mean particle diameter of l-2μm. The first and the second styrene compositions and other additives were mixed with the mixing ratios as illustrated in Table 1. Test pieces were manufactured by injection molding the mixed compositions under the temperature of 190-220 °C for the cylinder of the injection molder and 50 °C for the molder. with the injection pressure of 100 ton. The size of the test pieces were 450mm x 250mm. The test pieces were melted and filled up in the mold having one insert hole 18 at the center portion as one gate, as illustrated in FIG. I B. Table 1 In example 1 , the polystyrene-based resin composition was obtained by using only the first styrene composition. In examples 2-5. the added amount of the second styrene composition was gradually increased. It is shown that the swelling index is decreased as the cross linking ratio increases through increasing the mixing ratio of the styrene compositions having different particle sizes. The obtained testing results with respect to the test pieces manufactured by using two compositions in different mixing ratios and in the shape illustrated in FIG. I B. are illustrated in Table 2. Table 2 In example 1. the length is long and the width is wide for the weld line. The length and the width of the weld line are reduced in examples 2-5 which include the second styrene composition. And the width of the weld line increases in example 5 which includes a lot of the second styrene composition. The gloss characteristic is better for examples 2-5 than that for example 1. And the stain characteristic is not good for example 5 which includes a lot of the second styrene composition. The generating amount of the gas is measured by weighing the reduced amount of the test species after standing the test species at 230 °C for 30 minutes. The gas generating amount is little for examples 2 and 3. As known from the examples, the exterior view of the mould product can be improved by using the mixed composition of the first styrene composition having the large-sized rubber particles and the second styrene composition having the small- sized rubber particles at a predetermined ratio. From the examples, it could be misunderstood that the first and the second polystyrene composition were simply mixed. In fact, they were mixed and then injection-mould. Therefore, the large-sized rubber particles and the small-sized rubber particles contact closely and the exterior view of the mould product can be further improved. As described above, the exterior view 7 of the mould product can be improved through injection molding the composition obtained by mixing the polystyrene composition including the large-sized rubber particles and the polystyrene composition including the small-sized rubber particles in the present invention. Although the preferred embodiment of the invention has been described, it is understood that the present invention should not be limited to the preferred embodiment, but various changes and modifications can be made by one skilled in the art within the spirit and scope of the invention as hereinafter claimed.

Claims: WHAT IS_CL AIMED IS: 1. A polystyrene-based resin composition for an injection molding comprising a mixture of a first and a second styrene compositions having rubbers having different particle sizes. 2. The polystyrene-based resin composition for an injection molding as claimed in claim 1, wherein said first styrene composition includes A % of said rubber having the particle diameter of 3-5μm, and said second styrene composition includes B % of said rubber having the particle diameter of l-3μm. 3. The polystyrene-based resin composition for an injection molding as claimed in claim 2. wherein A corresponds to 6-8 and B corresponds to 7-9. 4. The polystyrene-based resin composition for an injection molding " as claimed in claim 2. wherein said polystyrene-based resin composition comprises 70- 90% of said first styrene composition. 9-29% of said second styrene composition, a thermo-stabilizer and a silicon oil.

Inventors: CHO CHAE SUNG, CHO, CHAE, SUNG
Applicants: DAEWOO ELECTRONICS CO LTD, DAEWOO ELECTRONICS CO., LTD.
Application year: 1999
Main classification: C08L25
IPC codes: B29C45, B29K25, B29K105, C08F279, C08F291, C08L21, C08L25, C08L83