JP2877944B2 - 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 obtaining molded articles such as tubes and rods.

Technical Background of the Invention As is well known, a polytetrafluoroethylene resin has a melt viscosity of 10
^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, extrusion, rolling and the like cannot be performed in a softened or fluidized state by heating as in ordinary plastics. Therefore, polytetrafluoroethylene resin (hereinafter PT
Tubes, rods and the like made of EF resin) are produced 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 a firing zone at 0 to 380 ° C., and finally cooled to finally obtain a dense fired PTFE resin molded article having sufficient mechanical strength.

However, especially when extruding a long object such as a tube or rod, the outer diameter of the molded product becomes uneven in the firing step after drying the auxiliary agent, or the roundness of the cross section of the tube or rod does not appear. Alternatively, there is a problem in that the long object is not fired in a straight line and warps in the length direction. Moreover, there has been a problem that a desired dimensional accuracy cannot be obtained in units of mm. This tendency was particularly remarkable in a molded product having a large outer diameter.

The product manufactured in this way not only impairs the commercial value, but also has the following inconvenience when the tube is lined in a steel pipe, for example.

As an example of a method for manufacturing a PTFE-lined steel pipe, there is a method in which an extruded PTFE tube is drawn into a steel pipe and lined. In this case, an appropriate clearance is required between the outer diameter of the lining tube and the inner diameter of the steel pipe to be lined, and this clearance is usually
About 2% of the inner diameter of the steel pipe is appropriate. However, the PTFE tube formed by the conventional manufacturing method is drawn into the steel pipe due to reasons such as the unevenness of the diameter at each part and the lack of roundness of the tube cross section. When,
Since the tension of the PTFE tube on the inner surface of the steel pipe is different at each part, it places a burden on the corner of the flange, which is the longitudinal stop end, and has the disadvantage that damage is likely to occur at this part. However, problems such as buckling were likely to occur.
In addition, since the cross section of the tube is not a perfect circle but is flat, the lining tube is warped when it is drawn into the steel pipe.

On the other hand, as means for preventing the occurrence of the above-mentioned inconveniences, a so-called annealing treatment of removing the distortion by directly heating the lining tube drawn into the steel pipe in an electric furnace or the like is known.

However, in order to perform such an annealing process,
It requires special equipment, requires a lot of labor and energy,
There is a possibility that the production cost will increase. Moreover, such an annealing treatment may cause a residual strain to be partially generated in the obtained molded article, and may decrease the mechanical strength of the molded article.

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 PTFE extruded product molded by this method, the variation in the outer diameter in the axial direction of the molded product was greatly improved as compared with the conventional example, but still about 4% (the maximum outer diameter). (A value obtained by dividing the difference by the outer diameter dimension).

The present inventors have conducted intensive studies on the reason why such a variation still exists, and presumed that the reason is as follows.

In other words, in the process of lowering the temperature of firing, the molded product that had been pressed against the inner wall of the firing tube by thermal expansion was cooled and shrunk,
It is presumed that the baking tube is separated from the inner wall and enters a free baking state from this moment, and that the weight of the molded product acts on the molded product, causing a trumpet-like gradient from the upper end to the lower end of the molded product. Such a gradient tends to occur particularly in a long molded product. For this reason, it is presumed that variations in the outer diameter dimension may occur along the longitudinal direction.

In order to eliminate such inconvenience, the present inventors have
An attempt was made to place the firing tube, in which the molded product was housed, in a horizontal position, and fire the product. However, it has been found that simply laying and sintering eliminates the variation in the outer diameter dimension in the axial direction, but increases the flatness in the cross section of the molded product. The reason is that the molded product is heated in the firing tube, expands thermally, and when pressed against the inner wall of the firing tube, is heated above the melting point of PTFE. It is conceivable that the portion hangs down due to its own weight and circumferential thermal stress. In the extreme case, there is a possibility that not only the flatness of the cross section is increased but also a depression or a crack is generated and the tube is broken. Such a phenomenon tends to occur when the thickness of the molded tube is small or when the outer diameter of the tube is large.

Object of the Invention The present invention provides a PTFE
It is made to solve problems at the same time when manufacturing molded products such as tubes and rods. It is possible to produce extruded products of desired shape and dimensions with little dimensional variation, with high accuracy, easily and at low cost. An object of the present invention is to provide a method for producing a PTFE molded article to be obtained.

SUMMARY OF THE INVENTION In order to achieve such an object, a PTFE according to the present invention is used.
The method for producing a resin molded product includes an extruding step of obtaining an unfired polytetrafluoroethylene resin paste extruded product by using an extruder, and extruding the extruded product with an inner diameter slightly larger than the outer diameter of the molded product. An insertion step of inserting the extruded product into a baking mold having the extruded product in a state where the extruded product is inserted into the baking mold, and the extruded product is polytetrafluoroethylene resin while horizontally arranging and rotating the baking mold. Heating at a temperature equal to or higher than the melting point of, the outer peripheral surface of the molded product is brought into close contact with the inner peripheral surface of the firing mold by thermal expansion, and then cooled by cooling, thereby firing the extruded product; Removing the fired polytetrafluoroethylene resin molded product from the firing mold.

In the present invention, after sizing the unfired polytetrafluoroethylene resin paste extruded product immediately after being extruded from the extruder by a sizing die, the sized product is inserted into the firing mold. Is also good. In the present invention, "sizing" means that a molded product having a smaller inner diameter is obtained by passing a molded product extruded from a die of an extruder through a die having a smaller internal diameter.

In the present invention, the inner peripheral surface of the firing mold may have a sliding layer having a small frictional resistance.

According to such a method for manufacturing a PTFE resin molded product according to the present invention, the fired mold is horizontally arranged while the PTFE resin paste extruded product extruded from the extruder is inserted into the fired mold. Since the baking mold is rotated while baking is performed, even if the molded product thermally expands during baking, its radially outward expansion is limited by the inner peripheral surface of the baking mold. Since the shrinkage of the molded article accompanying the cooling of the molded article becomes uniform and the mold is horizontally arranged upon shrinkage, the variation in the outer diameter dimension in the axial direction due to the weight of the molded article is reduced. Further, since the firing mold is rotated, the top of the horizontally arranged molded product does not collapse due to its own weight or the like, and the roundness of the molded product cross section does not decrease. Therefore, it is possible to manufacture a PTFE resin molded product having a desired shape and a desired dimension having a constant outer diameter dimension in the axial direction with high accuracy, easily, and at low cost.

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

1 to 4 are schematic cross-sectional views showing steps of a method for producing a PTFE resin molded product according to one embodiment of the present invention, and FIG. 5 is a method for producing a PTFE resin molded product according to another embodiment of the present invention. 6 to 9 are cross-sectional views of a main part of a firing mold used in another embodiment of the present invention, and FIG. 10 is a main part of a firing mold used in still another embodiment of the present invention. It is a perspective view.

The embodiment of the present invention shown in FIGS.
The case where a PTFE resin molded product is manufactured is shown. Although the present embodiment is an example of so-called upward extrusion in which an unfired PTFE tube is extruded upward, the present invention is not limited to this, and 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 obtain a preform. The particle size of the PTFE resin powder for obtaining the preform is not particularly limited, but is preferably a fine powder having an average primary particle size of 0.05 to 1.0 μ, which formed secondary particles and became aggregated particles. This is a so-called PTFE fine powder.

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, in this embodiment, the extruded tubular unfired PTFE
The hook 3 is locked to the tip of the resin paste extruded product 2 and the hook 3 is pulled up by the wire 4 and the pulley 5, so that the extrusion of the paste extruded product 2 can be smoothly performed.

In the present invention, the green PT extruded from the die 1 of the extruder is used.
The FE resin paste extruded product 2 has an outer diameter of
It is inserted into the firing mold 6 having a slightly larger inside diameter D than d _1. The inner diameter D of the firing mold 6 is set such that when the paste extruded molded article 2 is fired in a subsequent step, the molded article 2 expands radially outward and presses against the inner peripheral surface of the firing mold 6. It is determined. And the inner diameter D of the baking mold 6, and 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 tube constituting the firing mold 6 is preferably a thin metal seamless tube. Seamless tubes are preferred
This is because the workability is improved and the appearance of the fired PTFE resin molded product in the firing mold 6 is not adversely affected. In addition, a stainless steel seamless pipe is preferable also in a metal seamless pipe.

After the PTFE resin paste extruded product 2 is inserted into the firing mold 6 along the longitudinal direction, the paste positioned between the lower end of the firing mold 6 and the die 1 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 moving means 7 moves the firing mold 6 into which the paste extruded product 2 has been inserted into a heating furnace 8 as shown in FIG.
It is closed at 0,12 and mounted horizontally and rotatably on the stands 22,24. Ring portions 18 and 20 that engage with bearings 14 and 16 are formed on the outer periphery of both ends of the firing mold 6.
16 allows the firing mold 6 in the heating furnace 8 to rotate freely in the horizontal direction.

A drive shaft 26 is connected to one of the blind flanges 12 to forcibly rotate the firing mold 6 arranged in this manner, and the drive shaft 26 is rotated by a motor 30 and a reduction gear 28. It is designed to be driven.

In this state, while rotating the firing die 6, the temperature in the heating furnace 8 is increased, and the paste extruded product 2 in the firing die 6 is fired. The extrusion aid contained therein is dried and removed. Such a drying and removing step is performed by heating at a temperature of about 100 to 250 ° C., but may be performed simultaneously with firing or may be performed 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. The firing time is preferably 2 to 6 hours. The rotation speed of the firing die 6 varies depending on the type of the molded product to be fired, but is preferably about 1 to 50 rpm. Further, the rotation speed may be changed depending on the heating temperature.

In such a sintering step, the paste extruded product 2 during sintering expands radially outward and longitudinally, but a sintering die 6 exists radially outward, and the sintering die 6, the expansion of the molded product 2 is restricted, 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 firing mold 6 is made of metal, its thermal expansion can be ignored compared to the thermal expansion of the molded product 2. Therefore, the molded article 2 being fired 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 addition, since the firing mold is disposed horizontally and firing is performed while rotating the firing mold, even if the molded product thermally expands during firing, the radially outward expansion does not occur in the firing mold. The shrinkage of the molded product following cooling of the molded product becomes uniform, and the mold is arranged horizontally during shrinkage, so the outer diameter dimension in the axial direction due to the own weight of the molded product Is reduced. Further, since the firing mold is rotated, the top of the horizontally arranged molded product does not collapse due to its own weight or the like, and the roundness of the molded product cross section does not decrease.

Next, in the present invention, the firing mold 6 into which the fired molded article 2 is inserted is cooled while rotating until the molded article 2 recovers its strength. In this case, the molded article 2 may be cooled directly in the electric furnace 8 after firing, but immediately after firing, the electric furnace 8 may be cooled.
Alternatively, the molded product 2 may be taken out together with the firing mold 6 and cooled. Upon cooling to below the melting point of PTFE, sintered moldings starts contraction, is released thermal stress gradually, PTFE resin molded article 2a which eventually has a smaller outside diameter d ₃ than the inner diameter D of the baking molds 6 Is obtained. Outer diameter d ₃ of the PTFE resin molded article 2a as the final product, the outer diameter d ₁ of the PTFE resin paste extrudate 2 unsintered, the ratio D / d ₁ (the clearance between the inner diameter D of the baking molds 6 ), And the firing temperature. Also in the case of such a cooling step, it is preferable that the firing mold 6 is horizontally arranged and rotated.

 Thereafter, the molded product 2a is taken out of the firing mold 6.

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

For example, as shown in FIG. 5, a sizing die 11 is attached to an end of a die 1 of an extruder, and an unsintered 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. By passing through such a sizing die, the reduced-diameter molded product 2 recovers its outer diameter in the firing die immediately after entering the firing die, and the outer peripheral surface of the sized molded product becomes the firing die. Close to the inner peripheral surface. If firing is performed in this state, a tube with higher precision can be obtained. The following steps are the same as in the embodiment shown in FIGS.

In the example shown in FIG. 5, 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.
It is preferably 3. Thus, the die 1 of the extruder
By sizing the unfired PTFE resin paste extruded product 2 immediately after being extruded from the mold, it is possible to insert the molded product 2 into a fired mold having a smaller diameter, and after the insertion, recover the stress of the unfired PTFE molded product 2 As a result, a product having better dimensional accuracy than a molded product that does not sizing can be obtained because it adheres better to the inner surface of the firing tube.

Further, as shown in FIG. 6, the firing mold 6 used in the present invention may be one in which a sliding layer S having low frictional resistance is formed on the inner peripheral surface of the mold body 6a.

The sliding layer S is for reducing the frictional resistance with the molded article 2 and smoothly displacing the molded article 2 when the molded article 2 is fired or cooled. Therefore, this sliding layer S
May be any material having heat resistance at the firing temperature of the polytetrafluoroethylene resin and low sliding resistance. For example, as the sliding layer S, the mold body
A carbon layer 6b adhered to the inner peripheral surface of 6a may be used.

In this embodiment, since the sliding layer S is formed on the inner peripheral surface of the firing mold 6, even if the molded product 2 contracts and displaces during firing, the frictional resistance generated between the two is extremely small. As a result, the molded product 2 is always smoothly displaced along the sliding layer S, and the radially outer peripheral surface of the molded product 2 is not deformed. In particular, in the early stage of cooling of the firing die 6, 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 frictional resistance between them becomes extremely small even by the axial expansion displacement, and the molded article 2
The displacement is always smooth along the sliding layer S, and 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.

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 sintering 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 mold 6 itself may be formed of 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. By doing so, the contact area between the surface of the molded article 2 and the firing mold 6 is reduced, and the frictional resistance is reduced. The diameter of the through-hole is not particularly limited, but 0.1 to 3.0 mm, preferably 0.5
~ 1.5mm is good. In addition, the open area ratio is 20
~ 60% is preferred.

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 shown in FIGS. 7 to 10, the air permeability of the sliding layer S becomes excellent, so that air traps are partially formed between the molded product 2 and the sliding layer S. It is not formed, and the outer peripheral surface is not deformed by this air pocket.

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

According to the method for producing a PTFE resin molded product according to the present invention, the PTFE resin paste extruded product extruded from the extruder is inserted into the calcination mold, and the calcination mold is horizontally moved. The baking mold is rotated and baking is performed, so even if the molded product thermally expands during baking, its radially outward expansion is limited by the inner peripheral surface of the baking mold. The shrinkage of the molded product following the cooling of the molded product becomes uniform, and the mold is arranged horizontally and rotated at the time of shrinkage. Variations in diameter are reduced. Further, since the firing mold is rotated, the top of the horizontally arranged molded product does not collapse due to its own weight or the like, and the roundness of the molded product cross section does not decrease. Therefore, it is possible to manufacture a PTFE resin molded product having a desired shape and a desired dimension having a constant outer diameter dimension in the axial direction with high accuracy, easily, and at low cost.

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

Example 1 About 20% by weight of an extrusion aid (ISOPAR E) was added to PTFE powder (Teflon (registered trademark) 6J) and mixed, and the mixture was mixed at 5 kgf / c.
After preforming with m ² , an unfired PTFE tube having a nominal size of 100 A (outer diameter 114 mm, inner diameter 110 mm) and length 7.5 mm was extruded from an extruder die having a die having an inner diameter of 113 mm.

Next, this tube was inserted into a firing mold having an embossed shape as shown in FIG. 9 and an inner diameter of 117.0 mm and a length of 7.5 mm. The emboss was uniformly formed in an area of about 35% with respect to the entire area of the inner peripheral surface of the firing mold, and the height of the embossed peak was about 0.1 mm. After that, the firing mold
It was mounted horizontally and rotatably in the heating furnace in the state shown in FIG. Next, the motor 30 with a speed reducer provided outside the heating furnace is driven to rotate the drive shaft 26 continuously at about 4 rpm, and the inside of the heating furnace is heated in that state, and the unfired tube is fired at about 370 ° C. did.

Table 1-A and Table 2-A show the variation in the outer diameter and the flatness of the PTFE tube cooled to room temperature after firing.

Example 2 About 20% by weight of an extrusion aid (ISOPAR E) was added to PTFE powder (Teflon (registered trademark) 6J) and mixed, and the mixture was mixed at 5 kgf / c.
After preforming with m ² , an unfired PTFE tube having a nominal size of 200 A (outside diameter 216 mm, inside diameter 209 mm) and length 6.5 m was extruded from an extruder die having a die having an inside diameter of 217 mm.

Next, this tube was inserted into a firing mold having an embossed shape as shown in FIG. 9 and an inner diameter of 221.0 mm and a length of 6.5 mm. The emboss was uniformly formed in an area of about 35% with respect to the entire area of the inner peripheral surface of the firing mold, and the height of the embossed peak was about 0.1 mm. After that, the firing mold
It was mounted horizontally and rotatably in the heating furnace in the state shown in FIG. Next, the motor 30 provided outside the heating furnace was driven to continuously rotate the drive shaft 26 at about 12 rpm. In this state, the inside of the heating furnace was heated, and the unfired tube was fired at about 370 ° C.

Table 1-B and Table 2-B show the variation in the outer diameter and the flatness of the PTFE tube cooled to room temperature after firing.

Comparative Example 1 The unfired PTFE tube extruded in Example 1 was not inserted into the fired tube, but was hung on a hook. After drying the auxiliary, the tube was placed in a heating furnace, and the tube was suspended vertically.
Firing at a temperature of ° C.

Table 1-A and Table 2-A show the variation flatness and the like of the outer diameter of the fired PTFE tube after cooling to room temperature.

Example 3 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 the firing tube of m, the auxiliary agent was dried and the PTFE was fired under the same conditions as in Example 1.

Table 1-A and Table 2-A show the variation flatness and the like of the outer diameter of the fired TFPE tube after cooling.

Comparative Example 2 The unfired PTFE tube extruded in Example 2 was not inserted into the fired tube, but was hung on a hook. After drying the auxiliary agent, the tube was placed in a heating furnace, and the tube was suspended vertically.
Firing at a temperature of ° C.

Table 1-B and Table 2-B show the variation flatness and the like of the outer diameter of the fired PTFE tube after cooling to room temperature.

[Brief description of the drawings]

1 to 4 are schematic cross-sectional views showing steps of a method for producing a PTFE resin molded product according to one embodiment of the present invention, and FIG. 5 is a method for producing a PTFE resin molded product according to another embodiment of the present invention. FIGS. 6 to 9 are cross-sectional views of a main part of a firing mold used in another embodiment of the present invention, and FIG. 10 is a main part of a firing mold used in still another embodiment of the present invention. It is a part perspective view. Reference Signs List 1 die 2 paste extruded molded product 2a PTFE resin molded product 6 fired mold 6a fired mold body 6b carbon layer 6c wire mesh 6d uneven surface 6e ... emboss 6f ... through-hole, 8 ... heating furnace 11 ... sizing die, S ... sliding layer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-86427 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B29C 67/00-67/04

Claims (8)

(57) [Claims] 1. An extrusion step of obtaining an unfired polytetrafluoroethylene resin paste extruded product using an extruder, and a step of sintering the extruded product having an inner diameter slightly larger than the outer diameter of the molded product. An insertion step of inserting the extruded product into the mold, and, while the extruded product is inserted in the baking mold, horizontally arranging and rotating the baking mold, the extruded product is at least the melting point of the polytetrafluoroethylene resin. A heating step of heating the extruded product by heating at a temperature of 3 ° C. so that the outer peripheral surface of the molded product is brought into close contact with the inner peripheral surface of the firing mold by thermal expansion, and then cooled by cooling. Removing the polytetrafluoroethylene resin molded product from the firing mold. 2. The relation 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 claim 1, wherein: 3. A method of sizing an unfired polytetrafluoroethylene resin paste extruded product immediately after being extruded from an extruder with a sizing die, and inserting the sized molded product into the firing mold. A method for producing a polytetrafluoroethylene resin molded product according to any one of claims 1 and 2. 4. A molded article of polytetrafluoroethylene resin according to claim 1, wherein said inner peripheral surface of said firing mold has a sliding layer having a small frictional resistance. Manufacturing method. 5. The method according to claim 4, wherein said sliding layer is a carbon layer, a graphite layer or a boron nitride layer. 6. The method for producing a molded article of polytetrafluoroethylene resin according to claim 4, wherein said sliding layer is formed by a wire mesh. 7. The method for producing a polytetrafluoroethylene resin molded product according to claim 4, wherein the sliding layer is formed by making the inner peripheral surface of the mold an uneven surface. 8. The method according to claim 4, wherein said sliding layer is formed by forming a large number of through holes in a peripheral surface of a mold.