JP3189619B2 - Injection molding method for foamable plastic composition - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to a method for injection molding a foamable polypropylene composition. More specifically, injection molding for producing a foamed polypropylene molded article that is lightweight, has excellent heat insulating properties, and is more rigid, in which the surface of the molded article is a layer having a dense structure with no foaming or low foaming and the inside is a highly foamed layer. It relates to a molding method.

[0002]

2. Description of the Related Art As a conventional method of producing a foamed plastic molded article, a thermoplastic composition containing a foaming agent is injected into a cavity having the same volume as a target molded article in an amount smaller than the volume of a cavity. A method is known in which the composition is filled into a cavity by utilizing an increase in volume due to foaming of the composition. However, in this method, it is difficult to completely fill the composition when the shape of the end portion of the target molded product is thin. Furthermore, even when the thickness of the target molded product is 4 mm or more,
The expansion ratio is limited to 1.2 to 1.5 times, and at such a ratio, there remains a problem in the heat insulating property and weight reduction of the obtained foamed plastic molded article.

[0003] As another method for producing a foamed plastic molded product, Japanese Patent Application Laid-Open No. 62-246710 discloses a fixed die and a movable die which moves forward and backward with respect to the fixed die. The cavity formed between them can be enlarged and reduced by moving the movable mold forward and backward, and when the foaming agent is injected into the mold from the injection molding machine, the movable mold is retracted to form the cavity. A method of injection molding a foamed resin that expands to a predetermined size is disclosed. Japanese Patent Application Laid-Open No. 4-214311 discloses a mold in which the molds that fit into each other and are relatively movable in the direction of expanding or reducing the cavity volume are positioned at a predetermined cavity volume reduction position, and foaming properties are formed in the cavity. The resin is compressed by moving the mold in the cavity volume expansion direction while injecting the molten resin while maintaining it at a resin pressure that does not foam, and then moving the mold in the cavity volume reduction direction to compress the resin. Injection compression molding to cool and solidify, then move the mold again in the cavity volume expansion direction to reduce the resin pressure to start foaming, foam the resin inside, and take out the molded product after cooling A law is disclosed.

However, the conventional method as described above has a problem that a molded article having an excellent appearance cannot be obtained. In the former method, as the cause, the volume of the cavity is enlarged at the same time as the injection, so that rupture of air bubbles occurs on the surface of the molded article, and a non-foamed or low-foamed dense foam structure cannot be obtained, When a mold having a cavity structure having a thin portion at a position away from the gate or the gate is used, it is considered that complete filling is difficult. Further, in the latter method, a mold having a cavity structure having a thin portion at a position distant from the gate (for example, a mold for a molded article having a large flat portion and having a rib, a boss, or the like) and a normal mold It is considered that when using the molding machine of the above, it is difficult to maintain the pressure at which the expandable plastic does not foam. Of course, if injection is performed at high speed and high pressure using a large molding machine, or a counter pressure method is used in combination, the pressure at which the expandable plastic does not expand can be maintained, but there remains a problem in economics. In addition, economical problems remain because two steps of reducing and expanding the cavity volume are required, and labor, cost and time are required to determine the cavity volume when reducing and expanding.

Further, according to the conventional method, in the case of a molded product having a large flat surface portion and a thin rib, wall surface, boss, or the like at the end portion, the resin cannot be sufficiently filled up to the end of the molded product (a so-called short circuit). Shot) occurrence of defective products,
In addition, there remains a problem that a molded product having a dense surface and excellent appearance cannot be obtained.

It is considered that the reason for this is that any of the above methods expands the cavity simultaneously with or during the injection, that is, the volume of the cavity to be filled increases while the resin is being supplied. . In particular, when the operation is performed in a cavity having a large flat portion, a situation in which the resin supply speed cannot follow the volume expansion speed of the cavity may sufficiently occur. In such a situation, it is very difficult to keep the pressure of the supplied resin in the cavity constant, and it is impossible to prevent the resin from starting to foam. Furthermore, in a cavity having such a shape that requires a certain or more resin supply pressure to fill up to the end, a short shot is inevitable. Also, even if cavity compression is performed again after the cavity is expanded, most of the resin foaming has already been completed at the same time, so the cavity compression process only compresses the bubble generated by foaming and ends the No fluid filling effect can be expected.

For the same reason as described above, a decrease in the resin pressure in the cavity promotes foaming during the resin supply, and the bubbles generated by the foaming are successively generated at the resin tip portion being supplied. Since it is destroyed, the trace is left on the surface of the molded product, and a molded product having a good appearance cannot be obtained. Further, in order to prevent the pressure drop, a compressed gas is sealed in a mold in advance and a so-called counter pressure method of keeping the pressure at the tip portion of the flowing resin constant is also used. In the relation to the tee volume expansion rate, a phenomenon in which the flow of the resin apparently stops or recedes in the plane of the cavity occurs, and the trace appears as a so-called hesitation mark on the surface of the molded product, significantly impairing the appearance become.

In order to eliminate the drawbacks of the conventional method, the shape of the molded product must be limited to a bar, a column, or a shape sufficiently thick in the thickness direction with respect to the projected area of the plane portion. That is, when obtaining a good molded product by a method including a step of enlarging the cavity at the same time as or during the injection, it is not possible to escape from the restriction that the shape of the molded product is limited to a narrow range.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object thereof is to provide a conventional injection molding machine using a mold having a cavity structure having a thin portion at a position distant from a gate. Injection molding that is economical to produce lightweight, heat-insulating, and rigid foamed polypropylene molded products with a layer with a dense structure with no foaming or low foaming on the surface and a high foaming layer inside. Is to provide a way.

[0010]

According to the present invention: a) Injecting a molten expandable polypropylene composition into a cavity having a volume of 48.7-95% of the volume of the desired molded article. B) a step of cooling after completion of the injection while maintaining the above-mentioned cavity volume until a solidified layer in contact with the entire surface of the mold on the cavity side and a molten layer inside the mixture are mixed; c) a desired molded article And d) removing the molded article after further cooling, and a method for injection-molding the expandable polypropylene composition.

[0011] The foamable polypropylene composition used in the present invention contains polypropylene and a foaming agent. Examples of the polypropylene include a propylene homopolymer and a propylene / ethylene copolymer. Examples of the foaming agent include inorganic compounds such as ammonium carbonate and sodium bicarbonate, and organic compounds such as azo compounds, sulfohydrazide compounds, nitroso compounds, and azide compounds. As the azo compound, azodicarbonamide (ADC)
A) 2,2-azoisobutyronitrile, azohexahydrobenzonitrile, azoaminobenzene and the like can be exemplified. As the sulfohydrazide compound, benzenesulfohydrazide, benzene-1,3-disulfohydrazide, diphenylsulfone- Examples include 3,3-disulfohydrazide and diphenyloxide-4,4-disulfohydrazide. Examples of the nitroso compound include N, N-dinitrosopentamethylenetetramine (DNPT) and N, N-dimethylterephthalate. Examples of the azide compound include terephthalazide and p-tert-butylbenzazide.

[0012] The expandable polypropylene composition of the present invention comprises:
For example, it can be prepared by blending a foaming agent in an amount of 0.5 to 5% by weight with respect to polypropylene and dry blending using a tumbler or the like.

The cavity having a volume of 48.7 to 95% of the volume of the target molded product specified in the present invention is, for example, divided by the expansion ratio of the target molded product by the expansion ratio of the target molded product. Can be expressed as For example, when the size of the target molded product is 200 mm × 200 mm × 4 mm and the expansion ratio is 2, the cavity having a volume of 50% of the volume of the target molded product is 200 mm × 20 mm.
It can be expressed as 0 mm x 2 mm.

When a cavity having an excessively small volume is used, the gradient of the pressure applied to the expandable polypropylene composition at the time of filling becomes large, and therefore, uniform filling cannot be performed, and a foamed polypropylene molded article having an excellent appearance cannot be obtained. . On the other hand, when a cavity having a volume greater than 95% is used, the pressure applied to the foamable polypropylene composition temporarily decreases during injection, foaming starts before the filling is completed, and the foamed polypropylene molding having an excellent appearance is obtained. Unsatisfactory products cannot be obtained.

The expansion of the cavity volume according to the present invention
For example, it can be performed by moving a mold by a mold clamping mechanism of an injection molding machine or by moving a slide core provided on the mold. Such a change in the cavity volume can be performed by manual operation, but it is desirable that the change can be controlled by a molding machine or a mold mechanism itself. That is, when the cavity volume is increased by moving the mold by the mold clamping mechanism of the injection molding machine, it is desirable to have a control function capable of arbitrarily moving and stopping the mold during the molding operation. When the cavity volume is increased by moving the slide core, it is desirable to have a function that can arbitrarily control the movement of the slide core.

The cooling of the solidified layer in contact with the mold surface and the internal molten layer according to the present invention is performed, for example, using a cooling mechanism of an injection mold, and the filling of the composition by injection is completed. Then, the volume of the target molded product is 48.7-9.
This can be done by holding the state of the cavity having a volume of 5% for 1 to 20 seconds.

The foamable polypropylene composition used in the present invention may optionally contain an inorganic filler such as an antioxidant, a weathering agent, an ultraviolet absorber, an antistatic agent, a colorant, an olefinic elastomer and talc. It can be blended within a range that does not impair the purpose.

(Function and Effect) In the method of the present invention, the state of the cavity is in the reduced position while the expandable polypropylene composition is injected into the cavity, that is, the cavity is reduced or enlarged during the injection. It is a feature that the operation of expanding and recooling the cavity is performed after the injection is completed without changing the volume of the cavity. Therefore, the molded product obtained by this method is a layer having a dense structure with no foaming or low foaming on the surface and a highly foamed layer inside, and is excellent in appearance, heat insulating properties and the like.

The reason why a layer having a dense structure with no foaming or low foaming on the surface of the molded article is obtained is that the volume of the cavity is 48.7 to the volume of the molded article up to the step b).
95%, that is, since the volume of the cavity is in the reduced position and the volume of the cavity is not expanded, the injection pressure and the injection holding pressure are transmitted to the end of the melted expandable polypropylene composition injected and filled in the cavity. It is considered that the molten expandable polypropylene composition in this state is in a non-foamed or low-foamed state.
In that state, there is a step of cooling to a state where the solidified layer in contact with the mold surface and the internal molten layer are mixed, so that the molten polypropylene composition in contact with the mold is cooled in a non-foamed or low foamed state. Therefore, it is considered that foaming does not occur even if the surface of the mold is transferred and then the cavity volume is enlarged, so that a molded product having a layer having a dense structure with no foaming or low foaming is formed. That is, it is considered that the appearance is excellent. In addition, since it is considered that the injection pressure and the injection holding pressure are transmitted to the end of the molten foamable polypropylene composition injected and filled in the cavity as described above, the cavity having a thin portion at a position away from the gate. Even when a mold having a structure is used, it is considered that the foamable polypropylene composition in a molten state can be filled even in a thin portion, and complete filling is easy. Of course, since the cooling step is performed until the solidified layer in contact with the mold surface and the internal molten layer coexist, the internal molten layer is considered to foam in the subsequent step c), and the above d) Since the inside is cooled and solidified in the process, a molded product having a highly foamed layer inside can be obtained.

[0020]

The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples. The compositions, injection molding machines and molds used in the following Examples and Comparative Examples are shown below.

[Composition] The following composition A, composition B and composition C were used. Composition A: Test condition 14 of JIS K7210 (230
° C; 21.18N).
g / 10 minutes, 97 parts by weight of a crystalline propylene homopolymer having a crystal melting point of 165 ° C. and azodicarbonamide (AD
A composition obtained by mixing 3 parts by weight of CA) and mixing and stirring with a tumbler mixer. Composition B: JIS K7210 test condition 14 consisting of 85% by weight of a propylene homopolymer and 15% by weight of a propylene / ethylene copolymer having an ethylene content of 60% by weight.
(230 ° C .; 21.18 N), 97 parts by weight of a crystalline propylene / ethylene block copolymer having a melt flow rate of 2.5 g / 10 minutes and a crystal melting point of 165 ° C., and 3 parts of azodicarbonamide (ADCA). A composition obtained by mixing parts by weight and mixing and stirring with a tumbler mixer. Composition C: JIS K7210 test condition 14 (230
° C; 21.18N).
g / 10 minutes, 97 parts by weight of a crystalline propylene homopolymer having a crystal melting point of 165 ° C., azodicarbonamide (ADC
A composition obtained by mixing 2% by weight of A) and 1 part by weight of ammonium carbonate, and mixing and stirring with a tumbler mixer.

[Injection Molding Machine] An injection molding machine having a cylinder having a screw diameter of 90 mm and a mold clamping control mechanism and having a maximum clamping force of 650 T was used. [Mold] 410 mm in length and 295 m in width shown in FIG. 1 described later.
m, having a cavity for obtaining a box-shaped molded product with a height of 50 mm, and a parting part of the mold having a fitting shape, the volume of which can be changed at the movable mold position. A mold was used.

The characteristics used in the following Examples and Comparative Examples were evaluated by the following methods. [Filled state] The molded products obtained in the examples and comparative examples were compared with the standard products shown below, and G was evaluated when the shape of the end portion where underfilling appeared was the same as the standard product, and N when different. did. Standard product: JIS K7210 test condition 14 (230
C .; 21.18 N) and a melt flow rate of 2.5
g / 10 minutes of polypropylene, using the molding machine and the mold used in the examples, the cylinder setting temperature of the injection molding machine was set to 210 ° C., the cooling water temperature of the mold was set to 50 ° C. Inject 702 g into a cavity with a thickness of 5.0 mm and a total volume of 780 cc.
After cooling for 2 seconds, the mold was opened and the molded product was taken out. This molded product was used as a standard product.

[Appearance] The molded products obtained in Examples and Comparative Examples were compared with the following standard products, and the evaluation was performed. N was given when it was possible. [Expansion Ratio] A test piece of 100 mm × 100 mm was cut out from the molded product obtained in each of the examples and comparative examples, and the specific gravity of each test piece was measured.

[Heat insulation property] A 100 mm x 100 mm test piece was cut out from the molded product obtained in each of the examples and comparative examples.
The thermal conductivity was measured according to TM D2320 (probe method) and used as an index of thermal insulation (unit: Kcal / m · Hr ·
° C). The smaller the value of the thermal conductivity, the better the heat insulation. [Rigidity] From the molded products obtained in Examples and Comparative Examples, 15
A test piece of mm × 119 mm was cut out and subjected to JIS K7203.
A bending test was performed in accordance with. The test result indicates the maximum bending load and was used as an index of rigidity.

Example 1 Using the composition A, the injection molding machine and the mold, the cylinder set temperature of the injection molding machine was set to 230 ° C., and the cooling water temperature of the mold was set to 70 ° C. Molding was performed. The thickness between the cavities (between the movable mold and the fixed mold) in contact with the top surface of the molded product (hereinafter abbreviated as the thickness of the top surface) 2.
Inject into a cavity of 2mm, volume 440cc and 3
The injection of 396 g of the composition was completed in seconds. After cooling for 10 seconds after the injection filling was completed, the expansion of the volume was started, and after 1.5 seconds from the start, the expansion of the volume was completed so that the cavity thickness of the top surface was 5.0 mm and the total volume was 780 cc. . After cooling for 100 seconds after the completion of the volume expansion, the mold was opened and the molded product was taken out. A molded product having a length of 410 mm, a width of 295 mm and a height of 52.8 mm (a volume of 780 cc) was obtained. The properties of the obtained molded product are shown in Table 1 below.

Example 2 Using the above composition A, an injection molding machine and a mold, the cylinder set temperature of the injection molding machine was set to 230 ° C., and the cooling water temperature of the mold was set to 70 ° C. Molding was performed in the same procedure as in Example 1. After cooling for 15 seconds after the injection filling was completed, the expansion of the volume was started, and after 1.5 seconds from the start, the expansion of the volume was completed so that the cavity thickness of the top surface portion was 5.0 mm and the total volume was 780 cc. . Length 410mm x width 295mm x height 52.
A molded product of 8 mm (volume: 780 cc) was obtained. Table 1 shows the properties of the obtained molded product.

Example 3 Using the composition A, the injection molding machine and the mold, the cylinder temperature of the injection molding machine was set at 230 ° C., and the cooling water temperature of the mold was set at 70 ° C. Molding was performed in the same procedure as in Example 1. Inject into a cavity with a cavity thickness of 1.7mm on the top and a total volume of 380cc,
Injection of 2 g of the composition was completed. 410mm in height X 295 in width
A molded product having a size of mm × 52.8 mm in height (a volume of 780 cc) was obtained. Table 1 shows the properties of the obtained molded product.

Example 4 Using the composition B, the injection molding machine and the mold, the cylinder set temperature of the injection molding machine was set to 230 ° C. and the cooling water temperature of the mold was set to 70 ° C. Molding was performed using the same procedure as described above. A molded product having a length of 410 mm, a width of 295 mm and a height of 52.8 mm (a volume of 780 cc) was obtained. Table 1 shows the properties of the obtained molded product.

Example 5 Using the composition C, the injection molding machine and the mold, the cylinder set temperature of the injection molding machine was set to 230 ° C. and the cooling water temperature of the mold was set to 70 ° C. Molding was performed using the same procedure as described above. A molded product having a length of 410 mm, a width of 295 mm and a height of 52.8 mm (a volume of 780 cc) was obtained. Table 1 shows the properties of the obtained molded product.

Comparative Example 1 Using the composition A, the injection molding machine and the mold, the cylinder temperature of the injection molding machine was set to 230 ° C. and the cooling water temperature of the mold was set to 70 ° C. Molding was performed. The thickness of the cavity at the top is 5.0mm and the total volume is 78
Inject into a 0cc cavity and 396g in 3 seconds
The injection of the composition was completed. After cooling for 120 seconds after the injection was completed, the mold was opened and the molded product was taken out. Height 410mm
A molded article having a width of 295 mm and a height of 51.3 mm (a volume of 665 cc) was obtained. Table 1 shows the properties of the obtained molded product.

Comparative Example 2 Using the composition A, the injection molding machine and the mold, the cylinder temperature of the injection molding machine was set to 230 ° C., and the cooling water temperature of the mold was set to 70 ° C. Molding was performed. Injection was performed into a cavity having a top surface of 2.2 mm in thickness and a volume of 440 cc, and the injection of 396 g of the composition was completed in 3 seconds. The expansion of the volume was started 2 seconds after the start of the injection, and the expansion was completed 1.5 seconds after the start so that the cavity thickness at the top surface was 5.0 mm and the total volume was 780 cc. After cooling for 100 seconds after the completion of the volume expansion, the mold was opened and the molded product was taken out. A molded product having a length of 410 mm, a width of 295 mm and a height of 52 mm (volume 720 cc) was obtained. The properties of the obtained molded product are shown in Table 1 below.

Comparative Example 3 Using the composition A, the injection molding machine and the mold, the cylinder setting temperature of the injection molding machine was set to 230 ° C., and the cooling water temperature of the mold was set to 70 ° C. Molding was performed. Injection was performed into a cavity having a top surface of 2.2 mm in thickness and a volume of 440 cc, and injection of 396 g of the injection composition was completed in 3 seconds. The expansion of the volume starts at the same time as the start of injection.
After 5 seconds, the volume expansion was completed so that the cavity thickness at the top surface was 5.0 mm and the total volume was 780 cc. After cooling for 100 seconds after the completion of the volume expansion, the mold was opened and the molded product was taken out. A molded product having a length of 410 mm, a width of 295 mm and a height of 52 mm (volume of 700 cc) was obtained. The properties of the obtained molded product are shown in Table 1 below.

As shown in Table 1, Examples 1, 2, 3, 4
The molded articles obtained in Nos. 5 and 5 were excellent in all of the filling state, heat insulation, rigidity and appearance. On the other hand, the molded products obtained in Comparative Examples 1, 2 and 3 were inferior in any of the filling state, heat insulation properties, rigidity and appearance, and Comparative Examples 1, 2 and 3 were inferior. Is not suitable for the present invention.

[0035]

According to the present invention, it is possible to mold with a normal injection molding machine using a mold having a cavity structure having a thin portion at a position distant from the gate, and furthermore, the surface of the molded product has no foam or It is possible to provide an economically excellent injection molding method for producing a foamed polypropylene molded article having a low-foaming layer having a dense structure and a high-foaming layer inside, and having light weight, excellent heat insulation properties, and rigidity. The molded article obtained by such a method is extremely useful for automobiles, home appliances, and general industrial parts.

[0036]

[Table 1]

[Brief description of the drawings]

FIG. 1 is a diagram of a molded product (surface) obtained in an example.

FIG. 2 is a view of a molded product (back surface) obtained in an example.

[Explanation of Signs] 1 Gate position 2 Top surface and a part to cut out a test piece used for testing rigidity and heat insulation

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B29C 45/00-45/84

Claims (5)

(57) [Claims] 1. a) injecting the molten foamable polypropylene composition into a cavity having a volume of 48.7 to 95% of the volume of the desired molded article; b) after the injection is completed, a mold Cooling while maintaining the above-mentioned cavity volume until a solidified layer in contact with the entire surface on the cavity side and a molten layer inside coexist; c) expanding the cavity volume to the volume of the target molded article And d) removing the molded article after further cooling, and a method for injection-molding the expandable polypropylene composition. 2. The method according to claim 1, wherein the expandable polypropylene composition comprises polypropylene and a blowing agent. 3. The method according to claim 1, wherein the foamable polypropylene composition further contains an antioxidant, a weathering agent, an ultraviolet absorber, an antistatic agent, a coloring agent, an olefin elastomer, an inorganic filler and the like. 4. A blowing agent of 0.5 to polypropylene.
2. The method according to claim 1, wherein the amount is from 5 to 5% by weight. 5. A foamed polypropylene molded article produced by the method according to claim 1.