Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Patent
1994-06-13
1996-01-16
Heitbrink, Jill L.
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
26432818, 26433111, B29C 4500, C08L 5900
Patent
active
054845670
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates a method for producing a thin-walled molded article of a polyacetal resin having excellent dimensional stability to temperature and humidity changes.
BACKGROUND ART
A polyacetal resin is widely used as an engineering resin because of its excellent mechanical strength, fatigue resistance, electrical properties and the like.
However, since the polyacetal resin has high crystallizability, it has drawbacks that a molded article of it undergoes warpage or sink marks by heat dissipation immediately after injection molding and after heat dissipation (cooling), the molded article further undergoes dimensional changes such as warpage when it is exposed to a high temperature and humidity. Particularly, in case of a thin-walled molded article, remarkable warpage appears on it.
The present invention has been completed based on finding that warpage of a thin-walled molded article is reduced when the amorphous layer, which is the skin layer of the molded article, is made as thin as possible, compared to the total thickness, by using a specific boron nitride.
Japanese Patent Publication Unexamined No. 94185/1971 discloses that boron nitride is added to polyoxymethylene. However, it does not disclose the specific boron nitride used in the present invention and the thin-walled injection molded article of the present invention.
An object of the present invention is to provide a thin-walled molded article of a polyacetal resin, whose dimensions change very little at a high temperature and humidity.
DISCLOSURE OF THE INVENTION
The present invention relates to a method for producing a thin-walled molded article of a polyacetal resin comprising adding 10 to 1000 ppm of boron nitride, having an average particle diameter of 1 to 10 .mu.m and a content of diboron trioxide of 0.005 to 1.0% by weight, to a polyacetal resin and molding an article having a maximum wall thickness of 0.2 to 1 mm by injection molding.
In the present invention, the thin-walled molded article is a molded article having a maximum wall thickness of 0.2 to 1 mm. The molding method of the present invention is particularly useful for producing a thin-walled molded article having a maximum wall thickness of 0.25 to 0.6 mm.
The polyacetal resins to be used in the present invention include a polyacetal resin homopolymer consisting substantially of oxymethylene units, produced by using a formaldehyde monomer or a cyclic oligomer such as a trimer (trioxane), tetramer (tetraoxane) and the like as a raw material, and a polyacetal resin copolymer comprising 0.1 to 2% by weight of a oxyalkylene unit having 2 to 8 carbon atoms, produced by using the above raw material and cyclic ethers such as ethylene oxide, propylene oxide, epichlorohydrine, 1,3-dioxolane, a formal of a glycol, a formal of a diglycol and the like. The polyacetal resin further includes a branched polyacetal copolymer having a branched molecular chain and a block copolymer comprising 50% by weight or more of an oxymethylene repeating unit and other component blocks. In the present invention, if necessary, the above polyacetal resins can be used in combination. However, the polyacetal resin homopolymer or the polyacetal resin copolymer is preferably used alone in view of dimensional stability. The polyacetal resin homopolymer is more preferable.
The MFR measured under ASTM D1238-57 E condition of a polyacetal resin is preferably 40 g/10 min. or more, more preferably 60 g/10 min. or more, most preferably 70 to 100 g/10 min.
The present invention is characterized in injection molding a polyacetal comprising a specific boron nitride.
An average particle diameter of boron nitride to be used in the present invention is 1 to 10 .mu.m, preferably 2 to 7 .mu.m. A content of diboron trioxide is 0.005 to 1.0% by weight, preferably 0.05 to 0.7% by weight, more preferably 0.1 to 0.6% by weight. A mixing amount of diboron trioxide is 10 to 1000 ppm, preferably 10 to 300 ppm, more preferably 100 to 200 ppm, based on the amount of the polyacetal resin.
In the present
REFERENCES:
patent: 3976605 (1976-08-01), Matsunaga et al.
patent: 4051096 (1977-09-01), Koseki et al.
patent: 5131827 (1992-07-01), Tasaka
Ibe Sadao
Niino Masahiko
Asahi Kasei Kogyo Kabushiki Kaisha
Heitbrink Jill L.
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