JPH0156891B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for molding plastic bottles with excellent thermal stability and appearance.

In general, plastic bottles obtained by biaxially stretching parisons made of thermoplastic resins such as polyethylene and polypropylene have poor thermal stability, and when filled with high-temperature substances, the plastic bottles are susceptible to stress distortion caused during stretching and molding. May be deformed. especially,
This deformation was large in plastic bottles with irregularities on the outer circumferential surface.

Conventionally, biaxially stretched bottles were heat-treated to crystallize them to increase their thermal stability.However, in recent years, when filling plastic bottles with contents, it has been necessary to use high-temperature conditions immediately after high-temperature sterilization. It has become common practice to fill plastic bottles as they are, and the demand for thermal stability for plastic bottles is increasing.

The present invention provides a method for molding plastic bottles that meets such demands, and its gist is that a parison heated to an appropriate temperature for stretching is heated to a temperature above the crystallization temperature and below the melting temperature of the parison, The parison is then placed in a primary molding mold having a cavity approximately in the shape of the final bottle, and a pressurized fluid is blown into the parison to inflate it until it comes into close contact with the inner surface of the mold to form an intermediate bottle while further internal pressure is applied. The bottle is held as it is for a predetermined period of time to perform heat treatment, and then the pressure is removed, the mold is opened, and the heat-treated intermediate bottle is taken out, and the intermediate bottle is set in a secondary molding mold having a cavity in the shape of the final bottle. The method of molding a plastic bottle is characterized in that the intermediate bottle is then re-inflated by blowing pressurized fluid into the intermediate bottle.

When the plastic bottle obtained by blow-stretching is taken out from the mold, it undergoes natural shrinkage, and along with this, the stress and strain generated during stretching are eliminated, and the unevenness formed on the outer circumferential surface of the bottle becomes gentle. Therefore, the present invention divides the bottle stretching process into two stages. In the first stage, an intermediate bottle close to the desired final shape is formed in a primary mold and subjected to heat treatment, and in the second stage, it is shrunk and By blow-stretching the intermediate bottle in a softened state in a secondary molding mold having a cavity in the shape of the final bottle, a plastic bottle with excellent thermal stability and with sharp edges and irregularities formed on the outer circumferential surface is produced. It is in the molding.

The molding method of the present invention can be applied to plastic bottles obtained by blow-stretching parisons obtained by injection molding or extrusion molding of thermoplastic resins such as polyethylene, polypropylene, and polyethylene terephthalate. The molding method will be specifically explained with regard to blow molding of a parison made of polyethylene terephthalate resin.

FIG. 1 is a sectional view showing the first stage of the drawing process of the present invention, in which 1 is a primary mold, 2 is a blowing nozzle, 3 is a drawing rod, and P is a parison.

The molding die 1 has a half-split cavity substantially in the shape of a final bottle, and a plurality of unevenness 12 of a desired shape is provided on the wall surface of the die. It is preferable that the edges of the unevenness 12 be sharper than the unevenness of the secondary mold. Further, a heat medium such as heating oil is circulated through the circulation holes 11 provided in the mold wall, and the cavity wall surface is heated to a temperature above the crystallization temperature of the parison and below the melting temperature (130 to 230°C).

The blowing nozzle 2 has a blowing port 21 connected to a source of pressurized fluid, and a stretching rod 3 is provided in the blowing port 21 so as to be slidable vertically therethrough.

In the first stage of the stretching process of the present invention, the appropriate temperature for stretching (80
A parison P heated to 150 DEG C.) is set in the mold 1, and a blowing nozzle 2 is attached to the opening of the parison. Then, after the stretching rod 3 is protruded into the parison and stretched in the longitudinal direction, or while stretching is being performed in the longitudinal direction, pressurized fluid (10 to 30 kg/cm ² in absolute pressure) is blown into the mold surface through the blowing port 21. An intermediate bottle is formed by expanding the bottle until it comes into close contact with the bottle, and is then held for a predetermined period of time while applying internal pressure. This holding time is determined appropriately depending on the wall temperature of the mold. For example, when the wall temperature is set to 130°C, it is held for 5 seconds, and when it is set to 230°C, it is held for 2 seconds, and the intermediate bottle is held for 2 seconds. Heat treatment is applied to. At this time,
If the wall temperature of the mold is below 130°C, it will take time for crystallization to occur, making it impossible to obtain an effective degree of crystallinity.
If the temperature is higher than 230°C, the bottle surface will become rough and white, making it impossible to perform a good heat treatment.

After that, the mold 1 is opened to remove the pressure, or while applying a slight internal pressure, and the intermediate bottle is taken out.The intermediate bottle, which is in a contracted and softened state, is heated from the outside with a heater to solidify it. The film is placed in a secondary molding die and subjected to the second stage of blow stretching.

FIG. 2 is a cross-sectional view showing the second-stage stretching process of the present invention, in which the secondary forming mold 4 configured in half has a cavity in the shape of a final bottle, and the wall surface of the mold is is provided with unevenness 42. Further, the cavity wall surface is cooled by a cooling medium circulating within the circulation hole 41. The intermediate bottle is set in the mold 4, and pressurized fluid (absolute pressure: 15 to 30 kg/cm ² ) is blown into the bottle to re-expand it until it comes into close contact with the inner surface of the mold. Through this second-stage stretching process, the intermediate bottle is taken out from the primary molding die and shrunk, and the unevenness on the outer circumferential surface becomes gentle.The intermediate bottle is then stretched again to sharpen the edges of the unevenness to achieve the desired shape. A plastic bottle in its final shape is obtained.

As described in detail above, in the forming method of the present invention, in the first stage drawing step, an intermediate bottle having an uneven outer circumferential surface that is close to the final shape is stretch-molded and heat-treated, and in the second stage drawing step, the final bottle is The unevenness of the outer peripheral surface is sharpened by re-stretching it in a mold having a shaped cavity, and the plastic bottle obtained in this way is different from conventional plastic bottles that are simply heat-treated after being blow-stretched. It has significantly higher thermal stability than bottles, and is extremely less likely to deform or shrink when filled with high-temperature substances. Moreover, since the irregularities on the outer circumferential surface are formed sharply, when displayed at a store, the product is aesthetically appealing to consumers and significantly increases the product value.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view showing the first-stage stretching process of the present invention, and FIG. 2 is a vertical cross-sectional view showing the second-stage stretching process within a molding die. 1... Primary mold, 2... Blow nozzle, 3...
...Stretching rod, 4...Secondary mold, P...Parison.

Claims (1)

[Claims] 1. A parison heated to an appropriate stretching temperature is placed in a primary mold that is heated to a temperature above the crystallization temperature and below the melting temperature of the parison and has a cavity approximately in the shape of the final bottle, and pressurized fluid is introduced into the parison. The intermediate bottle is molded by blowing air into the mold to expand it until it comes into close contact with the inner surface of the mold, and is further heat-treated by holding it for a predetermined period of time while applying internal pressure.Then, the pressure is released and the mold is opened to form a heat-treated intermediate bottle. and set the intermediate bottle in a secondary molding mold having a cavity in the shape of the final bottle,
A method for molding a plastic bottle, characterized in that after that, pressurized fluid is again blown into the intermediate bottle to cause it to expand again.