JPH07195485A - Extrusion foaming mold - Google Patents

Abstract

PURPOSE:To obtain an extrusion foaming mold for obtaining a rectangular (platy) foam. CONSTITUTION:In an extrusion foaming mold used for an apparatus wherein a volatile foaming agent and thermoplastic resin, after mixing by melting with an extruder, is extruded into a low pressure area through a mold to make a rectangular foam, the mold 40 is composed of a tubular rectifying area part 40A having a smooth inner peripheral surface, and a squeezing in area part 40B continued to the rectifying part. The squeezing in area part 40B has a shape wherein a sectional area decreases gradually toward a rectangular outlet side. Further, a ratio of a sectional area X at an inlet of the mold to an sectional area Y at an outlet of the mold (squeezing in ratio) [(X-Y)/X] is 90-99.9%. A platy foam which is free from occurring of a void can be molded thereby.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an extrusion foaming mold for obtaining a rectangular (plate-like) foam, and particularly LDPE.
The present invention relates to a mold suitable for extrusion of a crystalline olefin resin such as (low density polyethylene) that requires precise temperature control of a foamable melt during foaming.

[0002]

2. Description of the Related Art There are various types of molding methods for extruded foams, and in particular, a fan die or a coat hanger die is generally used when an extruded foam having a rectangular cross section is extruded. These molds are designed in consideration of evenly distributing the flow passages in the width direction in order to change the melt discharged from the extruder from a nozzle shape to a rectangular shape.

[0003]

However, when a rectangular foam is formed by using a fan die or a coat hanger die for extrusion foaming, a die which causes almost no problem in extruding a non-foam body such as a film. It has been confirmed that the flow path shape in the interior greatly affects the quality of the extruded foam. That is, when a foam is extruded with such a die, the cross-sectional area of the outlet of the die is widened in order to obtain a foam having a large thickness and a large cross-sectional area with a particularly limited extrusion amount. It has been found that when a foaming agent having a low compatibility with a resin is used, defects such as voids are likely to appear inside the extruded foam when the foaming agent is used. It is an object of the present invention to provide an extrusion foaming mold that eliminates the above-mentioned disadvantages when a foam having a rectangular cross section is extruded.

[0004]

The present inventors have conducted many studies and experiments on a method of extruding a foam having a rectangular cross section. As a result, the most important points required when designing a mold for extrusion foaming are to make the temperature distribution of the molten resin in the mold as uniform as possible and to make its flow as smooth as possible. I found out that.

That is, when the foam is extruded from the mold, the uniformity of the temperature distribution of the foamable melt introduced into the mold and the dispersibility of the foaming agent are extremely important, and the temperature distribution is not uniform. It has been experimentally confirmed that if there is a uniform portion or a poorly dispersed portion of the foaming agent, the resin and the foaming agent are separated and progress at that portion, resulting in defects such as voids in the extruded foam.

In particular, polystyrene has a gentle viscosity curve in the proper foaming temperature range, and therefore, this temperature difference (nonuniformity of the temperature distribution of the foamable melt) is absorbed to obtain a good foamed molded article to some extent. However, in the case of crystalline polyolefin resin, the appropriate foaming temperature range is narrow and the gradient of the viscosity curve is steep, so if the temperature difference is large, the viscosity difference will be large and the flow will be uneven. Therefore, good foaming conditions cannot be obtained.

On the other hand, even if the temperature distribution of the foamable melt introduced into the mold is almost uniform and the uniformity is maintained, the mold flow path is introduced. If the design is not suitable, for example, if there is an irregular change in the cross-sectional area of the flow path in the mold, or if there is a portion that is susceptible to shearing, a non-uniform state of foaming agent dispersibility will occur there. It has also been experimentally confirmed that the resin and the foaming agent are separated from each other and appear as defects inside the foam.

In conventional structures such as a fan die and a coat hanger die, when the cylindrical portion at the die entrance is expanded into a rectangular shape, the cross-sectional area is changed so as to maintain a good balance of the thickness in the width direction and the flow path in the width direction. Is widened, and irregular changes in cross-sectional area occur in common. Therefore, when forming a foam having a rectangular extrusion cross-section using a conventional fan die or the like, it is unavoidable that defects such as voids appear inside the extruded foam. Therefore, when forming a rectangular foam using an extrusion foaming mold, in order to avoid defects such as voids appearing in the molded product, the temperature distribution of the melt flowing in the mold is almost It is essential that they are uniform and that sudden narrowing or opening phenomenon does not occur in the flow path in the mold.

The present invention is basically based on the above-mentioned findings. Basically, after the volatile foaming agent and the thermoplastic resin are melt-mixed by an extruder, they are passed through a mold to a low pressure region. A mold for extrusion foaming used in an apparatus for extruding a plate-shaped foam, comprising a straightening zone portion and a narrowing zone portion connected to the straightening zone portion, and the straightening zone portion has a smooth inner peripheral surface. The narrowed area has a rectangular outlet at the tip and has a shape in which the cross-sectional area gradually decreases toward the outlet side, and the cross-sectional area X of the die inlet and the die outlet The cross-sectional area Y and the ratio (narrowing ratio) ((X-Y) / X) are 9
Disclosed is a die for extrusion foaming, which is characterized by 0 to 99.9%. It is a particularly preferred embodiment that the diameter of the straightening zone is 0.8 to 1.5 times the diameter of the cylinder of the extruder.

[0010]

EXAMPLES The metal mold for extrusion foaming according to the present invention will be described in more detail below by describing the background of the present invention based on comparative examples and examples. FIG. 1 is a schematic view of an apparatus which melts and mixes a volatile foaming agent and a thermoplastic resin, which have been conventionally used, in an extruder 10 and then extrudes through a mold into a low pressure region to form a plate-shaped foam. Is shown. The volatile foaming agent and the thermoplastic resin are melted and mixed by the heater 1 and the screw 2, and are extruded by the screw 2 into the mold 20 through the breaker plate 3 provided at the tip of the extruder 10, and further inside the mold 20. It passes through the flow path 21 and is extruded as a plate-like foam from the rectangular die outlet 22.

The die 20 shown in the drawing is a so-called fan die, as shown in a perspective view of the die half as shown in FIG. 2, and has a thickness in the width direction when it is expanded from the cylindrical portion 23 at the die inlet into a rectangular shape. In order to maintain good balance, the cross-sectional area is changed to widen the flow passage in the width direction, and the cross-sectional area changes irregularly. LDPE (M
I = 0.3) 100 parts by weight + 0.4 parts by weight of talc mixed with 10 parts by weight of 1-chloro-1,1-difluoroethane as a foaming agent, and an experiment was conducted using an extruder with a diameter of 150 mm. It was

[Comparative Example 1] In the extruder 10 shown in FIG. 1, as a mold 20, as shown in Comparative Example 1 of Table 1, a mold inlet cross-sectional area X was 50.3 cm ² , and a mold outlet cross-sectional area. Y is 5.2 cm
^{2. The} experiment was conducted with the fan die having the shape shown in FIG. 2 in which the narrowing ratio ((X−Y) / X) was 89.7%.

The experimental conditions at that time, that is, the mold inlet shear rate, the mold outlet shear rate, the resin discharge amount, the radial resin temperature distribution at the mold inlet, etc., are shown in the column of Comparative Example 1 in Table 1. As shown. The foam molded product had a good appearance,
Many voids were observed. It was understood that this was due to the fact that the radial resin temperature distribution ΔT ° C. at the mold inlet was as large as 4 ° C. and was non-uniformly distributed as described above and the shape of the fan die itself.

Comparative Example 2 Therefore, an experiment was conducted by preparing a mold 30 as shown in FIG. 3 for the purpose of making the radial direction resin temperature distribution at the mold inlet uniform. The mold 30 is composed of a rectifying area portion 30A and a narrowing portion 30B. The narrowing portion 30B is a fan die similar to the fan die 20 used in Comparative Example 1, and the rectifying area portion 30A has a smooth inner peripheral surface. It is a cylindrical tube with. This rectifying area portion 30A is
This is intended to make the radial direction distribution of the temperature of the resin cooled by the extruder 10 uniform, and by installing a heater 31 to avoid supercooling from the surface, the wall surface portion and the central portion of the flow channel are provided. The temperature was made uniform. Also,
The rectification area portion had a diameter B of 150 mm (extruder diameter A / rectification area diameter B = 1). The extruder used was the same as that used in Comparative Example 1.

The shape of the mold, the experimental conditions, etc. are as shown in Comparative Example 2 of Table 1, and in this experiment, the rectifying area portion 3
The resin temperature distribution ΔT ° C. at the inlet of 0 A (that is, this corresponds to the resin temperature distribution at the mold inlet in Comparative Example 1) was 4 ° C., but ΔT = 2 ° C. was averaged at the outlet. Has been done. As described above, by sending the one whose temperature distribution is made uniform by passing through the rectifying region to the fan die 30B, the generation of voids in the foam molded article was reduced as compared with the case of Comparative Example 1, but Many voids were still found.

[Examples 1 to 4] Next, the inventors of the present invention prototyped a new narrowed region portion of the die. The shape is such as shown in FIGS. 4A and 4B, and the cross-sectional area is gradually reduced from the inlet having a circular cross section toward the outlet having a rectangular shape. The outlet width H of FIG. 4a (hereinafter referred to as the present invention die 1) is smaller than the inlet diameter R, and the outlet width H of FIG. 4b (hereinafter referred to as the present invention die 2) is The diameter is slightly larger than the diameter R of the inlet, but in each case, the cross-sectional area gradually decreases from the inlet toward the rectangular outlet.

FIG. 5 shows a state in which the extrusion foaming mold 40 according to the present invention having the above-mentioned newly designed die at the tip is attached to the extruder, and this mold 40 is the same as in the case of Comparative Example 2. And a die 40B according to the present invention (die 1 and 2 of the present invention). In addition, 41 is a heater. An experiment was conducted using the mold having the above shape under the same conditions as in Comparative Example 2. The results are shown in Examples 1 to 4 in Table 1. In addition, Example 1 and Example 2
In both cases, the die 1 of the present invention having the same size is used, but in the second embodiment, the diameter B of the rectifying area is different from that of the first embodiment. Further, both the third and fourth embodiments use the die 2 of the present invention, but the exit cross-sectional area Y is different, and the diameter B of the rectifying area portion is the same as that of the first embodiment.

As shown in Table 1, each of the molded rectangular foams had a clean appearance, and no void was observed in any of them. Therefore, with respect to the mold 40 used in the experiment of Example 1 and the mold 20 (fan die) used in Comparative Example 1, changes in the cross-sectional area in the mold flow direction and the length direction were measured. The result is shown in FIG. 6 (since the mold 20 does not correspond to the rectifying region, the position of 60 cm in FIG. 5 corresponds to the inlet X for the mold 20). Figure 6
Therefore, compared with the change in the cross-sectional area of the narrowed-down portion 40B of the mold 40 according to the present invention, the conventional fan die obviously has the irregular cross-sectional area in the narrowed-down area, which is the cause of the occurrence of voids. It is highly speculated that

[Embodiments 5 and 6] Next, in the mold 40 according to the present invention, those having a rectifying zone having a diameter B of 250 mm (Embodiment 5) and 90 mm (Embodiment 6) were prepared. The same caliber (A = 250) as in the other examples above.
(mm) was attached to an extruder and an experiment was conducted. The experimental conditions are as described in the columns of Example 5 and Example 6 in Table 1. In this case, some voids were observed in the molded product. In Example 5, the ratio of the diameter A of the extruder to the diameter B of the rectifying region was as small as 0.6, and as a result, the cross-sectional area expanded rapidly, resulting in non-uniform flow between the inner wall surface of the flow passage and the center. It is presumed that this is caused by the fact that the distribution of the resin temperature in the radial direction at the outlet of the rectification region is Δ4 ° C. Further, in the case of Example 6, the ratio of the diameter A of the extruder to the diameter B of the rectification area portion was as large as 1.7, and as a result, internal heat was generated due to the abrupt narrowing of the cross-sectional area, and the rectification area was increased. It is presumed that this is due to the fact that the distribution of the resin temperature in the radial direction at the outlet remained at the temperature difference Δ4 ° C. at the inlet. Therefore,
In the present invention, the diameter A of the extruder and the diameter B of the rectification area part
It can be seen that it is a very preferable embodiment that the ratio of is in the range of 0.8 to 1.5.

[Comparative Example 3] Next, using the same apparatus as in Example 1, the ratio of the sectional area X of the die inlet to the sectional area Y of the die outlet (narrowing ratio) ((XY) / X). However, 88.1% was prepared for the experiment. The experimental conditions and results are shown in Comparative Example 3 in Table 1. The appearance of the obtained molded product was slightly shrunk, and generation of voids was large. This is because the narrowing ratio is small, so that the pressure cannot be concentrated at the mold outlet and the foaming property becomes poor, foaming occurs inside the mold, voids are generated due to poor dispersion of the foaming agent, and the appearance also tends to shrink. It is speculated that This fact and Examples 1 to 6 above
In the present invention, the reduction ratio ((X−Y) / X) is approximately 90% or more.
However, it was found that it is essential to be 90 to 99.9%.

When a conventional fan die is used, even if the narrowing ratio is 90% or more, it is unavoidable that a void is generated for the above reason. Examples 1 to 6 and Comparative Examples 1 to 3 described above
From the description, the volatile foaming agent and the thermoplastic resin are melt-mixed by an extruder, and then the metal mold for extrusion foaming is used in a device that extrudes into a low-pressure region through a mold to form a rectangular foam. In, the die is composed of a rectifying area portion and a narrowing area portion connected to the rectifying area portion, and the rectifying area portion is a cylindrical tube having a smooth inner peripheral surface, and the narrowing area portion is a rectangular outlet. And has a shape in which the cross-sectional area gradually decreases toward the exit side, and the cross-sectional area X of the die inlet
And the cross-sectional area Y of the die outlet (narrowing ratio) ((X-Y)
It will be understood that by using a mold in which / X) is 90 to 99.9%, it is possible to mold a rectangular foam having a clean appearance without generation of voids.

[0022]

[Table 1]

[0023]

By using the extrusion foaming mold according to the present invention, it is possible to obtain a void-free rectangular (plate-shaped) foam.

[Brief description of drawings]

FIG. 1 is a schematic view of an extrusion foam molding apparatus using a fan die.

FIG. 2 is a perspective view showing a cross-sectional shape of a fan die.

FIG. 3 is a schematic view of an extrusion foam molding apparatus using a fan die-shaped mold having a rectifying area portion.

FIG. 4 is a perspective view showing a cross-sectional shape of a narrowed portion of a mold according to the present invention.

FIG. 5 is a schematic view of an extrusion foam molding apparatus using a mold according to the present invention.

FIG. 6 is a diagram showing a change in cross-sectional area of the mold in the flow direction.

[Explanation of symbols]

10 ... Extruder, 40 ... Mold, 40A ... Rectifying area part, 40
B ... Narrowed area part, 41 ... Heater

Claims (2)

[Claims] 1. A mold for extrusion foaming used in a device for melting a volatile foaming agent and a thermoplastic resin in an extruder and then extruding the mixture through a mold into a low pressure region to form a rectangular foam. A rectifying zone portion and a narrowing zone portion connected to the rectifying zone portion, the rectifying zone portion is a cylindrical tube having a smooth inner peripheral surface, and the narrowing zone portion has a rectangular outlet. And has a shape in which the cross-sectional area gradually decreases toward the outlet side, and the ratio (narrowing ratio) of the cross-sectional area X of the mold inlet to the cross-sectional area Y of the mold outlet ((X-Y) / X). But 90 ~
A mold for extrusion foaming, which is characterized by being 99.9%. 2. The extrusion foaming mold according to claim 1, wherein the diameter of the straightening zone is 0.8 to 1.5 times the diameter of the cylinder of the extruder.