JPH0544901B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a thermoforming method in which a plastic sheet is heated and molded using a mold, and is a novel method that uses air jetting to control the thickness of the molded product. related to plug-assisted molding. The molded products thus obtained are used as food packaging, various industrial containers, and parts. <Prior art and problems> The thermoforming method involves heating a plastic sheet and molding it using a mold.
Female mold molding, vacuum molding, pressure molding, vacuum-pressure molding, etc. are known and widely used. Male molding is a process in which a heated sheet surrounds a convex mold, and the bottom part that comes into contact with the mold first is thicker, and the side part near the flange that makes contact last is the thinnest part. There are some characteristics. In contrast, female molding uses a concave mold, so the heated sheet is placed at the flange, bottom of the mold,
The bottom corners tend to be in contact with each other in order, and the bottom corners tend to be the thinnest parts. In this way, the part of the sheet that first contacts the mold is cooled and stops elongating, and the other parts are stretched, so the part that first contacts the mold becomes thicker and the other parts thinner. As a means of correcting such unevenness in sheet thickness, there is a method of using a plug such as a push rod.The plug comes into contact with the sheet before the sheet contacts the mold, and stretches the sheet at an appropriate rate.
After this, uniformity is achieved by bringing the material into close contact with the mold using vacuum or compressed air. When a plug is used, this determines how the sheet is stretched, so various ideas have been made regarding the material and shape of the plug. In order to prevent defects caused by the thickness of the sheet called drag lines or chill lines, which occur when the sheet is cooled by contact with the plug, it is common practice to raise the temperature of the plug or to cover the plug surface with flannel, felt, etc.
The shape of the plug varies greatly depending on the shape of the mold, but it is determined by trial and error in order to obtain the desired thickness distribution, and usually a dedicated plug is used for each type of mold to obtain a versatile plug. Things were extremely difficult. As described above, there is a strong need for a molding method using a plug that does not cause the sheet to be locally cooled by the plug and is versatile. Furthermore, the plastic sheet referred to here refers to a sheet made of thermoplastic plastic, including polyolefin resins such as polyethylene and polypropylene, benzene resins such as styrene, polyamide, acrylonitrile, polyester resin, and ethylene/vinyl alcohol copolymer. , polyvinylidene resin, etc., and refers to a single substance or a composite. <Means for Solving the Problems> In order to solve the above problems, in the present invention, a flat plastic sheet is heated, and the plastic sheet is equipped with an air jetting function and is slid up and down. and a concave mold having a structure that has at least a suction function of both air injection and suction functions, or has a structure that does not have both functions, and fixes the plug. Controlling the air pressure injected or suctioned from the plug and the mold, or controlling the air pressure jetted or suctioned from the plug and the mold while moving the plug up and down, or keeping the air pressure constant; This problem was solved by molding the plastic sheet in contact with the mold without touching the plug. This molding method will be explained in more detail with reference to the drawings. Figures 1a, b, and c show the molding method according to the present invention using an air injection plug with a truncated cone-shaped or truncated pyramid-shaped surface as a porous injection port. It is a sectional view showing an example of the molding process when the second
The figure shows a case where a nozzle type air injection plug is used. In the thermoforming method of the present invention, a plug 1 and a mold 3 are provided with injection ports 5 and 8 that can adjust the amount, pressure, and temperature of air to be injected into the heated plastic sheet 2. It is interposed between the concave portion and the concave portion. Next, there are three possible methods for bringing the plastic sheet 2 into contact with the recesses of the mold, which will be explained below. First, with the plug fixed, air is injected from the injection port 5 of the plug 1, and by increasing the air pressure ejected from the injection port 5 of the plug 1, the plastic sheet 2 is inserted into the recess of the mold 3. When the air pressure of the plug 1 is further increased, the plastic sheet 2 is stretched into the mold 3.
It enters the recess while being further stretched, and finally reduces the injection air pressure from the injection port 8 of the mold 3,
Moreover, by exhausting and removing the air in the mold, the entire plastic sheet can be brought into contact with the mold 3 at almost the same time without the plastic sheet 2 partially touching the recesses of the mold 3 during the process. . The second method is to control the stretching of the plastic sheet by moving the plug 1 up and down, and after the plastic sheet 2 has been stretched to some extent and entered the recess of the mold 3, The plug 1 is inserted into the mold and the entire plastic sheet 2 is brought into contact with the recess of the mold 3 almost simultaneously. The third method is a combination of the first method and the second method, that is, the injection air pressure from the injection port 5 of the plug 1 is increased or decreased, and the entire plug 1 is moved up and down. This controls how the plastic sheet 2 is stretched. What is common to all three methods above is that in order to control the way the plastic sheet 2 is stretched, air may also be injected from the spout 8 of the mold 3; It is desirable that the air pressure ejected from the plug 1 is stronger than the air pressure ejected from the mold 3. By the above-mentioned method, it is possible to make the plug and the plastic sheet non-contact, to suppress the occurrence of chill lines caused by such contact, and to obtain a molded product with a well-balanced thickness. The plug 1 has a circular or polygonal support plate 4 supported from above by a vertically movable support shaft 6, and an air supply pipe 7 connected to the support plate 4 from above. The plug has a structure in which a porous truncated conical or truncated pyramid shaped outlet is attached with the bottom facing upward, and the shape of the plug is as shown in FIG.
A so-called general convex plug, which is porous and has a truncated cone-shaped or truncated pyramid-shaped outlet, was used at the bottom of the support plate 4. However, as shown in Fig. There is no problem in using an injection plug with an injection nozzle attached, and by doing so, it becomes possible to partially select the air injection part, and also when the shape of the mold 3 recess is complex. By locally changing the air pressure injected from the plug 1, complex shapes can be accommodated, and the resulting molded product can be prevented from becoming partially thick. In addition, the mold 3 is either a type that shares both of the injection function and suction function, has only the injection function, or does not share both functions. Even if it is a type,
It is necessary to have an exhaust function, and for this exhaust function, a dedicated exhaust port may be provided separately from the injection port 8. <Function> According to the present invention, by jetting air with a plug having an air jetting function, it is possible to mold the plastic sheet while controlling how it is stretched. Therefore, it is possible to form the plastic sheet in advance into a shape close to the mold recess, and the plastic sheet can be brought into contact with the mold recess substantially simultaneously. Example 1 A tray with a drawing ratio (depth/opening diameter) of 0.2 was formed using a vacuum forming apparatus under the forming conditions shown in Table 1.
Three types of molding were used: vacuum forming using only a metal mold, plug-assisted vacuum forming using a glass-based material called syntactic foam, and air jetting plug having a porous jetting port according to the present invention. Both plugs had the same shape, and the clearance with the mold was 5 mm. After molding, the thickness of each part of the tray obtained was measured using a micrometer, and the results are shown in FIG. In vacuum forming, the thickness at the bottom corners (measurement points D and F) became very thin, about 1/3 of the sheet thickness. On the other hand, plug-assisted vacuum forming shows an improvement in Goner thickness, but on the other hand, the center area (below the measurement point) tends to become too thick due to contact between the plug and the sheet. According to the air injection type plug of the present invention, the variation in thickness is reduced except for the flange portion (measurement points A and K), and
The thickness was ~180μ. Note that the temperature of the injection air was 100°C.

[Table] Example 2 A container with a drawing ratio of 0.67 was molded using a vacuum forming apparatus under the molding conditions shown in Table 2.

[Table] The injection nozzle type plug shown in Fig. 2 was used for molding. The thickness of the molded product when the blowout air thickness is set to 1/2 atmosphere and the air temperature is changed is shown in the fourth table.
As shown in the figure. Note that while air was being injected from the plug, air at the same pressure was injected from the mold side. When the air temperature is 20℃, the bottom corner (measurement point O,
The thickness of R) was 1/4 of the sheet thickness and 3/4 in the center, and there was a large variation, but as the air temperature was increased, a molded product with a good thickness balance was obtained. <Effects of the Invention> The thermoforming method of the present invention consists of the above steps. Therefore, the plastic sheet is gradually brought closer to the shape inside the mold, which is the mold, while adjusting the air volume, air pressure, etc., using the plug and the injection port provided in the mold recess, until the plastic sheet is shaped to a certain extent. Since the plastic sheet comes into contact with the mold only after it has been molded, it is possible to prevent the plastic sheet from partially touching the mold and being cooled and forming thick spots before molding. In addition, when a nozzle assembly type plug is used, the injection air supply location can be selected locally, so replacing the mold can change the shape of the molded object. However, since the location where air is injected from the plug can be selected accordingly, a plug with versatility can be obtained.

[Brief explanation of drawings]

Figure 1 shows an example of the molding method according to the present invention in which a porous air jetting plug is used, and Figure (b) shows a state in which air is jetted out from the plug and the mold at the same time, resulting in a non-contact state. ing. Figure c shows the state of molding by vacuuming. Figure 2 shows a case where a nozzle type air jet plug is used. FIG. 3 is a graph of the thickness distribution of a molded article molded by the molding method shown in FIG. 1, and the results of general vacuum forming and plug-assisted vacuum forming. FIG. 4 is a graph showing the thickness distribution of a molded article molded by the molding method shown in FIG. 2 when the air temperature is varied. 1...Plug, 2...Plastic sheet, 3
...Mold, 4...Support plate, 5...Injection port, 6...
Support shaft, 7... Air supply pipe, 8... Injection port, 9... Mold recess, 10... Nozzle shaped plug, 14... Graph when molded by vacuum forming, 15... By plug assist vacuum forming Graph when molded, 16...Graph when molded with an air jet plug, 17...Air temperature 20
Graph when molding at ℃, 18...Air temperature 60
Graph when molding at °C, 19...Air temperature
Graph when molded at 100℃.

Claims (1)

[Scope of Claims] 1. A flat plastic sheet is heated, and the plastic sheet is heated with a plug equipped with an air jetting function and capable of sliding up and down, and with at least one of air jetting and suction functions. The plug is placed between the plug and a concave mold having a structure that is equipped with a function or has neither function, and the plug is fixed to control the air pressure injected or sucked from the plug and the mold, Alternatively, the plastic sheet is brought into contact with the mold without touching the plug by controlling the air pressure injected or sucked from the plug and the mold while moving the plug up and down, or by keeping the air pressure constant. A thermoforming method characterized by 2. The thermoforming method according to claim 1, wherein the plug has an injection port having a porous surface in the shape of a truncated cone or a truncated pyramid. 3. The thermoforming method according to claim 1, wherein the plug is an assembly of nozzles, and air is ejected from the nozzles.