JPH0615663A - Cylindrical fluororesin molded piece and production thereof - Google Patents

Abstract

PURPOSE:To form a molded piece having an MD/CD ratio <=1.0 between a linear expansion coefficient MD in parallel to an axis of a cylinder and a coefficient CD vertical to the axis and a dispersion of + or -0.005 or less in apparent specific gravity by a method wherein fluororesin powder is charged between a mold and a cylindrical rubber mold, which are coaxially disposed, the powder is vertically pressurized to be preformed into a cylinder, and the molded piece. CONSTITUTION:A round bar-form mold 2 and a cylindrical rubber mold 3 are coaxially fitted into a lower mold 1. A space between the molds 2, 3 is filled with fluororesin powder 4, thereafter being sealed by fitting an upper mold 5. These are put in a pressurizing device 9 to be pressurized by a pressurizing medium 11 injected through a drain 10. Thus, the powder 4 is axially and vertically pressurized to be preformed into a cylinder. The preform is removed, heated, and calcined to form a cylindrical fluororesin molded piece. In this method, the fluororesin molded piece has an MD/CD ratio <=1.0 between a linear expansion coefficient MD in a direction (a) in parallel to a cylinder's axis X and a linear expansion coefficient CD in a direction (b) vertical to the axis and a dispersion of + or -0.005 or less in apparent density on the both ends in the direction (a), thus being improved in wearability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tubular fluoroplastic molding and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, among fluororesins, polytetrafluoroethylene, in particular, has been excellent in water repellency, oil repellency, non-adhesiveness, low friction, chemical resistance and safety. Products, sheets, paints, etc., processed into various forms, corrosion resistant materials for chemical plants, non-adhesive coated household products, mechanical parts such as bearings that require low friction, medical treatment, It is used in a wide range of fields such as cash register products.

Since this polytetrafluoroethylene has a very high melt viscosity, it cannot be processed by an injection molding method, but is generally processed by a compression molding method. The compression molding method is a method in which fluorine resin powder filled in a predetermined mold is compressed by a press to be preformed, and the molded product is then fired in a hot air drying furnace.

By the way, in recent years, along with the expansion of the field of use of fluorocarbon resin moldings, consideration of use under special conditions and strict physical properties have been demanded. Particularly, sliding members for pistons, There is a demand for tubular or ring-shaped fluororesin moldings applied to piston rings, seal rings and the like.

In the above compression molding method, such a molded body is produced by using a cylindrical metal mold as a core die and pressurizing it in the axial direction to form a cylindrical molded body. It is manufactured by cutting into a ring shape.
As a characteristic of this molded product, it is known that the linear expansion coefficient in a direction parallel to the cylindrical axis, that is, the compression direction in compression molding is larger than the linear expansion coefficient in the vertical direction.

[0006]

However, in this conventionally known fluorine resin molded body, when it is mounted on a piston or the like having a not so large axial clearance,
Due to the large linear expansion coefficient in the direction parallel to the axial direction, the temperature in the system during operation may cause axial expansion, causing problems such as abnormal wear and reduced seal.

In view of the above-mentioned conventional circumstances, the present invention has a cylindrical shape which does not cause problems such as abnormal wear due to thermal expansion and a decrease in seal even when mounted on a piston or the like having a small axial clearance. It is an object of the present invention to provide a fluorine resin molded body and a method for producing the same.

[0008]

Means for Solving the Problems As a result of intensive studies to achieve the above-mentioned object, the inventors of the present invention have adopted a specific pressurizing method to form a cylindrical shape in the compression molding of polytetrafluoroethylene powder. When preformed, the subsequent heating and firing,
Since the linear expansion coefficient has a different directionality from the conventional one, even if it is mounted on a piston with a small axial clearance, problems such as abnormal wear due to thermal expansion and seal decrease will not occur. As a result, I came to complete the present invention.

That is, the first aspect of the present invention is such that when the linear expansion coefficient in the direction parallel to the cylinder axis is MD and the linear expansion coefficient in the vertical direction is CD, MD / CD≤1.0. According to a tubular fluorocarbon resin molded body characterized in that, in this molded body, the variation of the apparent density between one end side and the other end side in the direction parallel to the cylindrical axis is especially A structure of ± 0.005 or less, and a structure in which this molded body is made of polytetrafluoroethylene,
Each is made into a suitable mode.

A second aspect of the present invention is to fill fluorine resin powder between a die and a cylindrical rubber die coaxial with the die,
This powder is pressed from the direction perpendicular to the axial direction of the mold through the rubber mold to preform it into a cylinder, and then heated and fired to release it from the mold. The present invention relates to a method for manufacturing a fluorine resin molded body.

[0011]

In the method for manufacturing a tubular fluoroplastic molding of the present invention, first, as shown in FIG.
Then, a cylindrical or round rod-shaped mold 2 made of iron or the like and a cylindrical rubber mold 3 coaxial therewith are fitted, and a fluorine resin powder 4 is filled between the mold 2 and the rubber mold 3. After that, the upper mold 5 is fitted and sealed.

Examples of the tubular rubber mold 3 include natural rubber, synthetic rubber, chloroprene rubber, urethane rubber, silicone rubber, nitrile rubber, and the like, and any material can be used as long as it is an elastic body capable of transmitting pressure. The rubber hardness (Hs) is usually 50 to 100,
Rubber elongation is 100% or more, rubber thickness is 0.1-3 mm
Those of are preferably used.

The fluorine resin powder 4 is either molding powder (based on compression molding powder) or fine powder (based on paste extrusion powder). But it's okay. As the type of resin, polytetrafluoroethylene is preferably used, but other fluorine resins such as tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-perfluoroalkylvinylether copolymer, ethylene -Tetrafluoroethylene copolymer, polychlorotrifluoroethylene, ethylene-chlorotrifluoroethylene copolymer, modified polymers thereof or blends of two or more thereof may be used.

This fluorine resin powder 4 is used according to the purpose.
One or two kinds of the above fluorocarbon resins such as various fillers, for example, reinforcing filler, sliding filler, conductive filler, heat conductive filler, adsorptive filler, functional filler, etc. Using the known mixers such as tumbler mixer, Hensiel mixer, and super mixer,
It may be a homogeneous mixture.

Examples of the reinforcing filler include glass fibers, carbon fibers, aramid fibers, alumina fibers, boron fibers, glass beads, silicon carbide whiskers, silicon nitride whiskers, potassium titanate whiskers and the like.

As the slidable filler, graphite, molybdenum disulfide, tungsten disulfide, boron nitride, mica, aromatic polyester resin, silicone resin, calcium fluoride, graphite fluoride, glass flake, car Examples include bombrads, grafites, and bronze.

Examples of the conductive filler include various metal powders, metal flakes and metal fibers, and examples of the thermal conductive filler include beryllium oxide, aluminum nitride, alumina, magnesia and titania.

Examples of the adsorptive filler include silica gel, zeolite, talc, bentonite and potassium titanate, and examples of the functional filler include calcium carbonate, barium titanate, kaolin and clay.

The fluorine resin powder 4 thus filled between the mold 2 and the rubber mold 3 and sealed in the upper and lower molds 1 and 5 is then, as shown in FIG. In the pressurizing device 9 consisting of 6 and the upper and lower lids 7 and 8, the pressure medium 11 such as water, oil or air, which is press-fitted from the drain 10 to which the corrosion inhibitor is added, is usually 100 to 1,000 kg
Pressurized with a pressure of about / cm ² .

The pressure applied to the fluorine resin powder 4 is
Since the axial direction of the mold 2 is restrained by the mold itself,
The powder 4 is applied only from the direction perpendicular to the axial direction through the rubber mold 3. By this application, the powder 4 is pressure-bonded to the peripheral surface of the mold 2 and preformed into a tubular shape. It

In FIG. 1, the cylindrical rubber mold 3 is arranged outside the mold 2 and pressure is applied from the outside of the rubber mold 3. On the contrary, the mold 2 is cylindrical. Then, the cylindrical rubber mold 3 is placed inside the mold, and the fluorine resin powder 4 filled between the molds 2 and 3 is pressed from the inside of the rubber mold 3,
You may make it obtain the cylindrical preform similar to the above.

After being preformed into a tubular shape in this manner,
It is taken out from the pressurizing device 9, and the rubber die 3 and the upper and lower die 1,
5 is removed and heated and baked at a temperature of usually 360 to 400 ° C., and after the baking, the mold 2 is released.
A target tubular fluoroplastic molded product is obtained.

As shown in FIG. 2, this tubular fluorocarbon resin molded product is composed of a molded product 40 having a hollow portion 2s, and the linear expansion coefficient in the direction a parallel to the cylindrical axis x is MD and the direction b is vertical.
It is characterized in that MD / CD ≦ 1.0, particularly preferably 0.80 ≦ MD / CD ≦ 1.0, where CD is the coefficient of linear expansion.

Therefore, when the molded body 40 is cut as it is or cut into a ring having a predetermined thickness and mounted on a piston or the like having a small axial clearance, the system temperature rises during operation. However, since the coefficient of linear expansion in the direction parallel to the axial direction is small, the coefficient of expansion in the same direction is small, so that problems such as abnormal wear and reduction in seal as in the past do not occur.

The molded body 40 is characterized in that the variation in the apparent density between the one end side and the other end side in the direction a parallel to the cylindrical axis x is ± 0.005 or less with respect to the average value. Is also equipped. This is because, as shown in FIG. 1, uniform compression is applied from a direction perpendicular to the axial direction to obtain a uniform apparent density over the entire length from one end side to the other end side. In the molding method, the apparent density of the center side becomes abnormally smaller than that of both ends, which causes a problem that deformation and warpage occur during heating and use.
According to this, such a problem can be avoided.

[0026]

As described above, according to the present invention, the fluorine resin powder is preformed into a cylindrical shape by a specific pressurizing method, so that a cylindrical shape having a linear expansion coefficient different in directionality from the conventional one. It is possible to provide a base resin molded body, and according to this molded body, it is possible to avoid problems such as abnormal wear and seal reduction due to thermal expansion when mounted on a piston or the like having a small axial clearance.

[0027]

EXAMPLES Next, examples of the present invention will be described to more specifically describe.

Example 1 Granulated powder of polytetrafluoroethylene (molding powder) (average particle size 350 μm, bulk density 0.9)
0 g / cc), as shown in Fig. 1, diameter 14 mm, length 3
30mm iron mold and 30m coaxial inner diameter (diameter)
m, thickness 1.5 mm, length 350 mm, elongation 500%, filled with a cylindrical rubber mold, the upper and lower molds form a closed system, and then set in a high pressure device, using water as a pressure medium. Then, pre-molding was performed under an applied pressure of 300 kg / cm ² and a holding time of 5 minutes.

After this pre-molding, the rubber mold and the upper and lower molds were taken out from the inside of the high-pressure apparatus and the temperature was changed to 370 ° C.
After heating and firing for a period of time, the mold was released from the mold to produce a cylindrical fluororesin molded product having an outer diameter (diameter) of 21 mm, an inner diameter (diameter) of 14 mm and a length of 300 mm as shown in FIG.

With respect to this cylindrical fluororesin molded article, the distance between one end and the other end in the direction parallel to the cylindrical axis is 1
The sample was divided into 5 equal parts, and an arbitrary portion in each of the parts was extracted as a sample, and the apparent density and linear expansion coefficient of these samples were measured by the following methods. The results are shown in Table 1 below.

<Apparent Density> The apparent density was measured by an underwater substitution method according to JIS K-7112. Numerical values were obtained up to 3 digits after the decimal point using a balance with an accuracy of 0.1 mg.

<Linear expansion coefficient> A sample was cut into a 5 mm square,
The linear expansion coefficient MD in the direction parallel to the cylindrical axis and the linear expansion coefficient CD in the vertical direction were measured with a thermomechanical analyzer (TMA20 manufactured by Seiko Denshi Kogyo Co., Ltd.) at a measurement temperature of 30- The measurement was carried out under the conditions of 250 ° C. and a temperature rising rate of 10 ° C./min, and was calculated according to the following formula.

[0033] K: apparatus constant L: sample length (= 5mm) T: Temperature beta _0: linear expansion coefficient of the quartz glass beta: average linear expansion coefficient in a temperature T ₁ -T ₂ ΔL: displacement of the sample length at a temperature T ₁ -T ₂ amount

[0034]

[Table 1]

From the results shown in Table 1 above, the cylindrical fluororesin molding of the present invention has an MD / CD ratio of 1.0 or less, and a variation in the apparent density over the entire length from one end to the other end. The average value (= 2.161) is ± 0.005, which is almost uniform over the entire length, and has excellent characteristics such that problems due to thermal expansion, thermal distortion and deformation do not easily occur. it is obvious.

Comparative Example 1 The same granulated powder of polytetrafluoroethylene as in Example 1 was used as a cylindrical metal mold having an inner diameter (diameter) of 21 mm and a length of 350 mm, and a diameter of 14 mm and a length arranged at the shaft core. It was filled with a steel mold having a length of 330 mm and was preformed through the upper and lower molds in the axial direction with an applied pressure of 300 kg / cm ² and a holding time of 5 minutes.

After this preforming, the upper and lower molds were removed, and the mixture was heated and baked at 370 ° C. for 4 hours, and then released from the iron mold to have an outer diameter (diameter) of 21 mm and an inner diameter (diameter) of 14 m.
A cylindrical fluororesin molding having a length of m and a length of 300 mm was produced.

With respect to this cylindrical fluororesin molded product, the distance between one end and the other end in the direction parallel to the cylindrical axis is 1
The sample was divided into 5 equal parts, an arbitrary portion in each of the parts was extracted as a sample, and the apparent density and the linear expansion coefficient of these samples were measured by the same method as described above. The results are shown in Table 2 below.

[0039]

[Table 2]

From the results shown in Table 2 above, the cylindrical fluororesin molded article of the comparative example has an MD / CD ratio of 1.1 or more, and an average value of variations in the apparent density from one end side to the other end side ( = 2.138), it is as large as ± 0.03, and in particular, the apparent density on the central side is extremely small, and thermal distortion and deformation are likely to occur.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing one structural example of an apparatus used in a method for manufacturing a tubular fluorine resin molded body of the present invention.

FIG. 2 is a perspective view showing an example of a tubular fluorine resin molded body of the present invention.

[Explanation of symbols]

 2 Mold 3 Cylindrical rubber type 4 Fluorocarbon resin powder 9 Pressurizing device 40 Cylindrical fluorocarbon resin molded product x Cylindrical axis a Direction parallel to cylindrical axis b Direction perpendicular to cylindrical axis

Claims (4)

[Claims] 1. When the linear expansion coefficient in the direction parallel to the cylindrical axis is MD and the linear expansion coefficient in the vertical direction is CD, M
A tubular fluorocarbon resin molded product having a relationship of D / CD ≦ 1.0. 2. The tubular fluorine according to claim 1, wherein the variation in the apparent density between the one end side and the other end side in the direction parallel to the cylindrical axis is ± 0.005 or less with respect to the average value thereof. Resin molding. 3. The cylindrical fluororesin molded article according to claim 1, which is made of polytetrafluoroethylene. 4. A fluorine resin powder is filled between a die and a cylindrical rubber die coaxial therewith, and the powder is passed through the rubber die from a direction perpendicular to the axial direction of the die. A method for producing a tubular fluororesin molded article, which comprises pressurizing and preforming into a tubular shape, followed by heating and firing to release from the mold.