JP2002361724A - Method for thermoforming polyethylene sheet, resin sheet for thermoforming, method for producing resin sheet for thermoforming, and method for producing thermoformed article using polyethylene sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a thermoformed article using a polypropylene sheet cannot have enough low temperature impact resistance nor can be used as a container for dairy products. SOLUTION: When an biaxially oriented polyethylene sheet S1 is heated, the sheet S1 is heat-softened by controlling heating to take out the stretch-back by an orientation effect of biaxial orientation, while controlling draw-down heat-softening within a prescribed range, and the heat-softened polyethylene sheet S1 is thermoformed by using a female mold for thermoforming (a prescribed thermoforming mold). By the stretch-back by biaxial orientation, the polyethylene sheet S1 can be held without the sheet being drawn down. In this way, a polyethylene sheet can be thermoformed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for thermoforming a polyethylene sheet, a resin sheet for thermoforming, a method for producing a resin sheet for thermoforming, and a method for producing a thermoformed product using a polyethylene sheet.

[0002]

2. Description of the Related Art Conventionally, containers used for frozen foods and the like are:
It is manufactured by performing thermoforming on a sheet of polypropylene or the like manufactured using a T-die. In the case of a tray for frozen foods, the production is performed by heating and softening the polypropylene sheet so that the polypropylene sheet does not draw down, and performing thermoforming using a predetermined female mold for thermoforming.
In the case of a wide mouth food container, since a thick polypropylene sheet is used, thermoforming is performed while a plug made of a material having a low dynamic friction coefficient is inserted into the thermoforming female mold. Here, since the polypropylene sheet is a slip-resistant material, it is necessary to use a material having a small dynamic friction coefficient for the plug, and the radius of curvature of the corner portion is set to be as large as about 15 mm. Further, there is a container using polystyrene or the like as a container used for food. In addition, containers for dairy products such as yogurt are made of paper or those obtained by laminating polyethylene on paper. When a polyethylene sheet manufactured by using a T-die is used instead of the polypropylene sheet, the temperature range from the softening by heating to the drawdown is narrow, so that the drawdown is caused by heating. Further, even if the thermoforming is performed at a temperature lower than the softening point of the polyethylene sheet, it is not sufficiently stretched. For this reason, thermoforming could not be performed stably with a polyethylene sheet.

[0003]

The above-mentioned prior art has the following problems. Polypropylene has a glass transition temperature of -20 to -10 ° C and is higher than polyethylene. That is, in a container obtained by thermoforming a polypropylene sheet, the impact strength in a frozen state is significantly reduced, and thus there is a problem that sufficient low-temperature impact resistance cannot be obtained. Further, even with polystyrene or the like, sufficient low-temperature impact resistance cannot be obtained. Furthermore, dairy containers are subject to ministerial ordinances concerning milk and dairy product standards (Ministerial Order for Milk, etc.), but polypropylene does not conform to the standards. Can not be. For this reason, there has been a desire to manufacture containers by thermoforming a polyethylene sheet that can sufficiently obtain impact resistance even in a frozen state and that complies with the product standards of the above ministerial ordinance. SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has a method for thermoforming a polyethylene sheet, a thermoforming resin sheet, and a thermoforming resin sheet capable of stably thermoforming a polyethylene sheet. It is an object of the present invention to provide a method and a method for producing a thermoformed product using a polyethylene sheet.

[0004]

Means for Solving the Problems In order to achieve the above object, the invention according to claim 1 is directed to a method of heating a biaxially stretched polyethylene sheet while keeping drawdown by heating softening within a predetermined range. The polyethylene sheet is heated and softened by controlling the heating so as to draw back by the stretching effect, and thermoforming is performed on the heated and softened polyethylene sheet using a predetermined thermoforming mold.

[0005] In the invention according to claim 1 configured as described above, when the biaxially stretched polyethylene sheet is heated, drawdown due to heat softening occurs.
Since the polyethylene sheet is biaxially stretched, the polyethylene polymer stretched in various directions on a plane tends to shrink in order to return to the original shape. That is, when the biaxially stretched polyethylene sheet is softened by heating, the polyethylene sheet will be turned back due to the stretching effect of the biaxial stretching.
Therefore, while heating and softening the biaxially stretched polyethylene sheet by controlling the heating so as to draw back by the stretching effect of the biaxial stretching while keeping the drawdown due to the heat softening within a predetermined range, the heat-softened polyethylene sheet is stretched. By turning it back, it can be held in a state where it is not drawn down, and thermoforming can be performed. Therefore, it is possible to stably perform thermoforming on the polyethylene sheet.

Various materials can be applied to the biaxially stretched polyethylene sheet. For example, a sheet made of a polymer of ethylene alone or a sheet of a copolymer of ethylene and another olefin may be used. Further, the sheet may be a sheet in which a resin other than polyethylene is mixed, or may be a sheet to which an additive is added. When a thermoformed product of a sheet is used for dairy products, a material such as milk may be selected so as to conform to a ministerial ordinance.

[0007] The invention according to claim 2 is an example of the polyethylene sheet to be thermoformed.
Wherein the polyethylene sheet is a sheet biaxially stretched at a crystallization temperature or higher and a melting point or lower of the polyethylene sheet. In the invention according to claim 2 configured as described above, since the biaxially stretched polyethylene sheet is biaxially stretched at a temperature equal to or lower than the melting point of the sheet,
Polyethylene molecules are solidified while being stretched in various directions on a plane. That is, the polyethylene sheet is suitable for performing thermoforming because the stretching effect of biaxial stretching is imparted and the polyethylene sheet is turned back. Further, the polyethylene sheet may be a sheet that is biaxially stretched at a temperature that is higher than or equal to the crystallization temperature and lower than or equal to the melting point of the polyethylene sheet and close to the crystallization temperature. In this case, since the polyethylene sheet is biaxially stretched at a temperature closer to the crystallization temperature than the melting point of the sheet, the polyethylene molecules are solidified while being further stretched. That is, since the polyethylene sheet is a sheet having a large stretching effect of biaxial stretching, the polyethylene sheet has a large rebound due to the stretching effect, and is suitable for thermoforming.

As an example of such a sheet, claim 3
According to a second aspect of the present invention, in the method for thermoforming a polyethylene sheet according to the second aspect, the polyethylene sheet is a sheet biaxially stretched by an inflation method. In the invention according to claim 3 configured as described above, since the polyethylene sheet is air-cooled while being biaxially stretched by the inflation method,
The film is biaxially stretched at a temperature close to the crystallization temperature. Here, an apparatus for producing a polyethylene sheet by an inflation method is an apparatus at a relatively lower cost than a molding apparatus by, for example, a tenter-type stretching method. That is, thermoforming can be performed on a polyethylene sheet that is biaxially stretched by a low-cost apparatus and has a large stretching effect of biaxial stretching. Obviously, it is only an example to obtain a biaxially stretched polyethylene sheet by molding by an inflation method, and the polyethylene sheet may be biaxially stretched by a tenter-type stretching method or the like.

[0009] The use of a polyethylene sheet having the above stretching effect makes it possible to carry out thermoforming by means of reversing. However, when the stretching ratio of biaxial stretching is very large, slight heating unevenness may occur. Phenomenon such as drilling may occur due to too strong upturn. Therefore, the invention according to claim 4 is the method of thermoforming a polyethylene sheet according to claim 3, wherein the polyethylene sheet is biaxially stretched on a low-magnification side among stretch ratios that can be stretched by the inflation method. It is configured as a sheet. That is, since the stretch ratio of the polyethylene sheet is low, the stretch is not too strong, and even if slight uneven heating occurs, no holes are formed.

[0010] As an example of the stretching ratio, the invention according to claim 5 is characterized in that, in the method for thermoforming a polyethylene sheet according to claim 4, the stretching ratio in the inflation method is 2 to 20 times. is there. That is, since the stretching ratio of the polyethylene sheet is as low as 2 to 20 times, the stretch is not too strong.

The invention according to claim 6 is an example of a polyethylene sheet suitable for thermoforming by biaxial stretching, as in the above-described polyethylene sheet.
The method for thermoforming a polyethylene sheet according to any one of claims 1 to 5, wherein the polyethylene as a raw material of the polyethylene sheet has a melt flow rate of 0.01 to
10 g / 10 min, the melt tension is 40 mN or more, and the strain hardening parameter is 1.2 or more. That is, since the melt flow rate is an appropriate value, the polyethylene sheet is stably biaxially stretched, and the drawdown does not become too large when softened by heating. Further, since the melt tension and the strain hardening parameter are not too small, the thickness of the thermoformed product becomes uniform.

As an example of a structure for controlling the heating of the polyethylene sheet, the invention according to claim 7 is directed to the method for thermoforming a polyethylene sheet according to any one of claims 1 to 6, wherein When heating, the temperature of the polyethylene sheet is higher than the softening point of the polyethylene sheet and the melting point is 50%.
The configuration is such that the heating is controlled so as to be lower than the temperature obtained by adding ° C. That is, since the biaxially stretched polyethylene sheet is heated and controlled between the softening point of the sheet and the temperature obtained by adding 50 ° C. to the melting point, the draw effect due to heat softening is kept within an appropriate range and the stretching effect is maintained. Is pulled out.

[0013] As another example, the invention according to claim 8 is a method for thermoforming a polyethylene sheet according to any one of claims 1 to 7, wherein the polyethylene sheet is preheated by a preheater roll. Then, it is configured to perform radiant heating after preheating. That is, since the polyethylene sheet to be heated and softened is preliminarily heated by the preheater roll, the sheet can be quickly brought to the target temperature during radiant heating, and the efficiency of thermoforming can be improved. Needless to say, a configuration in which the polyethylene sheet is preheated by the preheater roll and the radiation heating is performed is only an example when the heating control of the polyethylene sheet is performed. For example, hot air may be supplied to preheat the sheet, or only radiant heating without preheating may be performed.

There are times when it is desired to perform thermoforming using a thick sheet made of polyethylene. Therefore, the invention according to claim 9 is based on claims 1 to 8 described above.
The method for thermoforming a polyethylene sheet according to any one of the above, wherein the polyethylene sheet is a multilayer sheet in which a biaxially stretched polyethylene layer and another polyethylene layer or another resin layer are laminated. There is. In the invention according to claim 9 configured as described above, since the polyethylene sheet is a multilayer sheet, it can be a thick sheet. Then, when the multilayer sheet is heated and softened, the biaxially stretched polyethylene layer is turned back by the stretching effect of the biaxial stretching. Thick sheets tend to have a large drawdown due to their own weight, but drawback prevents the drawdown. In other words, even if there is a resin layer having no stretching effect of biaxial stretching in the multilayer sheet, thermoforming can be easily performed, so that it is suitable when performing thermoforming using a thick sheet using polyethylene. Configuration. In addition, if the polyethylene layer is biaxially stretched at a temperature close to the crystallization temperature of the polyethylene, rebound due to the stretching effect is large, which is suitable for thermoforming. Also,
When biaxial stretching on the low-magnification side is performed by the inflation method, the re-stretching is not too strong, and even if slight uneven heating occurs, perforation does not occur.

The thermoforming may be performed by inserting a plug into the thermoforming female mold. As one example of the configuration, the invention according to claim 10 is any one of claims 1 to 9 described above. In the thermoforming method for a polyethylene sheet according to any one of the above, the thermoforming mold is a female mold for thermoforming capable of closely contacting the heat-softened polyethylene sheet on the inner surface, and the heat-softened polyethylene sheet, The thermoforming is performed while a plug made of a material having a large coefficient of kinetic friction that can advance and retreat into the female mold for thermoforming enters the female mold for thermoforming. That is, since the polyethylene sheet is a slippery material, the thickness of the side surface of the thermoformed product can be made uniform by using a plug made of a material having a large dynamic friction coefficient.

Further, according to an eleventh aspect of the present invention, in the method for thermoforming a polyethylene sheet according to the tenth aspect, the plug has a corner having a radius of curvature of 5 mm or less at which the plug comes into contact with the heat-softened polyethylene sheet. There is a certain configuration. That is, since the polyethylene sheet is a slippery material, the thickness of the side surface of the thermoformed product can be made uniform by using a plug having a radius of curvature of 5 mm or less at a corner portion that comes into contact with the heat-softened polyethylene sheet. Becomes possible.

As an example of the configuration of the thermoforming mold having a vacuum suction hole, in the method of thermoforming a polyethylene sheet according to any one of claims 1 to 11, the thermoforming mold may be a vacuum mold. A thermoforming female mold having a vacuum suction hole capable of applying pressure and capable of closely contacting the heat-softened polyethylene sheet on the inner surface thereof, and having fine irregularities formed on the inner surface thereof, The thermoforming may be performed by applying a vacuum pressure to the vacuum suction hole. That is, since fine irregularities are formed on the inner surface of the thermoforming mold, when a vacuum pressure is applied to the vacuum suction hole, air is reliably transmitted from the closed space formed by the inner surface and the polyethylene sheet. It is sucked.

In the polyethylene sheet comprising the multilayer sheet according to the ninth aspect, a biaxially stretched polyethylene layer has a stretching effect. Therefore, the invention according to claim 12 is a thermoforming resin sheet in which a plurality of resin layers are laminated, wherein a biaxially stretched polyethylene layer and another polyethylene layer or another resin layer are formed. It is configured to be a laminated multilayer sheet. In the twelfth aspect of the present invention configured as described above, when the thermoforming resin sheet is softened by heating, the polyethylene layer undergoes reversion due to the biaxial stretching effect. That is, even if there is a resin layer having no stretching effect of biaxial stretching, the stretching effect of biaxial stretching can be imparted, and thermoforming can be performed. When the resin sheet for thermoforming is a thick sheet, the drawdown tends to be large. However, the drawdown is prevented because the biaxially stretched polyethylene layer is given a stretching effect by the biaxial stretching. Can come off. That is, it is also effective as a single thermoforming resin sheet, and is suitable when thermoforming is performed using a thick sheet using polyethylene.

According to a thirteenth aspect of the present invention, in the thermoforming resin sheet according to the twelfth aspect, the polyethylene layer is biaxially stretched by an inflation method and stretchable by the inflation method. It is configured to be stretched on the low magnification side among the various magnifications. In other words, the resin sheet for thermoforming is biaxially stretched at a temperature close to the crystallization temperature by the inflation method, and the stretching effect is imparted. There is no perforation. Needless to say, the present thermoforming resin sheet may be a sheet corresponding to the third, fifth, and sixth aspects.

Further, as an example of a structure for producing the present thermoforming resin sheet, the invention according to claim 14 is a sheet-like form in which polyethylene in a molten state is extruded from a predetermined first extruder and biaxially stretched. The multilayered thermoforming resin sheet is manufactured by extruding polyethylene or other resin in a molten state from a predetermined second extruder and laminating and bonding to a biaxially stretched sheet-like polyethylene. . Claim 14 configured as described above.
In the present invention, the polyethylene layer constituting the multilayer thermoforming resin sheet is extruded from the first extruder and biaxially stretched. Then, the polyethylene layer or another resin layer to be laminated on the polyethylene layer is extruded from the second extruder and laminated and bonded to the polyethylene layer. That is, even if there is a resin layer having no biaxial stretching effect on the thermoforming resin sheet, the stretching effect by biaxial stretching can be imparted, so that a thermoformed article provided with a polyethylene layer is molded. An example of a configuration for producing a thermoforming resin sheet suitable for the present invention can be provided.

According to a fifteenth aspect of the present invention, in the method for producing a thermoforming resin sheet according to the fourteenth aspect, when the molten polyethylene is formed into a biaxially stretched sheet, the inflation is performed. It is configured to be biaxially stretched by a method. That is, a thermoforming resin sheet having a large stretching effect of biaxial stretching can be manufactured. Further, the invention according to claim 16 is the invention according to claim 1.
5. The method for producing a resin sheet for thermoforming according to item 5, wherein the sheet-like polyethylene is biaxially stretched on a low-magnification side among magnifications that can be stretched by the inflation method. That is, since the re-stretching due to the stretching effect is moderate, it is possible to manufacture a thermoforming resin sheet that enables more reliable thermoforming.

The present invention is also applicable to the case where a thermoformed product is manufactured by the above-described thermoforming method for a polyethylene sheet. Therefore, the present invention is also applicable as a method for manufacturing a thermoformed product using a polyethylene sheet, and the invention according to claim 17 has basically the same operation. Further, it goes without saying that the thermoforming method described in claims 2 to 11 can be adapted to the manufacturing method.

[0023]

As described above, the present invention can provide a method for thermoforming a polyethylene sheet which can stably perform thermoforming on a polyethylene sheet. According to the second aspect of the present invention, the polyethylene sheet is given a stretching effect of biaxial stretching, so that the polyethylene sheet is turned back. Therefore, it is easy to perform thermoforming. Further, according to the third aspect of the invention, it is possible to perform thermoforming on a polyethylene sheet which is biaxially stretched by a low-cost apparatus and has a large stretching effect of biaxial stretching.
Furthermore, according to the fourth and fifth aspects of the present invention, it is possible to prevent perforation due to minute heating unevenness without excessively strong upsetting.

Further, according to the invention according to claim 6,
The polyethylene sheet is stably biaxially stretched and drawdown does not become too large when softened by heating, so that the thermoformed thermoformed product has a uniform thickness. Further, according to the seventh aspect of the invention, it is possible to heat-soften the polyethylene sheet by drawing out the rebound by the stretching effect while keeping the drawdown due to the heat softening within an appropriate range. Furthermore, according to the invention of claim 8, since the polyethylene sheet can be quickly brought to the target temperature when performing radiant heating, it is possible to improve the efficiency of thermoforming. Further, according to the ninth aspect, it is possible to perform thermoforming using a polyethylene sheet in which another polyethylene layer or another resin layer is laminated. At this time, even if the sheet is thick, it does not draw down, and even if there is a resin layer having no biaxial stretching effect in the multilayer sheet, thermoforming is easily performed by re-stretching due to the biaxial stretching effect. It becomes possible.

Further, according to the tenth and eleventh aspects of the present invention, it is possible to make the side wall thickness of the thermoformed product uniform. Further, according to the invention of claim 12,
Even if there is a resin layer having no stretching effect of biaxial stretching, a resin sheet for thermoforming that enables draw-down to be prevented by stretching back due to the stretching effect of biaxial stretching and enables easy thermoforming. Can be provided. Furthermore, according to the thirteenth aspect of the present invention, since the re-stretching due to the stretching effect becomes appropriate, it is possible to more reliably perform thermoforming.

According to the fourteenth aspect of the present invention, even if there is a resin layer having no biaxial stretching effect, the drawdown is prevented by the reversal due to the biaxial stretching effect and thermoforming is performed. And a method for producing a thermoforming resin sheet that can be easily performed. Further, according to the invention of claim 15, a thermoforming resin sheet having a large stretching effect of biaxial stretching can be manufactured, and according to the invention of claim 16, reversion by the stretching effect becomes appropriate. Further, according to the seventeenth aspect of the present invention, it is possible to provide a method for producing a thermoformed product using a polyethylene sheet, which can stably perform thermoforming on the polyethylene sheet.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in the following order. (1) Configuration of manufacturing apparatus for biaxially stretched polyethylene sheet: (2) Configuration of thermoforming apparatus for polyethylene sheet: (3) Action when thermoforming polyethylene sheet: (4) Second Embodiment:

(1) Configuration of an apparatus for producing a biaxially stretched polyethylene sheet: FIG. 1 shows a polyethylene sheet (hereinafter, referred to as PE) used for performing a thermoforming method of a polyethylene sheet according to the first embodiment of the present invention. 1 schematically illustrates an outline of a sheet manufacturing apparatus 100 for manufacturing a sheet. The sheet manufacturing apparatus 100 is for manufacturing a PE sheet biaxially stretched from polyethylene in a molten state by an inflation method, but an apparatus capable of manufacturing a resin sheet such as polypropylene in addition to polyethylene. But also. In the drawing, when polyethylene as a raw material is put into a hopper 102 attached to an upper portion of an extruder 101, the polyethylene is heated and melted by a heater (not shown) inside the extruder 101. The extruder 101 supplies the melted polyethylene to the inflation die 103. Then, the polyethylene is formed into a cylindrical shape and extruded upward from the inflation die 103.

The inflation die 103 is a so-called spiral die, and has an inner mandrel 103a and an outer ring 103b as shown in the perspective view of the main part of FIG. Of course, the use of a spiral die is merely an example, and other dies for inflation molding may be used. An annular slit 103c is formed above the inflation die 103 between the inner mandrel 103a and the outer ring 103b. A molten resin supply port (not shown) is formed on the lower side surface of the inflation die 103, and the polyethylene melted by the extruder 101 flows into the inside of the inflation die 103 from the molten resin supply port and is annularly formed from the slit 103c. It flows out upward. Although not performed in this embodiment, a multilayer die may be used. In this case, a biaxially stretched sheet composed of a plurality of layers can be obtained,
Thermoforming can be performed using this sheet.

The interval between the slits 103c can be adjusted by replacing the inner mandrel 103a or the outer ring 103b. Therefore, if the thickness is up to about 1 mm, the PE sheet to be manufactured can have a desired thickness. Further, the stretching ratio can be changed in accordance with the interval between the slits 103c, the temperature of the molten polyethylene, the flow rate of the polyethylene, and the like. Here, the stretching ratio is a ratio of the area of the PE sheet after solidification to the area of the polyethylene immediately after being extruded from the slit 103c. For example, the traveling direction of the PE sheet in the sheet manufacturing apparatus 100 is defined as a vertical direction,
Assuming that the stretch ratio of the PE sheet is 3 times in the vertical direction and 6 times in the horizontal direction, the draw ratio is 3 × 6 = 18 times. In the inflation method, the stretching ratio of polyethylene can be 200 times or more.

An air supply port 103d is formed below the inflation die 103, and air is supplied to the air supply port 103d by an air supply mechanism (not shown) at the start of the production. Then, the air flows out from the air outlet 103e provided in the upper part to the substantially central portion of the polyethylene which is formed into a ring and extruded. And
The polyethylene in the molten state extruded from the slit 103c is expanded by the air supplied from the air outlet 103e while being air-cooled by a blowing mechanism (not shown), and solidified into a cylindrical shape while rising. In addition, since the cylindrical polyethylene is sandwiched between the pair of pinch rolls 105, 105 arranged above the inflation die 103, air needs to be supplied to the inside of the cylindrical polyethylene only at the start of production.

Above the inflation die 103,
A pair of flat guide plates 104, 104 arranged in a substantially C shape are provided, and the ascending cylindrical polyethylene is continuously folded. The folded polyethylene is sandwiched between pinch rolls 105, 105, and the folded portion is cut open by a cutter 106. After that, the PE sheet S1 moves according to the plurality of guide rolls 107 and is wound up by a pair of winders 108. Then, the PE sheet S1 biaxially stretched by the inflation method is formed into a roll.

Incidentally, polyethylene is a crystalline polymer, and the crystallization temperature is lower than the softening point. For example, in the case of HDPE (high density polyethylene), the melting point is about 130 ° C., the softening point is about 126 ° C., and the crystallization temperature is about 115 ° C. For example, when polyethylene in a molten state is extruded from a T-die and cooled directly by a cooling roll, the polyethylene is cooled from a temperature higher than the melting point to a temperature lower than the crystallization temperature, so that the polyethylene polymer is crystallized in a state where it is hardly oriented. Is done. That is, since the solidified PE sheet hardly has a stretching effect and is not stretched particularly in the width direction of the PE sheet, even if drawdown occurs due to heating, almost no reversal occurs in the width direction. Therefore, it was not possible to make the PE sheet stretched back when heated. In addition, since the temperature range from the heat softening to the drawdown of the PE sheet is narrow, it is very difficult to control the heating so that the drawdown does not occur. Therefore, it is extremely difficult to stably perform thermoforming using the PE sheet manufactured by such a manufacturing method.

On the other hand, in the sheet manufacturing apparatus 100, the polyethylene is expanded while gradually cooling it with air.
The biaxial stretching is performed at a temperature not lower than the crystallization temperature and not higher than the melting point of the PE sheet and close to the crystallization temperature. In other words, the film is stretched in both the vertical and horizontal directions while crystallization proceeds, and is air-cooled, so that the polyethylene molecules are solidified while being stretched in various directions on the plane. In other words, the solidified PE sheet is provided with a stretching effect of biaxial stretching. When the PE sheet is heated and softened so as to be drawn down by heating, the polyethylene molecules shrink and sandwich the edge of the sheet as shown in FIG. Otherwise, the entire PE sheet will be small. When the edge is sandwiched, it is in a state where it is not drawn down, that is, it is in a stretched state. This phenomenon is reversal due to the stretching effect of biaxial stretching. Therefore, the heat-softened PE sheet can be held in a state in which it has not been drawn down by being stretched, and thermoforming can be performed.

Here, when the stretching ratio of the biaxial stretching is very large, when the PE sheet is heated, delicate unevenness in heating may cause a phenomenon such as excessive reversal and opening of holes, which may occur. Therefore, among the draw ratios that can be drawn by the inflation method, biaxial drawing is performed at a low draw ratio of 2 to 20 times to PE.
I'm going to manufacture sheets. As a result, the re-stretching due to the stretching effect becomes appropriate, so that the thermoforming can be performed favorably. The sheet manufacturing apparatus 100 for manufacturing a PE sheet by the inflation method is a relatively low-cost apparatus as compared with a manufacturing apparatus using a tenter-type stretching method or the like. A PE sheet having a large stretching effect in axial stretching can be manufactured. Needless to say, even a relatively expensive apparatus such as a manufacturing apparatus using a tenter-type stretching method can biaxially stretch polyethylene in a molten state and impart a biaxial stretching effect to a PE sheet.

By the way, polyethylene which is a raw material of a PE sheet suitable for thermoforming by being manufactured by the sheet manufacturing apparatus 100 is as follows. For MFR (melt flow rate), 0.01 to 10 g / 10
Minutes, preferably 0.02 to 5 g / 10
Minutes. The value of this MFR is JIS K6922-2.
2.16kg in accordance with the Annex to the polyethylene test method
It is obtained by measuring with a load. If the MFR is too small, it is difficult to extrude the molten polyethylene stably from the inflation die. If the MFR is too large, the drawdown becomes too large when the PE sheet is softened by heating. For density, 0.88 ~
It is preferably 0.97 g / cm ³ . The value of density is JI
It was measured and determined in accordance with SK692-2-2 Annex, Polyethylene Test Method.

[0037] The melt tension is preferably at least 40 mN, and more preferably at least 60 mN. The melt tension value was determined using a melt tension tester manufactured by Toyo Seiki Co., Ltd., so that the L / D of the soybean was 8.02 / 2.10 mm, the cylinder temperature was 190 ° C., the extrusion speed was 20 mm / min, and the take-up speed was 10 m / m.
It was obtained by measuring under the condition of minutes. The strain hardening parameter is preferably 1.2 or more, and more preferably 1.5 or more. The value of the strain hardening parameter was determined as follows. First, MELTON made by Toyo Seiki
RHEOMETER, temperature 150 ° C, strain rate 0.1
The change in elongational viscosity over time is measured under the conditions described above, and a logarithmic graph of a time-elongational viscosity curve is created with the horizontal axis representing the logarithm of time and the vertical axis representing the logarithm of extensional viscosity. This graph becomes a straight line having a gentle slope at the initial stage of the measurement, and deviates from the straight line at a later stage to become a curve. Here, an area where a straight line is drawn is called a linear area, and an area where a curve is drawn thereafter is called a non-linear area. In the case of a material exhibiting strain hardening, in the nonlinear region, the slope of the curve once increases, and at a certain stage, the elongational viscosity (logarithmic value) reaches a maximum value, and the slope thereafter becomes negative. The maximum value is defined as ηmax, and the elongational viscosity (logarithmic value)
The strain hardening parameter λmax was determined by the following equation (1), with the value (logarithmic value) of the elongational viscosity on the extension of the straight line in the linear region at the time (logarithm) at which the maximum value ηmax was reached as η1. λmax = ηmax / η1 (1) If the melt tension or the strain hardening parameter is too small, P
When thermoforming is performed on the E sheet, the thickness of the thermoformed product is not easily uniform, and a good thermoformed product cannot be obtained.
Further, when the PE sheet is heated and softened, the drawdown becomes too large.

Various materials can be applied to the PE sheet having the above-mentioned properties and capable of being thermoformed.
For example, a sheet made of a polymer of ethylene alone or a copolymer sheet of ethylene and another α-olefin may be used. Further, a sheet in which a resin other than polyethylene such as polypropylene, polystyrene, polyamide, etc. may be mixed, or a sheet to which additives such as an antioxidant, an ultraviolet absorber, an inorganic filler, and an antistatic agent are added. There may be. In order to improve the rigidity of the thermoformed product, 1 to 50% of talc may be added. further,
Although described in detail later, a multilayer PE sheet in which another polyethylene layer or another resin layer is laminated on a biaxially stretched polyethylene layer may be employed.

(2) Structure of Thermoforming Apparatus for Polyethylene Sheet: FIG. 4 shows a thermoforming apparatus 20 used for carrying out a thermoforming method for a polyethylene sheet according to an embodiment of the present invention.
0 is schematically shown. The thermoforming device 2
00 is for performing thermoforming on a biaxially stretched PE sheet, but it is also possible to perform thermoforming on a resin sheet such as polypropylene besides the PE sheet.
In the figure, a thermoforming apparatus 200 generally includes an unwinding machine 201 for unwinding a rolled PE sheet S1, a transfer machine 202 for transferring an unrolled PE sheet S1, and a PE.
A heater 210 for heating and softening the sheet S1, a transport mechanism (not shown) for holding the edge of the PE sheet S1 and transporting the PE sheet S1, a forming machine 220 for thermoforming the PE sheet S1, and trimming for trimming Machine 20
3. Take-out machine 20 for taking out the trimmed thermoformed product
4. Winding machine 20 for winding the scrap of PE sheet S1
5 is provided. Then, a process of continuously manufacturing a thermoformed product using the PE sheet S1 is performed.

In the above configuration, the unwinder 201 is provided with a roll-shaped PE sheet S1. The PE sheet S1 is a sheet that has been biaxially stretched by the inflation method in the sheet manufacturing apparatus 100. And P
The required amount of the E sheet S1 is sequentially unwound from the unwinder 201 while being sandwiched by the rollers of the transfer device 202. The unwound PE sheet S1 is transferred to the transfer machine 2
At 02, it is transferred to the heater 210.

The heating device 210 includes a pre-heater roll 211
And a heater 212. Preheater roll 211
Is a temperature lower than the softening point of the PE sheet S1 (for example, 1
(10 ° C.). Preheater roll 211
Is sandwiching the PE sheet S1 between a pair of rollers,
The PE sheet S1 is preheated by contact heating. The preheated PE sheet S <b> 1 is conveyed below the heater 212, with the edge portion sandwiched by the transport mechanism. Heater 212
Radiantly heats the pre-heated PE sheet S1. The temperature of the heater 212 is set so that the temperature of the PE sheet S1 is higher than the softening point and lower than the temperature obtained by adding 50 ° C. to the melting point. When the pre-heating of the PE sheet S1 is not performed, the temperature of the heater needs to be set to, for example, about 300 ° C. However, when preheating the PE sheet S1, the temperature may be set to a relatively low temperature of about 130 to 180 ° C.

Although the PE sheet S1 heated by the heater 212 has a melting point or higher and draws down once, it is held in a state in which it is not drawn down (stretched) due to reversal due to the stretching effect of biaxial stretching. You. Note that heaters may be disposed on both the upper surface and the lower surface of the PE sheet S1 to heat and soften the PE sheet S1. As described above, in the heater 210, since the PE sheet S1 is preheated by the preheater roll 211, the temperature of the heater 212 can be set low, and the heating can be performed more uniformly. In addition, the sheet can be quickly brought to the target temperature when performing radiant heating, and the production efficiency can be improved.

The heat-softened PE sheet S1 is transferred to the molding machine 220 by the transfer mechanism 206 shown in FIG.
FIG. 5 is a vertical sectional view showing a main part of the molding machine 220 as viewed from the position of AA in FIG. Also, the transport mechanism 2
06 and the like are shown in a simplified manner. The molding machine 220 includes an upper table 221 that can be vertically driven upward, a lower table 222 that can be vertically driven downward, and an upper table 221.
The upper clamp 223 is attached to the lower surface of the lower table 222, and the thermoforming female mold 224 and the lower clamp 225 are attached to the upper surface of the lower table 222. Then, as the upper table 221 and the lower table 222 approach in the vertical direction, the heat-softened PE sheet S1 is formed in close contact with the inner surface 224a of the thermoforming female mold 224 by vacuum forming. Has become. Although the present molding machine 220 employs a molding machine for molding only one thermoformed product in the width direction of the PE sheet S1, a molding machine for molding a plurality of thermoforming products in the width direction may be used. Good.

The upper table 221 and the lower table 222
It is fixed to a drive device (not shown), and can be moved up and down within a predetermined stroke range by the drive device. The lower surface of the upper clamp 223 attached to the upper table 221 comes into contact with the heated and softened PE sheet S1 when the two tables 221 and 222 approach each other, and clamps the PE sheet S1 between the upper surface of the lower clamp 225. Lower table 2
The lower clamp 225 attached to the mold 22 has a shape surrounding the thermoforming female mold 224 which is a thermoforming mold according to the present invention, and is located at a position facing the upper clamp 223.

The bottom of the thermoforming female mold 224 has a diameter of 0.3 mm.
A plurality of vacuum suction holes 224b of about 3 to 0.5 mm are provided. The vacuum suction hole 224b is connected to an air suction mechanism (not shown). When the air suction mechanism suctions air from the vacuum suction hole 224b at a predetermined timing, a vacuum pressure acts on the female mold 224 for thermoforming. . Fine irregularities are formed on the inner side surface 224a of the thermoforming female mold 224 by shot blasting. The fine irregularities are formed, for example, by accelerating a shot of a steel grain or the like by air pressure or the like and causing the shot to collide with the inner surface of the thermoforming female mold.

Although not performed in this embodiment, a heater may be provided on the lower surface of the upper table 221 to prevent the temperature of the transferred PE sheet S1 from dropping. Further, a cooling passage may be formed inside the lower clamp 225 so that the edge of the thermoformed product does not melt, and the lower clamp 225 may be cooled by passing cooling water. Further, the female mold 224 for thermoforming is used.
A cooling passage may be formed inside the thermoforming female mold 224 in order to quickly solidify the PE sheet S1 which is in close contact with the inner side surface 224a. As described above, various thermoforming machines can be employed for performing the thermoforming method of the present invention.

When the tables 221 and 222 approach each other,
The heat-softened PE sheet S1 is sandwiched between the lower surface of the upper clamp 223 and the upper surface of the lower clamp 225, and is held in the opening 224c of the thermoforming female mold 224. Thereafter, when a vacuum pressure is applied to the vacuum suction hole 224b, air in a space partitioned by the inner side surface 224a of the thermoforming female mold 224 and the PE sheet S1 is sucked into the vacuum suction hole 224b.
Then, the PE sheet S1 is brought into close contact with the inner side surface 224a. In addition, as in this embodiment, PE is formed by vacuum forming.
Performing thermoforming on a sheet is merely an example, and various modes are possible as long as thermoforming is performed. For example, compressed air may be supplied from above to press the PE sheet against the inner surface of the female mold for thermoforming so that the PE sheet is brought into close contact with the thermoforming female mold, or vacuum molding and compressed air molding may be used in combination.

Thereafter, when the two tables 221 and 222 are separated from each other in the vertical direction, the formed PE sheet S1 is transferred to the trimming machine 203. The trimming machine 203
The periphery of the formed PE sheet S1 is trimmed to separate a thermoformed product. The thermoformed product is continuously taken out of the trimming machine 203 by the take-out machine 204. On the other hand, the scrap sheet of the trimmed PE sheet S1 is wound up and collected while being pulled by the winder 205.

The above-described molding machine 220 operates according to, for example, a timing chart shown in FIG. The figure is
The operation sequence of the molding machine 220 is shown by a timing chart together with the operation sequence of the transport mechanism 206. When the PE sheet S1 is heated and softened by the heater 210, the transport mechanism 206 transfers the PE sheet S1 to the molding machine 220 (timing T1 to T2). Next, the upper table 221 is lowered and the lower table 222
Is raised, and the tables 221 and 222 are brought closer to each other (timing T3). At this time, the heat-softened PE sheet S1 is sandwiched between the lower surface of the lowered upper clamp 223 and the upper surface of the raised lower clamp 225, and is held in the opening 224c of the thermoforming female mold 224.

Then, air is sucked from the vacuum suction hole 224b to apply a vacuum pressure to the thermoforming female mold 224 (timing T4 to T5). Here, since a closed space is formed by the inner surface 224a of the female mold 224 for thermoforming and the PE sheet S1, as shown in the vertical sectional view of FIG. 7, the heat-softened PE sheet S1 is formed on the inner surface 224a. Be brought close together. In addition, since minute unevenness is formed on the inner surface 224a, when a vacuum pressure is applied to the vacuum suction hole 224b, air is reliably sucked from the closed space formed by the inner surface 224a and the PE sheet S1. You. Therefore, thermoforming can be performed more reliably.

At the timing when the PE sheet S1 solidifies,
The upper table 221 is raised and the lower table 22
2 is lowered, and both tables 221 and 222 are separated from each other in the vertical direction (timing T6). Then, the molded PE
The sheet S1 is held by the transport mechanism 206 and separates from the upper clamp 223 and the lower clamp 225. Thereafter, returning to the timing T1 to T2, the PE sheet S1 heated and softened by the heater 210 is transferred to the molding machine 220 by the transfer mechanism 206. At this time, the formed PE sheet S1 is transferred to the trimming machine 203. And, as mentioned above,
The thermoformed product is separated by the trimming machine 203 and the unloading machine 2
04. Thus, the thermoforming device 2
By means of 00, thermoforming can be performed on the biaxially stretched PE sheet. In this embodiment, the thermoforming is performed on the roll-shaped PE sheet. However, for example, the thermoforming can be performed on the PE sheet cut for one shot.

(3) Function when thermoforming a polyethylene sheet: The function of the present embodiment will be described below. The PE sheet S1 manufactured by the above-described sheet manufacturing apparatus 100 is a sheet biaxially stretched by an inflation method. P unwound from unwinder 201
The E sheet S <b> 1 is transferred to the heater 210 by the transfer device 202, and is preheated by the preheater roll 211. Here, the PE sheet S1 is heated to a temperature lower than the softening point by contact heating. Of course, at this point, the PE sheet S1 has not been softened by heating. Thereafter, the PE sheet S1 is radiantly heated by the heater 212, and the temperature of the PE sheet S1 becomes higher than the softening point. Then, since the temperature range from the heat softening to the drawdown of the PE sheet S1 is narrow, the PE sheet S1 is softened by heat and drawdown occurs as shown in the vertical sectional view of FIG. Although the drawing shows a state in which the PE sheet S1 is drawn down in the heater 210 by a vertical sectional view, illustration of the preheater roll 211 and the like is omitted. Here, the descending amount L1 of the substantially middle portion in the width direction of the PE sheet S1 is the drawdown amount.

Conventionally, PE manufactured using a T-die or the like
When a sheet is used, thermoforming cannot be performed stably using a thermoforming mold for drawdown, and a thermoformed product having good commercial value cannot be obtained. Also, PE
Even if the thermoforming is performed at a temperature lower than the softening point of the sheet, the thermoforming cannot be performed stably even with this method because the sheet is not sufficiently stretched. On the other hand, in the case of a polypropylene sheet manufactured using a T-die or the like, it can be softened by heating to such an extent that draw down does not occur, and the thermoforming is easily performed by using a thermoforming mold to easily form a thermoformed product. Manufacturing can be performed. However, thermoformed products using polypropylene sheets do not have sufficient low-temperature impact resistance, and containers for dairy products do not conform to ministerial ordinance standards for milk and dairy product standards. Can not be used for containers. For this reason, it has been desired to produce a thermoformed product using a PE sheet that can sufficiently obtain impact resistance even in a frozen state and conforms to the product standard.

In the present invention, in order to produce a thermoformed product using a PE sheet, a method for producing the PE sheet is devised, and the PE sheet produced by this production method is heated while controlling the heating appropriately. I try to soften it. That is, the PE sheet is biaxially stretched by an inflation method, and the PE sheet is heated and controlled so that the temperature of the sheet is higher than the softening point and lower than the temperature obtained by adding 50 ° C. to the melting point. I have. Here, the heating is controlled so as to be lower than the temperature obtained by adding 50 ° C. to the melting point. When the temperature of the biaxially stretched PE sheet is increased to the temperature or more, the PE sheet becomes a molten state and the effect of reversing is lost, and the drawdown Because it will.

In the state of FIG. 8, when the PE sheet S1 is further radiated and heated by the heater 212, as shown in the vertical sectional view of FIG. There is no state. At this time, the temperature of the PE sheet S1 is higher than the softening point of the PE sheet S1 and lower than the temperature obtained by adding 50 ° C. to the melting point. 8 is a vertical cross-sectional view showing a state in which the PE sheet S1 is turned back inside the heater 210. The drawdown position of the PE sheet S1 that has been drawn down in FIG. 8 is indicated by a dotted line.

Here, as described above, the overstretch occurs because the polyethylene molecules are solidified while being stretched in various directions on the plane by the biaxial stretching by the inflation method. This is because the drawn polyethylene molecules tend to shrink in an attempt to return to the original shape as shown. As a result, the PE sheet S1 that has been heated and softened by the reversal due to the biaxial stretching effect is transported by the transport mechanism 2 while the amount of drop L1 due to the drawdown due to the heat softening is within a predetermined amount, that is, the drawdown is within the predetermined range.
At 06, it can be held in a state where it has not been drawn down. Further, since the heating of the temperature of the PE sheet S1 is controlled within the above-mentioned temperature range, the drawdown due to the heating and softening can be kept within an appropriate range. In addition, since the stretching ratio of the biaxial stretching of the PE sheet S1 is a low-magnification side of 2 to 20 times of the stretching ratio that can be stretched by the inflation method, slight heating unevenness is generated in the radiant heating from the heater 212. However, such a phenomenon that a hole is formed due to excessively strong reversal due to the stretching effect does not occur.

Thereafter, the heat-softened PE sheet S1 is
At the timing T1 to T2 in FIG.
20. FIG. 5 shows this state.
That is, the heat-softened PE sheet S1 is not drawn down, but remains stretched. Therefore, when the vacuum pressure is applied to the thermoforming female mold 224 after the tables 221 and 222 are approached at timings T3 to T5,
As shown in FIG. 7, the inner surface 22 of the female mold 224 for thermoforming is used.
The heated and softened PE sheet S1 can be brought into close contact with 4a. Thereafter, when the two tables 221 and 222 are separated from each other in the vertical direction at the timing when the PE sheet S1 is solidified, the formed PE sheet S1 is held by the transport mechanism 206, transferred to the trimming machine 203, and the thermoformed product is removed. Separated.

As described above, in this embodiment, when the PE sheet biaxially stretched by the inflation method is heated, the drawback due to the biaxial stretching is performed while the drawdown due to the heat softening is kept within a predetermined range. The PE sheet is heated and softened by controlling the heating so as to draw out the sheet. Then, the heat-softened PE sheet is stretched back due to the stretching effect of biaxial stretching, so that the PE sheet is not drawn down. Can be held. Therefore, thermoforming can be performed on the heat-softened PE sheet using a predetermined thermoforming mold.

(4) Second Embodiment: In the above-mentioned thermoforming method, since a PE sheet manufactured by an inflation method is used, it is suitable for performing thermoforming on a relatively thin PE sheet. On the other hand, there are cases where it is desired to perform thermoforming on a thick sheet (for example, 1.5 mm in thickness) using polyethylene, such as a wide-mouth food container for dairy products. In this case, it is preferable that there be a multilayer PE sheet in which another polyethylene layer or another resin layer is laminated on the biaxially stretched polyethylene layer. FIG.
5 schematically shows a schematic diagram of a laminate sheet manufacturing apparatus 300 for manufacturing a thermoforming resin sheet (multi-layered PE sheet) used for performing a thermoforming method of a PE sheet according to a second embodiment of the present invention. The laminated sheet manufacturing apparatus 300 is configured to laminate and bond a PE sheet biaxially stretched by an inflation method to another polyethylene extruded from a T-die to form a two-layer sheet. Can be laminated to various resins.

In the figure, a roll-shaped PE sheet S1 is mounted on an unwinding roll 301. The laminated sheet manufacturing apparatus 300 unwinds the PE sheet S1 and a pair of laminating rolls 303, Transfer to 303. Here, the PE sheet S1 is a sheet that has been biaxially stretched at a stretching ratio of 2 to 20 times by the inflation method using the sheet manufacturing apparatus 100 described above. Therefore, the first extruder according to the present invention is the extruder 101 described above. On the other hand, the second extruder 304 according to the present invention extrudes another polyethylene to be laminated on the PE sheet S1 in a molten state. That is, when the raw material polyethylene is charged into the hopper 305 attached to the upper part of the extruder 304, the raw polyethylene is heated and melted inside the extruder 304 by a heater (not shown). The extruder 304 extrudes the molten polyethylene into a sheet. Then, the polyethylene is cooled by cooling rolls 306 vertically arranged in three rows.

Thereafter, the laminate sheet manufacturing apparatus 300
Transports the sheet-like polyethylene to the laminating rolls 303, 303 according to the guide rolls 307. Here, the heating burner 30 is located at a position slightly toward the guide roll 307 from the laminating rolls 303, 303.
8, and heats and softens the once cooled sheet-like polyethylene to form laminating rolls 303 and 30.
Supply 3 The surfaces of the laminating rolls 303, 303 are mirror-plated, and can be cooled to an appropriate temperature by flowing cooling water through a cooling passage (not shown) provided therein. Then, the PE sheet S1 and the heat-softened sheet-like polyethylene are sandwiched and laminated and bonded. The laminate-bonded thermoforming resin sheet S2 moves according to the pair of cooling rolls 309 and 309 and the plurality of guide rolls 310, and is wound up by the winder 311. Then, a multilayer thermoforming resin sheet S2 in which a biaxially stretched polyethylene layer and another polyethylene layer are laminated by the inflation method is formed into a roll.

As described above, by using the sheet manufacturing apparatus 100 and the laminated sheet manufacturing apparatus 300,
A polyethylene sheet in a molten state is biaxially stretched into a sheet, and a polyethylene sheet in a biaxially stretched state is laminated and bonded to another polyethylene in a molten state to produce a multilayer resin sheet for thermoforming. . In addition, the biaxially stretched polyethylene layer is biaxially stretched by an inflation method at a stretch ratio on the low-magnification side.

The thermoforming resin sheet suitable for thermoforming manufactured by the sheet manufacturing apparatus 100 and the laminate sheet manufacturing apparatus 300 is as follows. The MFR, density, melt tension, and strain hardening parameters for the biaxially stretched polyethylene layer may be the same as those described above. The material may be a layer made of a polymer of ethylene alone, a layer of a copolymer of ethylene and another α-olefin, or a sheet in which a resin other than polyethylene is mixed. Or a sheet to which an additive is added. When a thermoformed product of a sheet is used for a dairy product, a material that conforms to ministerial ordinances such as milk may be selected. The resin laminated on the biaxially stretched polyethylene layer may be polyethylene,
It may be polypropylene, polystyrene, polyamide, or the like. Further, using a laminate sheet manufacturing apparatus capable of manufacturing a multilayer sheet of three or more layers, a thermoforming resin sheet of three or more layers is manufactured, and thermoforming is performed on the thermoforming resin sheet. Is also good. At this time, the biaxially stretched polyethylene layer may be formed on one surface of the other resin layer, may be formed on both surfaces, or may be formed on an intermediate portion sandwiched between the other resin layers. Is also good.

The thermoforming apparatus for performing thermoforming on the thermoforming resin sheet S2 is the thermoforming apparatus 200 shown in FIG.
The same device as described above can be used. Here, when performing so-called deep drawing thermoforming on a thick thermoforming resin sheet, the thickness of the thermoformed product is made uniform only by applying a vacuum pressure to the thermoforming female mold. Therefore, a plug that can enter the female mold for thermoforming is also used. FIG. 11 is a vertical sectional view showing a main part of a molding machine 420 constituting a thermoforming apparatus for thermoforming a thick thermoforming resin sheet. In addition, the present molding machine 420 forms four rows of 4 × 4 = 16 pieces at a time in four rows in the width direction of the thermoforming resin sheet S2 and four pieces in the moving direction. Is what you do.

In the figure, a molding machine 420 is, like the molding machine 220 described above, an upper table 4 that can be driven up and down.
21, a lower table 422 that can be driven up and down below, a plurality of upper clamps 423 attached to the lower surface of the upper table 421, a plurality of female dies for thermoforming 424 attached to the upper surface of the lower table 422, and a lower clamp. 425. In addition, fine irregularities are formed on the inner side surface 424a of the thermoforming female mold 424 by shot blasting. Further, a plurality of vacuum suction holes 424b are provided at the bottom of the thermoforming female mold 424, and a vacuum pressure acts on the thermoforming female mold 424 by an air suction mechanism (not shown).

The molding machine 420 is provided with a plurality of plugs 426 that can move up and down by being supplied with vacuum or compressed air on the lower surface of the upper table 421. These plugs 426 are arranged above the openings 424c of the thermoforming female molds 424, corresponding to the respective thermoforming female molds 424. The plug 426 when a vacuum is supplied
Is lower than the lower surface of the upper clamp 423 and lower than the lower surface of the upper table 421. And
When both tables 421 and 422 approach, the plug 42
6 can enter the female mold 424 for thermoforming. The gap when the plugs 426 enter the thermoforming female mold 424 when the tables 421 and 422 approach each other is between 3 mm and 分 の of the depth of the thermoforming female mold 424. . By setting the space within this range, the thickness of the thermoformed product can be made uniform.

The plug 426 is made of a material having a large dynamic friction coefficient. For example, when thermoforming a polypropylene sheet, the plug must be made of a material having a low coefficient of kinetic friction because the polypropylene sheet is a non-slip material. Therefore, a plug made of a material having a small dynamic friction coefficient, such as a plug made of polyacetal, and having a large radius of curvature at a corner portion in contact with the sheet is used for the polypropylene sheet. When a plug made of a material with a large dynamic friction coefficient is used, the heat-softened polypropylene sheet is pulled by the plug that enters the female mold for thermoforming due to frictional force, and the thickness of the thermoformed product cannot be made uniform. It is. However, when the plug contacts the polyethylene layer of the thermoforming resin sheet, the PE sheet is a slippery material, so that a plug having a large dynamic friction coefficient can be used. In this embodiment, a polyurethane plug is used, but plugs of various materials can be used. Preferably, a plug made of a material exhibiting a higher coefficient of dynamic friction than the coefficient of dynamic friction of polypropylene is used. The coefficient of kinetic friction is 0.308 for polypropylene. However, the dynamic friction coefficient was measured by using a continuous sliding wear (Suzuki type) tester, the mating material S45C, the friction speed 61 cm / sec, and the inter-surface pressure 8.0.
It is a value measured under the condition of kgf / cm ² . Examples of the material whose dynamic friction coefficient is larger than that of polypropylene include polycarbonate, ABS resin, acrylic resin, and polyimide. In addition, when the test was performed by changing the slip resistance value of the plug, it was found that the plug made of a material having a large dynamic friction coefficient had a greater effect of making the side wall thickness of the thermoformed product using the PE sheet uniform. .

In the case of a polypropylene sheet, for the same reason, it is necessary to make the radius of curvature of the corner portion in contact with the heat-softened polypropylene sheet as large as about 15 mm. On the other hand, in the case of a PE sheet, it is possible to reduce the radius of curvature of the corner portion that comes into contact with the sheet. Therefore, a plug made of a material having a large dynamic friction coefficient and having a small radius of curvature at a corner portion in contact with the sheet is used for the PE sheet. In addition, when the test was performed while changing the radius of curvature of the corner portion, the radius of curvature was 5 mm.
It was found that the following effect was more effective in making the side wall thickness of the thermoformed product using the PE sheet uniform.
Therefore, the radius of curvature of the corner portion 426a that comes into contact with the heat-softened thermoforming resin sheet S2 is set to 5 mm or less. The multilayer thermosetting resin sheet S2 conveyed by the conveyance mechanism 406 has the biaxially stretched polyethylene layer on the upper side and the laminated and bonded polyethylene layer on the lower side. Since the resin-bonded layer of the thermoforming resin sheet S2 is also made of polyethylene, the polyethylene-bonded layer may be placed on the upper side.

In the thermoforming apparatus, the roll-shaped thermoforming resin sheet S2 is unwound from an unwinder and transferred to a heater by a transfer machine. In the heater, preheating is performed by a preheater roll, and the temperature is set lower than the softening point of the biaxially stretched polyethylene layer of the thermoforming resin sheet S2. afterwards,
When the thermoforming resin sheet S2 is radiantly heated by a heater,
The temperature of the thermoforming resin sheet S2 becomes higher than the softening point. Then, similarly to the case shown in FIG. 8, the material is softened by heating and drawdown occurs. When the thermoforming resin sheet S2 is further radiantly heated by a heater, as in the case shown in FIG. 9, it is in a state in which the biaxially stretched polyethylene layer is not drawn down due to the reversal of the layer. as a result,
The heat-softening thermoforming resin sheet S2 that has been heat-softened by the reversal due to the stretching effect of biaxial stretching can be held in a state in which the drawdown due to the heat softening is within a predetermined range, without being drawn down by the transport mechanism 406.

The biaxially stretched polyethylene layer is biaxially stretched at a temperature close to the crystallization temperature of the polyethylene, and biaxially stretched at a low magnification of 2 to 20 times by inflation. As a result, there is no excessive reversal, and even if slight uneven heating occurs, no holes or the like occur. Then, the heat-softened thermoforming resin sheet S2 is conveyed to the molding machine 420 by the conveyance mechanism 406. Then, as shown in FIG. 11, the heat-softened thermoforming resin sheet S2 is not drawn down,
It is still stretched.

The operation of the molding machine 420 will be briefly described below with reference to the timing chart shown in FIG. When the thermoforming resin sheet S2 is heated and softened by the heater, the transport mechanism 406 transfers the thermoforming resin sheet S2 to the molding machine 420 (timing T11 to T12). At this time, both tables 421 and 422 are in the separated position,
The plug 426 is in a raised position under vacuum pressure. Next, when both the tables 421 and 422 are brought close to each other (timing T13), the heat-softened thermoforming resin sheet S2 becomes
It is sandwiched between the upper clamp 423 and the lower clamp 425 and is held in the opening 424c of the female mold 424 for thermoforming.

In the molding machine 420, the plug 42
6, and is moved to the lowered position, and a vacuum pressure is applied from the vacuum suction hole 424b (timing T14). At this time, the plug 426 is connected to the female mold 4 for thermoforming.
24, and heat-softened thermoforming resin sheet S2
Is pressed into the thermoforming female mold 424. At the same time, by applying a vacuum pressure to the thermoforming female mold 424, the thermoforming resin sheet S2 to be pushed is drawn toward the inner side surface 424a. Then, as shown in the main part sectional view of FIG.
The heat-softened thermoforming resin sheet S2 has an inner surface 424a.
To be closely associated with. At this time, since the plug 426 is made of a material having a large dynamic friction coefficient and the radius of curvature of the corner portion 426a is 5 mm or less, the thickness of the thermoforming resin sheet S2 closely contacting the inner surface of the thermoforming female mold 424 is uniform. become.

The resin sheet S2 for thermoforming has an inner surface 424a.
At the timing of close contact, the vacuum pressure is applied to the plug 426 to return to the raised position, and the supply of the vacuum pressure from the vacuum suction hole 424b is stopped (timing T15). Then, at the timing when the thermoforming resin sheet S2 solidifies,
The two tables 421 and 422 are separated from each other in the vertical direction (timing T16). Then, the formed thermoforming resin sheet S2 is held by the transport mechanism 406, and is held by the upper clamp 42.
3 and the lower clamp 425, at timings T11 to T12.
Is transferred to the trimming machine. Then, the thermoformed product is separated by the trimming machine and taken out from the take-out machine. In this way, thermoforming can be performed on a multilayer sheet in which a biaxially stretched polyethylene layer is laminated with another polyethylene layer.

As described above, since the thermoforming resin sheet S2 using polyethylene is a multilayer sheet, it can be a thick sheet. A thick sheet tends to have a large drawdown due to its own weight. However, if the thermoforming resin sheet S2 is heated and softened, the layer of polyethylene biaxially stretched by the inflation method is turned back by the biaxial stretching effect. And draw down can be prevented. Thus, the resin sheet for thermoforming of the present invention has a biaxially stretched polyethylene layer, but has a biaxially stretched polyethylene layer. And thermoforming can be performed. In particular, it is suitable when thermoforming is performed using a thick sheet made of polyethylene.

As described above, according to the present invention, a stretching effect by biaxial stretching can be imparted to various polyethylene sheets including a multilayer sheet, and the drawdown by heat softening can be kept within a predetermined range. It can be maintained in a state where it is not drawn down due to the stretching back by the stretching effect. Therefore, it is possible to provide a method for thermoforming a polyethylene sheet and a method for manufacturing a thermoformed product using the polyethylene sheet, which can be thermoformed on the polyethylene sheet. Further, the present invention can also provide a thermoforming resin sheet and a method for producing a thermoforming resin sheet that can be easily thermoformed by preventing drawdown due to reversal due to the stretching effect of biaxial stretching.

[Brief description of the drawings]

FIG. 1 is a side view schematically showing an outline of a sheet manufacturing apparatus used for performing a thermoforming method of a PE sheet according to a first embodiment of the present invention.

FIG. 2 is a perspective view showing a main part of the inflation die in a partially sectional view.

FIG. 3 is a view schematically showing a state in which a PE sheet shrinks due to a stretching effect of biaxial stretching during heat softening.

FIG. 4 is a perspective view schematically showing an outline of a thermoforming device used for performing a thermoforming method of a PE sheet according to an embodiment of the present invention.

FIG. 5 is a vertical sectional view showing a main part of the molding machine as viewed from the position of AA in FIG. 4;

FIG. 6 is a timing chart showing the operation sequence of the molding machine together with the operation sequence of the transport mechanism.

FIG. 7 is a vertical sectional view showing a main part of the molding machine when both tables are close to each other and a vacuum pressure is applied in the female mold for thermoforming.

FIG. 8 is a vertical sectional view showing a state in which a PE sheet is drawn down in the heating machine.

FIG. 9 is a vertical cross-sectional view showing a state in which the PE sheet is stretched in the heater.

FIG. 10 is a side view schematically showing an outline of a laminate sheet manufacturing apparatus used for performing a thermoforming method of a PE sheet according to a second embodiment of the present invention.

FIG. 11 is a vertical sectional view showing a main part of a molding machine constituting the thermoforming device.

FIG. 12 is a timing chart showing the operation sequence of the molding machine together with the operation sequence of the transport mechanism.

FIG. 13 is a vertical sectional view showing a main part of the molding machine when a plug is inserted into the female mold for thermoforming and a vacuum pressure is applied to the female mold for thermoforming.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 sheet manufacturing device 101 extruder (first extruder) 102 hopper 103 inflation die 104 guide plate 105 pinch roll 106 cutter 107 guide roll 108 winding machine 200 thermoforming device 201 Unwinding machine 202 ... Transfer machine 203 ... Trimming machine 204 ... Unloading machine 205 ... Winding machine 206 ... Conveying mechanism 210 ... Heating machine 211 ... Preheater roll 212 ... Heater 220 ... Molding machine 221 ... Upper table 222 ... Lower table 223 ... Upper Clamp 224: Female mold for thermoforming 224a: Inner surface 224b: Vacuum suction hole 224c: Opening 225: Lower clamp 300: Laminate sheet manufacturing apparatus 301: Unwind roll 303: Laminate roll 304: Second extruder 306: Cooling Roll 308 ... Heating burner 420 ... Molding machine 424 ... Female mold for thermoforming 424a ... Inner surface 424b ... Vacuum suction hole 424c ... Opening 426 ... Plug 426a ... Corner S1 ... Polyethylene sheet S2 ... Resin sheet for thermoforming

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Akio Yoshikoshi 2-17-15 Tsukuda, Chuo-ku, Tokyo Tsukishima Kikai Co., Ltd. F-term (reference) 4F208 AA04C AC03 AR06 AR20 MA01 MB01 MC01 MC04 MG04 MG21 MG22 MH06 MH10

Claims (17)

[Claims] 1. When heating a biaxially stretched polyethylene sheet, the polyethylene sheet is heated and controlled so as to draw out rebound by the stretching effect of biaxial stretching while keeping a drawdown due to heat softening within a predetermined range. A thermoforming method for a polyethylene sheet, comprising heat-softening and heat-molding the heat-softened polyethylene sheet using a predetermined thermoforming mold. 2. The method of thermoforming a polyethylene sheet according to claim 1, wherein the polyethylene sheet is a sheet biaxially stretched at a temperature higher than a crystallization temperature and lower than a melting point of the polyethylene sheet. Thermoforming method of polyethylene sheet. 3. The thermoforming method for a polyethylene sheet according to claim 2, wherein the polyethylene sheet is a sheet biaxially stretched by an inflation method. 4. The method of thermoforming a polyethylene sheet according to claim 3, wherein the polyethylene sheet is a sheet biaxially stretched on a low-magnification side, out of magnifications stretchable by the inflation method. A method for thermoforming a polyethylene sheet. 5. The method of thermoforming a polyethylene sheet according to claim 4, wherein the stretching ratio of the inflation method is 2 to 20 times. 6. The method for thermoforming a polyethylene sheet according to any one of claims 1 to 5, wherein the polyethylene as a raw material of the polyethylene sheet has a melt flow rate of 0.01 to 10 g / 10 minutes. A thermoforming method for a polyethylene sheet, wherein a melt tension is 40 mN or more and a strain hardening parameter is 1.2 or more. 7. The method for thermoforming a polyethylene sheet according to claim 1, wherein the temperature of the polyethylene sheet is higher than the softening point of the polyethylene sheet when the polyethylene sheet is heated. A method for thermoforming a polyethylene sheet, wherein heating is controlled so as to be higher and lower than a temperature obtained by adding 50 ° C. to the melting point. 8. The method for thermoforming a polyethylene sheet according to any one of claims 1 to 7, wherein the polyethylene sheet is preheated by a preheater roll, radiantly heated after preheating. Of thermoforming polyethylene sheet. 9. The method for thermoforming a polyethylene sheet according to any one of claims 1 to 8, wherein the polyethylene sheet comprises a biaxially stretched polyethylene layer and another polyethylene layer or another polyethylene layer. A thermoforming method for a polyethylene sheet, which is a multilayer sheet in which resin layers are laminated. 10. The thermoforming method for a polyethylene sheet according to any one of claims 1 to 9, wherein the thermoforming mold is capable of closely contacting the heat-softened polyethylene sheet on an inner surface. A female mold, for the heat-softened polyethylene sheet, thermoforming while allowing a plug of a material having a large coefficient of kinetic friction that can advance and retreat into the thermoforming female mold to enter the thermoforming female mold. A method for thermoforming a polyethylene sheet. 11. The method of thermoforming a polyethylene sheet according to claim 10, wherein the plug has a radius of curvature of 5 mm or less at a corner portion abutting on the heat-softened polyethylene sheet. Thermoforming method. 12. A thermoforming resin sheet in which a plurality of resin layers are laminated, the multilayer sheet comprising a biaxially stretched polyethylene layer and another polyethylene layer or another resin layer. A thermoforming resin sheet. 13. The thermoforming resin sheet according to claim 12, wherein the polyethylene layer is biaxially stretched by an inflation method, and is stretched on a low magnification side among stretchable ratios by the inflation method. A resin sheet for thermoforming, characterized in that: 14. The molten polyethylene is extruded from a predetermined first extruder into a biaxially stretched sheet, and the molten polyethylene or other resin is extruded from a predetermined second extruder. A method for producing a thermoforming resin sheet, comprising producing a multilayer thermoforming resin sheet by laminating and bonding to biaxially stretched sheet-like polyethylene. 15. The method for producing a resin sheet for thermoforming according to claim 14, wherein when the polyethylene in the molten state is formed into a biaxially stretched sheet, the polyethylene is biaxially stretched by an inflation method. A method for producing a thermoforming resin sheet. 16. The method for producing a resin sheet for thermoforming according to claim 15, wherein, among the draw ratios that can be stretched by the inflation method,
A method for producing a thermoforming resin sheet, comprising biaxially stretching the sheet-like polyethylene on a low magnification side. 17. When heating the biaxially stretched polyethylene sheet, the polyethylene sheet is heated and controlled so as to draw out the rebound by the biaxial stretching effect while keeping the drawdown due to heat softening within a predetermined range. Heat-softening, heat-softening the polyethylene sheet, using a predetermined thermoforming mold to perform thermoforming, thereby producing a thermoformed product using the polyethylene sheet; Manufacturing method of molded article.