JP2017155776A - Gear and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gear having excellent durability with high productivity.SOLUTION: A gear comprises a polyacetal resin composition, the gear having a tooth tip density of 1.420 g/cm3 or more.SELECTED DRAWING: None

Description

  The present invention relates to a gear and a manufacturing method thereof.

  Resin gears are used in a wide range of applications as OA devices such as copiers and printers, digital home appliances such as VTRs and DVDs, automobile window glass, power transmission mechanism parts such as sheets and sunroofs, and drive transmission mechanism parts.

  Many of the gears used for such various purposes are required to have high accuracy. As a method for improving the accuracy of the gear, a technique of suppressing deformation by defining the thicknesses of the rim, the hub, and the web is disclosed (for example, see Patent Document 1).

  Recently, there has been a demand for a resin gear capable of transmitting a larger torque to a drive unit of an electric motorcycle. As a gear that transmits such a large torque and has high durability, a gear using a polyamide resin reinforced with fibers such as glass is disclosed (for example, see Patent Documents 2 to 4).

  Further, polyacetal resin is known as a material suitably used for resin gears. This is because the polyacetal resin has high mechanical strength, excellent oil resistance and organic solvent resistance, can be used in a wide temperature range, and is easy to process.

  Among polyacetal resins, it is known that polyacetal homopolymers are superior in mechanical strength and have a high thermal deformation temperature (see, for example, Patent Document 5). Furthermore, a technique for improving the durability and quietness of a gear by using a polyacetal homopolymer as a material and a gear having a specific shape is disclosed (for example, see Patent Document 6). This document also discloses a technique for further improving mechanical properties by using a polyacetal homopolymer containing crystal nucleation inorganic particles as a material.

Japanese Patent No. 3622109 JP 2003-201398 A Japanese Patent Laid-Open No. 2008-8404 Japanese Patent No. 4209666 JP 2008-291073 A JP 2011-5731 A

  When using a polyamide resin reinforced with fibers such as glass as a resin gear, it is difficult to control the fiber orientation in the teeth, and the mechanical strength and durability of the gear are not constant. There is a problem that it is difficult to obtain the desired performance.

  In addition, since the polyacetal homopolymer has a high crystallization speed, the resin solidifies in the mold in injection molding, and the flow length in the mold tends to be short. This tendency is particularly remarkable when crystal nucleation inorganic particles are included. When such a resin is used for a gear, there is a problem that filling of the resin into the tooth tip is particularly insufficient. In addition, it is necessary to have a mold structure with a large gate diameter, which causes a decrease in productivity, such as a long cycle time including a holding time, and difficulty in obtaining multiple pieces. There is.

  Therefore, the present inventors have intensively studied to solve the above problems. As a result, in a gear containing a polyacetal resin composition, it was found that by setting the tooth tip density to a specific value, durability was further improved and productivity at the time of molding could be improved, and the present invention was completed. I let you.

  That is, the present invention is as follows.

[1]
A gear containing a polyacetal resin composition, wherein the tooth tip density is 1.420 g / cm ³ or more.

[2]
The gear according to [1], wherein the polyacetal resin composition includes a polyacetal homopolymer.

[3]
The gear according to [1] or [2], wherein a melt flow rate of the polyacetal resin composition is 0.8 to 3.0 g / 10 minutes.

[4]
The gear according to [1] or [2], wherein the melt flow rate of the polyacetal resin composition is 0.8 to 1.4 g / 10 min.

[5]
The gear according to any one of [1] to [4], which is non-fiber reinforced.

[6]
The manufacturing method of the gear in any one of [1]-[4] including the process of injection-molding a raw material with the injection pressure of 160-300 MPa.

[7]
The gear manufacturing method according to [6], wherein in the injection molding, the raw material is compressed in a mold after injection.

[8]
The manufacturing method of the gear in any one of [6] or [7] including the process of heat-processing the molded object for 2 to 8 hours in the temperature range of 100-160 degreeC after injection molding.

  According to the present invention, a gear having excellent durability can be obtained with high productivity.

FIG. 1A is a cross-sectional view of an example of a gear according to the present embodiment. FIG. 1B is a top view of an example of a gear according to the present embodiment. FIG. 2 is a schematic diagram showing an example of a set of polymerization reactors for polyacetal homopolymer used in the present embodiment. FIG. 3 is a schematic diagram of an example of a gear strength tester used for evaluation of the gear according to the present embodiment.

  Hereinafter, a mode for carrying out the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. In addition, this invention is not limited to this embodiment, It can deform | transform and implement suitably within the range of the summary.

[gear]
The gear of this embodiment contains a polyacetal resin composition and has a tooth tip density of 1.420 g / cm ³ or more. When the tooth tip density is 1.420 g / cm ³ or more, the durability of the gear becomes more excellent. The tooth tip density is a value measured by the method described in Examples described later.

In the gear of the present embodiment, the upper limit of the tooth tip density is not particularly limited, but is preferably 1.430 g / cm ³ or less, more preferably 1.428 g / cm ³ or less, and further preferably 1.426 g / cm ³ or less. is there.

  The gear having the tooth tip density in the above range is not particularly limited, but can be obtained by, for example, a gear manufacturing method described later.

  The gear of this embodiment is preferably non-fiber reinforced from the viewpoint of dimensional stability and dimensional accuracy of a molded product. Here, non-fiber reinforcement means that the content of reinforcing fibers is 1 part by mass or less with respect to 100 parts by mass of the polyacetal homopolymer. The reinforcing fiber refers to a known reinforcing fiber such as glass fiber or carbon fiber, and is usually added in an amount of 10% by mass or more in order to increase the mechanical strength of the material.

[Polyacetal resin composition]
The polyacetal resin composition used in the present embodiment preferably has a melt flow rate (hereinafter also referred to as “MFR”) of 0.8 to 3.0 g / 10 minutes. Further, from the viewpoint of suppressing creep deformation due to a large load at a high temperature, the MFR of the polyacetal resin composition is preferably 0.8 to 2.5 g / 10 minutes, more preferably 0.8 to 1.4 g. / 10 minutes.

  When the MFR of the polyacetal resin composition is 0.8 g / 10 min or more, the production of the molded product is further stabilized and it can be remarkably easy to maintain the durability. On the other hand, when the MFR of the polyacetal resin composition is 3.0 g / 10 min or less, the durability is further improved. In addition, the method described in the below-mentioned Example is used for the measuring method of MFR in this specification.

  The polyacetal resin composition preferably contains a polyacetal homopolymer. The inclusion of the polyacetal homopolymer tends to further improve the durability of the gear and the quietness when driven for a long time.

  In the polyacetal resin composition, the content of the polyacetal homopolymer is preferably 80 to 100% by mass, more preferably 90 to 99% by mass, and further preferably 95 to 98% by mass.

[Polyacetal homopolymer]
In the present embodiment, the polyacetal homopolymer refers to a polymer in which the structure of the main chain is substantially composed only of — (CH ₂ O) —. Here, “substantially” means that the content of a repeating unit (comonomer) other than — (CH ₂ O) — is 0.1 mol% or less.

As a method for confirming that the polyacetal homopolymer is contained, the ¹ H-NMR method is used as shown below. The gear is dissolved with hexafluoroisopropanol (HFIP) over a period of 24 hours to a concentration of 1.5% by mass. Using this solution, ¹ H-NMR measurement was performed. From the ratio of the integrated values of peaks attributed to comonomer components (for example, oxyalkylene component) b other than oxymethylene component a and oxymethylene component a, The ratio (b / a) of comonomer component b to 100 mol% of methylene component can be determined.

[Production method of polyacetal homopolymer]
(Monomer raw material)
Although the polyacetal homopolymer used for this embodiment is not specifically limited, For example, it is obtained by superposing | polymerizing the formaldehyde which is a monomer (main monomer) as a main raw material. As formaldehyde, from the viewpoint of continuously obtaining a resin having a stable molecular weight, it is preferable to use a formaldehyde gas that is purified and has a low impurity concentration and is stable. A known method can be used as a purification method of formaldehyde. For example, methods disclosed in Japanese Patent Publication No. 5-32374 and Japanese Patent Publication No. 2001-521916 can be used.

  As the formaldehyde gas, it is preferable to use a gas that does not contain impurities such as water, methanol, formic acid, or the like that stops polymerization during the polymerization reaction or has a chain transfer action. When formaldehyde gas containing as little impurities as possible is used, an unexpected chain transfer reaction can be effectively prevented, and a substance having a target molecular weight can be more easily obtained. Among them, the content of water is preferably 100 ppm or less, and more preferably 50 ppm or less, from the viewpoint of making it easier to obtain a substance having the target molecular weight.

(Polymerization method)
The production of the polyacetal homopolymer used in the present embodiment is not particularly limited, but can be performed using, for example, a known slurry polymerization method. For example, the polymerization methods disclosed in Japanese Patent Publication Nos. 47-6420 and 47-10059 can be used. By using such a polymerization method, a crude polyacetal homopolymer having unstable terminal portions can be obtained. The crude polyacetal homopolymer is preferably subjected to a terminal stabilization treatment described later.

(Chain transfer agent)
As the chain transfer agent, alcohols and acid anhydrides are generally used. Further, in order to obtain a block copolymer or a branched polymer, a polyol, a polyether polyol, or a polyether polyol / alkylene oxide may be used. Also, as described above, it is preferable to use a chain transfer agent that does not contain impurities as much as possible from the viewpoint of effectively preventing an unexpected chain transfer reaction. Especially, it is preferable that content of the water in a chain transfer agent is 2,000 ppm or less, and it is more preferable that it is 1,000 ppm or less.

  The method for obtaining the chain transfer agent with very few impurities is not limited to the following, but for example, a method of bubbling the chain transfer agent with dry nitrogen and removing the impurities with an adsorbent such as activated carbon or zeolite and purifying it can be mentioned. It is done. In addition, a chain transfer agent may be used individually by 1 type, and may use 2 or more types together.

(Polymerization catalyst)
The polymerization catalyst used in the polymerization reaction, that is, the onium salt polymerization catalyst is represented by the following formula (1).

[R ₁ R ₂ R ₃ R ₄ M ⁺ ] X ⁻ (1)
In the above formula, R ₁ , R ₂ , R ₃ and R ₄ are each independently an alkyl group, M is an element having a lone pair, and X is a nucleophilic group.

  Among the onium salt-based polymerization catalysts represented by the above formula (1), since a high polymerization efficiency can be achieved as a polymerization catalyst, from the viewpoint that the amount of the catalyst can be reduced, a quaternary ammonium salt-based compound or A quaternary phosphonium salt compound is preferable, and tetramethylammonium bromide, dimethyl distearyl ammonium acetate, tetraethylphosphonium iodide and tributylethylphosphonium iodide are more preferable.

(Reactor)
Although the reactor used for superposition | polymerization is not restrict | limited to the following, For example, a batch-type reaction tank with a stirrer, a continuous type kneader, a twin screw type continuous extrusion kneader, and a twin screw paddle type continuous mixer are mentioned. It is preferable that the outer periphery of the barrels of these reactors has a structure capable of heating and cooling the reaction mixture.

(Terminal stabilization treatment)
Of the above-mentioned terminal stabilization treatment of the crude polyacetal resin, the method of blocking with an ether group is not limited to the following, and examples thereof include the method described in JP-B 63-452. Further, the method for blocking with an acetyl group is not limited to the following, but for example, the method described in US Pat. No. 3,459,709 in which a large amount of acid anhydride is used in a slurry state, and an acid Examples include the method described in US Pat. No. 3,172,736, which is carried out in the gas phase using an anhydride gas. Although it does not restrict | limit as an etherifying agent used in the method of blocking with said ether group, For example, orthoester is mentioned. An example is an ortho ester of an aliphatic or aromatic acid and an aliphatic, alicyclic or aromatic alcohol. Specific examples of such orthoesters include orthocarbonates such as methyl orthoformate, ethyl orthoformate, methyl orthoacetate, ethyl orthoacetate, methyl orthobenzoate, ethyl orthobenzoate, and ethyl orthocarbonate.

  The etherification reaction is not limited to the following, but includes, for example, Lewis acids such as p-toluenesulfonic acid, medium strength organic acids such as acetic acid and hydrobromic acid, and medium strength mineral acids such as dimethyl sulfate and diethyl sulfate. The catalyst can be introduced by introducing 0.001 to 0.02 parts by mass with respect to 1 part by mass of the etherifying agent. Solvents used for the etherification reaction are not limited to the following, but include, for example, low boiling point aliphatic, alicyclic and aromatic hydrocarbons such as pentane, hexane, cyclohexane and benzene, and methylene chloride, chloroform and And organic solvents such as halogenated lower aliphatic compounds such as carbon tetrachloride.

  On the other hand, when the terminal of the polymer is blocked with an ester group, the organic acid anhydride used for esterification is not limited to the following, and examples thereof include organic acid anhydrides represented by the following formula (2).

R ₅ COOCOR ₆ (2)
In the above formula, R ₅ and R ₆ each independently represent an alkyl group. R ₅ and R ₆ may be the same or different.

  Among the organic acid anhydrides represented by the above formula (2), propionic anhydride, benzoic anhydride, acetic anhydride, succinic anhydride, maleic anhydride, glutaric anhydride and phthalic anhydride are preferable, and acetic anhydride is more preferable. . An organic acid anhydride may be used individually by 1 type, and may use 2 or more types together.

  Further, the method for blocking with an ester group in the gas phase is not limited to the following, but for example, a method described in JP-A-11-92542 in which terminal blocking is performed after removing an onium salt polymerization catalyst. preferable. This is because by removing the onium salt polymerization catalyst in the polymer resin so that it does not remain as much as possible, the terminal decomposition can effectively suppress the promotion of the polymer decomposition reaction by the onium salt polymerization catalyst. This is because the polymer yield can be significantly improved and the coloring of the polymer can be effectively prevented.

It is preferable that the concentration of the terminal hydroxyl group is reduced to 5 × 10 ⁻⁷ mol / g or less by blocking the end of the polymer with an ether group and / or an ester group. More preferably, the concentration of the terminal hydroxyl group is 0.5 × 10 ⁻⁷ mol / g or less.

[Additive]
The polyacetal resin composition used in the present embodiment includes crystal nucleation inorganic particles, heat stabilizers, antioxidants, acid scavengers, weather resistance (light) stabilizers, mold release agents, as long as the gist of the present invention is not impaired. It may contain additives such as a lubricant, a conductive material / antistatic agent, a thermoplastic resin and a thermoplastic elastomer, and a pigment.

(Crystal nucleation inorganic particles)
The polyacetal resin composition may include crystal nucleation inorganic particles.

  The crystal nucleation inorganic particles are not limited to the following, for example, silicates such as talc, silica, quartz powder, glass powder, calcium silicate, aluminum silicate, kaolin, phyllite, clay, diatomaceous earth and wollastonite, Metal oxides such as iron oxide, titanium oxide and alumina, metal sulfates such as calcium sulfate and barium sulfate, carbonates such as calcium carbonate, magnesium carbonate and dolomite, silicon carbide, silicon nitride, boron nitride and various metal powders, etc. , Finely divided solids of crystal nucleation minerals commonly known in polyacetals (eg, polyacetal homopolymers). Among these crystal nucleation inorganic particles, as a crystallization nucleating agent for polyacetal (for example, polyacetal homopolymer), the crystallization speed is improved, and as a result, mechanical properties (such as mechanical strength) are improved, and polyacetal ( For example, from the viewpoint of preventing loss of thermal stability of the polyacetal homopolymer), it is preferable to include at least one of talc and boron nitride.

  The crystal nucleation inorganic particles preferably include 10 to 90 ppm with respect to the polyacetal resin composition. In order to quantify the crystal nucleation inorganic particles, for example, a resin composition containing polyacetal (for example, polyacetal homopolymer) is hydrolyzed with hydrochloric acid or the like, or a metal is obtained by high frequency inductively coupled plasma (ICP) emission analysis. A method for quantifying the components can be mentioned. Here, as the measurement of the concentration (ppm) in the present specification, a method of hydrolysis and quantification with the above-described hydrochloric acid is adopted.

  The average particle diameter of the crystal nucleation inorganic particles is 0.1 to 10 from the viewpoint of forming good crystal nuclei when mixed with polyacetal (for example, polyacetal homopolymer) and contributing to improvement in strength and durability. 0.0 μm, preferably 0.5 to 5.0 μm. The average particle diameter can be measured by a known method. For example, a method of cutting out the obtained resin mechanical part and measuring it by a microscopy (polarizing microscope or SEM-EDX) can be mentioned. Here, as the measurement of the average particle diameter in the present specification, the method described in Examples described later is employed.

  The content of the crystal nucleation inorganic particles is preferably 0.01 to 500 ppm, more preferably 0.05 to 100 ppm, based on the polyacetal resin composition (for example, polyacetal homopolymer). More preferably, it is -90 ppm.

(Heat stabilizer and antioxidant)
The polyacetal resin composition used in the present embodiment preferably contains a heat stabilizer and an antioxidant. The heat stabilizer preferably contains formaldehyde-reactive nitrogen. The antioxidant is preferably a hindered phenol type.

  Examples of the heat stabilizer containing formaldehyde-reactive nitrogen include, but are not limited to, polyamides such as nylon 4-6, nylon 6, nylon 6-6, nylon 6-10, nylon 6-12 and nylon 12 Resin, and these polymers (for example, nylon 6 / 6-6 / 6-10, nylon 6 / 6-12, etc.). Other examples include acrylamide and derivatives thereof, or copolymers of acrylamide and derivatives thereof with other vinyl monomers. Examples of such copolymers include, but are not limited to, poly-β-alanine copolymers obtained by polymerizing acrylamide and its derivatives and other vinyl monomers in the presence of metal alcoholates.

  In addition, amide compounds, amino-substituted triazine compounds, adducts of amino-substituted triazine compounds and formaldehyde, condensates of amino-substituted triazine compounds and formaldehyde, urea, urea derivatives, hydrazine derivatives, imidazole compounds, imide compounds, etc. Can be mentioned.

  Specific examples of the amide compound include, but are not limited to, polycarboxylic acid amides such as isophthalic acid diamide, and anthranilamides.

Specific examples of the amino-substituted triazine compound are not limited to the following.
2,4-diamino-sym-triazine,
2,4,6-triamino-sym-triazine,
N-butylmelamine,
N-phenylmelamine,
N, N-diphenylmelamine,
N, N-diallylmelamine,
Benzoguanamine (2,4-diamino-6-phenyl-sym-triazine),
Examples include acetoguanamine (2,4-diamino-6-methyl-sym-triazine) and 2,4-diamino-6-butyl-sym-triazine.

Specific examples of the adduct of the amino-substituted triazine compound and formaldehyde are not limited to the following.
N-methylol melamine,
N, N′-dimethylol melamine and N, N ′, N ″ -trimethylol melamine are mentioned.

  Specific examples of the condensate of the amino-substituted triazine compound and formaldehyde include, but are not limited to, a melamine / formaldehyde condensate.

  Examples of the urea derivative include, but are not limited to, N-substituted urea, urea condensate, ethylene urea, hydantoin compound, and ureido compound.

  Specific examples of the N-substituted urea include, but are not limited to, methylurea substituted with a substituent such as an alkyl group, alkylenebisurea, and aryl-substituted urea.

  Specific examples of the urea condensate are not limited to the following, and examples thereof include a condensate of urea and formaldehyde.

  Specific examples of the hydantoin compound include, but are not limited to, hydantoin, 5,5-dimethylhydantoin, and 5,5-diphenylhydantoin.

  Specific examples of the ureido compound include, but are not limited to, allantoin.

  Examples of the hydrazine derivative include, but are not limited to, hydrazide compounds.

Specific examples of the hydrazide compound include, but are not limited to, for example, dicarboxylic acid dihydrazide, and more specifically, but not limited to,
Malonic acid dihydrazide,
Succinic acid dihydrazide,
Glutaric acid dihydrazide,
Adipic acid dihydrazide,
Pimelic acid dihydrazide,
Superic acid dihydrazide,
Azelaic acid dihydrazide,
Sebacic acid dihydrazide,
Dodecanedioic acid dihydrazide,
Isophthalic acid dihydrazide,
Examples include phthalic acid dihydrazide and 2,6-naphthalenedicarbodihydrazide.

  Specific examples of the imide compound include, but are not limited to, succinimide, glutarimide, and phthalimide.

  These compounds containing formaldehyde-reactive nitrogen (including polymers) may be used alone or in combination of two or more.

On the other hand, the antioxidant is not limited to the following, but, for example, a hindered phenol antioxidant is preferable. Specific examples include, but are not limited to:
n-octadecyl-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) -propionate,
n-octadecyl-3- (3′-methyl-5′-t-butyl-4′-hydroxyphenyl) -propionate,
n-tetradecyl-3- (3 ′, 5′-di-t-butyl-4′-hydroxyphenyl) -propionate,
1,6-hexanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) -propionate],
1,4-butanediol-bis- [3- (3,5-di-tert-butyl-4-hydroxyphenyl) -propionate],
Triethylene glycol-bis- [3- (3-t-butyl-5-methyl-4-hydroxyphenyl) -propionate], and pentaerythritol tetrakis [methylene-3- (3 ′, 5′-di-t-butyl) -4′-hydroxyphenyl) propionate] methane and the like.

Among them, preferably
Triethylene glycol-bis- [3- (3-t-butyl-5-methyl-4-hydroxyphenyl) -propionate], and pentaerythritol tetrakis [methylene-3- (3 ′, 5′-di-t-butyl) -4′-hydroxyphenyl) propionate] methane.

  These antioxidants may be used individually by 1 type, and may be used in combination of 2 or more type.

  In this embodiment, it is preferable to contain 0.05-0.5 mass part of heat stabilizers and antioxidants with respect to 100 mass parts of polyacetal (for example, polyacetal homopolymer), respectively.

(Acid scavenger)
Examples of the acid scavenger include, but are not limited to, condensates of the above amino-substituted triazine compounds and amino-substituted triazine compounds with formaldehyde, and more specifically, melamine / formaldehyde condensates. Other acid scavengers include, but are not limited to, alkali metal and alkaline earth metal hydroxides, inorganic acid salts, carboxylate salts, and alkoxides. Specifically, but not limited to, for example, hydroxides such as sodium, potassium, magnesium, calcium and barium, and carbonates, phosphates, silicates, borates of the metals listed above, and Carboxylic acid salts, and layered double hydroxides can be mentioned.

  The carboxylic acid of the carboxylate is preferably a saturated or unsaturated aliphatic carboxylic acid having 10 to 36 carbon atoms, and these carboxylic acids may be substituted with a hydroxyl group. Specific examples of the saturated or unsaturated aliphatic carboxylate include, but are not limited to, for example, calcium dimyristate, calcium dipalmitate, calcium distearate, (myristic acid-palmitic acid) calcium, (myristin) Acid-stearic acid) calcium and (palmitic acid-stearic acid) calcium. Among these, calcium dipalmitate and calcium distearate are preferable.

  Examples of the layered double hydroxide include, but are not limited to, hydrotalcites represented by the following formula.

[(M ²⁺ ) _1-X (M ³⁺ ) _X (OH) ₂ ] ^{X +} [(A ⁿ⁻ ) _{x / n} · mH ₂ O] ^X-
In the above formula, M ²⁺ is a divalent metal, M ³⁺ represents an anion of trivalent metal, A ^n-n-valent (n is an integer of 1 or more), X is the range of 0 <X ≦ 0.33 M is a positive number.

In the above formula, examples of M ²⁺ are not limited to the following, but examples include Mg ²⁺ , Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Ni ²⁺ , Cu ²⁺ , Zn ^{2+ and the} like. examples of M ^3+, but are not limited to, for ^{example, Al 3+, Fe 3+, Cr} 3+, Co 3+, in 3+ , and the like, as examples of a ^n- is less Although not limited to, for example, OH ⁻ , F ⁻ , Cl ⁻ , Br ⁻ , NO ₃ ⁻ , CO ₃ ²⁻ , SO ₄ ²⁻ , Fe (CN) ₆ ³⁻ , CH ₃ COO ⁻ , oxalate ion, Salicylic acid ions and the like can be mentioned. Of these, CO ₃ ²⁻ and OH ⁻ are preferable. Specific examples include, but are not limited to, natural hydrotalcite represented by Mg _0.75 Al _0.25 (OH) ₂ (CO ₃ ) _0.125 · 0.5H ₂ O, and Mg _4.5 Al ₂ (OH) ₁₃ CO ₃ · 3.5H ₂ O, include synthetic hydrotalcite represented by _{Mg 4.3 Al 2 (OH) 12.6} CO 3 or the like.

  These acid scavengers may be used alone or in combination of two or more.

  The content of the acid scavenger is preferably 0.001 to 2.0 parts by mass, and 0.01 to 1.0 part by mass with respect to 100 parts by mass of polyacetal (for example, polyacetal homopolymer). It is more preferable, and it is still more preferable that it is 0.015-0.5 mass part.

(Weather-resistant (light) stabilizer)
The weather resistance (light) stabilizer is preferably at least one selected from the group consisting of benzotriazole-based and oxalic acid anilide-based UV absorbers and hindered amine-based light stabilizers.

The benzotriazole ultraviolet absorber is not limited to the following, for example,
2- (2′-hydroxy-5′-methyl-phenyl) benzotriazole,
2- (2′-hydroxy-3 ′, 5′-di-t-butyl-phenyl) benzotriazole,
2- [2′-hydroxy-3 ′, 5′-bis (α, α-dimethylbenzyl) phenyl] 2H-benzotriazole,
2- [2′-hydroxy-3 ′, 5′-bis- (α, α-dimethylbenzyl) phenyl] -2H-benzotriazole, and 2- (2′-hydroxy-4′-octoxyphenyl) benzotriazole Is mentioned.

The oxalic acid alinide ultraviolet absorber is not limited to the following, for example,
2-ethoxy-2′-ethyl oxalic acid bisanilide,
2-Ethoxy-5-t-butyl-2′-ethyl oxalic acid bisanilide and 2-ethoxy-3′-dodecyl oxalic acid bisanilide.

Among them, preferably 2- [2′-hydroxy-3 ′, 5′-bis- (α, α-dimethylbenzyl) phenyl] -2H-benzotriazole, and 2- (2′-hydroxy-3 ′, 5 ′ -Di-t-butyl-phenyl) benzotriazole.

  These benzotriazole-based ultraviolet absorbers may be used alone or in combination of two or more.

The hindered amine light stabilizer is not limited to the following, for example,
N, N ′, N ″, N ′ ″-tetrakis- (4,6-bis- (butyl- (N-methyl-2,2,6,6-tetramethylpiperidin-4-yl) amino) -triazine -2-yl) -4,7-diazadecane-1,10-diamine,
Dibutylamine, 1,3,5-triazine, N, N′-bis (2,2,6,6, tetramethyl-4-piperidyl-1,6-hexamethylenediamine and N- (2,2,6, 6, tetramethyl-4-piperidyl) butylamine polycondensate,
Poly [{6- (1,1,3,3-tetramethylbutyl) amino-1,3,5-triazine-2,4-diyl} {(2,2,6,6-tetramethyl-4-piperidyl ) Imino} hexamethylene {(2,2,6,6-tetramethyl-4-piperidyl) imino}],
A condensate of dimethyl succinate and 4-hydroxy-2,2,6,6, tetramethyl-1-piperidineethanol;
Reaction product of decanedioic acid bis (2,2,6,6-tetramethyl-1 (octyloxy) -4-piperidinyl) ester, 1,1-dimethylethyl hydroperoxide and octane;
Bis (1,2,2,6,6-pentamethyl-4-piperidyl) [[3,5-bis (1,1-dimethylethyl) -4-hydroxyphenyl] methyl] butyl malonate,
Methyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacate,
Bis (2,2,6,6-tetramethyl-4-piperidyl) -sebacate,
1,2,3,4-butanetetracarboxylic acid, 1,2,2,6,6-pentamethyl-4-piperidinol and β, β, β ′, β ′,-tetramethyl-3,9- [2, 4,8,10-tetraoxaspiro (5,5) undecane] diethanol condensate.

Among them, preferably bis (2,2,6,6-tetramethyl-4-piperidyl) -sebacate,
Bis- (N-methyl-2,2,6,6-tetramethyl-4-piperidinyl) sebacate and 1,2,3,4-butanetetracarboxylic acid and 1,2,2,6,6-pentamethyl- It is a condensate of 4-piperidinol and β, β, β ′, β ′,-tetramethyl-3,9- [2,4,8,10-tetraoxaspiro (5,5) undecane] diethanol.

  These hindered amine light stabilizers may be used individually by 1 type, and may be used in combination of 2 or more type.

  The content of the weather resistance (light) stabilizer is preferably 0.001 to 5.0 parts by mass, and 0.01 to 4.0 parts by mass with respect to 100 parts by mass of polyacetal (for example, polyacetal homopolymer). More preferably, it is 0.015 to 3.0 parts by mass.

(Release agent / lubricant)
Although it does not restrict | limit as a mold release agent and a lubricant below, For example, alcohol, fatty acid and those fatty acid ester, polyoxyalkylene glycol, the olefin compound whose average degree of polymerization is 10-500, and silicone are used preferably.

  The content of the release agent / lubricant is preferably 0.001 to 5.0 parts by mass, and 0.01 to 4.0 parts by mass with respect to 100 parts by mass of polyacetal (for example, polyacetal homopolymer). More preferably, it is 0.015 to 3.0 parts by mass.

(Conductive material / Antistatic agent)
Examples of the conductive agent include, but are not limited to, conductive carbon black, carbon nanotube, nanofiber, nanoparticle, metal powder, and fiber. Examples of the antistatic agent include, but are not limited to, for example, aliphatic polyethers (excluding compounds having a terminal fatty acid ester), aliphatic polyethers having a terminal fatty acid ester, fatty acids, and many Polyalkylenes based on fatty acid esters of polyhydric alcohols having free hydroxyl groups obtained from polyhydric alcohols, boric esters of glycerin mono fatty acid esters, ethylene oxide adducts of amine compounds, basic carbonates and their anion exchangers Examples thereof include an antistatic agent including a polyol and an alkali metal salt-dissolved polyalkylene polyol.

  The content of the conductive material / antistatic agent is preferably 0.001 to 5.0 parts by mass, and 0.01 to 4.0 parts by mass with respect to 100 parts by mass of polyacetal (for example, polyacetal homopolymer). More preferably, it is 0.015 to 3.0 parts by mass.

(Thermoplastic resin and thermoplastic elastomer)
Examples of the thermoplastic resin include, but are not limited to, polyacetal copolymer, polyolefin resin, acrylic resin, styrene resin, polycarbonate resin, polytetrafluoroethylene, polyamide, polyurethane, polylactic acid, and uncured epoxy resin. It is done. These modified products may also be included. Examples of the thermoplastic elastomer include, but are not limited to, polyurethane elastomers, polyester elastomers, polystyrene elastomers, and polyamide elastomers.

  The content of the thermoplastic resin and the thermoplastic elastomer is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and still more preferably 5 parts by mass or less with respect to 100 parts by mass of polyacetal (for example, polyacetal homopolymer).

(Pigment)
Examples of the pigment include, but are not limited to, inorganic and organic pigments, metallic pigments, and fluorescent pigments. The inorganic pigment refers to what is generally used for coloring a resin, and is not limited to the following. For example, zinc sulfide, titanium oxide, barium sulfate, titanium yellow, cobalt blue, a combustion pigment, Examples include carbonates, phosphates, acetates, carbon black, acetylene black, and lamp black. Examples of the organic pigment include, but are not limited to, for example, condensed zozo, inone, furothocyanin, monoazo, diazo, polyazo, anthraquinone, heterocyclic, pennon, quinacridone, thioindico , Berylene-based, dioxazine-based, and phthalocyanine-based pigments.

  The content of the pigment is preferably 0.001 to 5.0 parts by mass, and 0.01 to 4.0 parts by mass with respect to 100 parts by mass of polyacetal (for example, polyacetal homopolymer). More preferably, it is 0.015-3.0 mass parts.

(Surface treatment agent)
When the above-mentioned crystal nucleation inorganic particles are added, a known surface treatment agent may be used in order to further improve the affinity and dispersibility with the polyacetal (for example, polyacetal homopolymer) used in this embodiment. Examples of the surface treatment agent include, but are not limited to, silane coupling agents such as aminosilane and epoxysilane, titanate coupling agents, fatty acids (saturated fatty acids and unsaturated fatty acids), alicyclic carboxylic acids, and resin acids. As well as metal soaps. In the polyacetal resin composition used in the present embodiment, the addition amount of the surface treatment agent is preferably 3% by mass or less, more preferably 2% by mass or less, from the viewpoint of suppressing deterioration of physical properties and thermal stability. More preferably, it is not substantially added.

(Production method of polyacetal resin composition)
The polyacetal resin composition used in the present embodiment can be produced, for example, by the following method.

  The polymer powder subjected to the terminal stabilization treatment is generally pelletized using an extruder after drying for the purpose of improving handling. After mixing polyacetal (for example, polyacetal homopolymer) and crystal nucleation inorganic particles and other additives as necessary using a Henschel mixer, tumbler, V-shaped blender, etc., single-screw or multi-screw kneading extruder, etc. The polyacetal resin composition used in the present embodiment is obtained by melt-kneading using. Among these, a twin screw extruder equipped with a decompression device is preferable.

  Further, the polyacetal resin composition can also be produced by using a quantitative feeder or the like without mixing in advance, and by continuously feeding the crystal nucleation inorganic particles all at once or divided into an extruder. .

  In addition, a high-concentration master batch composed of polyacetal (for example, polyacetal homopolymer) and other components (hereinafter referred to as “additives”) is prepared in advance, and this master batch is used during extrusion melt kneading or molding. Can be added to polyacetal (for example, polyacetal homopolymer) to obtain a polyacetal resin composition.

[Gear manufacturing method]
The gear of the present embodiment includes, for example, polyacetal (for example, polyacetal homopolymer), crystal nucleation inorganic particles, heat stabilizer, antioxidant, acid scavenger, weather resistance (light) stabilizer, mold release as necessary. It is obtained by molding a polyacetal resin composition to which additives such as an agent / lubricant, a conductive material / antistatic agent, a thermoplastic resin and a thermoplastic elastomer, and a pigment are added.

  The manufacturing method of the gear according to the present embodiment is not limited to the following. For example, in addition to normal injection molding, injection compression molding, gas assist injection molding, foam injection molding, in-mold composite molding (metal insert molding, Examples thereof include a method of molding a raw material (for example, the polyacetal resin composition) by any molding method such as metal outsert molding. Among these, from the viewpoints of productivity, durability, and quality, the molding method is preferably injection molding and / or injection compression molding, or a combination of these and in-mold composite molding.

  The conditions for injection molding are not limited to the following, but preferably the molding temperature is 180 to 220 ° C, the mold temperature is 30 to 120 ° C, and the injection time is 0.1 to 10 seconds.

  The gear manufacturing method according to the present embodiment preferably includes a step of injection-molding the raw material with an injection pressure of 160 to 300 MPa. The injection pressure is more preferably 180 to 300 MPa, and further preferably 200 to 300 MPa. In injection molding, if the injection pressure is too high, there is a possibility of causing defective molding such as jetting. Therefore, the injection pressure is usually set to about 150 MPa or less at the highest. However, by setting the injection pressure within the above range, it is possible to fill the gear teeth with resin even when using an extremely high viscosity polyacetal resin composition having an MFR of 3.0 g / 10 min or less as a raw material. As a result, the tooth tip density can be controlled within a specific range, and a gear having excellent durability can be manufactured with high productivity. It has been found that the jetting problem can be improved by increasing the gate diameter.

  In the gear manufacturing method according to the present embodiment, it is preferable to set the holding pressure higher than usual from the viewpoint of improving the tooth tip density and improving the durability of the gear. The holding pressure is preferably 100 MPa or more, more preferably 120 MPa, and particularly preferably 150 MPa or more. Although the pressure holding time varies depending on the structure of the mold, it is preferable to apply sufficient pressure to the resin injected into the mold and maintain the molding cycle, and from the viewpoint of the above two points, 40 seconds or more are preferable. 50 seconds or more is more preferable, and 60 seconds or more is particularly preferable.

  In the gear manufacturing method according to this embodiment, from the viewpoint of improving the tooth tip density and improving the durability of the gear, it is preferable to further compress the raw material in the mold after injection in the injection molding. In normal injection molding, after the mold closing process, the resin is injected into the mold, followed by a pressure holding process, a cooling process, and a mold opening process to obtain a molded product. Here, the in-mold compression is a molding method characterized in that a resin is pressure-molded between the injection start time and the mold opening time in injection molding. The preferred timing for in-mold compression is to apply sufficient pressure to the resin injected into the mold and maintain the molding cycle, and press-mold the resin during the pressure-holding process from the above two points of view. It is preferable.

  The gear manufacturing method according to the present embodiment preferably includes a step of further heat-treating (annealing) the molded product from the viewpoint of removing residual distortion of the molded product and improving mechanical strength. The preferable conditions for the heat treatment are 100 ° C. or more and 160 ° C. or less, 2 hours or more and 8 hours or less, more preferably 120 ° C. or more and 160 ° C. or less, and 3 hours or more and 5 hours or less from the viewpoint of suppressing the variation in annealing.

[Use of gears]
The gear according to this embodiment is significantly superior in durability and productivity as compared with conventional polyacetal (for example, polyacetal homopolymer) and polyamide resin, and is used for various applications. be able to.

  In particular, the gear is not limited to the following, for example, a helical gear, a spur gear, an internal gear, a rack gear, a helical gear, a quick bevel gear, a helical bevel gear, a helical bevel gear, a crown gear , Face gears, screw gears, worm gears, worm wheel gears, hypoid gears, and Novikov gears. In addition, the helical gear and the spur gear may be combined with a single gear, a two-stage gear, and a drive motor in multiple stages, and may be a combination gear having a structure that reduces speed without causing uneven rotation.

  The parts for which such gears are preferably used are not limited to the following, for example, gears, cams, sliders, levers, arms, clutches, felt clutches, idler gears, pulleys, rollers, rollers, key stems, key tops, shutters, Examples include reels, shafts, joints, shafts, bearings, and guides. In addition, outsert-molded resin parts, insert-molded resin parts, chassis, trays, and side plates are also included.

  Specific applications of the gear are not limited to the following, but include, for example, mechanical parts for office automation equipment, mechanical parts for cameras or video equipment, mechanical parts for music, video or information equipment, mechanical parts for communication equipment, Mechanical parts for equipment, mechanical parts for electronic equipment, automotive mechanical parts (door peripheral parts, seat peripheral parts, air conditioner peripheral parts), bicycle mechanical parts (drive motor transmission, etc.), switch parts, stationery mechanical parts, housing Mechanical parts for equipment, mechanical parts for vending machines, mechanical parts for sports / leisure / recreational equipment, and mechanical parts for industrial equipment.

  Further, the gear according to the present embodiment can be applied to all electric vehicles from the viewpoint of maintaining significantly superior durability as compared with a conventional gear. Here, examples of the electric vehicle include, but are not limited to, senior four-wheels, motorcycles, and electric motorcycles. Among these, an electrically assisted bicycle is preferable from the viewpoint of demanding high durability as a power transmission gear.

  The gear according to the present embodiment is preferably used by applying grease. Thereby, durability and quietness can be greatly improved.

  Hereinafter, although the gear concerning this embodiment is explained still more concretely by an example and a comparative example, this embodiment is not restricted only to these examples.

[Preparation of evaluation sample]
<Preparation of polyacetal homopolymer powder>
FIG. 2 is a schematic view showing a set of polymerization reactors for polyacetal homopolymer. As shown in FIG. 2, a polyacetal homopolymer was produced using a jacketed 5 L tank polymerization vessel attached with a stirrer.

  FIG. 2 illustrates a polyacetal homopolymer polymerization reactor used in this embodiment, and includes a stirring blade-attached stirring motor (a), a jacketed reactor (b), a slurry circulation pump (c), and supply of formaldehyde gas. (1), supply of chain transfer agent and polymerization solvent (2), supply of polymerization catalyst (3), and slurry collection (4).

  The jacketed reactor (b) was filled with 2 L of n-hexane (solvent for etherification reaction), and a circulation line was provided. The length of the line was 6φ × 2.5 m, and the line was circulated at 20 L / hour by the slurry circulation pump (c). Into this, 200 g / hour of dehydrated formaldehyde gas was directly supplied (1). Dimethyl distearyl ammonium acetate (polymerization catalyst) is supplied to the circulation line immediately before the reactor (b) (3), and for the purpose of supplementing the amount of slurry equivalent to removing acetic anhydride (chain transfer agent) to the next step, n -Added to hexane. And the slurry containing a crude polymer was obtained by superposing | polymerizing at 58 degreeC, feeding these polymerization catalysts and a chain transfer agent continuously (2). The added polymerization catalyst and chain transfer agent were adjusted according to the MFR of the resin composition as the final target product. The obtained slurry was reacted in a 1: 1 mixture of hexane and acetic anhydride at 140 ° C. for 2 hours to acetylate the molecular terminals, thereby performing terminal stabilization. The polymer after the reaction was collected by filtration, depressurized to 2 mmHg or less, and dried in a vacuum dryer set at 80 ° C. for 3 hours to obtain a polyacetal homopolymer powder.

<Preparation of polyacetal resin composition pellets>
Polyacetal homopolymer powder and additives (0.3 parts by mass of heat stabilizer, 0.3 parts by mass of antioxidant, 0, 20 or 50 ppm of crystal nucleation inorganic particles with respect to 100 parts by mass of polyacetal homopolymer), Henschel Mix for 1 minute using a mixer. Then, using a screw type twin screw extruder with a vent set to 200 ° C. (BT-30, manufactured by Plastic Industry Co., Ltd .; L / D = 44), the screw speed was set to 100 rpm, and melt kneading was performed at 24 amps. Thus, polyacetal resin composition pellets were obtained. The operation was carried out while avoiding oxygen contamination (degassing) as much as possible from the charging of the raw material to the collection of the pellet sample.

<Manufacture of gears>
The polyacetal resin composition pellets obtained above were set to a cylinder temperature of 200 ° C. and a mold temperature of 100 ° C. using an injection molding machine equipped with a helical gear mold (see FIGS. 1A and 1B). Injection molding was performed to obtain a JIS class 4 test gear.

  Regarding the setting of the holding pressure, the standard condition was 120 MPa · 60 seconds (retaining pressure: expressed as “medium”), and in some examples, 150 MPa · 60 seconds (retaining pressure: expressed as “high”).

  Further, in some examples, a step of applying a compression force of 10 kN to the web surface of the gear for 2 seconds was performed 10 seconds after the start of holding pressure (compression molding: expressed as “present”).

<Evaluation of polyacetal resin composition>
1. MFR Measurement of Polyacetal Resin Composition After drying the polyacetal resin composition pellets obtained above at 80 ° C. for 3 hours, MFR (according to ASTM D-1238-57T under conditions of a load of 2.16 kg and a cylinder temperature of 190 ° C. Compliant). In general, a large MFR value is an indicator that fluidity is high and moldability is excellent.

<Evaluation of gears>
1. Test apparatus For evaluating durability and quietness, a power absorption type gear strength tester as shown in FIG. A suitable drive motor and power absorber (powder clutch / brake) were selected according to the test conditions.

  FIG. 3 illustrates a gear strength tester used for the evaluation of the gear according to the present embodiment, and includes a drive motor (a1), a bearing support (a2), a tachometer (a3), and a power absorption device (b1). , A bearing support (b2), a torque meter (b3), a microphone (c1), a sound level meter (c2), a soundproof box (d1), a metal gear (A), and a test gear (B).

  As shown in FIG. 3, the gear strength tester includes a drive motor (a1) and a metal gear (A) via a shaft (module: 0.8, pitch circle diameter: 100 mm, tooth width: 30 mm, helix angle. : 20 degree) helical metal gears are installed, and can be operated at an arbitrary rotational speed. The metal gear (A) and the test gear (B) mesh with each other with a backlash amount of 0.1 mm. The test gear (B) is connected to the power absorption device (b1) via a shaft and has a structure capable of applying an arbitrary load torque.

2. Test Gear The test gear (B) in FIG. 3 is a helical gear, which has a module 0.8, a pitch circle diameter of 100 mm, a helix angle of 15 degrees, and a gate having a diameter of 1.5 mm at three points on the boss portion. (See FIG. 1B), and the intervals are uniform every 120 degrees in the circumferential direction. Moreover, as shown to FIG. 1A, the upper rib height g1 and the lower rib height g2 are the same dimensions.

3. Measurement of the tooth tip density of the gear The tooth tip density of the gear is determined by cutting the tooth portion G at the boundary with the rim F shown in FIG. 1B and using an electronic hydrometer (ELEKTRON TEK, SD-200L) Measured by the method. Measurement was performed on arbitrary five teeth, and the average value was defined as the tip density.

4). Test method using gear strength tester A torque of 2 N · m was applied, and a test was continuously performed for 100 hours. The rotation speed was fixed at 100 rpm. The test was performed in a constant temperature room at 23 ° C. and 50% humidity.

(1) Durability As the durability, the appearance of the gear before and after the test was evaluated according to the following criteria.

  A: Before and after the test, the gear was not deformed with the naked eye, and there was no particular problem.

  ○: Before and after the test, deformation of the gear was confirmed with the naked eye, but there was no problem in operability.

  Δ: Some problems in operability.

  ×: Malfunction due to tooth breakage or shaft hole deformation.

  Examples and Comparative Examples will be described below with reference to Table 1 below. Experiments were conducted according to the above-described preparation and evaluation methods unless otherwise specified.

[Examples 1-4, Comparative Examples 1-3]
Under the conditions shown in Table 1, a polyacetal resin composition was produced by the method described above, and a gear (injection molded resin gear) was produced by injection molding the polyacetal resin composition.

From Table 1, Examples 1-4 which made the tooth-tip density of the injection-molded resin gear within a specific range (1.420 g / cm ³ or more) due to the effects of injection pressure, annealing, etc., have durability (gear durability) ) And excellent productivity in a well-balanced manner. In particular, Examples 1 and 2 using a polyacetal resin composition having an MFR of 0.8 to 1.5 g / 10 min were also confirmed to be significantly superior in durability under high torque. On the other hand, Comparative Examples 1 to ^{3 in} which the tooth tip density of the injection-molded resin gear is out of a specific range (less than 1.420 g / cm ³ ) are compared with Examples 1 to 4 having the same gear shape, It was confirmed that the durability was significantly inferior.

[Examples 5 to 8]
Under the conditions shown in Table 1, a polyacetal resin composition was produced by the method described above, and a gear (injection molded resin gear) was produced by injection molding the polyacetal resin composition.

As shown in Table 1, in the manufacturing methods of the gears of Examples 5 to 8, the density of the tooth tip is 1.420 g / cm ³ by compression molding, pressure holding effect, and a combination thereof, instead of annealing conditions. It differs from the manufacturing method of the gear of Example 1 by the above point. It was confirmed that the gears of Examples 5 to 8 had the same gear shape and the same durability was obtained as compared with the gear of Example 1 in which the polyacetal resin composition had the same MFR value. Above all, it was confirmed that the gear of Example 7 which combined compression molding and annealing conditions was remarkably excellent in the balance between durability and productivity.

[Examples 9 to 11]
Under the conditions shown in Table 1, a polyacetal resin composition was produced by the method described above, and a gear (injection molded resin gear) was produced by injection molding the polyacetal resin composition.

  Furthermore, as shown in Table 1, the gear manufacturing methods of Examples 9 to 11 are the gears of Examples 1 to 8 in that a polyacetal resin composition containing 50 ppm of talc is used as crystal nucleation inorganic particles. This is different from the manufacturing method. The gears of Examples 9 to 11 have the same gear shape and the balance of durability and productivity compared to the gears of Example 4 and Comparative Example 3 in which the polyacetal resin composition has the same MFR value. It was confirmed that it was excellent. Especially, it was confirmed that the gear of Example 9 in which the MFR value of the polyacetal resin composition is 0.8 to 1.5 g / 10 minutes is remarkably excellent in durability.

  The gear according to the present invention has an excellent balance between durability and productivity as compared with a conventional fiber reinforced polyamide resin or the like. Therefore, the gear according to the present invention can be suitably used in the automobile field, the electric / electronic field, and other industrial fields.

a stirring motor with stirring blade,
b Reactor with jacket,
c Slurry circulation pump,
1 Supply of formaldehyde gas,
2 Supply of chain transfer agent and polymerization solvent,
3 Supply of polymerization catalyst,
4 Slurry collection,
a1 drive motor,
a2 bearing support,
a3 Tachometer,
b1 power absorber,
b2 bearing support,
b3 Torque meter,
c1 microphone,
c2 Sound level meter,
d1 soundproof box,
A metal gear,
B test gear,
C boss,
D web,
E rib,
F rim,
G tooth,
H shaft hole,
e Web thickness,
f Rim height,
g1 Upper rib height,
g2 Lower rib height,
h Boss thickness,
i Rim thickness.

Claims (8)

A gear containing a polyacetal resin composition, wherein the tooth tip density is 1.420 g / cm ³ or more.   The gear according to claim 1, wherein the polyacetal resin composition includes a polyacetal homopolymer.   The gear according to claim 1 or 2, wherein a melt flow rate of the polyacetal resin composition is 0.8 to 3.0 g / 10 minutes.   The gear according to claim 1 or 2, wherein a melt flow rate of the polyacetal resin composition is 0.8 to 1.4 g / 10 minutes.   The gear according to any one of claims 1 to 4, which is non-fiber reinforced.   The manufacturing method of the gear of any one of Claims 1-4 including the process of injection-molding a raw material with the injection pressure of 160-300 MPa.   The gear manufacturing method according to claim 6, wherein in the injection molding, the raw material is compressed in a mold after injection.   The manufacturing method of the gear in any one of Claim 6 or 7 including the process of heat-processing a molded object for 2 to 8 hours in the temperature range of 100-160 degreeC after injection molding.