JP7245117B2 - Polyacetal powder, method of use thereof, and additive manufacturing method - Google Patents

Description

The present invention relates to polyacetal powders and methods of use thereof, as well as additive manufacturing methods.

In recent years, additive manufacturing technology has been widely used as one of the techniques for producing a modeled product having a complicated structure.

Since this additive manufacturing technology is a method that does not use a mold, it has the advantage of being able to produce prototypes in a short period of time, and is increasingly being used, for example, for functional confirmation of prototypes. Moreover, there is an increasing need for application not only to trial production but also to direct production of small-lot, high-variety products. Against this background, among the additive manufacturing technologies, 3D printers that use powdered resin as a modeling material, such as selective laser sintering, are attracting attention.

In the selective laser sintering method, a powder material is spread on a modeling table with a roller or blade, and the powder material is selectively irradiated with a laser, heated and sintered, and the process is repeated in sequence to create a laminate. It is a method of making (Patent Document 1). Selective laser sintering is a method that can be used with thermoplastic resins that are currently used for resin-made industrial parts, so it can be used for other additive manufacturing technologies (liquid bath photopolymerization, material injection, etc.). In comparison, it is superior in that the strength, reliability, and dimensional stability of the molded product are high. Another feature of the selective laser sintering method is that it enables the production of shaped articles with rather complex shapes and contours, which cannot be achieved by conventional methods such as machining.

Polyamide 11, polyamide 12, polypropylene and the like are widely known as resins constituting powder materials that can be used in the selective laser sintering method (Patent Document 2). Further, Patent Document 3 discloses a powder composed of polyacetal that can be used in selective laser sintering.

U.S. Pat. No. 4,863,538 WO 1996/006881 WO2011/051250

However, in Patent Documents 1 to 3, there is still room for improvement because the problem of warpage during molding of polyacetal, warpage of molded articles, and mechanical properties have not been sufficiently studied.

Therefore, the problem to be solved by the present invention is to provide a polyacetal powder that is capable of obtaining a molded product with less warpage and excellent mechanical properties. Another object of the present invention is to provide a method of using the polyacetal powder and an additive manufacturing method using the polyacetal powder.

As a result of extensive research to solve the above problems, the present inventors have found that polyacetal powder having specific powder characteristics and containing polyethylene in a specific mass ratio has less warpage and excellent mechanical properties. The present invention was completed by discovering that it is possible to manufacture products.

That is, the present invention is as follows.
[1]
The average particle diameter (D50) is 5 μm or more and 70 μm or less,
100 parts by mass of polyacetal and 0.01 to 5 parts by mass of polyethylene ,
A polyacetal powder , wherein the value obtained by dividing the solid bulk density by the loose bulk density (solid bulk density/loose bulk density) is 1.45 or less .
[2]
The polyacetal powder according to [1], which has a crystallization speed of 40 seconds or more.
[3]
The polyacetal powder according to [1] or [2], which has a crystallization temperature of 130°C or higher and 145°C or lower.
[4]
The polyacetal powder according to any one of [1] to [3], wherein polyethylene has a density of 0.88 to 0.93 g/cm ³ .
[5]
The polyacetal powder according to any one of [1] to [4], wherein MFR of polyethylene is 0.2 to 100 g/10 minutes.
[6 ]
The polyacetal powder according to any one of [1] to [ 5 ], wherein the proportion of particles whose diameter is twice or more the D50 is 10 vol% or less.
[ 7 ]
The polyacetal powder according to any one of [1] to [ 6 ], wherein the proportion of particles having a diameter of 0.2 times or less of D50 is 10 vol% or less.
[ 8 ]
The polyacetal powder according to any one of [1] to [ 7 ], wherein the D50 is 65 μm or less, and the proportion of particles having a diameter of 0.2 times or less of the D50 is 3 vol % or less.
[ 9 ]
The polyacetal powder according to any one of [1] to [ 8 ], containing 0.05% by mass or more and 1.0% by mass or less of hydrophobic silica with respect to 100% by mass of the polyacetal powder.
[ 10 ]
The polyacetal powder according to [ 9 ], wherein the hydrophobic silica is dry-process silica.
[ 11 ]
The polyacetal powder according to [ 9 ] or [ 10 ], wherein the hydrophobic silica has an average primary particle size of 1 nm or more and 20 nm or less.
[ 12 ]
The polyacetal powder according to any one of [ 9 ] to [ 11 ], wherein the hydrophobic silica has a BET specific surface area of 50 m ² /g or more and 300 m ² /g or less.
[ 13 ]
The polyacetal powder according to any one of [ 9 ] to [ 12 ], wherein the hydrophobic silica is hydrophobized with one or more selected from the group consisting of dimethyldichlorosilane, trimethylchlorosilane, and silicone oil.
[ 14 ]
The polyacetal powder according to any one of [1] to [ 13 ], which has a weight loss rate of 5% or less when held for 1 hour at a temperature 10°C lower than the melting point of the polyacetal powder.
[ 15 ]
The polyacetal powder according to any one of [1] to [ 14 ], which is used for additive manufacturing.
[ 16 ]
A method for using the polyacetal powder according to any one of [1] to [ 15 ], which is characterized in that it is used for additive manufacturing.
[ 17 ]
[1] An additive manufacturing method, wherein the polyacetal powder according to any one of [1] to [ 15 ] is used as a molding material.

ADVANTAGE OF THE INVENTION According to this invention, it is possible to provide a polyacetal powder with which it is possible to obtain a shaped article with less warpage and excellent mechanical properties. Further, according to the present invention, it is possible to provide a method of using the polyacetal powder and an additive manufacturing method using the polyacetal powder.

EMBODIMENT OF THE INVENTION Hereinafter, the form (henceforth this embodiment) for implementing this invention is demonstrated in detail. It should be noted that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present invention.

[Polyacetal powder]
The polyacetal powder of the present embodiment has an average particle diameter (D50) of 5 μm or more and 70 μm or less, and is characterized by containing 100 parts by mass of polyacetal and 0.01 to 5 parts by mass of polyethylene.
The polyacetal powder of the present embodiment may be a polyacetal powder containing only at least one polyacetal and at least one polyethylene as resin components, or a powder containing only one polyacetal and one polyethylene as resin components. It may also be some polyacetal powder.
According to the polyacetal powder of the present embodiment, it is possible to obtain a molded product with less warpage and excellent mechanical properties.

Moreover, the polyacetal powder in the present embodiment may further contain hydrophobic silica, other resins, additives, etc., if necessary.
In particular, by including hydrophobic silica, the polyacetal powder of the present embodiment can be improved in high-temperature lamination properties and powder application to a modeled portion, and a modeled article having higher mechanical properties can be obtained.

<Polyacetal>
Polyacetal contained in the polyacetal powder of the present embodiment includes, for example, polyacetal homopolymers and polyacetal copolymers, and polyacetal copolymers are preferred from the viewpoint of moldability and thermal stability. On the other hand, polyacetal homopolymer is preferred from the viewpoint of the mechanical strength of the resulting shaped article.
The above polyacetal may be used alone or in combination of two or more.

[Polyacetal homopolymer]
A polyacetal homopolymer is a polymer having only oxymethylene units in its main chain.
A polyacetal homopolymer can be obtained, for example, by a known slurry polymerization method (for example, the method described in JP-B-47-6420 and JP-B-47-10059).
In addition, commercially available polyacetal homopolymers include Tenac (trademark) manufactured by Asahi Kasei Corporation.

[Polyacetal Copolymer]
A polyacetal copolymer is a copolymer having, in the main chain, oxymethylene units derived from a main monomer and comonomer units derived from a comonomer. The comonomer unit is not particularly limited as long as it is a unit that can be copolymerized with an oxymethylene unit, but an oxyalkylene unit having 2 or more carbon atoms is preferable. Both ends or one end of the polyacetal copolymer may be blocked with an ester group and/or an ether group.

The polyacetal copolymer preferably contains 0.3 mol or more, more preferably 0.4 mol or more, still more preferably 0.5 mol or more of the comonomer unit per 100 mol of the oxymethylene unit. It is more preferable to contain 6 mol or more, and it is particularly preferable to contain 1.2 mol or more. In addition, the polyacetal copolymer preferably contains 3.0 mol or less, more preferably 2.0 mol or less, and even more preferably 1.5 mol or less of the comonomer unit. By setting the content ratio of the comonomer unit to 100 mol of the oxymethylene unit within the preferred range described above, there is a tendency to obtain a polyacetal powder having a good balance between sculptability by a 3D printer and thermal stability.

The content of comonomer units in the polyacetal copolymer can be obtained by the following procedure using ¹ H-NMR method.
The procedure below describes a polyacetal copolymer in which the comonomer unit is an oxyalkylene unit having 2 or more carbon atoms.
That is, the polyacetal copolymer was dissolved in hexafluoroisopropanol to a concentration of 1.5% by mass over 24 hours, and ¹ H-NMR analysis was performed using this solution, and an oxymethylene unit and a carbon number of 2 or more were detected. From the ratio of the integral value of the oxyalkylene unit and the peak attributed to, the content ratio of the oxyalkylene unit having 2 or more carbon atoms (bmol) to the oxymethylene unit (a = 100mol) (b/a: mol/100mol) can be asked for.

1) Main Monomer Examples of main monomers used in the production of polyacetal copolymers include cyclic oligomers such as formaldehyde, its trimer trioxane, and its tetramer tetraoxane.
In the present embodiment, "main monomer" refers to a monomer unit containing 50% by mass or more of the total amount of monomers.

2) Comonomer The comonomer used in the production of the polyacetal copolymer is not particularly limited, but examples thereof include cyclic ether compounds having an oxyalkylene unit with 2 or more carbon atoms in the molecule.

The cyclic ether compound is not particularly limited, but examples include ethylene oxide, propylene oxide, 1,3-dioxolane, 1,3-propanediol formal, 1,4-butanediol formal, 1,5-pentanediol formal, 1, Examples include 6-hexanediol formal, diethylene glycol formal, 1,3,5-trioxepane, 1,3,6-trioxiocane, and mono- or di-glycidyl compounds capable of forming a branched or crosslinked structure in the molecule.
A cyclic ether compound may be used individually by 1 type, and may be used in combination of 2 or more type.

The method for producing the polyacetal copolymer is not particularly limited, but conventionally known methods such as U.S. Pat. , No. 1720358, No. 3018898, JP-A-58-98322 and JP-A-7-70267. Specifically, a cationic initiator is used as a polymerization catalyst, and a formaldehyde monomer or a cyclic oligomer of formaldehyde, a cyclic ether compound as a comonomer, a chain transfer agent (molecular weight modifier), and an organic solvent are used as raw materials for a polymerization reaction. and a method of producing by a continuous bulk polymerization reaction. In addition, the obtained polyacetal copolymer (crude polyacetal copolymer whose terminals are not stabilized) is subjected to a known method (for example, the method described in JP-B-63-452, etc.) to add an ether group, an ester group, or the like to the end. It may be sealed and stabilized.
Commercially available polyacetal copolymers include Tenac-C (trademark) manufactured by Asahi Kasei Corporation.

The polyacetal content in 100% by mass of the polyacetal powder of the present embodiment is preferably 90% by mass or more, more preferably 95% by mass or more, and particularly preferably 97 to 99.7% by mass.

<Polyethylene>
The polyethylene contained in the polyacetal powder of the present embodiment is not particularly limited, and generally commercially available high-density polyethylene (e.g., polyethylene having a density of 0.942 g/cm ³ or more) or low-density polyethylene (e.g., density Polyethylene of less than 0.930 g/cm ³ ) may also be used, but from the viewpoint of compatibility with polyacetal, and from the viewpoint of slowing the crystallization speed of polyacetal powder and improving moldability with a 3D printer, low-density polyethylene is more preferred.

The density of the polyethylene is preferably 0.88 to 0.93 g/cm ³ , and 0.91 to 0.93 g/cm 3 from the viewpoint of obtaining a molded product with less warpage and more excellent adhesion between layers. ³ is more preferred.

As the low-density polyethylene, high-pressure low-density polyethylene and/or linear low-density polyethylene can be used. Density polyethylene is preferred.
High-pressure low-density polyethylene can be produced, for example, by radical polymerization under high pressure of 1000 to 3000 kg/cm ² . Short-chain branching (ethyl branching or butyl branching) is generated by an intramolecular hydrogen abstraction reaction due to back biting during polymerization, resulting in a low density. The high-pressure low-density polyethylene preferably has branches (long-chain branches) comparable to the main chain due to an intermolecular hydrogen abstraction reaction. The density of the high-pressure low-density polyethylene is preferably 0.91-0.93 g/cm ³ .
Linear low-density polyethylene is, for example, polyethylene produced by ionic polymerization, and α-olefins such as 1-butene, 1-hexene, 4-methylpen-1, and 1-octene are added at a few percent to a few percent relative to the weight of ethylene. It can be obtained by introducing short chain branches by polymerizing 10%. The density of the linear low-density polyethylene is preferably 0.88-0.93 g/cm ³ .

The melt flow index (MFR) of polyethylene is preferably 0.2 to 100 g/10 minutes, more preferably 3 to 12 g/10 minutes, still more preferably 5 to 10 g/10 minutes.
The MFR can be measured under the conditions of 190° C. and a load of 2.16 kg according to the ISO1133 A method.

The blending amount of polyethylene is 0.01 to 5 parts by mass, preferably 0.05 to 3 parts by mass, more preferably 0.06 to 3 parts by mass, and still more preferably 0.13 parts by mass with respect to 100 parts by mass of polyacetal. ~3 parts by mass. If it is less than 0.01 part by mass, the effect of slowing down the crystallization speed to improve the warpage of the shaped product and the adhesion between layers is insufficient, and if it is added in excess of 5 parts by mass, peeling occurs on the surface and inside of the shaped product. It is not desirable in some cases.

<Hydrophobic silica>
The polyacetal powder of this embodiment may further contain hydrophobic silica.
The content of hydrophobic silica in 100% by mass of the polyacetal powder of the present embodiment is preferably 0.05% by mass or more and 1.0% by mass or less.
In additive manufacturing the flowability of the polyacetal powder is very important. The polyacetal powder is melted and dented in the molding area by heating with a laser, but if powder with poor fluidity is used, the roughness of the surface of the dent becomes large, and the powder does not adhere to the molding area, resulting in voids. There is a risk that the mechanical properties of the molded product will be impaired or the molding will be stopped. In this regard, the present inventors have found that the polyacetal powder further contains hydrophobic silica in a content of 0.05% by mass or more and 1.0% by mass or less, and the hydrophobic silica is adhered (exposed) to the surface It has been found that when the powder is in the form of particles, the fluidity of the powder can be improved, the layering property at high temperatures and the powder application to the molded part can be improved, and the mechanical properties of the molded product can be improved.
Furthermore, as the polyacetal powder, a powder having a narrow particle size distribution curve (hereinafter simply referred to as "particle size distribution curve") expressed as a number-based frequency distribution is usually preferred, but the width of the particle size distribution curve is wide. The inventors have found that the same effect as described above can be obtained even when the powder is used. Thereby, the productivity of the polyacetal powder (for example, the yield after the classification step, etc.) can also be increased.

On the other hand, when the content of hydrophobic silica in the polyacetal powder is less than 0.05% by mass, the effect of improving the fluidity of the powder is insufficient. In addition, if the content of hydrophobic silica exceeds 1.0% by mass, the laser absorption of the polyacetal powder is inhibited by the hydrophobic silica, so that the amount of heat required for heating and sintering cannot be obtained, and the voids ( Voids) may occur, which may impair the mechanical properties and molding accuracy of the molded product. Also, even if hydrophilic silica is used instead of hydrophobic silica, the hydrophilic silica does not adhere (expose) to the surface, and the effect of improving the fluidity of the powder cannot be exhibited.
From the same point of view, the content of hydrophobic silica in the polyacetal powder of the present embodiment is more preferably 0.1% by mass or more and 0.7% by mass or less with respect to 100% by mass of the polyacetal powder. More preferably, the content is not less than 0.5% by mass and not more than 0.5% by mass.

In the polyacetal powder of the present embodiment, the particulate with hydrophobic silica attached (exposed) to the surface means that at least a portion of one hydrophobic silica particle is exposed to the surface of the powder particle, and the remaining A portion may be embedded in the particles. On the other hand, when the hydrophobic silica particles are not exposed on the surface of the powder particles, such as when hydrophobic silica is kneaded into a polyacetal composition containing polyacetal and polyethylene to produce polyacetal powder, it is not included.
Whether or not the polyacetal powder is particulate with hydrophobic silica adhered (exposed) to the surface can be confirmed by observing the surface of the powder particles with a scanning electron microscope (SEM).
The polyacetal powder described above can be obtained by adding and mixing a predetermined amount of hydrophobic silica (hydrophobic silica mixing step) before or after the classification step, or at the same time as the classification step, as described later.

In the present embodiment, the hydrophobic silica is not particularly limited, and those commonly used in industrial applications can be used. For example, both dry process silica and wet process silica can be used. Dry process silica is preferably used. Wet-process silica may contain impurities that decompose polyacetal during its manufacturing process, and may impair the resin characteristics of the polyacetal powder. On the other hand, dry-process silica is unlikely to contain such impurities during its manufacturing process, and there is no risk of impairing the resin characteristics of the polyacetal powder.

In the present embodiment, the average primary particle size of the hydrophobic silica is not particularly limited, but is preferably 1 nm or more and 20 nm or less. If the average primary particle size of the hydrophobic silica is within the above range, the fluidity of the polyacetal powder can be further improved, and the layering properties at high temperatures, the powder placement on the molded part, and the mechanical properties of the molded product can be improved. can be improved.
From the same point of view, the average primary particle size of the hydrophobic silica is more preferably 5 nm or more and 15 nm or less, and even more preferably 7 nm or more and 13 nm or less.
The average primary particle size of hydrophobic silica can be measured by a wet method (solvent: water) by a laser diffraction/scattering method.

In the present embodiment, the BET specific surface area of the hydrophobic silica is not particularly limited, but is preferably 50 m ² /g or more and 300 m ² /g or less. If the BET specific surface area of the hydrophobic silica is within the above range, the fluidity of the polyacetal powder can be further increased, and the lamination property at high temperatures, powder placement on the molded part, and the mechanical properties of the molded product can be further improved. can.
From the same point of view, the BET specific surface area of the hydrophobic silica is more preferably 100 m ² /g or more and 250 m ² /g or less, and still more preferably 130 m ² /g or more and 200 m ² /g or less.
The BET specific surface area of hydrophobic silica can be determined according to the BET method.

In the present embodiment, the dry-process silica is not particularly limited, and one commonly used in industrial applications can be used. It is preferable that the Hydrophobic silica that has been hydrophobized with a surface modifier has a reduced number of surface hydroxyl groups, thereby increasing adhesion to the surface of particles of the polyacetal composition and further increasing the fluidity of the polyacetal powder.
The surface modifier is not particularly limited as long as it is commonly used for surface modification treatment of silica, and examples thereof include dimethyldichlorosilane, trimethylchlorosilane, silicone oil, octylsilane, hexamethylsilazane, and the like. . Above all, it is preferable to hydrophobize with at least one surface modifier selected from the group consisting of dimethyldichlorosilane, trimethylchlorosilane, and silicone oil.
One of the above surface modifiers may be used alone, or two or more thereof may be used in combination.
As the hydrophobic silica hydrophobized with the surface modifier, a commercially available product may be used, or a hydrophobic silica obtained by hydrophobization using a surface modifier may be used.

<Other resins>
The polyacetal powder of the present embodiment may contain resins other than polyacetal and polyethylene as resins.
Specific examples of other resins that can be contained as a resin component are not particularly limited and can be appropriately selected depending on the purpose. Examples include polyamide 6, polyamide 11, polyamide 12, polyamide 66, polypropylene, polyether Crystalline resins such as ether ketone (PEEK), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT) are included. These may be used individually by 1 type, and may use 2 or more types together.
The content of other resins in the polyacetal powder may be 0.05 to 5 parts by mass with respect to 100 parts by mass of polyacetal.

<Additive>
Further, the polyacetal powder of the present embodiment includes, for example, antioxidants, stabilizers (thermal stabilizers, etc.), weathering (light) agents (ultraviolet absorbers, etc.), lubricants (release agents, etc.), crystal nucleating agents, Additives such as inorganic/organic fillers, conductive agents/antistatic agents, and appearance modifiers (pigments, dyes, etc.) may also be included. In particular, the polyacetal powder of the present embodiment preferably contains an antioxidant and a heat stabilizer as additives. The antioxidant is preferably a hindered phenolic antioxidant, and the heat stabilizer preferably contains formaldehyde-reactive nitrogen.
An additive may be used individually by 1 type, and may use 2 or more types together.

<<Antioxidant>>
As the antioxidant, a hindered phenol-based antioxidant is preferably used.
Examples of hindered phenol antioxidants include 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-t-butyl-4-hydroxyphenyl)-propionate], 1,4-butanediol-bis-[3-(3,5-di-t-butyl -4-hydroxyphenyl)-propionate], triethylene glycol-bis-[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)-propionate], pentaerythritol tetrakis[methylene-3-(3' , 5′-di-t-butyl-4′-hydroxyphenyl)propionate]methane and the like. 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.
An antioxidant may be used individually by 1 type, and may use 2 or more types together.

<<Thermal Stabilizer>>
Examples of heat stabilizers include 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, and the like. .

Specific examples of amino-substituted triazine compounds include 2,4-diamino-sym-triazine, 2,4,6-triamino-sym-triazine, N-butylmelamine, N-phenylmelamine, N,N-diphenylmelamine, N , N-diamino-6-butyl, N-diallylmelamine, benzoguanamine (2,4-diamino-6-phenyl-sym-triazine), acetoguanamine (2,4-diamino-6-methyl-sym-triazine) -sym-triazine and the like.

Specific examples of adducts of amino-substituted triazine compounds and formaldehyde include N-methylolmelamine, N,N'-dimethylolmelamine and N,N',N''-trimethylolmelamine.

Specific examples of the condensates of amino-substituted triazine compounds and formaldehyde include melamine/formaldehyde condensates.

Examples of urea derivatives include N-substituted urea, urea condensates, ethylene urea, hydantoin compounds, ureido compounds and the like. Specific examples of N-substituted urea include methyl urea, alkylenebis urea, and aryl-substituted urea substituted with a substituent such as an alkyl group. Specific examples of urea condensates include condensates of urea and formaldehyde. Specific examples of hydantoin compounds include hydantoin, 5,5-dimethylhydantoin, 5,5-diphenylhydantoin and the like. Specific examples of ureido compounds include allantoin and the like.

Examples of hydrazine derivatives include hydrazide compounds. Specific examples of the hydrazide compound include dicarboxylic acid dihydrazide, and more specific examples include malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, pimelic acid dihydrazide, sperate dihydrazide, azelaic acid dihydrazide, Sebacic acid dihydrazide, dodecanedioic acid dihydrazide, isophthalic acid dihydrazide, phthalic acid dihydrazide, 2,6-naphthalenedicarbodihydrazide and the like can be mentioned.

Specific examples of imidazole compounds include imidazole, 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole and the like.

Specific examples of imide compounds include succinimide, glutarimide, and phthalimide.

A heat stabilizer may be used individually by 1 type, and may use 2 or more types together.

Stabilizers other than heat stabilizers, weathering (light) agents (ultraviolet absorbers, etc.), lubricants (mold release agents, etc.), crystal nucleating agents, inorganic/organic fillers, conductive agents/antistatic agents, appearance improvers As (pigments, dyes, etc.), commonly used ones can be appropriately used.

The content of the additive is not particularly limited, and can be appropriately set within a range that does not impair the effects of the present invention.

<Average particle size (D50)>
The polyacetal powder of the present embodiment is characterized by having an average particle size (D50) of 5 μm or more and 70 μm or less. If the average particle size (D50) of the polyacetal powder is within the above range, sufficient workability can be ensured, for example, it can be sufficiently uniformly spread to a thickness of about 100 μm, and the resulting molded product has minute voids. It is possible to sufficiently suppress the generation of (voids).
When the D50 of the polyacetal powder is less than 5 μm, the workability deteriorates. is likely to occur frequently.
From the same point of view, D50 of the polyacetal powder is preferably 10 μm or more and 60 μm or less, more preferably 20 μm or more and 55 μm or less.

The D50 of the polyacetal powder of the present embodiment is the 50% particle diameter (median diameter) accumulated from the fine particle side, which is obtained from the volume-based particle size distribution measured by the wet method (solvent: water) by the laser diffraction/scattering method. More specifically, it can be measured by the method described in Examples below.

<Particle size distribution>
In the polyacetal powder of the present embodiment, the proportion of particles whose diameter is two times or more the average particle diameter (D50) (for example, when D50 is 50 μm, particles whose diameter is 100 μm or more) is 10 vol% or less. is preferably When the above ratio is 10 vol% or less, it is possible to improve the fluidity of the polyacetal powder, increase the density of the molded product, improve the mechanical properties, and make the surface of the molded product smoother. Since the molding accuracy is improved, it becomes easier to manufacture precise parts.
From the same point of view, the proportion of particles having a diameter of at least twice the D50 in the polyacetal powder is more preferably 5 vol% or less, further preferably 3 vol% or less, and substantially 0 vol%. is even more preferable.

In the polyacetal powder of the present embodiment, the proportion of particles whose diameter is 0.2 times or less the average particle diameter (D50) (for example, when D50 is 50 μm, particles whose diameter is 10 μm or less) is 10 vol. % or less.
The laser beam used for shaping can have a very small irradiation spot size, so that the contour resolution of the shaped article can be very sharp. However, due to heat conduction from the point fused by heating with the laser, the powder outside the point scanned by the laser fuses directly to the part to be sintered, exceeding the laser scanning area, and thus exceeding the design dimensions. There is a risk that the powder will fuse beyond the limit. In addition, if the heat from the sintering remains, even if the powder is simply laid on top of the layer, the powder will sinter into the sintered portion of the previous layer, causing so-called inter-layer growth. There is fear. When such inter-layer growth occurs, the molding precision is lowered, and it becomes difficult to remove the unsintered powder from the finished product. In this regard, the present inventors have found that particles with a diameter of 0.2 times or less of D50 are particularly prone to fusion, affecting modeling accuracy, and that the ratio is 10 vol% or less. , It was found that the molding accuracy was improved and it became easy to manufacture precise parts. Furthermore, when the above ratio is 10 vol % or less, it is possible to improve the fluidity of the polyacetal powder, increase the density of the molded article, and improve the mechanical properties.
From the same point of view, the proportion of particles having a diameter of 0.2 times or less of D50 in the polyacetal powder is more preferably 5 vol% or less, further preferably 3 vol% or less, and substantially 0 vol%. is even more preferable.

In addition, the polyacetal powder of the present embodiment preferably has a D50 of 65 μm or less and a proportion of particles having a diameter of 0.2 times or less of the D50 of 3 vol % or less. When the D50 and the ratio of particles having a diameter of 0.2 times or less of the D50 are each within the above ranges, the layering property of the polyacetal powder can be further improved, and the mechanical properties of the resulting shaped article can be further improved. .
From the same point of view, it is more preferable that the proportion of particles having a D50 of 63 μm or less and a diameter of 0.2 times or less of the D50 be 2 vol % or less, more preferably 2 vol % or less of particles having a D50 of 62 μm or less and a diameter of 0.2 times the D50. It is more preferable that the proportion of particles having a ratio of 2 times or less is 1 vol % or less.

The proportion of particles with a diameter of 2 times or more D50 and the proportion of particles with a diameter of 0.2 times or less than D50 can be obtained from the particle size distribution and D50 measured by a laser diffraction/scattering method.
The polyacetal powder in which the ratio of particles having a diameter of 2 times or more D50 and/or the ratio of particles having a diameter of 0.2 times or less than D50 is within the above range is obtained by the pulverization step and/or classification of the method for producing polyacetal powder. It can be obtained by adjusting various conditions in the process.

<Melting point>
The polyacetal powder of the present embodiment preferably has a melting point of 160° C. or higher and 173° C. or lower. If the melting point is within the above range, the temperature difference (process window) from the crystallization temperature of the polyacetal powder can be sufficiently widened, and the tolerance of the polyacetal powder to the temperature difference in the molding area during molding is widened (for example, (Even if there are parts where the temperature of the molding area is lower than the set temperature due to heat radiation to the surroundings, it is possible to suppress the occurrence of warping during molding), and improve the molding accuracy of the resulting molded product. be able to. In addition, when mass-producing a large number of shaped products at once, it is possible to widen the tolerance of the polyacetal powder to the temperature difference in the shaping apparatus, and to suppress variations in shaping accuracy between shaped products.
From the same point of view, the melting point of the polyacetal powder is more preferably 162° C. or higher and 171° C. or lower.

Here, the melting point of the polyacetal powder means the intrinsic melting point of the polyacetal powder, which can be measured as the second scan melting point in differential scanning calorimetry (DSC). The melting point of the first scan measured by DSC is affected by the thermal history of the polyacetal powder to be measured, so it may be lower than the intrinsic melting point of the polyacetal powder, such as when freeze-pulverized during the manufacturing process. On the other hand, the melting point of the second scan measured by DSC is measured as the melting point when the polyacetal powder melts and solidifies once during the first scan measurement, and then melts again. can have an intrinsic melting point.

A polyacetal powder having a melting point within the above range can be obtained by controlling the crystallinity of the polyacetal copolymer by adjusting the comonomer unit content within the above range.
Generally, increasing the comonomer content in the polyacetal copolymer lowers the degree of crystallinity and lowers the melting point of the polyacetal powder. Conversely, decreasing the comonomer content in the polyacetal copolymer increases the degree of crystallinity and can increase the melting point of the polyacetal powder.

<Difference between First Scan Melting Point and Second Scan Melting Point>
In the polyacetal powder of the present embodiment, the difference between the melting point of the first scan and the melting point of the second scan in differential scanning calorimetry (DSC) measurement is preferably 0° C. or more and 5° C. or less.
The melting point of polyacetal powder, which determines the width of the process window, which is important in additive manufacturing using resin powder as a modeling material, corresponds to the melting point of the first scan in DSC measurement. The fast scan melting point reflects the thermal history that the polyacetal powder was subjected to before it was manufactured.
On the other hand, the melting point of the second scan is the melting point of the resin itself, and the higher the better. Even if the melting point of the first scan is low, if the melting point of the second scan is high, it is possible to raise the melting point of the first scan by performing a post-treatment such as annealing, which will be described later.

If the difference between the melting point of the first scan and the melting point of the second scan is 0° C. or more and 5° C. or less, a wide process window close to the capabilities of the resin can be achieved.
From the same point of view, the difference between the melting point of the first scan and the melting point of the second scan in the DSC measurement of the polyacetal powder is more preferably 0° C. or higher and 4° C. or lower, and more preferably 0° C. or higher and 3° C. or lower. preferable.
In addition, the difference between the melting point of the first scan and the melting point of the second scan in DSC measurement is the melting point of the higher one to the lower melting point of the melting point of the first scan and the melting point of the second scan measured by DSC measurement. is subtracted from the value.
Polyacetal powder having a difference between the melting point of the first scan and the melting point of the second scan within the above range should be annealed (lower than the melting point and It can be obtained by holding for a certain period of time at a high temperature that allows crystal growth (for example, 120° C. for 1 hour) to allow crystal growth.

<Crystallization temperature>
The polyacetal powder of the present embodiment preferably has a crystallization temperature of 130°C or higher and 145°C or lower.
The temperature in the modeling area and in the modeling apparatus during additive manufacturing is always kept higher than the crystallization temperature of the resin powder. However, if there is some temperature difference (temperature unevenness) within the modeling area or within the modeling device, the polyacetal in the portion cooled to the crystallization temperature in the heated and sintered modeling layer will crystallize, especially in the stacking direction. Warpage occurs, and the molding accuracy is reduced in the resulting molded product. In this regard, the present inventors have found that by increasing the temperature difference between the melting point and the crystallization temperature of the polyacetal powder, the tolerance for temperature differences (temperature unevenness) within the molding area and within the molding apparatus can be increased. , it was found that warping during molding can be suppressed, and the molding accuracy of the resulting molded product can be improved. They also found that when mass-producing a large number of shaped products at one time, variations in modeling accuracy between shaped products can be suppressed.
From the same point of view, the crystallization temperature of the polyacetal powder is more preferably 135°C or higher and 143°C or lower.
The crystallization temperature of the polyacetal powder can be measured by differential scanning calorimetry (DSC), and more specifically by the method described in Examples below.
A polyacetal powder having a crystallization temperature within the above range can be obtained by adjusting the content of the comonomer unit in the main chain of the polyacetal copolymer and/or the amount of the crystal nucleating agent.

<Crystallization rate>
The polyacetal powder of the present embodiment preferably has a crystallization speed of 40 seconds or more. When the crystallization speed is 40 seconds or more, it is possible to allow temperature unevenness in the molding area and between layers during molding, further suppress warpage during molding, and improve the molding accuracy of the resulting molded product. From the same point of view, the crystallization speed of the polyacetal powder is more preferably 45 seconds or longer, and even more preferably 50 seconds or longer.
Here, the crystallization rate is determined by differential scanning calorimetry (DSC), for example, using "DSC-2C" manufactured by Parkin Elmer Co., Ltd., and rapidly lowering the temperature of the once melted polyacetal powder to a predetermined temperature near the crystallization temperature. After holding and observing the exothermic peak due to crystallization, the time from the start of holding at the predetermined temperature to reaching the maximum temperature of the exothermic peak (peak top temperature) is taken. More specifically, it can be measured by the method described in Examples below.

Methods for controlling the crystallization speed of the polyacetal powder include adjusting the content of comonomer units in the polyacetal copolymer, the presence/absence, type and amount of the crystal nucleating agent, and the like. Specifically, if the content of the comonomer unit is increased, the degree of crystallinity is lowered, so the crystallization rate tends to be slowed down. The crystallization rate can also be slowed by not including additives that act as crystal nucleating agents.
In addition, the crystallization speed can be adjusted, for example, by the type, density, content, etc. of polyethylene, and by using a specific proportion of low-density polyethylene (especially linear low-density polyethylene, high-pressure low-density polyethylene), etc. can be within the above range.

In addition, in general, as a method for suppressing the warp of a molded product in injection molding, the crystallization speed of the molding material is increased to sufficiently crystallize it in the mold, so that the warp occurs after it is removed from the mold. There are ways to reduce the
On the other hand, in modeling using a 3D printer or the like, warping during modeling can be suppressed by sufficiently slowing down the crystallization rate, contrary to the above method.

<Weight reduction rate>
The polyacetal powder of the present embodiment preferably has a weight loss rate of 5% or less when held for 1 hour at a temperature 10° C. lower than the melting point of the polyacetal powder. When the weight loss rate of the polyacetal powder is 5% or less, high thermal stability can be maintained, and long-term modeling is possible stably, and volatile organic substances (VOCs) such as formaldehyde due to heating are released. can be sufficiently suppressed.
From the same point of view, the weight loss rate of the polyacetal powder is more preferably 3% or less, and even more preferably 1% or less.
The weight loss rate of the polyacetal powder can be measured using a thermogravimetry device (manufactured by Perkin Elmer, trade name "Pyris 1 TGA", etc.), and more specifically, described in the examples below. method.
A polyacetal powder having a weight reduction rate within the above range can be obtained by adjusting the type and/or amount of components other than the polyacetal copolymer, such as polyethylene, hydrophobic silica, other resins, and various additives. can be done.

<MFR>
The polyacetal powder of the present embodiment preferably has a melt flow rate (MFR) of less than 70 g/10 minutes. If the polyacetal itself is of such a high molecular weight that the MFR is less than 70 g/10 minutes, shaped articles with excellent creep properties can be obtained.
Moreover, the polyacetal powder of the present embodiment preferably has a melt flow rate (MFR) of 2.0 g/10 minutes or more. When the MFR of the polyacetal powder is 2.0 g/10 minutes or more, the polyacetal flows when melted by a laser, so that the gaps between the particles can be filled more efficiently. A shaped article with fewer voids and excellent mechanical properties such as tensile strength is obtained.
From the same point of view, the MFR of the polyacetal powder is more preferably 2.5 g/10 min or more and 55 g/10 min or less, and still more preferably 3 g/10 min or more and 40 g/10 min or less.
Here, the MFR of the polyacetal powder is measured according to ISO1133 (condition D, temperature 190°C).
A polyacetal powder having an MFR within the above range can be obtained by adjusting the amount of the chain transfer agent added in the polymerization process of the polyacetal copolymer.

<Fixed Bulk Density/Loose Bulk Density>
In the polyacetal powder of the present embodiment, the value obtained by dividing the solid bulk density by the loose bulk density (solid bulk density/loose bulk density) is preferably 1.45 or less. If the value obtained by dividing the compacted bulk density by the loosened bulk density is within the above range, the polyacetal powder has excellent fluidity and stackability, and the generation of voids can be further suppressed. Physical properties can be further improved.
From the same point of view, the value obtained by dividing the solid bulk density by the loose bulk density is more preferably 1.40 or less, and even more preferably 1.36 or less.
The loose bulk density is the density measured when the powder is loosely packed (untapped) in a container of a given volume. The compacted bulk density is measured by repeatedly dropping (tapping) a container of a specified volume filled with powder from a constant height at a constant speed, and filling the container tightly until the bulk density of the powder in the container becomes almost constant. is. Specifically, the hardened bulk density and loose bulk density can be measured by the methods described in Examples below.

Polyacetal powder whose value obtained by dividing the solid bulk density by the loose bulk density is within the above range has a particle size distribution (for example, the proportion of particles whose diameter is 2 times or more of D50, particles whose diameter is 0.2 times or less of D50 ratio, etc.), particle shape (for example, the ratio of the average short diameter to the long diameter, the average surface smoothness shape factor, etc., which will be described later).

<Particle shape>
From the viewpoint of fluidity, the particles constituting the polyacetal powder of the present embodiment are preferably spherical. In general, it is difficult to directly form fine particles of resins such as polyacetal as a particle material by polymerization, and in order to produce powder using such resins, operations such as pulverizing pellets obtained by any method are required. necessary. Therefore, at that time, the pulverization method and pulverization conditions are appropriately adjusted to directly obtain nearly spherical particles, or the pulverized powder is stirred while being heated in post-processing, and the corners of the particles are rounded off. to obtain spherical particles.
The shape of the particles constituting the polyacetal powder of the present embodiment will be described in more detail below.

<<Ratio of average short diameter to long diameter of particles>>
It is preferable that the particles constituting the polyacetal powder of the present embodiment have an average minor axis to major axis ratio of 0.50 or more. When the average minor axis to major axis ratio of the particles is 0.50 or more, irregularities can be avoided, fluidity can be improved, and the powder can be laid out without gaps.
From the same point of view, the average minor axis to major axis ratio of the particles is more preferably 0.65 or more, and still more preferably 0.70 or more.
Here, the average short diameter to long diameter ratio refers to the ratio of the short diameter to the long diameter of the particles (short diameter / long diameter), and the average short diameter to long diameter ratio is the ratio of the short diameter to the long diameter of 100 arbitrarily selected particles. mean value. The minor axis and major axis respectively refer to the short side and long side of the circumscribing rectangle having the smallest circumscribing area on the boundary pixel of the particle.
The average short diameter to long diameter ratio of the particles constituting the polyacetal powder was obtained by using a digital microscope (manufactured by Keyence Corporation, "VH-7000 type" and "VH-501 lens") to obtain an image of 1360 × 1024. It can be obtained as an average value of short diameter/long diameter of 100 particles by saving in pixels and TIFF file format and using image processing analysis software ("Image HyperII" manufactured by Digimo Co., Ltd.).
A polyacetal powder having particles with an average ratio of minor axis to major axis within the above range can be obtained by adjusting various conditions in the pulverization step and/or the classification step of the method for producing the polyacetal powder.

<<Average surface smoothness shape factor of particles>>
The particles constituting the polyacetal powder of the present embodiment preferably have an average surface smoothness shape factor of 0.80 or more. When the average surface smoothness shape factor of the particles is 0.80 or more, the particles have a more regular shape with less unevenness, and as a result, the fluidity of the particles can be further improved. From the same point of view, the average surface smoothness shape factor of the particles is more preferably 0.85 or more, more preferably 0.90 or more.
Here, the surface smoothness shape factor is an index representing the circularity or ellipticalness of particles, and is expressed by the following formula using the measured values of the minor axis, major axis and peripheral length of particles.
Surface smoothness shape factor = [π × (long diameter of particle + short diameter of particle)] / [2 × circumference length of particle]

The surface smoothness shape factor is 1 if the particle is a perfect circle and nearly 1 if the particle is a perfect ellipse. The surface smoothness shape factor becomes smaller than 1 when the shape is not smooth circle or ellipse but irregular shape, because the perimeter of the particle becomes longer.
The average surface smoothness shape factor is the average value of surface smoothness shape factors of 100 arbitrarily selected particles.
The average surface smoothness shape factor of the particles constituting the polyacetal powder is obtained by saving the image taken using the above-mentioned digital microscope in 1360 × 1024 pixels in TIFF file format and using the above-mentioned image processing analysis software. The major axis, minor axis and perimeter of 100 particles are measured, the surface smoothness shape factor of each is obtained according to the above formula, and the average value can be obtained.
A polyacetal powder whose particles have an average surface smoothness shape factor within the above range can be obtained by adjusting various conditions in the pulverization step and/or classification step of the method for producing the polyacetal powder.

[Method for producing polyacetal powder]
The polyacetal powder in the present embodiment is produced by, for example, a step of granulating a resin material such as polyacetal and polyethylene (granulation step), a step of pulverizing the granulated resin material (pulverization step), and a step of pulverizing the pulverized resin material. A step of sieving to obtain a powder having the desired powder properties (classification step) and, optionally, a step of mixing with hydrophobic silica to obtain a particulate powder with hydrophobic silica attached (exposed) on the surface (hydrophobic silica mixing step).
Each step will be specifically described below.

[[Granulation step]]
In the granulation step, for example, pellets can be obtained by melt-kneading resin materials such as polyacetal and polyethylene using a kneader, roll mill, single-screw extruder, twin-screw extruder, multi-screw extruder, or the like. .

Here, when the resin material is melt-kneaded, it is preferable to replace the atmosphere with an inert gas or perform degassing by single-stage or multi-stage venting in order to maintain quality and work environment. The temperature during melt-kneading is preferably above the melting point of the resin material and below 250° C., and may be 20 to 50° C. higher than the melting point of the resin material.

In addition, in the granulation step, the above-described additives may be added to the resin material and mixed, and the resulting resin composition may be melt-kneaded. At this time, the additive may be divided into several times and mixed with the resin material.

[[Pulverization process]]
In the pulverization step, the pellets obtained in the granulation step can be pulverized to obtain powder. The pulverization method is not particularly limited, and examples thereof include ordinary temperature pulverization and freeze pulverization. Moreover, after dissolving the pellet obtained by the granulation process in a solvent, the method of precipitating is also mentioned.

[[Classification process]]
In the classification step, by sieving the powder obtained in the pulverization step, the average particle size (D50), the ratio of particles with a diameter of 2 times or more of D50, and the particles with a diameter of 0.5 times or less than D50 By appropriately adjusting the ratio of and the like, the polyacetal powder of the present embodiment can be finally obtained.

In addition, in the method for producing the polyacetal powder of the present embodiment, the degree of crystallinity may optionally be adjusted by heat treatment before or after the classification step.

[[Hydrophobic silica mixing step]]
In the method for producing the polyacetal powder of the present embodiment, a hydrophobic silica mixing step can optionally be carried out before the classification step, simultaneously with the classification step, or subsequent to the classification step.
In the hydrophobic silica mixing step, powder of a polyacetal composition containing polyacetal and polyethylene is mixed with hydrophobic silica to obtain particulate polyacetal powder having hydrophobic silica adhered (exposed) to the surface, and polyacetal Fluidity of powder can be improved.
The mixing method is not particularly limited, and may be mixed using an ordinary powder mixer such as a Nauta mixer, a Henschel mixer, a tumbler mixer, or the like. Alternatively, the mixture may be mixed by manual stirring or the like.

[Applications of polyacetal powder]
The polyacetal powder of the present embodiment is suitable for use in additive manufacturing techniques, and particularly suitable for selective laser sintering. Since the polyacetal powder in the present embodiment has optimized powder properties and particle properties, it can be uniformly laid out to a thickness of, for example, about 100 μm. Therefore, by performing additive manufacturing using this resin powder in the selective laser sintering method, it is possible to sufficiently suppress minute voids, reduce warpage, and achieve excellent interlayer adhesion and high mechanical properties. It is possible to obtain a shaped article having Such molded products are widely applicable to mechanical parts such as automobile parts, electric/electronic parts, industrial parts, and medical parts.

[How to use polyacetal powder]
A method of using the polyacetal powder of the present embodiment includes using the polyacetal powder of the present embodiment described above for additive manufacturing.
There are no particular restrictions on the additive manufacturing technology used for additive manufacturing, such as the selective laser sintering method, or applying a heat-absorbing agent to the desired part of the molding area covered with resin powder and applying heat from the top surface with a heater. Modeling, heat melting method, and the like can be mentioned.
In the method of using the polyacetal powder of the present embodiment, specific usage conditions are not particularly limited, and are appropriately selected according to the additive manufacturing technology to be used, the application and purpose of the molded product, etc., and the polyacetal powder is used. can be used.

[Additive manufacturing method]
The additive manufacturing method of this embodiment includes using the polyacetal powder of this embodiment described above as a modeling material. According to the additive manufacturing method of the present embodiment, since the polyacetal powder of the present embodiment described above is used, it is possible to obtain a molded article having high molding accuracy and mechanical properties.
The additive manufacturing technique used in the additive manufacturing method of the present embodiment is not particularly limited as long as the resin powder can be used as a modeling material, and examples thereof include selective laser sintering and heat melting. be done.
Specific modeling conditions and the like in the additive manufacturing method of the present embodiment are not particularly limited, and can be appropriately selected and modeled according to the additive manufacturing technique to be used and the application of the modeled product.

EXAMPLES The present invention will be described below with reference to examples, but the present embodiment is not limited to the following examples. Various measurement methods for the polyacetal powders and shaped articles of Examples and Comparative Examples, and raw material components of the polyacetal powders and shaped articles used in Examples and Comparative Examples are shown below.

(1) Comonomer Unit Content Ratio of Polyacetal Copolymer The comonomer unit content ratio of the polyacetal copolymer was obtained by the following procedure using the ¹ H-NMR method.
That is, the polyacetal copolymer was dissolved in hexafluoroisopropanol to a concentration of 1.5% by mass over 24 hours, and this solution was subjected to ¹ H-NMR analysis (magnetic field strength: 400 MHz, measurement solvent: hexafluoroisopropanol , reference material: tetramethylsilane, temperature 55° C., number of times of accumulation 500).
The integral value of the attributed peak of the oxymethylene unit obtained by ¹ H-NMR analysis (δ (ppm): 4.8 to 5.4) and the attributed peak of the oxyalkylene unit having 2 or more carbon atoms which is the comonomer unit ( δ (ppm): For example, when the comonomer unit is an oxyethylene unit, the content of the comonomer unit (bmol) with respect to the oxymethylene unit (a = 100 mol) from the ratio with the integrated value of 3.6 to 4.0) A ratio (b/a: mol/100 mol) was obtained.

(2) Particle size, etc. Average particle size (D50), ratio of particles with a diameter of 2 times or more than D50, ratio of particles with a diameter of 0.2 times or less of D50, using a Malvern Mastersizer , D50 of 65 μm or less, and the proportion of particles having a diameter of 0.2 times or less of D50 was measured.

(3) Melting point, crystallization temperature, crystallization speed Using a differential scanning calorimeter (manufactured by Parkin Elmer: DSC-2C), the DSC curve obtained by performing the measurement according to the following procedures 1 to 8, the first Scan melting point, second scan melting point, crystallization temperature, and crystallization rate were determined.
1: 3 mg of polyacetal powder to be measured is enclosed in an aluminum pan for measurement, and placed at a predetermined position in a heating furnace of a differential scanning calorimeter.
2: The temperature is raised at a heating rate of 10°C/min until it reaches 220°C, and the highest point of the endothermic peak (melting point peak) at that time is defined as the "first scan melting point".
3: Hold the temperature for 2 minutes after reaching 220°C.
4: Then, the temperature is lowered at a rate of 80°C/min until it reaches 149°C, and the temperature is maintained for 10 minutes to measure the exothermic peak due to crystallization.
5: The time from the start of holding at 149° C. to the highest point of the exothermic peak is defined as the “crystallization speed”. It should be noted that the longer the time, the slower the crystallization rate.
6: Next, the temperature is raised at a rate of temperature increase of 20°C/min until it reaches 210°C, and the temperature is maintained for 5 minutes.
7: Then, the temperature is lowered from 210°C to 100°C at a rate of 20°C/min. The exothermic peak (crystallization peak) at this time is defined as "crystallization temperature".
8: Next, the temperature was raised from 100°C to 210°C at a heating rate of 20°C/min, and the maximum endothermic peak (melting point peak) at that time was taken as the "second scan melting point".

(4) Melting point difference between the first scan and the second scan in DSC measurement By subtracting the melting point of the first scan from the melting point of the second scan obtained by DSC measurement as described above, the melting point difference between the first scan and the second scan (Table 1) (referred to as "melting point difference between 1st and 2nd scans").

(5) Weight reduction rate Using a thermogravimetry device (manufactured by Perkin Elmer, trade name "Pyris1 TGA"), sample weight: 5 mg, air flow rate: 10 mL / min, temperature increase rate: room temperature at 10 ° C. / min From there, the temperature was raised to a temperature 10° C. lower than the melting point of the polyacetal powder to be measured, and the weight loss rate was measured after holding at that temperature for 1 hour.

(6) Melt flow rate (MFR)
Using "P-111" manufactured by Toyo Seiki Seisakusho, the MFR was measured in accordance with ISO 1133 (Condition D) under conditions of 190°C and a load of 2.16 kg.

(7) Hardened Bulk Density/Loosen Bulk Density Powder tester PT-X type (manufactured by Hosokawa Micron) was used to measure the hardened bulk density and loose bulk density.
Specifically, the sample feeder was vibrated until the powder piled up in a stainless steel 100 cm ³ cylindrical container, causing the powder to flow down. The resulting density was defined as loose bulk density (g/cm ³ ).
In addition, a stainless steel 100 cm ³ cylindrical container was capped, the sample supply device was vibrated to let the powder flow down, and tapping was performed with a stroke length (tapping height) of 18 mm, a tapping speed of 60 times/minute, and a tapping frequency of 180 times. gone. After that, the excess powder on the container was scraped off with a blade, and the measured density was taken as the compacted bulk density (g/cm ³ ).
The hardened bulk density measurement, measured as described above, was divided by the loosed bulk density measurement to obtain the hardened bulk density/loosen bulk density value.

(8) Warp and turn-up during shaping Observe the strip-shaped shaped test piece during shaping, and evaluate warp and turn-up according to the following criteria.
(Evaluation criteria)
⊚ (excellent): No curling occurred in the molded portion, and the strip could be molded neatly.
◯ (Good): In the first few layers, the edges of the strip were slightly curled, but soon settled, and the strip was formed neatly.
x (poor): The ends of the strip were curled, and part of the strip could not be shaped neatly. Alternatively, the laser-irradiated portion was greatly curled, and the shaped portion was dragged by the recoater, making it impossible to shape the strip.
The less curling occurs, the less warpage occurs during molding, and the better the molding accuracy.

(9) Mechanical Properties of Shaped Product The shaped strip-shaped shaped test piece was subjected to a tensile test at a tensile speed of 5 mm/min in accordance with ISO527-1 to measure the tensile breaking stress. The measured values obtained were evaluated according to the following criteria.
(Evaluation criteria)
◎ (excellent): 40 MPa or more ○ (good): 20 MPa or more and less than 40 MPa Δ (poor): 10 MPa or more and less than 20 MPa × (poor): less than 10 MPa The higher the tensile breaking stress value, the better the mechanical properties.

(10) Evaluation of Warpage of Modeled Product Observing the shaped strip-shaped modeled test piece, the following criteria were used to evaluate the warpage of the modeled product.
(Evaluation criteria)
⊚ (excellent): Almost no warp.
◯ (Good): Warpage of at least one end is within 2 mm when placed on a flat surface.
x (inferior): Warp at both ends of 2 mm or more when placed on a flat surface.

<Polyacetal>
Polyacetal copolymer (Tenac-C (trademark) 3510 manufactured by Asahi Kasei Corp.)

<Polyethylene>
A: High-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 7 g/10 minutes, density 0.96 g/cm ³ )
B: High-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 15 g/10 minutes, density 0.95 g/cm ³ )
C: Linear low-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 0.5 g/10 minutes, density 0.92 g/cm ³ )
D: Linear low-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 9 g/10 minutes, density 0.92 g/cm ³ )
E: Linear low-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 16 g/10 minutes, density 0.92 g/cm ³ )
F: High-pressure low-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 7 g/10 minutes, density 0.92 g/cm ³ )
G: High-pressure low-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 0.5 g/10 minutes, density 0.92 g/cm ³ )
H: High-pressure low-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 15 g/10 minutes, density 0.92 g/cm ³ )
I: High-pressure low-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 90 g/10 minutes, density 0.92 g/cm ³ )
J: High-pressure low-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 0.1 g/10 minutes, density 0.92 g/cm ³ )
K: High-pressure low-density polyethylene (manufactured by Asahi Kasei Corporation; MFR = 120 g/10 minutes, density 0.91 g/cm ³ )

<Silica>
Dry process silica hydrophobized with dimethyldichlorosilane (Reolosil (registered trademark) DM20-S manufactured by Tokuyama Co., Ltd.)
Dry process silica hydrophobized with trimethylchlorosilane (HM-20L manufactured by Tokuyama Corp.)

(Examples 1 to 14, Comparative Examples 1 and 2)
<Granulation process>
Each component was melt-kneaded (cylinder temperature: 165° C. to 210° C.) with a twin-screw kneader according to the formulation shown in Table 1 to obtain a pellet-like molding material.

<Pulverization/classification process>
While freezing the obtained pellets with liquid nitrogen, they were freeze-pulverized with a Rinrex Mill (registered trademark) (manufactured by Hosokawa Micron Corporation), and sieved to appropriately adjust the powder characteristics to obtain a polyacetal powder.

(Examples 15-17)
For the examples containing silica, after the pulverization and classification process described above, the following silica mixing process was performed to obtain a polyacetal powder containing silica.
In addition, the surface of the obtained polyacetal powder was observed with an SEM to see whether or not silica particles adhered (exposed) to the surface.
<Silica mixing step>
Silica was added to the powder obtained in the pulverization and classification process according to the compounding recipe shown in Table 1, and the mixture was mixed with a tumbler mixer to obtain a silica-containing polyacetal powder.

(Rectangular shaped test piece)
Using the polyacetal powders obtained in Examples and Comparative Examples, each was shaped into a strip shape having a length of 5 cm, a width of 1 cm, and a thickness of 4 mm in the stacking direction using a commercially available selective laser sintering molding apparatus. A strip-shaped shaped test piece was used.

Table 1 shows the results of each measurement and evaluation performed in Examples and Comparative Examples.

As shown in Table 1, the polyacetal powder according to the example has less warping and turning during molding, and the molded product formed by the selective laser sintering method using the polyacetal powder according to the example has less warping. , It was found to have excellent mechanical properties and high molding accuracy.

ADVANTAGE OF THE INVENTION According to this invention, it is possible to provide a polyacetal powder with which it is possible to obtain a shaped article with less warpage and excellent mechanical properties. In addition, according to the present invention, it is possible to provide a method of using the polyacetal powder and an additive manufacturing method capable of obtaining a molded article with less warpage and excellent mechanical properties.

Claims (17)

The average particle diameter (D50) is 5 μm or more and 70 μm or less,
100 parts by mass of polyacetal and 0.01 to 5 parts by mass of polyethylene ,
A polyacetal powder , wherein the value obtained by dividing the solid bulk density by the loose bulk density (solid bulk density/loose bulk density) is 1.45 or less . 2. The polyacetal powder according to claim 1, wherein the crystallization speed is 40 seconds or more. The polyacetal powder according to claim 1 or 2, having a crystallization temperature of 130°C or higher and 145°C or lower. Polyacetal powder according to any one of the preceding claims, wherein the polyethylene has a density of 0.88-0.93 g/cm ³ . Polyacetal powder according to any one of claims 1 to 4, wherein the polyethylene has an MFR of 0.2 to 100 g/10 min. 6. The polyacetal powder according to any one of claims 1 to 5 , wherein the proportion of particles whose diameter is twice or more the D50 is 10 vol% or less. The polyacetal powder according to any one of claims 1 to 6 , wherein the proportion of particles having a diameter of 0.2 times or less of D50 is 10 vol% or less. The polyacetal powder according to any one of claims 1 to 7 , wherein the D50 is 65 µm or less, and the proportion of particles having a diameter of 0.2 times or less of the D50 is 3 vol% or less. The polyacetal powder according to any one of claims 1 to 8 , containing 0.05% by mass or more and 1.0% by mass or less of hydrophobic silica with respect to 100% by mass of the polyacetal powder. 10. The polyacetal powder according to claim 9 , wherein said hydrophobic silica is dry process silica. The polyacetal powder according to claim 9 or 10 , wherein the hydrophobic silica has an average primary particle size of 1 nm or more and 20 nm or less. The polyacetal powder according to any one of claims 9 to 11 , wherein the hydrophobic silica has a BET specific surface area of 50 m ² /g or more and 300 m ² /g or less. The polyacetal powder according to any one of claims 9 to 12 , wherein the hydrophobic silica is hydrophobized with one or more selected from the group consisting of dimethyldichlorosilane, trimethylchlorosilane, and silicone oil. The polyacetal powder according to any one of claims 1 to 13 , wherein the weight loss rate is 5% or less when held for 1 hour at a temperature 10°C lower than the melting point of the polyacetal powder. Polyacetal powder according to any one of claims 1 to 14 , for use in additive manufacturing. A method of using the polyacetal powder according to any one of claims 1 to 15 , characterized in that it is used for additive manufacturing. Additive manufacturing method, characterized in that the polyacetal powder according to any one of claims 1 to 15 is used as a building material.