JPH0649334B2 - Thermoforming method for polyester sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for thermoforming a polyester sheet, which can be used, for example, for producing a plastic packaging container.

[Background Art] A transparent polyester sheet obtained by performing rapid cooling in a film formation using a polyester resin has good thermoformability, and thus can be easily formed into a container.
However, this molded polyester container is
Since it shrinks by heat at the above temperature, it cannot be used as a container for high temperature filling around 90 ° C.

Then, conventionally, various methods have been proposed as a method for increasing the heat shrinkage temperature of a molded product obtained by thermoforming a transparent polyester sheet.

For example, a sheet is uniaxially stretched, then heated to a predetermined temperature to shrink it, and then thermoformed (Japanese Patent Laid-Open No. 50-210).
51), a sheet is biaxially stretched, heated in an inert gas and deep-drawn in a mold, and then rapidly cooled to a temperature below the second transition temperature (JP-A-57-53316). , The sheet is biaxially stretched, Tc (crystallization temperature) or less and (Tc-
A method of heat-treating with a mold of 70 ° C. or higher and then cooling with a cooling mold (Japanese Patent Publication No. 1-285050, US Pat. No. 4,388).
No. 356), a sheet is biaxially stretched to have a shrinkage ratio at 150 ° C. of 3% or less, and pressure-formed at 180 to 240 ° C. (Japanese Patent Laid-Open No. 58-67411).

[Problems to be Solved by the Invention] As described above, various methods for increasing the heat shrinkage temperature of a polyester sheet molded product have been proposed. No satisfactory results have been obtained.

When a polyester sheet is thermoformed by an existing thermoforming method, rapid drooping (drawdown) occurs as the heating temperature rises. Therefore, the upper limit of the heating temperature, which is the drooping limit at which molding is possible, is around 100 ° C., and thus it is actually impossible to thermoform a transparent polyester container having a heat resistance of 65 ° C. or higher.

It is an object of the present invention to provide a thermoforming method for a polyester sheet, which has a high heat shrinkage temperature and can give a molded article having good drawability and transparency.

[Means and Actions for Solving the Problems] A method for thermoforming a polyester sheet according to the present invention is a method in which a transparent polyester sheet is fixed to a peripheral portion of a molding die, and this sheet is heated to a low temperature crystallization temperature or higher to produce a cavity. 5 to 50% of the diameter of the mold opening, preferably 10 to 50% of the vertical distance from the upper end of the opening to the maximum hanging point of the sheet, and then cooled to 70 ° C or lower, preferably 40 ° C or lower. It is characterized in that the sheet is molded by a molding die.

As the transparent polyester sheet, an amorphous polyester sheet (non-oriented), a stretched polyester sheet (oriented),
There are copolymerized amorphous polyester sheets (non-oriented) and the like.
With this copolymerized amorphous polyester sheet (non-oriented),
For example, it is a copolymer of a monomer such as 1,4-cyclohexanedimethanol and a polyester. The crystalline polyester sheet (non-oriented) is opaque, and a transparent molded product cannot be obtained even by this molding method.

If the polyester sheet is not fixed to the peripheral edge of the molding die, wrinkles occur on the flange of the molded product. Further, in the case of vacuum forming, the degree of vacuum cannot be increased. As a specific means for fixing the sheet to the peripheral edge of the molding die, for example, a ring-shaped clamp that presses the entire circumference of the peripheral edge of the die can be used.

Heating the sheet above the low temperature crystallization temperature (cold crystallization temperature) droops below the low temperature crystallization temperature (drawdown).
Is not generated. This low temperature crystallization temperature is 135 in the case of polyethylene terephthalate (PET), for example.
~ 140 ° C.

When the amount of droop of the sheet is set to 5 to 50% of the diameter of the mold opening, the heat shrinkage temperature of the molded product becomes low when it is less than 5%, and when it exceeds 40%, the sheet droops too much and holes are formed in the sheet. This is because molding becomes impossible. When the shape of the die opening is not circular but square, the length of the diagonal line is used instead of the diameter.

Cooling the molding die to 70 ° C. or lower is because when it is higher than 70 ° C., the transparency of the obtained molded product becomes poor, and the molded product adheres to the molding die, resulting in poor releasability. Because.

Conventional molding methods such as nerve molding, pressure molding, and plug molding can be arbitrarily used as the molding method in the molding die, but when performing pressure molding or plug molding, the heater on the die is manually operated. Therefore, vacuum forming is preferable from the request of simplification of the apparatus. In the case of vacuum forming, the degree of vacuum is preferably 500 mmHg or more.

In the present invention, the sheet also includes a thin film.

[Examples] Examples of the thermoforming method for a polyester sheet according to the present invention will be described with reference to the drawings.

First, the structure of the thermoforming apparatus used in this embodiment will be described.

As shown in FIG. 1, the thermoforming apparatus includes a vacuum forming die 11, a heater 12 provided above the vacuum forming die 11, and a sheet supply side of the vacuum forming die 11. It comprises a pair of upper and lower preheating heaters 13A and 13B provided.

The vacuum molding die 11 has a circular cavity 14 in which the polyester 1 is shaped, and the bottom portion 11A of the cavity 14 and the flange molding portion 11 of the container 2 to be molded.
Vacuum communication holes 15 are formed between B and the peripheral portion 11C. The mold 11 is provided with a cooling water passage, which is a cooling means (not shown), and the temperature of 70 ° C. or lower is maintained during molding. The vacuum forming mold 11 is movable in the vertical direction. In addition, a ring-shaped clamp 16 for fixing the polyester sheet 1 is provided above the peripheral edge portion 11C along the entire circumference of the peripheral edge portion 11C.

As the heaters 12, 13A, 13B, any heating means such as an infrared heater can be selected.

Using the thermoforming device, the polyester sheet 1 is thermoformed into the container 2 as follows.

First, as shown in FIG. 1 (A), a polyester resin was extruded from a general extruder at 295 ° C., and a transparent polyester sheet 1 having a thickness of 0.15 to 1.5 mm formed through a T die was supplied to a thermoforming device. To do. Polyester sheet 1
After being preheated by the preheat heaters 13A and 13B, it is sent to the stage where the mold 11 is positioned, and is fixed by being sandwiched between the clamp 16 and the peripheral portion 11C of the mold 11 by the rising of the mold 11. . The preheating temperature is preferably a temperature at which the sheet 1 does not droop, specifically, 90 ° C. or lower.

Next, as shown in FIG. 1 (B), the polyester sheet 1 is heated to a temperature lower than the low temperature crystallization temperature by a heater 12 above the die 11 and hung down by a predetermined amount. This predetermined hanging amount L is a value obtained by measuring the vertical distance from the upper end of the opening of the cavity 14 to the maximum hanging point of the sheet 1, and heating so that this L becomes 5 to 50% of the diameter of the mold opening. To do.

Next, as shown in FIGS. 1 (C) and (D), when the sheet 1 is heated and droops by a predetermined amount and reaches 140 to 170 ° C., vacuum suction is performed from the vacuum communication hole 15 of the mold 11. Then 7
Sheet 1 in cavity 14 cooled to below 0 ° C
Shape.

Next, as shown in FIG. 1D, the mold 11 is lowered to release the molded container 2 from the cavity 14.

Hereinafter, the same steps as above are repeated to continuously mold the container 2.

In the above embodiment, the preheating heaters 13A and 13B are provided to preheat the polyester sheet 1 before molding. However, without providing the preheating heaters 13A and 13B, the sheet 1 is heated only by the heater 12 above the mold 11. May be. However, by providing the preheaters 13A and 13B, the heating process can be speeded up.

Further, the vacuum forming in this embodiment may be plug assist vacuum forming using a plug.

Experimental Example In the above examples, the following polyethylene terephthalate sheet was used as the transparent polyester sheet 1, and the thermoforming conditions were set as follows to mold the container 2 according to each experimental example. The drawability and transparency of each of these containers 2 were evaluated, and the results of measuring the heat shrinkage temperature are shown in Table 1 below.

As a comparative example, the container according to each comparative example was molded under thermoforming conditions different from those of the experimental example. Table 1 below also shows the results of the same evaluations and measurements as in the experimental examples for each of these containers.

I. Types of transparent polyester sheet (a) Amorphous non-oriented polyethylene terephthalate sheet: A polyethylene terephthalate resin was extruded from an extruder, formed into a film through a T die, and then rapidly cooled to 60 ° C. by an electrostatic application method.

(B) Crystalline oriented polyethylene terephthalate sheet ...
The amorphous non-oriented polyethylene terephthalate sheet was uniaxially stretched at 100 ° C. by a factor of 2 to produce it.

(C) Crystalline non-oriented polyethylene terephthalate sheet: A polyethylene terephthalate resin added with a crystallization agent was used and produced in the same manner as in the above (a).

II. Thermoforming conditions (a) Mold: Two kinds of molds 11 having a drawing ratio (depth / opening diameter) of the cavity 14 of 0.5 and 1.0 were used. The diameter of each mold opening is 80 mm.

(B) Mold temperature ... The temperature was kept at 20 ° C. or 80 ° C. by the cooling means provided in the mold 11.

(C) Temperature of heater above the mold ... Heating to 450 ° C.

(D) Distance between the heater above the die and the sheet ... The distance between the heater 12 above the die 11 and the sheet 1 before hanging is 150.
mm.

(E) Sheet heating temperature: The surface temperature of the sheet 1 immediately before molding, which is heated by the heater 12 above the mold.

(F) Vacuum forming vacuum degree: 750 mmHg.

III. Evaluation method (a) Drawing property: It was evaluated whether the drawing ratio (depth / opening diameter) of the molded container 2 was the same as the drawing ratio of the cavity 14.

(B) Transparency ... Transparency was visually evaluated.

(C) Thermal shrinkage temperature ... The container 2 was left in an air oven for 30 minutes, and the temperature at which about 3% shrinkage occurred was measured visually.

As shown in Table 1, according to Experimental Examples 1 to 5, an amorphous non-oriented or crystalline oriented polyethylene terephthalate sheet was used as the transparent polyester sheet 1, and the hanging amount was 5 to 5 times the diameter of the mold opening. 50%, that is, within the range of 4 to 40 mm, and since the temperature of the mold 11 was kept at 70 ° C. or lower,
It can be seen that the container 2 having excellent drawability and transparency and having a high heat shrinkage temperature can be obtained. Therefore, this container 2
By using, it becomes possible to perform high temperature filling at around 90 ° C., which was impossible in the past. Further, since the thermoforming process is simple, high productivity can be obtained.

On the other hand, according to Comparative Examples 1, 2, 6, and 7, a transparent amorphous non-oriented or crystalline oriented polyethylene terephthalate sheet is used as the sheet, and the mold temperature is equal to or lower than the predetermined temperature of the present invention. However, it can be seen that when the amount of drooping is outside the predetermined range of the present invention, good characteristics cannot be obtained with respect to any one or more of drawability, transparency and heat shrinkage temperature. In Comparative Example 6, there was a hole in the sheet during hanging, and it could not be molded into a container.

According to Comparative Example 3, the mold temperature was not higher than the predetermined temperature of the present invention, but in addition to the sheet being an opaque crystalline non-oriented polyethylene terephthalate sheet, since it was not drooped, drawability and transparency were obtained. Good properties were not obtained.

According to Comparative Example 4, the droop amount and the mold temperature were within the predetermined ranges of the present invention, but the transparency was poor because the sheet was an opaque crystalline non-oriented polyethylene terephthalate sheet.

According to Comparative Example 5, the sheet is a transparent amorphous non-oriented polyethylene terephthalate sheet, and the hanging amount is also within the predetermined range of the present invention, but the mold temperature exceeds the predetermined temperature range of the present invention. Therefore, the transparency was poor.

[Effects of the Invention] According to the present invention, a molded product having a high heat shrinkage temperature as well as good drawability and transparency can be obtained.

[Brief description of drawings]

FIGS. 1A to 1E are process diagrams showing a molding method according to an embodiment of the present invention. 1 ... Polyester sheet, 2 ... Container, 11 ... Mold, 12 ... Heater, 14 ... Cavity, 15 ... Vacuum communication hole, 16 ... Clamp.

Claims (1)

[Claims] 1. A transparent polyester sheet is fixed to a peripheral portion of a molding die, and the sheet is heated to a temperature lower than a crystallization temperature at a low temperature so that a die opening is formed at a vertical distance from an upper end of an opening of a cavity to a maximum hanging point of the sheet. 5% to 50% of the diameter of the polyester sheet, and then the sheet is molded by a molding die cooled to 70 ° C. or less.