JP3190918B2 - Resin material for gas assisted injection molding and molding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a molded article in a hollow shape by injecting a gas into an injected molten resin, that is, a gas-assisted injection molding (hereinafter referred to as GAI molding) method and a resin used for the method. It is about materials.

[0002]

2. Description of the Related Art GAI molding is a process in which a molten resin injected into a mold cavity is filled with nitrogen or the like by a nozzle provided at the tip of an injection machine or a needle arranged at a part of the mold. This is a method of forming a molded product into a hollow shape by injecting gas, and it is a molding technology that has attracted particular attention recently because of its features such as not causing sink marks and warpage, and being able to produce molded products lightly and inexpensively. . Conventionally, as a material for this GAI molding, emphasis was placed on cheapness and easy handling,
Although resins such as PE, PS, PP, ABS, and modified PPO have been used, in recent years, demands for GAI molded articles using resins having high mechanical strength and heat resistance have increased.

[0003]

SUMMARY OF THE INVENTION However, GAI
Molding is a method of injecting an amount of molten resin smaller than the volume of the mold cavity or in anticipation of shrinkage upon cooling, and expanding it with gas to mold it into a desired shape. Due to the timing of injection and flow caused by gas, the resin flow velocity changes and the surface appearance of the molded product becomes poor, or solidification starts before a sufficient flow length is obtained, so that the expected voids cannot be obtained. There was a problem. In particular, when a resin having high mechanical strength and heat resistance is used, this problem becomes conspicuous, which has been a major obstacle to GAI molding.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems, and makes it possible to produce a GAI molded product having a good surface appearance, no defective portions, and high mechanical strength and heat resistance. The present invention relates to a resin material for GAI molding, and its constitution is that it is made of a polyacetal resin having a crystallization time of 5 minutes or more. The crystallization time referred to in the present invention means that the polyacetal resin is kept at 200 ° C. for 5 minutes using a heat flux type DSC (differential scanning calorimeter) and then cooled to 151 ° C. at a rate of 10 ° C./min. , when held at that temperature refers to a time from the holding initiation to crystallization exotherm peak, the present invention is a resin to be subjected to GAI molding, by allowed to prepare the crystallization time to 5 minutes or more, surface appearance It is possible to manufacture a molded article having a good quality. If the crystallization time of the polyacetal resin to be used is shorter than 5 minutes, the effect of the present invention cannot be obtained because the resin is hardened at the time of gas injection. The upper limit of the crystallization time is not particularly limited. However, if the crystallization time is long, the time until the resin is cured may be long, and the molding efficiency may be lowered. Therefore, the crystallization time of the polyacetal resin used is 500.
Minutes or less.

In the present invention, either a homopolymer or a copolymer can be used as the polyacetal resin, but the oxymethylene unit (—OCH ₂ —) is the main constituent unit and the comonomer component is contained in an appropriate amount . Copolymers (including block copolymers) are preferred,
Particularly, a polyacetal copolymer obtained by copolymerizing a comonomer component at 3 to 30% by weight is preferable because the crystallization time can be easily adjusted to 5 minutes or more and excellent mechanical strength can be maintained. A particularly preferred amount of comonomer is 5 to 20% by weight. The acetal copolymer may have not only a linear molecule but also a branched structure or a crosslinked structure, and the viscosity of the acetal resin is not particularly limited.

The comonomer component used in the production of the polyacetal copolymer is not particularly limited, but one having the following structure is generally used.

Embedded image (However, R ₁ , R ₂ , R ₃ , and R ₄ are the same or different substituents, and include a hydrogen atom, an alkyl group (1 to 5
Those to three hydrogens are replaced by halogen atoms, hereinafter the same), or refers to an alkyl group substituted with halogen, R ₅ is substituted with a methylene group, an oxymethylene group, each alkyl group or halogenated alkyl Methylene group or oxymethylene group (p = 0 to 3 in this case),-
(CH ₂ ) _q -OCH _2- (q = 1 to 4) or-(O-CH ₂ -CH ₂ ) _q -OCH
_A divalent group represented by 2-(q = 1 to 4) (in this case, p =
1) is shown. If the comonomer component used is at least one selected from the group consisting of ethylene oxide, epichlorohydrin, 1.3-dioxolan, diethylene glycol formal, 1.4-butanediol formal, 1.3-dioxane and propylene oxide, It is preferable because the time can be easily adjusted to a desired range.

A GA comprising the polyacetal resin of the present invention
The resin material for molding I is provided with conventionally known additives, for example, various stabilizers, lubricants, release agents, coloring agents, charging agents, in order to impart desired properties according to the purpose within a range that does not impair the effect. Inhibitors and other surfactants, or organic polymeric materials, inorganic,
One or two types of organic fibrous, powdery, and plate-like fillers
More than one kind can be added and contained.

[0008]

The polyacetal having a crystallization time of 5 minutes or more has high mechanical strength and heat resistance, does not cure immediately when injected into a mold, and expands sufficiently by gas injection, so that it has a large porosity. Even if it is a molded product, it is sufficiently transferred to a mold, and a molded product having a good surface appearance can be obtained even if the flow rate fluctuates after injection and before gas injection.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments. [Example 1] Trioxane and ethylene oxide were copolymerized, and the fraction of ethylene oxide as a comonomer component was 4.0% by weight.
Acetal copolymer A was obtained. Specifically, biaxial weight
Using a joint machine, let the warm water of 80 ° C pass through the jacket,
While rotating the two rotating shafts at 120 rpm,
96.0 weight of trioxane from the supply port provided at the end
%, Ethylene oxide 4.0% by weight and a chain transfer agent
Of 500 ppm of methylal (to monomer ratio)
While supplying trifluoride from the catalyst supply port.
50 ppm (to monomer ratio) in terms of the amount of iodine
Polymerization was performed while adding iodine dibutyl etherate,
The polymer was discharged from the discharge port located on the opposite side of the supply port
(Average residence time about 10 minutes). Polymerization discharged from the polymerization machine
The body is immediately ground and touched with aqueous triethylamine.
The medium was deactivated, followed by separation, washing and drying. Next
Then, 100 parts by weight of this polymer was
3% by weight aqueous solution and pentaerythrityl-tetrakis
[3- (3,5-di-tert-butyl-4-hydroxy)
[Ciphenyl) propionate] (Ciba Geigy Corporation)
0.3% by weight of Irganox 1010)
Unstable due to melt kneading at 210 ° C with twin screw extruder
The terminal portion was removed. The polymer is then used as a stabilizer.
Pentaerythrityl-tetrakis [3- (3,5-di
-Tert-butyl-4-hydroxyphenyl) propyl
Onate] 0.03% by weight and melamine 0.1% by weight
And melt-kneaded at 200 ° C in a twin-screw extruder to stabilize
The obtained acetal copolymer A was obtained. In addition, ethylene oxide
6. 1,3-dioxolane instead of 4.0% by weight of oxide
Trioxane and 1,3-dioxolane were copolymerized in the same manner as above except that 0% by weight was used, and the fraction of 1,3-dioxolane as a comonomer component was 7.0% by weight.
Acetal Copolymer B was obtained. As a result of measuring the crystallization times of the acetal copolymers A and B using a DSC measurement device, the crystallization times were 40 minutes and 42 minutes, respectively. Next, the obtained acetal copolymer was melted in a cylinder of an injection molding machine and injected into a cavity. At this time, the temperature of the cylinder and the temperature of the mold were 190
C. and 80.degree. C., and the injection amount of the resin was about 80% of the volume of the cavity. The outline of the mold cavity shape is shown in FIG.
It is as follows. Then, 0.5 seconds after the injection of the resin, a compressed nitrogen gas at a pressure of 150 kg / cm2 was injected from a needle (not shown) arranged in the mold cavity to expand the resin and transfer the resin into the mold cavity. . Thereafter, the compressed state by the gas was released, and the mold was further cooled to take out the molded product. In the case of acetal copolymer A and in the case of B, the resulting molded article was excellent in mechanical strength and heat resistance, had a good appearance, and did not have sink marks or warpage.

Comparative Example 1 Same as Example 1 except that the amount of ethylene oxide was changed.
The same procedure as in Example 1 was carried out using an acetal copolymer obtained by copolymerizing trioxane and ethylene oxide and having a fraction of ethylene oxide as a comonomer component of 2.4% by weight and a crystallization time of 1.2 minutes. Molding was performed.
A wrinkled hesitation mark was formed at the center of the obtained molded product, which was not practical. This mark is presumed to have been formed before the injection speed of the resin became slow and gas injection started.

[0011]

When the resin material of the present invention is used for GAI molding, since the resin material does not rapidly solidify after being injected into a mold, a molded article having sufficient voids can be obtained. Further, even if the flow velocity of the resin changes due to the timing of the injection of the molten resin and the flow of the gas, a molded article having a good surface appearance can be obtained. Moreover, this molded product has no sink mark or warpage and is excellent in mechanical strength and heat resistance.

[Brief description of the drawings]

FIG. 1 is a reference view showing a state of a cross section of a mold used in Examples and Comparative Examples.

[Explanation of symbols]

 1 ... mold, 3 ... U-shaped cylindrical cavity.

Continuation of front page (56) References JP-A-6-207080 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C08G 2/00-2/38

Claims (3)

(57) [Claims] 1. A gas-assisted injection molding resin material comprising a polyacetal resin having a crystallization time of 5 minutes or more. 2. The resin material for gas-assisted injection molding according to claim 1, wherein the polyacetal resin is a polyacetal copolymer obtained by copolymerizing 3 to 30% by weight of a comonomer component. 3. A gas-assisted injection molding method using the resin material according to claim 1 or 2.