WO1991004847A1

WO1991004847A1 - Seal joint and process of forming the same

Info

Publication number: WO1991004847A1
Application number: PCT/BR1990/000016
Authority: WO
Inventors: Isidor Huber; Adorno Marcello Cattaneo
Original assignee: Asberit Ltda.
Priority date: 1989-09-26
Filing date: 1990-09-26
Publication date: 1991-04-18

Abstract

A process for producing an expanded PTFE type flanged joint seal comprises the steps of cutting a strip of expanded PTFE to a suitable length, removing material from the end portions of the cut strip so that, on bending the strip to the desired shape of the seal with such end portions overlapped, the overlapped region has substantially the same cross section as the remainder of the strip and then compacting at least one circumferential portion of the strip. The seal so formed has an annular sealing zone having upper and lower sides and inner and outer edges, and also an annular reinforcement zone (5; 18) of a width which extends beyond the said outer edge of the sealing zone (4; 14). Preferably, the reinforcement zone comprises a separate annular base sheet (18) of a card material or the like fixed to the lower side of the expanded PTFE strip and arranged to extend outwardly of the sealing zone (14).

Description

"SEAL JOINT AND PROCESS OF FORMING THE SAME"

The present invention refers to a joint seal to be used in flanged joints and to a process of forming the same. The joint of the present invention is of the type made of expanded polytetrafluorethylene (PTFE).

The use of PTFE based materials in the manufacturing of joint seals is well known in industry and specially in the chemical industry where corrosive products are processed, thus making PTFE a preferred material due to its high resistance to chemical attack.

Traditionally PTFE joints have been formed by machining from compacted bushings, or by stamping from molded PTFE plates. These techniques had the inconvenience that the entire operative surface of the joint was highly compacted, causins the loss of most of the sealing characteristics of the PTFE joints due to the fact that the material did not present flexibility and softness after formation of the joint. Moreover, joints thus formed had a tendency to creeping when submitted to the tightening action of the flanges of the joint to be sealed.

A considerable improvement introduced in the manufacturing of such seal joints was the use of PTFE strips as the starting material for forming the joint. Fxamples of such improvement can be found in US patent 4,187,390 assigned to W. L. Gore & Associates, Inc. and Brazilian patent 004.740/74, also in name of W . L. Gore & Associates, Inc.. Products similar to those of the said patents are produced and commercialized by Gore itself under the trademark "GORE-TEX JOINT SEALANT" and also by other companies such as, for example, the product commercialized by Sumitomo under the trademark "SUPERSEAL" and the product commercialized under the trademark "SEAL-LON 24B" by the Austrian company Teadit.

The joints formed from such PTFE strips have several advantages over those formed by the conventional machining and stamping processes mentioned above, in that they have greater mechanical strength due to the expanded material, when compared to that of cured and pressed materials, greater resistance to creeping when the material is tightened between the flanges of the joint to be sealed and greater pressure resistance, as well as presenting greater flexibility, softness and shapeability, thus ensuring improved sealing efficiency in that the material easily conforms to any irregularities of the flange surfaces.

However, even presenting the above explained advantages, the joints made with expanded PTFE strips do have some disadvantages when compared with those conventional joints made by machining and stamping, since, contrary to the joints made by the latter processes, the joints made from strips have to be formed into the desired shape at the moment of their installation, thus resulting in a risk of misalignment of the seal joint.

Another problem of the joints formed from expanded

PTFE strips resides in the fact that the ends of the strip have to be overlapped, so as to form the joint, resulting in a lump which, when tightened between the flanges of the joint, could cause damage to the flanges, which, in the chemical industry, are often made of glass, ceramics or plastics.

Finally, the processes of forming such prior art joints, when made from strips, are extremely time consuming because they have to be formed "in situ" and all the bolts fixing the flanges have to be removed so that the joint can be manually shaped by bending the PTFE strip and placing it in position, after which the bolts of the flanged joint have to be repositioned and tightened.

The joints formed by machining or stamping, on the other hand, allow all the bolts to be removed with the excep tion of the lowermost two, and the joint to be supported on such two remaining bolts, making it unnecessary to open the entire flanged joint.

Summary of the Invention

It constitutes a first object of the invention to provide a process of forming a joint seal from expanded polytetrafluorethylene (PTFE) strips which does not present the problems of the prior art, as above commented, and also by means of which a joint seal can be be formed which can be installed in a flanged joint without the necessity of removing all the bolts thereof.

This object has been achieved in the present invention by the provision of a process comprising the steps of:

a) cutting the strip to a length which is equal to or slightly greater than the perimeter of the area to be sealed;

b) removing a given amount of material from the ends of the strip, so that, when overlapping those ends, the cross section area in the overlapped portion is substantially identical to the cross section area of the remainder of the strip;

c) overlapping the said end portions of the strip so as to define an overlapped portion having substantially the same length as those end portions of the strip from which the said given amount of material was removed; and

d) compacting the said strip along at least a circumferential portion thereof.

Another object of the invention, as above indicated, is to provide a joint seal having an annular sealing zone of expanded PTFE, which does not present the problems inherent to the joint seals of the prior art, as above explained. This objective has been achieved in the present invention by the provision of a joint seal for flanged joints, comprising an expanded PTFE annular element having an annular sealing zone having upper and lower sides and inner and outer edges, the seal joint being characterized by further comprising an annular reinforcing zone having a width which extends beyond the said outer edge of the sealing zone.

Preferably, the said reinforcing zone comprises an annular base sheet fixed to the lower side of the expanded PTFE element, the base sheet comprising, for example, cardboard or gasket card.

In one of the embodiments of the present invention, the expanded PTFE element has an at least partially compacted outer annular zone whereas the sealing zone itself is not compacted.

Apart fro this, the expanded PTFE element may also have an at least partially compacted inner annular zone.

In another embodiment of the invention, the sealing zone is also partially compacted, that is to say, the joint comprises an expanded and then at least partially compacted annular PTFE element associated with the the reinforcing base sheet. The present invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which

Figure 1 is a perspective view of an expanded PTFE strip from the end portions of which material has been removed to permit an annular element to be formed;

Figure 2 is a perspective view of the strip of Figure 1 after shaping to form the annular element with the end portions overlapped;

Figure 3 is a section taken along the line A-A of Figure 2 after partial compaction to define reinforcement zones, of a joint seal formed in accordance with the present invention;

Figures 4 and 5 are two possible variations of the cross section, after partial compaction, of joints made in accordance with this invention;

Figure 6 is a cross section of another joint seal made in accordance with the invention, in which the reinforcement zone comprises an outer ring of another material;

Figure 7 is an illustrative cross sectional view of a seal making mould during the manufacture of the seal joint shown in Figure 6; and Figure 8 is a cross section of another seal in which the reinforcement zone is formed from a material other than PTFE.

As can be seen from the accompanying drawings, the seal joints are made from a strip of expanded PTFE 1, predetermined quantities of material being removed from the end portions 2 of strip 1, as shown more specifically in Figure 1.

After removal of the material from end portions 2, the strip 1 is bent to the shape illustrated in Figure 2. As can be seen, during bending the end portions 2 of the expanded PTFE sbrip 1 are overlapped by the length from which material was removed so that the cross section of the overlapped region 3 is substantially the same as that of the rest of the strip. This avoids the formation of a lump in the overlapped region 3.

After bending to the shape shown in Figure 2, the strip is partially compacted along a circumferential portion thereof so that the seal assumes the cross sectional shape shown in Figure 3 with a sealing zone 4 in high relief having the same thickness as the original strip 1, and two at least partially compacted reinforcement zones 5 and 5' which provide the seal with rigidity and stability. In this manner, the seal joint enjoys the advantages that were previously exclusive to the conventional stamped and machined seals whereas, at the same time, the sealing zone 4 has the flexibility and softness inherent to the expanded PTFE strip. In other words, the advantages of both techniques are obtained and their respective disadvantages avoided.

Figures 4 and 5 show alternative shapes for the cross sections of seals made in accordance with the present invention, such shapes depending upon the given practical application of the seal and in practice being determined by persons versed in the art when applying the teachings of the invention. In both of the cases illustrated reference number 4 shows a sealing zone (there are two narrower such zones in Figure 4) and numbers 5 and 5' indicate reinforcement zones (in Figure 4, there are two reinforcement zones whereas, in Figure 5, there is only one).

Further in accordance with a preferred embodiment of the invention, an adhesive of any suitable type may be applied to the end portions 2 of strip 1, in the region of overlap 3 shown in Figure 2, with a view to facilitate the forming of the seal although it will be appreciated that this is purely optional and should not be considered as essential for carrying out the invention. Also, instead of using an adhesive, this auxiliary fixing of the end portions 2 may be obtained by hot pressing the said end portions so that they stick to each other.

Referring once more to Figure 1, the removal of material from the end portions 2 is preferably effected by cutting across such portions 2 at an angle of preferably less than 80° with respect to the plane of the seal. It is further preferred to make the cut at an angle of 3° to 70° and even more preferred for the angle to be between 5° and 20° with respect to the plane of the seal.

Finally, with respect to the compacting step, it should be noted that compaction may be effected by any suitable means, pressing in a mould and calendering being preferred.

In the embodiments shown in Figures 3 to 5, the reinforcement zones are formed from the same PTFE of the functional part (sealing zone 4) of the seal. This, however, could be considered to make the final product too expensive, depending on its final application, in view of the relatively elevated cost of the material, especially when bearing in mind the fact that zones 5 and 5' have a merely structural as opposed to a sealing function. Figure 6, however, shows another embodiment of the present invention - which is presently preferred - in which the reinforcement zone consists of a sheet of less expensive material, such as gasket card or even conventional card or cardboard, that has sufficient rigidity for the desired purpose. Thus, Figure 6 shows that the seal has a sealing zone 14 of expanded PTFE with narrow inner and outer flange zones 16 and 17 which are highly compacted and represent portions of the original strip. A separate ring 18 of card material is fixed to the under side of compacted zone 17, substituting zone 5, for example, of Figure 3. In this manner, one obtains a seal having the same (or better) sealing and structural characteristics as that illustrated in Figure 3 but with a considerable economy of PTFE. Figure 7 shows part of a mould used for producing the seal of Figure 6 as well as the seal itself as it is being formed. The mould comprises a base plate 19 formed with an annular cavity 20 and an upper shaping plate 21 having a smooth circular lower surface 22 formed with an annular cavity 23 having a depth substantially equal to the thickness of the expanded PTFE strip being used. Such strip is for all intents and purposes similar to that shown in Figure 2 but narrower. The ring 13 of card material is then placed in cavity 20 followed by the annular strip which is partially overlapped with respect thereto. The seal is then formed by lowering shaping plate 21 so that its surface 22 is forced into cavity 20. As this occurs that part of the strip will form the sealing zone 14 penetrates cavity 23 without undergoing any compaction whereas its edges which correspond to flange zones 16 and 17 in Figure 6 will be highly compacted, the outer edge corresponding to edge 17 being further squeezed and fixed against the ring 18 of card material. The fixing may be aided by the use of a suitable glue.

Figure 8 shows another alternative for the seal using the ring of card material. In this case the whole strip, including the sealing zone 14', is compacted to a lesser degree depending on the desired properties of the seal joint.

It will be understood that the present invention has been described with reference to certain preferred embodiments thereof and that many other minor structural variations may be made within the basic concept of the invention that is defined in the following claims.

Claims

1. Process for forming a flanged joint seal from a strip made from an expanded PTFE based polymer, characterized by comprising the steps of:

a) cutting the strip (1) to a length which is equal to or slightly greater than the perimeter of the area to be sealed;

b) removing a given amount of material from the end portions (2) of the strip, so that, when overlapping said end portions, the cross sectional area of the overlapped portion is substantially identical to the cross section area of the remainder of the strip;

c) overlapping the said end portions (2) of the strip so that said overlapped portion has substantially the same length as said end portions of the strip from which the said given amount of material was removed; and

d) compacting the said strip along at least a circumferential portion thereof.

2. Process according to claim 1, characterized in that said compacting step is effected by calendering.

3. Process according to claim 1, characterized in that said compacting step is carried out under pressure in a Mould.

4. Process according to claim 1, 2 or 3, characterized in that said step of removing material is carried out by cutting across said end portions (2) of said strip (1).

5. Process according to claim 1, 2 or 3, characterized in that said compacting step is carried out along two different zones (5) of said strip (1).

6. Process according to claim 1, 2 or 3, characterized in that said compacting step is carried out along three different zones (5) of said strip (1).

7. Flanged joint seal comprising an expanded PTFE annular element having an annular sealing zone having upper and lower sides and inner and outer edges, characterized by further comprising an annular reinforcement zone (5; 18) having a width which extends beyond the said outer edge of the sealing zone (4; 14).

8. Flanged joint seal according to claim 7, characterized in that said reinforcement zone comprises an annular base sheet (18) fixed to the lower side of the expanded PTFE element and externally of said sealing zone (14).

9. Flanged joint seal according to claim 8, characterized in that said base sheet (18) comprises a card material.

10. Flanged joint seal according to claim 8 or 9, characterized in that said expanded PTFE element has an at least partially compacted outer annular zone (17) terminating externally of said outer edge, said annular base sheet (18) being fixed to the outer annular zone (17) of the element.

11. Flanged joint seal according to claim 10, characterized in that said sealing zone (14) is not compacted.

12. Flanged joint seal according to claim 11, characterized in that said expanded PTFE element has an at least partially compacted inner annular zone (16) terminating internally of said inner edge.

13. Flanged joint seal according to claim 10, characterized in that said sealing zone (14') is also partially compacted.

14. Flanged joint seal according to claim 7, characterized in that said annular reinforcement zone comprises an outer zone (5) of the annular PTFE element, said outer zone (5) being at least partially compacted.

15. Flanged joint seal according to claim 14, characterized by further comprising an inner annular zone (5') of the annular PTFE element, said inner zone being at least partially compacted.