JP7487582B2 - Blow molding die and method for producing blow molded article using same - Google Patents

Description

The present invention relates to a blow molding die and a method for producing a blow molded product using the same.

Blow molded products made of synthetic resins (hereafter referred to as resins) are produced as containers for daily necessities, medical products, etc., and the most widely used resins are polyethylene, polypropylene, and polyethylene terephthalate.

Blow molding is a molding method in which a thermoplastic resin (hereafter referred to as the parison) is extruded from a double cylindrical gap (hereafter referred to as the annular gap) formed between an outer tube and a core, and then sandwiched in a mold and air is blown into it to obtain a molded body.

In blow molding, surface roughness of the molded body called sharkskin or melt fracture occurs during molding, which can cause problems in the appearance of the product. Among surface roughness, sharkskin is known as a type of surface roughness in which the surface is periodically roughened and is known to occur easily when a high-viscosity resin is used or when extrusion is performed at high speed. There are various theories about the cause of sharkskin, and the mechanism of its occurrence has not yet been identified. However, it is known that sharkskin occurs at or near the discharge part of the thermoplastic resin, and is likely to occur when the lubricity of the thermoplastic resin is poor, that it is more likely to occur the higher the shear stress that the thermoplastic resin receives at the discharge part of the thermoplastic resin, and that it is more likely to occur in resins with a linear molecular structure with low melt elasticity, such as high-density polyethylene and linear low-density polyethylene, because they cannot withstand the sudden elongation stress received at the discharge part and melt and break locally.

Increasing the molecular weight is effective in improving the mechanical strength of containers made from high-density polyethylene or linear low-density polyethylene, but high-molecular-weight polyethylene has a high viscosity in the molten state, which makes it more likely for sharkskin to occur during molding for the reasons mentioned above. In addition, in order to reduce production costs, it is desirable to improve productivity through high-speed molding, but as the extrusion speed increases, sharkskin becomes more likely to occur during molding.

Under these circumstances, a method is known in which the outer cylinder and core for blow molding are coated with plating or other coatings to improve the slipperiness of the resin in the annular gap and suppress sharkskin. A method has also been proposed in which the temperature of the mold that sandwiches the parison is set high in order to eliminate sharkskin that occurs in the parison (see, for example, Patent Document 1). Furthermore, a method has been proposed in which the molecular weight and molecular weight distribution of the resin are set within a specific range, thereby suppressing sharkskin during molding while maintaining mechanical strength (see, for example, Patent Document 2). A method has also been proposed in which a lubricant is added to the resin to improve the slipperiness of the resin composition (see, for example, Patent Document 3).

JP 2004-269864 A JP 2009-138122 A Japanese Patent Application Laid-Open No. 6-328549

However, in the known coating of the outer tube and core, durable metal plating has poor sharkskin suppression effect, and Teflon (registered trademark) coating, which has excellent sharkskin suppression effect, has poor durability, so currently no method has been found that achieves both durability and sharkskin suppression effect at a high level. In addition, the method proposed in the above Patent Document 1 is unsuitable as a method for improving productivity because heating is performed in the mold process where the cooling effect should be increased to improve productivity. In addition, the methods proposed in the above Patent Documents 2 and 3 result in resin properties (e.g., impact resistance, cleanliness, etc.) that must be reduced in order to suppress sharkskin, resulting in problems such as limited applications.

As a result of intensive research to solve the above problems, the inventors have found that a blow molded article free from sharkskin can be easily obtained by blow molding using a die having a shape that satisfies certain requirements, and have completed the present invention. That is, the present invention is the following aspects [1] to [5] of the present invention.
[1] A blow molding die having an outer tube attached to a die head and a core attached to the lower end of a mandrel, an annular flow path through which a thermoplastic resin passes between the outer tube and the core, and an annular gap through which the thermoplastic resin is discharged onto a die lip surface, wherein at least a portion of the gap in the annular flow path is 0.5 to 1.5 mm, the length of the gap in the flow path direction is 10 to 50 mm, the annular gap is equal to or wider than the gap in the annular flow path, and the diameter of the annular gap is 10 to 100 mm.
[2] The blow molding die according to [1], wherein the core tip is tapered.
[3] The blow molding die according to [1], wherein the core tip has an inverted tapered shape.
[4] A method for producing a blow molded article using a blow molding machine having the blow molding die according to any one of [1] to [3].
[5] The method for producing a blow-molded article according to [4], characterized in that the parison has a drooping speed of 5 to 50 mm/sec and a swell of 1.3 to 2.5.

The method for producing blow molded articles using the blow molding die of the present invention can suppress roughness of the surface of the blow molded article, and the resulting blow molded article can be suitably used for food containers, medical containers, cosmetic containers, etc.

The blow molding die according to one embodiment of the present invention has an outer cylinder attached to a die head and a core attached to the lower end of a mandrel, and has an annular gap between the outer cylinder and the core, through which the thermoplastic resin passes and through which the thermoplastic resin is discharged onto the die lip surface. At least a part of the gap of the annular flow path through which the thermoplastic resin passes is 0.5 to 1.5 mm, and the length of the gap in the flow path direction is 10 to 50 mm, preferably 15 to 25 mm. If the length of the annular flow path in the flow path direction is less than 10 mm, the deformation stress of the thermoplastic resin received inside the molding machine is not fully relaxed while being discharged from the annular gap on the die lip surface, and the parison swells greatly. If it exceeds 50 mm, the pressure loss due to the viscosity of the thermoplastic resin becomes very large, and the resin pressure in the flow path at the top of the die head may exceed the upper limit of the machine. If at least a part of the gap of the annular flow path is less than 0.5 mm, the pressure loss due to the viscosity of the thermoplastic resin becomes very large, and the resin pressure in the flow path at the top of the die head may exceed the upper limit of the machine. Furthermore, if the diameter exceeds 1.5 mm, the deformation stress of the thermoplastic resin received inside the molding machine is not fully relaxed while it is discharged from the annular gap on the die lip surface, and the parison swells significantly. Furthermore, the annular gap through which the thermoplastic resin is discharged is equal to or wider than the gap of the annular flow path through which the thermoplastic resin passes, and has a diameter of 10 to 100 mm. If the annular gap through which the thermoplastic resin is discharged is smaller than the gap of the annular flow path through which the thermoplastic resin passes, the parison is strongly affected by the elongation deformation that occurs when it swells, and sharkskin due to melt fracture is likely to occur. If the diameter of the annular gap through which the thermoplastic resin is discharged is less than 10 mm, the pressure loss due to the viscosity of the thermoplastic resin becomes very large, and there is a risk that the resin pressure in the flow path above the die head will exceed the machine upper limit. If the diameter exceeds 100 mm, only blow molded bodies of a size larger than the practical upper limit can be obtained for the use of the blow molded body produced using the die.

In one embodiment of the blow molding die of the present invention, the tip of the core used in combination is preferably tapered or inverted tapered.

The reason sharkskin is suppressed by using a die that forms a specific annular flow passage is as follows:

A typical blow molding machine is equipped with a parison control mechanism that makes it possible to change the parison thickness during molding. Specifically, the tip of the die lip surface (core tip) as shown in Figure 3 is a tapered (divergence type) or inversely tapered (convergence type) core that is mechanically moved up and down relative to a fixed outer cylinder, thereby varying the annular gap on the die lip surface and changing the parison thickness. This is called a parison control mechanism. In this way, if the parison thickness is thick, a thick-walled molded product is obtained, and if the parison thickness is thin, a thin-walled molded product is obtained.

When the parison is extruded from the annular gap into the free space, the thickness of the parison becomes thicker than the width of the annular gap. This phenomenon occurs because the thermoplastic resin is a viscoelastic fluid, and is generally called swell. For this reason, the parison thickness is also affected by swell. Basically, swell strongly depends on the primary structure of the thermoplastic resin, but the size of the swell when the same resin is used is strongly affected by the shape of the annular flow path formed by combining the outer tube and the core. Specifically, if the deformation stress of the thermoplastic resin received inside the molding machine is relaxed while it is discharged from the annular gap on the die lip surface, the swell becomes small. In other words, if the discharge speed is very slow or the annular flow path from the annular gap on the die lip surface to the discharge is narrow and long, the deformation stress received by the thermoplastic resin is relaxed, and the swell becomes small. Here, when trying to obtain a molded body of the same thickness using the same resin, if a die that reduces the swell is used, it becomes necessary to adjust the parison thickness by widening the gap compared to when a die that increases the swell is used. In other words, by widening the annular gap on the die lip surface, the shear stress on the thermoplastic resin at the discharge section is reduced. As mentioned above, it is known that sharkskin occurs when the thermoplastic resin is subjected to high shear stress at the discharge section, so it is self-evident that sharkskin can be suppressed by using a die designed to reduce swell.

The method for producing a blow molded body, which is one aspect of the present invention, is characterized by using a blow molding machine having the blow molding die described above. As molding conditions, it is preferable to produce a blow molded body under conditions where the parison drooping speed is 5 to 50 mm/sec and the swell is 1.3 to 2.5. When a parison is blow molded such that the parison swell is in the range of 1.3 to 2.5 when the parison is extruded at a parison drooping speed of 5 to 50 mm/sec, it is possible to widen the annular gap on the die lip surface and mold the parison with a swell exceeding 2.5, which is preferable because it is possible to obtain a blow molded body with a smooth surface without sharkskin.

It is also preferable to manufacture the blow molded article under conditions where the shear stress of the thermoplastic resin discharged from the annular gap on the die lip surface is 80 kPa to 220 kPa. It is preferable to extrude the parison and blow mold it when the shear stress of the thermoplastic resin discharged from the annular gap on the die lip surface is in the range of 80 kPa to 220 kPa, because this produces a blow molded article with a smooth surface free of sharkskin.

The resin used in the manufacturing method of blow molded articles, which is one aspect of the present invention, is not particularly limited as long as it is a thermoplastic resin, but preferred are polyethylene-based resins such as high-density polyethylene, linear low-density polyethylene, and high-pressure low-density polyethylene, polypropylene-based resins, crystalline thermoplastic resins such as polyamide, polyester, and polyacetal, and non-crystalline thermoplastic resins such as polystyrene, acrylonitrile-butadiene-styrene copolymer (ABS), polycarbonate, polyphenylene oxide, and polyacrylate.

There are no particular limitations on the blow molded article obtained by the above manufacturing method, so long as it is used as a container, but it is preferably a container with a storage section for storing liquid, for example, the storage section being made of a polyethylene-based resin. The average thickness of the container body is practically 0.2 to 1.0 mm, and preferably 0.3 to 0.9 mm. However, due to the nature of blow molding, thin and thick parts are formed, and thickness deviations of about 0.1 to 0.5 mm may occur at the corners of the container bottom compared to the average thickness of the container body.

Applications of the blow molded articles obtained by the above manufacturing method include food containers, medical containers, food containers, cosmetic containers, etc. Examples of food containers include various beverage containers, concentrated beverage containers, seasoning containers, side dish containers, dressing containers, mayonnaise/ketchup containers, various retort food containers, baby bottles, etc. Examples of medical containers include infusion preparation containers, ampoule preparation containers, kit preparation containers, eye drop containers, etc. Examples of food containers include various beverage containers, concentrated beverage containers, seasoning containers, side dish containers, dressing containers, mayonnaise/ketchup containers, various retort food containers, baby bottles, etc. Examples of cosmetic containers include containers for hair styling products, hair products, perfumes, hair dyes, eye shadows, nail polish, lotions, creams, milky lotions, skin lotions, perm solutions, etc.

The present invention will be described in more detail below by showing examples, but the present invention is not limited to these examples.
～Resin～
Polyethylene FY31 (MFR=1.0 g/10 min, density=930 kg/m ³ ) manufactured by Tosoh Corporation was used.
~Blow molding evaluation~
Using a direct-type blow molding machine (manufactured by Tahara Co., Ltd.) equipped with a 500 ml square bottle mold, a 40 mmφ extrusion screw, and an outer cylinder and core, the above resin was extruded from the annular gap on the die lip surface at a molding temperature of 210°C and an extrusion rate of 5 to 10 kg/h. The core used was a fixed divergence type with a die lip surface of 18 mmφ. The outer cylinder used had a die lip surface of 20 mmφ, and several types of outer cylinders were used that, when combined with the above core, changed the length and gap in the flow path direction of the annular flow path through which the thermoplastic resin passed. The extruded parison was sandwiched between molds to mold a 500 ml square bottle with an average thickness of the container body of 0.3 mm. In addition, the swell of the parison changes depending on the die and discharge amount used, and the thickness of the parison also changes. Also, the weight of the parison causes thickness deviation in the parison's drooping direction (vertical direction) during drooping. Therefore, in order to make the thickness of the molded bottle 0.3 mm, the molding machine's parison control mechanism was used to change the annular gap (hereinafter, die clearance) through which the thermoplastic resin on the die lip surface is discharged, as in general blow molding. The swell and drooping speed of the parison during blow molding, and the surface roughness of the bottle body obtained by molding were evaluated. The evaluation methods used in the examples and comparative examples are shown below.

Since a divergence core with a die lip surface of 18 mmΦ and an outer cylinder with a die lip surface of 20 mmΦ are used, the diameter of the annular gap through which the thermoplastic resin is extruded onto the die lip surface is 20 mm, and the gap of the annular gap is a minimum of 0.5 mm, and can be made arbitrarily wide by the parison control mechanism.
～Swell evaluation～
In the above-mentioned blow molding evaluation, when the parison was extruded without using the parison control mechanism of the molding machine and with the die clearance fixed at 1.5 mm, the parison hanging down 50 mm from the die lip surface was cut off from the die lip surface and its weight was measured. Based on the obtained parison weight, the swell was calculated using the following formula.

Swell (-) = parison volume / ideal volume Here, the parison volume was calculated by dividing the obtained parison weight by the density of the molten parison (750 kg/m ³ ). The ideal volume was calculated as the volume of a double cylinder with an outer diameter of 20 mm, an inner diameter of 18 mm, and a height of 50 mm.
~Dropping speed~,
In the above-mentioned blow molding evaluation, when molding a 500 ml square bottle with an average container body thickness of 0.3 mm, the time it took for the parison to sag 50 mm from the die lip surface was measured, and the sagging speed was calculated using the following formula.

Drooping speed (mm/sec) = 50 (mm) / time (sec) until the parison droops 50 mm from the die lip surface
-Evaluation of surface roughness-
In the above-mentioned blow molding evaluation, a 10 mm square test piece was cut out from the container body of a 500 ml square bottle molded with an average thickness of 0.3 mm. The surface of the test piece was image-analyzed using a digital microscope (manufactured by Keyence Corporation) to measure the line roughness (Rz). Note that Rz is expressed as the difference between the highest and lowest points of the measured line, so a smaller value indicates a smaller surface roughness and better smoothness.
~Example 1~
In the above-mentioned blow molding evaluation, a die having an annular flow passage with a length in the flow passage direction of 20 mm and a gap of 1.5 mm was used, and the extrusion speed was set to 5.0 kg/h to evaluate the swell and droop speed. The results are shown in Table 1.
~Examples 2 and 3~
The same procedure as in Example 1 was carried out except that the extrusion speed was changed as shown in Table 1. The results are shown in Table 1.

~Example 4~
In the above-mentioned blow molding evaluation, a die with an annular flow path having a length of 20 mm in the flow path direction and a gap of 1.5 mm was used to mold a 500 ml square bottle with an average thickness of the container body of 0.25 mm at an extrusion rate of 8.0 kg/h. The surface roughness of the bottle body obtained by molding was evaluated.
~Examples 5 and 6~
The same procedure as in Example 4 was carried out except that the average thickness of the container body was changed as shown in Table 2.
The results are shown in Table 2.

Comparative Example 1
The same procedure as in Example 1 was carried out except that the die was changed to one in which the length of the annular flow path in the flow path direction was 20 mm and the gap was 2.5 mm. The results are shown in Table 3.
Comparative Examples 2 and 3
The same procedure as in Comparative Example 1 was carried out except that the extrusion speed was changed as shown in Table 3. The results are shown in Table 3.
~Comparative Example 4~
The same procedure as in Example 1 was carried out except that the die was changed to one in which the length of the annular flow path in the flow path direction was 20 mm and the gap was 3.5 mm. The results are shown in Table 3.
Comparative Examples 5 and 6
The same procedure as in Comparative Example 4 was carried out except that the extrusion speed was changed as shown in Table 3. The results are shown in Table 3.
~Comparative Example 7~
The same procedure as in Example 1 was carried out except that the die was changed to one in which the length of the annular flow path in the flow path direction was 5 mm and the gap was 3.5 mm. The results are shown in Table 3.

~Comparative Example 8~
The same procedure as in Example 4 was carried out except that the die was changed to one in which the length of the annular flow path in the flow path direction was 20 mm and the gap was 2.5 mm. The results are shown in Table 4.
Comparative Examples 9 and 10
The same procedure as in Comparative Example 8 was carried out except that the average thickness of the container body was changed as shown in Table 4. The results are shown in Table 4.
Comparative Example 11
The same procedure as in Example 4 was carried out except that the die was changed to one in which the length of the annular flow path in the flow path direction was 20 mm and the gap was 3.5 mm. The results are shown in Table 4.
Comparative Examples 12 and 13
The same procedure as in Comparative Example 11 was carried out except that the average thickness of the container body was changed as shown in Table 4. The results are shown in Table 4.
~Comparative Example 14~
The same procedure as in Example 4 was carried out except that the die was changed to one in which the length of the annular flow path in the flow path direction was 5 mm and the gap was 3.5 mm. The results are shown in Table 4.

FIG. 1 is a diagram showing a blow molding machine. FIG. 2 shows a blow molding die. FIG. 1 shows a blow molding die with a tapered core. FIG. 13 illustrates the widening of the annular gap on the die lip face due to the lowering of the tapered core. FIG. 1 shows a blow molding die using an inverted tapered core. FIG. 13 shows how the annular gap on the die lip surface narrows as the inverted tapered core lowers.

Reference Signs List 1 Thermoplastic resin 2 Extruder 3 Die head 4 Mandrel 5 Core 6 Outer cylinder 7 Die 8 Annular flow passage through which thermoplastic resin passes 9 Annular gap through which thermoplastic resin is discharged 10 Die lip surface 11 Gap in annular flow passage 12 Length of annular flow passage in flow passage direction 13 Tapered outer cylinder 14 Tapered core (divergence type)
15 Die clearance 16 Reverse tapered outer cylinder 17 Reverse tapered core (convergence type)

Claims (3)

The die head has an outer cylinder and a core attached to the lower end of the mandrel.
Between the outer cylinder and the core is an annular flow passage through which a thermoplastic resin passes, and a thermoplastic resin is placed on a die lip surface.
A blow molding die having an annular gap through which resin is discharged,
Some of the gaps are 0.5 to 1.5 mm, and the length of the gaps in the flow path direction is 10 to 50 mm.
The annular gap is equal to or wider than the gap of the annular flow passage, and its diameter is 10 to 100 mm
The present invention relates to a method for producing a blow-molded article, the method being characterized in that a parison drooping speed is 5 to 50 mm/sec and a swell is 1.3 to 2.5 , using a blow molding machine having a blow molding die . 2. The method for producing a blow molded article according to claim 1, wherein a tip of the core of the blow molding die is tapered. 2. The method for producing a blow molded article according to claim 1, wherein a tip of the core of the blow molding die has an inverse tapered shape.