JP2007218588A - Method of measuring bulk density of prefoamed particles - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for measuring the proper bulk density of prefoamed particles by suppressing the shrinkage of the prefoamed particles in a manufacturing method of the prefoamed particles. <P>SOLUTION: A part of the prefoamed particles, which are prefoamed by a prefoaming device and fed to a prefoamed particle storage tank from the prefoaming device, is sampled by a sampling container through sampling piping and sampled prefoamed particles are discharged to a dryer from the sampling container by a flower to be dried by air streams while the bulk density of the dried prefoamed particles is subsequently automatically measured by a bulk density measuring instrument. After the prefoamed particles are sampled by the sampling container to be held in the sampling container for a definite time, they are discharged to the dryer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for automatically measuring the bulk density of pre-expanded particles of a foamable thermoplastic resin to be foamed with a pre-foaming device.

  In order to produce a foamed resin product from a foamable thermoplastic resin, first, the thermoplastic resin particles (raw material resin) as a raw material are foamed to a predetermined bulk density by a pre-foaming device, and further foamed by a molding machine to be molded. The body is molded. In the production of the foamed resin product, to keep the bulk density of the prefoamed particles constant in order to stabilize the amount of the prefoamed particles supplied to the molding machine and to stabilize the quality of the molded body. is important.

  In general, the step of producing the above pre-foamed particles is to disperse a certain amount of raw material resin particles made of a thermoplastic resin into a pre-foamed pressurized container, such as water, a dispersant, and if necessary a surfactant. It is charged with an auxiliary agent, a foaming agent is charged, the inside of the pressurized container is adjusted to a predetermined pressure, and steam is passed through a jacket attached to the pressurized container to adjust the mixture in the pressurized container to a predetermined temperature. When the predetermined temperature and pressure are reached, the pressurized container discharge valve is opened, and the resin particles are discharged from the pressurized container through the nozzle together with water containing the dispersant and the like to cause foaming to obtain pre-expanded particles. The pre-expanded particles thus obtained are separated from water by a separator and sent to a pre-expanded particle storage tank.

Conventionally, as a method of adjusting the bulk density of the pre-foamed particles, the pre-foamed particles are collected, the bulk density is measured, and the pre-foaming device operating conditions (for example, based on the deviation from the target value) The amount of raw material particles supplied to the pre-foamed pressurized container, the pressure of the pre-foamed pressurized container, the temperature, etc.) are changed. The bulk density of the pre-expanded particles is determined by dehydrating the pre-foamed particles wetted with water vapor or water after pre-foaming with the pre-foaming apparatus, and measuring this with a bulk density measuring apparatus. For example, in Patent Document 1, a part of the pre-foamed particles pre-foamed by the pre-foaming device and sent from the pre-foaming device to the pre-foamed particle storage tank is collected in a sample collection container through the sample collection pipe, The pre-expanded particles are discharged from the sampling container to the dryer by a blower, the pre-expanded particles are air-dried, and then the bulk density of the dried pre-expanded particles is automatically measured by a bulk density measuring machine. Based on the measurement results of the bulk density of the particles, a method is known in which the foaming conditions are adjusted during the foaming time to produce pre-foamed particles having a uniform bulk density, that is, a stable foaming ratio.
WO2005 / 87475

  In the conventional well-known automatic magnification measuring apparatus and method for pre-expanded particles, when using a foaming agent that exhibits a liquid state at room temperature and atmospheric pressure, the pre-expanded particles are collected by collecting the pre-expanded particles from the process and drying them immediately. There is a problem of shrinking.

  This is presumably because the moisture on the surface of the pre-expanded particles evaporates in the drying step, the surface is cooled by the latent heat of evaporation, and the bubbles in the pre-expanded particles contract. When this shrinkage occurs, there is no correlation between the bulk density of the pre-foamed particles actually produced in the process and the pre-foamed particles collected by automatic measurement, and the correct bulk density cannot be measured during the production of the pre-foamed particles. There is a problem.

  Therefore, the problem to be solved by the present invention is to prevent the shrinkage of the pre-foamed particles in the production process of the pre-foamed particles using a foaming agent that exhibits a liquid state at room temperature and atmospheric pressure, such as water. It is to measure the bulk density of the pre-foamed particles, and further, based on the measurement result of the bulk density of the obtained pre-foamed particles, the foaming conditions are adjusted during the foaming time, This is to produce pre-expanded particles having a stable expansion ratio.

  In order to achieve the above object, the method for measuring the bulk density of pre-expanded particles according to the present invention is a pre-expansion of a foamable thermoplastic resin pre-foamed by a pre-foaming device and sent from the pre-foaming device to a pre-foamed particle storage tank. Part of the particles are collected in a sample collection container through a sample collection pipe, and the pre-foamed particles are discharged from the sample collection container to the dryer by a blower, and the pre-foamed particles are air-dried, and then dried pre-foam. In the method of automatically measuring the bulk density of particles with a bulk density measuring device, the present invention has found that the above-mentioned problems can be solved by pre-expanded particles being collected in a sample collection container and then discharged to a dryer after being held for a certain period of time. It was completed.

  That is, the first of the present invention is a method for measuring the bulk density of pre-expanded particles, in which a part of the pre-expanded particles that are pre-expanded by the pre-expanding device and sent from the pre-expanding device to the pre-expanded particle storage tank are sampled. After collecting the pre-expanded particles collected in a sample-collecting container through a sampling pipe, discharging the pre-expanded particles from the sample-collecting container to the dryer by a blower and air-drying the pre-expanded particles, the bulk density of the dried pre-expanded particles is increased. In the method of automatically measuring by a density measuring machine, after pre-expanded particles are collected in a sample collection container, the pre-expanded particles are held in the sample collection container for a certain period of time and then discharged to a dryer. The present invention relates to a method for automatically measuring the bulk density of expanded particles.

  The second aspect of the present invention is to measure the bulk density of the discharged pre-expanded particles by the above-described method while continuously discharging the pre-expanded particles from the pre-expanding device, and to calculate the measurement result as a bulk density comparison calculation device. The method for producing pre-foamed particles is characterized by adjusting the foaming conditions of the pre-foamed particles pre-foamed by the pre-foaming device by feeding back the result to the pre-foaming device in comparison with the target value About. As a preferred embodiment, the present invention relates to the manufacturing method described above, wherein the set pressure of the pre-foaming device is adjusted by feedback from the bulk density comparison calculation device.

  According to the method for measuring the bulk density of pre-expanded particles according to the present invention, the shrinkage of the pre-expanded particles collected from the pre-expanding apparatus is also used in the production of pre-expanded particles using a foaming agent that is liquid at room temperature and atmospheric pressure. It is possible to measure the bulk density with high accuracy in a short time while suppressing the above. Further, by feeding back the measurement result of the bulk density to the operating conditions of the pre-foaming apparatus, it is possible to produce pre-foamed particles having a predetermined bulk density.

  Hereinafter, the present invention will be described with reference to FIG. FIG. 1 schematically shows an entire pre-expanded particle manufacturing apparatus in which an automatic bulk density measuring apparatus 20 is connected to a pre-expanding apparatus 10.

  The pre-foaming apparatus 10 is an example of an apparatus for obtaining pre-foamed particles by a conventionally known method. For example, the pre-foaming apparatus 10 includes a pre-foaming pressurized container 1, a nozzle 5, and a separator 6. The pre-foamed pressurized container 1 includes an input port 2, a pressure setting device 3, and a pressurized container discharge valve 4. The separator 6 includes a drain pump 7.

  The process of producing the pre-foamed particles by the pre-foaming apparatus 10 is as follows. First, thermoplastic resin particles (hereinafter sometimes referred to as raw material particles), water, a dispersant, and, if necessary, a dispersion aid such as a surfactant are charged into the pre-foamed pressure vessel 1. Next, the pre-foaming pressure vessel 1 is heated to adjust the mixture in the pre-foaming pressure vessel 1 to a predetermined temperature, and the pressure in the pre-foaming pressure vessel 1 is adjusted to a predetermined pressure through the pressure setting device 3. . In this way, after the raw material particles are impregnated with the foaming agent, the pressure vessel discharge valve 4 is opened, and the raw material particles are expanded by the method of releasing into the low-pressure atmosphere through the nozzle 5 (depressurization foaming method). can get.

  Here, the thermoplastic resin particles are not particularly limited, and a known raw material resin for producing the pre-foamed particles can be used. Examples thereof include polyolefin resins such as polyethylene and polypropylene, and polystyrene resins such as polystyrene. If necessary, for example, cell nucleating agents such as talc, UV absorbers, antioxidants, antistatic agents, heat stabilizers, flame retardants, colorants, fillers, lubricants, crystal nucleating agents, etc. Agents may be blended.

  Here, the dispersant is not particularly limited, and a known dispersant for producing pre-foamed particles can be used. Examples thereof include inorganic salts such as tricalcium phosphate, basic magnesium carbonate, basic zinc carbonate, and calcium carbonate, and clays such as bentonite and kaolin.

  Here, the dispersion aid is not particularly limited, and a known dispersion aid for producing pre-expanded particles can be used. For example, anionic surfactants such as sodium dodecylbenzene sulfonate, sodium n-paraffin sulfonate, higher alcohol sodium sulfate, sodium alkylnaphthalene sulfonate, cations such as benzalkonium chloride, alkyltrimethylammonium chloride, dialkyldimethylammonium chloride Surfactants.

  In the present invention, the foaming agent is not particularly limited, and known ones such as a volatile foaming agent, an inorganic gas, and water can be used, but the foaming agent is particularly liquid at room temperature and atmospheric pressure. In some cases, for example, aliphatic hydrocarbons such as hexane, heptane and octane, cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane, cycloheptane and cyclooctane, halogenated carbons such as trichlorofluoroethane and dichloropentafluoropropane In the case of hydrogen, water, etc., the method of the present invention can be suitably used, and further, water is preferably used as a blowing agent.

  The pre-expanded particles thus obtained are separated from the water containing the dispersant and the like by the separator 6. The pre-expanded particles are sent to the pre-expanded particle storage tank 30 through the pipe 8 connecting the separator 6 and the pre-expanded particle storage tank 30.

  The automatic bulk density measuring device 20 of the present invention is provided with a sample collecting pipe 9 for sampling pre-foamed particles, which is branched from the middle of a pipe 8 connecting the separator 6 and the pre-foamed particle storage tank 30, and one end of the pipe A fixed-volume sample collection container 12 having a structure that does not allow the pre-expanded particles to pass therethrough and a dryer 15 are provided, and these are connected to the three-way valve 11. The sample collection container 12 includes a blower 13 including a heater 14. The dryer 15 is connected to a bulk density measuring machine 16 provided with a bulk density comparison calculation device 17.

  The measurement of the pre-expanded particles by the automatic bulk density measuring device 20 is performed as follows. During the pre-foaming, the three-way valve 11 causes the sample collecting pipe 9 and the sample collecting container 12 to communicate with each other, the sample collecting pipe 9 and the dryer 15 are disconnected, and the pipe between the separator 6 and the pre-foamed particle storage tank 30. The pre-expanded particles are collected from the sample collection pipe 12 through the sample collection pipe 9 and the three-way valve 11.

  The three-way valve 11 can be a commonly used three-way ball valve. There are two types of commercially available three-way ball valves, an L port type and a T port type. In the present invention, it is preferable to use an L port type. This is a ball valve for changing the direction of fluid in three directions. By rotating a ball with an L-shaped flow path by 90 degrees, instead of using three two-way valves, The direction can be switched by one. The diameter of the three-way valve only needs to be large enough to prevent the pre-expanded particles from being blocked. If the diameter of the pre-expanded particles is about 2 mm, the diameter of the pre-expanded particles is about 10 mm or more. , 50A (inner diameter approximately 50 mm) or more, can be collected without clogging. Three-way ball valves include a type in which the diameter of the ball portion is smaller than the diameter of the pipe, and a type in which both have the same diameter (referred to as a full bore). Either type can be used as long as the pre-expanded particles are not blocked.

  On the other end side of the sample collection container 12 (the side opposite to the side where the three-way valve 11 is connected), a mesh for preventing the pre-expanded particles from passing therethrough is provided. As long as the mesh captures the pre-expanded particles but can pass water and gas, the wire diameter and the mesh are not limited. Due to the internal pressure of the pre-foaming device 10 (pre-foaming pressurized container 1), the pre-foamed particles, water and gas pass through the three-way valve 11 and flow into the sample collection container 12, and the pre-foamed particles that flow in are captured by the mesh, Part of the water and gas pass through the mesh, and then flow backward through the heater 14 and the blower 13 and are discharged to the outside. In this way, the space A in the sample collection container 12 partitioned by the mesh is filled with a certain amount of pre-expanded particles by the internal pressure of the pre-expanding device 10. When the internal pressure of the pre-foaming device 10 is insufficient to fill the pre-expanded particles into the sample collection container 12, the pressure inside the sample collection container 12 is reduced through the mesh to The pre-expanded particles can be filled smoothly. The capacity of the sample collection container 12, that is, the volume of the space A partitioned by the mesh is about 0.05 to 10L, more preferably about 0.3 to 3L. If the capacity of the sample collection container 12 is too large, it becomes difficult to quickly dry the pre-expanded particles, and the bulk density may not be measured during the foaming time and the measurement results may not be fed back. If it is too small, the measurement accuracy of the bulk density decreases, which is not preferable.

  Next, after switching the three-way valve 11 to shut off the sample collection pipe 9 and the sample collection container 12, the collected pre-expanded particles are held as they are for a certain period of time for the purpose of suppressing shrinkage during drying in the next step. Is done.

  The predetermined time is the time when the sample collection pipe 9 and the sample collection container 12 are shut off, and the time when the air flow is generated by the blower 13 to send the pre-expanded particles to the dryer 15, which will be described later, is ended. It is time.

  The length of time to be retained depends on the temperature of the pre-foamed particles, the resin composition, and the boiling point of the foaming agent (gas), but when the foaming agent is water, it is preferably 0.5 minutes or more, More preferably, it is 1 minute or longer. For the purpose of suppressing shrinkage, it is preferable that this holding time is long. On the other hand, in order to feed back the measurement result, if this holding time is lengthened, the response may be delayed, and the pre-expanded particles having the desired bulk density may not be obtained. 3 minutes or less.

  The pre-expanded particles immediately after being collected in the sample collection container 12 contain a gasified foaming agent. When the pre-expanded particles immediately after collection are immediately dried by the method described in WO 2005/87475, the surface of the pre-expanded particles is cooled by latent heat of evaporation when the water on the surface evaporates. The agent is liquefied and the inside of the pre-expanded particles is depressurized, and the cell structure forming the pre-expanded particles is crushed and contracted. On the other hand, if the pre-expanded particles are held for a certain period of time as described above, the surrounding air gradually enters the pre-expanded particles during that time, so the degree of vacuum inside the pre-expanded particles is low when drying thereafter. Therefore, it is considered that shrinkage is suppressed.

  The pre-expanded particles thus held in the sample collection container for a certain period of time are then pushed out by the air flow generated by the blower 13 and sent to the dryer 15 and dehydrated and dried.

  The airflow may be heated through the heater 14 in order to shorten the time required for drying. The heater 14 can be a commonly used air heater. Examples of the heater 14 include a shell-type heater, a duct heater, a screw heater, and the like. Depending on the air volume and temperature required to send the pre-foamed particles from the sample collection container 12 to the dryer 15 for dehydration and drying. Select the one with the appropriate method and ability. The amount of air from the blower 13 used for drying and the temperature of the drying air adjusted by the heater 14 are the moisture content on the surface of the pre-foamed particles, the humidity of the dry air, the resin composition of the pre-foamed particles, and the type and amount of the foaming agent. What is necessary is just to set according to etc. Since drying time will become long if drying temperature is low, it is good to increase the air volume from the air blower 13 in this case.

  As the dryer 15, a dryer of a system generally used for drying powder particles can be used. The residence time of the pre-expanded particles in the dryer 15 may be a time required for drying. When the drying speed is fast and the pre-expanded particles are dried in a few seconds from the sample collection container 12 to the dryer 15, a dryer called air flow drying or flash drying can be used. When the drying speed is low, a cyclone dryer or a fluidized bed dryer can be used. If the dryer 15 has a structure in which dry air and pre-expanded particles rotate inside, such as a cyclone dryer, the pre-expanded particles stay in the dryer as long as the air is blown, and the drying is completed. The necessary residence time can be secured freely. However, when excessively dried, the pre-expanded particles whose surface has no moisture are charged inside the dryer 15 and the particles repel at the time of measuring the bulk density described later, so that the correct bulk density may not be measured. In this case, the bulk density can be measured normally by adding (spraying) an antistatic agent to the pre-expanded particles and grounding the device before drying. As the antistatic agent, commercially available antistatic agents, surfactants and the like can be used. As a countermeasure against electrification of dry particles, it is also effective to humidify the air used for drying.

  The dried pre-expanded particles are put into the bulk density measuring device 16 from the discharge valve 18 at the bottom of the dryer 15. The bulk density measuring device 16 can calculate the bulk density by collecting a sample in a container having a constant volume and measuring the weight of the sample. Further, the bulk density measuring device 16 includes a mechanism for discharging the measured sample to the outside. The measurement result of the bulk density is calculated by the bulk density comparison calculation device 17. The bulk density comparison calculation device 17 inputs the electrical signal of the measurement result of the bulk density output from the bulk density measuring device 16 into a personal computer or a programmable controller, and compares it with the target value. When it is calculated by the bulk density comparison calculation device 17 and is different from the target bulk density, a signal for setting a new pressure set value is sent to the pressure setting device 3 of the preliminary foaming device 10. Known devices can be used for the bulk density measuring device 16 and the bulk density comparison calculation device 17.

  EXAMPLES The present invention will be further described below with reference to examples, but the present invention is not limited to these examples. In the following examples, “part” represents “part by weight”.

(Examples 1-4)
The example which applied this invention to manufacture of polyolefin resin pre-expanded particle is given. Ethylene-propylene random copolymer (melting point 145 ° C., MI value 7 g / 10 min) 100 parts of polypropylene resin, ethylene- (meth) acrylic acid copolymer as a hydrophilic polymer crosslinked with potassium ion 2 parts of ionomer resin (trade name High Milan SD100 manufactured by Mitsui DuPont Polychemical Co., Ltd.), 1 part of melamine (trade name made by melamine BASF) as a compound having a triazine skeleton and a molecular weight per unit triazine skeleton of 300 or less, and inorganic filling Add 0.15 part of talc (average particle size 8 μm) as an agent and 2.6 parts of carbon black as a colorant, melt and knead with a 50 mmφ single screw extruder, and then form a strand from a cylindrical die with a diameter of 2.2 mmφ Extruded into water, cooled with water, cut with a cutter, from a cylindrical polyolefin resin composition To obtain resin particles (pellets) (1.8 mg / particle).

FIG. 1 shows 100 parts (800 kg) of the obtained resin particles, 0.75 part of tricalcium phosphate as an inorganic dispersant, 0.01 part of sodium n-paraffin sulfonate as a dispersion aid, and 200 parts of water. The pre-foaming pressure vessel 1 (capacity 3.0 m ³ ) of the pre-foaming device 10 shown in FIG. 2 was charged, and the temperature in the pre-foaming pressure vessel 1 was heated to 151 ° C. with stirring. Thereafter, compressed air was supplied from the pressure setting device 3 to raise the pressure of the pre-foamed pressurized container 1 to 2.3 MPa, and the temperature was kept at the internal temperature for 30 minutes. Thereafter, while adjusting the pressure in the pre-foamed pressurized container 1 with compressed air from the pressure setting device 3 so that the bulk density becomes 65 to 71 g / L, the pressurized container discharge valve 4 is opened and the nozzle 5 (opened) is opened. Pre-expanded particles were obtained by discharging into a steam saturated atmosphere at 100 ° C. through an orifice having a diameter of 3.6 mmφ and a numerical aperture of 5 holes. The foamed pre-expanded particles were separated from water by the separator 6 and sent to the pre-expanded particle storage tank 30 through the pipe 8 (diameter 250 mm).

  At this time, the pre-expanded particles were sampled through the 50 A sample collection pipe 9 connected in the middle of the pipe 8 connecting the separator 6 and the pre-foamed particle storage tank 30. The sampled pre-expanded particles pass through the sample collection pipe 9 and the three-way valve 11 when the 50A three-way valve 11 is opened to the sample collection pipe 9 and the sample collection container 12, and the sample collection container 12 having a capacity of 1.5L. The space A was filled. Ten seconds later, the filling of the pre-expanded particles was completed, and the three-way valve 11 was switched to shut off the sample collection pipe 9 and the sample collection container 12.

  Here, the pre-expanded particles were held for the time shown in Table 1.

Next, air of 5.0 m ³ / min and static pressure of 4.0 kPa is discharged from the blower 13, the heater 14 is heated so that the air temperature becomes 110 ° C., and the three-way valve 11 is connected to the dryer 15 and the sampling container. 12 was open. The pre-expanded particles filled in the sample collection container 12 with the static pressure of the air passed through the three-way valve 11 and were sent to the dryer 15. The blower 13 is stopped 60 seconds after the start of air blowing, the discharge valve 18 at the bottom of the dryer 15 is opened, and the pre-expanded particles that have been dried in a state in which the shrinkage is suppressed as shown in Table 1 are known structures. The product was dropped into the bulk density measuring machine 16 and charged.

  In the bulk density measuring device 16, the pre-expanded particles were put into a measuring cup having a capacity of 1.0 L, and a certain volume was collected. Excess pre-expanded particles 0.5 L overflowed from the measuring cup and discharged to the outside. The measuring cup was suspended by a measuring instrument, the weight W (g) of the pre-expanded particles was measured, and the bulk density of the pre-expanded particles was determined as W / 1.0 = W (g / L). . Next, the pre-expanded particles for which measurement was completed were discharged to the outside from a discharge valve provided at the bottom of the measuring cup, and the measurement of the bulk density was completed. Table 1 shows the error relative to the true bulk density.

さらに、アナログ信号入出力端子を内蔵したパソコンからなる公知の嵩密度比較演算装置１７により測定結果と目標値を比較し、設定圧力を調節する信号を圧力設定器３に送りながら、予備発泡粒子の製造を続けた。製造開始から終了までに測定した嵩密度の測定結果は、いずれも目標とする範囲である６５〜７１ｇ／Ｌにおさまっていた。

Further, the measurement result and the target value are compared by a known bulk density comparison arithmetic unit 17 composed of a personal computer having a built-in analog signal input / output terminal, and a signal for adjusting the set pressure is sent to the pressure setter 3, Production continued. The measurement results of the bulk density measured from the start to the end of the production were all within the target range of 65 to 71 g / L.

(Comparative Example 1)
Except for holding the pre-expanded particles collected in the sample collection container 12 for a predetermined time, immediately sending them to the dryer 15 and drying them, the bulk of the pre-expanded particles was obtained by the same method using the same apparatus as in Example 1. Density was measured.

  With this method, the pre-expanded particles contracted remarkably, and there was no correlation between the bulk density of the prepared pre-expanded particles and the automatically measured bulk density.

1 is an overall flow diagram of a pre-foamed particle manufacturing apparatus in which an automatic bulk density measuring apparatus according to an embodiment of the present invention is applied to a pre-foaming apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Prefoaming pressurization container 2 Input port 3 Pressure setting device 4 Pressurization container discharge valve 5 Nozzle 6 Separator 7 Drain pump 8 Piping 9 Sample collection piping 10 Prefoaming device 11 Three-way valve 12 Sample collection container 13 Blower 14 Heater 15 Dryer 16 Bulk density measuring device 17 Bulk density comparison calculation device 18 Dryer discharge valve 20 Automatic bulk density measuring device 30 Pre-foamed particle storage tank m mesh

Claims (3)

  A method for measuring the bulk density of pre-expanded particles, wherein a part of the pre-expanded particles that are pre-expanded by a pre-expanding device and sent from the pre-expanding device to a pre-expanded particle storage tank is sampled through a sample collection pipe. The pre-expanded particles collected are discharged from the sample collection container to the dryer by a blower and air-dried, and the bulk density of the dried pre-expanded particles is automatically measured by a bulk density measuring machine. In this method, after pre-expanded particles are collected in a sample collection container, the pre-expanded particles are automatically retained in the sample collection container for a certain period of time and then discharged into a dryer. How to measure.   While continuously discharging the pre-expanded particles from the pre-expanding device, the bulk density of the discharged pre-expanded particles is measured by the method according to claim 1, and the measurement result is set to a target value by a bulk density comparison calculation device. The method for producing pre-expanded particles is characterized in that the foaming conditions of the pre-expanded particles preliminarily expanded by the pre-expanding device are adjusted by feeding back the result to the pre-expanding device.   The manufacturing method of Claim 2 which adjusts the setting pressure of a preliminary | backup foaming apparatus with the feedback from the said bulk density comparison calculating apparatus.

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