JP2837701B2 - Method for producing polytetrafluoroethylene resin molded article - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a molded article of polytetrafluoroethylene resin, and more particularly, to a particularly long polytetrafluoroethylene resin having no dimensional variation and no warp. The present invention relates to a manufacturing method for extruding a molded product such as a tube and a rod.

Technical background of the invention and its problems As is well known, polytetrafluoroethylene resin has a melt viscosity of 10 even at a melting point (327 ° C.) or higher of 380 ° C.
^At about ¹¹ poise, the viscosity at the molding temperature of ordinary plastic is much higher than 10 ³ -10 ⁴ poise.

For this reason, unlike ordinary plastics, it is not possible to perform extrusion, rolling, or other forming after softening or flowing by heating. Therefore, molded articles such as tubes and rods made of polytetrafluoroethylene resin (hereinafter, PTFE resin) are molded by a paste extrusion method.

This paste extrusion method usually employs solvent naphtha, to give unfired PTFE resin powder fluidity at around room temperature,
It is performed by blending an extrusion aid such as white kerosene or toluene, preforming this into a cylindrical shape, placing the obtained preformed product (a billet) in a cylinder of an extruder, and extruding by pressing with a ram. . According to this extrusion method, PT
The FE resin particles are plastically deformed by the aid of the auxiliary agent, and are extruded from a die attached to the tip of the cylinder, whereby a continuous unfired PTFE resin molded article having a predetermined shape can be obtained. next,
The unfired PTFE molded article enters a long cylindrical furnace body in the next step. Here, the extrusion aid contained in the unfired PTFE resin molded article in the drying zone at about 100 to 250 ° C. in the first stage. Is removed and then at a temperature above the melting point of the PTFE resin, for example 36
The unfired PTFE resin is fired in the firing zone at 0 to 380 ° C., and finally cooled to finally obtain a dense fired PTFE resin molded product having sufficient mechanical strength.

However, especially when extruding a long object such as a tube or a rod, the desired dimensional accuracy cannot be obtained in the unit of mm, and the outer diameter of the molded article becomes non-uniform in the firing step after the auxiliary agent is dried, There is a problem that the roundness of the rod or the cross section of the rod is not obtained, or the long object is not fired in a straight line, causing warpage in the length direction and significantly deteriorating the commercial value.

Therefore, the present inventors have developed a method for manufacturing a PTFE extruded product which can solve the problems caused by the extrusion molding method at once, and which can produce an extruded product having a desired size accurately, easily and at low cost. This has already been proposed (Japanese Patent Application
63-238,426).

This manufacturing method comprises the steps of: firing an unfired polytetrafluoroethylene resin paste extruded product extruded from an extruder into a firing mold having an inner diameter slightly larger than the outer diameter of the molded product; The cooling is performed to produce a polytetrafluoroethylene resin molded product.

According to this method, even if the extruded product thermally expands during firing, its radially outward expansion is limited by the inner peripheral surface of the firing mold, and the subsequent molding accompanying the cooling of the molded product. The shrinkage of the product becomes uniform, and a desired PTFE extruded product having a constant outer diameter dimension in the axial direction can be obtained with high accuracy.

However, in the case of extruded PTFE products formed by this method, especially in the case of long tubular thin tubes (approximately 2.0 mm or less) with a large inner diameter, after firing and cooling, Protrusions rarely occurred on the surface. Such a bamboo node-shaped annular projection not only impairs the commercial value of the molded product, but is also undesirable from the viewpoint of the physical properties of the extruded PTFE product.

The present inventors have conducted intensive studies on the cause of the occurrence of the knot-shaped annular protrusion of bamboo, and as a result, have presumed that the cause is as follows.

In other words, when a molded product obtained by extruding an unfired polytetrafluoroethylene resin into a paste and then firing is inserted into a firing mold, the molded product undergoes thermal expansion in the radial direction. To adhere to the inner peripheral surface of the firing mold,
It contracts in the axial direction and moves and displaces along the inner peripheral surface of the firing mold. When this is cooled, it shrinks slightly in the radial direction, shrinks in the length direction, and moves and displaces along the inner peripheral surface of the firing mold.
At about 330 ° C., conversely, expansion expansion in the axial direction of about 7% was observed.

When the radial expansion and contraction do not coincide with the axial expansion and contraction, the outer peripheral surface of the molded product is still in contact with the inner peripheral surface of the firing mold. If the friction between the molded product and the firing mold is not uniform when the molded product is displaced in the axial direction due to expansion and contraction, the axial expansion and contraction of the molded product will not be uniform throughout, and will be uneven. The present inventors have presumed that the above-mentioned bamboo knot-shaped annular projection will be partially generated, and have completed the present invention.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a method for manufacturing a PTFE extruded product capable of accurately, easily, and at low cost producing a PTFE extruded product having a desired outer diameter and a smooth shape. The purpose is to provide.

SUMMARY OF THE INVENTION In order to achieve the above object, a method for producing a polytetrafluoroethylene resin molded product according to the present invention comprises: extruding an unfired polytetrafluoroethylene resin paste extruded from an extruder; Baking and cooling are performed in a state of being inserted into a firing mold having an inner diameter slightly larger than the outer diameter dimension of the above and having a sliding layer with a small frictional resistance on the inner peripheral surface to obtain a polytetrafluoroethylene resin molded product. It is characterized by:

The sliding layer is formed by using a carbon layer, a graphite layer, a boron nitride layer, and a wire mesh, or by forming an inner peripheral surface of a mold with an uneven surface, or by forming a large number of through holes in the peripheral surface. Preferably, it is formed.

The relationship between the inner diameter (D) of the fired mold and the outer diameter (d ₁ ) of the unfired polytetrafluoroethylene resin paste extrudate is preferably 1 <D / d ₁ ≦ 1.2.

According to such a method for producing a PTFE resin molded product according to the present invention, when a PTFE resin paste extruded product extruded from an extruder is inserted into a firing mold and fired and cooled, the molded product is Will slide along the sliding layer on the inner peripheral surface of the firing mold. Further, even if air enters between the molded product and the firing mold, it can escape through the sliding layer. Therefore, the axial displacement of the molded product accompanying the firing and cooling is smoothly performed, and the nodal annular protrusion of the bamboo does not occur on the surface, the outer peripheral surface of the molded product becomes smooth, and the outer diameter is further reduced. The dimensions are also constant in the axial direction.

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, the present invention will be described based on embodiments shown in the drawings.

FIGS. 1, 3, 4, and 5 are schematic cross-sectional views showing steps of a method of manufacturing a PTFE resin molded product according to one embodiment of the present invention, and FIG. 2 is a cross-sectional view of a main part of a firing mold used in the embodiment. FIG.

The embodiment of the present invention shown in FIGS.
The case where a PTFE resin molded product is manufactured is shown. In the present embodiment, an example of so-called upward extrusion in which an unfired PTFE tube is extruded upward is used. However, the present invention is not limited to this. For example, a PTFE tube can be extruded downward.

In the method of the present invention, first, an extrusion aid such as solvent naphtha, white kerosene, or toluene is blended in order to impart fluidity near room temperature to the unfired PTFE resin powder, and this is preformed into a cylindrical or columnar shape. To form a preform. The particle size of the PTFE resin powder for obtaining this preform is not particularly limited, but preferably has an average primary particle size of 0.05.
This is a so-called PTFE fine powder in which fine particles of up to 1.0 μm are formed into secondary particles to form aggregated particles.

Next, put this preform into the cylinder of the extruder,
It is extruded into a tube shape from a die 1 of an extruder by pressurizing with a ram, for example, as shown in FIG. At this time, the hook 3 is engaged with the tip of the extruded tubular unfired PTFE resin paste molded product 2, and the hook 3 is pulled up by the wire 4 and the pulley 5, thereby forming the paste extruded molded product 2. Extrusion molding is smooth.

The unsintered PTFE resin paste extrudate 2 is extruded from a die 1 of the extruder is inserted into the firing mold 6 having a slightly larger inside diameter D than the outer diameter d ₁ of the molded product 2.

In particular, in the firing die 6 in this embodiment, as shown in detail in FIG. 2, a sliding layer S having a low frictional resistance is formed on the inner peripheral surface of the die main body 6a. The sliding layer S is for reducing frictional resistance with the molded article 2 and smoothly displacing the molded article 2 when the molded article 2 is fired or cooled as described later. Therefore, the sliding layer S may be any material as long as it has heat resistance at the firing temperature of the polytetrafluoroethylene resin and has low sliding resistance. With a carbon layer 6b adhered to the inner peripheral surface of the substrate.

The inner diameter D of the firing mold 6, that is, the inner diameter D of the sliding layer S
Is determined such that, when the molded product 2 expands radially outward when the extruded molded product 2 described later is fired, the molded product 2 is pressed against the inner peripheral surface of the firing mold 6. Firing mold 6
The inner diameter D of the outer diameter d ₁ of the paste extrusion molded product 2, preferably in the represented range following equation.

1 <D / d ₁ ≦ 1.2 Further, the mold main body 6a constituting the firing mold 6 is originally intended to improve the workability and the PT to be fired in the firing mold 6
From the viewpoint of not adversely affecting the appearance of the FE resin molded product, a thin metal seamless pipe, particularly a stainless steel seamless pipe, is preferable. However, in the case of the firing mold 6 having the sliding layer S described above, the mold is used. The main body 6a is formed using an inexpensive tube,
A carbon layer may be formed on the inner peripheral surface, which is advantageous in cost. The sliding layer S may be a graphite layer or a boron nitride layer other than the carbon layer. Further, the entire firing mold 6 can be formed integrally with the sliding layer S using the material constituting the sliding layer S.

After the PTFE resin paste extruded product 2 is inserted into the firing mold 6 along the longitudinal direction, the paste located between the lower end of the firing mold 6 and the die 1 is inserted as shown in FIG. The extrudate 2 is cut. Then, the hook 3 and the wire 4 are unlocked, and the hook 3 is locked by the moving means 7, and the paste extruded product 2 and the firing die 6 are held by the moving means 7.

Next, the baking mold 6 into which the paste extruded product 2 has been inserted is moved into the furnace 8 shown in FIG. 4 by the moving means 7, and the baking mold 6 is held by the wire 9. At the same time or before that, the lower end of the firing mold 6 is covered with a blind plate 10.

In such a state, the temperature in the furnace 8 is increased, and the paste extruded product 2 in the firing mold 6 is fired. Before that, the extrusion aid contained in the paste extruded product 2 is dried and removed. Let it. Such a drying and removing step is performed at 100 to 250 ° C.
It is carried out by heating at a temperature of the order, but may be carried out at the same time as firing, or may be carried out in a separate step.

The temperature at which the paste extruded product 2 is fired is a temperature equal to or higher than the melting point of PTFE (327 ° C.), and usually 340 to 380 ° C. is appropriate. Also. The firing time is preferably 2 to 6 hours.

In such a firing step, the paste extruded product 2 during firing expands radially outward, but the die body 6a of the firing die 6 exists radially outward. As a result, the expansion of the molded product 2 is limited, and the outer peripheral surface of the molded product 2 and the inner peripheral surface of the firing mold 6 are brought into pressure contact with each other. That is, the outer diameter d ₂ of the molded article 2 during firing is equal to the inner diameter D of the firing mold 6.
(D ₂ = D). In this case, since the mold body 6a of the firing mold 6 is made of metal, its thermal expansion is negligible compared to that of the molded product 2. Therefore, the molded article 2 during firing presses the inner peripheral surface of the firing mold 6 and expands in a state where uniform thermal stress is applied to each part.

In particular, in this embodiment, the sliding layer S is formed on the inner peripheral surface of the firing mold 6.
Therefore, even if the molded article 2 shrinks and displaces during firing, the frictional resistance generated between the two becomes extremely small, and the molded article 2 is constantly displaced smoothly along the sliding layer S. And the radially outer peripheral surface of the molded product 2 is not deformed.

Next, the fired molded article 2 is taken out of the furnace 8 together with the firing mold 6, and cooled to room temperature as shown in FIG. When cooled to below the melting point of the PTFE, the fired molded product 2 starts to shrink in the radial direction, the thermal stress is gradually released, and the PTFE having an outer diameter d ₃ smaller than the inner diameter D of the firing mold 6 is soon. A resin molded product 2a is obtained.

In the initial stage of cooling, the molded product 2 contracts in the radial direction and expands and displaces in the axial direction. However, since the sliding layer S is formed on the inner peripheral surface of the firing die 6, the expansion displacement in the axial direction causes the molded product 2 to expand. However, the frictional resistance between them becomes extremely small, and the molded product 2 is always smoothly displaced along the sliding layer S, so that the radially outer peripheral surface of the molded product 2 is not deformed. In addition, since the shrinkage in the radial direction is performed uniformly and smoothly in the axial direction, it is possible to accurately obtain a PTFE extruded product having a desired dimension whose outer diameter is constant in the axial direction.

The outer diameter of the PTFE resin molded product 2a as the final product
d ₃ is the outer diameter of the unfired PTFE resin paste extruded product 2
and d _1, is determined by various factors of the ratio D / d _1, and the like cooling rate of the inner diameter D of the baking mold 6.

The present invention is not limited to the embodiments described above, and can be variously modified within the scope of the present invention.

For example, as shown in FIG. 6, a sizing die 11 is attached to the end of the die 1 of the extruder, and the unfired PTFE resin paste extrudate 2 immediately after being extruded from the die 1 of the extruder.
After sizing (diameter reduction) with the sizing die 11,
This sized molded product 2 is taken as the outer diameter of the molded product 2.
It may be inserted into the firing mold 6 having a slightly larger inner diameter than d _1. Other steps are the same as those in the above-described embodiment.

In the example shown in FIG. 6, the relationship between the diameter a of the die 1 of the extruder and the diameter b of the sizing die 11 is (ab) / a <0.
08 is preferred.

As described above, when the unfired PTFE resin paste extruded product 2 immediately after being extruded from the die 1 of the extruder is sized, the molded product 2 can be inserted into a fired mold having a smaller diameter. Due to the stress recovery of the unfired PTFE molded product 2, a product having better dimensional accuracy than a molded product that is not sized can be obtained by better adhering to the inner surface of the fired tube.

Further, the PTFE resin molded product is not limited to the tube shape, and may be any shape such as a solid columnar shape, a bellows shape, and a laminated shape with a reinforcing material.

FIG. 7 shows another embodiment of the present invention, in which the sliding layer S is formed by a wire mesh 6c. In such a wire mesh 6c, since each wire has a circular cross section, if this wire is used as the sliding layer S, the contact state with the molded product 2 becomes point contact, and the dynamic friction or sliding resistance is reduced. It can be made smaller and has no frictional resistance with the molded article 2 like the carbon layer 6b. The mesh size of the wire mesh 6c is preferably, for example, about 30 to 60 mesh. In the present invention, the firing tube 6 may be constituted only by such a wire mesh.

FIG. 8 shows still another embodiment of the present invention.
The inner peripheral surface of the firing die body 6a is subjected to blast finishing to form a fine uneven surface 6d as the sliding layer S. Such an uneven surface 6d is also in a point contact state with the molded product 2 in a substantially point contact state, the dynamic friction or the sliding resistance is reduced, and there is no frictional resistance with the molded product 2.

Further, as shown in FIG. 9, the wire mesh 6 itself may be constituted by an embossed cylinder, and a large number of embosses 6e may be formed on the inner peripheral surface. The shape of the emboss 6e is preferably a semicircle so as to make point contact with the molded product. Further, the pitch of the embossment 6e is preferably 0.4 to 5.0 mm, and the area where the embossment 6e is formed is 20 to
60% is preferred. The height of the peak of the emboss 6e is preferably about 0.1 to 2.0 mm.

Further, as shown in FIG. 10, a large number of through holes 6f may be provided on the peripheral surface of the mold 6 to form the sliding layer S. The diameter of the through hole is not particularly limited, but is preferably 0.1 to 3.0 mm, and more preferably 0.5 to 1.5 mm. The porosity is preferably 20 to 60% with respect to the entire inner peripheral area.

In the present invention, a tube in which such an embossment 6e or a through hole 6f is formed may be used as a firing mold itself,
By separately preparing a mold body and inserting it into the mold body, a firing mold may be configured.

In particular, in the case of the sintering mold having the sliding layer S shown in FIGS. 7 to 10, the air permeability of the sliding layer S is excellent because the air permeability of the sliding layer S is excellent. Is not formed, and the air pocket does not deform the outer peripheral surface.

According to the method for manufacturing a PTFE resin molded product according to the present invention, since the sliding layer having a low frictional resistance is formed on the inner peripheral surface of the firing mold, the molded product is fired or cooled. Even when the expansion and contraction displacement is performed in the axial direction, the displacement in the axial direction is performed smoothly, so that the outer peripheral surface of the molded product does not have, for example, a knot-shaped uneven portion of bamboo, and the outer peripheral surface of the molded product is smooth. become. In addition, according to the present invention, an extruded PTFE product having a desired outer diameter dimension that is constant in the axial direction can be obtained with high accuracy, and its production is easy and cost-effective.

In addition, when the sliding layer is formed by the metal mesh and the uneven surface, the air permeability of the sliding layer is improved, and the deformation of the molded product due to the air pocket between the molded product and the mold can be effectively prevented.

Hereinafter, the present invention will be described based on more specific examples.

Example 1 PTFE powder (Teflon 6J) with extrusion aid (Isopar
E) was added to about 20% by weight and mixed.^TwoSpare
After molding, a die for an extruder with a die with an inner diameter of 113 mm
Nominal size 100A Outside diameter 114mm, inside diameter 110mm, length 7.2m
The fired PTFE tube was extruded.

Next, the tube was inserted into a firing tube with an inner diameter of 118 mm and a length of 7.2 m having a carbon layer 6a formed on the inner peripheral surface, and then the extrusion aid was removed at about 150 ° C and dried.

Next, the unfired PTFE tube after the drying step was inserted into the firing tube, placed in a furnace, and fired at 370 ° C. After firing, the furnace was cooled to room temperature. Tables 1 and 2 show the variation in the outer diameter of the PTFE tube after cooling and the degree of flatness.

Example 2 An inner diameter of 118 m with a 30-60 mesh wire mesh formed on the inner peripheral surface
After inserting the unfired PTFE tube into a firing tube having a length of m, the auxiliary agent was dried and PTFE was fired under the same conditions as in Example 1.

Tables 1 and 2 show the unevenness and the like of the variation in the outer diameter of the fired PTFE tube after cooling.

Comparative Example 1 The unfired PTFE tube extruded in Example 1 was not inserted into the firing tube, but was hung on a hook. The auxiliary was dried, and then fired under the same conditions as in Example 1.

Tables 1 and 2 show the variation in the outer diameter of the fired PTFE tube after cooling, and the flatness and the like.

[Brief description of the drawings]

FIGS. 1, 3, 4, and 5 are schematic cross-sectional views showing steps of a method of manufacturing a PTFE resin molded product according to one embodiment of the present invention, and FIG. 2 is a cross-sectional view of a main part of a firing mold used in the embodiment. Fig. 6, Fig. 6 is a schematic sectional view showing another embodiment of the present invention, Figs. 7 to 9 are schematic sectional views showing still another embodiment of the present invention, and Fig. 10 is still another embodiment of the present invention. It is a perspective view showing an example. 1 ... die, 2 ... paste extruded product, 2a ... PTFE resin molded product, 6 ... fired mold, 6a ... fired mold body, 6b ... carbon layer, 6c ... wire mesh, 6d ... Uneven surface, 6e: embossed 6f: through hole, 8: furnace, S: sliding layer.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-86427 (JP, A) JP-A-54-106573 (JP, A) JP-A-59-178228 (JP, A) (58) Investigation Field (Int.Cl. ⁶ , DB name) B29C 67/00-67/04 B29C 33/00-33/76

Claims (7)

(57) [Claims] An extruded product of an unfired polytetrafluoroethylene resin paste extruded from an extruder is provided with a sliding layer having an inner diameter slightly larger than an outer diameter of the molded product and having a small frictional resistance on an inner peripheral surface. A method for producing a molded article of a polytetrafluoroethylene resin, wherein the molded article is fired and cooled in a state of being inserted into a sintering mold having a polytetrafluoroethylene resin molded article. 2. The method according to claim 1, wherein the sliding layer is a carbon layer, a graphite layer or a boron nitride layer. 3. The method for producing a polytetrafluoroethylene resin molded product according to claim 1, wherein said sliding layer is formed by a wire mesh. 4. The method for producing a polytetrafluoroethylene resin molded article according to claim 1, wherein said sliding layer is formed by forming an inner peripheral surface of a mold with an uneven surface. 5. The method for producing a molded article of polytetrafluoroethylene resin according to claim 1, wherein said sliding layer is formed by forming a large number of through holes in a peripheral surface of a mold. 6. An unfired polytetrafluoroethylene resin paste extruded product immediately after being extruded from an extruder is sized by a sizing die, and the sized product is fired and cooled in the firing mold. The method for producing a polytetrafluoroethylene resin molded product according to any one of claims 1 to 4, wherein: 7. The relationship between the inner diameter (D) of the fired mold and the outer diameter (d ₁ ) of the unfired polytetrafluoroethylene resin paste extruded product is 1 <D / d ₁ ≦ 1.2. The method for producing a polytetrafluoroethylene resin molded product according to any one of claims 1 to 5, characterized in that: