JPS5923536B2 - Plastic container manufacturing method and manufacturing equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a container-like thin-walled molded article from a plastic sheet.

More specifically, when manufacturing a thin-walled molded product from a plastic sheet by the plug-assisted pressure forming method, by improving the order of inserting a plug into the preheated sheet and squeezing the sheet into the opening of the female mold. , relates to a method and apparatus for manufacturing a thin-walled container with improved heat resistance, rigidity, etc.

The purpose of the present invention is to have excellent shape retention when heated to the extent that it can be used for applications requiring heat resistance such as general plastic sheets and retort food containers, and also to have good wall thickness distribution and stiffness. The object of the present invention is to provide a simple and advantageous method for manufacturing a hollow container with improved shrinkage after molding.

As a thermoforming method for producing thin-walled containers with a deep molding depth from plastic sheets, pressure forming using plugs as an aid has been known for a long time, but recently polyolefins such as polypropylene have been developed using this plug-assisted pressure forming method. A solid state air pressure forming method for sheets has been proposed.

In this molding method, a polyolefin sheet is introduced in a solid state below its melting point between a mold plug and a mold having an end opening, and the molding plug is moved together with the sheet relative to the mold opening to raise the sheet. The sheet is supported in a mold, and a pressurized fluid is introduced into the raised part of the sheet to form the product in the mold.
489, etc.). By this solid phase pressure forming method, polypropylene containers can be manufactured at a high production rate without generating wrinkles at the opening of the container due to sagging of the preheated sheet. However, polypropylene containers manufactured by the above-mentioned solid phase air forming method have poor heat resistance, and
It has the disadvantage of causing large dimensional shrinkage when heated to 0°C or higher, making it difficult to use in applications that require heat resistance of 100°C or higher, such as for retort foods.

The present inventors have investigated various ways to obtain thin-walled polypropylene containers with excellent heat resistance suitable for retort food containers, etc., without reducing production speed or deteriorating quality such as transparency of molded products. In the plug-assisted pressure forming method, the heat resistance of the molded product can be greatly improved by performing the steps of squeezing the preheated sheet from the top with a forming plug and from the bottom using the female mold in this order. They discovered that the present invention could be improved, and conducted further research to complete the present invention.

The container manufacturing method of the present invention involves heating a plastic sheet whose ends are fixed with a fixture such as a chuck in advance to a molding temperature, and inserting a heated auxiliary plug placed in a molding box and an opening of a female mold into the plastic sheet. The sheet is introduced between a sheet retaining plate which has a large opening and is installed above the periphery of the female opening, and the plug is first advanced in the direction of the female opening to squeeze the sheet. , then advancing the holding plate in the direction of the forming box to a height equivalent to the clamping surface of the sheet,
The hem of the raised sheet protruded by the plug is squeezed by the open end of the holding plate, and then the cooled female mold is advanced toward the molding box, and the raised sheet portion protruded by the plug is inserted into the female mold opening. After the periphery of the sheet is pressed between the molding box and the periphery of the female mold, pressure fluid is immediately introduced into the molding box to mold the sheet onto the surface of the female mold, and the sheet is cooled.

Furthermore, in the method for manufacturing a container of the present invention, a plastic sheet whose ends are fixed with a fixture such as a chuck is heated in advance to a molding temperature, and a heated auxiliary plug placed in a molding box and an opening of a female mold are formed. Introducing the sheet between a sheet holding plate having a larger opening and installed above the periphery of the female opening, first bringing the holding plate close to the same height as the clamping surface of the sheet. The plug is then advanced from above the sheet through the opening in the retaining plate toward the female mold, contacts the open end of the retaining plate to squeeze the sheet, and then moves the cooled female mold toward the molding box. advance, put the raised sheet portion protruded by the brag into the opening of the female mold, further squeeze the hem of the raised sheet with the opening end of the female mold,
Next, after the periphery of the sheet is compressed between the molding box and the female mold, a pressure fluid is immediately introduced into the molding box, and the sheet is molded onto the surface of the female mold due to the pressure difference, and is then cooled.

The method of the present invention will be explained below with reference to the drawings.

FIG. 1 shows a molding apparatus embodying the present invention, which consists of a lower mold section and an upper mold box section. In the lower mold section, a female mold 3 for molding a sheet 2 fixed by a fixture 1 such as a chuck has a cooling water circulation path 4 inside thereof, and is moved up and down by an air cylinder 5. The female mold 3 is provided with a sheet holding plate 6 around its opening, and the sheet holding plate 6 is initially pushed up to the highest position by a spring 7. The upper molding box part moves up and down with an air cylinder 8, and forms a molding box 10 having a compressed air supply port 9 and a molding box 1.
Auxiliary plug 1 installed in 0 and having a built-in heater 11
It consists of 2. Auxiliary plug 12 is air cylinder 1
3, it can be moved underground. Molding is performed as follows.

First, a plastic sheet whose peripheral part is fixed with a fixture such as a chuck is preheated to the molding temperature, and then introduced between the molding box part and the mold part as shown in Fig. 2. Air cylinder 8 molded box 1
0 is lowered and the auxiliary plug 1 is heated with the heater 11.
2 to tighten the seat 2, then air cylinder 5if
C. Therefore, the female mold 3 is brought close to the same height as the clamping surface of the sheet 2, and the hem of the raised sheet pushed out by the plug 12 is squeezed by the open end of the sheet holding plate 6 pushed up by the spring 7, and further cooled. The raised sheet portion protruded by the plug 12 is put into the opening of the female mold 3, and the periphery of the sheet is raised between the molding box 10 and the periphery of the opening of the female mold 3, as shown in FIG. Tighten.

In this case, during the process in which the sheet is pressed by the molding box 10 and the female die 3, the sheet holding plate is pressed by the opening of the molding box, and the spring is compressed. Immediately after the sheet is pressed between the molding box and the female mold, compressed air is introduced into the molding box through the compressed air supply port 9, and the sheet is shaped on the surface of the female mold as shown in Fig. 4, and then cooled. This completes the molding of the container. In the present invention, it is necessary to fix the end of the plastic sheet with a chuck, and the chuck and other fixing devices are installed outside the molding box and female mold as illustrated in FIG. is necessary.

In the case of multi-piece molding, it is necessary to fix the sheet with chucks or the like further outside the female molds at both ends of a plurality of female molds lined up in a row in the width direction of the sheet.

Therefore, in this case, a preheated sheet with both ends fixed with chucks or the like is introduced between the plurality of auxiliary plugs and a sheet holding plate having an opening for each female opening, and the sheet holding plate is first clamped onto the sheet. the plurality of auxiliary plugs are each advanced from above the sheet in the female die direction through the plurality of openings in the holding plate, and the sheet is squeezed at the open end of the holding plate; The plurality of female molds are then advanced toward the molding box, the raised sheet portions protruded by the plugs are inserted into the cavities of the pair of female molds, and the hems of the raised sheets are inserted into the open end of each female mold. After squeezing the periphery of the sheet between the molding box and the area around the opening of the female mold, pressure fluid is immediately introduced into the molding box, and the pressure difference causes the sheet to be pressed onto the surface of the female mold. Shape and cool. The present invention is clearly different from the known plug-assisted pressure forming method in that an auxiliary plug is first inserted from above the preheated sheet to squeeze the sheet, and then a female die is moved toward the sheet from the opposite side of the sheet, and the plug is plugged. 1. The hem of the raised sheet is squeezed by the opening end of the female die. In this case, after inserting the plug into the sheet, move the sheet holding plate installed over the female die opening in the direction of the forming box. The hem of the raised sheet protruded by the plug is brought close to the same height as the clamping surface of the sheet and squeezed by the open end of the sheet holding plate, and then the female mold is advanced toward the sheet. If the hem of the raised sheet protruded by the plug is squeezed directly with the opening end of the female die, the appearance of the mouth of the container may vary depending on the material of the sheet and the design of the opening of the female die. This is because it may damage the The surface of the sheet holding plate that comes into contact with the preheated sheet is preferably coated with a material having excellent heat insulating properties and slipperiness, such as fluororesin, in order to improve the drawing properties of the sheet.

Depending on the material of the plastic sheet, it is preferable to incorporate a heater into the sheet holding plate to keep it warm. It is preferable that the seat holding plate is capable of moving forward and backward independently of the female mold, and therefore it is preferable that the seat holding plate is actuated by a spring as shown in FIG. 1 or moved independently by an air cylinder.

The timing of movement of the sheet holding plate is important for improving the finish of the molded product, and the timing is preferably adjusted by opening and closing a solenoid valve for the air cylinder. Although the size of the opening in the sheet holding plate also has a large effect on the filling of the molded product, it is preferable that the size of the opening is about the same size as the periphery of the ear of the container to be molded. In the method of the present invention, the forming box and the auxiliary plug are moved in the direction of the preheated sheet, the sheet holding plate is moved in the direction of the sheet,
The timing of the movement of the female mold toward the seat and the supply of compressed air are important, and a mechanical or electrical timer is required to accurately control this timing.

The auxiliary plug is preferably a chromium-plated metal plug or a metal plug whose surface is coated with fluororesin, and it is preferable to use a built-in heater to heat the plug to a temperature close to the seat preheating temperature. Preferably, the auxiliary plug installed within the molding box can be moved forward and backward independently of the molding box by means of an air cylinder. The female mold is preferably a metal mold with a built-in cooling water circulation path in order to improve the molding cycle.

The method of the invention is suitable for manufacturing containers from thermoplastic sheets with a thickness of 0.2 Tm to 3.0 m. By the method of the present invention, strong containers with uniform wall thickness distribution can be manufactured from sheets of vinyl chloride resin, styrene resin, polyethylene terephthalate resin, etc.

When a container is manufactured from polypropylene, polyethylene, or a single-layer or multi-layer sheet containing these resins by the method of the present invention, the container has a uniform wall thickness distribution, is strong, and has significantly improved heat resistance. For sheets made mainly of 4VVc polypropylene resin, the temperature of the sheet is below the melting point!
Containers with excellent heat resistance and rigidity can be manufactured by both solid-state air pressure forming, which is formed by IC bithermal heating, and melt air forming, which is formed by heating to a temperature equal to or higher than the melting point.

The so-called solid phase pressure forming method, in which polypropylene is heated to a temperature below its melting point and pressure molded, has the advantage of improving transparency while preventing wrinkles from forming at the mouth of the formed hollow container. , has come into practical use in recent years.

However, three polypropylene containers manufactured by the well-known Bragg-assisted solid-phase air forming method undergo significant shrinkage and deformation when heated to a temperature of around 100°C, which is far lower than the crystalline melting point of approximately 165°C. , the heat resistance of polypropylene resin is not effectively utilized. However, when the polypropylene sheet 5 is solid-state air-formed using the method of the present invention,
The heat resistance of the molded product is improved to 130°C or higher while maintaining its transparency, allowing it to be used in heat-resistant applications such as retort food containers.

In solid state air pressure forming of a polypropylene sheet, the molding temperature is below its crystal melting point, preferably 140°C to crystal melting point, and the temperature of the auxiliary plug is 120°C to 160°C.
Preferably, the temperature of the female mold is 5°C to 40°C, and the pressure of the air supply during molding is 2 to 6K. In the known polypropylene solid state air pressure molding method, the optimum molding temperature range is narrow and the sheet temperature range is limited to about 2 to 5°C, whereas in the method of the present invention, the optimum molding temperature range is narrow. This has been greatly expanded, and molding can be performed in a continuous temperature range from solid phase air pressure forming below the melting point to melt air forming above the melting point.

In the case of melt air forming of polypropylene sheets, the forming temperature is above the crystal melting point, the temperature of the auxiliary plug is preferably 100°C to 200°C, and the transparency of the container is slightly lower than that of solid state air forming products. However, the heat resistance is further improved.

In pressure forming of polypropylene sheets, it is preferable to keep the temperature of the female mold low in order to increase the molding speed. However, in the known solid-state air forming method, if the temperature of the female mold is kept low, the female mold will collapse during the molding process. Cooling occurs in the portion of the sheet that is close to or in contact with the container, which reduces the heat resistance of the container.

However, in the method of the present invention, even if the female mold is sufficiently cooled, cooling of the sheet due to approaching or contacting the female mold during molding is unlikely to occur, and therefore the heat resistance of the container does not deteriorate. In this manner, the method of the present invention can improve the molding speed without reducing the heat resistance of the container. The polypropylene solid-phase pressure-formed container manufactured by the method of the present invention has a more uniform wall thickness distribution, is stronger, has excellent heat resistance, and has good transparency compared to the polypropylene solid-phase pressure-formed container manufactured by the conventional method. It is. In the conventional method, the plug is inserted into the sheet after the periphery of the preheated sheet is pressed between the opening end of the female mold and the molding box. The stretching ratio of the elongation that occurs during plug insertion of the sheet part between the sheet part that contacts the end and the sheet part that contacts the tip of the plug is larger than that of other parts, so that the molded Containers tend to have thinner walls at the top of the side walls than at the bottom.

As a result, the compressive strength of the container is low, making it weak. In contrast, in the method of the present invention, a plug is first inserted into a preheated sheet whose ends are fixed with a chuck, etc., and then the female mold is advanced toward the sheet, the protruding part of the sheet is inserted into the female mold, and the Since the hem of the plug is squeezed by the female die, there is no large local stretching of the sheet formed onto the surface of the female die, and the difference in thickness between the part of the sheet that contacts the tip of the plug and the hem of the sheet is small. Therefore, the wall thickness of the molded container is uniform, and the compressive strength of the container is significantly increased.

When molded by the method of the present invention, the preheated sheet tends to be plugged into the female mold from around the edge of the female mold opening, so the average wall thickness of the container is 10% lower than that of a container molded by the conventional method. % to 40% increase.

Therefore, with the method of the present invention, it is possible to manufacture a container with the same wall thickness as the conventional method using an original fabric sheet that is 10% to 40% thinner than the conventional method, thereby achieving savings in material costs. I can do it. The reason why the heat resistance of the container manufactured by the method of the present invention is superior is not clear at present, but the following reasons can be considered as compared to the conventional method.

In the known plug-assisted solid state air forming method, a preheated sheet is compressed around the edges of the forming box and the female mold opening, and then a plug is inserted into the female mold to pre-stretch the sheet. Localized high elongation occurs.

When inserting the plug, the sheet portion that is stretched while contacting the open end of the female mold is locally highly stretched while being cooled on the surface of the female mold. For this reason, when a molded container is reheated, large dimensional shrinkage begins to occur at a relatively low temperature, for example, around 100° C. in the case of a polypropylene solid phase pressure molded product. However, in the molding method of the present invention, a plug is inserted into the preheated sheet, the sheet holding plate is further moved to the clamping surface, and the squeezed sheet is then placed in a female mold and subjected to pressure forming.

This mechanism eliminates the local low temperature and high stretching of the sheet that comes into contact with the female mold opening end when the plug is inserted, which is the case with conventional methods. Further, even when the sheet is squeezed at the open end of the shape, localized high stretching does not occur. Therefore, even if a container manufactured by the method of the present invention is reheated, local dimensional shrinkage of the container is unlikely to occur; for example, in the case of a polypropylene solid phase air-formed product, shrinkage deformation begins to occur at a temperature around 130°C. . The present invention will be explained in detail below with reference to Examples.

EXAMPLE Using a pressure molding machine with a molding area of 300 x 250 wm, a pot-shaped round container with a diameter of 90 m and a molding depth of 53 TW1 was molded by the method of the present invention.

The molding device is shown in Fig. 1, and includes a molding box,
The female mold, auxiliary plug, and seat holding plate are all made of iron, and the inner surface of the female mold and the plug surface are chrome-plated and mirror-finished.

The temperature of the plug was regulated using a built-in heater. Water at 20°C was circulated in the female cooling water circulation path. The upper surface of the sheet holding plate was coated with fluororesin. Molding was carried out by the method of the present invention using an extruded polypropylene sheet having a thickness of 0.8 TrrIrL and a coextruded sheet having a composition of polypropylene/saponified ethylene vinyl acetate copolymer/polypropylene.

The ends of the sheet were fixed with a chuck, and the sheet set between the molding box and the female mold was heated from above with a far-infrared heater for about 15 minutes.
After preheating to 5°C, remove the heater from the sheet soil, then lower the forming box onto the preheated sheet as shown in Figure 2, squeeze the sheet with a brag heated to 150°C,
Next, the female mold is raised to the same height as the clamping surface of the sheet using an air cylinder, and the hem of the raised sheet protruded by the plug is first squeezed by the open end of the sheet holding plate supported by a spring, and then the plug is tightened. Put the protruded raised sheet part into the opening of the female mold, press the sheet between the molding box and the area around the opening of the female mold, and immediately supply 5z4d of compressed air from the compressed air supply port to tighten the sheet. was molded onto the female mold surface and cooled.

For comparison, containers of the same shape were molded using the same molding apparatus and using a conventional method.

In the conventional method, a female mold is raised from below a sheet preheated to 155°C to a height equivalent to the clamping surface of the sheet, and at the same time the molding box is lowered to clamp the sheet between the female mold and the molding box. A plug heated to 140°C is inserted into the female mold to squeeze the sheet into the female mold opening, and immediately compressed air at a pressure of 5 Kf//Cd is introduced from the compressed air supply port to mold the sheet onto the female mold surface. , cooled. The evaluation results of the heat resistance temperature, shrinkage start temperature, container weight, transparency, and rigidity of polypropylene containers and multilayer containers having a polypropylene/saponified ethylene vinyl acetate copolymer/polypropylene structure manufactured by the method of the present invention and the conventional method are as follows. Shown in Table 1.

The heat-resistant temperature is determined by immersing a cup-shaped container in silicone oil, applying a compressive load of 540 fm from above to the open end of the cup, and heating the silicone oil at a heating rate of 2°C/min. It was defined as the temperature at which the amount of compressive deformation of a cutlet with a molding depth of 53 mm reached 2wnVC.

The temperature at which the shrinkage starts is determined by placing the cup in an air thermostat set at a constant temperature for 10 minutes, then taking it out and measuring its dimensions.The depth of the cup under no load is 1m in shrinkage! defined as the temperature reached. The container weight is the weight of one cup, and transparency and rigidity were evaluated based on appearance and feel. As is clear from Table 1, the molded products made by the method of the present invention have improved heat resistance by about 30°C, better transparency and rigidity, and have better transparency and rigidity than the comparative molded products made by the conventional method. Despite being molded from scratch, the weight of the container increases by approximately 30%.

[Brief explanation of the drawing]

FIG. 1 shows a container molding apparatus according to the present invention, and in the figure, 1 is a fixture such as a chuck for fixing the sheet end, 2 is a plastic sheet, 3 is a female mold, 4 is a cooling water circulation path, 5
1 is an air cylinder, 6 is a seat holding plate, 7 is a spring, 8 is an air cylinder, 9 is a compressed air supply port, 10 is a molded box, 11 is a heater, 12 is an auxiliary plug, and 13 is an air cylinder. Figures 2 to 4 show the molding process using the molding apparatus shown in Figure 1, with Figure 2 showing the sheet squeezed by the auxiliary plug, and Figure 3 showing the female mold being raised and forming the molding box. FIG. 4 shows the state in which the sheet is pressed with the female mold, and the sheet is molded onto the surface of the female mold by supplying compressed air and cooled.

Claims (1)

[Claims] 1. A plastic sheet whose ends are fixed with a fixing device such as a chuck is heated in advance to a molding temperature, and a heated auxiliary plug is placed in a molding box and an opening larger than the opening of the female mold. The sheet is introduced between a sheet retaining plate having a section and a sheet retaining plate installed above the peripheral part of the female mold opening, the plug is first advanced in the direction of the female mold opening to squeeze the sheet, and then the retaining plate is The plate is advanced in the direction of the forming box to a height equivalent to the clamping surface of the sheet, the skirt of the raised sheet protruded by the plug is squeezed with the open end of the holding plate, and the cooled female mold is then inserted into the forming box. The protruding sheet portion protruded by the plug is inserted into the female mold opening, and the peripheral portion of the sheet is compressed between the molding box and the peripheral portion of the female mold, and then the pressurized fluid is immediately introduced into the molding box. A method for manufacturing a plastic container, comprising: forming a sheet onto the surface of a female mold, and cooling the sheet. 2 A plastic sheet whose ends are fixed with a fixing device such as a chuck is preheated to the molding temperature, and a heated auxiliary plug installed in the molding box and a female mold with an opening larger than that of the female mold. The sheet is introduced between the sheet holding plate installed above the periphery of the mold opening, and the holding plate is first brought close to the same height as the clamping surface of the sheet, and then the plug is inserted from above the sheet. The sheet is advanced toward the female mold through the opening in the retaining plate to contact the open end of the retaining plate and squeezed, and the cooled female mold is then advanced toward the mold box to remove the raised sheet ejected by the plug. The part is placed into the opening of the female mold, the hem of the raised sheet is squeezed by the opening end of the female mold, the periphery of the sheet is compressed between the molding box and the female mold, and then the pressure is immediately put into the molding box. A method for manufacturing a plastic container, characterized by introducing a fluid, forming a sheet onto the surface of a female mold using a pressure difference, and cooling the sheet. 3. A molded box containing an auxiliary plug operated by an air cylinder and arranged facing the molded box,
A sheet pressure forming apparatus comprising a female mold having a cooling water circulation path, and a sheet holding plate that is operated by a moving mechanism independent of the female mold and has an opening larger than the female mold opening on the periphery of the female mold opening. A plastic container manufacturing device comprising: