JPS6049911A - Formation of inflation film - Google Patents

Abstract

PURPOSE:To obtain a film having good accuracy of thickness with a high efficiency by a method in which a polyolefinic resin having a specific melt tension is used, and a mandrel of a specific structure is attached to die. CONSTITUTION:A polyoelfinic resin used includes polyethylene, polypropylene, ethylene-propylene copolymer, etc., having a melt tension of 3g or more. A cylindrical film 8 extruded from the annular slit 3 of a die 1 goes up along a mandrel 3 provided above the die 1, expanded by air confined in a cylinder, and drawn. Since the mandrel 3 has a diameter larger than the annular slit 2 but is less than 1.3 times that of the annular slit 2 and also the diameter is the same from upper part to lower part, the cylindrical film 8 can be smoothly moved along the mandrel 3.

Description

[Detailed description of the invention] The present invention relates to a method for forming a blown film.

More specifically, the present invention relates to a method for forming a blown film in a highly efficient and stable manner to form a thin synthetic resin film with well-balanced strength.

In recent years, packaging bags using synthetic resin films, etc.
Thinner walls are being considered in order to save on materials.

When forming a blown film, there is a method of increasing the draft rate and/or blow ratio in order to make the film thinner.

However, because inflation molding involves blowing air into the interior of a synthetic resin extruded into a cylindrical shape to cause it to expand, the inflated bubble is in an unstable state. If molding is attempted with a large blow ratio and/or a high blow ratio, the bubbles will swing or break, making stable molding very difficult.

In particular, when a synthetic resin with a high melt tension is used, the cylindrical film extruded from the annular slit of the die may cause so-called neck-in, where it becomes thinner to a diameter smaller than the diameter of the annular slit, and then expands. Therefore, the part that supports the bubble becomes substantially smaller, and there is a strong tendency for it to become unstable.

The present inventors have conducted various studies on methods for stably molding thermoplastic synthetic resins with high melt tension at high speed, and as a result, using polyolefin resins with specific melt tension,
The present inventors have discovered that by attaching a mandrel with a specific structure to a die and performing molding, a film that is extremely stable and has good thickness accuracy can be obtained with high efficiency, and the present invention has been completed.

That is, the gist of the present invention is to perform inflation molding of a polyolefin resin having a melt tension of 32 or higher, extruding the polyolefin resin into a cylindrical shape from a die having an annular slit, and extruding the extruded molten cylindrical film into a cylindrical film.
Inflation characterized by moving the annular slit along a mandrel having substantially the same diameter from the lower part to the upper part, which has a diameter less than 5 times the diameter of the annular slit, and then taking it off while being expanded. It consists in the film forming method.

An example of the method of the present invention will be described below with reference to the drawings.

FIG. 1 is an explanatory diagram showing an example of a blown film forming apparatus used in the method of the present invention.

In the figure / is the die, a is the annular slit, 3 is the mandrel, da is the mesh groove, and the left is the air introduction hole.

ri is an air outlet, g is a cylindrical film, ri is an air ring, and /θ is an on-off valve.

Examples of the polyolefin resin used in the method of the present invention include polyethylene, polypropylene, ethylene-propylene copolymer, polybutene-/, ethylene-vinyl acetate copolymer, linear low-density polyethylene, etc. Among these, those having a melt tension of 32 The above are used.

The melt tension is 76θC1 from the nozzle used in the JIS K t7AO melt index measurement method.
8. Extrude at 2.2 m/1 nin, /,! r2F
From the nozzle when pulled at a speed of IL /MAR, 2
This is the tension (f) measured at a distance of s.

When a polyolene-in resin having a melt tension of less than 3 f is used, molding is possible, but the strength of the obtained film, particularly tear strength and impact strength, cannot be obtained.

Among the above-mentioned polyolefin resins with a melt tension of 37 or more, high-density polyethylene has a density of 0.93 & f 7m or more, a melt index of /y/io min or less, easy to balance the vertical and horizontal orientation of the film, and has high ultimate strength. A film is obtained and can be suitably used in the molding method of the present invention.

The cylindrical film extruded from the annular slit of the die rises along the mandrel 3 provided above, is expanded by the air sealed inside the cylindrical body, and is taken off.

The diameter of the mandrel 3 is greater than or equal to the diameter of the annular slit and less than 7.3 times the diameter of the annular slit, and is substantially the same diameter from the bottom to the top.

Since the cylindrical film moves along the mandrel 3 having such a diameter, no matter how fast the cylindrical film is taken up, the diameter of the cylindrical film g becomes smaller than the diameter of the annular slit (J) and no bubbles are formed. Once it is stable, when the cylindrical film, once thinned, expands, the change in diameter will not be too large and it will not burst.

If the surface of the mandrel 3 is too smooth, the molten cylindrical film g will stick and cause the cylindrical film g to break, so it is preferable to form irregularities on the surface.

The unevenness provided on the surface of the mandrel 3 has a depth of 0.5mm over the entire side surface of the mandrel. Preferably, the unevenness is formed by a mesh-like groove in which grooves of A rren or more intersect with each other.

It is desirable that the width and pitch of the grooves be in the order of three.

Further, the direction in which the grooves are provided is /Ar~q! with respect to the direction in which the cylindrical film g is taken. It is preferable that it be inclined by about r'.

By forming such grooves, in addition to the above-mentioned effect of preventing the cylindrical film g from adhesion, an air layer is formed between the cylindrical film sheet and the mandrel 3, thereby reducing the pulling resistance.

It is preferable to use heat for the mandrel 3 to improve cooling efficiency, and specific examples thereof include aluminum, iron, copper, and the like. Among them, aluminum can be preferably used from the viewpoint of workability, thermal conductivity, corrosion resistance, etc.

In addition, if the mandrel 3 is placed too far away from the die/die, the cylindrical film g extruded from the die may become thin and then expand again, which may cause breakage. Therefore, it is desirable that the distance from the surface of the die to the position where substantially the same diameter portion of the mandrel 3 begins is at least /10 or less of the diameter of the annular slit.

When performing inflation molding according to the method of the present invention, molding is performed with a blow ratio of about 2 or more and a draft rate (take-up speed/extrusion speed) of about 2 or more to ensure molding stability.
This is desirable from the viewpoint of the strength balance of the product film.

The method of the present invention will be further explained below with reference to Examples, but the present invention is not limited to the following Examples unless the gist thereof is exceeded.

Examples 1 to 3 High-density polyethylene (manufactured by Mitsubishi Chemical Corporation, Patek'f
LII000F, melt index 20, 031/1
0 minutes, density: O, de54tf/屹fused Zhang Crab 9r, )(
Uses horns (Tick is a registered trademark of Mitsubishi Chemical Industries ■)! r
An inflation molding device equipped with a die having an annular slit of SO φ and a mandrel of S φ is used to form a cylinder at one temperature.

Inflation molding Lt under the conditions of temperature: tooc, die temperature, 200C.

Mesh-like unevenness is provided that intersects with each other in a row/ran.

The distance from the die surface to the position where the 81st minute of substantially the same diameter of the mandrel begins was set at Cθ. /(Pull expansion started from the die surface at 3!rOrtanO position (capacity 1).

The blow-up ratio is constant q and o, and the take-up speed is xom
1 minute (Example/), 30rn/min (Example 2), 60m
/min (Example 3), and 3011, θμ, and 10μ, respectively.
A film with a thickness of 100 ml was obtained, and the stability of the bubbles and the strength balance of the obtained film were evaluated.

Bubble stability was evaluated by visually observing the bubbles and grading them into three grades: good, somewhat unstable, and unstable.

Regarding the strength balance, tear the obtained film by hand in the pulling direction and check whether the cut goes vertically, horizontally, or diagonally.If the cut goes diagonally, it is considered to have good strength. Those that were uneven in the vertical or horizontal direction were considered defective.

The results are shown in Table 1.

Except for using the apparatus in which the mandrel of Comparative Example/, Example 2 was replaced with a 35-proof one (Comparative Example/), and the mandrel of Example λ was removed (Comparative Example, 2). Inflation molding was performed and evaluated in the same manner as in Example A.

The results are shown in Table 1.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an example of a blown film forming apparatus used in the method of the present invention. In the figure, / indicates gou, Y indicates an annular slit, 3 indicates a mandrel, G indicates a mesh groove, and za indicates a cylindrical film. Applicant Mitsubishi Chemical Industries, Ltd. Agent Patent Attorney Hase - (1 other person) 0 Figure 1

Claims (6)

[Claims] (1) When performing inflation molding of a polyolefin resin with a melt tension of 3 f or more, the polyolefin resin is extruded into a cylindrical shape from a die having an annular slit, and the extruded molten cylindrical film is shaped into an annular film with a diameter greater than or equal to the annular slit. A method for forming a blown film, characterized in that the film is moved along a mandrel having substantially the same diameter from a lower part to an upper part having a diameter of 3 times or less than the diameter of a slit, and then taken off while being expanded. (2) Polyolefin resin has a density o, q 3s t
8. The method according to claim 7, wherein the high-density polyethylene has a melt index of /cr/1 or more and a melt index of /1//θ min or less. (3) Mandrel has thermal conductivity / Okca1/77L
A method according to claim 1 or 1, characterized in that the method is made of a thermally good conductor with a thermal conductivity exceeding hr C. (4) A depth of 0.0 mm is applied to the entire side of the mandrel. A rra
The method according to any one of claims 1 to 3, characterized in that a mesh-like groove is provided in which n or more grooves intersect with each other. (5) Claims 1 to 5, characterized in that the distance from the die surface to the position where a portion of the mandrel of substantially the same diameter begins is at least //70 of the diameter of the annular slit. The method described in any of Section 7. (6) The method according to any one of claims 1 to 4, wherein the inflation molding is performed at a blow ratio of - or more and a draft rate of - or more.