JPS636347B2 - - Google Patents

Description

[Detailed description of the invention]

Conventionally, methods for manufacturing containers from sheet thermoplastic resins include vacuum forming methods and pressure forming methods. Especially when increasing production speed,
Pressure molding seems to be the preferred method. The present invention relates to improvements in air pressure forming methods. In the pressure forming method, if the melting point or melting point of the resin is not clear, there are cases where it is carried out by heating above its softening temperature, and cases where it is carried out below the melting point of the resin or the softening temperature, and the mechanical properties of the resin,
For example, when trying to increase the tensile strength, impact strength, and transparency beyond the original resin properties, the latter method is used. However, when molding is attempted below the melting point or softening temperature of the resin, the stretching strain during molding tends to remain inside the resin, and the elastic elements of the so-called viscoelastic resin are constrained by the viscous elements, resulting in molding. Afterwards or when the container is heated, the internal strain is released and elastic recovery occurs. Therefore, there are problems such as not being able to obtain the desired shape, or deformation when heated in a boil retort, etc., and as the wall thickness becomes thinner during molding and stretching, the surface cools, resulting in so-called uneven stretching. The thickness of the molded product tends to be thicker or thinner in some areas, resulting in poor molding yield. In order to solve this problem, we have developed a method in which a softened thermoplastic resin sheet is stretched in advance using a so-called temperature-controlled plug.In particular, the shape of the plug is adapted to the shape of the molded product, and the wall thickness is thin. There was a molding method (Japanese Patent Publication No. 53-1790) in which the plug was pressed against the sheet and then molded using pressurized air so that the plug was only in contact with the part where it was expected to be. However, this method has problems such as difficulty in controlling the plug temperature, expensive mold equipment costs in the case of multi-cavity molding, and poor yield. Therefore, the present invention was made as a result of intensive studies on air pressure forming methods in order to obtain molded products with good wall thickness distribution, low internal distortion, and good heat resistance without using these plugs. . That is, the present invention heats a thermoplastic resin sheet to a temperature lower than its melting point or softening temperature,
Similarly, pressurized air is heated to a temperature lower than the melting point or softening point of the resin used, and the heated air is used to mold a thermoplastic resin sheet. Here, the melting point is unique to crystalline resins, and polypropylene, polyethylene, etc. have melting points. On the other hand, amorphous resins that do not have a clear melting point, such as acrylonitrile-butadiene-styrene copolymer, polyvinyl chloride, and polystyrene, are expressed by their softening temperature. Softening temperature is BS−
2782. When performing pressure molding using these resin sheets,
When heat molding is carried out at a temperature higher than the above-mentioned melting point or softening temperature of the resin, desired improvements in mechanical properties, transparency, etc. cannot be observed, and there are disadvantages such as a long heat molding-cooling cycle. On the other hand, when thermoforming is performed at a temperature lower than the melting point or softening temperature of the resin, the so-called biaxial orientation of molecules is emphasized, and the tensile strength and
Impact strength and transparency are improved, and molding cycles can be shortened. However, in this case, residual strain of the resin tends to exist, and problems such as elastic recovery after molding or deformation during heating are likely to occur. Therefore, the present invention can eliminate these drawbacks due to internal distortion by molding using heated air at a temperature lower than the melting point or softening point of the resin instead of the conventionally used low-temperature air. This was done by discovering. Thereby, the resin sheet can be stretched without being cooled during molding and stretching, and the completed molded product can have less internal distortion. In conventional molding using low-temperature air, a pressure of 2 to 6 kg/cm ² is normally blown directly into the sheet whose surroundings are sealed with clamps. Because it is cooled considerably and stretched at high speed, internal strains tend to remain, and the molded product tends to be partially thick and thin, resulting in poor wall thickness distribution. The temperature of the heated air used in the present invention is lower than the above-mentioned melting point or softening temperature of the resin used, but sometimes it is possible to mold well even if it exceeds this temperature. This is thought to be due to internal strain remaining in the sheet itself, thermal history, crystalline state, etc. However, in order to reliably achieve the objective, the temperature should be lower than the melting point or softening temperature of the resin used, preferably as close to these temperatures as possible, but depending on the desired mechanical properties and transparency. Then, the temperature of the heated air is appropriately selected and determined. These heated air can be produced through heaters provided with a spongy or fin shape. If this method does not raise the desired temperature, a predetermined amount of air may be preheated in the accumulator in advance. When heated air is introduced into a thermoplastic resin sheet that has been softened by heating, for example, in the first step, the pressure of the heated air is made equal to or lower than the yield stress of the resin at the resin temperature at that time, and the sheet is pre-stretched. This makes it possible to smooth out wrinkles in the heated and expanded sheet. In the second stage, the pressure necessary to stretch the resin above the yield stress at the resin temperature at that time is applied. This allows stretching without leaving any internal strain.
Finally, the pressure is increased all at once until it comes into close contact with the mold. This operation makes it possible to improve the wall thickness distribution of the molded product and to reduce molding internal distortion as much as possible. In this molding method, the time required only for molding is approximately 5% in the first stage, 90% in the second stage, and 5% in the third stage. In particular, it is important to slowly stretch the molded product in the second step until just before it comes into close contact with the mold, and in some cases, the third step can be omitted. By slowly stretching, sufficient stress relaxation is achieved and internal strain can be reduced. In this second stage, the tensile strength of the molded article increases as it is stretched, so it is preferable to gradually increase the pneumatic pressure accordingly. Further, the space sealed between the sheet and the mold is gradually evacuated depending on the degree to which the sheet is squeezed against the mold by compressed air pressure. It is important to adequately control these timing and pressure controls. Resins that can be used in the present invention include crystalline resins such as polypropylene and polyethylene, acrylonitrile-butadiene-styrene copolymers,
Non-crystalline resins such as polyvinyl chloride and polystyrene can be used without particular limitation. EXAMPLES In order to further clarify the present invention, Examples will be described. Example Melt index (MI) is 5, melting point (DSC
A homopolypropylene sheet (0.8 mm thick) with a value measured at 165°C was heated at a furnace temperature of 250°C and held in the furnace until the surface temperature of the sheet reached 160°C.
This sheet was introduced into a molding zone, clamped with a compressed air box and a mold, and pre-stretched by applying a pressure of 0.2 kg/cm ² to a temperature of 156°C. Next, the heating and compressed air pressure was gradually increased from 0.2 Kg/cm ² , and when the final pressure reached 6 Kg/cm ² , the molding was completed. During this time, the mold side was gradually evacuated. Table 1 shows the properties of the molded product thus produced. Comparative Example The same sheet as used in the example was similarly heated until the surface temperature reached 160°C, and the sheet was introduced into a molding zone and clamped between a compressed air box and a mold, and cooled air at a pressure of 6 kg/cm ² was applied. (18°C) and one mold was evacuated and molding was performed. Table 1 shows the properties of the molded product thus produced.

【table】

[Table] In Table 1, the measurement points for wall thickness distribution are the positions indicated by numbers 1 to 5 in the drawings. Also,
Mold shrinkage was measured by how much the product shrank relative to the mold dimensions. The use of the pressure forming method according to the present invention has the following advantages compared to conventional pressure forming. (1) Variations in wall thickness distribution of molded products can be reduced. (2) Minimizes molding shrinkage and reduces changes in molded product shape over time. Thereby, for example, when punching a molded product, it has good dimensional stability and can be punched easily. (3) Heat resistance can be improved. (4) Since the thickness distribution is good, the apparent rigidity of the molded product increases, and the thickness of the sheet used can be reduced accordingly. (5) The elastic recovery after molding is small and molded products that are faithful to the mold can be obtained.

[Brief explanation of the drawing]

The drawing is an explanatory cross-sectional view showing the locations where the wall thickness of the molded product is measured.

Claims (1)

[Claims] 1 In pressure forming of thermoplastic resin sheets,
Air is heated in advance to a temperature lower than the melting point or softening temperature of the thermoplastic resin, and the thermoplastic resin sheet is heated to a temperature lower than the melting point or softening temperature of the resin and stretched by the pressure of the heated air. , as the stretching process, the pressure of the heated air is lowered to below the yield stress of the resin at the resin temperature at that time, and the sheet is pre-stretched, and then the pressure of the heated air is gradually increased to a level higher than the yield stress at the resin temperature at that time. A pressure forming method that is characterized by closely contacting the stretched sheet to the mold surface.