WO2005105404A1

WO2005105404A1 - Molding method and apparatus for expandable polymer resin

Info

Publication number: WO2005105404A1
Application number: PCT/KR2004/001038
Authority: WO
Inventors: Min-Gi Jeong; Gi-Hong Kim; Young-Mi Eum; Dong-Hyun Kim; Bong-Kuk Park
Original assignee: Polma Co., Ltd.
Priority date: 2004-05-03
Filing date: 2004-05-03
Publication date: 2005-11-10
Also published as: KR20060038906A; KR100589433B1

Abstract

Problems to be solved. A conventional molding method of an expandable polymer resin employs a high-temperature heat transfer medium to perform high-temperature fusion of expandable polymer resin particles and thus suffers from high energy and time consumption due to heating and cooling of a mold and thus reduced production efficiency. Solutions. As such, the present invention provides a method for molding an expandable polymer resin-molded material at a low temperature and a molding apparatus for use in practicing the same. In accordance with the present invention, pre-expanded particles of expandable polymer resin are coated with a solution of a coating resin having a softening temperature lower than that of the expandable polymer resin to form skin layers on surfaces of expanded particles and expanded resin is then molded by fusion of these skin layers. Additional expansion of the expandable polymer resin, or provision of external physical pressurizing force, or simultaneous use of both obtains pressure necessary for the molding process. In any case, it is possible to perform the molding process at significantly lower temperature than in conventional methods.

Description

MOLDING METHOD AND APPARATUS FOR EXPANDABLE POLYMER RESIN

Technical Field

The present invention relates to a method for molding an expandable polymer resin-molded material at a low temperature and an apparatus for the same. More particularly, it relates to a method of preparing an expansion-molded product involving expanding expandable polymer resin particles in the form of beads or cylindrical pellets and utilizing the resulting expanded material to obtain an expansion-molded product, improved in that a molding process of the expandable polymer resin is capable of being performed at a low temperature, and an apparatus for the same.

Background Art

Expandable polymer resins, such as expandable polystyrene (EPS), expandable polyethylene (EPE) and expandable polypropylene (EPP), have advantageous characteristics including being light-weight, exhibiting high buffering capacity and heat insulating ability, due to their expanded structures having air trapped in foams of plastic resins. Molding these expandable polymer resins is generally effected by subjecting expandable polymer resin particles, containing expanding agents in the form of beads having a diameter of about 0.2 to 1.8 mm or cylindrical pellets having a length of about 2.5 mm and a diameter of about 0.6 mm, to pre-expansion in an expander, and subjecting the resulting expanded particles to secondary expansion^5' and molding in a molding apparatus (mold) following drying, aging and storage. More specifically, illustrating such a molding process by way of expandable polystyrene (EPS), a representative expandable polymer resin, the expanding agent such as pentane or butane is first introduced to a suspension polymerization process of a styrene monomer in a raw resin production factory to obtain spherical particles (beads) having a diameter of about 0.2 to 0.3 mm. The thus-obtained particles are washed, dried and screened according to sizes thereof. Then, the screened particles are subjected to a coating process and the like depending upon desired uses and they are shipped as expandable polystyrene resin particles for marketing. In a molding factory, the particulate raw resins (EPS) are charged to the expander, heated to a temperature of about 103 ^°C to 105 ^°C by saturated steam while stirring, and expanded to a desired specific gravity, thereby effecting pre-expansion. The thus-expanded particles are dried/aged, and temporally stored in a silo. Upon molding, the thus-stored expanded polystyrene particles are charged to a mold via use of air or vacuum and high-temperature steam ranging from 110^°C to 120 °C is injected to warm the internal temperature of the mold to a range of 108 °C to 116°C, thereby expanding the expanding agents contained in the expanded resin particles, thereby effecting molding. At this time, as the particles are swollen by the expansion process, the voids between particles, which were between about 40 to 50%, are filled in, and surfaces of the particles are softened and fused to each other, thereby obtaining a molded material having a desired shape. After completion of molding, the mold is cooled via use of cooling water or vacuum and the molded material is released from the mold, thereby obtaining a finished molded material. Meanwhile, in the context of the molding process, depending upon kinds of molded articles, for example in the case where it is desired to mold products having a predetermined shape such as packaging materials or boxes for electric or electronic products, the molded material is molded to have the desired shape of the finished product by means of a shape-molding machine. Whereas, for products such as heat insulating materials or blocks for civil engineering and construction, the molding material is molded into a large block form by means of a block-molding machine, and is then cut into a desired thickness and width for use. However, upon performing the molding process via use of conventional molding methods as mentioned above, since steam injection is employed to cause fusion between particles, the internal temperature of the mold should be elevated above the temperature at which surface of the raw material resin is softened and at least partially melted. Therefore, there are significant problems associated with large consumption of time and energy due to repetitive heating and cooling of the mold each time the molding process is performed. Further, in the case of block molded material, the molded material can be released from the mold once the surface of the resulting material is cooled after completion of molding, but the released article, i.e., the block is very large and thus a large quantity of heat still remains inside the block. Therefore, it is necessary to wait for several hours to several days, until internal heat of the molded material is completely dissipated leading to solidification, in order to cut the resulting molded material and ship as a final product for marketing even after completion of the molding process. Consequently, in the case of such block molding, since the course of from molding to manufacturing of the final product takes a prolonged period of time, a vast area is needed for loading and storing molded materials discharged from the molding apparatus and a period of time for producing the products becomes long, thus lowering production efficiency.

Disclosure of the Invention

Technical Problem Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a novel molding method of an expandable polymer resin-molded material, which is capable of reducing energy and at the same time, shortening the time consumed for cooling, leading to improved productivity for shape molding, and, for block molding, is capable of providing the above-mentioned advantages and is capable of being shipped in the form of a final product by cutting the resulting material immediately after molding; and a molding apparatus for the same.

Technical Solution In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a novel molding method having a configuration which follows, involving molding expandable polymer resin particles that were subjected to pre-expansion and charged to a mold, into a molded material without heating the resin particles to a temperature at which the raw material resin is softened and melted to cause fusion between resin particles. In accordance with a first embodiment of the present invention, there is provided a method for low-temperature molding of an expandable polymer resin, comprising: heating to expand expandable polymer resin particles containing an expanding agent to a relevant expansion ratio, and drying and aging the resulting expanded material to prepare expanded resin particles (expansion step); coating and drying the expanded resin particles with a solution of a coating resin having a lower softening temperature than the expandable polymer resin to form skin layers on the surfaces of the expanded resin particles (skin layer formation step); and charging the expanded resin particles having the skin layers formed thereon to a mold, injecting a low-temperature heat transfer medium thereto and warming to soften the coating resin, followed by physically pressurizing the contents using an external compression device, such that expanded resin particles are bound to each other by fusion between skin layers on surfaces of the expanded resin particles (compression molding step). In accordance with a second embodiment of the present invention, there is provided a method for low-temperature molding of an expandable polymer resin, comprising: heating to pre-expand expandable polymer resin particles containing an expanding agent to a predetermined expansion ratio, and drying and aging the resulting expanded material to prepare expanded resin particles (pre-expansion step); coating and drying the expanded resin particles with a solution of a coating resin having a lower softening temperature than the expandable polymer resin to form skin layers on the surfaces of the expanded resin particles (skin layer formation step); and charging the expanded resin particles having the skin layers formed thereon to a mold, injecting a low-temperature heat transfer medium thereto and warming the contents to an expansion temperature of the expanding agent to thereby minimize the voids between particles via expansion of expanded resin particles, followed by physically pressurizing the contents using an external compression device to cause binding between expanded resin particles via fusion of skin layers on surfaces of the expanded resin particles (expansion-compression molding step). In accordance with a third embodiment of the present invention, there is provided a molding method of an expandable polymer resin at a low temperature, comprising: heating to pre-expand expandable polymer resin particles containing an expanding agent to a predetermined expansion ratio, and drying and aging the resulting expanded material to prepare expanded resin particles (pre-expansion step); coating and drying the expanded resin particles with a solution of a coating resin having a lower softening temperature than the expandable polymer resin to form skin layers on the surfaces of the expanded resin particles (skin layer formation step); and charging the expanded resin particles having the skin layers formed thereon to a mold, injecting a low-temperature heat transfer medium thereto and warming the contents to an expansion temperature of the expanding agent to thereby minimize the voids between particles via expansion of expanded resin particles while binding between expanded resin particles via fusion of skin layers on surfaces of the expanded resin particles (expansion-molding step). Meanwhile, a molding apparatus of an expandable polymer resin utilized in practicing a low-temperature molding method in accordance with the first and second embodiments of the present invention, includes an inlet for expandable polymer resin particles, an open-and-shut mode outlet for discharging a finished molded material and a transfer device for a molded material, wherein a heat transfer medium is injected to the inside of the mold formed in the apparatus and thus the expanded resin particles are fusion-molded by warming and pressurizing,

wherein at least a pair of oppositely facing sides inside the mold are provided with a pair of compression plates installed in a structure capable of being transferred within a given distance in close vicinity to each other, or is provided with a compression plate installed in a structure capable of being transferred within a given distance in close vicinity relative to the fixed side, such that the expanded resin particles filled inside the mold are pressurized to be compressed via transfer ofthe plate(s).

Advantageous Effects Use of a method and apparatus for low-temperature molding of an expandable polymer resin in accordance with the present invention can effect reduction of energy consumption due to capability to perform a molding process at a temperature significantly lower than in conventional methods. Performing a molding process at a substantially low temperature obviates a cooling process after completion of molding, leading to great reduction in production time and thus enhanced productivity. In addition, in accordance with the present invention, it is advantageously possible to produce a variety of functional products by addition of functional additives such as a flame retardant to a coating resin solution that forms skin layers. As such, the molding method and apparatus in accordance with the present invention can be widely applied to molding of expandable polymer resins such as EPS, EPE and EPP, leading to a reduction in production costs.

Brief Description the Drawings The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a cross-sectional view of main parts of a block-molding apparatus in accordance with one embodiment of the present invention; FIGS. 2 through 6 sequentially show processes for molding an expandable polymer resin utilizing a block-molding apparatus as shown in FIG. 1 ; FIG. 7 is a cross-sectional view of main parts of a shape-molding apparatus in accordance with another embodiment of the present invention; FIG. 8 is a photograph taken at a magnification of 400X for a cross section of molded material (Bl) obtained in step B of example 1 using an image analyzer; and FIGS. 9 and 10 are SEMs taken at magnifications of 30X and 400X, respectively, for a cross section of expanded polystyrene particles (A2) having skin layers formed thereon obtained in step A of example 2.

Best Mode for Carrying Out the Invention Respective steps of a method for low-temperature molding of an expandable polymer resin in accordance with the present invention and preferred embodiments of a molding apparatus will now be described in more detail.

Method for low-temperature molding of expandable polymer resin in accordance with a first embodiment (Compression molding)

Expansion step Expansion is not significantly different from pre-expansion as in conventional methods. That is, small particulate expandable raw resins (for example, EPS, EPE and EPP) in the form of spheres or pellets, containing an expanding agent, are charged to an expander, and stirred while heating to a temperature of about 103 ^°C to 105 ^°C using saturated steam, thereby expanding the expandable polymer resin particles to a desired specific gravity. Provided, since a secondary expansion is omitted at molding step in this embodiment, it is preferred to achieve maximal expansion such that there remains no expanding agent in the resin particles. After completion of expansion, the expanded particles are dried and aged under temporary storage in a silo. In addition, similar to conventional pre-expansion, this expansion step may be performed in two steps composed of first and second expansion depending upon an expansion ratio. In this case, after performing the first expansion, the expanded particles containing moisture are aged for a predetermined period of time in a silo by means of gravity or air such that internal pressure of the expanded particles is stabilized to the state equal to atmospheric pressure. This is followed by secondary expansion to a desired expansion ratio and re-aging.

Skin layer formation step In this step, surfaces of the expanded resin particles aged and stored after expansion are coated with a solution of a coating resin having a softening temperature much lower than the expandable polymer resin and are dried to form skin layers on the surfaces of the expanded resin particles. In this connection, the coating resin forming the skin layers is selected from those that have affinity for the expandable polymer resin, are soluble in solvents not eroding the surface of the expandable polymer resin, and have a softening temperature significantly lower than the expandable polymer resin, i.e., EPS, EPE or EPP and thus are partially softened at a temperature of 50^°C to 60^°C or lower, and therefore are fusible with one another upon pressurizing. As examples of such coating resin, mention may be made of natural resins such as dextrin and casein, and synthetic resins such as acrylic resins, vinyl acetate resins, polyester resins, polyurethane resins, polyvinyl alcohol resins and polyamide resins. However, since EPS resins exhibit lower chemical resistance to various organic solvents, it is particularly preferred to employ vinyl acetate resins that are soluble in solvents that do not impair the surface of the expandable polymer resin. Vinyl acetate resins used herein are selected from a vinyl acetate homopolymer or copolyer of vinyl acetate containing one or more vinyl esters such as vinyl caproate and vinyl stearate, acrylic esters such as ethyl acrylate, butyl acrylate and octyl acrylate, esters of fumaric acid such as dibutyl maleate, those having carboxyl groups such as maleic acid, acrylic acid and itaconic acid, and vinyl alcohols, butadiene, caprolactone, ethylene or mixtures or blends thereof having a degree of polymerization (DP) ranging from 10 to 100,000. Preferred are those having vinyl acetate monomer contents of 55% or more. As the solvents that can be used to dissolve the vinyl acetate resins, mention may be made of water and organic solvents such as alcohols, esters, ketones, carboxylic acids, aromatics, and halogenated hydrocarbons. It is preferred to select solvents having relatively high volatility, low harmfulness, and particularly less action to erode or dissolve surface of the expanded resin particles. As an example of such solvents, mention may be made of alcohols. The concentrations of coating resin components in the coating resin solution are determined depending upon their adhesiveness to the surface of the expanded resin particles and workability upon treatment. It is preferred to use the resin solution in which the coating resin is generally dissolved in a concentration of 3 to 80% by weight. In order to produce molded articles having improved functionality, the resin solution may include a variety of functional additives such as nucleating agents, lubricating agents, antioxidants, heat stabilizers, ultra violet (UN) stabilizers, fillers, reinforcing agents, plasticizers, colorants, anti-shock agents, flame retardants, antistatic agents, cross linking agents, fluorescent brighteners, heat conductivity- imparting agents, electrical conductivity-imparting agents, permeability control agents, magnetic agents, surfactants, stabilizers, excipients, medicaments, solvents, curing agents, desiccants, fortifying agents, fragrances and antibacterial agents. Meanwhile, the coating resin solution should be uniformly applied to surfaces of the expanded resin particles. Therefore, if the coating resin solution exhibits low viscosity, the resin solution is usefully applied by spraying it on surfaces of the expanded resin particles while stirring well. In contrast, if the coating resin solution exhibits high viscosity, the expanded resin particles are added to the resin solution and the mixture is stirred and mixed using a mixer, thereby achieving application. Application of the coating resin solution to the surfaces of the expanded resin particles causes formation of a lump of the expanded resin particles due to the viscosity of the resin solution, thus making it unsuitable for molding. Therefore, after applying the resin solution, a release agent is preferably added to separate individual expanded resin particles. That is, if coated expanded resin particles are likely to agglomerate due to high viscosity of the resin solution, the resin solution is applied to the particles, then coated particles are treated with the release agent so as to separate individual particles from one another, and dried to obtain coated expanded resin particles. As the release agent, liquid materials or solid powder other than solvents for the coating resin solution used may be utilized. Use of the solid powder results in reduced moldability and production of dust in the course of a drying process. In addition, if the liquid material used exhibits excessively higher affinity for the expandable polymer resin than that between the coating resin and expandable polymer resin, this causes separation of the coating resin from expanded resin particles. Therefore, the liquid release agent should be selected from those having affinity for the expandable polymer resin that is not excessively higher. For example, where the vinyl acetate resin solution is employed as the coating resin solution, considering the fact that expandable polymer resin particles such as EPS, EPE and EPP exhibit strong hydrophobicity, the liquid material having some hydrophilicity is preferred. In addition, after uniform application to the outside of the coated resin, it is necessary that the release agent remain until the solvent is appropriately removed. As the release agent simultaneously satisfying the above-mentioned requirements, mention may be made of water and other hydrophilic liquid materials such as ethylene glycol and glycerin having more than two hydroxyl (- OH) groups in its molecular structure, or water having improved polarity in which soluble salt compounds such as sodium chloride, borax, boric acid and sodium hydroxide have been dissolved. In addition, the release agent may be used alone or in admixture with two or more materials. The release agent may be suitably used in an amount of 1 to 40% by weight of the coating resin used, depending upon surface area of the expanded resin particles, the composition of the coating resin and the like. On the other hand, after application of the coating resin solution, or after addition of the release agent, the expanded resin particles are dried with stirring, thereby being separated into individual particles. Drying may be performed by a method of stirring the expanded resin particles while introducing warm air of 70 ^°C or less, a method of drying the expanded resin particles with stirring in a vessel heated below 70 °C , a method of drying the expanded resin particles under reduced pressure by application of vacuum, or a method involving combined use of warming and vacuum. The expanded resin particles, coated and dried in this skin layer formation step, are stored in a silo and then transferred to a subsequent molding process. Compression molding step In this step, the expanded resin particles having skin layers formed thereon are molded at a low temperature to form a molded material. That is, the expanded resin particles having skin layers formed thereon are first transferred and charged to a mold via use of vacuum or air and a low- temperature heat transfer medium is injected to the inside of the mold, thereby warming to a temperature of less than about 65 "C, such that the coated resins of the skin layers are softened. Of course, the internal temperature of the mold employed at this time may be varied somewhat depending upon kinds of the coating resin utilized. The expanded resin particles, in which the skin layers were softened, are physically pressurized by an external compression device, such that fusion between skin layers on surfaces of the expanded resin particles leads to binding between expanded resin particles, thereby form single molded material. After completion of molding, the molded material is cooled to a temperature below a softening point of the coating resin via use of cooling water or vacuum and released from the mold. According to the method of the present invention, since the molding temperature is significantly lower than that employed in conventional methods, namely, higher than 100^°C, the cooling process can be rapidly affected. In particular, in the case of block molding, since the internal temperature of the molded material is not particularly high even immediately after releasing from the molding apparatus, the molded material can be directly subjected to a cutting process to obtain a final product, without prolonged storage necessary for solidifying the molded material by lowering the internal temperature thereof. Method for low-temperature molding of expandable polymer resin in accordance with second embodiment (Expansion compression molding)

Pre-expansion step This pre-expansion step may be carried out using the same procedure as in conventional molding of expandable polymer resin particles. That is, an expanding agent for expansion in the molding process remains even after pre-expansion.

Skin layer formation step This step of skin layer formation is identical to that in the method for low- temperature molding of an expandable polymer resin in accordance with the first embodiment. Therefore, details thereof are not provided herein. Provided, since the temperature is raised to about 70 "C for expansion, it is sufficient if the softening temperature of the coating resin is below such temperature.

Expansion compression molding step In this expansion compression molding step, a low-temperature heat transfer medium is injected to a mold such that the internal temperature of the mold is elevated only to the expansion temperature of the expanding agent contained in the expanded resin particles, thereby causing expansion. That is, the expanding agent, for example butane or pentane, contained in the expandable polymer resin such as EPS may expand even when it is warmed only to about 70 °C, and the coated resin of the skin layer softens, as such, further increase of the temperature is not necessary. At this step, the voids between particles are minimized by expansion and at the same time, fusion between skin layers on the surfaces of the expanded resin particles by physical pressurization via the external compression device leads to binding of expanded resin particles, thereby forming single molded material. After completion of molding, a molded material is cooled to a temperature below a softening point of the coating resin via use of cooling water or vacuum and is released from of the mold.

Method for low-temperature molding of expandable polymer resin in accordance with third embodiment (Expansion molding)

Skin layer formation step This step of skin layer formation is identical to that in the method for low- temperature molding of an expandable polymer resin in accordance with the second embodiment. Therefore, details thereof are not provided herein.

Expansion molding step This expansion molding step is the same as in the conventional expansion molding method, except that a low-temperature heat transfer medium is injected to increase the internal temperature of the mold only to the expansion temperature of the expanding agent contained in the expanded resin particles, thereby achieving molding at a low temperature. That is, the expanding agent, for example butane or pentane, contained in the expandable polymer resin such as EPS may expand even when it is warmed only to about 70 ^°C and therefore warming is effected within the limit in which the pressure as required is provided by expansion without separate external pressurization. Within such a temperature range, the coated resin of the skin layers formed in the previous step softens, resulting in fusion between skin layers, thereby effecting molding. Similarly, after completion of molding, a molded material is cooled to a temperature below the softening point of the coating resin via use of cooling water or vacuum, and is released to the outside.

Molding apparatus A molding apparatus of an expandable polymer resin according to the present invention utilized in practicing a low-temperature molding method in accordance with the first and second embodiments of the present invention, includes, in a mold, at least a pair of oppositely facing sides as described above, a pair of compression plates installed in a structure capable of being transferred within a given distance in close vicinity relative to each other, or a compression plate installed in a structure capable of being transferred within a given distance in close vicinity relative to the fixed side, such that the expanded resin particles filled inside the mold are compressed via movement of the plate(s). Upon using the molding apparatus in accordance with the present invention, internal pressure produced during expansion of the expandable polymer resin particles and pressure artificially applied from the outside fill the voids between the expanded resin particles, thereby forming expanded particles being maximally closely fused to form a single molded material. Therefore, regardless of whether the secondary expansion is performed in the mold or not, it is possible to perform molding at or below the expansion temperature. The above-mentioned constitution can be commonly applied to both a shape-molding apparatus and a block-molding apparatus. In the case of a block molding apparatus, the compression plate(s) in the above-mentioned constitution are preferably formed on upper and lower sides of an approximately hexahedral mold, but may be formed on a pair of opposite side walls of right and left sides or front and rear sides. In addition, the compression plate(s) may also be formed on one surface of upper/lower, right/left and front/rear surfaces alone, if necessary. It is preferred to have a pair of opposite compression plates such as upper/lower or right/left plates in order to easily obtain a molded material having uniform density. Whereas, when using a shape molding apparatus, since molds having various shapes for shape molded articles are required, depending upon desired uses, shapes of compression plates or positioning thereof may become complex. Preferably, the compression plates are formed on one side surface of the mold, i.e. the surface opposite the surface having a complicated shape. However, if desired, compression plates may also be formed on a pair of opposite side walls, for example right and left side walls.

Block molding apparatus A block molding apparatus for use in the low-temperature molding method in accordance with the first and second embodiments of the present invention will now be described with reference to FIGS. 1 through 6. FIG. 1 is a cross-sectional view of main parts of a block-molding apparatus in accordance with one embodiment of the present invention, and FIGS.

2 through 6 sequentially show processes for molding expandable polymer resins utilizing this molding apparatus, h this connection, since the apparatus for molding an expandable polymer resin in accordance with the present invention may take the constitution identical to that of a conventional apparatus except for the compression plates, the description of the detailed constitution is greatly simplified and main constitutional parts are only shown for clarity. First, referring to FIG. 1, a block molding apparatus 1 of an expandable polymer resin in accordance with the present invention includes a mold 10 having a general hexahedral internal shape, and an open-and-shut gate 20 serving as an outlet for a finished molded material, an inlet (not shown) for charging expandable polymer resins and a steam inlet (not shown) for injecting steam serving as a heat transfer medium into the mold, systemically connected each other. Inner surfaces of the mold 10 are composed of molding plates 11 having smooth surfaces, such that molding of expandable polymer resins is affected. The molding plate 11 at one side of the mold 10 is connected to the open-and-shut gate 20 by which the molding plate 11 can be opened and closed. At the lower part of the molding plate 11 opposite the open-and-shut gate 20 is formed a pushing bar 30 which is a transfer device for transferring and discharging the molded material. The open-and-shut gate 20 and pushing bar 30 are, respectively, configured to be connected to pneumatic cylinder rods 21 and 31 such that open-and-shut action and transfer operation are performed by transfer force provided from pneumatic cylinders 22 and 32. Meanwhile, shafts 23 are formed on the upper and lower parts at both sides of the open-and-shut gate 20, which support the open-and-shut gate 20 to thereby provide a transfer path, upon opening and shutting. In the present invention, the upper and lower surfaces of the mold 10 are configured to be movable upward and downward without being fixed. That is, outside surfaces of the compression plates 40 and 40', which are molding plates forming the top and bottom surfaces of the mold 10, are connected to the cylinder rods 41 and 41' of the hydraulic cylinders 42 and 42', such that outside surfaces of the compression plates 40 and 40' are movable inward by transfer force of hydraulic cylinders 42 and 42'. With such a constitution, the compression plates 40 and 40' move inward by a predetermined distance, and apply pressure to expanded resin particles contained therein, thereby being capable of molding expanded resins. Briefly, a process for molding an expandable polymer resin-molded material utilizing an apparatus for molding an expandable polymer resin in accordance with the present invention will be described with reference to FIGS. 2 through 6. Firstly, to the mold 10, of an empty molding apparatus 1 as shown in FIG.

2, the open-and-shut gate 20 of which is shut, are charged expanded resin particles, which have been previously expanded and coated with a coating resin having a low softening point to form skin layers, via a separate inlet (not shown), as shown in FIG. 3. Steam is then injected thereto to soften the coated resin of skin layers. Next, as shown in FIG. 4, the compression plates 40 and 40' are moved inward by action of the upper and lower hydraulic cylinders 42 and 42' and thereby the expandable polymer resin charged therein is compressed to perform molding via fusion between skin layers. After completion of molding, the mold 10 is cooled and the upper and lower compression plates 40 and 40', as shown in FIG. 5, are returned to their initial positions. At this time, the resulting molded material rests on the lower compression plates 40'. Finally, as shown in FIG. 6, the open-and- shut gate 20 is opened by action of the pneumatic cylinders and cylinder rods 22, 32, 23 and 33 and the pushing bar 30 moves forward to discharge the molded material to the outside of the mold 10. After discharging, the molded material can be immediately cut into a final product using a cutting apparatus connected to the molding apparatus 10.

Shape molding apparatus A shape molding apparatus for use in the low-temperature molding method in accordance with the first and second embodiments of the present invention will now be described with reference to FIG. 7. FIG. 7 is a cross-sectional view of main parts of a shape molding apparatus in accordance with one embodiment of the present invention. This apparatus also may take the constitution identical to that of a conventional shape molding apparatus except for the compression plates, and therefore the detailed description of the constitution thereof is greatly simplified and main constitutional parts are only shown for clarity. As shown in FIG. 7, a shape molding apparatus 100 of an expandable polymer resin in accordance with the present invention includes a mold 110, an open-and-shut gate 120 connected to the mold 110, an inlet (not shown) for introducing raw material and an inlet (not shown) for injecting steam. Inner shape of the mold 110 is configured to have a shape mold 111 such that expandable polymer resins are molded into a molded article having a desired shape. One side of the shape mold 111 is connected to a cylinder rod 141 of a hydraulic or pneumatic cylinder 142, fixed to the open-and-shut gate 120. Thereby, the compression plate 140 is configured to move inward and pressurize expandable polymer resin particles charged therein, via transfer force provided by the above cylinder 142. Meanwhile, at the middle part of the shape mold 111 opposite the open- and-shut gate 120 is formed a pushing bar 130 which is a transfer device for transferring and discharging a finished molded article. The open-and-shut gate

120 and pushing bar 130 are, respectively, connected to pneumatic cylinder rods

121 and 131 such that open-and-shut action and transfer operations are performed by transfer force provided by pneumatic cylinders 122 and 132. Additionally, shafts 123 are provided on the upper and lower parts at both sides of the open-and- shut gate 120, which then support the open-and-shut gate 120 to thereby provide a transfer path, upon opening and shutting. Processes for obtaining an expandable polymer resin-molded material with the low-temperature molding method in accordance with the first and second embodiments of the present invention, utilizing the shape molding apparatus 100, can be carried out similar to those as in the block molding apparatus 1. That is, expanded resin particles, which were previously expanded and coated with a coating resin having a low softening point to form skin layers, are charged to the inner space of the shape mold 111 via a separate inlet (not shown), as shown in FIG. 7. Steam is then injected thereto to soften the coated resins of skin layers. Next, the compression plate 140 is moved inward (to the left in FIG. 7) to compress the expandable polymer resin particles contained therein by action of the right cylinder 142, thereby effecting molding via fusion between skin layers. After completion of molding, the mold 110 is cooled and the compression plate 140 is returned to its initial position. At this time, the resulting molded material maintains close contact with the left side of the shape mold 111. Then, the open- and-shut gate 120 is opened by action of the pneumatic cylinders 122 and 132 and cylinder rods 123 and 133, and the pushing bar 130 advances to the right to push the molded material to be discharged to the outside of the mold 110. The discharged molded material is placed on and transferred by the transfer device (not shown) installed at the lower part outside the molding apparatus 100.

EXAMPLES Example 1: Preparation of expandable polystyrene- molded material via compression molding

A. Preparation of expanded polystyrene particles having skin layers 450 g of vinyl acetate resin having a degree of polymerization of 500 was dissolved in 550 g of methanol having a purity of more than 98%>. 1 kg of aluminum hydroxide particles having an average particle size of 7.5 μm were homogeneously dispersed in the resulting resin solution, thereby obtaining 2 kg of vinyl acetate resin solution containing a functional additive. 2 kg of expanded polystyrene particles, which were completely expanded at an average expansion ratio of about 100 and were then aged, were charged to a

0.2 M³ ribbon mixer. 2 kg of vinyl acetate resin solution was added thereto and mixed for 1 minute while stirring at 100 rpm, such that the coating resin solution was uniformly applied to surfaces of the expanded polystyrene particles. After continued stirring for about 10 seconds while introducing warm air of 60 ^°C to 65 ^°C to the ribbon mixer, 50 g of ethylene glycol was sprayed with continuous stirring to separate coated particles into individual granules. Then, the thus-separated particles were dried with continued stirring for 3 minutes, thereby preparing expanded polystyrene particles (Al) having skin layers.

B. Compression molding The expanded polystyrene particles (Al) having skin layers thus prepared were charged to a self-fabricated molding apparatus as shown in FIG. 1, were heated for about 10 sec with steam at a pressure 3 kgf/cm² to soften coated resins of the skin layers. Then, compression plates were transferred by upper and lower hydraulic cylinders to apply pressure, thereby fusing expanded polystyrene particles. Thereafter, the resulting material was cooled for about 30 sec via use of water and a vacuum pump and discharged to obtain a molded material (Bl). The thus-obtained molded material (Bl) was cut using a knife. FIG. 8 shows a photograph of the cross section of the molded material (Bl) taken at a magnification of 400X using an image analyzer. As can be seen from FIG. 8, expanded polystyrene particles were uniformly fused to form a molded material without changes in the morphology of the particles.

Example 2: Preparation of expandable polystyrene-molded material via expansion compression molding A. Preparation of expanded polystyrene particles having skin layers Expanded polystyrene particles (A2) having skin layers were prepared using the same procedure as in Example 1, except that polystyrene particles were expanded at an expansion ratio of about 80 in a pre-expansion step, and expanded polystyrene particles, which contain the remaining expanding agent and are thereby capable of being additionally expanded in a molding step, were used. FIGS. 9 and 10 show, respectively, SEMs of expanded polystyrene particles (A2) having a skin layer formed thereon thus obtained. FIG. 9 is a photograph of a boundary with the skin layer, taken at a magnification of 3 OX. FIG. 10 is a photograph of a boundary with the skin layer, taken at a magnification of 400X. It was possible to confirm skin layers formed on surfaces of expanded polystyrene particles

B. Expansion compression molding The expanded polystyrene particles (A2) having skin layers thus prepared were charged to a self- fabricated block molding apparatus as shown in FIG. 1, were heated and expanded to 0.5 kgf/cm for 20 sec by introduction of steam at a pressure of 3 kgf/cm². Then, compression plates were transferred by upper and lower hydraulic cylinders to apply pressure, thereby fusing expanded polystyrene particles. Thereafter, the resulting material was cooled for about 30 sec via use of water and a vacuum pump and discharged to obtain a molded material (B2).

Example 3: Preparation of expandable polystyrene-molded article via expansion molding A. Preparation of expanded polystyrene particles having skin layers Expanded polystyrene particles (A3) having skin layers were prepared

^"using the same procedure as in Example 1, except that polystyrene particles were expanded at an expansion ratio of about 80 in a pre-expansion step, and expanded polystyrene particles, which contain an expanding agent in particles and thereby are capable of being additionally expanded in a molding step, were used.

B. Expansion molding 4 kg of expanded polystyrene particles (A3) having skin layers prepared in step A were charged to a conventional block molding apparatus, and were heated and expanded to 0.5 kgf/cm² for 30 sec by introduction of steam at a pressure of 3 kgf/cm², followed by cooling to obtain a molded material (B3). Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

Claims:

1. A method for low-temperature molding of an expandable polymer resin, comprising: heating to expand expandable polymer resin particles containing an expanding agent to a relevant expansion ratio, and drying and aging the resulting expanded material to prepare expanded resin particles (expansion step); coating and drying the expanded resin particles with a solution of a coating resin having a lower softening temperature than the expandable polymer resin to form skin layers on the surfaces of the expanded resin particles (skin layer formation step); and charging the expanded resin particles having the skin layers formed thereon to a mold, injecting a low-temperature heat transfer medium thereto and warming to soften the coating resin, followed by physically pressurizing the contents using an external compression device, such that expanded resin particles are bound to each other by fusion between skin layers on surfaces of the expanded resin particles (compression molding step).

2. A method for low-temperature molding of an expandable polymer resin, comprising: heating to pre-expand expandable polymer resin particles containing an expanding agent to a predetermined expansion ratio, and drying and aging the resulting expanded material to prepare expanded resin particles (pre-expansion step); coating and drying the expanded resin particles with a solution of a coating resin having a lower softening temperature than the expandable polymer resin to form skin layers on the surfaces of the expanded resin particles (skin layer formation step); and charging the expanded resin particles having the skin layers formed thereon to a mold, injecting a low-temperature heat transfer medium thereto and warming the contents to an expansion temperature of the expanding agent to thereby minimize the voids between particles via expansion of expanded resin particles, followed by physically pressurizing the contents using an external compression device to cause binding between expanded resin particles via fusion of skin layers on surfaces of the expanded resin particles (expansion-compression molding step).

3. A method for low-temperature molding of an expandable polymer resin, comprising: heating to pre-expand expandable polymer resin particles containing an expanding agent to a predetermined expansion ratio, and drying and aging the resulting expanded material to prepare expanded resin particles (pre-expansion step); coating and drying the expanded resin particles with a solution of a coating resin having a lower softening temperature than the expandable polymer resin to form skin layers on the surfaces of the expanded resin particles (skin layer formation step); and charging the expanded resin particles having the skin layers formed thereon to a mold, injecting a low-temperature heat transfer medium thereto and warming the contents to an expansion temperature of the expanding agent to thereby minimize the voids between particles via expansion of expanded resin particles while binding between expanded resin particles via fusion of skin layers on surfaces of the expanded resin particles (expansion-molding step).

4. A method according to any one of claims 1 to 3, wherein the coating resin is at least one selected from the group consisting of natural resins such as dextrin and casein, synthetic resins such as acrylic resins, vinyl acetate resins, polyester resins, polyurethane resins, polyvinyl alcohol resins and polyamide resins, their copolymers, and mixtures or blends thereof.

5. A method according to claim 4, wherein the vinyl acetate resin is selected from a vinyl acetate homopolymer, or a copolymer of vinyl acetate with one or more vinyl esters such as vinyl caproate and vinyl stearate, acrylic esters such as ethyl acrylate, butyl acrylate and octyl acrylate, esters of fumaric acid such as dibutyl maleate, those having carboxyl groups such as maleic acid, acrylic acid and itaconic acid, and vinyl alcohols, butadiene and caprolactone or mixtures or blends thereof and is dissolved in solvents such as water or alcohols, esters, ketones, carboxylic acids, aromatics and halogenated hydrocarbons.

6. A method according to any one of claims 1 to 3, wherein, after coating with the coating resin solution, a release agent is applied to separate individual expanded resin particles and is then removed by drying.

7. A method according to any one of claims 1 to 3, wherein functional additives such as nucleating agents, lubricating agents, antioxidants, heat stabilizers, ultra violet (UN) stabilizers, fillers, reinforcing agents, plasticizers, colorants, anti-shock agents, flame retardants, antistatic agents, cross linking agents, fluorescent brighteners, heat conductivity-imparting agents, electrical conductivity-imparting agents, permeability control agents, magnetic agents, surfactants, stabilizers, excipients, medicaments, solvents, curing agents, desiccants, fortifying agents, fragrances and antibacterial agents are further added to the coating resin solution.

8. A method according to any one of claims 1 to 3, wherein the expandable polymer resin is selected from the group consisting of expandable polystyrene, expandable polyethylene and expandable polypropylene, their copolymers, and mixtures or blends thereof.

9. A molding apparatus of an expandable polymer resin, comprising: an inlet for expandable polymer resin particles, an open-and-shut mode outlet for discharging a finished molded material and a transfer device for a molded material, wherein a heat transfer medium is injected to the inside of the mold formed in the apparatus and thus the expanded resin particles are fusion-molded by warming and pressurizing, wherein at least a pair of oppositely facing sides inside the mold, are provided with a pair of compression plates installed in a structure capable of being transferred within a given distance in close vicinity to each other, or is provided with a compression plate installed in a structure capable of being transferred within a given distance in the close vicinity relative to the fixed side, such that the expanded resin particles charged inside the mold are pressurized to be compressed via transfer of the compression plate(s).