Method of manufacturing a boot seal

The present invention resides in a boot seal for use in sealing a joint. The joint comprises first and second relatively movable parts. The boot seal is configured in the form of a sleeve to extend between and around the vehicle relatively movable parts. The sleeve has a laminate wall which comprises at least two layers of thermoplastic elastomeric material which are bonded together. At least one of the layers is a stretch-toughenable polyester thermoplastic elastomer which has been diametrically stretched in an amount effective to increase the toughness of said one layer. The laminate wall has a percent elongation to rupture of at least 100, and a flexural modulus which is less than 100,000 psi. The laminate wall preferably has a puncture resistance of at least 50 Newtons/mm wall thickness.

FIELD OF THE INVENTION

The present invention relates to a vehicle steering or suspension system, and particularly relates to a boot seal for sealing a movable joint in a vehicle steering or suspension system.

BACKGROUND OF THE INVENTION

Boot seals are used to protect the bearings in joints between relatively movable parts of vehicle steering and suspension systems. The boot seals are typically formed of a thermoplastic or thermosetting elastomer. Examples of prior art elastomers are neoprene rubber, a blend of ethylene-propylene rubber and polypropylene marketed by Monsanto Company of St. Louis, Mo. under the trademark SANTOPRENE, and HYTREL polyester marketed by E.I. DuPont de Nemours Co.

The boot seals are typically blow molded into the configuration in which they are to be installed, and are snapped into place in an interference fit which is secured by metal clips.

Boot seal failure can be caused by fatigue, punctures, cuts or tears, and abrasive wear. Boot seal failures are a common cause of joint failure. If a boot seal fails, water and dirt can get into the joint, and/or grease can leak out. It is desirable to increase the resistance of boot seals to failure.

SUMMARY OF THE INVENTION

The present invention resides in a boot seal for use in sealing a joint. The joint comprises first and second relatively movable parts. The boot seal is in the form of a sleeve which extends between and around the vehicle relatively movable parts. The sleeve has a laminate wall which comprises at least two layers of thermoplastic elastomeric material which are bonded together. At least one of the layers is a stretch-toughenable polyester thermoplastic elastomer which has been diametrically stretched in an amount effective to increase the toughness of the one layer. The laminate wall has a percent elongation to rupture of at least 100, and a flexural modulus which is less than 100,000.

Preferably, the laminate wall has a puncture resistance of at least 50 Newtons/mm wall thickness.

Preferably, the stretch toughened one layer is a stretch-toughenable polyester material which is resistant to dimensional change when exposed to heat.

Preferably, the boot seal is a laminate of a first outer layer which is formed of a stretch-toughened polyester material and a second inner layer which is formed of a thermoplastic elastomer, wherein the outer layer polyester material has a generally higher degree of toughness and a greater resistance to hydrocarbon chemicals than the inner layer thermoplastic elastomer, and wherein the inner layer thermoplastic elastomer has a higher degree of flexibility and softness than the outer layer polyester thermoplastic material.

In an embodiment of the present invention, the inner layer thermoplastic elastomer is a polyolefin.

The present invention also resides in a process for making a boot seal. Two thermoplastic elastomeric raw materials are obtained and fed separately into an extruder. The raw materials are separately extruded from the extruder as coaxial tubular molten streams having an outside diameter D1into a mold cavity comprising a corrugated inner wall having an inside diameter D2. The molten streams of thermoplastic elastomeric material are vacuum expanded against the mold cavity inner wall and then cooled to a semi-solid state while vacuum held against the mold cavity inner wall. The amount of expansion (D2/D1) preferably is in the range of about 200% to about 700%. One of the raw materials is a polyester thermoplastic elastomer which is stretch toughenable by said vacuum expansion and which when cooled has a percent elongation to rupture of at least 100 and a flexural modules less than 100,000 psi.

The boot seal preferably has a puncture resistance of at least 50 Newtons/mm thickness of the boot seal.

The thicknesses of the molten streams of thermoplastic elastomeric material are preferably controlled to achieve a boot seal wall thickness in the range of about 0.6 to about 2 mm.

In a preferred embodiment of the present invention, the mold cavity is defined by an endless series of movable molds, each mold having an open clamshell configuration prior to the point of extrusion, and a closed clamshell configuration after extrusion, the extrusion being continuous.

DESCRIPTION OF ONE SPECIFIC PREFERRED EMBODIMENT OF THE INVENTION

Referring toFIG. 1, the boot seal12of the present invention is a sleeve-like member formed of a flexible thermoplastic material. The boot seal12has a small diameter first end14and a larger diameter second end16. The boot seal12between ends14and16has an intermediate portion18which is corrugated. The diameters of the first and second ends and the distances between the peaks and valleys of the corrugations are not critical, and are dictated by the particular dimensions of the application with which the boot seal is used.

FIG. 2shows that the boot seal12has a laminate construction comprising a first ply20, a second ply22, and an adhesive bonding layer24between the first and second plies20and22. The adhesive bonding layer24is optional and dependent upon the compositions of the elastomeric materials used in the first and second plies20and22. Certain plastics when in a molten state can bond together without the need of an intermediate adhesive bonding layer. This laminate construction extends the full length of the boot seal12.

The boot seal12ofFIG. 1is particularly useful with a ball and joint construction for a vehicle steering or suspension apparatus30, as illustrated in FIG.3. Referring toFIG. 3, the apparatus30comprises a ball stud32and a housing34. The stud32has a ball end36located in a socket38of the housing34. The stud32further has a shank40projecting longitudinally from the ball end36. The shank40is connectable with a movable part of a vehicle steering or suspension apparatus in a known manner. The housing34has a shank42which is connectable with another movable part of the steering or suspension apparatus in a known manner. A bearing44is located in the socket38, and supports the ball end36of the stud32for limited movement relative to the housing34.

The boot seal12in the form of a sleeve around the apparatus30shields the bearing44, the housing34, and the ball end36of the stud32from dirt and other foreign substances. A first clamp50holds the small end14of the boot seal12firmly against the stud shank40. A second clamp52similarly holds the large end16of the boot seal12firmly against a seal ring54which is operatively associated with housing shank42. The flexible intermediate portion18of the boot seal12deflects between the boot seal ends14and16upon movement of the ball stud32relative to the housing34. The relative movement is pivotable but can also be longitudinal to a limited extent depending upon the particular design of the steering or suspension apparatus. A lubricant (not shown) for the bearing44, such as grease or the like, may be contained within the space56enclosed by the intermediate portion18of the boot seal12.

The corrugated boot seal12(FIGS. 1 and 2) is formed of a laminated flexible thermoplastic elastomeric material. Preferably, the boot seal laminate structure, shown inFIG. 2, in one embodiment of the present invention, comprises an inner ply20which may be formed of a selected thermoplastic for flexibility and softness. The outer ply22is formed of a polyester thermoplastic elastomer for toughness and resistance to hydrocarbon chemicals.

A preferred polyester thermoplastic elastomer for the outer ply22is a copolyester resin marketed by Eastman Chemical Products, Inc. under the trademark ECDEL. ECDEL is believed to be a cycloaliphatic thermoplastic copolyester (a copolyester-ether); more specifically, a condensation product of the trans isomer of 1,4-dimethyl-cyclohexanedicarboxylate units, of cyclo-hexanedimethanol units and hydroxy terminated polytetramethylene ether glycol units. It is related to polyethylene terephthalate (PET).

A preferred grade of ECDEL for the present invention is 9967. ECDEL 9967 has a melt temperature of 205° C. to 230° C. (400° F. to 445° F.).

ECDEL 9967 is related to and has many of the same properties as polyethylene terephthalate (PET). Polyethylene terephthalate (PET) is stretch-toughenable which makes it suitable for use in the manufacture of blow molded bottles. Primarily, ECDEL 9967 is also stretch-toughenable. Stretching the plastic, for instance about 200% to about 700%, allows the formation of thinner, more uniform side walls, but in addition causes a molecular orientation in the plastic which dramatically increases the strength and barrier properties of ECDEL 9967.

Unlike polyethylene terephthalate (PET), however, ECDEL was found to have more flexibility. Stretched polyethylene terephthalate (PET) is a very rigid material, as it has to be for use in blow molded bottles. Its flexural modulus (as determined by ASTM method D790) is about 450,000 psi. ECDEL 9967 in contrast has a flexural modulus of about 21,750 psi.

ECDEL 9967, in addition, has other beneficial properties. It has a percent elongation to break of about 400. The percent elongation to break is determined using ASTM method D638. In this test, specimens about 3 mm (⅛ in.) thick are tested using a crosshead speed of 508 mm (20 in.) per min. The percent elongation test is conducted at about 23° C. (73° F.) and 50% RH.

The flexibility and stretchability of ECDEL 9967 make this polyester particularly useful for the manufacture of boot seals.

ECDEL 9967 also has and a high degree of puncture resistance (PR) depending upon the amount stretched.

The resistance to puncture is measured on 50 mm×50 mm (2×2 inch) specimens of boot seal samples using a load cell and a steel rod probe. The probe has a working end which is finished to a radius of 3.28 mm (0.134 inch). The load cell is assembled with an Instron tensile testing machine. A 760 gram ram is allowed to free fall 400 mm to force the test samples to be punctured by the steel rod probe. The maximum tensile force exerted by the probe free fall on the specimens is recorded in Newtons. This force is divided by the wall thickness (the minimum wall thickness if the specimen wall thickness varies), to obtain the puncture resistance.

Extrusion molded samples of ECDEL stretched about 320% were found to have a puncture resistance of about 130 to about 150 Newtons per mm. Even moderate stretching of about 20% was found to provide beneficial stretch properties.

There are a number of thermoplastic polymers that are stretch-toughenable. For instance, polyethylene terephthalate (PET) mentioned above, and also polypropylene, styrene acrylonitrile and polyvinyl chloride (PVC) are stretch-toughenable. However, polypropylene and styrene acrylonitrile, as with polyethylene terephthalate (PET), are very stiff following stretching and have flexural moduli of about 245,000 and 490,000 psi, respectively. Polyvinyl chloride following stretching retains some flexibility, but its percent elongation properties prevent it from being used successfully in boot seal applications.

Based on the above information and other data, it has been determined that the laminated boot seals of the present invention should be made using at least one ply of a stretch-toughenable polyester thermoplastic elastomer in which the boot seal is stretched an amount effective to achieve a puncture resistance of at least 50 Newtons/mm wall thickness, the elastomer at the same time having a flexural modulus which is less than 100,000 psi and a percent elongation to rupture of at least 100.

The inner ply20of the boot seal12preferably is a material which is softer than the outer ply22. A softer inner ply20better accommodates surface micro-roughness and imperfections of the linkage with which the boot seal12is used, thereby improving the sealability of the seals12with the linkage. A preferred inner ply is a polyolefin. Polyolefins also have more flexibility than polyesters. This enhances the flexibility of the boot seal12.

A preferred polyolefin for the inner ply is a polyether resin marketed by Eastman Chemical Products, Inc. under the trademark MXSTEN. This polyolefin is a polyethylene resin used primarily in the packaging field. However, other polyolefins which are extrusion moldable, or any flexible extrudable film forming material which is soft and bondable with a polyester resin such as a polyurethane, can be used as the inner ply20.

An advantage in the use of MXSTEN is that it is stretch toughenable similar to ECDEL.

The inner ply20need not be a polyolefin or similar soft extrudable material, such as a polyurethane. For instance, boot seals in accordance with the present invention can be made wherein both plies20and22are ECDEL polyester. However, this may require linkage surfaces essentially free of surface micro-roughness.

It is also possible to use an inner ply20of ECDEL wherein the ECDEL is blown or sponged. This is accomplished using conventional blowing agents and procedures. By using blown ECDEL in the inner ply20, the inner ply is made softer so that it has softness properties similar to those of a polyolefin. Thus, it is usable with surfaces having micro-roughness and imperfections. At the same time, the inner ply20has strength properties of stretch-toughened ECDEL.

Similarly, the outer ply22need not be 100% polyester thermoplastic elastomer. Boot seals in accordance with the present invention have been successfully made wherein the outer ply22contains a substantial weight percentage MXSTEN. The ECDEL present in the outer ply22even in small amounts, when stretch-toughened, offers superior puncture resistance.

The advantage in incorporating an amount of MXSTEN in the outer ply22as well as in the inner ply20is that it enhances the bonding strength of the outer ply22to the inner ply20.

When the outer ply22is substantially a polyester thermoplastic elastomer and the inner ply20is substantially a polyolefin, it may be desirable to co-extrude with the plies20and22an intermediate adhesive bonding layer, designated layer24in FIG.2. Suitable extrudable thermoplastic adhesives are well known. One is TIE BOND TL-905 marketed by Shell Chemical Company. Another is ADMER QB520A marketed by Mitsui Chemical Company. When both plies20and22contain compatible plastics, for instance substantial amounts of MXSTEN in both plies, or substantial amounts of ECDEL in both plies, then no adhesive may be necessary. The plies may be self-bonding.

The corrugated boot seal30preferably is formed in a continuous extrusion/molding process as disclosed in FIG.4. Both ECDEL and MXSTEN are extrudable plastics. The process ofFIG. 4will be described for the manufacture of a laminate comprising an outer ply of ECDEL and an inner ply of MXSTEN, bonded together by an adhesive bonding layer. Referring toFIG. 4, the ECDEL resin is fed into one hopper110for introduction into the process, and MXSTEN resin is fed into a second hopper112for introduction into the process. The resins flow to a heated extruder114in separate chambers (not shown in FIG.4), and then as separate flows of molten plastic into an extruder die116. The extruder die116comprises concentric separate pathways, to be described, which introduce concentric layers of molten plastic into a corrugator118.

Simultaneous with the above steps, an adhesive is fed into a third hopper120, and from there into the heated extruder114for flow through the extruder die116as a molten stream between the concentric layers of ECDEL and MXSTEN.

The corrugator118is a continuous vacuum corrugator manufactured by Cullom Machine Tool & Die, Inc. of Cleveland, Tenn. The machine is disclosed in U.S. Pat Nos. 4,439,130 and 5,257,924 incorporated by reference herein. Cullom Machine Tool & Die, Inc. is also the owner of U.S. Pat Nos. 4,486,929; 4,718,844; 5,494,430; 5,645,871; 5,059,109; 5,489,201; and 5,531,583; all disclosing subject matter relating to that of the '130 and '924 patents, also incorporated by reference herein. Another patent containing relevant subject matter is U.S. Pat No. 4,319,872 incorporated by reference herein.

The corrugator118comprises a continuous series of mold blocks152which travel in a counterclockwise direction, in the view ofFIG. 4, on an inner track124. The track124has a forward run122which extends from near the extruder116for essentially the full length of the lower area of the corrugator, and a return run126which extends for essentially the full length of the upper area of the corrugator. The corrugator118comprises transition areas130and132between the forward and return runs122and124.

As shown inFIG. 4, molded plastic tubing127exits continuously from the forward run122and is passed to a cutter134which cuts the tubing into boot seals12of desired lengths.

Further details of the corrugator118are shown inFIGS. 5,6and7.

Referring toFIG. 5, which is an enlarged, detailed section view of the corrugator118in the forward run122(FIG.4), the track124of corrugator118(FIG. 5) comprises a pair of internal rails142and144that extend continuously around the inside of the corrugator118. Carriage rollers146and148are received into the rails142and144. The carriage rollers146and148are mounted on the ends of a shaft150which in turn supports mold block152. Multiple mold blocks152are connected together in a continuous series around the corrugator, as shown in FIG.4. The mold blocks152are each comprised of clam-shaped mold halves154and156. InFIG. 5, the mold halves154,156are in a closed position with the halves being brought together by the camming action of guide rollers158and160against cam surfaces162and164.

Referring toFIG. 6, which is an enlarged, detailed section view of the corrugator118in the return run126(FIG.4), the clam-shaped mold halves154and156are pivoted apart, on pivot center158(FIG.6), so that each mold block152is in an open position. InFIG. 6, the mold halves154and156are pivoted into the open position by cam surfaces162and164acting on guide rollers158and160.

Referring back toFIG. 4, the mold halves154and156are in the closed position ofFIG. 5for essentially the full extent of the forward run122, and in the open position ofFIG. 6for essentially the full extent of the return run126. In the transition areas130and132, the mold halves pivot from the closed position ofFIG. 5to the open position ofFIG. 6, and vice versa, respectively.

Details of one embodiment of the extruder die116are shown in FIG.7. The extruder die116in the embodiment ofFIG. 7is adapted for the co-extrusion of two layers, an inner ply20and an outer ply22. Instead of two layers, the extruder die116can be adapted readily for the extrusion of three layers which would include an intermediate adhesive bonding layer24between the inner and outer plies20and22.

Referring toFIG. 7, the extruder die116comprises a die block170. The die block has a first passageway172for the outer ply22, and a second passageway174for the inner ply20. Passageways172and174are coaxial. Molten plastic introduced in ports176feeds the first passageway172and molten plastic introduced into port178feeds the second passageway174.

Referring toFIG. 7A, it can be seen how coaxial molten plastic streams exit from the first and second passageways172and174of the extruder die116.

FIG. 8shows the interaction of the extruder die116with mold blocks152. Portions of three mold blocks152are shown inFIG. 8, from left to right, mold blocks152a,152b, and152c.In the closed position ofFIG. 5, the clam-shaped mold halves154and156are closed to define a mold cavity180. The leftmost mold block152ais cammed to an open position so that the clam-shaped mold halves154and156(invisible inFIG. 8) embrace the extruder116which extends axially into the corrugator forward run122(FIG.4), on axis122a(FIG. 8) of the forward run. The mold block152bis cammed to a partially closed position, and mold block152cto a fully closed position. Molten plastic is introduced into the mold block cavity180when the clam-shaped mold halves154and156are nearly fully closed.

When the mold blocks152are fully closed, a vacuum is drawn in the mold block inner wall182(FIGS. 5 and 8) to expand the extruded plastic diametrically against the inner wall182. The mold block halves154and156have a plurality of slits184(FIG. 6) disposed in the grooves186(FIG. 6) of the corrugated inner walls182thereof. Each of the slits184communicates with one of a plurality of bores188. The bores188extend longitudinally through the mold halves154and156and communicate with a continuous circular vacuum header190(FIG.6). The vacuum header190is, in turn, in communication with a vacuum manifold192(FIG. 5) which is maintained under vacuum. This communication is maintained for the entire lower run of the corrugator along which the mold blocks152are cammed to a closed position. The vacuum transmitted to the slits184of the mold halves154and156expands the extruded tube of plastic outwardly against the mold block inner wall182into the configuration of a continuous corrugated tubular member, as shown in FIG.1.

At the point of extrusion, the thermoplastic as received is at an elevated temperature, dependent upon the plastic used, in order to make the thermoplastic pliable and susceptible to molding. It is desirable to cool the thermoplastic while it is in its expanded state. This is accomplished by means of air plenums194(FIG. 5) which extend along the sides of the corrugator118, for the full length of the forward run122. The air plenums194communicate with a source of pressurized air (not shown). The plenums194lead to a pair of arcuate shields196which embrace the mold blocks152moving in the forward run, in a spaced relationship with the mold blocks152, to define an annular air chamber198. Cooling air is introduced continuously into the annular air chamber198to cool the mold blocks152.

The ECDEL and MXSTEN resins are particularly advantageously used in the vacuum molding process of the apparatus ofFIGS. 4-8, as they are continuously extrudable, are stretch-toughenable in the vacuum expansion process, and form a rigid enough ply, when cooled, to cut.

The following examples illustrate the present invention.

A boot seal12(FIG. 1) was manufactured using the apparatus ofFIGS. 4-8. The boot seal had a laminate construction comprising an outer ply of ECDEL, an inner ply of MXSTEN, and an intermediate adhesive ply marketed by Shell Chemical Company under the tradename TIE BOND TL-905.

In the manufacturing step, the pelletized materials were introduced separately into the extruder114where they were reduced to a molten state. The molten materials were extruded as a 3-ply hollow laminate at a temperature slightly above 225° C. (437° F.). The melting point of ECDEL is 225° C. (437° F.). MXSTEN and the adhesive TIE BOND melt at much lower temperatures.

The hollow laminate following extrusion had an outside diameter of about 0.5 inch (about 12.7 mm). The MXSTEN ply had an outside diameter of about 0.3 inch (about 7.62 mm). The thickness of the adhesive layer was about 0.05 inch (about 1.27 mm), and that of the outer ECDEL layer about 0.15 inch (about 3.81 mm).

The corrugator had a linear speed of about 60′/min, and a forward run of about 4′. Expansion of the molten plastic laminate occurred in about the first few inches of travel; i.e., in about the first second following extrusion, while the plastics were still molten. The molds had an interior configuration identical to the exterior configuration of the boot seals ofFIG. 1

Referring toFIG. 1, the manufactured (expanded) boot seals12had a large end internal diameter of about 2 inches (about 47-50 mm), a small end internal diameter of about 0.66 inch (about 16.9 mm), and a corrugated intermediate section between the large and small ends. The corrugated intermediate section had an outside diameter (peak-to-peak) of about 2.5 inches (about 63.4 mm) for most of its length except where tapered at the end closest to the boot seal small end.

The distance between the peaks and valleys in the corrugated intermediate section, and also in the tapered area, was about 0.5″ (about 13.4 mm). This means that the boot seal laminate was expanded in the corrugator118(FIG. 4) while molten about 400% for most of its length, and to a minimum of about 130% at the boot seal small end.

The following Table gives approximate boot seal wall thicknesses at various points along the length of each seal.

The reduced wall thicknesses of the expanded boot seal result primarily from the diametrical expansion in the corrugator but also from some longitudinal lengthening, particularly near the small end. The reductions in wall thicknesses were greater in the areas of higher expansion.

The molds of the corrugator functioned as a heat sink in the corrugator forward run. The continuous extruded laminate had a temperature of about 180°-200° F. at the time the molds were opened and the extruded laminate was expelled from the corrugator. At this temperature, the laminate was self-supporting, and was cooled in air to about 130° F., at which point the extruded laminate was cut into about 10 inch lengths suitable for use in the apparatus of FIG.2.

The manufactured boot seals have a flexural modulus which is about the same as that of ECDEL, about 21,750 psi, well below the parameter of 100,000 psi, and a percent elongation to break which is about the same as that of ECDEL, about 400, well above the parameter of 100.

These data illustrate the excellent flex and elongation properties of boot seals made according to the present invention.

At the same time, the boot seals have excellent toughness imparted primarily by the expansion of the ECDEL ply, but also by the added boot seal thickness provided by the MXSTEN ply. As indicated above, MXSTEN stretch-toughens as does ECDEL. This improved toughness is illustrated in Examples 2-4.

Corrugated tubular laminates were mold formed. The outer ply of each corrugated laminate contained an amount of ECDEL. The corrugated laminates were laboratory assembled and then laboratory stretched (at ambient temperature) to evaluate the effect of stretch. They were tested for strength using the puncture resistance test.

As shown in the following Table 2, both plies of the laminates contained amounts of MXSTEN. The purpose of this was to observe certain properties unrelated to the scope of the present invention.

The stretch procedure was carried out so that in the valleys of the corrugations, the stretch was about 20%. At the peaks of the corrugations, the stretch was about 320%.

Comparative data were obtained on mold formed samples composed of SANTOPRENE. The comparative samples were not laminates and were not stretched as SANTOPRENE does not stretch-toughen.

The tubular samples of the present invention had the following compositions and ply-dimensions prior to stretching.

The plies in Examples 2, 3 and 4 were bonded together by Mitsui Chemical Company adhesive ADMER QB502A.

The following test data were obtained. The data given in the following Table 3 are average data obtained from six samples in each Example.

TABLE 3Puncture Resistance (PR of MXSTEN/ECDEL Laminates)20% Stretch320% StretchExampleTmmcPR N/mmTmmPR N/mm21.13560.4315031.19520.4812841.14530.47129SANTOPRENE1.639.91.639.9cTmm is the average laminate wall thickness following expansion. In the case of SANTOPRENE, Tmm is the wall thickness of the samples tested.

Even moderate stretching of samples in which a ply contains ECDEL (e.g., 20%) achieves an improvement in puncture resistance (PR) compared to SANTOPRENE. Substantial stretching (e.g., 320%) achieves a dramatic increase in puncture resistance. Example 2 in which the outer ply was 50% ECDEL provided better puncture resistance than Examples 3 and 4 which contained 10% and 30% ECDEL respectively.

An advantage of the present invention is that the boot seal12of the present invention is resistant to dimensional changes induced by temperature. Conventional stretch-strengthenable plastics which have been stretched tend to shrink when exposed to high temperatures. Power steering linkages, and the boots installed to protect the linkages, get hot. The temperature can reach 175° C. The amount of shrink caused by high temperatures can cause the boots made of many plastics to interfere with the ball joints and/or other linkages over which they are installed. Interference between the boot and linkage protected by the boot likely accounts for a percentage of the failure modes observed with prior art boots.

Samples of boots made in accordance with the present invention were exposed to different temperature for different periods of time. The outside diameter of the boots was measured by laser beam. The results are given in the following Table.

From Table 4, it can be seen that the boots of the present invention are very heat-stable and did not to distort from heat-induced shrinkage.

Advantages of the present invention should be apparent. Primarily, the use of a stretch-toughenable polyester thermoplastic elastomer which has a flexural modulus of at least 100,000 psi and a percent elongation to rupture of at least 100 provides a boot seal having greatly improved properties, particularly puncture resistance, compared to SANTOPRENE. Preferably, the resin is stretch-toughened to a puncture resistance of at least 50 Newtons/mm of boot seal wall thickness. By using a stretch-toughenable polyester resin such as ECDEL, the stretch-toughened boot seal additionally is resistant to temperature induced shrinkage and thus failures caused by interference of the boot seal with the linkage being protected. Further, a polyester resin such as ECDEL advantageously can be extruded and vacuum molded in a continuous process such as that described with reference toFIGS. 4-8. This dramatically reduces the cost of manufacture compared to conventional blow molding procedures which are batch procedures.

By using the stretch-toughenable polyester resin as the outer ply in a laminated structure, wherein the inner ply is an extrudable resin which is softer than the polyester resin, a boot seal is obtained which is both strong and readily sealable with the linkage being protected.

An embodiment of the present is illustrated in FIG.7B.FIG. 7Billustrates an extruder head214of an extruder die216positioned in a mold cavity218. The extruder head214is adapted for the co-extrusion of two-layers, an inner ply and outer ply. Instead of two layers, the extruder head214can be readily adapted for the extrusion of three layers which include an intermediate bonding layer between the inner and outer plies.

The extruder head214includes a nozzle220. The nozzle has a cylindrical side wall222that extends from the die block (not shown) of the extruder die216along a central axis224to an open end226. The cylindrical side wall222has a cylindrical inner surface228and a beveled surface230at the open end226of the cylindrical side wall222.

An outer mandrel232is located in substantially coaxial relationship within the cylindrical side wall222and is spaced from the cylindrical side wall222. The outer mandrel232has a tube portion234, a cone portion236, and a flange portion238. The tube portion234extends from the cone portion236of the outer mandrel232to the flange portion238. The tube portion234has an outer cylindrical surface240and an inner cylindrical surface242. The outer cylindrical surface240of the tube portion234and the inner cylindrical surface228of the cylindrical side wall222define a first passageway244through which a first molten plastic stream flows from feed ports (not shown) of the extruder die216. The inner cylindrical surface242of the tube portion234of the outer mandrel232defines a second passageway246through which a second molten plastic stream flows from feed ports (not shown) of the extruder die216.

The cone portion236of the outer mandrel232extends through the open end226of the cylindrical side wall222. The cone portion236of the outer mandrel232has an outer frustoconical surface248and an inner frustoconical surface250. The outer frustoconical surface248of the outer mandrel232and beveled surface230of the cylindrical side wall222define a first annulus252. The first annulus252is in communication with the first passageway244so that, during extrusion, the first molten plastic stream that is in the first passageway244flows from the first passageway244through the first annulus252.

The flange portion238of the outer mandrel232is connected to a first actuating means260that oscillates the outer mandrel232axially (left and right directions as shown inFIG. 7B) relative to the cylindrical side wall222. The first actuating means260can be any actuating means known in the art such as a pneumatic pressure cylinder mechanism.

Oscillation of the outer mandrel232causes the gap of the first annulus252to increase or decrease. As the first molten plastic stream passes from the first passageway244through the increasing and decreasing gap of the first annulus252, an outer ply262(FIG. 7C) is formed with a variable annular thickness. The outer ply262has alternating thinner regions262aand thicker regions262b.

The extruder head214further includes an inner conical shaped mandrel266that is located in substantially coaxial relationship within the cone portion236of the outer mandrel232and is spaced from the inner frustoconical surface250of the outer mandrel232. The inner conical shaped mandrel266is attached to rod268that extends in a coaxial relationship through the tube portion234of the outer mandrel232. The inner conical shaped mandrel266has a conical outer surface270. The conical outer surface270of the inner conical shaped mandrel232and the inner frustoconical surface250of the outer mandrel232define a second annulus274. The second annulus274is in communication with the second passageway246so that during extrusion, the second molten plastic stream in the second passageway246flows from the second passageway246through the second annulus274and forms an inner ply276.

The rod268is connected to a second actuating means278that oscillates the inner conical shaped mandrel266. The second actuating means278oscillates the inner conical shaped mandrel266axially (left and right directions as shown inFIG. 7B) relative to the outer mandrel232. The second actuating means278can be any actuating means278known in the art such as a pneumatic pressure cylinder mechanism.

Oscillation of the inner conical shaped mandrel266causes the gap of the second annulus274to increase or decrease. As the second molten plastic stream passes from the second passageway246through the increasing and decreasing gap of the second annulus274, the inner ply276(FIG. 7C) is formed with a variable annular thickness. The inner ply276has alternating thinner regions276aand thicker regions276b.

The outer ply262and the inner ply276are extruded from the first annulus252and second annulus274, respectively, into the mold cavity218. During co-extrusion of the outer ply262and the inner ply276, the oscillations of the outer mandrel232and the oscillations of the inner conical shaped mandrel266are synchronized. By synchronizing the oscillations of outer mandrel232and oscillations of the inner conical shaped mandrel266, the outer ply262and the inner ply276are extruded with the thicker regions262bof the outer ply262radially aligned with the thicker regions276bof the inner ply276. During co-extrusion, the oscillations of the outer mandrel232and the oscillations of the inner conical shaped mandrel266are also synchronized with the linear speed of the mold blocks284that travel along the inner track (not shown) of the corrugator. By synchronizing the oscillations of the outer mandrel232and inner conical shaped mandrel266with the linear speed of the mold blocks284, the thicker regions262band276bof the outer ply262and inner ply276can be radially aligned with the valleys288of the corrugated inner walls290of the mold blocks284.

FIG. 7Cshows the extruded inner ply276and outer ply262in the mold cavity218prior to a vacuum being drawn in the mold cavity218. The thicker regions262bof the outer ply262are in contact with the thicker regions276bof the inner ply276, and the thinner regions262aof the outer ply262are in contact with the thinner regions276aof the inner ply276. The thicker regions276band262bof the inner ply276and the outer ply262are radially aligned with the valleys288of the corrugated inner wall290, and the thinner regions276aand262aof the inner ply276and the outer ply262are radially aligned with the peeks292of the corrugated inner wall290.

After the outer ply262and the inner ply276are extruded into the mold cavity218, a vacuum is drawn in the corrugated inner wall290of mold block284. The extruded outer ply262and inner ply276expand radially against the peaks292and valleys288of the corrugated inner wall290. The wall thicknesses of the outer ply262and inner ply276decrease as the outer ply262and inner ply276expand against the peaks292and valleys288of the corrugated inner wall290. The degree of expansion of the outer ply262and inner ply276is greater along the valleys288of the corrugated inner wall290and less along the peaks292of the corrugated inner wall290. Hence, the wall thicknesses of the outer ply262and inner ply276along the valleys288of the corrugated inner wall290is thinned more than the wall thicknesses of the outer ply262and inner ply276along the peaks292of the corrugated inner wall290.

The wall thicknesses of the expanded outer ply262and inner ply276are shown in FIG.7D.FIG. 7Dshows that outer ply262and the inner ply276have uniform wall thicknesses longitudinally in the peaks292and valleys288along the corrugated inner wall290.

The advantage of this aspect of the method ofFIGS. 7B,7C, and7D is that the method provides a better means of control towards achieving more uniform or desired flexural, elongation, and strength properties longitudinally along the length of the boot seal.

Although the extruder head214in Example 6 is illustrated with two actuating means that oscillate both the outer mandrel232and the inner conical266shaped mandrel, the extruder head214may have only one actuating means that oscillates the outer mandrel or the inner conical shaped mandrel.