Background Art [2] In general, a related art boundary stone is classified into a sidewalk-roadway boundary stone that is installed between a roadway and a sidewalk to define the define the boundary between the roadway and sidewalk and a roadway boundary stone that is installed within the roadway to define the boundary between the roadway and flower garden or central site of a rotary intersection. Among the boundary stones, the sidewalk-roadway boundary stone is configured in such a manner that a concrete form is manufactured in the form of a roughly rectangular pillar by generally pressing concrete by means of a press, concrete is poured on the ground between the roadway and sidewalk, and a plurality of the concrete forms are then installed on the ground to sequentially abut each other in a line and to be bounded with the poured concrete.

[3] The related art concrete boundary stone should be maintained at a certain degree of strength or weight in order to prevent a vehicle from entering the sidewalk or to prevent the boundary stone from being damaged when the vehicle collide against the boundary stone. However, there is a problem in that the heavy weight results in poor workability.

[4] To prevent a boundary stone from being damaged, it has been recently man- ufactured by processing a stone material such as granite. The boundary stone made of a stone material has better appearance, strength and durability than the boundary stone made of concrete, but it is difficult to manufacture the boundary stone, so that the boundary stone has a high unit cost and too excessively heavy weight. Consequently, the workability is further deteriorated.

[5] In addition, to clearly fix the boundary, the boundary stone made of concrete or stone material may be painted or attached with a reflection tape. However, if the boundary stone is wetted with rainwater or snow, the paint or reflection tape is spon- taneously peeled off. Thus, it spoils the beautiful appearance of the boundary stone and does not sufficiently exhibit the effect of the painting or reflection tape. Moreover, there is a further problem in that a large amount of the distribution and installation costs are required, due to the weight of the boundary stone.

[6] In order to solve the aforementioned problems, a boundary stone made of plastics has been recently developed. Since the boundary stone made of plastics is light in weight, there is a still further problem in that the static stability is lowered when the boundary stone has been installed. To solve the above problem, a fixture for fixing the boundary stone onto the ground should be additionally installed. Therefore, the in- stallation stability is deteriorated and the installation process is difficult or complicated.

Disclosure of Invention Technical Problem [7] The present invention is conceived to solve all the aforementioned problems. An object of the present invention is to provide a boundary stone made of sand and plastics and a method of manufacturing the same, wherein the boundary stone comprises 60 to 72 parts by weight of sand and 28 to 40 parts by weight of plastics, whereby its weight is adequate, workability is good and the distribution and in- stallation costs are low.

Technical Solution [8] To achieve the object, the present invention provides a boundary stone made of sand and plastics and constructed to continuously abut each other along a border of the roadway and to define a boundary of the roadway, comprising 60 to 72 parts by weight of sand and 28 to 40 parts by weight of plastics.

[9] Further, the present invention provides a method of manufacturing a boundary stone made of sand and plastics and constructed to continuously abut each other along a border of the roadway and to define a boundary of the roadway, comprising a plastic- melting step of melting plastics; a composition-mixing step of simultaneously supplying 60 to 72 parts by weight of sand and 28 to 40 parts by weight of plastics to a mixing bath and mixing them with each other to disperse the sand within the plastics in a gel state, after the plastic-melting step; a composition-filling step of filling a mold with the mixed composition, after the composition-mixing step; a composition- pressurizing step of applying pressure to the composition to compensate for volume reduction due to shrinkage when the molten plastics is cooled, after the compositio n- filling step; a molded-product completing step of causing the molten plastics to be solidified and plasticized by cooling the mold, after the composition-pressurizing step; and a molded-product separating step of separating a molded product from the mold, after the molded-product completing step.

Advantageous Effects [10] As described above, the boundary stone made of sand and plastics according to the present invention has good bending fracture strength and a certain amount of its own WO 2005/080688 PCT/KR2005/000478 weight. Therefore, the boundary stone of the present invention can be used semipermanently in the same manner as the concrete boundary stone without need for repairing for several years once it has been constructed. Further, there are advantages in that a fixture or the like for fixing the boundary stone onto the ground is not required. In addition, the distribution costs are reduced and the workability is improved, because the boundary stone is light in weight. Furthermore, there is another advantage in that rupture and dew condensation do not occur even in a severe winter since the boundary stone does not absorb moisture.

Brief Description of the Drawings [11] Fig. 1 is a perspective view showing a boundary stone made of sand and plastics and its use state according to the present invention.

[12] Fig. 2 is a flowchart illustrating a method of manufacturing the boundary stone made of sand and plastics according to the present invention.

Best Mode for Carrying Out the Invention [13] Hereinafter, a preferred embodiment of the present invention will be described in detail.

[14] Fig. 1 is a perspective view showing a boundary stone made of sand and plastics and its use state according to the present invention. The boundary stone 100 made of sand and plastics according to the present invention comprises 60 to 72 parts by weight of sand 110, and 28 to 40 parts by weight of plastics 120.

[15] The reason for limiting the mixture ratio is that if the amount of the sand 110 is less than 60 parts by weight, the boundary stone made of sand and plastics is light in weight due to the small amount of sand so that there is a problem in that a vehicle enters the sidewalk or the boundary stone is broken when the vehicle collides against the boundary stone, resulting in reduction in the stability. On the other hand, if the amount of the sand is greater than 72 parts by weight, there is another problem in that a great amount of distribution and installation costs is required as well as the workability is deteriorated due to the heavy weight of the boundary stone made of sand and plastics.

[16] Meanwhile, if the amount of the plastics is less than 28 parts by weight, there is a disadvantage in that the binding force of the sand 110 is lowered and the bending fracture load of the boundary stone 100 made of sand and plastics is lowered. On the other hand, if the amount of the plastics is greater than 40 parts by weight, there is a disadvantage in that the bending fracture strength is rapidly decreased and the weight of the boundary stone 100 made of sand and plastics is also reduced.

[17] When the dimension of the boundary stone is 150 x 170 x 200 x 1,000 (upper thickness x lower thickness x height x length), the weight of the boundary stone made WO 2005/080688 PCT/KR2005/000478 of concrete is about 73 kilograms and the weight of the boundary stone made of a stone material is about 100 kilograms, whereas the weight of the boundary stone 100 made of sand and plastics is about 49 kilograms. Thus, the weight of the boundary stone according to the present invention can be reduced by about 33% as compared with the prior art.

[18] Therefore, the boundary stone 100 made of sand and plastics according to the present invention can maintain a certain degree of its own weight such that its dis- tribution cost can be reduced and its workability can also be improved.

Mode for the Invention [19] Fig. 2 is a flowchart illustrating a process of manufacturing the boundary stone made of sand and plastics according to the present invention. The process of manu- facturing the boundary stone made of sand and plastics will be explained with reference to Fig. 2.

[20] First, a step of melting the plastics (step 10 in Fig. 2) is performed and corresponds to a step of melting the plastics 120 in a plastic-melting machine.

[21] In the plastic-melting step, the plastic-melting machine such as an injection molding machine is used. The temperature of a cavity, i. e. the temperature of a space in which the plastics 120 are melted, should be maintained at 200 to 350 oC. The reason is that the temperature is suitable to the molding conditions, including melting, for polyethylene (PE), polypropylene (PP), polycarbonate (PC), poly vinyl chloride (PVC) and the like.

[22] If a colored plastic material is used in the plastic-melting step, a finished product of the boundary stone 100 made of sand and plastics has a color. Thus, it is possible to manufacture the boundary stones 100 made of sand and plastics, which have a variety of colors.

[23] A step of mixing the composition (step 20 in Fig. 2) is performed after the plastic- melting step. This step is a step of simultaneously supplying a mixing bath with the ad- ditionally screened sand 110 and the plastics 120 melted in the plastic-melting step and mixing the sand and plastics.

[24] The sand 110 is injected according to an air-jet principle employed in sandblasting and the like. The body of an injection nozzle comprises a linear sand injection port and sand suction port, which are integrally formed with each other, and a Y-shaped air suction port between the sand injection port and the sand suction port.

[25] The sand suction port of the injection nozzle comprises a sand suction tube that has an open distal end and an inner space. The air suction port of the injection nozzle comprises an air suction tube that has an inner space and a distal end coupled to a blower. The sand injection port of the injection nozzle is placed in the mixing bath and WO 2005/080688 PCT/KR2005/000478 one side of the sand suction tube is placed in sand. When the blower is operated, air with high pressure generated by the blower is introduced into the air suction tube and the introduced air is injected through the sand injection port of the injection nozzle. At this time, sand is sucked through the sand suction tube and then injected together with the air into the mixing bath.

[26] The molten plastics 120 in the cavity are caused to flow into the mixing bath through a nozzle by a pressurizing device such as a piston or a screw.

[27] The sand 110 and the molten plastics 120 are fed into and mixed in the mixing bath.

At this time, to solve nonuniformity of the mixture due to difference in specific gravities of the sand 110 and the plastics 120, the sand 110 and the molten plastics 120 are mixed with each other while the plastics 120 are in a gel state, so that the sand 110 can be uniformly distributed in the plastics 120.

[28] Meanwhile, to keep the interior of the mixing bath at a predetermined temperature, the mixing bath is heated by a heater using a liquid such as water or oil. The liquid heater may absorb or uniformly apply heat from or to the mixing bath even though the temperature of the mixing bath increases above the predetermined temperature during the heating, thereby heating and cooling the mixing bath on an average.

[29] The mixed composition comprises 60 to 72 parts by weight of sand 110 and 28 to 40 parts by weight of plastics 120. The reason for limiting the composition ratio to the foregoing is that if the amount of the sand 110 is less than 60 parts by weight, the boundary stone 100 made of sand and plastics is light in weight due to the small amount of sand so that there is a problem in that its stability is deteriorated since a vehicle enters the sidewalk or the damage of the boundary stone cannot be prevented when the vehicle collides against the boundary stone. On the other hand, if the amount of the sand is greater than 72 parts by weight, there is another problem in that a great amount of distribution and installation costs is required as well as the workability is de- teriorated due to the heavy weight of the boundary stone made of sand and plastics [30] Meanwhile, if the amount of the plastics 120 is less than 28 parts by weight, there is a disadvantage in that the binding force of the sand 110 is lowered and the bending fracture strength of the boundary stone 100 made of sand and plastics is thus lowered.

If the amount of the plastics is greater than 40 parts by weight, there are another dis- advantage in that the bending fracture strength is rapidly decreased and the boundary stone 100 made of sand and plastics also becomes light in weight.

[31] After the composition-mixing step, a composition-filling step (step 30 in Fig. 2) is performed. This step is a step of filling a mold with the mixed composition, wherein the mixture of the sand 110 and the molten plastics 120 flows from the mixing bath into the mold to fill a filling cavity of the mold therewith. When the pressurizing device such as a piston or a screw advances, the mixture of the sand 110 and the WO 2005/080688 PCT/KR2005/000478 melted 120 is caused to flow from the mixing bath through a sprue, a runner and a gate of the mold and filled into the filling cavity.

[32] In other words, the mixture of the sand and the molten plastics 120 flows from a conical nozzle of the mixing bath into the runner, which is a short trough or passage having a cross section in the form of a circle or the like, through the conical sprue, which is in contact with the nozzle of the mixing bath and has a small width and a taper angle of about 2 to 4 oC. At this time, the mixture of the sand and the molten plastics 120 flows into the filling cavity through the gate, which has a cross section in the form of a circle or the like and is installed at a position where the mixture of the sand and the molten plastics can rapidly flow from an inlet of the filling cavity to an opposite position.

[33] After the composition-filling step, a composition-pressurizing step (step 40 in Fig.

2) is performed. This step is a step of applying pressure to the composition so as to compensate for volume reduction due to shrinkage while the molten plastics 120 is cooled. In this step, after the filling cavity has been filled with the mixture of the sand and the plastics, the pressurizing device such as a piston or a screw further advances to apply pressure to the mixture. A shrinkage of about 25% generated when the molten plastics are changed from the molten state to a solid state, i. e. volume reduction due to the shrinkage during the cooling of the plastics, is compensated.

[34] After the composition-pressurizing step, a molded-product completing step (step 50 in Fig. 2) is performed. This step is a step of completing a molded product by cooling the mold to solidify and plasticize the molten plastics 120 so that the grains of the sand are positioned in place.

[35] Meanwhile, this step is a step of solidifying the molten plastics 120 by cooling the mold, wherein cooling water is caused to pass through cooling holes or grooves formed in the molding machine or mold to remove heat from the molten plastics through the mold and to provide the molten plastics with plasticity that is a property by which de- formation produced under stress above an elastic limit is not restored to its original state.

[36] After the molded-product completing step, a molded-product separating step (step 60 in Fig. 2) is performed. This step is a step of separating the molded product from the mold, i. e. a step of separating the completed, molded product from the mold using an ejector while separating a movable (upper) mold half from a fixed (lower) mold half, wherein the molded product is separated from the filling cavity using ejector pins that are fixed on an ejector plate and placed in the mold and advance together with the ejector plate when the mold is opened.

[37] Preferably, a sand-screening step may be performed before the plastic-melting step.

This sand-screening step is a step of screening sand to obtain sand with a proper grain WO 2005/080688 PCT/KR2005/000478 size and removing impurities contained in the screened sand 110.

[38] As for the sand 110 employed in the present invention, sand 110 with relatively uniform grains is required. To this end, the sand 110 with relatively uniform grains is obtained by screening the sand gathered in a river or sea using a sand-screening machine in which a vibrating sieve is used. For example, the gathered sand 110 is transferred to a screening unit of the sand-screening machine by a conveyor or the like thereof and caused to pass through the vibrating sieve with meshes of 0.4 mm. The remaining sand that did not pass through the sieve is separately collected and the sand that passed through the sieve is again caused to pass through another vibrating sieve with meshes of 0.2 mm. The sand that passed through the other sieve is separately collected and the remaining sand that did not pass through the other sieve, i. e. sand with a grain size of 0.2 to 0.4 mm, is used in the present invention.

[39] If a grain size of the sand 110 is to be limited, the amount of gathered sand 110 to be used, the manufacturing costs, the surface roughness of the boundary stone, and its merchantability should be considered.

[40] This sand-screening step may further comprise the step of removing impurities contained in the sand and washing and drying the sand from which the impurities have been removed. The gathered sand may include a great deal of impurities. Particularly, sand gathered in a sea contains much salt. Therefore, to remove the impurities from the sand, the sand is washed and then dried to remove moisture added to the sand during the washing process so that the sand can contains moisture below a certain level.

[41] As an example of a method of washing the sand, there is a method of spraying tap water or underground water over the sand. In the drying step, it is possible to use a hot air drying method of causing the washed sand to pass through a hot air drying chamber, or a natural drying method of drying the washed sand by a wind or sunrays.

[42] <Example> [43] The boundary stones 100 made of sand and plastics according to the present invention were manufactured and subjected to comparison tests in view of bending fracture strength according the KSF 4006 of Korean Standards. As for a test method, a test sample cut at a size of 200 x 60 x 60 mm was placed horizontally in a direction of the width on a test support stand and brought into close contact with an upper surface of a tester. In a state where a span is set to be 530 mm, a supporting rod is then placed at the center of the span and load was uniformly applied at a certain loading rate. At this time, the bending fracture strength was measured as maximum load when the test sample is broken down.

[44] Boundary stones made of sand and plastics according to the present invention were manufactured to have the same weight as conventional boundary stones available on the market, and were subjected to comparison tests in view of absorption rate according to KSF 4006 of Korean Standards. As for a test method, a test sample was collected after the bending strength test has been completed, and dried in a dryer at 100 to 115 oC for 24 hours and until the difference in weights measured within 2 hours does not exceed 0.2%. Further, the test sample was cooled at a room temperature for 4 hours, and weighed. Then, the test sample is dipped into water at 5 to 30 oC for 24 hours, taken out and wiped off by a piece of cloth to remove water on a surface thereof, and weighed. Then, the absorption rate was calculated according to the following equation: [45] Absorption rate (%) = {(A-B)/B} x 100, [46] wherein A is a weight when the test sample is wet and B is a weight when the sample is dry.

[47] As for test instruments, since a well-known material tester and freezing chamber were used, detailed descriptions thereof will be omitted herein.

[48] The test results of the boundary stones made of sand and plastics according to the present invention are shown in Table 1 below.

[49] Table 1 Sand (parts by weight) Bending fracture strength (kgf) Absorption rate (%) 53 1610 0.1 56 1677 0.1 59 1736 0.1 62 1773 0.2 65 1811 0.1 68 1786 0.2 71 1762 0.1 74 1659 0.2 77 1644 0.2 80 1628 0.3 [50] * The remainder except for the parts by weight of sand corresponds to the parts by weight of plastics. 65 parts by weight of sand indicates a boundary stone made of sand and plastics comprising 35 parts by weight of plastics mixed therewith.

[51] Consequently, it is found that the boundary stone 100 made of sand and plastics according to the present invention exhibits that bending fracture strength is maximized when it comprises 65 parts by weight of sand, but that the bending fracture strength is gradually lowered as the amount of the sand further increases or decreases. It is believed that the sand was combined with the plastics at proper intervals.

Industrial Applicability [52] Therefore, the boundary stone 100 made sand and plastics according to the present invention can provide reduced distribution costs and improved workability while maintaining a certain degree of its own weight.