JP2002154130A - Polyacetal resin molding and method for injection molding the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable an application to a thicker molding having excellent dimensional stability by supplying the molding in which a fault generated mainly after molding such as a warpage or a sink mark is suppressed. SOLUTION: The molding of a polyacetal resin is obtained by dissolving or absorbing a carbon dioxide pressurized to 1 MPa or more in or to the polyacetal resin of a molten state and having a viscosity ηa, when a shearing speed γa is a range of 1.0×103 to 1.0×106, of >=30,000×γa-0.69 at a temperature higher by 15 deg.C than a melting point of the polyacetal resin, and then filling the resin in a mold cavity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molded article made of polyacetal (hereinafter abbreviated as "POM") resin and an injection molding method thereof.

[0002]

2. Description of the Related Art Conventionally, injection molded articles made of a crystalline resin such as POM have a large molding shrinkage, a large volume shrinkage due to crystallization, a low viscosity at the time of melting, and a low melting point as compared with an amorphous resin. The fluidity changes dramatically at the boundary, and the volume of the molded product gradually decreases with the progress of crystallization after molding. For this reason, there are many restrictions on product design, mold design, and molding conditions, and it is difficult to maintain dimensional accuracy at a high level.

For example, POM has a molding shrinkage of 1.8 to
This is a relatively large class of about 2.2% among thermoplastic resins, which is one of the reasons why it is difficult to maintain a high level of dimensional accuracy of a molded product by POM.
In addition, it can be said that there are many parts which are regarded as problems when applied to precision parts represented by gears, for example, such that the molding shrinkage ratio greatly varies depending on molding conditions, and it takes a long time until the dimensions are stabilized after molding. In the injection molding of POM, as a method of suppressing the molding shrinkage rate, a resin having a small molecular weight is used to improve flowability, increase the number of gates, shorten a flow distance to a flow terminal, a resin temperature or a metal. By taking measures such as increasing the mold temperature to decrease the speed at which the viscosity of the resin increases in the mold cavity, it is often observed that pressure is easily applied to the entire resin filled in the mold.

[0004] In general, a POM having a small molecular weight improves rigidity as compared with a crystalline resin having a large molecular weight.
In addition to a decrease in toughness and impact resistance, there is a tendency that the life for fatigue due to repeated loading and the like is shortened. In addition, increasing the number of gates means increasing the volume of the sprue and runner parts. As a result, in addition to increasing the parts that do not become products in each molding cycle, there are problems such as complicated mold structures. Therefore, it can be said that it is not preferable from the viewpoint of manufacturing. In addition, the method of increasing the resin temperature causes the decomposition of the resin to be promoted, and the method of increasing the mold temperature causes a problem that the molding cycle becomes longer.

[0005] On the other hand, in POM injection molding, as a method for ensuring dimensional stability after molding, a random copolymer is used. Methods such as reducing the volume, taking a long cooling time, taking out after sufficiently shrinking in the mold, and performing annealing treatment after molding to promote crystallization in a short time and reduce the volume are adopted. ing. POM random copolymers are
・ Compared to homopolymer, tensile elongation at break is large,
Although it is excellent in heat aging resistance and the like, it has a low rigidity, a low solidification rate, and a low heat deformation temperature.

[0006] The method of increasing the mold temperature and increasing the cooling time causes problems such as a long molding cycle.
Since the annealing treatment is a post-processing after the molding, the number of molding steps is increased, and when the annealing treatment itself varies, it is feared that it may cause a variation in the dimension of the molded product. On the other hand, when a thick molded product is obtained, the surface of the molded product is liable to sink due to volume shrinkage after molding. Therefore, the product is formed into a shape in which the back side of the design surface (the opposite side of the design surface) is hollowed out. In many cases, a design method such as reducing the average thickness of the sample was selected.

However, ribs and the like provided to compensate for the decrease in strength while reducing the thickness of the molded product cause linear sink marks on the design surface, which complicates the mold structure. Depending on the shrinkage of the resin and the magnitude and direction of the load applied to the molded product, stress concentrates on the rib itself, which may cause destruction, and there are cases where it is difficult to obtain a product having a desired shape. On the other hand, in the conventional injection molding method, the injection pressure and the filling pressure change in proportion to the melt viscosity of the resin used. A resin having a high melt viscosity requires a high injection pressure at the time of resin injection, which results in a large amount of distortion remaining in a molded product. The molding distortion remaining in the molded article is also called “residual distortion”. This residual strain is gradually relaxed after molding, but this is often due to deformation and shrinkage of the molded product. This is also seen when the mold structure, molding conditions, etc., are not appropriate.

The pressure applied to the resin filled in the mold cavity is preferably uniform, but the pressure distribution may be uneven near the gate and at the end of the flow. This means that it is difficult to transmit a sufficient pressure to the end portion of the flow, which causes poor appearance of the end portion of the flow, generation of voids, sink marks, expansion of molding shrinkage and unevenness. Therefore, it can be said that when filling the resin cavity into the mold cavity, it is preferable that an appropriate pressure that hardly causes residual strain to remain is transmitted uniformly to the entire cavity.

As an injection molding method for improving the dimensional accuracy with less distortion of the molded article, it is conceivable to increase the resin temperature during injection molding to lower the melt viscosity of the resin. Usually, the setting range of the resin temperature when molding the crystalline resin is narrower than that of the crystalline resin. Usually 5 to 3 from melting point
It is carried out in the range 0 ° C. higher, often in the range 10-20 ° C. above the melting point. This can be said to be a temperature range in which the viscosity of the resin is high up to a temperature range higher by about 5 ° C. than the melting point, so that the filling is difficult and the melting of the resin is not sufficient, and the melted portion and the unmelted portion are easily mixed. When the unmelted portion is mixed in the molded product, there is a concern that a problem such as a decrease in strength may occur.

On the other hand, the resin temperature during molding is 30 ° C. below the melting point.
In the above high temperature range, the decomposition of resin is promoted, and the appearance of the molded product surface is called silver (or “silver streak”),
Discoloration of the molded product itself may occur, and the generated decomposition gas may easily cause stains on the mold. This is not preferable because it may cause deterioration of the resin, may deteriorate the working environment, and may cause the mold to be disassembled and cleaned. Therefore, it can be said that there is a limit to the method of increasing the resin temperature in order to improve the fluidity of a crystalline resin having a high viscosity.

[0011] In addition, when the resin temperature is set high, the volume change of the resin itself during cooling and solidification becomes large, which is likely to cause sink marks, voids, etc., and it takes time to cool the resin. However, there is a concern that productivity may decrease. On the other hand, by increasing the mold temperature, a decrease in resin temperature and an increase in viscosity in the mold cavity can be suppressed. However, when the mold temperature is increased,
Since water cannot be used as a medium for controlling the temperature of the mold, water vapor or oil is often used, which makes the handling complicated and increases the cooling time of the resin filled in the mold. In addition, the molding cycle time is prolonged, and the size of the molded product at the time of removal is easily reduced.

In addition, in the case of injection molding in which the temperature of the mold is increased, if the cooling time is not sufficient and the cooling of the resin is insufficient, the temperature of the molded product at the time of removal is high. For this reason, after the molded article is taken out from the mold, there is a possibility that volume shrinkage or deformation due to its own weight may occur before the temperature of the molded article itself gradually decreases to the ambient temperature. This causes the dimensional accuracy to deteriorate, which is not preferable. Japanese Patent Application Laid-Open No. Sho 62-58287 discloses an injection molding method in which the surface temperature of the mold is extremely increased only when the resin is filled into the mold cavity.
No. "Injection molding method of rubber reinforced polystyrene resin",
Each is disclosed in JP-A-62-58288, "ABS resin injection molding method". Injection molding to obtain a crystalline resin molded product that has a smooth surface and good mold transferability by inserting an inductor between the molds and heating the mold surface with both molds open. Is the law.

In these injection molding methods, a step of inserting an inductor or a high-frequency induction heating coil between the molds during a molding cycle, heating the mold surface, and pulling out the inductor or the high-frequency induction heating coil from between the molds. is necessary. When this injection molding method is applied to a polyacetal resin, the mold temperature is set to a heat deformation temperature (P) under a load of 1.82 MPa.
About 110 ° C. for OM random copolymer, about 130 ° C. for POM homopolymer), preferably above the melting point (about 165 ° C. for POM random copolymer, POM
It is necessary to heat to about 175 ° C with a homopolymer).

In addition, since the molding cycle is extended due to the step of heating the mold surface, there is a problem in productivity, and in the case of high-frequency induction heating, the amount of electricity consumed is excessive, which is not preferable from the viewpoint of energy saving. . In addition, the injection molding method involving high-frequency induction heating is not preferable because it is limited to a molded product having a relatively flat shape because it limits the degree of freedom in product design of the molded product. On the other hand, new molding methods such as a high-speed injection molding method and a gas assist molding method have been proposed as injection molding methods for crystalline resins with improved dimensional accuracy and dimensional stability.

In the high-speed injection molding method, by injecting a crystalline resin at a high speed, the melt viscosity is prevented from rising due to cooling from a mold, the melt viscosity is reduced by a high shear force, and the pressure difference in the cavity is reduced. Has the effect of doing Further, the effect of reducing the injection time is obtained, and the productivity is also improved. However, it is necessary to pay attention to the deterioration of the resin due to the heat generated by shearing, the generation of burrs due to high-speed injection, and the occurrence of resin burn due to adiabatic compression in the gas reservoir at the end of the mold cavity.

The gas-assist molding method generally forms a hollow portion in a molded product by injecting a compressed gas into a resin. The compressed gas has a pressure-holding effect from the inside of the molded article, and has the effect of suppressing the occurrence of sink marks on the molded article. The pressure-holding effect of the compressed gas is lower than the pressure-holding effect in the ordinary injection molding method, and the pressure-holding effect can be expected up to the end of the flow. For this reason, the residual distortion is small, the deformation of the molded product such as warpage can be reduced, and the dimensional accuracy can be expected to be improved. However, depending on the shape of the molded product, there are cases where the installation location of the gas injection port is limited, and the effect may not be sufficiently exhibited.

On the other hand, J.I. Appl. Polym. Sc
i. , Vol. 30, 2633 (1985), it is known that when carbon dioxide is absorbed by a resin, it acts as a plasticizer for the resin and lowers the glass transition temperature. It has not been widely applied to processing. Japanese Unexamined Patent Publication No. Hei 5-318541 discloses a method in which a gas such as carbon dioxide or nitrogen is contained in a thermoplastic resin, and the resin is filled in the cavity while removing the gas in the cavity. To obtain a molded article without deterioration in strength and appearance. However, in this method, when carbon dioxide is used as the gas, the amount of the gas contained in the resin is small at a maximum of about 0.18% by weight, and it is difficult to obtain a sufficient effect of improving the fluidity. It can be said that it is difficult to obtain accuracy and dimensional stability.

Further, WO 98/52734 discloses that
In injection molding of thermoplastic resin,
Shows a method of filling molten resin whose viscosity has been reduced by dissolving more than 1% by weight into a mold cavity that has been kept pressurized with a gas such as carbon dioxide at a pressure higher than the pressure at which foaming does not occur at the flow front of the molten resin. The mold surface reproducibility, glossiness, and weld lines are less noticeable.
It is described that it is effective for the reproducibility of the sharp edge of the mold surface, the reproducibility of the unevenness of the fine mold surface, and the like. After filling the cavity, there is a step of holding the resin under pressure. The pressure at this time (hereinafter referred to as “holding pressure”)
Is in the range of 89-93% of the filling pressure. However, the holding pressure equivalent to 89% to 93% of the filling pressure may cause burrs and hardly form a foamed portion inside the molded product, and may cause defects such as sink marks and warpage which mainly occur after molding. Difficult to solve.

[0019]

DISCLOSURE OF THE INVENTION The present invention has a foamed portion inside and uses the apparent specific gravity of a molded product without limiting the resin composition of a polyacetal resin and without impairing the degree of freedom in product design. An object of the present invention is to improve the precision of a molded product required for a crystalline resin by adjusting the specific gravity of the POM-based resin to be smaller than the specific gravity of the POM-based resin. Specifically, the present invention provides a molded product that can be easily filled into a mold cavity, has excellent dimensional stability of the molded product, and suppresses the occurrence of defects such as warpage and sink that occur mainly after molding. In particular, it is to enable application to a thick molded product.

[0020]

Means for Solving the Problems The present inventors have found that a molded product has a foamed portion and an apparent specific gravity of the molded product without limiting the resin composition of the polyacetal resin and without impairing the degree of freedom in product design. An object of the present invention is to improve the precision of a molded product required for a crystalline resin by adjusting the specific gravity of the POM resin to be smaller than the specific gravity of the POM resin used. Specifically, the present inventors have found that mold cavities are easy to fill, are excellent in long-term dimensional accuracy and dimensional stability of molded products, and are mainly generated after molding such as warpage and sink marks generated in molded products. In order to obtain a molded product in which the occurrence of defects is suppressed, and to make it possible to apply it to a thick molded product, we have studied.

As a result, when the polyacetal resin has a shear rate γa of 1.0 × 10 ³ to 1.0 × 10 ^{6 at a} temperature 15 ° C. higher than the melting point of the polyacetal resin.
Is within the range, ηa ≧ 30000 × γ
a-a polyacetal resin in the range of ^0.69 , characterized in that it is obtained by dissolving or absorbing carbon dioxide in the polyacetal resin, and then obtained by filling the mold cavity, It has been found that it improves long-term dimensional accuracy and dimensional stability of molded products, facilitates filling into mold cavities, and solves problems that mainly occur after molding, such as warpage and sink marks on molded products. The invention has been completed.

That is, the present invention provides: When the polyacetal-based resin has a shear rate γa of 1.0 × 10 ³ to 1.0 at a temperature 15 ° C. higher than the melting point of the polyacetal-based resin.
When the viscosity ηa is in the range of 0 × 10 ⁶ , ηa ≧ 30
A polyacetal resin in the range of 000 × γa ^−0.69 , and 1M in the molten polyacetal resin.
1. A polyacetal-based resin molded product obtained by dissolving or absorbing carbon dioxide pressurized to Pa or more and filling the mold cavity. (1) The molded article made of a polyacetal resin has a foamed portion inside and a non-foamed layer which is not substantially foamed on a surface layer of the molded article.
Polyacetal resin molded article according to,

3. 3. The molded article of the polyacetal resin according to the above 1 or 2, wherein the molded article made of the polyacetal resin has a non-foamed layer having a thickness of 500 μm or more on the surface layer of the molded article. 4. The injection molding method for a polyacetal-based resin molded product according to claim 1, wherein carbon dioxide is dissolved or absorbed in the polyacetal-based resin in a molten state and then filled into a mold cavity. The polyacetal resin in the molten state has a
After dissolving or absorbing 2% by weight or more of carbon dioxide,
The injection molding method for a polyacetal resin molded article according to the above item 4, wherein the injection molding is performed by filling the mold into a mold cavity.

6. The polyacetal-based resin molding according to the above item 4 or 5, wherein the polyacetal-based resin is obtained by filling the molten polyacetal-based resin into a mold cavity adjusted or maintained at a pressure higher than the atmospheric pressure by a pressurized gas. 6. injection molding method of the product By a pressure that is 30% or more of the filling pressure, a certain time,
7. The injection molding method for a polyacetal-based resin molded product according to any one of the above items 4 to 6, further comprising a step of holding under pressure. (4) The method according to (4), further comprising a step of maintaining the pressure under a pressure that is 100% or less of the filling pressure for a predetermined time.
7. A method for injection-molding a polyacetal-based resin molded product according to any one of the above items.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. In the present invention, polyacetal (hereinafter referred to as “PO
M) is not limited, and the POM homopolymer represented by the following chemical formula 1 may be used.
A crystalline resin having a molecular structure called a POM / random copolymer represented by the following chemical formula 2 is preferably used.

The terminal group of the POM molecule is composed of a POM chemically bonded with other components such as a lubricating polymer and silicon.
It may be an M block copolymer. — (CH ₂ —O) _n — (Formula 1) — (CH ₂ —O) _n1 —X _n2 — (CH ₂ —O) _n3 —X _n4 — (Formula 2) (where X is a monomer component) Further, the present invention can be practiced even with a mixture of these POM / polymers or a mixture of POM / polymers having different molecular weights and molecular weight distributions.

The POM-based resin in the present invention may be a polymer alloy obtained by mixing one or more resins having the above-described POM components as main components and having different characteristics. There is no particular limitation on the resins having different properties that can be used by being mixed with the POM resin as the main component, as long as they are compatible with the POM resin as the main component.
For example, polyamide, polyethylene terephthalate, polybutylene terephthalate, various polyethylenes, polyetheretherketone, polypropylene, polystyrene, ABS resin, polyvinyl chloride, polycarbonate,
Modified polyphenylene ether, polyphenylene sulfide, polyimide, polyamideimide, polyetherimide, polyarylate, polysulfone, polyethersulfone, polyetheretherketone, liquid crystal polymer, polytetrafluoroethylene, thermoplastic elastomer, polytetrafluoroethylene, polyvinyl Alcohol and the like can be mentioned.

An inorganic or organic filler can be added to the POM resin used in the present invention for the purpose of imparting specific gravity and strength. Examples of the specific gravity imparting agent include inorganic salts, oxides, and metal powders such as barium sulfate, red iron oxide, and tungsten powder. Further, as a strength imparting agent, glass fiber, carbon fiber, metal fiber, aramid fiber, potassium titanate, asbestos, silicon carbide, ceramic, silicon nitride, barium sulfate, calcium sulfate, kaolin, clay, pyrophyllite, bentonite, Sericite, zeolite, mica, mica, nepheline sinite, talc, atalpargite, wollastonite, slag fiber, ferrite, silicon, calcium, calcium carbonate, magnesium carbonate, dolomite, zinc oxide, gypsum, glass beads,
Glass powder, glass balloon, quartz, quartz glass,
Alumina and the like can be considered.

The shape of these inorganic or organic fillers is not limited, and fibrous, plate-like, spherical and the like can be arbitrarily selected. In addition, two or more of the above-mentioned inorganic or organic fillers can be used in combination. Further, if necessary, it can be used after being pre-treated with a silane-based or titanium-based coupling agent. The amount of the inorganic or organic filler added to the POM-based resin of the present invention is not limited, but the specific gravity of the POM-based resin is adjusted, rigidity is improved, and dimensional accuracy is secured. In order to sufficiently obtain the effect of adding an additive, such as suppressing deformation such as warpage, the amount is preferably 5% by weight or more, more preferably 10% by weight or more. When the amount is less than 5% by weight, the effect of adding the above-mentioned filler is small.

Here, the added amount of the filler means a ratio when the total amount of the POM-based resin is 100% by weight, and when two or more fillers are used, the total added amount. In the present invention, the amount of the inorganic or organic filler added,
When one kind of inorganic filler is added, it indicates the amount added, and when two or more kinds are added, it indicates the total added amount thereof.
Further, the amount of the inorganic or organic filler added indicates a ratio when the total amount of the resin component, the inorganic or organic filler, and other additives is 100% by weight.

The POM-based resin in the present invention may contain additives usually used, for example, antioxidants, flame retardants, release agents, lubricants, heat stabilizers, weather resistance stabilizers, rust inhibitors, fillers. , A coloring agent, an antibacterial agent, a fungicide, etc.
More than one kind can be added. Further, by selecting one or more of carbon fiber, metal fiber, and graphite as other additives, the electric resistance value of the crystalline resin can be reduced. This is because small powder such as dust
This is preferable because it can prevent the molded article made of the resin from adhering to the molded article due to static electricity.

In the present invention, the POM resin has a shear rate γa of 1.0 × 10 ³ to 1.0 × 10 ^{6 at a} temperature 15 ° C. higher than the melting point of the POM resin used.
When in the range, the viscosity ηa of the POM resin is
It is characterized by being within the range shown in the following equation 1. ηa ≧ 30000 × γa ^−0.69 (Equation 1) Here, the melting point of the POM-based resin is a temperature at which POM in a solid state under a constant pressure keeps a balance with a liquid phase, and has a melting point. It is also called.

In general, the melting point of POM is about 175 ° C. for POM homopolymer, and about 165 ° C. for POM random copolymer, although it fluctuates slightly depending on the amount of comonomer inserted into the polymer. As a method of measuring the melting point of the crystalline resin in detail, for example, differential scanning calorimetry (DSC:
Differential Scanning Cal
orimetry). In the present invention, the melting point of the POM-based resin is measured by the following procedure using a differential scanning calorimeter ("DSC-7" manufactured by PerkinElmer). (1) Hold about 5 mg of a sample at 50 ° C. for 2 minutes. (2) After the temperature is raised to 200 ° C. at a rate of 200 ° C./min, the temperature is maintained at 200 ° C. for 2 minutes. (3) After the temperature is lowered to 50 ° C. at a rate of 10 ° C./10 min, the temperature is reduced to 50 ° C. for 2 minutes. Hold. (4) The temperature is raised to 200 ° C. at a rate of 10 ° C./min. (5) The temperature of the top peak of the endothermic peak appearing at the time of the temperature rise in (4) is defined as the melting point.

In the present invention, the measurement of the viscosity ηa of the POM resin was carried out according to the following procedure using a viscosity meter shown in FIG. (1) The temperature of the heating cylinder and the plunger of the viscometer is set and maintained at a temperature 15 ° C. higher than the melting point of the POM resin to be measured. (2) An appropriate amount of the molten POM-based resin is measured on a plunger portion, and the POM is injected at an injection speed Sa appropriately set.
Inject through the orifice at (mm / sec). At this time, the resin pressure Pa (Pa) is measured by a pressure sensor. The length La of the orifice used here is 10 mm, and the hole diameter Da of the orifice is 1.0 mm.

(3) From equations 2 and 3 shown below, τ
a and γa are calculated. Shear stress τa (Pa) = (Pa × Da) ÷ (4 × La) (Formula 2) Shear rate γa (/ sec) = (4 × Qa) ÷ (π × Da ³ ÷ 8) (Formula 3) , Qa (mm ³ / sec) = (π × Da ² ) ÷ 4 ×
Sa. (4) Calculate the viscosity ηa from Equation 4 shown below. Viscosity ηa (Pa · sec) = shear stress ÷ shear rate = τa ÷ γa (Equation 4) (5) The shear rate γa (/ sec) is 1.0 × 10
An τa value of any number, preferably 5 or more, is measured in the range of ³ to 1.0 × 10 ⁶ .

By calculating the ηa value at each shear rate and drawing the calibration curve calculated from each ηa value distribution or each ηa value, the following equation (5) is obtained.
Are determined, and the shear rate dependence of the viscosity ηa of the POM resin is determined. ηa1 = A × γa1- ^B (Formula 5)
At a temperature 15 ° C. higher than the melting point of the base resin, the viscosity ηa (Pa · s) when the shear rate γa (/ sec) is in the range of 1.0 × 10 ³ to 1.0 × 10 ⁶ is ηa ≧ 300.
00 × .gamma.a ^-0. After dissolving or absorb carbon dioxide in the polyacetal resin in the range of ^69, characterized in that it is obtained by filling the mold cavity.

On the other hand, at a temperature 15 ° C. higher than the melting point of the POM resin, the shear rate γa (/ sec) is 1.0 × 1
0 viscosity when ³ from a range of 1.0 × 10 ⁶ ηa (P
The POM resin in which a · s) is in the range of ηa <30000 × γa− ^0.69 means that the average molecular weight is small. A POM-based resin having a small molecular weight can be converted to a POM resin having a large
Compared with the OM-based resin, the rigidity is improved, but the toughness and the impact resistance are decreased, and the life against fatigue due to a repeated load or the like tends to be shortened.

In the present invention, a method of dissolving or absorbing carbon dioxide in a POM resin is characterized in that carbon dioxide pressurized to 1 MPa or more is kneaded into a molten POM resin. By dissolving or absorbing carbon dioxide pressurized to 1 MPa or more in the POM-based resin, the fluidity when filling the POM-based resin into the mold cavity is improved. When carbon dioxide is dissolved or absorbed into carbon dioxide, it is presumed that carbon dioxide is efficiently dispersed as a plasticizer. As a result, there is no need to raise the resin temperature, so there is no need to worry about thermal decomposition and deterioration of the resin, and the mold temperature does not need to be raised more than necessary.

Further, since the filling pressure at the time of filling the mold cavity with the POM-based resin is reduced, deformation of a molded product such as warpage after molding is smaller than that of a conventional molding method. This is considered to be due to the fact that the filling pressure during filling into the mold cavity is lower than that of the conventional molding method, so that residual strain hardly remains in the molded product. This facilitates injection molding with POM resin, which has high viscosity when molten, improves the quality of molded products, increases the degree of freedom in product design, and can be realized to date due to the high viscosity when molten. The realization of an injection-molded article using the missing resin composition can be expected.

The method of dissolving or absorbing carbon dioxide pressurized to 1 MPa or more in the POM resin in a molten state is not limited.
Easy to disperse uniformly in resin, easy to dissolve or absorb in a short time, easy to adjust absorption or dissolution amount, easy setup before molding, pressure-resistant structure of molding machine hopper, etc. Considering that it is not necessary to consider the inside of the heating cylinder of the injection molding machine, the nozzle of the molding machine,
A facility for supplying carbon dioxide is provided at any position between the nozzle of the molding machine and the mold, and the POM in a molten state is provided.
A method of dissolving or absorbing carbon dioxide in the system resin is preferable.

A method of dissolving or absorbing carbon dioxide in a molten POM-based resin in a heating cylinder of an injection molding machine, at a nozzle of the molding machine, or at any position between the nozzle of the molding machine and a mold. As a method, a method in which carbon dioxide pressurized to 1 MPa or more is supplied to a POM resin in a molten state from an intermediate portion or a tip portion of a screw of a molding machine or a heating cylinder is considered. When supplying carbon dioxide from the screw of the molding machine or the middle of the heating cylinder, increase the depth of the screw groove near the carbon dioxide supply, as in the vent of a vent-type screw, to increase the heating cylinder. It is preferable that the resin pressure in the inside is lowered and the resin transfer is starved. In order to uniformly dissolve, absorb and disperse carbon dioxide in the POM resin after supplying carbon dioxide, it is conceivable to provide a mixing mechanism such as a dalmage or kneading pin for the screw, and to provide a static mixer in the resin flow path. Can be

As a method of dissolving or absorbing carbon dioxide in a POM resin, a method is known in which a resin pellet is granulated in a state where carbon dioxide is mixed with a POM resin in a molten state in advance and injection molding is performed using the pellet. Although it is conceivable, carbon dioxide is gradually released from the pellets after granulation, and the amount of carbon dioxide in the resin may be unstable, which is not preferable. In addition, a method of absorbing carbon dioxide into resin pellets in a closed container such as a hopper of a molding machine in advance is conceivable, but it is not easy to adjust the amount of absorption or dissolution, and the setup before molding is complicated. It is not preferable because the hopper of the molding machine needs to have a pressure-resistant structure.

In the present invention, a molded article made of a POM-based resin is characterized in that it has a foamed portion inside.
It refers to a portion where a void due to foaming or a whitening phenomenon is observed when the image is magnified and observed 20 times. Since the molded product made of the POM-based resin in the present invention has a foamed portion inside, the thickness of the resin portion is substantially reduced with respect to the product thickness, and the volume shrinkage is reduced. It is considered that long-term dimensional accuracy and dimensional stability are excellent.

Further, the volume shrinkage of the POM-based resin is supplemented from the inside of the molded article by forming a foamed portion inside the molded article, so that the occurrence of sink marks on the surface of the molded article is suppressed. It seems to have been. In the present invention, P
The molded product made of the OM-based resin has a foamed portion inside, and a non-foamed layer having a thickness of 500 μm or more on the surface layer of the molded product. If the thickness is less than 500 μm, the surface of the molded article may be swollen, and the mechanical strength may be lowered.

The thickness of the non-foamed layer can be adjusted by the holding pressure and the holding time. The higher the holding pressure and the longer the holding time, the thicker the non-foamed layer tends to be. But,
If the holding pressure is too high or if the holding time is too long,
When the POM-based resin is cooled and solidified in the mold cavity, the carbon dioxide dissolved in the POM-based resin hardly forms a foamed portion inside the molded product, and may cause sink on the surface of the molded product. It is not preferable because there is.

By dissolving or absorbing carbon dioxide in the POM-based resin and filling it in the mold cavity, the POM-based resin is cooled and solidified in the mold cavity, causing volume shrinkage. This is because carbon dioxide dissolved or absorbed in the POM-based resin is formed by appropriately foaming. By having a foamed portion inside a molded article made of a POM-based resin, it is possible to apply the molded article made of a thermoplastic resin composition to a thicker molded article, and it is expected that the degree of freedom in product design is increased. .

The amount of carbon dioxide dissolved or absorbed in the POM resin is not limited.
By dissolving or absorbing carbon dioxide in the POM-based resin, the fluidity when filling the POM-based resin into the mold cavity is improved, and an increase in the filling pressure can be suppressed. Is preferably 0.2% by weight or more, more preferably 0.4% by weight or more. If the amount of carbon dioxide dissolved or absorbed is less than 0.2% by weight, it is difficult to obtain the effect of improving the fluidity by dissolving or absorbing carbon dioxide, and sufficient dimensional accuracy and dimensional stability are obtained. It is not preferable because it becomes difficult to obtain.

In the present invention, the amount of carbon dioxide dissolved or absorbed is measured by the following method. (1) Immediately after molding, measure the weight of the molded article (M1). (2) The molded article is left in a hot-air dryer kept at 100 ° C. for 48 hours or more to allow carbon dioxide to diffuse, and then the weight of the molded article taken out of the hot-air dryer is measured (M2). (3) The amount of carbon dioxide dissolved or absorbed (% by weight)
It is calculated from (M1−M2) ÷ M2 × 100.

In general, the injection molding method has a step of filling the resin into the mold cavity and then holding the resin in the cavity under pressure. This step is called a “holding step”, and the degree of the pressure is called “holding pressure”. In the injection molding method of the POM resin according to the present invention, after filling the POM resin into the mold cavity, the filling pressure is reduced. It is preferable that the resin in the mold cavity is held under pressure by a pressure corresponding to 30 to 100%. In the present invention, when the holding pressure is less than 30% of the filling pressure, the thickness of the non-foamed layer formed on the surface layer of the molded product becomes thin, and the proportion of the foamed portion in the arbitrary cross section increases, so that the mechanical strength is increased. Is likely to decrease.

On the other hand, if the holding pressure exceeds 100% of the filling pressure, burrs may be generated, and a foamed portion may not be easily formed inside the molded product, and sink and warpage may easily occur after molding. 0% by weight or more of carbon dioxide.
Injection molded products of crystalline resin dissolved or absorbed by 2% by weight or more form a non-foamed layer with an appropriate thickness on the surface layer of the molded product and have an appropriate foamed part inside the molded product The preferable range of the holding pressure in the injection molding process is a range of 30 to 100%, more preferably a range of 30 to 90% with respect to the filling pressure, and most preferably, 30 to 85%.

Here, the filling pressure refers to a resin pressure generated when the molten resin is charged into the mold cavity. Specifically, the screw pressure in the in-line type injection molding machine and the plunger position in the pre-plasticization type injection molding machine indicate the resin pressure generated when moving from the measurement position to the PV switching position. Also, the pressure holding time is not limited, but when the pressure holding time is extremely short, the carbon dioxide dissolved or absorbed in the crystalline resin before filling into the mold cavity expands, It is not preferable because a swelling phenomenon may occur in the molded product.

Specifically, the dwell time is preferably 3 seconds or more, more preferably 5 seconds or more,
Most preferably, it is at least 7 seconds. The molded article made of the POM-based resin of the present invention is a molded article, part, or product in the minimum unit constituted by the POM-based resin,
Electronic products, containers, household goods, electrical products, general machinery, plumbing parts, precision machinery, tools, industrial parts, transportation units, etc. Includes molded products and products that require secondary processing, such as sheets and plates.

In the present invention, the POM-based resin injection molding method is a usual thermoplastic resin molding and processing method. In addition to the most common injection molding method, hollow injection molding method, gas assist molding method Method, blow molding method, injection / compression molding method and the like. In the injection molding method of a molded article using the POM-based resin of the present invention, when filling the mold cavity with the POM-based resin having dissolved or absorbed carbon dioxide, the amount of dissolved or absorbed carbon dioxide is not less than a certain value. There is a possibility that a foamed pattern may be generated on the surface of the molded product.

In order to suppress the occurrence of a foaming pattern on the surface of the molded product, the inside of the mold cavity is adjusted or held by a pressurized gas at a pressure higher than the pressure at which foaming does not occur at the flow front of the POM resin. It is necessary.
The pressure of the pressurized gas may be a minimum pressure at which the foaming pattern on the surface of the molded product disappears, the amount of gas used during the molding cycle is minimized, and the structure of the mold cavity seal and gas supply device is reduced. It is preferable that the gas pressure is low for simplification. If the gas pressure exceeds 15 MPa, the mold may open due to the gas pressure, and problems such as difficulty in sealing the mold cavity may easily occur. Therefore, the pressure of the gas for pressurizing the mold cavity is higher than the atmospheric pressure and 15 MPa.
It can be said that the following is preferable.

At this time, as the gas for adjusting or holding the inside of the mold cavity at a constant pressure, a single substance or a mixture of various gases inert to the POM resin can be used. POM
Carbon dioxide and hydrocarbons having high solubility in the system resin and a gas in which hydrogen is partially replaced with fluorine are preferred. Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to the examples.

The resin used for the injection molding was a POM resin homopolymer (“Tenac 30” manufactured by Asahi Kasei Corporation).
10, 4010, 7010 "), POM-based resin random copolymer (" Tenac-C "manufactured by Asahi Kasei Corporation)
3510, 4520, 7520, 8520 ").
Both are in the form of pellets before molding. The molding machine is “TUPARL TR5” manufactured by Sodick Plustech Co., Ltd.
0S2 ”,“ SG125-H ”manufactured by Sumitomo Heavy Industries, Ltd.
A "P" molding machine was used.

The cylinder temperature during the injection molding was POM ·
The temperature was set to 200 ° C. when molding the homopolymer, and 190 ° C. when molding the POM / random copolymer. The mold temperature was 80 ° C. The filling pressure during molding was a value obtained by reading the filling pressure during injection on a monitor screen of a molding machine. The holding pressure was a value corresponding to 70% of the filling pressure, the holding time was 7 seconds, and the cooling time was 15 seconds.

The sink amount of the molded product was measured by measuring the flow end portion of the injection-molded ISO dumbbell molded product using a contact-type surface roughness meter "Surface Roughness Profiler (Surfcom 570A manufactured by Tokyo Seimitsu Co., Ltd.). ")". The dimensional accuracy is
Using a three-dimensional measuring machine “AE122 manufactured by Mitutoyo Corporation” and a measurement program “Geopak 400 manufactured by the company”, measurement was performed according to a multipoint flatness measurement method.

[0059]

[Reference Examples 1-4] Tenac 30 was used as a sample.
The melting point was measured using 10, 4010 and Tenac-C 3510, 4520. Using the viscosity measuring device shown in FIG. 1, the temperature of the heating cylinder and the plunger of the viscosity measuring device
Set and maintain a temperature 15 ° C. higher than the melting point of each sample,
The viscosity was measured under an arbitrary shear rate condition in the range of 1.0 × 10 ³ to 1.0 × 10 ⁶ (/ sec). Table 1 shows the results. For each sample, the viscosity value is
ηa ≧ 30000 × γa− ^0.69 .

The tensile elongation characteristics, creep characteristics, and fatigue characteristics (at 40 MPa and 30 MPa) of each sample were measured. The tensile elongation characteristics were measured according to the ISO shown in FIG.
Prepare a dumbbell test piece conforming to the standard, and follow JIS K7
162 (ISO 527-2), a tensile test was performed, and the elongation at break was measured. The creep characteristic is a creep characteristic when a load of 13.7 MPa is prepared in an environment of an ambient temperature of 100 ° C. by preparing a tensile dumbbell test piece having a thickness of 3 mm shown in FIG. The time was measured.

[0061] The fatigue characteristics were measured by ASTM D shown in FIG.
A cantilever bending fatigue test piece according to 671 TYPE1 standard was prepared, and the load condition was 40M according to the method B of the standard.
The test was performed at two levels of Pa and 30 MPa. The repetition rate is 1800 times / min. The number of repetitions at which the test piece was broken or the number of repetitions before the test piece was broken and the maximum deflection amount exceeded ± 8 mm was measured.

[0062]

[Reference Comparative Examples 1-3] Tenac 70 was used as a sample.
10, the melting point was measured using Tenac-C 7520, 8520. As in Comparative Examples 1 to 4, 15
The viscosity was measured at an elevated temperature of ° C. under any shear rate conditions within the range of 1.0 × 10 ³ to 1.0 × 10 ⁶ (/ sec). Table 1 shows the results. The viscosity value of each sample was ηa <30000 × γ
a It is in the range of ^-0.69 . Moreover, similarly to Comparative Examples 1 to 4, the tensile elongation characteristics, creep characteristics, and fatigue characteristics of each sample were measured. Table 1 shows the results.

[0063]

[Table 1]

[0064]

Embodiments 1 to 4 Carbon dioxide adjusted to an arbitrary pressure of 4.2 to 12.4 MPa using a Tenac 3010 or 4010 is supplied from a gas supply unit provided at the center of the heating cylinder of the molding machine to the inside of the heating cylinder. After being supplied to the molten POM resin, dissolved and absorbed, and filled in a mold cavity, a flat molded article with a boss shown in FIG. 5 was obtained. In the obtained flat molded article with a boss, the amount of sink and the amount of warpage generated at the base of the boss on the surface where the boss was not installed were measured. Table 2 shows the results.

[0065]

[Comparative Example 1] Using Tenac 3010, 0.8 MPa
The POM which is in a molten state in the heating cylinder is supplied from the gas supply unit provided at the center of the heating cylinder of the molding machine to the carbon dioxide adjusted to the temperature.
The resin was supplied to the resin, dissolved and absorbed, and then filled into a mold cavity to obtain a flat molded article with a boss shown in FIG. In the same manner as in Examples 1 to 4, the sink mark amount and the warpage amount were measured. Table 2 shows the results.

[0066]

COMPARATIVE EXAMPLES 2 AND 3 Using Tenac 3010 and 4010, a flat molded article with a boss was obtained by a normal injection molding step without supplying carbon dioxide. In the same manner as in Examples 1 to 4, the sink mark amount and the warpage amount were measured. Table 2 shows the results.

[0067]

[Table 2]

[0068]

Embodiments 5 to 8 Using Tenac-C 3510 and 4520, carbon dioxide adjusted to an arbitrary pressure of 5.2 to 12.8 MPa is heated from a gas supply unit provided at the center of a heating cylinder of a molding machine. The POM resin in a molten state in the cylinder was supplied, dissolved and absorbed, and then filled into a mold cavity to obtain a box-shaped model molded product shown in FIG. In addition, the filling pressure when filling the resin into the mold cavity was measured.

The molded product was heated at 23 ° C. for 24 hours after molding.
The condition was adjusted in a 50% RH atmosphere, and an annealing treatment was performed for 8 hours in a hot-air dryer adjusted to a temperature of 80 ° C. After the annealing, the condition was adjusted again for 16 hours in an atmosphere of a temperature of 23 ° C. and a humidity of 50% RH. The flatness of the molded product was compared based on the flatness of the opening of the box-shaped model molded product shown in FIG. Specifically, the end face of the opening of the box-shaped model molded product is used as the measurement surface, and the “three-dimensional measuring device (AE12
2, Mitutoyo Corporation) ”and“ Measurement program (Ge
opak400, manufactured by Mitutoyo Corporation) according to a multipoint flatness measurement method. The flatness of the opening was measured. Table 3 shows the results.

[0070]

Comparative Examples 4 and 5 Carbon dioxide adjusted to an arbitrary pressure of 0.4 to 0.6 MPa using Tenac-C 3510 and 4520 was supplied from a gas supply unit provided at the center of a heating cylinder of a molding machine. The POM resin in a molten state in the heating cylinder was supplied, melted and absorbed, and then filled into a mold cavity to obtain a box-shaped model molded product shown in FIG. The filling pressure was measured in the same manner as in Examples 5 to 8. Further, the molded article was subjected to annealing treatment and condition adjustment, and then the flatness of the opening was measured. Table 3 shows the results.

[0071]

Comparative Examples 6 and 7 Using Tenac-C 7520, 8520, carbon dioxide was not supplied from the gas supply unit provided at the center of the heating cylinder, and the same process as in ordinary injection molding was performed. Was obtained. Example 5
The filling pressure was measured in the same manner as in No. 8 to No. 8. Further, the molded article was subjected to annealing treatment and condition adjustment, and then the flatness of the opening was measured. Table 3 shows the results.

[0072]

[Table 3]

[0073]

Embodiments 9 to 12 Carbon dioxide adjusted to an arbitrary pressure of 1.8 to 12.8 MPa using a Tenac 3010 or 4010 is supplied from a gas supply unit provided at the center of the heating cylinder of the molding machine to the inside of the heating cylinder. After being supplied to the molten POM resin, dissolved and absorbed, and then filled into a mold cavity, a box-shaped model molded article shown in FIG. 7 was obtained. The molded article was conditioned for 24 hours after molding under an atmosphere of a temperature of 23 ° C. and a humidity of 50% RH, and annealed for 8 hours in a hot-air dryer adjusted to a temperature of 80 ° C. After the annealing treatment, the condition was adjusted again in an atmosphere of a temperature of 23 ° C. and a humidity of 50% RH for 16 hours, and then the amount of warpage was measured. The amount of warpage of the molded product was determined by measuring the dimensions A, B, C, and D shown in the figure of the box-shaped model molded product shown in FIG. The amount a was calculated from the difference between the measured values A and B, and the amount of warpage b on the long side was calculated from the difference between the measured values C and D. In addition, the amount of carbon dioxide absorbed was measured. Table 4 shows the results.

[0074]

COMPARATIVE EXAMPLES 8-9 Using Tenac 3010, 4010, carbon dioxide adjusted to an arbitrary pressure of 0.5-0.8 MPa was supplied from a gas supply unit provided at the center of the heating cylinder of the molding machine to the inside of the heating cylinder. To the molten POM resin,
After being dissolved and absorbed, the mold was filled into a mold cavity to obtain a box-shaped model molded product shown in FIG. Example 9
After performing the annealing treatment and the state adjustment as in
The amount of warpage a, the amount of warpage b, and the amount of carbon dioxide absorbed at the opening were measured. Table 4 shows the results.

[0075]

Comparative Example 10 Using the Tenac 7010, carbon dioxide was not supplied from the gas supply unit provided at the center of the heating cylinder, and the box-shaped model shown in FIG. A molded product was obtained. As in Examples 9 to 12,
After performing the annealing treatment and the state adjustment, the warpage amount a and the warpage amount b of the opening were measured. Table 4 shows the results.

[0076]

[Table 4]

[0077]

According to the molded article of the present invention, the occurrence of defects mainly occurring after molding such as warpage and sink is suppressed without limiting the resin composition of the POM resin and without impairing the degree of freedom in product design. It is a molded product, and can be applied to thicker molded products.

[Brief description of the drawings]

FIG. 1 is a model diagram of a viscosity measuring machine.

FIG. 2 shows a dumbbell-shaped test piece.

FIG. 3 shows a dumbbell-shaped test piece.

FIG. 4 shows a cantilever bending fatigue test piece.

FIG. 5 shows a flat molded product with a boss.

FIG. 6 shows a box-shaped model molded product and a flatness measurement point.

FIG. 7 shows a box-shaped model molded product and a dimension measurement location.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Orifice 2 Pressure sensor 3 Plunger 4 Dumbbell type test piece 5 Gate 6 Cantilever bending fatigue test piece 7 Flat molded product with boss 8 Boss 9 sink mark measurement position 10 Box model molded product 11 Flatness measurement surface

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C08L 59:00 C08L 59:00

Claims (8)

[Claims] The viscosity ηa of the polyacetal resin at a temperature 15 ° C. higher than the melting point of the polyacetal resin when the shear rate γa is in the range of 1.0 × 10 ³ to 1.0 × 10 ⁶ is as follows: ηa ≧ 30000 × γa ^−0.69 A polyacetal resin in a range of ^−0.69 , wherein carbon dioxide pressurized to 1 MPa or more is dissolved or absorbed in the molten polyacetal resin, and then filled into a mold cavity. A molded product of a polyacetal resin characterized by being obtained by: 2. A molded article made of a polyacetal resin,
The polyacetal-based resin molded article according to claim 1, wherein the molded article has a foamed portion and a non-foamed layer which is not substantially foamed on a surface layer portion of the molded article. 3. A molded article made of a polyacetal resin,
The polyacetal-based resin molded article according to claim 1 or 2, wherein the molded article has a non-foamed layer having a thickness of 500 µm or more on a surface layer portion. 4. The injection molding method for a polyacetal resin molded article according to claim 1, wherein carbon dioxide is dissolved or absorbed in the polyacetal resin in a molten state, and then filled into a mold cavity. . 5. The polyacetal resin according to claim 4, wherein 0.2% by weight or more of carbon dioxide is dissolved or absorbed in the polyacetal resin in a molten state and then filled into a mold cavity. Injection molding method for resin molded products. 6. The method according to claim 4, wherein the polyacetal resin in a molten state is obtained by filling a mold cavity maintained or maintained at a pressure higher than the atmospheric pressure with a pressurized gas. An injection molding method of the polyacetal resin molded article according to the above. 7. A pressure which is 30% or more of the filling pressure,
The injection molding method for a polyacetal resin molded product according to any one of claims 4 to 6, further comprising a step of maintaining a pressure for a predetermined time. 8. The injection molding of a polyacetal-based resin molded product according to claim 4, further comprising a step of maintaining the pressure under a pressure of 100% or less of the filling pressure for a predetermined time. Method.