Patent Description:
Fluid sealing systems for plastic, fluid conveying pipes are used in a variety of industries. The pipes used in such systems are typically formed from thermoplastic materials including polyolefins and PVC. In forming a joint between sections of pipe, the spigot or male pipe end is inserted within the female or socket pipe end. An annular, elastomeric ring or gasket is typically seated within a groove formed in the socket end of the thermoplastic pipe. As the spigot is inserted within the socket, the gasket provides the major seal capacity for the joint. Various types of sealing technologies have been employed to assure the sealing integrity of the pipe joint. It is important that the sealing gasket not be dislodged during the joint make up and that the gasket not become twisted or otherwise compromised in field applications.

Earlier gasketed sealing systems are known in which a homogeneous rubber gasket was generally deformable, allowing it to be flexed or bent by hand, accepting inverse curvature, and inserted within a mating internal raceway formed in the female, belled pipe end. The raceway in the female pipe bell end was pre-formed using a collapsible mandrel belling tool at the pipe manufacturing facility. A prior art attempt to insure the integrity of such pipe joints involved the use of a pipe gasket having a first distinct body region formed of an elastically yieldable sealing material, such as rubber, bonded to a second distinct body region formed of a more rigid material, such as a rigid plastic. The intent was that the rigid body region of the gasket would assist in holding the gasket in place within the pipe groove. Other approaches to the problem included the use of a homogeneous rubber ring with a stiffening band which was inserted into a mating groove provided on the internal diameter of the rubber ring.

In the early <NUM>'s, a new technology was developed by Rieber & Son of Bergen, Norway, referred to in the industry as the "Rieber Joint. " The Rieber system employed a combined mould element and sealing ring for sealing a joint between the socket end and spigot end of two cooperating pipes formed from thermoplastic materials. In the Rieber process, an elastomeric gasket was installed within an internal groove in the socket end of the female pipe as the female or belled end was simultaneously being formed. Rather than utilizing a preformed groove, the Rieber process provided a prestressed and anchored elastomeric gasket during the belling operation. Because the gasket was installed simultaneously with the formation of the belled pipe end, a rigid, embedded reinforcing ring could be supplied as a part of the gasket. Because the pipe groove was, in a sense, formed around the gasket with its embedded reinforcing ring, the gasket was securely retained in position and did not tend to twist or flip or otherwise allow impurities to enter the sealing zones of the joint, thus increasing the reliability of the joint and decreasing the risk of leaks or possible failure due to abrasion. The Rieber process is described in the following issued United States patents, among others: <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; and <CIT>.

Despite the advances offered by the Rieber process, the belling operation was somewhat complicated and costly. Also, certain situations exist in which it would be desirable to manually install a gasket in the field or at the manufacturing plant, or to remove one gasket and reinstall another within a preformed raceway in the selected pipe end, rather than utilizing an integrally installed gasket in which the groove in the pipe is formed around the gasket. So, in some instances, it may be desirable to have a gasket which can be installed by hand by simply bending and installing the gasket in the pipe raceway. A further seal is shown in <CIT> and discloses a sealing ring for the spigot and socket connection of two pipes, in particular discharge pipes, having an outwardly radially curved bearing surface, the bearing surface and lip seal being formed from soft flexible rubber or plastics material.

Accordingly, one object of the present invention is to provide an improved gasket which is securely retained within a preformed pipe groove without the necessity of a separate retaining band.

Another object of the invention is to provide such a sealing gasket with attributes which allow it to seal under low pressure or non-pressure conditions without being twisted or extruded, or displaced during field assembly, and yet which can be installed by hand in the bell raceway of a plastic pipe.

Another object of the invention is to provide an improved sealing gasket of the plastic/rubber variety which optimizes the sealing surface contact of the gasket with the pipe bell raceway and with the spigot end of the joining pipe while minimizing on the amount of the rubber material used. For instance in the case of a PP-TPE gasket, the object would be to minimize the amount of the TPE used while compensating by using more PP.

Another object of the invention is to provide such a sealing gasket of the PP-TPE type which is less than <NUM>% TPE by volume.

A method of manufacturing a sealing gasket is defined in independent claim <NUM>.

The sealing gaskets of the invention meet the foregoing objectives of a rubber/plastic gasket design which is especially useful in sealing PVC pipes in low pressure or non-pressure applications, such as sewer lines, where its minimalistic lightweight design can be used advantageously. The preferred gaskets of the invention include a ring shaped, hard plastic band made, for example, of a suitable polyolefin, e.g., polypropylene (PP). The hard plastic band supports two separate rubber or thermoplastic elastomer (i.e., TPE, preferably a TPV) sealing regions, the regions making up an outer ring and an inner lip, A main objective of the design is to reduce to the amount of TPV required and to compensate by using more PP instead. In a particularly preferred form, the sealing gaskets of the invention are comprised of approximately <NUM>% PP and <NUM>% TPV by volume. The unique design, which features an outer sealing ring, an inner sealing lip and a sort of plastic cup-like band also inherently achieves the goal of minimizing the total volume of TPV used.

As will be more fully described, a slender PP body supports both TPE components (ring and lip) providing most of the necessary stiffness to develop adequate contact pressure against the sealing surfaces. The outer ring is selectively sized to provide the proper function as a sealing body. The contact pressure on the outer diameter of the sealing gasket during joint make-up comes from the ring compression (due to interference) and from compression and bending of the PP body. The outer ring and inner lip components effectively absorb all joint dimensional variations. In the case of the inner diameter, the main source of contact pressure comes from the circumferential stress of the stretched lip.

There is no a direct compression line from the outer to the inner sealing surfaces through the soft material as in most seals. In the design of the invention, the internal PP bending stress is the means for transmitting the reaction forces through the seal. The gaskets can be easily installed in a pre-formed raceway of a plastic pipe by bending the gasket by hand. The ease of installation and sealing performance can be adjusted by making slight changes in the PP material properties or body geometry to meet requirements and standards. The V-type seal shape promotes self-energizing behavior when hydrostatic pressure is applied.

A unique molding operation is used to injection mold the sealing gaskets of the invention. The TPV is injected over the PP insert from the seal axis through two gates into the inner lip. The outer TPV ring injection is made through several ribs located on the outer circumferential surface of the seal which are left incorporated into the final product. Once in the final product, these ribs or runners serve as bumpers that help the seal fit laterally into the pipe raceway groove with less material being required.

in a particularly preferred form, a pipe sealing gasket is shown which is designed for receipt within a raceway provided within a female bell socket end of a thermoplastic pipe, the female bell socket end having a given internal diameter which is designed to receive a given outer diameter of a mating male thermoplastic pipe end to form a pipe joint. The gasket is made up of a hard plastic ring-shaped band having an outer circumferential surface and an inner circumferential Surface. The band has two separated elastomer portions, a first of the separated elastomer portions forming an outer ring which circumscribes the outer circumferential surface of the hard plastic band, and a second of the separated elastomer portions forming an inner lip which circumscribes the inner circumferential surface of the hard plastic band. The two separate elastomer portions are connected by a series of spaced ribs which form a continuous body of elastomer connecting the first and second separated elastomer portions at spaced intervals around the band.

The hard plastic ring shaped body portion, together with the supported outer elastomer ring portion and inner elastomer lip portion form a V-shaped profile in cross section, the V-shape itself acting to promote a self-energizing behavior when hydrostatic pressure is applied to the pipe joint. The hard plastic band supports both the elastomer outer ring and the elastomer inner lip, providing adequate stiffness to develop contact pressure between the outer ring and the raceway of the bell socket end of the pipe and between the inner lip and the mating male pipe end as a pipe joint is being made up.

The elastomer outer ring portion of the preferred gasket of the invention has an outer ring surface which is selectively sized to function as a sealing body, whereby contact pressure with the pipe belled end on the ring outer surface comes from ring bending and compression due to interference with the pipe belled end and from bending of the polyolefin body. As explained, the elastomer outer ring and inner lip portions are selectively sized to absorb any dimensional variation in the pipe male and female members. In other words, the outer ring absorbs variations in the raceway ID. The remaining dimensional variations such as spigot OD, joint misalignment and deflection and belled end ID (affecting joint misalignment) are absorbed by the inner lip. In the case of the elastomer inner lip portion, the main source of contact pressure comes from circumferential stress of the lip being stretched by the mating male pipe end as the pipe joint is made up.

The gaskets of the invention have further unique features which separate them from gaskets of the prior art. The fact that there is no direct compression line from the outer elastomer ring portion to the inner elastomer lip portion of the gasket through soft material, as is the case in most sealing gaskets, is one distinct difference in the gasket design of the invention. The gaskets of the invention rather rely upon internal bending stress of the hard plastic band as the means for transmitting reaction forces through the seal to the outer elastomer ring portion and the inner elastomer lip portion.

A manufacturing method is also shown for forming the pipe sealing gaskets having the characteristics previously described. The steps in the method in simplest form include the steps of:.

Additional objects, features and advantages will be apparent in the written description which follows.

The invention described herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting examples which are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processes and manufacturing techniques are omitted so as to not unnecessarily obscure the workings of the invention. The examples used herein are intended merely to facilitate an understanding of ways in which the invention herein may be practiced and to further enable those of skill in the art to practice the invention. Accordingly, the examples should not be construed as limiting the scope of the claimed invention.

Turning now to <FIG>, there is shown a pipe sealing gasket <NUM> which embodies the advantageous features of the invention. The gasket <NUM> is shown installed within a raceway <NUM> provided within the belled end <NUM> of a female pipe section of thermoplastic pipe <NUM>. The female pipe section <NUM> can be formed of any of a variety of commercially available thermoplastic materials, such as the polyolefin family including polyethylene and polypropylene as well as polyvinyl chloride and similar materials, most typically PVC.

Thermoplastic pipes of this general type are used in a variety of industrial settings including water, sewage and chemical industries. The belled end <NUM> of the thermoplastic pipe section has a mouth opening <NUM> which is engageable with a spigot end <NUM> of a mating male pipe section <NUM> to form a pipe joint. The gasket receiving raceway <NUM> has been pre-formed in the pipe mouth opening <NUM> at the pipe manufacturing facility, as by using a collapsible mandrel belling tool. The gasket of the invention is flexible enough to be hand installed in the raceway <NUM>, or by using automated installation equipment.

Certain of the advantages of the gasket design of the invention can perhaps be best understood with reference to the same general style gaskets of the prior art. <FIG> and <FIG> show one such typical design, designated generally as <NUM>. The prior art gasket is shown in profile in <FIG> and in cross-section in <FIG>. The gasket <NUM> can be seen to be an annular, ring-shaped member having a main gasket body <NUM> formed of a flexible elastomeric material, such as a suitable natural or synthetic rubber. The elastomeric material used to form the body <NUM> of the gasket will vary in composition depending upon the end application but may encompass a number of different natural and synthetic rubbers including, for example, styrene butadiene rubber (SBR), ethylene propylene diene rubber (EPDM), acrylonitrile-butadiene rubber (NBR), nitrile rubber, etc..

Turning now to <FIG>, the main gasket body <NUM> includes an outer sealing surface <NUM> which, in this case, is provided with a series of ribs or serrations. The main gasket body also includes a lower, primary sealing surface <NUM>. As will be understood by those skilled in the relevant arts, the primary sealing surface <NUM> is an evenly sloping face of the gasket body which forms a combination lip and compression seal region for the gasket. The lip region is separated from the outer sealing surface <NUM> by a V-shaped recess (shown generally as <NUM> in <FIG>). The V-shaped recess allows the lip region of the gasket body to bend inwardly as the mating male, spigot end of a mating pipe section encounters the primary sealing surface <NUM> of the gasket.

As is further evident from <FIG>, the main gasket body <NUM> of the prior art gasket is reinforced by a hard plastic band <NUM>. The gasket body can thus be thought of as having a rubber element and as having a hard plastic element, the hard plastic element serving as the reinforcing element for the gasket body. Significantly, however, as with typical two-component seals, the inner and outer sealing surfaces <NUM> , <NUM>, from the spigot to the raceway, are part of the same continuous block of rubber or TPV. That is to say the inner sealing surface <NUM> (against the spigot) is connected to the outer sealing surface <NUM> (raceway) by a continuous mass of TPV injected in the same mold cavity, where a PP retaining ring is already placed. Extra TPV is used to fill in the space between the two critical and functional contact surfaces of the sealing lip, and raceway. This also necessarily implies that the PP portion of the gasket has to be fairly wide and completely fill in its portion of the mold cavity (in order that it does not distort), creating a wide bonding surface, and using a large volume of PP.

This leads to various complications or limitations which are inherent in the prior art molding techniques. For example, if the PP band was left sticking up in the typical mold cavity to create two separate areas in the mold for the TPV(thereby minimizing the amount of TPV required), the TPV would fill around it and the PP would be distorted by the high pressure TPV as it flowed into the mold cavity. It would be pushed by the high pressure TPV to one side or the other of cavity, depending on the position of the mold gate. There would be no way to create separate sealing surfaces of TPV on the lip and raceway that are interrupted with lower cost PP.

With reference now to <FIG> and <FIG>, there is shown an improved sealing gasket of the invention, designated generally as <NUM>. The gasket <NUM> is intended for receipt within a raceway provided within a female bell socket end of a thermoplastic pipe, the female bell socket end having a given internal diameter which is designed to receive a given outer diameter of a mating male thermoplastic pipe end to form a pipe joint (see <FIG>). The gasket of the invention can be seen to have a hard plastic ring shaped band <NUM> having an outer circumferential surface <NUM> and an inner circumferential surface <NUM>, and two separated elastomer portions <NUM>, <NUM> (see <FIG>). A first of the separated elastomer portions <NUM> forms an outer ring which circumscribes the outer circumferential surface of the hard plastic band, and a second of the separated elastomer portions <NUM> forming an inner lip which circumscribes the inner circumferential surface of the hard plastic band <NUM>. The two separate elastomer portions <NUM>, <NUM>, are connected by a series of spaced ribs (see, e.g., ribs, <NUM>, <NUM>, <NUM> in <FIG>), which form a continuous body of elastomer connecting the first and second separated elastomer portions at spaced intervals.

It will be appreciated that the hard plastic band <NUM> supports both the elastomer outer ring and the elastomer inner lip (surfaces <NUM>, <NUM>), providing adequate stiffness to develop contact pressure between the outer ring and the raceway (<NUM> in <FIG>) of the bell socket end of the pipe and between the inner lip and the mating male pipe end.

With reference again to <FIG>, it can be seen that the elastomer outer ring portion <NUM> comprises an outer ring surface that is selectively sized to function as a sealing body, whereby contact pressure with the pipe belled end (<NUM> in <FIG>) on the ring outer surface comes from ring bending and compression due to interference with the pipe belled end and from bending of the polyolefin body, and wherein the elastomer inner lip portion <NUM> and outer ring portion <NUM> are sized to absorb any dimensional variation in the pipe male and female members. In the case of the elastomer inner lip portion <NUM>, the main source of contact pressure comes from circumferential stress of the lip being stretched by the mating male pipe end (<NUM> in <FIG>) as the pipe joint is made up.

It will also be appreciated from <FIG> that there is no direct compression line from the outer elastomer ring portion <NUM> to the inner elastomer lip portion <NUM> of the gasket through soft material. Rather the internal bending stress of the hard plastic band <NUM> is the means for transmitting reaction forces through the seal to the outer elastomer ring portion 47and the inner elastomer lip portion <NUM>. This is to be distinguished from the prior art gasket design shown in <FIG> and <FIG> where the rubber areas <NUM>, <NUM> are continuous.

As can be seen in <FIG>, the hard plastic band <NUM>, together with the supported outer elastomer ring portion <NUM> and inner elastomer lip portion <NUM> form a V-shaped profile in cross section, the V-shape itself acting to promote a self-energizing behavior when hydrostatic pressure is applied to the pipe joint. In the particular example of the gasket shown in <FIG>, both the elastomer outer ring portion and the elastomer inner lip portion of the gasket have exposed circumferential sealing surfaces which are provided with a series of circumferential lands and grooves (e.g., lands <NUM> and grooves <NUM>) for engaging the female pipe socket end and the mating male pipe as the pipe joint is made up.

As was previously mentioned, the rubber portions of the gaskets of the invention can be formed of a rubber, such as for example, a thermoplastic elastomer such as a thermoplastic vulcanizate, or a more traditional rubber material such as a styrene butadiene rubber, ethylene propylene diene monomer rubber or nitrile rubber. The durometer of the rubber may vary depending on the end application but will typically be in the range from about <NUM>-<NUM> Shore A hardness, preferably about <NUM>-<NUM> Shore A. The hard plastic band <NUM>, on the other hand, is formed of a synthetic plastic material having a durometer which is greater than the durometer of the rubber portions of the gasket. The synthetic plastic material used for the band <NUM> is preferably a material which shows an appropriate stiffness for the application at hand while allowing flexing during installation.

The preferred material for the rubber portion of the gaskets of the invention are "Thermoplastic Vulcanizates", referred to as TPVs. These materials are part of the thermoplastic elastomer (TPE) family of polymers. However, these materials have the characteristic of being closest in elastomeric properties to EPDM thermoset rubber, combining the characteristics of vulcanized rubber with the processing properties of thermoplastics. TPVs offer a combination of elastomeric properties, such as compression and tension set, coupled with aging performance and chemical resistance. TPV's are typically ready to use in conventional thermoplastic processes such as injection molding and extrusion and does not need to be compounded with different ingredients such as reinforcing fillers (carbon black, mineral fillers), stabilizers, plasticizing oils, and curing systems. Compared to processing rubber, thermoplastic processing of TPV can often deliver shorter cycle times, a higher part output per hour, and the reuse of scrap produced during processing. This can result in part cost reduction, less tooling/machinery, lower scrap costs, and optimization of material logistic costs compared to rubber.

Various hard plastic type materials may be suitable candidates for use as the hard plastic band. These materials include such materials as the polyolefins, such as polypropylene, as well as other materials such as polyvinylchloride and various "engineered plastics. " The preferred material for the instant application is a suitable polypropylene. The preferred sealing gaskets are thus a PP-TPV composite.

One advantage of the gasket design of the invention is that less hard plastic is required, thereby resulting in a cost savings. The gaskets of the invention are comprised of greater than <NUM>% synthetic polyolefin. In a particularly preferred form, the gaskets of the invention are, for example, comprised of approximately <NUM>% poly propylene and <NUM>% thermoplastic elastomer by volume.

The sealing gasket design of the invention, which has been described, is only achieved by certain unique techniques used in the molding operation. As has been briefly discussed, there are various complications or limitations which are inherent in the prior art molding techniques. Attempting to create two separate areas in the mold by leaving the PP band sticking up would leave the plastic band subjected to distortion forces caused by the high pressure TPV as it flowed into the mold cavity. There was no convenient way to create separate sealing surfaces of TPV on the lip and raceway contact areas of the gasket that are interrupted with lower cost PP.

The unique molding technique of the invention uses the previously described ribs or runners (<NUM>, <NUM>, <NUM>, in <FIG>) and, more specifically, their mirror image spaces or cavities in the mold to essentially create two cavities in the same mold on either side of the continuous PP band, and use the mold halves to maintain the PP in position. In this way, the TPV can be injected on both sides of the PP band without causing it to distort from the high pressure TPV, as happens with other designs. When the PP band is placed in the mold, and the two halves of the mold are closed, creating two unfilled areas in the mold. One area is on the inside of the PP band, the lip area, and the other is on the outside of the PP band, the raceway area. TPV is injected into the first open cavity and fills this cavity creating the sealing lip. Simultaneously the TPV pushes the PP band against the outside half of the mold, maintaining it in place. The small rib spaces in the outside half of the mold are not wide enough to allow the PP band to be pushed into and fill them, but are wide enough to allow the TPV to flow around the PP, an up the rib spaces (which are grooves in the outside cavity), to the second unfilled cavity. Here, the PP band is held in place by the inside cavity. As the inside cavity fill with TPV, the outside raceway sealing surface is formed, and the PP band is held against the inner mold cavity, so that is does not distort.

By using these techniques, it is possible to use the continuous PP band or ring and have the TPV (or rubber) be separated and used only in the amount needed for the sealing surfaces, without causing any distortion in the PP band. By producing a gasket having two rubber sealing surfaces and by separating the sealing surfaces in this way, the lip seal portion can be used as a lever to rotate the PP continuous band, and help the sealing surface on the raceway side of the PP band to be forced against the outer pipe's raceway, ensuring improved performance in actual field operations.

The above method of manufacturing the sealing pipe gasket of the invention will now be described primarily with respect to <FIG>. <FIG> shows the lower one half of an injection molding die <NUM> of the type that will be familiar to those skilled in the relevant arts. The mating upper half of the mold <NUM> is raised for ease of illustration. As can be seen in <FIG>, the first mold half <NUM> has a first mold face <NUM> with a circumferential recess <NUM>. As shown in <FIG>, in the first step of the manufacturing process, the hard plastic band <NUM> is placed within the circumferential recess <NUM>.

The second mold half <NUM> has a mold face which is essentially a mirror image of the first mold face. The first and second mold faces are then united and the moldable rubber compound (in this case a TPV) is injected into the circumferential recess. As has been briefly described, the TPV is injected over the PP insert from the seal axis through two gates into the inner lip region. The outer TPE ring injection is made through the mold areas which end up being the series of ribs or runners that go around the front of the seal area and are left incorporated into the final product. Once in the final product, these runners serve as bumpers that help the seal fit laterally into raceway groove of the plastic pipe with less material being required than with the gaskets of the prior art. In the known manner, heat and pressure are applied to the mold to form a ring gasket body. The heat and pressure in the mold cures the rubber sealing areas and adheres them to the hard plastic band portion.

<FIG> are simplified, partly schematic illustrations of the steps in the molding operation showing the flow path of the TPV (rubber) relative to the hard plastic band <NUM>. in the first part of the manufacturing operation shown in <FIG>, the rubber is flowed through the gate <NUM> from the rubber source (not shown) through the cavity between the mold halves to the lip region <NUM> of the hard plastic band (insert).

As shown in <FIG>, the rubber then travels circumferentially in two directions (illustrated by the arrows in <FIG>) around the lip cavity of the mold.

<FIG> then shows the rubber (TPV) passing through the ribs or runner regions of the mold, allowing it to pass to the outer ring cavity (illustrated schematically by the bottom curved arrow in Figure <NUM>).

As again illustrated in somewhat simplified fashion in <FIG>, the rubber continues to travel circumferentially in the inner and outer mold cavities until the melt fronts of the rubber meet.

<FIG> shows the mold halves <NUM>, <NUM> being separated at the end of the molding operation and the completed gasket <NUM> being removed from the mold.

Claim 1:
A pipe sealing gasket (<NUM>) designed for receipt within a raceway (<NUM>) provided within a female bell socket end (<NUM>) of a thermoplastic pipe (<NUM>), the female bell socket end (<NUM>) having a given internal diameter which is designed to receive a given outer diameter of a mating male thermoplastic pipe end (<NUM>) to form a pipe joint, the gasket comprising:
a hard plastic ring shaped band (<NUM>) having an outer circumferential surface (<NUM>) and an inner circumferential surface (<NUM>), and two separated elastomer portions (<NUM>, <NUM>), a first of the separated elastomer portions (<NUM>) forming an outer ring which circumscribes the outer circumferential surface (<NUM>) of the hard plastic band, and a second of the separated elastomer portions forming an inner lip which circumscribes the inner circumferential surface (<NUM>) of the hard plastic band, the two separate elastomer portions (<NUM>, <NUM>) being connected by a series of spaced ribs (<NUM>, <NUM>, <NUM>) which form a continuous body of elastomer connecting the first and second separated elastomer portions (<NUM>, <NUM>)at spaced intervals;
wherein there is no direct compression line from the outer elastomer ring portion (<NUM>) to the inner elastomer lip portion of the gasket through soft material;
and wherein the hard plastic band (<NUM>) supports both the elastomer outer ring (<NUM>) and the elastomer inner lip (<NUM>), providing adequate stiffness to develop contact pressure between the outer ring (<NUM>) and the raceway (<NUM>) of the bell socket end (<NUM>) of the pipe and between the inner lip (<NUM>) and the mating male pipe end (<NUM>).