JP2005125504A - Injection molding method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To easily obtain a good molded product free from appearance failure by extending the resin flowing length of the molten resin within a mold in an injection molding method for injecting the molten resin in a mold cavity especially through a film gate to fill the mold cavity. <P>SOLUTION: The mold cavity is preliminarily filled with a pressurized gas and the molten thermoplastic resin is subsequently injected so that the moving speed of a flow front becomes higher than a slide causing speed to generate a plug flow. <P>COPYRIGHT: (C)2005,JPO&amp;NCIPI

Description

  The present invention relates to a molding method for facilitating filling of a resin mold cavity in injection molding of a thermoplastic resin.

  In the injection molding of a thermoplastic resin, the resin is usually heated and melted to a temperature having sufficient fluidity to fill the mold cavity and molded. The flowability of the molten resin not only determines the ease of filling the mold cavity, but also determines whether sufficient pressure is transmitted to the resin in the cavity after filling, so only the dimensional accuracy and appearance of the molded product It is also an important factor that determines the processability of the resin by affecting the advanced transfer of fine information on the mold surface required for molded products such as optical disks. One index representing fluidity is the viscosity of the molten resin.

  A thermoplastic resin has a high melt viscosity and is inferior in fluidity as a molding material. For this reason, it is easy to cause appearance defects such as uneven gloss of the molded product, weld line, and fine shape transfer of the mold surface typified by optical disk pits, and thin parts cannot be completely filled with resin. There was a problem.

  Conventionally, there are the following three types of resin reforming means for improving fluidity.

  The first is to lower the molecular weight of the resin, which lowers the average molecular weight, broadens the molecular weight distribution, especially increases the low molecular weight components, but reduces the impact strength and chemical resistance, while increasing the fluidity. There is a problem.

  The second is a method of introducing a comonomer into a molecule, but there is a problem that the stiffness during heating is lowered.

  The third is a method of adding low molecular weight oily substances such as mineral oil and plasticizers such as higher fatty acid esters. The plasticizer reduces the stiffness during heating, or the plasticizer adheres to the mold during molding. There were problems such as fouling.

  Further, as a molding condition for improving fluidity, it is effective to increase the resin temperature and the mold temperature. However, the high resin temperature causes thermal decomposition of the resin itself and additives, and it tends to cause problems such as reduced strength of molded products, generation of foreign substances due to resin degradation products, mold contamination, and discoloration. When the value is increased, there is a problem that the cooling of the resin in the mold is delayed and the molding cycle time is increased.

  On the other hand, as shown in many documents such as Non-Patent Document 1 listed below, it is known that when carbon dioxide is absorbed into a resin, it acts as a plasticizer for the resin and lowers the glass transition temperature, For example, Patent Document 1 and Patent Document 2 disclose a method for improving the fluidity of a molten resin by including a gas such as carbon dioxide in a thermoplastic resin. This method using carbon dioxide has an advantage that carbon dioxide is released from the molded product after molding, so that physical properties are not impaired, and mold contamination and molding cycle delay do not occur.

J. et al. Appl. Polym. Sci. , Vol. 30, 2633 (1985) Japanese Patent Laid-Open No. 5-318541 Japanese Patent No. 32189797

  However, in the case of conventional injection molding using carbon dioxide, for example, in the case of molding a thin plate-shaped molded product, the fluidity of the molten resin is enhanced with carbon dioxide, but in the mold. If the resin flow length of the molten resin is not sufficiently obtained and a large amount of carbon dioxide is dissolved in the resin, there is a problem that appearance defects are likely to occur, such as it is difficult to prevent the occurrence of a foaming pattern generated on the surface of the molded product.

  The present invention has been made in view of the above problems, and in an injection molding method for injecting and filling molten resin into a mold cavity, the resin flow length of the molten resin in the mold is extended, and there is no appearance defect. An object of the present invention is to easily obtain a simple molded product.

  In order to solve the above-mentioned problems, the present inventors have studied, and in a specific fluid and resin combination, the fluid is present in the mold cavity in advance, and the fluid enters the interface between the resin and the mold surface during resin filling. When the resin flows in the mold with the fluid layer interposed between the resin and the mold surface, the resin flows while sliding on the mold surface, and the apparent flow resistance of the resin decreases, and the mold The present inventors have found that the resin flow length of the molten resin is extended and completed the present invention.

  That is, the first aspect of the present invention is an injection molding method in which a molten thermoplastic resin is injected and filled into a mold cavity, a pressurized gas is previously filled into the mold cavity, and then the molten thermoplastic resin is added to the flow front. The injection molding method is characterized in that the injection is performed so that the moving speed is equal to or higher than the slip generation speed, and the preferred embodiment includes that the pressurized gas is carbon dioxide.

  The second aspect of the present invention is an injection molding method in which a molten thermoplastic resin is injection-filled into a mold cavity. A lubricant is previously applied to the inner surface of the mold cavity, and then the molten thermoplastic resin is moved to the flow front. The injection molding method is characterized in that the injection is performed so that the speed is equal to or higher than the slip generation speed.

  Further, the first and second aspects of the present invention include that the movement speed of the flow front is 10 m / sec or more as a preferable mode.

  According to the present invention, the molten resin injected into the mold slides with respect to the mold surface, becomes a plug flow state, and the resin flow length is extended. Therefore, a good filling state is obtained, and there is no appearance defect. Can be easily obtained.

  The present invention will be described in further detail.

  Examples of the thermoplastic resin used in the injection molding method of the present invention include polyethylene, polypropylene, polyvinyl chloride, acrylic resin, styrene resin, polyethylene terephthalate, polybutylene terephthalate, polyarylate, polyphenylene ether, modified polyphenylene ether resin, One kind of wholly aromatic polyester (liquid crystal polymer), polyacetal, polycarbonate, polyetherimide, polyethersulfone, polyamide resin, polysulfone, polyetheretherketone, polyetherketone, or a blend of two or more of these, Furthermore, the compound which mix | blended various fillers with these can be mentioned.

  Styrenic resin here is a polymer blend obtained from homopolymers, copolymers, and these polymers and other resins using styrene as an essential raw material, and is preferably polystyrene or ABS resin. Polystyrene is styrene homopolymer or rubber-reinforced polystyrene in which rubber is distributed in the resin phase.

  The first injection molding method of the present invention is a method of injecting a molten resin after filling the mold cavity with the pressurized gas in advance, and the second is applying the lubricant to the inner surface of the mold cavity in advance. This is a method of injecting a molten resin after placing it.

  The gas and lubricant include a resin, a mold and a molding machine in addition to improving the slipperiness of the molten resin with respect to the mold surface by being interposed between the injected molten resin and the mold surface. It is preferable that the raw material is not deteriorated, that there is no danger to the molding environment, is inexpensive, and that it quickly volatilizes from the surface of the molded product after molding.

  As the gas filled in the mold cavity, for example, an inert gas such as carbon dioxide or nitrogen, or a saturated hydrocarbon having 1 to 5 carbon atoms or a gas such as chlorofluorocarbon in which part of the hydrogen is substituted with fluorine is used. These can be used in combination of two or more, but they are easy to dissolve in the molten resin and have a large plasticizing effect by being dissolved in the molten resin, improving the transferability to the molded product in the mold surface state. Since the effect is high, carbon dioxide is most preferable, and it is preferable to fill so that the carbon dioxide concentration in the mold cavity is 80% by weight or more.

  Gases of various temperatures can be used. Not only atmospheric temperature gas but also heated gas can be used well. In the case of a heated gas, a mixed gas of liquid vapor and carbon dioxide that easily dissolves carbon dioxide can be used favorably.

  Examples of the lubricant include water, alcohol, hydrocarbon-based, silicon-based, fluorine-based solvents, and liquids such as oil. These can be used alone or in combination of two or more. Moreover, this lubricant and the above pressurized gas can be used in combination.

  In the first injection molding method of the present invention, prior to the injection of the molten resin, the pressurizing gas is filled in the mold cavity in advance. That is, counter pressure molding using the pressurized gas is performed.

  The pressure of the pressurized gas in the mold cavity varies depending on the type of gas used and the resin, but is preferably 1 to 15 MPa. If the pressure of the pressurized gas in the mold cavity is too low, the moving speed of the flow front of the molten resin that flows in the mold cavity at the time of injection becomes higher than the slip generation speed described later, so that the injection is performed at a very high speed. It becomes necessary and the burden on facilities increases. In addition, when the gas pressure exceeds 15 MPa, the force for opening the mold cannot be ignored, and problems such as difficulty in sealing the mold cavity are likely to occur.

  In the second injection molding method of the present invention, a lubricant is previously applied to the mold surface prior to the injection of the molten resin.

  The amount of the lubricant applied varies depending on the type of lubricant and the resin used, but is preferably an amount sufficient to wet the mold surface. When the coating amount is too small, it becomes difficult to obtain the slip of the molten resin with respect to the mold surface, and it becomes difficult to make the flow front speed of the molten resin higher than the slip generation speed described later. Moreover, when it is set as an excessive application amount, the residual amount of the lubricant on the surface of the obtained molded product increases.

  In the present invention, after filling the pressurized gas or applying the lubricant, the molten resin is injected and filled into the mold cavity. At this time, the injection speed is set such that the moving speed of the flow front of the molten resin flowing in the mold cavity is equal to or higher than the slip generation speed. The slip generation speed is a speed at which the molten resin flows while sliding against the mold surface by entering a pressurized gas or a lubricant into the interface between the molten resin flowing into the mold cavity and the mold surface. This is the speed at which plug flow occurs.

  The injection speed at which the flow front speed of the molten resin becomes the slip generation speed differs depending on the resin used, the gas pressure in the mold cavity, the amount of lubricant applied, the structure of the mold, etc. When the flow length in the mold is measured by injecting molten resin at different injection speeds under pressurized gas filling conditions or under the same lubricant application conditions, this should be understood as a point where the flow length suddenly increases. Can do.

  When the mold cavity is preliminarily filled with the pressurized gas, it is preferable to release the pressurized gas in the mold to the outside of the mold during injection filling of the molten resin or immediately after completion of the injection filling to promote filling of the molten resin. Moreover, after injection-filling molten resin as mentioned above, the normal pressure-holding method of cooling and solidifying resin under high pressure can be used together. As a pressure holding method, a resin pressure holding method in which a molten resin replenishment pressure is applied to the mold cavity, a pressure fluid injection method in which a pressure fluid such as a gas is injected into the resin or the resin mold interface, and the mold cavity volume is reduced Examples include injection compression.

  In the present invention, a plasticizer such as carbon dioxide can be dissolved in the molten resin to be injected to improve fluidity. When injecting a molten resin in which carbon dioxide is dissolved as a plasticizer, it is preferable to add a counter pressure so as not to cause foaming at the flow front during injection filling, the method using the pressurized gas of the present invention Since it is also counter pressure molding, it is possible to suppress the foaming at the same time. In the case of a method using a lubricant, the foaming can be suppressed by simultaneously pressurizing the mold cavity with a pressurized gas.

  Examples of molded articles that are favorably molded according to the present invention include thermoplastic resin injection molded articles such as housings for optical equipment parts, weak electrical equipment, electronic equipment, office equipment, various automobile parts, and various daily necessities. Particularly preferred are thin parts that require a long flow length, such as connectors, housings, battery packs, memory packs, etc. used in electrical / electronic equipment and office equipment. In applications such as thin-walled housings for handy personal computers, it can be expected that molding will be easier, the quality of molded products will be improved, and the degree of freedom in product design will be increased.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

  First, materials and devices used in Examples and Comparative Examples will be described.

  Resin used for injection molding is liquid crystal polymer (“Siberus L204G35H” manufactured by Toray), rubber reinforced polystyrene (“PSJ polystyrene 433” manufactured by PS Japan), glass fiber reinforced polyamide resin (“Leona 1402G” manufactured by Asahi Kasei), polypropylene (Japan) Polychem "Novatec BC8") and before forming it is in the form of pellets.

  Nitrogen was used in addition to carbon dioxide having a purity of 99% or more as a gas previously filled in the mold cavity. Further, as a lubricant, a silicon release agent (“KF96SP” manufactured by Shin-Etsu Silicone) and a rust preventive oil (“CRC3-36” manufactured by Kure Industries) were used.

  SG125M-HP manufactured by Sumitomo Heavy Industries, Ltd. was used as the molding machine. The screw diameter is 32 mm.

  As the mold, a flow length measuring mold having a test part (mold cavity) having a thickness of 0.2 to 1.0 mm, a width of 5 mm, and a length of 400 mm was used.

  The mold has a mirror surface, the nozzle touch part diameter is 3.5 mm, the root part diameter is 6 mm, the sprue is 75 mm long, the thickness is 3 mm, the width is 4 mm, and the runner is 20 mm long. The structure is connected to the test section while gradually decreasing. At the end of the test section opposite the runner, a hole with a diameter of 1 mm for gas supply and release is connected to the gas supply device, and an O-ring is attached to the outer periphery of the test section for gas sealing. Provided, and the test section had an airtight structure.

As the gas supply device, a gas cylinder filled with liquefied carbon dioxide is kept at 35 ° C., and the gas supply source is passed through a heater and adjusted to a predetermined pressure by a pressure reducing valve. A gas reservoir capable of storing gas in a gas reservoir having an internal volume of 100 cm ³ and maintained at about 40 ° C. was used. The gas supply to the test unit is performed by opening the supply solenoid valve downstream of the gas reservoir and simultaneously closing the open solenoid valve that opens the inside of the test unit to the outside air. At the same time as the resin filling was completed, the supply solenoid valve was closed and the release solenoid valve was opened, so that the gas in the test part could be released out of the mold.

  The cylinder set temperature during injection molding was 310 ° C. for liquid crystal polymer (LCP), 290 ° C. for glass fiber reinforced polyamide resin (PA-GF), and 240 ° C. for rubber reinforced polystyrene (HIPS) and polypropylene (PP).

Example 1
A liquid crystal polymer dried at 140 ° C. for 5 hours in a hot air dryer and a mold having a mold surface temperature of 110 ° C. and a thickness of a test portion of 0.2 mm were filled in the mold with carbon dioxide at 8 MPa. The melted liquid crystal polymer was filled at a filling time of 0.026 seconds and an in-cylinder resin pressure of 200 MPa. The resin flow length at that time was 280 mm.

Comparative Example 1
A liquid crystal polymer was injection filled in the same manner as in Example 1 except that the mold was not filled with carbon dioxide. As a result, the resin flow length was 43 mm.

Example 2
Instead of filling with carbon dioxide, the liquid crystal polymer was injected and filled in the same manner as in Example 1 except that the mold was filled with nitrogen at 8 MPa. As a result, the resin flow length was 250 mm.

Example 3
Instead of filling with carbon dioxide, the liquid crystal polymer was injected and filled in the same manner as in Example 1 except that a silicon release agent was applied to such an extent that the mold surface was sufficiently wetted. As a result, the resin flow length was 180 mm.

Example 4
Instead of filling with carbon dioxide, liquid crystal polymer was injected and filled in the same manner as in Example 1 except that rust preventive oil was applied to such an extent that the mold surface was sufficiently wetted. As a result, the resin flow length was 200 mm.

  The resin flow lengths in Examples 1 to 4 and Comparative Example 1 are collectively shown in Table 1. The improvement rate in the table indicates the degree of improvement in the resin flow length, and the resin in the case where the injection of gas before injection or the application of the lubricant to the mold surface was not performed (Comparative Example 1 in Table 1). This indicates how many times the resin flow length is based on the flow length.

Experimental Example 1 and Comparative Example 2
A glass fiber reinforced polyamide resin and a mold having a mold surface temperature of 85 ° C. and a test section thickness of 0.5 mm were used. After filling the mold with carbon dioxide at 8 MPa, the resin filling time was 0.024 to 0.00. The melted glass fiber reinforced polyamide resin was filled at a resin pressure in the cylinder of 200 MPa in a range of 28 seconds (Experimental Example 1). Further, a molten glass fiber reinforced polyamide resin was injected and filled in the same manner except that carbon dioxide was not filled (Comparative Example 2). Each resin flow length is shown in Table 2 and FIG. The moving speed in the table is the moving speed of the resin flow front in the test section.

  As can be seen from Table 2 and FIG. 1, when the thickness of the test part is 0.5 mm, an improvement effect appears when the resin filling time is 0.077 seconds or less, and particularly when the resin filling time is 0.040 seconds or less. The effect of improvement is remarkable. In this case, it is considered that the slip generation speed exists between the resin filling time of 0.140 seconds and 0.077 seconds.

  Here, the flow when it is assumed that the screw of the injection molding machine moves at a constant speed during the filling time of -0.005 seconds (1/2 of the molding machine operation response time) and there is no resin compressibility. When the front moving speed is calculated from the screw moving time and the resin flow length, the slip generation speed is estimated to be about 10 m / sec.

Experimental Example 2 and Comparative Example 3
The resin flow length was measured in the same manner as in Experimental Example 1 and Comparative Example 2 except that a mold having a thickness of 1.0 mm was used and the resin filling time was changed in the range of 0.029 to 0.082 seconds. Each resin flow length is shown in Table 3 and FIG.

  As can be seen from Table 3 and FIG. 2, when the thickness of the test part is 1.5 mm, the improvement effect appears when the resin filling time is 0.060 seconds or less. In this case, the slip generation speed is estimated to be about 10 m / second, with the resin filling time existing between 0.082 seconds and 0.060 seconds.

Experimental Example 3 and Comparative Example 4
Using a glass fiber reinforced polyamide resin and a mold having a mold surface temperature of 85 ° C. and a test section thickness of 1.0 mm, filling the mold by changing from atmospheric air to 8 MPa carbon dioxide, and then filling each resin Was set to a constant value of approximately 0.030 seconds, and the melted glass fiber reinforced polyamide resin was filled at a resin pressure in the cylinder of 200 MPa. Each resin flow length is shown in Table 4 and FIG. Note that the CO ₂ pressure when the inside of the test section is atmospheric air (Comparative Example 4) is set to zero.

  As can be seen from Table 4 and FIG. 3, the slip generation speed varies depending on the carbon dioxide pressure in the test section, and in the case of the flow front moving speeds of this Experimental Example 3 and Comparative Example 4, slip occurs at a carbon dioxide pressure of 6 MPa or more. It is thought that.

Example 5 and Comparative Example 5
Using rubber reinforced polystyrene and a mold having a mold surface temperature of 60 ° C. and a test part thickness of 0.2 mm, the mold was filled with carbon dioxide at 8 MPa, then the resin filling time was 0.026 seconds, and the resin pressure in the cylinder was Filled with molten rubber reinforced polystyrene at 200 MPa (Example 5). The resin flow length was 90 mm. Further, a molten rubber reinforced polystyrene was injected and filled in the same manner except that carbon dioxide was not filled (Comparative Example 5). The resin flow length was 53 mm. Each result is shown in Table 5.

Comparative Examples 6 and 7
Using polypropylene and a mold having a mold surface temperature of 60 ° C. and a test section thickness of 0.2 mm, the mold was filled with carbon dioxide at 8 MPa, then the resin filling time was 0.026 seconds, and the resin pressure in the cylinder was 200 MPa. And filled with molten polypropylene (Comparative Example 6). The resin flow length was 42 mm. Further, polypropylene was injected and filled in the same manner except that carbon dioxide was not filled (Comparative Example 7). The resin flow length was 47 mm. Table 6 shows the results.

  In any case, it is considered that no slip occurred between the injected polypropylene and the mold.

Example 6 and Comparative Example 8
Without filling with carbon dioxide, after applying rust preventive oil to the mold surface, polypropylene was injected and filled in the same manner as in Comparative Example 6 except that the injection time was changed in the range of 0.033 to 2.4 seconds. did. Table 7 shows the resin flow length at that time.

6 is a graph showing the results of Experimental Example 1 and Comparative Example 2. 6 is a graph showing the results of Experimental Example 2 and Comparative Example 3. 6 is a graph showing the results of Experimental Example 3 and Comparative Example 4.

Claims (4)

In an injection molding method in which molten thermoplastic resin is injected and filled into a mold cavity, pressurized gas is prefilled into the mold cavity, and then the flow rate of the flow front of the molten thermoplastic resin exceeds the slip generation speed. An injection molding method characterized by injecting so that The injection molding method according to claim 1, wherein the pressurized gas is carbon dioxide. In an injection molding method in which a molten thermoplastic resin is injected and filled into a mold cavity, a lubricant is applied in advance to the inner surface of the mold cavity, and then the molten thermoplastic resin has a moving speed of the flow front higher than the slip generation speed. An injection molding method characterized in that the injection is performed. The injection molding method according to any one of claims 1 to 3, wherein a moving speed of the flow front is 10 m / sec or more.

Images