JPH07237261A - Injection stretch blow molding method of polyethylene - Google Patents

Abstract

PURPOSE:To enable mold release at a surface temperature, which is capable of performing both of mold release of a preform and stretch blow molding after that and provide a fresh method enabling the stretch blow molding within a suitable temperature range as polyethylene, by preceding separation of a core mold and the preform from each other by making use of pressure of gas. CONSTITUTION:Molten polyethylene is molded into a necessary preform by filling into an injection mold by injection, after a mouth part of the preform is held by a lip mold and released from a cavity mold and core mold, it is transferred to a blow mold and stretch blow molding is adopted to a thin hollow molded product. Prior to release of the preform from an injection mold, gas is injected into a boundary between the core mold and preform from an injection mold and the inside of the preform is separated from the core mold. the release of the mold is performed within a temperature range wherein internal cooling of the preform is incomplete, the preform is under a high temperature and surface temperature of the preform directly after mold release becomes 80-90 deg.C at the normal temperature. Stretch blow molding is performed within a time when the surface temperature after mold release is during rising by inner heat of the preform and before arriving at 120 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for molding polyethylene into a hollow molded product such as a thin container by injection stretch blow molding.

[0002]

2. Description of the Related Art In a molding method in which an injection mold is injection-filled with a molten resin to form a preform, and the preform is stretch blow-molded into a hollow molded article such as a container in a blow mold,
It is said that most of thermoplastic resins can be molded into a thin hollow molded product. However, at present, blow molding is employed for polyethylene, which is in high demand as a hollow molded product, because it is limited to polyethylene terephthalate, polypropylene, polycarbonate, vinyl chloride and the like.

[0003]

The molding method in which the injection molding of the preform is followed by the stretch blow molding into the hollow molded article is carried out by the injection mold before the injection-molded preform is hot and solidified. From the mold, the temperature of the hot preform is adjusted, and then the blow molding is performed by stretch blow molding into a hollow molded product, and the temperature control of the preform is disclosed in JP-A-4-214322. There is a method in which the stretch blow molding is carried out immediately by omitting.

In any of these injection stretch blow molding, the preform is molded by an injection mold composed of a cavity mold forming the outside of the preform and a core mold molding the inside of the preform, and the mouth of the preform. Using a lip mold that forms the outside of the cavity, through the cavity mold, the opening of the cavity mold and the closed lip mold, into the cavity between the core mold inserted into the cavity mold,
A molten resin is injected and filled from the bottom of the cavity mold.

In releasing the injection-molded preform from the injection mold, both the core mold and the lip mold are moved upward,
Alternatively, the core mold is moved upward and the cavity mold is moved downward, and the preform after release is transferred to the blow mold while the lip mold holds the periphery of the mouth.

The reason why the preform is extracted from the core mold and released from the mold in this manner is that it is extremely difficult to stretch the preform in the axial direction by the stretching rod when the preform is tightly held and closely adhered to the core mold by shrinkage due to cooling. Due to the difficulty, the lip mold is also used as the transfer member, and the preform is extracted from both the cavity mold and the core mold and transferred to the blow mold.

[0007] Usually, in releasing an injection-molded product from a mold, it is more difficult to extract the injection-molded product from the core mold side than to extract it from the cavity mold side. This is because the cavity side shrinks due to cooling of the injection-molded product, causing the outer surface of the molded product to separate from the cavity surface, while the core mold side conversely shrinks so that the inner part of the molded product adheres to the core mold. Because it will be.

The strength of a flexible hot preform capable of stretch blow molding is different from that of a normal injection molded product which has rigidity as a whole by cooling and solidification, and the morphology of the preform depends on the skin layers on the inner and outer surfaces. The strength required to cut the edge between the skin layer inside the preform and the surface of the core mold, even when the core mold is pulled out by holding the periphery of the cooled and solidified mouth with the lip mold because it has only enough strength to be retained. Without it, the preform is pulled out of the cavity mold while being in close contact with the core mold, and is squeezed by the lip mold in the fixed state, so that the lantern is deformed as if it is contracted and loses the form of the preform.

Therefore, in injection stretch blow molding, the preform is cooled to a temperature at which the stretch blow molding is possible and the skin layer on the surface of the preform has a hardness enough to withstand the extraction force of the core mold. . This cooling temperature varies depending on the wall thickness and design of the preform, but in the case of a wide-mouthed container preform with a not so high blow-up ratio, the draft of the cavity type and core type can be set to a large degree, so the shrinkage The adhesion force on the inside of the preform can be relaxed by the draft, and mold release at high temperature is also possible, so that the mold release is easier than the narrow-mouth container because of synergistic effects such as shrinkage due to cooling.

However, in the case of a preform for a narrow-necked container such as a bottle, which is required to have a small diameter and a long length and a large blow-up ratio, there is a limitation in the draft of the cavity type and the core type, and the limitation is the length. The longer it gets, the more severe it becomes. For this reason, it is necessary to cool the preform to a compatible temperature that enables both mold release and subsequent stretch blow molding to perform mold release from the injection mold. Regarding this compatible temperature, in the case of polyethylene terephthalate, it is 60 ° to 70 at room temperature.
C., 90 to 100.degree. C. in the case of polypropylene, and within that temperature range, both mold release and stretch blow molding are established.

However, in the case of a polyethylene preform which has a higher thermal conductivity than polyethylene terephthalate and polypropylene and a high molding shrinkage, when it is cooled to a temperature at which it can be withdrawn, the skin layer becomes thicker than necessary. The internal high temperature region becomes narrow, and even if the preform is immediately transferred to a blow mold and stretch blow-molded, it does not swell due to air pressure. Further, at the temperature at which stretch blow molding is presumed possible, the preform remains in close contact with the core mold, and when the core mold is extracted in that state, it becomes significantly deformed.

[0012] Therefore, it is more difficult to set the compatible temperature for both the mold release and the subsequent stretch blow molding with the polyethylene preform, as compared with the case of polyethylene terephthalate, and the mold releasing means that has been adopted so far is used. It was extremely difficult to perform stretch blow molding as it is, even in the case of a wide-mouthed container.

It is also conceivable that the preform after release is returned to a moldable temperature by reheating and then stretch blow molding is carried out, but in this case, it takes experience and time, and temperature unevenness is likely to occur. Therefore, there is a technical difficulty in injection stretch blow molding of polyethylene even when temperature control is adopted.

Regarding polyethylene, besides the above-mentioned mold release, there is a difficulty in the temperature at which the preform is stretch blow-molded. The resin temperature during molding of polyethylene in blow molding is 175 ° to 200 ° C. However, in injection stretch blow molding, such a resin temperature is the molding temperature of the preform, and the temperature possessed by the preform cooled to be self-sustaining is extremely lower than the resin temperature.

In the case of stretch blow-molding a preform that has been demolded at high temperature into a container such as a bottle before the surface temperature reaches the peak temperature due to the internal heat possessed by itself, the time until the polyethylene reaches the peak temperature is Since it is earlier than in the case of polyethylene terephthalate, stretch blow molding is more difficult than that of polyethylene terephthalate, and it is difficult to obtain a molded product having a good wall thickness distribution near the peak temperature. The difficulty of this stretch blow molding is that it has a higher thermal conductivity than polyethylene terephthalate or polypropylene, so the heating of the skin layer on the front side due to the internal heat after mold release is relatively fast, and the internal high temperature that contributes to stretch blow molding is high. It is presumed that the range occupied by the part is narrowed early and the internal heat quantity required for stretch blow molding tends to be insufficient. Therefore, even in the case of a wide-mouthed container that is easier to release than a narrow-mouthed container, as for the stretch-blowing temperature, if stretch-blowing is not performed within the limited time (within the temperature range), a good molded product will be obtained. It means that you cannot get it.

In addition, the arrival of the peak temperature varies to some extent depending on the wall thickness of the preform, the design, the molding conditions, etc., and if stretch blow molding is not performed accordingly, a good product cannot be obtained. It requires more severe molding operation than polyethylene terephthalate.

The present invention was conceived in order to solve the problems in the above-mentioned injection stretch blow molding of polyethylene, and its purpose is to precede the cutting of the edge between the core mold and the preform by utilizing the pressure of gas. Provides a new method that enables mold release at a surface temperature that enables both mold release of the preform and subsequent expansion blow molding, and enables stretch blow molding in a temperature range suitable for polyethylene. Especially.

[0018]

According to the present invention for the above object, a molten polyethylene is injection-filled into an injection mold to be molded into a required preform, and the preform is held in a cavity by a lip mold. After releasing from the core mold and the core mold, transfer to the blow mold and adopt stretch blow molding to the thin hollow molding, before releasing the preform from the injection mold, the core mold and preform A gas is press-injected into the boundary of the preform to cut the inside of the preform from the core mold, and then the internal cooling of the preform is incomplete and the temperature is high, and the surface temperature of the preform immediately after demolding is 8 at room temperature.
Mold release is performed in a temperature range of 0 ° to 90 ° C., and the stretch blow molding is performed within a time period before the surface temperature after mold release reaches 120 ° C. while rising due to internal heat of the preform. It is in.

The temperature of the molten polyethylene at the time of molding the above preform needs to be 200 ° C. or higher at the front temperature of the injection cylinder. This molten polyethylene is injected and filled into the cavity of an injection mold formed by a cavity mold whose temperature is set to about 90 ° to 105 ° C. and a core mold whose temperature is set to about 80 ° C. It is molded into a reform.

When the injection filling of the molten polyethylene is completed, the injection pressure is switched to the secondary pressure lower than the primary pressure required for the filling to shift to the holding pressure and then to the cooling. Air is the most preferable as the edge-cutting gas from the viewpoint of economy or handling, but an inert gas such as nitrogen gas may be used in some cases. Although the press-fitting does not affect the form of the preform even if it is started immediately after the completion of injection filling, it is most preferable to start the press-fitting immediately after the completion of the holding pressure. When the press-fitting is started immediately before the completion, the filling pressure is generated due to the increase of the internal pressure due to the press-fitting gas, and the predetermined amount of the molten resin cannot be filled, and the form of the preform is impaired.

The above-mentioned press-fitting of gas is performed by blowing gas from the root or tip of the core mold to the boundary between the core mold and the preform. The press-fitting position can be selected depending on the length and thickness of the preform, but it is preferable to press-fit from the tip of a long preform. Also, since the purpose of gas injection is to cut the edge between the core mold and the inside of the preform, the injected gas is kept at the boundary as it is for the time required, and the inside of the preform is isolated from the surface of the core mold and preformed to a temperature at which it can be released. However, the press-in (blowing) time requires at least 1 second, and the blowing pressure is 6
~15kg / cm ² range, and most preferably it is 9 kg / cm ² before and after.

The mold release of the preform is carried out in the above temperature range, but the stretch blow molding can be carried out in the temperature range in which the surface temperature of the preform is increased by 20 to 30 ° C. as compared with the mold release. If the surface temperature is 100 ° C. or less, the temperature is too low to achieve the entire extension due to the pressure of the gas, making the molding extremely difficult. Further, when the temperature rise is 30 ° C. or higher, the temperature is close to the peak temperature, and the temperature rise becomes slow, and the temperature condition for the preform becomes an environment in which the crystallization easily occurs. Therefore, when the surface temperature after releasing exceeds 120 ° C., it becomes difficult to perform stretch blow molding of a molded product having a good wall thickness distribution.

From the above, the most preferable surface temperature at the time of stretch blow molding is 2 times higher than the surface temperature at the time of mold release.
The temperature at which the temperature rises by about 5 ° C. is good, and the time is in the range of 4 to 7 seconds after releasing the mold. Further, the temperature at which the surface temperature at the time of mold release reaches a peak due to internal heat causes a certain level difference due to the wall thickness, but does not cause a large difference in the time required to reach the peak temperature. Therefore, when the internal heat quantity is adjusted by changing the wall thickness distribution so that the preform expands according to the blow ratio, even when a container with a good wall thickness distribution is obtained, when the wall thickness difference is extreme Except for the above, it is possible to perform stretch blow molding in a generally preferable temperature range within a predetermined elapsed time after release.

It is preferable that the blowing pressure of the air during the stretch blow molding is divided into a primary pressure and a secondary pressure in stages, and the secondary pressure is set to a pressure twice or more the primary pressure. The stretching can be performed up to about 2 times in the length direction and about 3 times in the width direction.

[0025]

[Operation] In the above method, when the polyethylene preform is released from the mold, gas is forced into the boundary between the core mold and the inside of the preform, and cooling is performed with the edge cut inside the preform. The gas existing at the boundary with the inside of the preform suppresses the cooling inside the preform, and the inner skin layer is formed in a thinner layer than when it adheres tightly. There is little, and adhesion is also eased.

On the other hand, on the front side of the preform, the surface of the preform is pressed against the cavity surface by the pressure-injected gas and is easily cooled, and the separation due to shrinkage due to cooling is prevented, so that the skin layer maintains the form of the preform. The hardness becomes quicker, and the mold can be released at the surface temperature at which stretch blow molding can be performed later. In addition, because of the rapid cooling, crystallization is extremely small.

Further, in the process in which the surface temperature is raised by the internal heat, crystallization is suppressed by heating from the inside and the growth is slow, so that there is no hindrance to stretching due to crystallization, and even polyethylene can be stretch blow-molded. It can be smoothly performed, and a molded product such as a container having an excellent wall thickness distribution can be obtained.

[0028]

[Example] NO1 material Polyethylene Hi-Zex 5300B Mitsui Petrochemical Co., Ltd. Molded product Milk bottle (500 cc) Dimensions Overall height 165 mm, mouth inner diameter 32 mm, neck length 147.5 mm, trunk outer diameter 73 mm, Body thickness 0.5
mm, weight 32g, preform dimension total height 137.5mm, mouth inner diameter 32mm, neck length 120mm, body thickness 3mm, upper body outer diameter 3
4.68mm, outer diameter of the lower end of the trunk 31.62mm, draft 0.766 °,

Preform molding conditions Injection cylinder temperature Nozzle 175 ° C, front 215 ° C, middle 215 ° C, rear 185 ° C, injection mold temperature (set value) Cavity mold upper 10 ° C, cavity 95 ° C,
Lower part 10 ℃, Core type 80 ℃, Injection pressure (holding pressure) 42kg / cm ² , Filling pressure holding time 6.5 seconds, Cooling time 1.8 seconds, Pressurization (blowing) gas Air, pressurization (blowing) Pressure 9kg / cm ² , press-in timing Immediately after holding pressure, press-in (blowing) time 1.8 seconds, mold release temperature 80 ° ~ 90 ° C (preform surface temperature),

Stretch blow molding conditions Mold temperature (set temperature) 60 ° C., stretch blow temperature 105 ° to 115 ° C. (preform surface temperature), blow pressure (stretch) Primary pressure 4 to 5 kg / cm ² , secondary pressure 12kg / cm ² , Blowing time 6 seconds, Stretching ratio Vertical (axial direction) 1.2 times, Horizontal (radial direction) 2.2 times,

As a result, a milk bottle of polyethylene was obtained which was milk white and had a uniform thickness distribution without uneven thickness in the stretched portion. No damage was observed when the contents were filled and dropped from a height of 2 m.

NO2 material Resin Polyethylene Hi-Zex 5100B Mitsui Petrochemical Co., Ltd. Molded product Narrow mouth round straight bottle (120cc) Dimensions 126.7mm in height, 11 under neck length
4.7 mm, mouth inner diameter 17.14 mm, body outer diameter 45.5 mm, body wall thickness 0.5 mm, weight 15.4 g, preform dimension overall height 107.0 mm, mouth inner diameter 1
7.14mm, under neck length 95mm, body thickness 3.
2mm, outer diameter of the upper part of the body 22.03mm, outer diameter of the lower end of the body 1
8.71mm, draft 1.0 °,

Preform molding conditions Injection cylinder temperature Nozzle 175 ° C., front 210 ° C., middle 210 ° C., rear 195 ° C., injection mold temperature (set value) Cavity mold upper 10 ° C., cavity 102
℃, lower 10 ℃, core type 80 ℃, injection pressure (holding pressure) 40kg / cm ² , filling holding time 5.45 seconds, cooling time 4.25 seconds, pressurization (blowing) gas air, pressurization ( Blow pressure 9kg / cm ² , press fit timing Immediately after holding pressure is complete, press fit (blown) time 4.25 seconds, release temperature 80 ° to 90 ° C (preform surface temperature),

Stretch blow molding conditions Mold temperature (set temperature) 60 ° C. Stretch blow temperature 105 ° to 115 ° C. (preform surface temperature), blow pressure (stretch) Primary pressure 5 kg / cm ² , secondary pressure 12
kg / cm ² , blowing time 6 seconds, draw ratio longitudinal (axial direction) 1.16 times, lateral (radial direction) 2.2 times,

As a result, a polyethylene narrow-neck round straight bottle was obtained which was milky white and had a uniform thickness distribution without uneven thickness in the stretched portion. No damage was observed when the contents were filled and dropped from a height of 2 m.

All of the above examples were carried out using an injection stretch blow molding machine model BS0530 manufactured by Aoki Koken Co., Ltd. Also, injection stretch blow molding is disclosed in JP-A-4-21.
The method described in Japanese Patent No. 4322 was adopted.

[0037]

According to the present invention, a molded product such as a container having a thin body made of polyethylene, which has been extremely difficult until now, can be easily molded by stretch blow molding, and a large number of molded products can be manufactured at a time. Production becomes possible. Further, since it can be formed into a thin wall, it is economically superior to a blow-molded product and has sufficient drop strength, so that it has a great industrial utility value.

Claims (1)

[Claims] 1. A molten polyethylene is injection-filled into an injection mold to be molded into a desired preform, and the preform is held by a lip mold to release from a cavity mold and a core mold of the injection mold. After that, when transferring to a blow mold and performing stretch blow molding into a thin hollow molded product, before releasing the preform from the injection mold, gas is pressed into the boundary between the core mold and the preform. The inside of the preform is cut off from the core mold, and then the internal cooling of the preform is incomplete and at a high temperature, and the surface temperature of the preform immediately after release is 80 ° C to 90 ° C at room temperature. Mold release is performed, and the stretch blow molding is performed within a time period before the surface temperature after the mold rise is increased by internal heat of the preform and reaches 120 ° C. Injection stretch blow molding of polyethylene. Method.