JPH0813494B2 - Method for producing foamed molded article - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a foamed molded article using a thermoplastic resin as a raw material.

[Prior art]

Conventionally, the so-called bead molding method and foam injection molding method have been typical methods for producing a foamed thermoplastic resin molded product using a mold. However, since the bead molding method includes a two-stage foaming step in which beads obtained by pre-expanding the raw material resin particles are heated in the mold to be fused while being secondary-foamed and molded, the operation is complicated. Not only that, but also the shape of beads and traces of steam holes remain on the molded product,
It has the drawback of exhibiting poor appearance. Furthermore, in the bead molding method, especially when a polyolefin resin is used,
It is difficult to obtain a molded product with low foaming, and even if it is obtained, it will be inferior in fusion between beads, and compared to extruded products, only bending strength and compression strength can be obtained. I also have. In addition, although the injection molding method can obtain a foamed molded product in a single step, this method usually requires a very high pressure at the time of injection, and the equipment such as the mold and the extruder is not structured to withstand the high pressure. Must be avoided, and thus enormous equipment costs are required. This tendency becomes more remarkable as the obtained molded product is made larger.
Further, in the foam injection molding method, there is a problem that only a molded product having a foaming ratio of at most about 2 can be obtained due to the mechanism. Further, the extrusion molding method has a drawback that a molded product cannot be obtained.

Therefore, as a foam molding method which does not have the defects found in the conventional in-mold foam molding method, the present inventors first melt-knead a thermoplastic resin and a foaming agent under pressure to form a foam After being made into a melt, the melt was once held at a temperature and pressure conditions at which foaming did not occur, and then extruded into a low-pressure region due to an interim defect, and the obtained extruded melt did not yet complete foaming. In the meantime, a method for producing a foamed molded article has been proposed, which comprises molding with a mold and completing foaming in the mold (Japanese Patent Application No. 61-86730).

However, even in this method, there are problems such as (1) it takes a long time for molding, (2) it is difficult to sharply form the corners of the molded product, and (3) it is difficult to smooth the surface of the molded product, and the improvement is still unsolved. There was room for it.

〔Purpose〕

It is an object of the present invention to provide a foam molding method that does not have the problems found in the foam molding method.

〔Constitution〕

According to the present invention, a thermoplastic resin and a foaming agent are melt-kneaded under pressure to form a foamable melt, and the melt is once held in a temperature and pressure condition where foaming does not occur, and then in a low pressure range. Production of a foamed molded article characterized by being extruded and impacted on the extruded melt obtained in the foaming process before the extruded melt has completed foaming to complete foaming in a mold. A method is provided.

As the thermoplastic resin used as a raw material in the present invention, conventionally known resins, for example, various polyethylenes, polypropylenes, copolymers of ethylene and other monoolefins (for example, propylene), ethylene and vinyl monomers (for example, acetic acid). Vinyl) and other polyolefin resins such as copolymers, silane modified products thereof, polystyrene, styrene resins such as ABS resins, other polyvinyl chloride resins, various nylon resins, polycarbonate resins, polyhydroxy ether resins, etc., These are properly used depending on the intended use. In particular, when obtaining a structural material, it is preferable to use high-density polyethylene or polypropylene. The foaming agent for foaming the resin may be either a decomposing type foaming agent or a solvent type foaming agent, but a solvent type one is preferable, and a cycloaliphatic carbonization such as cyclobutane or cyclopentane is particularly preferable. Aliphatic hydrocarbons such as hydrogen, propane, butane, pentane, hexane, heptane, halogenation of chlorodifluoromethane, trichlorofluoromethane, dichlorodifluoromethane, dichlorotetrafluoroethane, methyl chloride, ethyl chloride, methylene chloride, etc. There are hydrocarbons. A foaming aid such as a metal salt or hydrogen metal salt of an organic acid, a metal salt or hydrogen metal salt of carbonic acid, or talc may be used together with these foaming agents.

Next, steps for obtaining the foam-molded article of the present invention will be described with reference to the drawings.

In the drawings, I indicates a step of uniformly melt-kneading a thermoplastic resin and a foaming agent under pressure, which can be carried out using an ordinary extruder. In this case, the melt-kneading temperature and pressure are the temperatures and pressures commonly used for melt-kneading thermoplastic resins, and are determined by the type of resin. For example, in the case of high density polyethylene or polypropylene, the temperature
Temperatures and pressures in the range of 120-200 ℃ and pressure of 30-200kg / cm ² G are adopted.

The foamable melt obtained in the melt-kneading step I is then sent to a melt-holding extrusion step II, in which the melt is once held at a temperature and pressure conditions at which foaming does not occur, and Is intermittently extruded into a low pressure zone such as atmospheric pressure to obtain a melt extrudate F. When the foamable melt is sent from the melt-kneading step I to the melt-holding extrusion step II, the foamable melt is usually sent to the melt-holding extrusion step II with its temperature adjusted to be lower than the melting temperature by cooling. preferable. For example, in the case of high density polyethylene, the above temperature should be 120-170.
The temperature is preferably set to ° C. In the melt-holding extrusion step II, an extrusion ram 5 having a die 4 at its tip and reciprocating inside is formed.
It can be carried out by using a cylindrical accumulator 6 provided with. That is, the melt from the melt-kneading step is pressed into the accumulator through the communication hole provided in the accumulator, and the pressure at that time causes the alum to retreat within the accumulator by a predetermined distance and melts a predetermined amount in the accumulator. The thing is filled. After completing this filling operation, open the gate that closed the die,
A separately provided hydraulic device presses the ram through a piston rod provided between the ram and the hydraulic device, and advances the ram so that the melt held in the accumulator is externally passed through a die. Extrude into the low pressure zone. After this extrusion is completed, the gate is closed, the pressure of the hydraulic device is reduced, and the pressing force of the ram is reduced to a predetermined pressure, whereby the melt is charged into the accumulator again as described above. In this way, the melt extrudate F is intermittently extruded from the accumulator. The pressure at which the melt is filled in the accumulator is a pressure in the range where foaming does not occur, and is adjusted by the pressing force applied to the ram.

The melt-kneading step I and the melt-holding extrusion step II
For specific equipment for carrying out
It is described in detail in Japanese Patent No. 1317.

As described above, the melt extrudate F is intermittently extruded from the melt-holding extrusion step II.
Are sequentially taken by a take-up device if necessary, and sent to a forming step IV through an impact step III, where they are formed into a desired shape by a mold.

The impact step III is carried out by any method as long as it is a method capable of giving a physical impact force to the melt extrudate F, and sonic impact, impact with a compressed gas, mechanical impact or the like can be used, but particularly mechanical It is preferable to use a physical impact. For example, as shown in FIG. 1, the molten extrudate F can be placed on the fixed plate 8 and the striking plate 7 can be moved downward to strike. This striking operation is preferably performed instantaneously so that the area of the melt extrudate F does not remarkably increase.
The striking plate 7 is quickly raised to its original position. The impact on the melt extrudate F can be performed once or multiple times. The sonic impact is performed by applying air vibrations due to the acoustic waves to the melt extrudate, and the impact by the compressed gas is performed by blowing the compressed gas onto the surface of the melt extrudate. It is preferable that the strength of the impact force is as large as possible, but the impact force may be such that bubbles in the melt extrudate F in the foaming process can be made fine. The impact step can be performed at any time after the formation of the melt extrudate F and before the completion of molding. For example, as shown in FIG.
Can be performed on the way to the molding step IV, or can be performed by pressing pressure when tightening with a mold as shown in FIG. Due to this impact treatment, a large number of fine bubbles are generated in the melt extrudate in the foaming process, and the large bubbles are divided into fine bubbles to increase the foaming power and form a melt extrudate having excellent foam moldability. be able to.

In the molding step IV, the impacted melt extrudate F is molded into a required shape in the mold. In this case, the mold used is usually moldability, mold release property, productivity cost, etc. A mold is used. FIG. 2 shows an explanatory diagram for forming the melt extrudate F in the mold. In Figure 2, the mold
12 is composed of a male mold 9 and a female mold 10, and the space formed when these molds are combined corresponds to the shape of the molded product. First female type 10
The melt extrudate F is put into the inside (FIG. 2 (a)). Then female
10 is raised at a pressure of, for example, 1 to 10 kg / cm ² (G),
The extrusion melt is pressed with the male mold 9 to fill the space (FIG. 2 (b)). At this time, by setting the pressing pressure high, the foaming force of the extruded melt F can be increased from the time of pressing. After that, the female mold is gradually spread by the expansion and expansion of the extruded melt F (FIG. 2 (c)). At the time of foam expansion of this extruded melt, the press pressure applied to the female mold 10 in the process of foam expansion is maintained as it is or in a reduced state, and is cooled to a low pressure after completion of foam expansion, or foam expansion is performed. A method is employed in which the pressing pressure of the female mold 10 is gradually reduced according to the force to cool the female mold 10 after completion of foam expansion. In such a method, by adjusting the set pressure of the female mold 10 when the extruded melt F expands and expands, the expansion ratio of the obtained molded product can be easily controlled. Next, after cooling, the mold is opened to take out the molded product from the mold (Fig. 2 (d)). In FIG. 2, reference numeral 20 indicates an exhaust pipe communicating with the outside and reference numeral 21 indicates a communication hole.

When carrying out the method of the present invention, as apparent from the above, since the foam molding of the melt extrudate F is performed in the mold due to its molding method, the melt extrudate F charged into the mold is
It must still have sufficient foamability. Therefore, the foaming speed of the foamable melt extrudate F extruded from the melt-holding extrusion step II should be low, and the melt extrudate F should be low.
Is rapidly sent to the impact step III to increase the foaming force by applying an impact to make the bubbles fine, and then sent to the molding step IV to complete the foaming and molding. Other factors that affect the size of the bubbles in the melt extrudate F and the foaming rate include the type of resin and the melt index value (MI value), the type of the foaming agent, and the amount added,
Particularly related to the foaming speed are the type and amount of the foam control agent, the shape and size of the cross section of the die discharge part, and in order to reduce the foaming speed, the cross section of the die discharge port should be prepared immediately after extrusion of the melt extrudate. Examples include selecting a shape and size that reduce the difference between the pressure in the accumulator and the pressure in the atmosphere under extrusion, and setting the amount of the cell regulator to be 0 or the necessary minimum amount. For example, the MI value of resin
When high density polyethylene (HDPE) of 0.5 to 20 is used, for example, 5 to 20 parts by weight of a foaming foaming agent such as trichlorotrifluoromethane or / and dichlorodifluoromethane as a foaming agent with respect to 100 parts by weight of HDPE. After adding 0 to 0.05 parts of talc as a foam control agent to obtain a melt-kneaded product in an extruder, the melt-kneaded product is filled in an accumulator and extruded to obtain a melt-extruded product having a preferable foaming rate. it can.

In the impact step III of the present invention, the melt extrudate subjected to impact treatment generally has a foaming rate of 80% or less, preferably
It is preferable to keep it at 50% or less, particularly preferably at 30% or less. Note that the foaming rate here means that the melt extrudate F causes a foaming action due to the difference between the internal pressure of the accumulator immediately after extrusion of the melt extrudate and the extruded atmosphere pressure, and the volume is expanded. It means the relative volume ratio when the maximum value is 100.

The shape of the extrudate obtained from the melt-holding extrusion step is not particularly limited, and it does not necessarily have to be a plate-shaped product, and may be a lump-shaped product or a rod-shaped product. Such lumps are
By subjecting to impact treatment, promoting foaming, and shaping, it is possible to obtain a high-quality foam-molded article according to the space shape of the mold.

〔effect〕

According to the present invention, it is possible to easily manufacture a large-sized foam structural material such as a pallet, a flooring material, a wall material, and a large container, which requires strength. Moreover, in the case of the present invention, since the foamable melt is foam-molded in the mold, a foam-molded product having a complicated shape can be easily obtained. Further, according to the present invention, in general, a molded article having a foaming ratio of 3 to 10 times, which is difficult to obtain by the foaming injection molding method or the bead molding method, is 1.2.
Molded products having a wide expansion ratio of up to 50 times can also be obtained. Also, since the melt extrudate in the foaming process was subjected to impact treatment, many fine bubbles are generated in the melt extrudate in the foaming process, and the large bubbles are divided into fine bubbles to improve the foaming force. In addition, the melt-extruded product having excellent foam moldability can be obtained, and thus the obtained molded product has an extremely excellent surface condition.

In addition, since the extruded melt that has been subjected to the impact treatment stops the foaming rate, the production efficiency can be improved.

〔Example〕

 Next, the present invention will be described in more detail with reference to Examples.

Example 100 parts by weight of high density polyethylene having a density of 0.96 g / cm ³ and a melt index (MI) of 5 and 4 parts by weight of trichlorotrifluoromethane were melt-kneaded in an extruder having a discharge capacity of 10 kg / hr, and then the resin was heated to 140 ° C. The temperature was adjusted and once extruded into the accumulator. Next, the foamable melt extruded into this accumulator is 300kg / hr (0.83kg / sec)
At the discharge speed of, a die (opening cross-sectional area of 15 cm ² ) provided in the accumulator was extruded into a plate shape under atmospheric pressure, and the extrudate 380 g obtained was vertically movable as shown in FIG. 1.5kg / cm after putting it in the inner bottom area 20cm × 20cm)
² By increasing the pressure with (G), the extrudate is impacted in conformity with the male mold, and then the pressure of the female mold is gradually reduced (the extrudate is reduced in volume by its foaming expansive force as the pressure decreases). , And the mold was cooled at the place of 0.5 kg / cm ² (G), and the molded product was taken out. The obtained molded product has a density
It was a rectangular parallelepiped with a sharp corner at 0.24 g / cm ³ and a volume of 1600 cc. Further, this product showed a good surface condition, and the bubbles in the surface portion (skin layer) were fine and had good uniformity.

Further, for comparison, when an experiment was conducted in the same manner except that the hitting treatment was not performed in the above, the resulting molded product lacked the sharpness of the corner portion, and the surface condition of the molded product was the same as that of the present invention. It was inferior to the molded product, and the bubbles in the surface portion (skin layer) were coarse and non-uniform.

[Brief description of drawings]

FIG. 1 is a process diagram for carrying out the present invention. FIG. 2 is an explanatory view of the forming process of the melt extrudate. I ... Melt-kneading process, II ... Melt-holding extrusion process, III
…… Impact process, IV …… Molding process, 7 …… Striking plate, 12 ……
Mold, F ... Melt extrudate.

Claims (5)

[Claims] 1. A thermoplastic resin and a foaming agent are melt-kneaded under pressure to form a foamable melt, and the melt is once held at a temperature and pressure conditions where foaming does not occur, and then extruded into a low pressure region. A method for producing a foamed molded article, characterized in that the obtained extruded melt in the foaming process is subjected to impact while the extruded melt has not yet completed foaming to complete foaming in a mold. . 2. The production method according to claim 1, wherein the thermoplastic resin is a polyolefin resin. 3. The manufacturing method according to claim 1, wherein the impact is applied by a striking plate or compression during mold clamping. 4. The manufacturing method according to claim 1 or 2, wherein the impact is performed by a sound wave or a compressed gas. 5. A high-density polyethylene is used as the thermoplastic resin, and 5 to 20 parts by weight of a volatile foaming agent is used with respect to 100 parts by weight of the high-density polyethylene, any of claims 1 to 4. That way.