JPH04214322A - Injection stretch blow molding method - Google Patents

Abstract

PURPOSE:To form a container without unevenness in a short time by adjusting the temperature of an preform injection mold by means of an injection mold, releasing the same during the high temperature and carrying out stretch blow molding immediately. CONSTITUTION:Releasing out of an injection mold of a reform molded by injection filling molten resin in the injection mold is carried out while the shape is in the state of being able to retain by means of a skin layer formed on the surface by quenching and also in the state of high temperature because of the external cooling not being completed. The stretch blow molding of the preform is performed within the time (t) in which the surface temperature of the preform is risen by its own external temperature to the peak temperature.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molding method in which injection molding of a synthetic resin preform and stretch blow molding of a thin-walled hollow molded product are successively performed.

0002

[Prior Art] One of the molding methods generally referred to as injection stretch blow molding involves holding an injection molded preform by a lip mold and immediately transferring it to a blow mold. There is a three-station molding method that performs molding.

[0003] In the three-station molding method described in JP-A No. 63-296921, the internal and external temperatures of the preform, which has been released from the mold at a high temperature, are made uniform by the internal heat of the preform itself, thereby eliminating temperature differences. Since then, stretch blow molding has been carried out.

[0004] Furthermore, the technical concept of releasing the preform at high temperature is disclosed in Japanese Patent Application Laid-open No. 77536/1983.
A station-type molding method has already been disclosed.

[0005] In this molding method, an injection-molded polyethylene terephthalate preform is released from the mold at a temperature range that maintains its shape at a high temperature, and the temperature between the inner and outer surfaces and the inner center of the same plane cross section of the preform is After equalizing the difference, the preform temperature is increased to 95% by external energy.
Blow molding is performed by adjusting the temperature to a high temperature of ℃ or higher.

[0006]

[Problems to be Solved by the Invention] In the three-station method, the molding process consists of three steps: "injection molding of preform - stretch blow molding - removal of molded product". In the 4-station method, which requires the four steps of stretch blow molding and removal of the molded product, temperature adjustment of the preform immediately before stretch blow molding, which was considered indispensable, is no longer necessary.

[0007] Therefore, the preform temperature control device and its ancillary equipment used in the 4-station system can be omitted, and the number of neck molds that also serve as preform transfer members is reduced by one. In addition to structural advantages such as these, there are also economic advantages such as shorter molding cycle times and lower machine costs.

However, molded products molded using the three-station method tend to be limited to wide-mouthed containers. The preform has a large diameter, and there are no technical difficulties in designing the draft from the injection mold or core.
This is because high-temperature mold release can be easily performed.

[0009] Compared to the case of wide-mouth containers, when molding narrow-mouth containers such as bottles, which require a significantly smaller preform diameter, a longer stretched portion, and a higher stretching ratio, it is difficult to control the temperature of the preform and there are problems with punching. Due to slope limitations, four stations are used that allow temperature control just before blow molding.

The difficulty in controlling the temperature of the preform in the three-station system is that even if the surface layer is heated by internal heat and the temperature of the preform becomes uniform, there is no means to accurately detect this.

[0011] For this purpose, a rough estimate is made from the elapsed time after the mold is released, and trial shots are repeatedly performed at that point to determine the stretching blowing timing. This test shot requires experience and time, and when the material resin is polypropylene, the molding conditions often differ slightly depending on the manufactured rod.
Conditions must be set each time, which inevitably increases product loss.

[0012] This invention was devised in order to solve the molding problem of high-temperature mold release using the three stations described above, and its purpose is to release the preform at high temperature, There are no restrictions on the shape of the reform, draft angle, wall thickness distribution, etc., just like in the case of wide-mouthed containers.
It is an object of the present invention to provide a new injection stretch blow molding method capable of molding narrow mouth synthetic resin containers such as bottles.

[0013]

[Means for Solving the Problems] Molten resin is injected and filled into a mold to injection mold a preform 11 having the cross-sectional structure shown in FIG. When the preform is molded and left at room temperature, the surface temperature of the preform changes as shown in FIG.

[0014] Most of the thermoplastic resins used for molding containers show a similar change in surface temperature over time, although there are some differences in the time it takes to reach the peak temperature. The reason for this initial increase in surface temperature is that in preforms that have been released from the mold at high temperatures, the preform surface that is in contact with the mold cavity surface and core solidifies as the mold cools and forms a skin layer. The internal cooling is incomplete and the skin layer is in a semi-molten state at a high temperature, and after the cooling is cut off by mold release, the skin layer is heated from the inside.

Of course, in such a state, the temperature of the preform is not uniform except for the mouth part, which is completely cooled and solidified at the time of mold release, and stretch blow molding is performed with a temperature difference between the inside and outside of the preform. If this is done, cloudiness will occur due to crystallization or crazing, so in the conventional method described above, the temperature of the preform is made uniform before stretching and blowing.

According to the research conducted by the present inventors, clouding of a molded product during stretch blow molding is more likely to be due to the temperature of the stretch blow molding than to the temperature difference between the inside and outside.

[0017] In previous experiments, the surface temperature of the preform was 80°C for polyethylene terephthalate.
If it is less than that, clouding is likely to occur. It has also been found that when the surface temperature of the preform immediately after release from the mold is 60° C. or higher and stretch blow molding is performed after a very short period of time, crazing hardly occurs.

However, even in such a case, if the cooling time is long and the temperature immediately after demolding is below 60°C, even if the stretch blow molding temperature is 80°C or higher before the peak, the stretch blow molding will not be carried out there. It was also revealed that molded products tend to become cloudy.

[0019] In addition, with polyethylene terephthalate, if the cooling time is set short and the surface temperature immediately after mold release is set to 70°C or higher, the peak temperature often becomes 95°C or higher. , uneven thickness tends to occur, and rigidity is also lost.

[0020] Therefore, the cooling time of the preform in the injection mold is limited to a certain period of time, but the cooling time varies depending on the wall thickness even for the same resin, and also depends on the temperature of the cooling water used. However, the allowable range for the same wall thickness is around 1 second for polyethylene terephthalate.
Within this allowable range, it is possible to obtain a surface temperature immediately after release from the mold that makes it possible to mold a transparent, well-shaped, narrow-mouthed container.

Similarly to the case of polypropylene preforms, the surface temperature rises rapidly from the temperature at the time of mold release at room temperature and reaches a peak, as shown in Figure 2, after which the peak temperature is maintained for a long time and then gradually decreases. descended.

[0022] From the change in surface temperature over time, it is unclear when the temperature of the entire preform becomes uniform due to the internal temperature, but in conventional stretch blow molding using a high-temperature wide-mouth preform, it takes 17 seconds after release from the mold. Since stretch blow molding was performed at about 10 seconds, I tried stretch blow molding around the diagonal line in Figure 2 using this as a guide.As for the preform of a wide mouth container, it took around 17 seconds to complete the stretch blow molding with a transparent body. It was possible to mold a wide mouth container.

However, in preforms for narrow-mouth containers where the stretching ratio is higher than that for wide-mouth containers, even if the mold is released with a large draft angle, uneven thickness and poor bottom formation are likely to occur, resulting in poor performance. The molded product was not in good condition.

[0024] Even in the case of polypropylene, if the surface temperature immediately after release from the mold is 90°C or higher and the temperature during stretch blow molding is 110°C or higher, narrow-mouthed containers can be formed, and the allowable cooling time with the same wall thickness can be improved. The range was around 3 seconds.

It is clear from this attempt at stretch blow molding that in order to make the temperature of the preform uniform after demolding,
If stretch blow molding was performed after a certain period of time had elapsed,
Since the preform is subjected to slow cooling, whitening due to crystallization is likely to occur, and it is naturally difficult to mold a narrow-mouthed container.

[0026] Therefore, the difficulty in stretch blow molding of a preform that has been released from a high temperature mold is not only due to temperature unevenness, but is also greatly affected by the composition of the high temperature preform that changes over time and the stretch blow molding timing. It is.

Immediately after the preform 11 is released from the injection mold, the surface temperature is low, as shown in FIG. 1, so the surface side has a hard skin layer. It is 12. However, the state of formation of the skin layer 12 differs depending on the cooling rate.

In addition, in high-temperature mold release, the cooling does not reach the center, so although the internal resin 13 has fluidity in the range at high temperatures, drawdown is prevented by the skin layer 12 on the surface, and even after mold release, the internal resin 13 has fluidity in the range. The form of the renovation 11 is maintained.

[0029] Then, as time passes, the internal temperature is released to the outside, and at the same time, the skin layer 12 forming the surface is heated from within, so the surface temperature rapidly rises and the skin layer 12 also softens. On the other hand, as the internal temperature decreases, the flowing portion shrinks toward the center.

Skin layer 1 when the surface temperature reaches its peak
2 is thin enough to form a skin, and the inside is becoming semi-hardened. After the peak, the surface temperature slowly decreases over time, and as the temperature becomes equal throughout the preform, crystallization progresses.

In such a high-temperature preform 11, until the surface temperature reaches a peak, the surface hardens to form the skin layer 12, but near the peak temperature, the skin layer 12 also softens and is in a stretchable state. It is in.

[0032] Also, the skin layer 1 is heated from inside.
In No. 2, the softening occurs first in the thick portion where the internal heat is high. Until the surface temperature reaches its peak and a certain amount of time has passed,
There is a temperature difference between the thick and thin parts, and the difference is especially obvious before the peak.

[0033] When stretch blow molding is performed in such a state, the skin layer on the side of the thicker part that receives more heat, that is, the side with a higher surface temperature, stretches first while surrounding the softened internal resin. go.

However, as the surface area naturally increases due to the elongation, the heat dissipation area becomes larger and the temperature decreases, and the temperature difference with the thin wall side disappears, and furthermore, the temperature on the thin wall side becomes relatively high. Next, the elongation of the thinner side will take precedence. Such mutual stretching is repeated in an extremely short period of time, and during this time the internal temperature, which had a high amount of heat, drops to a temperature suitable for stretching, and the internal resin 13, which had been stretched along with the skin layer 12, , skin layer 12 from the middle
In the same way, it becomes thin and elongated, and a molded product with a uniform thickness distribution is formed there.

Therefore, when injection molding the preform 11, first, the shape of the container 14, which will be the molded product, is determined.
The preform 11 is molded by taking into consideration the amount of elongation of each part of the preform 11 in advance and intentionally adjusting the thickness distribution of each part, while maintaining the temperature of the injection mold constant, and injecting and filling the cavity. It is desirable that the cooling be carried out without any height or depression in any part.

[0036] Furthermore, for the high-temperature preform 11 on which the skin layer 12 was formed by rapid cooling, the best results were obtained when the molding was performed before the surface temperature reached its peak. At the point when it seems to have reached its peak, uneven thickness tends to occur,
Not very good results are obtained.

The surface temperature during stretch blow molding was 80° C. or higher for polyethylene terephthalate, and the time after mold release was around 8 seconds, and for polypropylene, it was 110° C. or higher and the time was around 14 seconds.

However, even within the above time, the skin layer 1
It is clear from past attempts that it is difficult to obtain good results unless 2 is formed by rapid cooling. This is thought to be due to the difference in the crystalline state that occurs in the skin layer 12 due to cooling; the crystals produced by rapid cooling become microcrystals, but the crystals grow larger in slow cooling, and the bonds between the crystals are weaker than in microcrystals. It's for a reason.

Therefore, one feature of the present invention is that molten resin is injected and filled into an injection mold to form a desired preform, and the preform is held at the mouth by a lip mold and blown from the injection mold. When the preform is transferred to a mold and stretch blow molded into the required thin-walled hollow molded product in the blow mold, the preform is released from the injection mold and its shape is maintained by a skin layer formed on the surface by rapid cooling. is in a state where it is possible to
In addition, the preform should be stretch-blown while the internal cooling is incomplete and the preform is still at a high temperature, and the stretch blow molding of the preform should be carried out within the time until the surface temperature of the preform, which increases due to its own internal temperature, reaches its peak temperature. .

Another feature of the present invention is that the preform made of polyethylene terephthalate is released from the injection mold when the surface temperature immediately after release is 60° C. or higher and 70° C. or higher at room temperature.
C. or less, and stretch blow molding is carried out within the time required for the surface temperature of the preform to reach its peak temperature in a temperature range of 80.degree. C. or higher and 95.degree. C. or lower.

Furthermore, another feature of the present invention is that the polypropylene preform is released from the injection mold at a temperature range such that the surface temperature immediately after release is from 90°C to 100°C at room temperature, and the stretching Blow molding is performed within the time required for the surface temperature of the preform to reach its peak temperature in a temperature range of 110°C or higher and 122°C or lower.In addition to the above synthetic resins, polyethylene, polycarbonate, etc. can be used as the crystalline resin. Preforms made of thermoplastic synthetic resin can be stretch-molded into containers by similar means.

[0042]

[Operation] The molten resin in the injection mold is rapidly cooled, and the resin in contact with the mold surface hardens in an extremely short period of time, forming a skin layer. Furthermore, as the surface separates from the mold surface due to shrinkage due to curing, cooling of the interior becomes poor and the temperature remains high in an incompletely cooled state.

However, the hard skin layer allows the preform to be released from the mold, and the shape is maintained even after the preform is released from the mold. Therefore, early release from the mold is possible regardless of the draft angle.

[0044] In such a high-temperature preform, even after the mold is released, the semi-molten internal resin remains wrapped in the skin layer until the surface temperature reaches its peak.
When the skin layer is stretched once it has softened, the skin layer first stretches thinly, taking the internal resin with it.

During this stretching process, the temperature of the entire preform decreases due to an increase in surface area, etc., and the temperature of the internal resin reaches a temperature suitable for stretching. The internal resin, which had previously extended along with the skin layer, begins to extend as thinly as the skin layer, and as a result, the preform becomes a molded product with a uniform wall thickness distribution.

[0046]

【Example. 1) Molten resin of polyethylene terephthalate was injected and filled into an injection mold, and rapidly cooled to form a narrow-mouth preform 11 as shown in FIG.

[0047] Three examples of preforms with different wall thicknesses were
Each sample was injection molded with different cooling times, and the change in surface temperature over time was measured.

[0048] The preform is for a 1 liter container, the total length is 124 mm, and the temperature measurement is 30 mm and 60 m above the bottom.
The temperature measured at three locations of 100 mm and 100 mm is the average value. The temperature measuring device is a digital radiation thermometer IR-AHOT (
(manufactured by Chinon Co., Ltd.) was used.

The injection molding conditions are as follows. (Cooling begins after the injection filling time has elapsed)

[0050] Figures 3 to 5 show the changes over time (average values) in the surface temperature of the following samples at room temperature (22°C).
The main points are shown in Table 1 below.

Table 1

For each sample above, the blowing air pressure 14
kg/cm2 to form a bottle-shaped container 14 as shown by the chain line in FIG.
It has been found that it is best to perform stretch blow molding within the time shown in the figure, that is, within the time t between time t1 before the surface temperature reaches its peak and time t2 when it is thought to have reached its peak. .

Table 2

However, for preforms whose surface temperature immediately after mold release is outside the range of 60°C to 70°C at room temperature, or whose surface temperature during stretch blow molding is outside the temperature range of 80°C to 95°C, the above table does not apply. As shown in No. 2, no good molded product was obtained. In addition, the elapsed time in the table refers to the time from releasing the preform to starting stretch blow molding, and the results were obtained by molding several bodies within a range of 1 second before and after that elapsed time. is shown as the molded state.

[0055]

【Example. 2) Molten polypropylene resin was injected and filled into an injection mold and rapidly cooled to form a narrow-mouth preform 11 as shown in FIG. 1 similar to that in Example 1.

Eight preforms having the same wall thickness were injection molded with different cooling times, and the changes in surface temperature over time at room temperature were measured.

The injection molding conditions are as follows.

FIG. 7 is a diagram showing the change over time (average value) of the surface temperature of each of the following samples at room temperature (22° C.), and the main points thereof are shown in Table 3 below.

Table 3

Sample No. 4 was stretch blow molded at a blowing air pressure of 12 kg/cm2 for the time shown in FIG. 6, as in the case of polyethylene terephthalate, to form a bottle shape as shown by the chain line in FIG. When the container 14 was molded, the stretch blow molding was carried out within the time shown in FIG. I found it best to do this.

However, the surface temperature immediately after release from the mold was 90°C at room temperature.
As shown in Table 4 below, good results were not obtained with preforms outside the range of .degree. C. to 100.degree. C. or with preforms whose surface temperature during stretch blow molding was 110.degree.

Table 4

[0063]

[Effects of the Invention] As described above, the present invention enables release of an injection molded preform from an injection mold by maintaining its shape by a skin layer formed on the surface by rapid cooling, and by internal cooling. The preform is stretch blow molded while it is still unfinished and at a high temperature, and the stretch blow molding of the preform is performed within the time it takes for the surface temperature of the preform, which rises due to its own internal temperature, to reach its peak temperature. Less stress and strain that tends to occur when stretch blow molding renovations. Therefore, shrinkage deformation during high-temperature filling due to stress strain is less likely to occur, and containers made of polyethylene terephthalate have improved heat resistance.

Furthermore, since the preform is stretched while the inside is in a semi-molten state, there is almost no influence from temperature fluctuations, and the forming is completed before the inside crystallizes, so a transparent, thin-walled container with no uneven thickness can be obtained. It will be done.

Furthermore, since the skin layer is formed by rapid cooling, it is possible to release the mold even if the inside is soft. Narrow-mouth bottle-like containers that require molding can also be molded using three stations in the same way as wide-mouth containers.

Moreover, since the time required for stretch blow molding is significantly shorter than before, the molding cycle becomes faster, and there are also advantages such as increased production per hour.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a high-temperature preform.

FIG. 2 is a graph showing changes over time in the surface temperature of a high-temperature preform injection-molded with a crystalline resin.

FIG. 3 is a graph showing changes over time in the surface temperature of a high-temperature preform of sample No. 1 injection-molded with polyethylene terephthalate.

FIG. 4 is a diagram of the surface temperature over time of a high temperature preform of sample NO2 injection molded with polyethylene terephthalate.

FIG. 5 is a graph showing changes over time in the surface temperature of a high-temperature preform of sample NO3 injection-molded with polyethylene terephthalate.

FIG. 6 is a graph showing changes in surface temperature over time during stretch blow molding of a preform made of polyethylene terephthalate.

FIG. 7 is a graph showing changes over time in the surface temperature of a high-temperature preform of sample NO4 injection-molded with polypropylene.

FIG. 8 is a graph showing changes in surface temperature over time during stretch blow molding of a preform made of polypropylene.

[Explanation of symbols] 11 Preform 12 Skin layer 13 Internal resin 14 Container

Claims (4)

[Claims] Claim 1: Inject and fill a molten resin into an injection mold to form a desired preform, and transfer the preform from the injection mold to a blowing mold while holding the mouth part with a lip mold. When stretch-blow molding the preform into the required thin-walled hollow molded product in the mold, the preform must be released from the injection mold in such a way that its shape can be maintained by a skin layer formed on the surface by rapid cooling, and Stretch blow molding of the preform is carried out while the internal cooling is incomplete and the temperature is still high.
An injection stretch blow molding method characterized in that the process is carried out within a period of time until the surface temperature of the preform, which increases due to its own internal temperature, reaches a peak temperature. [Claim 2] The release of the polyethylene terephthalate preform from the injection mold is carried out at a temperature range such that the surface temperature immediately after release is from 60°C to 70°C at room temperature, and during stretch blow molding, the preform is released from the injection mold. 2. The injection stretch blow molding method according to claim 1, wherein the injection stretch blow molding method is carried out within the time period until the surface temperature reaches a peak temperature in a temperature range of 80° C. or higher and 95° C. or lower. [Claim 3] When releasing a preform made of polypropylene from an injection mold, the surface temperature immediately after releasing the mold is room temperature.
Stretch blow molding is carried out at a temperature range of 0°C or higher and 100°C or lower, and stretch blow molding is performed when the surface temperature of the preform is 110°C or higher and 122°C.
An injection stretch blow molding method characterized in that the process is carried out within the time required to reach the peak temperature in a temperature range below ℃. 4. The injection stretch blowing apparatus according to claim 1, wherein the preform molding resin is made of a crystalline resin such as polyethylene or polycarbonate.