12. The vertical reciprocating motion of the valve pin enables opening and closing of the gate 13.

Therefore, given that the gate 13 is configured to be openable and closable by the opening and closing mechanism such as a valve pin, the resin composition from the gate 13 can be favorably cut off when the discharge of the resin composition from the gate 13 is stopped. This makes it possible to more precisely control the discharge amount of the resin composition from the runner 12.

In the present mold 3, a value obtained by dividing a total volume of the sprue 11, the runner 12, and the gate or gates 13 by a total volume of the cavity or cavities 31 is preferably 0.5 to 5.0, more preferably 0.8 to 4.0, even more preferably 1.0 to 3.0, and particularly preferably 1.0 to 2.0.

The above configuration can shorten a residence time of the resin composition within the sprue 11 and the runner 12. A short residence time of the resin composition is likely to provide a uniform temperature, so that the resin composition is less prone to being subjected to pyrolysis. Thus, it is possible to obtain an injection-molded product with less flash. The “total volume of the cavity or cavities” can also be expressed as the “total volume of a product or products”.

The present mold 3 preferably includes a pressure reducing portion 71. Via the pressure reducing portion 71, a gage pressure inside the cavity can be reduced so as to be not more than a predetermined value. A pressure reducing step in the present manufacturing method will be described later.

The present mold 3 preferably includes a vent portion 41. Further, since the maximum width of the parting line 21 of the present mold 3 is not more than 50 μm, the vent portion 41 is preferably provided at a position different from a position of the parting line 21. With the configuration in which the present mold 3 includes the vent portion 41 at the position different from the position of the parting line 21, it is possible to efficiently discharge a gas inside the cavity 31 to the outside. The following will describe details of the vent portion 41 with reference to FIG. 4. FIG. 4 is a plan view and a cross-sectional view schematically illustrating a closed state of the vent portion 41 of the present mold. The position of the vent portion 41 is not limited to the one depicted in FIG. 4, and may be any position in the present mold 3. Furthermore, there is no particular limitation on the number of the vent portion(s) 41. For example, the number of the vent portion(s) 41 may be one, two, three, four, or five or more.

The present mold 3 preferably has a mechanism for drawing out a gas from the vent portion 41. There is no particular limitation on the mechanism for drawing out a gas. It is preferable that the mechanism be a vacuum pump, since the vacuum pump has excellent sucking performance. Examples of the vacuum pump include a water-sealed vacuum pump and a dry roots-type vacuum pump.

The present mold 3 including, in addition to the pressure reducing portion 71, the vent portion 41 provided with the mechanism for drawing out a gas can more efficiently discharge a gas inside cavity 31 (i.e., carry out pressure reduction). This enhances productivity of the injection-molded product.

An opening of the vent portion 41 has a maximum width of 1 μm to 50 μm, preferably 2 μm to 40 μm, more preferably 3 μm to 30 μm, and even more preferably 3 μm to 15 μm. Setting the maximum width of the opening of the vent portion 41 so as to be not more than 50 μm provides the effect of drawing out a gas sufficiently and the effect of reducing flash. Further, setting the width of the vent portion so as to be not less than 1 μm makes it possible to discharge a gas inside the cavity 31 sufficiently.

Furthermore, the opening of the vent portion 41 may have a structure of at least one type selected from the group consisting of a slit type, a porous type, a nested type, and an opening/closing type. With the vent portion 41 having the opening of the above structure, it is possible to discharge a gas inside the cavity 31 easily.

The slit type refers to a type according to which the cavity 31 has, on its surface, a slit 42 having a length of 1 μm to 50 μm and allows a gas to be discharged to the outside of the cavity 31 through the slit 42. An example of the slit-type structure is illustrated in FIG. 4. In a case where the vent portion 41 has a slit-type opening, the vent portion 41 includes, for example, a slit 42, as illustrated in FIG. 4. The porous type refers to a type that utilizes a porous substance whose pores are connected to each other so as to allow a gas to be ultimately discharged to the outside of the cavity 31. The nested type refers to a type according to which the cavity 31 is partially nested so as to allow a gas to be discharged from the cavity through a gap formed thereby. An example of the nested-type structure is depicted in FIG. 5. In a case where the vent portion 41 has a nested-type opening, the vent portion 41 includes, for example, a nested portion 43, as illustrated in FIG. 5. The gas inside the cavity 31 is discharged through the (nested type) opening 44 of the vent portion at which the nested portion 43 and the cavity 31 are in contact with each other. The opening/closing type refers to a type according to which, between a stage at which a molten resin is supplied into the cavity 31 and a stage at which the molten resin reaches the opening/closing portion, an opening/closing portion is open so as to allow a gas to be discharged to the outside of the cavity 31 and the opening and closing portion is closed just before the molten resin reaches the opening and closing portion. With the vent portion 41 of any type, the maximum width of the opening portion is preferably 1 μm to 50 μm.

In a case where multiple vent portions 41 are provided, the opening portions of the respective vent portions 41 may have identical maximum width or different maximum widths. The structures of the opening portions of the respective vent portions 41 may be obtained by combining two or more types.

The present mold 3 may further include a mechanism for taking out an injection-molded product from the cavity 31. Examples of such a mechanism include an ejection pin 51 illustrated in FIG. 4. In a case where the present mold 3 includes this mechanism, the vent portion 41 may be provided at the same position as that of the ejection pin 51.

(Pressure Reducing Step)

The present manufacturing method includes the step (pressure reducing step) of: before filling the mold with the resin composition by injection, carrying out pressure reduction by reducing the gage pressure inside the cavity of the mold so as to be not more than −0.02 MPa. The expression “before filling the mold with the resin composition by injection” is not limited to any particular timing, as long as the timing is before injecting the resin composition into the mold. Examples of the timing include: (1) a timing before injection of the resin composition; (2) a period between a timing immediately after starting injection of the resin composition and a timing at which the mold is filled with the resin composition; and (3) a period between a timing before injection of the resin composition and a timing at which the mold is filled with the resin composition. Preferably, the pressure reducing step is carried out before injection of the resin composition. Carrying out the pressure reducing step hardly causes the above-described shortshot even if the maximum width of the parting line is set so as to be not more than 50 μm. This enhances continuous productivity of the injection-molded product.

A method for reducing the gage pressure inside the cavity is not limited to any particular one. For example, the reduction of the gage pressure inside the cavity may be carried out with use of a machine such as a pump. The pressure reduction can be carried out via the ejection pin 51 in the cavity, the vent portion 41 in the cavity, the parting line 21 being in contact with the cavity in the mold, and/or the like. From the viewpoint of efficient pressure reduction, the pressure reduction is preferably carried out via the ejection pin 51 and the vent portion 41.

The “gage pressure” herein means a difference pressure between an atmospheric pressure (ordinary pressure) of an environment outside the cavity and an atmospheric pressure inside the cavity. That is, the expression “the gage pressure is −0.02 MPa” means that the atmospheric pressure inside the cavity is smaller than the atmospheric pressure outside the cavity by 0.02 MPa.

The gage pressure in the pressure reducing step is not more than −0.02 MPa, preferably not more than −0.03 MPa, and more preferably not more than −0.05 MPa. There is no particular limitation on a lower limit of the gage pressure. For example, the lower limit is not less than −0.1 MPa. In a case where the pressure reduction is carried out with use of the above-described pump, the lower limit of the gage pressure may be a limit value determined by the performance of the pump.

(P3HA-Based Resin)

The resin composition for use in the present manufacturing method includes a P3HA-based resin. As used herein, the “P3HA-based resin” means a biodegradable aliphatic polyester (preferably, polyester with no aromatic ring). The P3HA-based resin is a polyhydroxyalkanoate that contains a 3-hydroxyalkanoate repeating unit as a repeating unit, represented by the general formula [—CHR—CH₂—CO—O—] (where R is an alkyl group represented by C_nH_2n+1and n is an integer of not less than 1 and not more than 15).

Further, the P3HA-based resin may contain at least one kind of copolymer of a 3HB unit and another hydroxyalkanoate unit. With respect to the total repeating units (100 mol %), the 3-hydroxybutyrate unit in the poly(3-hydroxyalkanoate)-based resin is preferably 94.5 mol % to 98.5 mol %, more preferably 95.0 mol % to 98.5 mol %, even more preferably 96.0 mol % to 98.5 mol %, and particularly preferably 96.5 mol % to 98.0 mol %.

The P3HA-based resin containing the 3HB repeating unit at a composition ratio of not less than 94.5 mol % tends to achieve improved rigidity, increased crystallization speed, decreased flash, and improved productivity. Meanwhile, the P3HA-based resin containing the 3HB repeating unit at a composition ratio of not more than 98.5 mol % achieves a melting point lower than the thermal decomposition temperature, and therefore enables stable continuous production. The monomer composition ratio of the P3HA-based resin can be determined by gas chromatography or the like (see International Publication No. WO 2014/020838, for example).

Preferably, the resin composition contains, as the P3HA-based resin, a homopolymer constituted by a 3HB unit or a poly(3-hydroxybutyrate)-based resin, which is a copolymer of a 3HB unit and another hydroxyalkanoate unit. Examples of the poly(3-hydroxybutyrate)-based resin include poly(3-hydroxybutyrate) (P3HB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P3HB3HV), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P3HB3HH), poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-3-hydroxyhexanoate) (P3HB3HV3HH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3HB4HB), poly(3-hydroxybutyrate-co-3-hydroxyoctanoate), and poly(3-hydroxybutyrate-co-3-hydroxydecanoate).

P3HA-based resins produced by microorganisms (microbial P3HA-based resins) are a P3HA-based resin that is normally constituted only by a polyhydroxyalkanoic acid monomer unit of the D-configuration (R-configuration). Among the microbial P3HA-based resins, P3HB, P3HB3HH, P3HB3HV, P3HB3HV3HH, and P3HB4HB preferable, and P3HB, P3HB3HH, P3HB3HV, P3HB4HB are more preferable, from the viewpoint of ease of industrial production.

The microorganisms that produce the microbial P3HB-based resin are not limited to any particular one, provided that the microorganisms are capable of producing the P3HA-based resins. For example, the first P3HB-producing bacteria discovered was Bacillus megaterium (discovered in 1925), and other examples of the P3HB-producing bacteria include natural microorganisms such as Cupriavidus necator (previously classified as Alcaligenes eutrophus), Ralstonia eutropha, and Alcaligenes latus. These microorganisms are known to accumulate P3HB within bacterial cells of the microorganisms.

Known bacteria that produce a copolymer of hydroxybutyrate and another hydroxyalkanoate include Aeromonas caviae, which is a bacterium capable of producing P3HB3HV and P3HB3HH, and Alcaligenes eutrophus, which is a bacterium capable of producing P3HB4HB. Particularly, regarding P3HB3HH, Alcaligenes eutrophus AC32 (FERM BP-6038) (T. Fukui, Y. Doi, J. Bateriol., 179, pp. 4821-4830 (1997)), into which genes of P3HA-based resin synthetases have been introduced in order to increase P3HB3HH productivity, is more preferable, for example. Bacterial cells of these microorganisms which have been cultured under appropriate conditions to accumulate P3HB3HH in the bacterial cells are used. In addition to the above, depending on the P3HA-based resins to be produced, genetically modified microorganisms into which various genes associated with synthesis of the P3HA-based resins have been introduced can be used, and culture conditions including substrate type can be optimized.

A molecular weight of the P3HA-based resin is not limited to any particular one, provided that it exhibits substantially sufficient physical properties for the intended use. A weight average molecular weight range of the P3HA-based resin is preferably 100,000 to 1,000,000, more preferably 150,000 to 700,000, even more preferably 200,000 to 500,000, and particularly preferably 250,000 to 450,000. With a weight average molecular weight of not less than 100,000, it is possible to achieve appropriate mechanical strength. Meanwhile, with a molecular weight of not more than 1,000,000, it is possible to reduce increase in melt viscosity, thereby providing superior moldability.

A method of determining the weight average molecular weight is carried out with use of gel permeation chromatography (GPC) (“Shodex GPC-101” available from Showa Denko K.K.). That is, a polystyrene gel (“Shodex K-804” available from Showa Denko K.K.) can be used for a column and chloroform can be used for a mobile phase. A weight average molecular weight may be determined as a molecular weight in terms of polystyrene. At this time, a calibration curve is made by using polystyrene with a weight average molecular weight of 31,400, 197,000, 668,000, and 1,920,000. As a column for the GPC, any column appropriate for determining the molecular weight may be used.

(Other Resins)

The resin composition in the present manufacturing method may contain, in addition to the P3HA-based resin, a second P3HA-based resin. The second P3HA-based resin contains at least one kind of copolymer of a 3HB unit and another hydroxyalkanoate unit. The 3HB unit in the poly(3-hydroxyalkanoate)-based resin is preferably 65.0 mol to 90.0 mol, more preferably 68.0 mol to 88.0 mol, and even more preferably 70.0 mol to 85.0 mol. The resin composition further containing the second P3HA-based resin provides a molded article having excellent toughness.

The second P3HA-based resin is not limited to any particular one, provided that it differs from the P3HA-based resin. Examples of the second P3HA-based resin may include resins mentioned above as the examples of the P3HA-based resin.

There is no particular limitation on a content of the second P3HA-based resin. The content of the second P3HA-based resin is preferably not more than 50 parts by weight, more preferably not more than 45 parts by weight, and even more preferably not more than 40 parts by weight, with respect to 100 parts by weight of the total P3HA-based resin. There is no particular limitation on a lower limit of the content of the second P3HA-based resin. The lower limit may be 0 part by weight. As the second P3HA-based resin, the P3HA-based resin described above can be used. The “total P3HA-based resin” herein means all P3HA-based resins contained in the resin composition in accordance with the present manufacturing method.

The resin composition may contain another resin other than the P3HA-based resins to an extent that the another resin does not impair the advantageous effects of the present invention. Examples of such another resin may include: aliphatic polyester-based resins such as a polybutylene succinate adipate, a polybutylene succinate, a polycaprolactone, and a polylactic acid; and aliphatic aromatic polyester-based resins as a polybutylene adipate terephthalate, a polybutylene sebatate terephthalate, and a polybutylene azelate terephthalate. The another resin may include only one kind of resin, or may include two or more kinds of resins.

There is no particular limitation on a content of the another resin. The content of the another resin is preferably not more than 50 parts by weight and more preferably not more than 40 parts by weight, with respect to 100 parts by weight of the total P3HA-based resin. Even more preferably, the content is not more than 30 parts by weight. There is no particular limitation on a lower limit of the content of the another resin, and the lower limit may be 0 part by weight.

(Inorganic Filler)

It is preferable that the resin composition further contain an inorganic filler. The resin composition containing the inorganic filler achieves the effects of increased crystallization speed, decreased flash, improved productivity, and the like.

There is no particular limitation on the inorganic filler. Examples of the inorganic filler include talc, calcium carbonate, mica, silica, glass fibers, and glass particles.

A content of the inorganic filler is, for example, more than 0 part by weight and not more than 60 parts by weight, preferably 5 parts by weight to 50 parts by weight, more preferably 10 parts by weight to 40 parts by weight, and particularly preferably 15 parts by weight to 35 parts by weight, with respect to 100 parts by weight of the total P3HA-based resin. Setting the content of the inorganic filler so as to fall within the above range can provide both a sufficient crystallization speed and toughness.

The resin composition may also contain an additive that is usable together with the P3HA-based resin, provided that the advantageous effects of the present invention are not impaired. Examples of such an additive may include: coloring agents such as pigments and dyes; deodorizing agents such as activated carbon and zeolite; perfumes such as vanillin and dextrin; plasticizers; antioxidants; antioxidative agents; weather resistance improving agents; ultraviolet ray absorbing agents; crystal nucleating agents; lubricants; mold releases; water repellent agents; antibacterial agents; and slidability improving agents. Contained as the additive may be only one kind of additive. Alternatively, two or more kinds of additives may be contained as the additive. A content of the additive(s) can be set by a person skilled in the art as appropriate according to the purpose of use.

(Injection-Molded Product)

A maximum thickness of flash generated in an injection-molded product obtained by the present manufacturing method is preferably not more than 80 μm, more preferably not more than 70 μm, and further preferably not more than 60 μm. The lower the maximum thickness of the generated flash is, the better it is. Although there is no particular limitation on a lower limit of the maximum thickness, the lower limit of the maximum thickness is, for example, not less than 5 μm. A maximum flash thickness of the injection-molded product can be measured by a method described in the later-presented examples.

The present invention is not limited to the embodiments, but can be altered by a skilled person in the art within the scope of the claims. The present invention also encompasses, in its technical scope, any embodiment derived by combining technical means disclosed in differing embodiments.

That is, embodiments of the present invention are as follows.

- <1> A method for manufacturing, with use of a mold apparatus, an injection-molded product of a resin composition containing a poly(3-hydroxyalkanoate)-based resin,
- the mold apparatus including a mold which has a parting line being in contact with a cavity in the mold and having a maximum width which is not more than 50 μm when the mold is closed, the method including the step of:
- before filling the mold with the resin composition by injection, carrying out pressure reduction by reducing a gage pressure inside the cavity of the mold so as to be not more than −0.02 MPa.
- <2> The method described in <1>, wherein:
- the mold in the mold apparatus has the parting line being in contact with the cavity in the mold and having the maximum width which is not more than 30 μm when the mold is closed.
- <3> The method described in <1> or <2>, wherein:
- a vent portion is provided at a position in the mold which position is different from a position of the parting line being in contact with the cavity, the vent portion having an opening whose maximum width is 1 μm to 50 μm.
- <4> The method described in any one of <1> to <3>, wherein:
- the resin composition contains a poly(3-hydroxybutyrate)-based resin.
- <5> The method described in <1> to <4>, wherein:
- the poly(3-hydroxyalkanoate)-based resin contains at least one kind of copolymer of a 3-hydroxybutyrate unit and another hydroxyalkanoate unit; and
- the 3-hydroxybutyrate unit in the poly(3-hydroxyalkanoate)-based resin is 94.5 mol % to 98.5 mol % with respect to 100 mol % of total repeating units.
- <6> The method described in any one of <1> to <5>, wherein:
- a thickness of flash generated in the injection-molded product is not more than 80 μm.
- <7> The method described in any one of <1> to <6>, wherein:
- in the step of carrying out the pressure reduction, the pressure reduction is carried out via an ejection pin and/or a vent portion.
- <8> The method described in any one of <1> to <7>, wherein:
- the resin composition further contains an inorganic filler.
- <9> The method described in <8>, wherein:
- a content of the inorganic filler in the resin composition is more than 0 part by weight and not more than 60 parts by weight.

EXAMPLES

The following description will discuss embodiments of the present invention in further detail on the basis of Examples. Note, however, that the present invention is not limited to Examples.

Materials

In Examples and Comparative Examples, the materials shown in Table 1 were used.

TABLE 1

Classification
Abbreviation
Product name
Contents
Manufacturer

P3HA
PHA-L
PHBH4203
PHBH, 3HB = 98.2 mol %,
Kaneka

Mw = 350,000
Corporation

PHA-M
PHBH4303
PHBH, 3HB unit = 94.1
Kaneka

mol %, Mw = 350,000
Corporation

Nucleating
PETL
Noilizer P
Pentaerythritol
Nihon Gosei

agent

Kako Co. Ltd.

Lubricant
BA
BNT-22H
Behenic acid amide
Nippon Fine

Chemical Co. Ltd.

Inorganic
TALC
MICRO
Talc
Nippon Talc Co.

filler

ACE K1

Ltd.

PHBH: Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

3HH: 3-hydroxyhexanoate

3HB: 3-hydroxybutyrate

Mw: Weight-average molecular weight calculated in terms of polystyrene

[Measuring Method and Evaluating Method]

In Examples and Comparative Examples, determination and evaluation were carried out by the following method.

(Flash Thickness)

Spoons obtained at 100th shot, 200th shot, 300th shot, 400th shot, and 500th shot in injection molding were collected. Then, the spoons were observed by a microscope at 400× magnification. For a spoon with the largest flash, a maximum flash thickness was measured by software in the microscope. If the maximum flash thickness was not more than 80 μm, it was determined that the flash thickness was sufficiently small.

(Continuous Productivity)

The mold was washed with 99.5% ethanol, and thereafter molded products obtained at first 10 shots were discarded in order to make the molded products stable in shape. Then, the number of shots carried out until shortshot occurs was evaluated as continuous productivity. The number of shots more than 1000 was determined as sufficient continuous productivity.

Examples 1 to 6 and Comparative Examples 1 and 2

Injection-molded products in accordance with Examples 1 to 6 and Comparative Examines 1 and 2 were manufactured in the following manner. First, pellets of compounds R1 to R4 were obtained in the following manner.

(Preparation of PHBH Blend)

For the compounds R1 to R4, feeding amounts Ce (kg) of raw materials were determined according to the following formula (1), on the basis of “We” and “Wt”. “We” denotes an added amount of each raw material for a PHBH blend (shown in Table 2), expressed in terms of parts per hundred. “Wt” denotes a total added amount of the PHBH blend, expressed in terms of parts per hundred.

Ce
=

10

×

We
/
Wt

(
1
)

The measured raw materials were fed into a 75 L super mixer (available from Kawata Mfk. Co., Ltd.), and were mixed at 300 rpm for 3 minutes, to obtain a PHBH blend.

(Pelletization)

TEM26SS (L/D=60, available from Toshiba Machine) was used. A barrel temperature was set at 140° C. and a screw speed was set at 100 rpm. A feed rate F1 (kg/hr) for the PHBH blend at the screw root was determined according to the following formula (2). A feed rate F2 (kg/hr) for the inorganic filler at the side feed was determined according to the following formula (3). The determination of the feed rates were carried out on the basis of “Wt” and “Wf”. “Wt” denotes a total added amount of the PHBH blend, expressed in terms of parts per hundred. “Wf” denotes an added amount of the inorganic filler, expressed in terms of parts per hundred.

F
⁢
1

=

10

×

Wt
/

(

Wt
+
Wf

)

(
2
)

F
⁢
2

=

10

×

Wf
/

(

Wt
+
Wf

)