JPS6248568B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention comprises inserting and disposing a stretching temperature control mandrel and a stretching mandrel in a connected manner in a thermoplastic resin unstretched tubular film heated to an appropriate stretching temperature.
The thin parts of the unstretched tubular film and the parts that are easily stretched due to their material properties are cooled by selectively bringing them into contact with a stretching temperature control mandrel. The present invention relates to a method for producing a thermoplastic resin stretched tubular film that prevents locally uneven stretching and uniformly stretches in the circumferential direction.

Stretched films are known to have improved mechanical and physical properties compared to unstretched films, and are widely used in the packaging field.

Conventionally, stretched films have been produced by two methods: stretching in a flat state and stretching in a tubular state.When stretching in a flat state, both ends of the film after being stretched The unstretched part of the film cannot be used as a product, so it must be removed after stretching, resulting in higher manufacturing costs. death,
It has the disadvantage that secondary processing steps such as heat sealing to form a bag shape are complicated.

One of the stretching methods to improve the drawbacks of such a flat stretched film is to insert a mandrel such as a truncated conical shape into an unstretched tubular film, and to run the tubular film along the circumferential side of the mandrel, at least A method of stretching in the circumferential direction has been proposed, but in this case, non-uniformity in the wall thickness of the unstretched tubular film, i.e. non-uniformity in the gap between the annular orifices when extruded from the annular orifice of an annular die, or Non-uniformity in wall thickness due to non-uniform cooling during the process in which a molten tubular film extruded from an orifice is cooled and solidified by an air ring or cooling mandrel, and non-uniformity in material as described below. Therefore, in the initial stage of stretching, the parts that are easily stretched are locally stretched non-uniformly in the circumferential direction, and the locally stretched parts are in close contact with the mandrel, while the unstretched parts are stretched on the circumferential side of the stretching mandrel. This shows the phenomenon of rising from the surface. At the initial stage of stretching, when a portion that is in close contact with the circumferential surface of the stretching mandrel and a portion that is raised are formed, the central axis of the unstretched tubular film and the central axis of the stretching mandrel become eccentric. The eccentricity causes uneven heating in the preheating mandrel that heats the unstretched tubular film to an appropriate stretching temperature immediately before stretching, and makes stretching in the circumferential direction increasingly uneven.

This non-uniform stretching at the initial stage of stretching affects the final stretched film, resulting in a stretched film with non-uniform wall thickness. In addition, non-uniformity in the thickness of the finally obtained stretched film not only causes sagging and wrinkles in the stretched film, reducing its commercial value, but also when printing on the film surface. This can cause printing defects and greatly reduce suitability for secondary processing, so that the stretched film itself may even lose its value.

By the way, as a method for improving the non-uniformity of stretchability caused by the non-uniformity of the thickness of the unstretched film, as described in Japanese Patent Publication No. 34-442,
By stretching an unstretched film with uneven thickness that has been heated to an appropriate stretching temperature while cooling the entire surface of the unstretched film, the thinner part is cooled and the stretching resistance of the thinner part is increased. A method has been proposed in which the thick portions of the film are actively stretched to improve the uniformity of the thickness of the stretched film.

However, although this method is effective to some extent against non-uniformity in stretchability caused by non-uniformity in the thickness of the unstretched film, although the thickness is uniform, the material is non-uniform. No improvement effect was observed on the non-uniformity of stretchability of the unstretched film.

That is, before the thermoplastic resin is extruded from the annular orifice due to non-uniformity in the process of being plasticized and kneaded by the extruder barrel and screw in the extruder, and non-uniformity in the flow of the molten resin within the annular die. Material non-uniformity based on the non-uniformity of , there is no improvement in the non-uniformity of the stretchability of the unstretched film caused by the non-uniformity of the material due to the non-uniformity of heating when heating the unstretched tubular film that has been cooled and solidified to the appropriate stretching temperature. No effect has been obtained.

The present invention has been made in view of these circumstances, and an object of the present invention is to solve the problem of non-uniformity in wall thickness and material problems when stretching an unstretched thermoplastic resin tubular film at least in the circumferential direction using a stretching mandrel. It is an object of the present invention to provide a method for producing a stretched tubular film that can prevent local non-uniform stretching at the initial stage of stretching due to non-uniformity, and can uniformly stretch the film in the circumferential direction.

That is, in the present invention, the thermoplastic resin unstretched tubular film F ₄ heated to an appropriate stretching temperature by a previously known unstretched tubular film manufacturing method has a diameter smaller than the diameter of the unstretched tubular film F ₄ , In addition, there is a cylindrical stretching temperature control mandrel 6 provided with a heat medium cavity 6a in which a heat medium circulates to enable temperature control, and an end surface connected to the stretching temperature control mandrel 6 that is closer to the other end surface. A truncated cone-shaped stretching mandrel 7 having a small diameter and provided with a heat medium cavity 7a in which a heat medium circulates to enable temperature control is inserted and disposed in connection with the above-mentioned stretching mandrel 7. Immediately before stretching, portions of the unstretched tubular film F ₄ that are likely to be stretched due to non-uniformity in wall thickness and non-uniformity of material are removed from the unstretched tubular film F ₄ heated to an appropriate stretching temperature. The film is cooled by selectively contacting the circumferential side surface of the stretching temperature-controlled mandrel 6, which is set at a temperature lower than the temperature of the stretching mandrel 6, and then stretched along the stretching mandrel 7 at least in the circumferential direction. The present invention provides a method for producing a stretched thermoplastic resin tubular film.

Here, the method for manufacturing the unstretched tubular film heated to the above-mentioned appropriate stretching temperature is not limited to the manufacturing method described below, and any other known manufacturing method may be employed.

In addition, thermoplastic resins include low-density polyethylene,
Olefin homopolymers and copolymers such as high-density polyethylene, polypropylene, and ethylene-propylene copolymers, polyesters, polyamides, and the like may be used, but any type of resin may be used as long as it is a stretchable thermoplastic resin. Furthermore, the polymers or copolymers can be used alone or in a mixed state.

The appropriate stretching temperature is the temperature at which molecular orientation occurs due to so-called stretching, and as is well known, it usually has a relatively wide temperature range and can be easily determined in the film processing industry. Generally, in the case of polyolefins, the temperature range is slightly lower than the melting point.

The stretching temperature control mandrel 6 is a cylindrical mandrel having _{a diameter smaller than that of the unstretched tubular film F 4 .}
It is desirable to have a diameter that is 1% to 10% smaller. If the diameter of the stretching temperature control mandrel 6 is equal to or larger than the diameter of the film F ₄ , the film F ₄ will come into close contact with the entire circumferential direction of the mandrel 6 and will be cooled over the entire contact area. In addition, if the non-uniformity of the drawing property caused by the non-uniformity of the material of the film F ₄ is not improved at all, and conversely, if the diameter of the drawing temperature control mandrel 6 is too small than the diameter of the film F ₄ , the drawing mandrel 7 If the local stretching due to the non-uniformity of the stretchability has progressed considerably and has not reached the stage where the final stretched film has uneven thickness, the film F ₄ will be placed on the stretching temperature control mandrel 6. This is undesirable as there is no contact between the film and the film during the stretching process.
This is because the center axis of F ₄ and the center axis of the stretching mandrel 7 are significantly eccentric, and heating becomes uneven when heating the film F ₄ to the appropriate temperature for stretching.

Although the length of the side surface of the stretching temperature control mandrel 6 is arbitrary, it is preferably less than or equal to the diameter of the unstretched tubular film F ₄ and 1/5 or more of the diameter. If the length of the side surface of the stretching temperature control mandrel 6 is too long, the portions of the film F ₄ other than those that are easily stretched will be stretched due to slight radial deflection of the unstretched tubular film F ₄ when heated to the appropriate stretching temperature. If the temperature control mandrel 6 is in contact with the temperature control mandrel 6 and is cooled, on the other hand, if the temperature control mandrel 6 is too short, local stretching due to non-uniformity of drawability on the drawing mandrel 7 will not reach a considerably advanced stage. This is because the film _F4 will no longer be in contact with each other.

Heat medium cavity 6a inside drawing temperature control mandrel 6
Although not shown in the drawings, it is desirable to have a single or multiple spiral pipe structure and to arrange the piping so that the surface temperature of the circumferential side surface is uniform.

The surface temperature of the stretching control mandrel 6 can be set to any desired temperature by controlling the temperature and flow rate of the heat medium circulating inside. Although the set temperature cannot be determined unambiguously depending on the type of thermoplastic resin to be stretched, the molding speed, and the degree of non-uniformity of stretchability, it is at least the temperature of the unstretched tubular film F ₄ that has been heated to the appropriate stretching temperature. You need to set it lower than that. More specifically, immediately before being stretched at least in the circumferential direction on the stretching mandrel 7, a portion having non-uniform stretchability, that is, a portion that is easily stretched, comes into contact with the stretching temperature control mandrel 6. It is desirable to set the temperature of the surface of the mandrel 6 so that it is lower than other parts by about 1° to 10°C. Normally, the desired temperature can be easily set by changing the temperature of the heating medium circulating inside the stretching temperature control mandrel 6 during molding.

The drawing mandrel 7 is connected to the drawing temperature control mandrel 6, and is a truncated cone whose end face on the side to be connected has a smaller diameter than the other end face. It is desirable to have a diameter equal to or smaller than the diameter.
The large diameter of the stretching mandrel 7 normally restricts the stretching ratio of the unstretched tubular film F ₄ in the circumferential direction, but this is not the case when further stretching in the circumferential direction after the stretching mandrel 7 is performed.

If the small diameter of the stretching mandrel 7 becomes larger than the diameter of the stretching temperature control mandrel 6, the stretching temperature control will be stopped until the local stretching due to non-uniformity of stretching on the stretching mandrel 7 has progressed considerably. This is not desirable because the unstretched tubular film F ₄ of the mandrel 6 no longer comes into contact with it. The small diameter of the stretching mandrel 7 is preferably 0% to 10% smaller than the diameter of the stretching temperature control mandrel 6.

The heating medium cavity 7a of the drawing mandrel 7 is preferably provided with a spiral pipe, similar to that of the drawing temperature control mandrel 6. The heat medium of the drawing mandrel 7 is circulated by a circulation system different from the circulation system of the heat medium circulation body of the drawing temperature control mandrel 6.

The surface temperature of the stretching mandrel 7 is naturally within the appropriate stretching temperature range, but in order to obtain a film with uniform thickness, it is desirable to set it slightly lower than the appropriate stretching temperature of the unstretched tubular film _F4 . .

The angle between the extension line of the side surface of the stretching mandrel 7 and the central axis may be arbitrary, but is preferably 20 to 70.
Good range of degrees. If the angle is too small, locally non-uniform stretching start portions due to non-uniform stretchability, i.e., portions other than those that are easily stretched, will also adhere tightly to the stretching mandrel 7, causing the stretching temperature control mandrel 6 to On the other hand, if the angle is too large, the cooling effect of the portions that are easily stretched cannot be relatively enhanced. On the other hand, if the angle is too large, even if the portions that are easily stretched locally, the stretching mandrel 7 This is because it is undesirable because it floats higher than the top and loses its cooling effect.

Since the circumferential surfaces of the stretching temperature control mandrel 6 and the stretching mandrel 7 come into contact with the film, it is desirable to reduce the coefficient of friction with the film.
It is desirable to cover with Teflon, glass cloth, etc.

Although the connecting surfaces of the stretching temperature control mandrel 6 and the stretching mandrel 7 are not shown, it is desirable that they be connected via a heat insulating material.

Between the stretching mandrel 7 and the high-speed nip roll 10, there is a stretching device in order to transmit the force generated by the nip roll 10 to the stretched tubular film _F6 on the stretching mandrel 7 via the stretched film _F7 . Stretched tubular film immediately after mandrel 7
It is desirable to make the diameter of F ₇ approximately equal to the diameter of the stretching mandrel 7, and for this purpose, the diameter of the stretched tubular film is
It is desirable to control the internal pressure of F ₇ by air pressure piping A ₃ . Furthermore, it is desirable to install a water cooling ring 8 in order to cool the stretched tubular film F ₇ and fix the molecular orientation effect due to stretching, and furthermore, the stretched tubular film F ₇ is taken up by a high-speed nip roll to form the final stretched tubular film F _{8 .} Guide plate 9 is used to prevent wrinkles from occurring on the film when cutting.
It is desirable to install a

In the figure, 11 is the mandrel support fixed pipe, _a2 is the air pressure pipe between the cooling mandrel 3 and the stretching mandrel 7, _o1 and _o2 are the coolant pipes for the cooling mandrel, and _h1 and _h2 are the preheating pipes. Heat medium pipes for the mandrel 5, h ₃ and h ₄ are heat medium pipes for the drawing temperature control mandrel, and h ₅ and h ₆ are heat medium pipes for the drawing mandrel.

Next, the operation will be explained based on the above configuration.

As shown in the drawing, the molten thermoplastic resin that has been plasticized and kneaded in an extruder (not shown) is passed through an annular die 1.
The molten tubular film F ₁ is extruded from the annular orifice, and this molten tubular film F ₁ is
It is inflated while being melted and oriented by the air pressure from the air pressure pipe _A1 , cooled to near the crystallization start temperature by the air ring 2, and then rapidly cooled to below the crystallization temperature by the cooling mandrel 3 to form an unstretched tubular film _F2. becomes. At this stage the unstretched tubular film
F ₂ is due to the non-uniformity of the inner thickness due to the non-uniformity of the gap between the annular orifice of the annular die 1 and the non-uniformity of cooling due to the air flow of the air ring 2, and the non-uniformity of plasticization and kneading in the extruder. uniformity, non-uniformity of flow within the annular die 1 and discrepancies between the melt orientation state and the crystallization state that occur while the molten tubular film F ₁ is cooled between the annular orifice of the annular die 1 and the cooling mandrel 3. It has material non-uniformity based on uniformity.

The unstretched tubular film F ₂ is then continuously taken up and transferred by a low-speed grooved nip roll 4 driven by an independent drive device (not shown) to become an unstretched tubular film F ₃ , and further passed over a preheating mandrel 5 . The unstretched tubular film _F4 is heated to an appropriate stretching temperature.

The film F ₄ thus obtained has further material non-uniformity due to non-uniform heating.

At the initial stage of stretching, the portions that are easily stretched are locally stretched unevenly in the circumferential direction, and the locally stretched portions are in close contact with the mandrel 7, while the unstretched portions are on the circumferential side surface of the stretching mandrel 7. Indicates the phenomenon of floating from above. At the initial stage of stretching, when a portion that is in close contact with the circumferential side of the stretching mandrel 7 and a portion that is raised are formed, the central axis of the unstretched tubular film and the central axis of the stretching mandrel 7 become eccentric.
At this time, the portions of the unstretched tubular film having non-uniform wall thickness and material non-uniformity are usually
Since it exists continuously in the forming direction of the film,
Due to the eccentricity caused by the non-uniform stretching described above, when the unstretched tubular film F ₄ passes over the stretching temperature control mandrel 6, in the process, the portion that is relatively easily stretched comes into close contact with the stretching control mandrel 6, The film is selectively cooled until the stretchability equivalent to that of the portions that are difficult to stretch is obtained, resulting in a cooled unstretched tubular film _F5 .

Subsequently, when reaching the stretching mandrel 7, uniform stretching properties in the circumferential direction have been obtained by selective cooling of the cooled unstretched tubular film F ₅ , so that the cooling unstretched tubular film F ₅ is locally stretched. The initiation is prevented and the stretching is carried out uniformly on the circumferential side of the stretching mandrel 7 at least in the circumferential direction.

The obtained stretched tubular film F ₇ is stretched at least in the circumferential direction by a high-speed nip roll 10 driven by an independent drive device (not shown) in the longitudinal direction (in which the film is continuously formed). The stretched tubular film F 7 is pulled out while transmitting a force (in the flow direction) to the stretched tubular film F ₇ to become a final stretched tubular film F ₈ which is wound up by a known method (not shown).

Note that the stretching ratio in the longitudinal direction is determined by stretching mandrel 7.
This can be easily changed by changing the rotational speed of the high-speed side nip roll 10 located behind. That is, the final stretched tubular film _F8 may be a uniaxially stretched film in the circumferential direction that is stretched only in the circumferential direction, or a biaxially stretched film that is stretched in the circumferential direction and the longitudinal direction sequentially or simultaneously.

Since the method of the present invention has the above-mentioned structural action, it is possible to avoid localized areas in the initial stage of stretching caused by the ununiformity of wall thickness and material nonuniformity that thermoplastic resin unstretched tubular films inevitably have. This makes it possible to prevent uneven stretching, and to obtain a tubular film that is uniformly stretched in the circumferential direction.

[Brief explanation of the drawing]

The drawing is a sectional view of a manufacturing apparatus for implementing the present invention. 1... Annular die, 2... Air ring, 3... Cooling mandrel, 4... Nip roll with low speed side grooves, 5... Preheating mandrel, 6... Stretching temperature control mandrel, 7
...Stretching mandrel, 8...Water cooling ring, 9...Guide plate, 10...High-speed side nip roll, 11...Mandrel support fixed pipe, _a1 ...Air pressure piping between the annular die and cooling mandrel, _a2 ...Cooling mandrel and drawing Air pressure piping between the mandrel, a ₃ ... Air pressure piping between the drawing mandrel and the high-speed side nip roll, o ₁ , o ₂ ... Refrigerant piping for the cooling mandrel,
h ₁ , h ₂ …heat medium piping for preheating mandrel, h ₃ , h ₄ …
Heat medium piping for drawing temperature control mandrel, _h5 , _h6 ...
Heat medium piping for stretching mandrel, F ₁ ...melting tubular film, F ₂ ...unstretched tubular film, F ₃ ...transferring unstretched tubular film, F ₄ ...unstretched tubular film heated to appropriate stretching temperature, F ₅ ...cooling Unstretched tubular film, F ₆ ...Stretched tubular film, F ₇ ...Stretched tubular film, F ₈ ...Final stretched tubular film.

Claims (1)

[Claims] 1. A heat medium cavity that has a diameter smaller than the diameter of the unstretched tubular film and that enables temperature control by circulating a heat medium inside the thermoplastic resin unstretched tubular film heated to an appropriate stretching temperature. A cylindrical drawing temperature control mandrel with a cylindrical drawing temperature control mandrel, the end face of which is connected to the drawing temperature control mandrel has a smaller diameter than the other end face, and a heating medium that circulates inside to enable temperature control. Immediately before stretching the unstretched tubular film by inserting and arranging it in conjunction with a truncated conical stretching mandrel provided with a medium cavity, the unstretched tubular film is checked for nonuniformity in wall thickness and nonuniformity in material. The portions that are easily stretched due to this are cooled by selectively bringing them into contact with the circumferential side surface of the stretching temperature control mandrel, which is set at a temperature lower than the temperature of the unstretched tubular film that has been heated to an appropriate stretching temperature. . A method for producing a thermoplastic resin stretched tubular film, which comprises subsequently stretching at least in the circumferential direction along the stretching mandrel.