JP3694775B2 - Manufacturing method of transparent resin container - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is mainly a container for storing food such as a lunch box or a lid of the container, and has a heat resistance that can be used for a microwave oven while maintaining transparency so that the inside can be seen through. The present invention relates to a method for producing a polyethylene terephthalate container generally called PET.
[0002]
[Prior art]
In general, polyethylene terephthalate (hereinafter abbreviated as PET) is known as a saturated polyester that is a linear thermoplastic polymer having an ester bond in the main chain and does not contain an unsaturated bond involved in curing in the molecule. Yes. This PET is widely used in PET bottles and the like because of its excellent surface properties such as high surface smoothness, low water absorption and thermal expansion coefficient and good dimensional stability.
[0003]
[Problems to be solved by the invention]
However, PET usually has a crystallization characteristic such that it reaches the glass transition point at 70 ° C., and beyond this transition point, it becomes white turbid due to crystallization shrinkage and the transparency is impaired. Therefore, when molding a lunch box container or the like with PET, if the transparency is maintained so that the internal contents can be seen through, it is limited to use in a low temperature region below the glass transition temperature, and the container is heated in a microwave oven, etc. Cannot be used. In addition, in order to have heat resistance so that it can be used in a microwave oven (normally the warming temperature in a microwave oven is 100 to 110 degrees C), it must be molded at 120 degrees C or more at the time of molding. The molded product becomes cloudy and the transparency is impaired.
[0004]
Therefore, the object of the present invention is to produce a container with heat resistance that can be used by being put in a microwave oven together with the container, while eliminating the above-mentioned problems and maintaining transparency that allows internal see-through. There is a way to do that.
[0005]
[Means for Solving the Problems]
In order to achieve the above object, the present invention takes the following technical means. That is, in the production method according to the present invention, a transparent polyethylene terephthalate sheet molded under a temperature condition of 70 ° C., which is a temperature lower than the crystallization region of polyethylene terephthalate , is 1.5 times in each direction. A biaxial stretching is performed at a stretch ratio of 2 times , and the stretched sheet material is molded by vacuum molding means under a temperature condition of 70 ° C. where the mold temperature is less than the crystallization region, and then the molded product is rapidly cooled. To do.
[0006]
A preferable mold temperature at the time of forming the sheet is 65 to 70 ° C. of the glass transition point, but the crystallization speed is slow up to a region exceeding 120 ° C. Therefore, even a little over 70 ° C. is sufficient for internal fluoroscopy. Transparency is obtained. In addition, transparency is promoted by biaxial stretching after the sheet is formed, and sufficient flexibility and bending strength are maintained, so that the formability when the sheet is formed into a container is not impaired.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Examples of the present invention will be described below. The container formed by the above method mainly comprises a container main body 1 and a lid 2 as shown in FIG. 1, and a box-shaped food container that can contain food such as a lunch box and can be heated in a microwave oven. Or at least the lid 2 in the container.
[0008]
First, a transparent sheet is formed by injection molding of PET under a temperature condition lower than the crystallization region. In this case, the mold temperature is preferably around 70 ° C. Next, this sheet is biaxially stretched at stretch ratios of 1.5 to 2 times in the longitudinal and lateral directions . This biaxial stretching promotes transparency, strength and heat resistance.
Next, the stretched sheet is formed into a container by vacuum forming means under a temperature condition less than the crystallization region, and then the molded product is rapidly cooled. The mold temperature during vacuum forming is preferably around 70 ° C. of the glass transition temperature of PET, similar to the temperature during sheet forming, but crystallization up to 120 ° C. by biaxial stretching of 1.5 to 2 times. Since the crystallization speed in the region becomes slow, even if the molding is performed at around 80 to 90 ° C., it is possible to sufficiently secure the transparency necessary for internal fluoroscopy in combination with the effect of the rapid cooling immediately after molding, and the warming in the microwave oven. Heat resistance that can withstand the region temperature of 100 to 110 ° C. can be maintained.
[0010]
According to the inventor's experiment, the mold temperature during sheet molding was set to 70 ° C., biaxial stretching was doubled, the mold temperature during molding the container was set to 70 ° C., and the container was rapidly cooled after molding. In the molded product, complete transparency was obtained, and even when it was repeatedly used in a warming region in a microwave oven, no clouding or deformation was observed. In addition, there was no problem in transparency even in a molded product in which the mold temperature when molding a container from a sheet was set to 85 ° C. by the same means as described above.
[0011]
Although the embodiments considered to be representative of the present invention have been described above, the present invention is not necessarily limited to only the structures of these embodiments, and has the structural requirements and achieves the above object. The present invention can be implemented with appropriate modifications within a range having an effect.
[0012]
【The invention's effect】
Since the present invention is constructed as described above, it retains the excellent properties of polyethylene terephthalate and the transparency that can be seen through, but has sufficient heat resistance to be used at the warming temperature of the microwave oven. The effect is that the container can be molded.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an example of a container formed by the forming method of the present invention.
[Explanation of symbols]
1 Container body 2 Lid

Claims (1)

A transparent polyethylene terephthalate sheet molded under a temperature condition of 70 ° C., which is a temperature lower than the crystallized region of polyethylene terephthalate , is biaxially stretched at stretch ratios of 1.5 to 2 times in the longitudinal and lateral directions. A method for producing a transparent resin container, comprising: forming a sheet material by vacuum forming means under a temperature condition of 70 ° C. in which a mold temperature is lower than the crystallization region, and then rapidly cooling the molded product.

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