JPH0136776B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to a method for thermoforming a thermoplastic resin molded article, and particularly to a thermoforming method for obtaining a predetermined molded article from a thermoplastic resin sheet by vacuum forming or pressure forming. Prior Art In thermoforming as described above, the molding material sheet is generally polymethyl methacrylate,
Thermoplastic resin sheets such as polycarbonate, acrylonitrile-butadiene-styrene copolymer, and polyvinyl chloride are used, but when these material sheets are handled with their surfaces exposed, the surface often deteriorates during the handling process. It is easy to get scratched, and there is a great risk that it will spoil the aesthetic appearance and reduce the product value. For this reason, protective paper or a protective film is usually attached to the surface. The above-mentioned protective paper is generally a paper material such as kraft paper coated with a water-soluble adhesive, and the protective film is mainly a polyethylene film coated with an adhesive, or a polyethylene film coated with an adhesive. ethylene·
A two-layer film made by integrating vinyl acetate copolymer films into a laminated structure is used. However, when thermoforming a molded product by vacuum forming or pressure forming using a synthetic resin sheet covered with a protective film as described above, in the conventional technology, the protective film must be peeled off before molding. I didn't get it. The reason for this is that in the case of thermoforming, heating at a relatively high temperature (150 to 200°C) changes the physical properties of the film itself and promotes adhesion, which makes it difficult for the protective film to peel off after molding. This is due to the fact that it becomes extremely difficult. Problems to be Solved by the Invention However, in the conventional thermoforming method, since the protective film is removed before thermoforming, for example, polycarbonate, acrylonitrile, butadiene, and When molding a material such as a styrene copolymer sheet, scratches caused during the pre-drying process may remain on the molded product, and if the thermoplastic resin sheet that has been softened by heat comes into contact with the mold, the outside of the sheet may be damaged. There were drawbacks such as the appearance of irregularities called fine bumps on the surface, which caused the molded product to exhibit a rough skin condition, and furthermore, the molded product was easily scratched during trimming. In particular, when the thermoplastic resin sheet is transparent, this surface quality defect causes diffused reflection in reflected and transmitted light, significantly reducing the appearance of the product, and there is a strong demand for its improvement. I was about to get hit. Means for Solving the Problems The present inventors focused on the fact that thermoforming is possible when the protective film is a polymer film, and after repeating numerous experiments and research on various protective films, found that If a protective film consisting of a two-layer film consisting of a low-density polyethylene film and an ethylene/vinyl acetate copolymer film is used and vacuum or air pressure molding is performed with this film adhered to a material sheet, the surface of the molded product may be scratched or the above-mentioned problems may occur. We have discovered that it is possible to obtain a molded product with a photosensitive surface that does not appear and has an extremely smooth surface texture, and that the protective film can be peeled off even after molding,
completed this invention. Therefore, in the thermoforming method for thermoplastic resin molded products according to the present invention, when thermoforming a molded product using a thermoplastic resin sheet as a material by vacuum or pressure forming, the material sheet is brought into contact with at least the mold in advance. A protective film made of a laminate of a low-density polyethylene film and an ethylene/vinyl acetate copolymer film is attached to the surface, and the material sheet is heated and softened to perform thermoforming while the protective film is attached. It is characterized by this. The material sheet for the above-mentioned molding is made of thermoplastic resins conventionally commonly used for vacuum or pressure molding, such as polymethyl methacrylate, polycarbonate, acrylonitrile-butadiene-styrene resin, polyvinyl chloride, etc. used. As a protective film, low-density polyethylene film with a thickness of about 40 to 100 μ (density 0.910 to 0.925) is used.
An ethylene/vinyl acetate copolymer film with a thickness of approximately 20 to 50μ is laminated on both sides or at least one side of the molded product, and the overall thickness is approximately 100 to 150μ, especially when considering the surface texture of the molded product. It is preferable to use a prepared protective film. The thickness ratio of low density polyethylene film (LDPE) and ethylene vinyl acetate copolymer film (EVA) is generally about 2/1 (LDPE/EVA). By the way, the use of low-density polyethylene as a base material for the protective film is particularly limited because, compared to the use of other thermoplastic resins, the drop in elongation after heat molding is small and the molded product is This is because the protective film can be easily peeled off without causing unnecessary breakage when the protective film is peeled off. Incidentally, when a high-density polyethylene film is used, the reduction in elongation rate makes it extremely difficult to peel off the protective film after molding, as shown in Example 1 below. On the other hand, the adhesive layer for attaching the protective film is limited to ethylene/vinyl acetate copolymer film when other adhesives, such as rubber-based, acrylic-based resin materials, etc. are used. This is based on the fact that it can achieve better adhesion or adhesion to the molding material sheet as well as easier peelability from the surface of the molded product after molding. Therefore, the film also includes:
Those having a vinyl acetate content of 5 to 15% are preferably used. In other words, if the vinyl acetate content is less than 5%, the adhesive strength to the material sheet will be weak, resulting in the risk of the protective film peeling off when the material sheet is handled, and if it exceeds 15%, the adhesive strength will be poor when the material sheet is heat-formed. This is because the film becomes too high and cannot be peeled off from the surface of the molded product. The present invention, as shown in FIG. 1, consists of a protective film 1 as described above, that is, a low-density polyethylene film 1a and an ethylene-vinyl acetate copolymer film 1b, which are laminated by coextrusion or lamination. The formed two-layer film is applied to one side (in the case of FIG. 1A) or both sides (in the case of FIG. 1B) of a molding material sheet 2 made of a thermoplastic resin, and is applied with the adhesive strength of the ethylene/vinyl acetate film 1b. Use and paste. Thereafter, as shown in FIG. 2, the molding sheet 2 is held in the clamp 4 of the molding machine with both sides of the protective film 1 attached facing the inside of the mold 3, and thermoformed using the heater 5. After heating to a certain temperature, the material sheet 1 is pressed against the inner surface of the mold by vacuum suction inside the mold or by supplying compressed air from outside the mold, as shown in Fig. 3, to form a predetermined molded product. . Note that during this molding, it is desirable to keep the mold in advance at a temperature of about 80 to 90°C, since this allows the surface texture of the molded product to be further finished. Effects of the Invention As described above, when thermoforming a molded article using a thermoplastic resin as a material by vacuum or pressure forming, a protective film is pasted on one side of the material sheet in advance, and the protective film is not peeled off. Since this is a method of thermoforming the material sheet while it is still attached, it is possible to avoid scratches on the surface due to external force during the handling process before molding, such as the drying process before molding the material sheet and the process of setting it on the molding machine. Not only can it be prevented, but also during molding, since the protective film is interposed between the mold and the material sheet, the protective film acts as a cushion layer and covers the surface of the molded product. It is possible to prevent formation of minute irregularities or generation of irregularities due to residual air. Therefore, it is possible to obtain a molded product having an extremely smooth and glossy surface without scratches or so-called bumps appearing on the surface of the molded product. Furthermore, since the protective film layer remains on the surface of the molded product, it is possible to prevent the surface from being scratched even during storage and handling of the molded product. When using the protective film, despite undergoing a thermoforming process, the elongation rate of the low-density polyethylene film does not decrease significantly, and the peel strength of the ethylene/vinyl acetate film does not increase significantly. This makes it easy to remove from the surface of the molded product.
Moreover, it can be peeled off cleanly without leaving any adhesion residue of the adhesive component. Examples Example 1 A polycarbonate resin sheet with a thickness of 1.5 mm was used as a molding material sheet, and a protective film (vinyl acetate content of EVA film of 10%) consisting of various two-layer films shown in Table 1 below was coated on both sides of the sheet. , 60
Pressure bonding was carried out under the conditions of ℃ x 2 kg/cm. Then, this protective film pasting material sheet was heated and softened to 190°C, and then molded into a box-shaped product by vacuum forming at a mold temperature of 90°C and a development magnification of 1.5 times. After cooling, the peelability of the protective film covering the surface of the molded product was examined, and the condition of the surface of the molded product after peeling was visually inspected. The result is the first
Shown in the table. In the table, HDPE means high-density polyethylene, LDPE means low-density polyethylene, and EVA means ethylene-vinyl acetate copolymer. The same applies to Table 2 and below.

【table】

[Table] Example 2 A polymethyl methacrylate resin sheet with a thickness of 2.0 mm was used as a molding material sheet, a protective film was attached to only one side of the sheet, the heating temperature of the material sheet was 175°C, and the mold temperature was A box-shaped molded product was formed using various protective film-applied material sheets in the same manner as in Example 1, except that the temperature was set at 80° C. and the air pressure molding method was adopted. The removability and surface appearance of the protective film in the molded article thus obtained were similar to the results of Example 1 shown in Table 1. Based on the results of Examples 1 and 2 above, when a two-layer film of low-density polyethylene film and ethylene/vinyl acetate copolymer film is used as a protective film, it can be heated while attached to the material sheet. It was confirmed that molding was possible and that the protective film could be easily peeled off after molding. Example 3 A material sheet made of the same polycarbonate resin as in Example 1 was used, and two types of protective films with different base layer materials were crimped onto one side of the sheet (sheet temperature: 60°C, pressure: 2 kg/cm). After pasting, it was once softened by heating at 190°C, and then cooled.
Then, we measured the peel strength and elongation of the protective film before and after heating, and the results were as follows.
It was as shown in the table.

[Table] According to the results in the table above, the base layer of the protective film has
By using a low-density polyethylene film, the decrease in elongation of the protective film after heating is significantly smaller than when using a high-density polyethylene film, and therefore, when peeling from the surface of the molded product, there is no waste in the film. It was found that it could be easily peeled off without causing any rupture or tearing. Example 4 Next, several types of protective films were prepared in which the vinyl acetate content in the ethylene/vinyl acetate group polymer film was varied, and these were made of polycarbonate resin in the same manner as in Example 3 above. The protective film was attached to a material sheet, heated and then cooled, and the peel strength of the protective film was examined. The results were as shown in Table 3 below.

[Table] From the results shown in the above table, it was confirmed that it is preferable to use an ethylene/vinyl acetate film having a vinyl acetate content in the range of 5 to 15% to form the adhesive layer of the protective film.

[Brief explanation of drawings]

Figure 1 A and B are cross-sectional views showing the state in which the protective film is attached to the material sheet, Figure 2 is a cross-sectional view showing the state during heat treatment immediately before thermoforming, and Figure 3 is the state during thermoforming. FIG. 1... Protective film, 1a... Low density polyethylene film, 2a..., 3... Molding mold.

Claims (1)

[Claims] 1. When thermoforming a molded article using a thermoplastic resin sheet by vacuum or pressure forming,
A protective film made of a laminate of a low-density polyethylene film and an ethylene/vinyl acetate copolymer film is affixed in advance to at least the surface of the material sheet that will be in contact with the mold, and the material sheet is placed as it is with the protective film affixed. 1. A method for thermoforming thermoplastic resin molded products, the method comprising thermoforming by heating and softening. 2 Ethylene/vinyl acetate copolymer film is
The method for thermoforming a thermoplastic resin molded article according to claim 1, wherein the vinyl acetate content is 5 to 15%.