JP2007130826A - Method for producing injection-foamed molded article - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an injection-foamed, weight-reduced article in which a surface condition is good and the thickness of a skin layer is reduced. <P>SOLUTION: The method for producing the injection-foamed article includes a process in which a foamable molten resin containing a thermoplastic resin and a chemical foaming agent is injected/packed in the cavity of a mold and a process in which while a heating gas heated at a temperature of the melting point of the thermoplastic resin ±50°C is packed in the cavity at a pressure of 0.1-5 MPa, the foamable molten resin is injected/packed in the cavity. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing an injection foam molded article.

  As a method of manufacturing an injection foam molded body, a mold connected to an injection port of an injection machine is composed of a fixed mold and a movable mold that moves forward and backward with respect to the fixed mold, and a cavity formed between them is movable. The mold can be enlarged and reduced by the advance and retreat of the mold, and the movable mold is advanced before the molten resin containing the foaming agent (hereinafter referred to as “foamable molten resin”) is injected into the mold cavity from the injection machine. And a molding method in which the movable mold is retracted substantially simultaneously with the injection of the foamable molten resin to enlarge the cavity to a predetermined size (see, for example, Patent Document 1), A mold that is movable relative to the shrinking direction is positioned at a predetermined cavity volume reduction position, and a mold is injected while injecting foamable molten resin into the cavity while maintaining a resin pressure that does not foam. Resin that starts foaming by moving the mold in the cavity volume expansion direction after compressing the resin by cooling the resin surface by solidifying the resin surface by moving the mold in the cavity volume reduction direction after expanding the volume There has already been proposed a molding method (see, for example, Patent Document 2) in which a molded body is taken out after the pressure is lowered to foam the internal resin and cooled.

  However, in the case of the former molding method, since the movable mold is moved in the expansion direction simultaneously with injection filling of the foamable molten resin, foaming starts with a decrease in the resin pressure, and the speed of foaming and mold movement cannot be balanced. In addition to the problems, there is a problem that a molded article having an excellent appearance cannot be obtained due to coarse bubbles or bursting of bubbles.

  On the other hand, in the case of the latter molding method, the cavity volume is increased while injecting the foamable molten resin into the cavity while maintaining the resin pressure so as not to foam with the movable mold positioned at the cavity volume reduction position. However, the expansion of the cavity volume has an element that induces a decrease in the resin pressure, and coarse bubbles and bubble bursts are likely to occur. Since there is a resin cooling step in the process of expanding or reducing the cavity volume, even if the mold temperature is high, a thick skin layer (solidified layer) is formed on the surface of the resin, and a molded article having an excellent appearance cannot be obtained. In addition, in these molding methods, the molten resin pressure around the gate becomes very high, and the resin around the gate is cooled and solidified in a high resin pressure state. Therefore, the expansion ratio around the gate is limited to about 1.01 to 1.2 times. On the other hand, in the part far from the gate, since it becomes equal to the atmospheric pressure, the molten resin foams sufficiently, and it is possible to obtain a foaming ratio of about 1.5 to 2.0 times. In addition, non-uniformity of the expansion ratio is likely to occur.

A molding method called a so-called gas counter pressure method has been proposed as a molding method that does not cause bubble breakage on the surface. This is because the mold cavity is filled with a gas having a pressure equal to or higher than the foaming pressure in advance, and the mold cavity is held with this gas, and the foaming agent is injected from the injection machine into the held mold cavity. The molten resin is injected and filled, and then the gas is extracted from the mold cavity.
That is, in the gas counter pressure method, immediately before adding a foaming agent to a resin and injecting it into the mold, the inside of the mold is pressurized with a gas higher than the foaming pressure, and then the resin containing the foaming agent is injected into the mold. Therefore, bubbles of foaming gas do not pop out at the front end of the resin flow, and the surface of the molded body is smooth, and after filling the molten resin containing the foaming agent, the pressurized gas in the mold is removed from the mold. Therefore, the foaming force corresponding to the volume shrinkage is generated inside the molded body, and there is an advantage that the surface of the molded body can be prevented from sinking.

  However, in this gas counter pressure method, when the gas is extracted from the cavity, the molten resin contacts the cavity wall surface and is cooled to form a thick skin layer on the surface of the molded body. There is a problem that can not be expected.

JP 62-246710 A JP-A-4-14311

  In view of the above circumstances, an object of the present invention is to provide a method for producing an injection foam molded article that can be formed into a lighter molded article having a good surface state and a thin skin layer. .

  In order to achieve the above object, a method for producing an injection foam molded body according to the present invention includes an injection foaming step including injection filling a foamable molten resin containing a thermoplastic resin and a chemical foaming agent into a cavity of a mold. A method for producing a molded article, wherein the foamable molten resin is placed in a cavity in a state where a heating gas heated to a melting point of ± 50 ° C. of the thermoplastic resin is filled in the cavity at a pressure of 0.1 MPa to 5 MPa. It is characterized by comprising a step of injection filling.

  Also, immediately after injection filling of the foamable molten resin into the cavity, the cavity is temporarily reduced to pressurize the foamable molten resin, and then the heated gas is discharged from the cavity and the cavity is expanded. Is preferred.

In the present invention, the thermoplastic resin is not particularly limited, and examples thereof include polyolefin resins such as polypropylene, polyethylene, polystyrene, and propylene / ethylene copolymers.
Although it does not specifically limit as a chemical foaming agent, Bicarbonate (inorganic compound), such as azodicarbonamide (organic compound) and sodium bicarbonate, etc. are mentioned.
The gas injected into the cavity as the heating gas is not particularly limited as long as it does not affect the resin, and examples thereof include inert gases such as carbon dioxide, nitrogen, argon, neon, and helium.

  In addition to the chemical foaming agent, the foamable molten resin of the present invention includes, as necessary, known foaming aids, foaming nucleating agents, foaming stabilizers, stabilizers, ultraviolet absorbers, antioxidants, Antistatic agents, lubricants, colorants, flame retardants, crosslinking agents and / or fillers can be blended.

Incidentally, as the foaming aid, for example, stearates such as sodium stearate, calcium stearate, magnesium stearate, potassium stearate, zinc stearate, montanic acid (which is octacosanoic acid) calcium, zinc montanate, etc. Higher fatty acid metal salts such as montanates, urea or urea compounds, paraffin, and other stearamides.
Examples of the foam nucleating agent include inorganic fillers such as talc, silica, calcium carbonate, and calcium silicate.

  In the present invention, “immediately after injection filling” means within 5 seconds from completion of injection filling, and preferably within 2 seconds.

  In the present invention, the temperature of the heating gas is limited to the melting point of the thermoplastic resin ± 50 ° C., because the heating gas temperature is less than the melting point of the thermoplastic resin −50 ° C. This is because there is no reduction effect, and when the heating gas temperature exceeds the melting point of the thermoplastic resin + 50 ° C., the appearance of the molded product is inferior, for example, the thermoplastic resin is yellowed by resin burn.

  Moreover, although the filling pressure of heating gas is limited to 0.1-5MP, it is because there is no gas filling effect if the filling pressure of heating gas is less than 0.1 MPa. Although there is no particular upper limit, it is necessary to increase the injection pressure of the foamable molten resin as the gas pressure increases. Therefore, it is preferable to be about 5 MP or less.

As described above, the method for producing an injection-foamed molded article according to the present invention is the above-described method in which the heating gas heated to the melting point ± 50 ° C. of the thermoplastic resin is filled in the cavity at a pressure of 0.1 MPa to 5 MPa. Since the foamable molten resin is injected and filled into the cavities, it is possible to obtain a molded article having a good surface condition and a thin skin layer and a lighter weight.
In other words, since the heated gas is filled, the compression and expansion of the foamable molten resin in the cavity suppresses the generation of coarse bubbles and the bursting of the bubbles, making the skin layer almost non-foamed and improving the appearance performance. In addition, since the surface layer of the expandable molten resin is kept at a high temperature until the completion of foaming by the heated gas, a thin skin layer is formed, and a lightweight molded body can be obtained.

  Further, immediately after injection filling of the foamable molten resin into the cavity, the cavity is once reduced and the foamable molten resin is pressurized, and then the heated gas is discharged from the cavity and the cavity is expanded. It becomes a fine and highly foamed layer, and the foaming ratio of the whole molded body is improved, and the appearance is also improved.

Hereinafter, the present invention will be described in detail with reference to the drawings showing embodiments thereof.
FIG. 1 shows one embodiment of a method for producing an injection foam molded body according to the present invention.

The injection foam molded product is manufactured using a mold 1 shown in FIG.
That is, the mold 1 includes a fixed mold 11 and a movable mold 12. The fixed mold 11 includes two movable blocks 14 provided to face each other in the cavity 13.
The fixed mold 11 is provided with a filling port (injection pin) 15 for the heated gas 3. The filling port 15 is connected to the heated gas supply path 2. The heated gas supply path 2 has one end connected to the filling port 15 and the other end connected to a heated gas supply source (not shown), and a remote control on-off valve 21 and a flow rate / speed control valve 22 on the way. I have.

And an injection-foaming molded object is manufactured as follows using this metal mold | die 1. FIG.
(1) As shown in FIG. 1A, the fixed mold 11 and the movable mold 12 are closed, and the movable block 14 provided in the fixed mold 11 is moved in a direction in which the internal volume of the cavity is reduced. In this state, the filling port 15 is opened, the remote control on-off valve 21 is opened, and the flow rate and speed of the heated gas 3 heated to the melting point of the thermoplastic resin ± 50 ° C. are controlled by the flow rate / speed control valve 22. However, the cavity 13 is filled with a gas pressure of 0.1 to 5 MPa.
(2) As shown in FIG. 1B, after the filling port 15 is closed in an airtight manner, the foamable molten resin 4 is filled into the cavity 13 from the injection machine 6 (only the nozzle portion is shown in the figure).
(3) As shown in FIG. 1 (c), immediately after the filling of the foamable molten resin 4, the movable mold 12 and the movable block 14 are once moved in the shrinking direction of the cavity 13 to expand the foamable melt to the pressure for suppressing the generation of foaming gas. Pressurize the resin. In addition, nucleation of fine bubbles can be performed in the foamable molten resin by this pressurization.
(4) As shown in FIG. 1 (d), the movable mold 12 and the movable block 14 are moved in the cavity expansion direction, and are expanded so as to coincide with the volume of the molded body to obtain the inner volume of the cavity 13. A foam molded body 5 is obtained. The heated gas is discharged from the mold parting surface to the outside of the cavity 13 before and after the enlargement of the cavity 13.
(5) After cooling the mold 1, the mold 1 is opened and the foamed molded body 5 molded is removed from the mold 1.
Next, specific examples of the present invention will be described in comparison with comparative examples.

Example 1
Using the mold 1 as shown in FIG. 1 in which the cavity 13 has a box shape to obtain a molded body having a length of 300 mm, a width of 200 mm, a height of 30 mm, and a thickness of 5 mm, and the maximum volume is 422.5 cm ³ , Thus, a box-shaped polypropylene foam molded article was obtained.
As shown in FIG. 1 (a), the temperature of the fixed mold 11 and the movable mold 12 is closed and the movable block 14 is moved in the shrinking direction to a 25% volume with a cavity inner volume of 422.5 cm ^3. The cavity 13 was filled with carbon dioxide gas heated to 180 ° C. as the heated gas 3 so that the pressure became 2 MPa from the filling port 15.

Next, as shown in FIG. 1 (b), after closing the filling port 15, polypropylene resin (MFR 30 g / 10 min, melting point 160 ° C., manufactured by Nippon Polypro Co., Ltd.) and bicarbonate, which is a chemical foaming agent, are used. A foamable molten resin obtained by melting and kneading sodium with 100 parts by weight of polypropylene resin at a blending ratio of 6 parts by weight of sodium bicarbonate with the injection machine 6 was injected and filled into the cavity 13 at an injection speed of 150 mm / sec. . At this time, the pressure in the cavity 13 (resin pressure) was 30 MPa.
Immediately after the completion of filling, as shown in FIG. 1 (c), the movable mold 12 and the movable block 14 are moved in the shrinking direction of the cavity 13 to compress the foamable molten resin 4, and the resin pressure in the cavity 13 is compressed. Was set to 105 MPa corresponding to 3.5 times the injection filling. Further, until the compression was completed, the temperature of the heated gas 3 was maintained at 160 ° C. or higher and the pressure was maintained at 2 MPa.
Next, as shown in FIG. 1 (d), the heated gas 3 is gradually exhausted from the cavity 13 through the mold parting surface before and after the expansion starts, and at the same time, the movable mold 12 and the movable block 14 are cavityd. The volume in the cavity was expanded to 422.5 cm ³ by moving in 13 expansion directions.

Then, after cooling the metal mold | die 1, the metal mold | die 1 was open | released and the foaming molding 5 was obtained.
When the cross section of the obtained molded body 5 was observed, the skin layer thickness was 0.1 mm. The bubble diameter in the vicinity of the skin layer is 4 to 10 microns and the average diameter is 8 microns. The bubble diameter in the foam layer was 12 to 40 microns and the average diameter was 25 microns. In addition, the foam of the molded body obtained by comparing the specific gravity of the molded body with the specific gravity of the unfoamed molded body (standard product) obtained by the method that does not reduce or enlarge the cavity in the same mold. When the magnification was determined, it was 4 times.

(Comparative Example 1)
A foamed molded article was obtained in the same manner as in Example 1 except that normal temperature carbon dioxide gas was used in place of the heated gas.
When the cross section of the obtained molded object was observed, the skin layer thickness was 0.35 mm. The bubble diameter in the vicinity of the skin layer is 25 to 45 microns and the average diameter is 35 microns. The bubble diameter in the foam layer was 50 to 90 microns and the average diameter was 80 microns. The expansion ratio was 3.6 times.

  Although the manufacturing method of this invention is not specifically limited, For example, it is suitable for manufacture of a motor vehicle interior material, household appliance parts, household goods, an industrial component, etc.

It is sectional drawing explaining the movement of the metal mold | die used for the manufacturing method of the injection foaming molding concerning this invention to process order.

Explanation of symbols

1 Mold 13 Cavity 3 Heated Gas 4 Expandable Molten Resin 5 Foam Molded Body

Claims (2)

A method for producing an injection foam molded article comprising a step of injection filling a foamable molten resin containing a thermoplastic resin and a chemical foaming agent into a cavity of a mold,
A step of injecting and filling the foamable molten resin into the cavity in a state in which the cavity is filled with a heating gas heated to the melting point ± 50 ° C. of the thermoplastic resin at a pressure of 0.1 to 5 MPa. A method for producing an injection foam molded article.   2. The method of claim 1, further comprising the step of temporarily reducing the cavity immediately after injection filling the foamable molten resin into the cavity and pressurizing the foamable molten resin, and then discharging the heated gas from the cavity and expanding the cavity. The manufacturing method of the injection-foaming molding as described in 2.

Images