FIELD OF THE INVENTION

This invention relates to a bobbin basket for a lock-stitch sewing machine, and more particularly to a bobbin basket that comprises an injection molded, thermoplastic amide-imide resinous polymer, and to the process of making such a bobbin basket.

BACKGROUND OF THE INVENTION This invention supplements the inventions disclosed and claimed in applicant\'s U.S. patent No. 4,858,543 issued August 22, 1989 and in his U.S. patent No. 5,188,046 issued February 23, 1993, both assigned to the assignee of the present invention. The bobbin basket covered by the patent just referred to is adapted for use with a horizontal rotary loop taker (or "hook") , but the invention of patent No. 5,188,046 and the present invention are both useful with bobbin baskets adapted for use with a horizontal hook, a vertical hook or a zigzag hook.

In the past, sewing machine bobbin baskets have been made of metal, typically steel. The inventions covered by patents Nos. 4,858,543 and 5,188,046 mentioned above went a long way toward providing a bobbin basket that avoids the problems long associated with steel bobbin baskets. The present invention goes still farther in increasing the usefulness of a plastic bobbin basket and extending its life.

The many advantages that a plastic bobbin basket has over a conventional all-steel basket are clear. Molding of the basic plastic piece is far more convenient and less expensive than the casting or forging — followed by various machining, polishing and hardening steps — that is involved in the production of an all-steel bobbin basket. Chattering of the bearing rib in its raceway — which produces chipping and burring, with resulting thread breakage — is avoided with a plastic bobbin basket. The

low coefficient of friction between the bearing rib and its raceway produces a longer life for the plastic bobbin basket, and avoids the problems of messy lubrication, galling, bluing of a rotary loop taker, and puckering of the goods being sewn that accompany the use of an all-steel basket.

There are several plastic materials of a high wear and self-lubricating type that may be considered as being more or less suitable for the manufacture of a bobbin basket. Of these, by far the most suitable are the poly(amide-imide) resins, in which the polymer chain comprises amide linkages alternating with imide linkages. Poly(amide-imide) resins have exceptional mechanical strength and dimensional stability, and a low coefficient of thermal expansion. Furthermore, they maintain these properties at very high temperatures.

However, until applicant made his present invention, it was believed by skilled workers in this art that the high melt viscosity of poly(amide-imide) resins combined with the extremely complicated structure of bobbin baskets made it impossible to use these polymers in producing such baskets by injection molding. If this were actually true, it would of course not be feasible to try to manufacture bobbin baskets from this resin, no matter how exceptional its engineering properties are.

There are several hundred rotary loop takers (or "hooks") that are currently being used in industrial lock¬ stitch sewing machines. These fall into four general categories: (1) vertical hooks generally, (2) horizontal hooks, (3) zigzag hooks and (4) vertical cap bobbin hooks. Each of the several hundred specific types of hooks in these four categories is used with a bobbin basket that is specially designed for that type of hook.

An examination of the structure of these bobbin baskets for the hundreds of types of rotary loop takers that are currently in use will demonstrate the extremely

complex arrangement of parts that is characteristic of bobbin baskets. This complexity has led skilled workers, in both the industrial sewing machine art and in the injection molding art, to conclude that the high melt viscosity of poly(amide-imide) resins makes it impossible as a practical matter to use such plastics for injection molding of bobbin baskets. In considering the problems that are presented by the use of poly(amideimide) resins for this purpose, it should be remembered that: 1. All the thin walls and other thin members in the bobbin basket make it difficult for this high viscosity melted resin to flow into the various cavities in the mold that define those members.

2. Every time the high viscosity melted resin must flow around an insert in the mold to produce a hole or aperture in the product that is being injection molded, the melted resin must not only flow around the insert but when it reaches the other side of the insert both branches of the flow must join together in a manner that is possible only if quite high pressure is applied to the melted resin being introduced into the mold. If sufficient pressure is not applied, a faulty junction of the two branches of the high viscosity melted resin flow will result.

3. During the injection molding of a bobbin basket, the mold defining the shape of the various elements of the bobbin basket-is positioned upside down (as shown in certain Figures of the accompanying drawing) so that the force of gravity will assist the downward flow of the melted resin to form the side wall of the bobbin basket after it has flowed outward from the centrally located inlet to form the bottom wall. The sharp turn that the melted resin must follow when flowing downward in this way from the space defining the bottom wall into the space defining the cylindrically shaped side wall makes it difficult for a polymer that has a high melt viscosity to fill the necessary cavities fully and completely.

4. Adequate flow of the high viscosity melted poly(amide-imide) resin is made even more difficult when the cavity to be filled veers out radially to define the bearing rib extending outward from the side wall, and then continues upward to the top of the side wall.

5. Adequate flow of the high viscosity melted poly(amide-imide) resin is made still more difficult when the cavity to be filled takes another sharp turn at the top of the wall as the outwardly extending flange or flanges are formed.

6. In the case of the sharp turn in the flow path of the high viscosity melted poly(amide-imide) resin at the junction of the bottom wall and the side wall, the right angle corner is on the inner side of the turn. The shape of the outer side of the turn is determined by two things. First, the outer side of the turn must be smoothly curved in order to reduce the friction between the outside of the bobbin basket and the needle thread loop which is cast around the bobbin basket by the rotating hook and slides across the outside surface of the bobbin basket during the formation of each lock-stitch. Second, the curve of the surface is selected to be as gradual and as even as possible, in order to keep the friction between the needle thread loop and the bobbin basket, when the loop is being cast around the basket, as low as possible.

As will be seen from the cross-sectional views given in the accompanying drawing, these latter two facts necessarily mean that the walls of the bobbin basket will be thinnest (measured in a plane that contains the longitudinal axis of the bobbin basket) at the exact corner that is being turned, and are thicker on both sides of the sharp corner. Thus, as the high viscosity melted resin approaches the thinnest part of the corner during injection molding of the bobbin basket, the melted resin faces a narrowing of the channel through which it must flow. On the other side of the thinnest point, the channel again

widens. This sequence of a wide channel, a narrow channel and a wide channel makes it very difficult for the high viscosity melted resin to fill the wider downstream channel reliably. 7. The same difficulty in the flow of the high viscosity melted resin is present when the resin must flow around any insert that is included in the mold in order to produce a hole or aperture, whether in the bottom wall or in the side wall. To form any hole or aperture, the flow path must always follow the sequence of a wide channel, a narrow channel and a wide channel, as measured in the plane of the opening. This fact can be readily seen by examining the accompanying drawing and visualizing the shape and dimensions of the flow path of the melted resin, when measured in the plane of the opening that is being formed, as that flow divides into two paths in order to pass around the insert in the mold, to be rejoined on the far side of the insert.

8. A plastic structure in the form of a complete cylinder will of course be stronger than a structure that comprises only a portion (whether large or small) of a complete cylinder, such as the bobbin basket side wall above the bearing rib in all bobbin baskets currently in use. This fact has undoubtedly tended to lead away from the use of plastic of any kind whatsoever for the complete body of the bobbin basket, and has added to the other factors just listed that have specifically discouraged the use of a high melt viscosity plastic, such as poly(amide- imide) resins, for the injection molding of bobbin baskets. A brief description of the structures of bobbin baskets currently in use will show that to a greater or lesser extent the problems just described will always be present when a bobbin basket in any of the four general categories is formed by injection molding of a thermoplastic amide-imide resinous polymer.

First of all, every currently used bobbin basket

has the following features:

The bobbin basket has an upwardly extending side wall of some kind that is substantially perpendicular to the bottom wall, with the inside surfaces of the bottom and side walls forming a substantially square corner and the outside surfaces of the two walls forming a smoothly curved junction. The bottom wall usually has at least one aperture extending through it. The side wall has the shape of at least a portion of a cylinder. — A bearing rib extends outwardly from this cylindrically shaped side wall, around a substantial portion but not all of the circumference of the bobbin basket. The bearing rib has a top wall, an exterior side wall, a bottom wall, a first end wall that forms a needle thread stop and a second end wall that forms a "V ^π -shaped needle thread pick-up notch. The rib is adapted to fit into a raceway located on the inner wall of the rotary loop taker with which it is to be used, to guide the raceway as the hook rotates about the relatively stationary bobbin basket.

The portion of the side wall below the bearing rib bottom wall extends around substantially the entire circumference of the bobbin basket. In contrast to this, the top of that portion of the side wall above the bearing rib extends uninterrupted around the bobbin basket for no more than a portion of the circumference of the bobbin basket.

There are one or more flanges extending outward from the top of the side wall for no more than a portion of the circumference of the bobbin basket, and the outside surface of the side wall and the bottom surface of each flange forms a generally square corner. The flange or flanges perform one or more of the functions of preventing the needle thread from becoming entangled with the bobbin case or pinched between the bobbin basket and the bobbin basket restraining finger, or preventing relative rotation

of the bobbin basket and the bobbin case.

More particularly, in a bobbin basket for use with vertical hooks generally (i.e.. not including vertical cap bobbin hooks) , the outwardly extending flange that is located at the top of the side wall extends around at least about one-half of the circumference of the bobbin basket. In such bobbin baskets, the first seven of the problems listed below are present. However, the eighth problem is not present to any significant extent because the portion of the side wall above the bearing rib extends substantially all the way around the circumference of the bobbin basket except for a relatively narrow slot through the side wall (in the vicinity of the needle thread pick-up and release area) that runs from the top of the wall to the midsection of the wall.

All bobbin baskets for use with horizontal hooks present the first seven problems listed above, and to a lesser extent the eighth problem. In these bobbin baskets, the portion of the side wall located above the bearing rib extends at its top for at least about one-half the circumference of the bobbin basket, and at its base for a greater portion of that circumference. The side wall contains an elongated needle-receiving aperture of substantial size with its longitudinal axis parallel to the bottom wall, and also contains at least one other hole. The outwardly extending flange that is located at the top of the side wall extends around the bobbin basket for at least about one-half of the circumference of the bobbin basket. Bobbin baskets for use with zigzag hooks and vertical cap bobbin baskets present all eight molding problems listed above. The portion of the side wall located above the bearing rib extends for a relatively small portion of the circumference of the bobbin basket at the top of the side wall, and for a larger portion — but not more than about one-half — of the same circumference

at the base of the side wall. A single outwardly extending flange at the top of the side wall extends around the bobbin basket for a relatively small portion of the circumference of the bobbin basket. In some of these bobbin baskets, the outwardly extending flange at the top of the side wall extends downward at its outer end.

All the complexities in the construction of bobbin baskets that are discussed above inevitably led to the conclusion that it would be impossible to use poly(amid in the pr molding,

Applicant has surprisingly discovered that the generally held belief just referred to is altogether erroneous. For it is in fact possible to utilize poly(amide-imide) resins, with their many vastly superior engineering properties, in injection-molded bobbin baskets.

The bobbin basket of this invention is injection molded and is comprised of post-cured, thermoplastic, amide-imide resinous polymers. It thus combines these polymers• many advantages of very high strength and exceptional wear resistance at extremely high temperatures with the only practical method of mass producing a plastic bobbin basket, which is by injection molding.

Satisfactory, improved and preferred chemical structures of the amide-imide resinous polymers are disclosed. Likewise, satisfactory, improved and preferred proportions of amide-imide resinous polymer, with graphite powder and a fluorocarbon polymer as additives, are disclosed.

A process of producing an injection molded bobbin basket from a thermoplastic, amide-imide resinous polymer, which includes a post-curing step, is also disclosed. BRIEF DESCRIPTION OF THE DRAWING

The present invention is described in detail

below with reference to the accompanying drawing, in which:

FIG. 1 is a top plan view of one embodiment of the bobbin basket of this invention adapted for use with a horizontal rotary loop taker in a sewing machine that is equipped with an undertrimmer;

FIG. 2 is a front elevation view of the embodiment of FIG. 1, as seen from the top of the latter Figure;

FIG. 3 is a cross-sectional view of the embodiment of FIG. 1 (after it has been rotated about its vertical diameter in the plane of the paper, to bring the left side in FIG. 1 to the right-hand side in FIG. 3) taken along line 3-3 in FIG. 1;

FIG. 4 is a similar view of the embodiment of FIG. 1 taken along line 4-4 in FIG. 1; and

FIG. 5 is a similar view of the embodiment of

FIG. 1 taken along line 5-5 in FIG. 1.

DETAILED DESCRIPTION OF BEST MODES OF PRODUCT AND PROCESS A detailed description of the best modes of practicing the present product and process inventions will now be provided by reference to the accompanying drawing.

General Configuration Of Bobbin Basket Plastic bobbin basket 20, seen in top plan view in FIG. 1, includes cylindrical side wall 22, crosswise support member 24 extending across the bottom of the bobbin basket, bobbin spool support post 26 extending axially from crosswise support member 24 into the space defined by side wall 22, and flange 28 extending radially outward from the top portion of the side wall. Center post 26 carries notch 27 near its upper end to receive the latch for retaining the bobbin case (not shown) in which the bobbin thread spool is contained. Apertures 25 in the bottom wall of the bobbin basket, which are smaller in area than the corresponding apertures of a conventional steel bobbin

basket, help to define crosswise support member 24.

Portions 28a and 28b of flange 28 form oppositely facing side walls 30a and 30b that are positioned generally radially to form rotation-restraining notch 32 in the top surface of flange 28. First notch side wall 30a forms the downstream side of rotation-restraining notch 32, relative to the direction of rotation 31 (in FIG. 1, counterclockwise) of the rotary loop taker with which bobbin basket 20 is adapted to be used. Second notch side wall 30b forms the upstream side, relative to the direction of hook rotation, of rotation-restraining notch 32. Notch 32 is adapted to receive the stud portion of a conventional stationary positioning finger (not shown) with a secure but sufficiently loose fit to permit limited back-and-forth rotational movement of the bobbin basket.

A second notch 33 is provided at the inside perimeter of flange 28, on the right-hand side of FIG. 1. In use, a projection from the bobbin case that contains the bobbin spool is seated in this notch to align the case properly and to keep the case from rotating on post 26.

Annular bearing rib 34 extends radially outward from cylindrical side wall 22. The rib extends substantially around the perimeter of the bobbin basket except for a portion of the rib that is omitted at 36 to provide two oppositely facing rib end portions 38 and 40 that define a needle thread pick-up and release area.

Rib end portion 38 forms "V"-shaped pick-up notch 42 at the downstream end, relative to the direction of hook rotation 31, of needle thread pick-up and release area 36 (FIGS. 1 and 2) . Rib end portion 40, located adjacent rotation-restraining notch 32, forms a needle thread stop at the upstream end, again relative to the direction of hook rotation, of needle thread pick-up and release area 36 (FIGS. 1 and 2) . Needle thread loop 41 comes into contact with stop 40 as the loop is pulled around the bobbin basket and tightened around the bobbin thread (FIG. 2) .

Needle guard plate 46 (shown in FIG. 2) is of the type disclosed and claimed in applicant\'s U.S. patent No. 4,858,543 referred to above. It is included in the plastic bobbin basket to prevent damage to cylindrical wall 22 if a needle used with the horizontal hook with which this basket is adapted to be employed should be accidentally deflected as it moves toward and into needle-receiving space 48. Plate 46 is omitted from FIG. 1 for clarity.

Damage Resistant Members Damage resistant members such as are disclosed and claimed in applicant\'s copending application Ser. No. 647,343 (now U.S. patent No. 5,188,046) are shown in FIG. 1, but are omitted for clarity from FIG. 2. Wear resistant pin 50 protects rib end portion 38, which forms "V"-shaped pick-up notch 42 at the downstream end of the needle thread pick-up and release area. Pin 54 minimizes wear that tends to occur on bearing rib end portion 40 adjacent the upstream end of the pick-up and release area.

Insert 56 is provided at the upstream side wall 30b of rotation-restraining notch 32 to minimize impact damage to that wall that would otherwise be caused by the stud portion of the stationary positioning finger referred to above. Pin 60, together with pins 50 and 54, helps prevent wear on the body of bearing rib 34 as the raceway of the associated rotary loop taker slides around the rib at the high rotational speeds at which sewing machines, especially industrial sewing machines, are operated.

Complicated Structure Of Bobbin Basket There are hundreds of different types of bobbin baskets employed with industrial sewing machines. Many are used with horizontal hooks, many with vertical hooks and some with zigzag hooks. Each one has a very complicated form that is dictated by the functions that are performed by the bobbin basket and by the basket\'s relationship with the rest of the sewing machine, especially the rotary loop

taker (or "hook") that is used in the machine.

Some bobbin baskets have even more complicated forms than the basket shown in the accompanying drawing, and others have a somewhat less complicated form. As regards molding problems, however, practically all bobbin baskets have very complicated structures. This has been explained above, and is discussed further below.

As mentioned above, FIGS. 3-5 are cross-sectional views of the bobbin basket of FIG. 1 taken along the respective lines indicated in that Figure. Each of these is shown in an inverted position, to illustrate how the inner and outer portions of a mold, together with various inserts and slides, as required, can be used to produce a plastic bobbin basket by the injection molding process if the polymeric material is in fact moldable. These three Figures also illustrate how complicated the structure of the typical bobbin basket of FIG. 1 is.

Flange 28, which extends outwardly from the top of cylindrical wall 22 in the lower right-hand corner of FIG. 1, is seen extending from the side wall in the lower left-hand portion of FIG. 3. Bearing rib 34 is shown extending outward from the side wall. Needle thread pick¬ up and release area 36 is not indicated in FIG. 3, but is located on the far side of cylindrical wall 22. Rotation-restraining notch 32, with its downstream wall 30a undercut at 62, is seen at the bottom of FIG. 3. Needle-receiving space 48 is located a short distance above notch 32 as seen in this inverted Figure. Aperture 70 is located adjacent the needle-receiving space, and apertures 72 are provided at each end of that space, to receive attaching means for securing metal needle guard plate 46 (seen in FIG. 2) to the plastic bobbin basket.

Holes are provided for each of the damage resistant members referred to above. Hole 50a in side wall 22 is adapted to receive wear-resisting insert 50, and hole 54a is adapted to receive wear-resisting insert 54. A hole

is provided in bearing rib 34 and side wall 22 to receive wear resistant member 60 at the location indicated in FIG. 1. (Pin 60 is of course not visible in the cross-sectional views of FIGS. 3-5.) The holes for these three pins just mentioned — 50, 54 and 60 — preferably extend through both bearing rib 34 and side wall 22. They are preferably insert molded, both for accuracy and to reduce the number of separate mechanical steps required. If the holes are drilled after the basic molding is completed, a diamond or carbide drill must be used.

The sectional views seen in FIGS. 3-5 are considerably larger than the actual size of the bobbin basket. In the actual life-size basket, the dimensions are very much smaller than they appear in the drawing. In addition to this, a number of thin walls, small ridges, narrow undercuts, large and small holes and sharp corners are present in the structure of this basket. As will be discussed below, these complexities make for an extremely difficult molding problem at best. Exceptional Mechanical Properties

Of Poly(amide-imide) Resins At High Temperatures

Plastics that may be considered acceptable for the manufacture of a bobbin basket include high-wear, self- lubricating resins such as ZYTEL 101 (nylon resin) and DELRIN (acetal resin), both marketed by E. I. du Pont de Nemours and Company, ULTM 4001 (polyether amide resin) marketed by General Electric Company and TORLON (poly(amide-imide) resin) marketed by Amoco Performance Products, Inc. Of these plastics, the heat-cured poly(amide-imide) resins are vastly better than the other available resins in those engineering properties that are critical for any part that is subjected to the extreme conditions under which bobbin baskets are used. In use, a bobbin basket is subjected (1) to severe harmonic vibrations caused by the needle thread

loops that are pulled forcibly around and along the outer surface of the bobbin basket thousands of times a minute (as a result of the high rotational speed of the associated rotary loop taker) , and (2) to serious impact forces on the upstream wall of the rotation-restraining notch that are created by the stud portion of the stationary positioning finger when the bobbin basket is jerked back and forth by the needle thread loop as tension is applied to that loop in one direction and then in the other. For these reasons, among others, the plastic of which a bobbin basket is formed must have excellent strength and rigidity.

The raceway of the rapidly rotating rotary loop taker rubs against the outwardly extending bearing rib of the bobbin basket and tends to create very substantial frictional heat and resulting wear. As the needle thread loop passes around the bobbin basket and is pulled up tight against it, the pressure of the thread as it repeatedly rubs across and along certain portions of the bobbin basket likewise tends to cause frictional heat and resulting wear. It is believed that the frictional heat produced in these two ways may raise the temperature imposed on certain narrowly defined portions of the bobbin basket to several hundred degrees F. or C. , or even higher. Thus it is a tremendous advantage that poly(amide-imide) resins, when mixed with the proper proportions of graphite powder and a fluorocarbon polymer such as polytetrafluorethylne (sold by du Pont under the trademark TEFLON) , have a very low coefficient of friction with both the steel of the rotary loop taker and the thread of the needle thread loop. The addition of these two additives significantly enhances the inherent lubricity of poly(amide-imide) resins.

To the extent that high temperatures do unavoidably develop, the strength and rigidity of poly(amide-imide) resins at elevated temperatures — as indicated by their high tensile strength, flexural strength, flexural modulus, compressive strength, fatigue

strength and heat deflection temperature — as well as their low coefficient of thermal expansion, produce an exceptionally high level of performance compared to other moldable polymers. Indeed, their exceptional high temperature performance in all these respects places the polyamide-imide polymers in a class by themselves among moldable engineering resins.

Some other engineering resins may perform well at elevated temperatures, but those high temperature plastics have the disadvantage of not being injection moldable. This makes it altogether impossible as a practical matter to produce the intricate and complicated structure of a typical bobbin basket such as illustrated in the accompanying drawing, much less the hundreds of different types of bobbin baskets that are in widespread use in industrial sewing machines, by using those other engineering resins.

Belief That Injection Molding of Poly(amide-imide) Bobbin Basket Not Possible

Over the years since the late 1960\'s, amide-imide polymers have been developed for use in molding and producing various products, such as wire coatings, enamels, films, impregnating materials and cooking utensils. One of the earliest of many patents issued on such products is U.S. patent No. 3,546,152 issued in 1970.

Beginning in the early 1980\'s, various injection molded engine parts were developed from post-cured poly(amide-imide) resins and were used successfully in internal combustion engines despite the conditions of severe frictional wear, stress and high temperatures present in such engines. These parts were developed for use in gasoline and diesel powered automotive engines, truck engines, aircraft engines, marine engines, lawnmower engines, and portable generators. One of the earliest of the patents issued on such products is U.S. patent No.

4,430,970 issued to Holtzberg et al. in 1984. Parts for the space shuttle, the engine of a world-class race car, and many other critical components have been molded from poly(amide-imide) polymers. The component parts of both domestic and industrial sewing machines could be another full line of products comparable in importance to the series of component parts for internal combustion engines. However, the injection molded, post-cured internal combustion engine parts are typically relatively thick-walled pieces having rather simple configurations, and can therefore be injection molded without any great difficulty. Injection molding of bobbin baskets presents an entirely different situation. The complicated structure of a typical bobbin basket such as that shown in the accompanying drawing makes it necessary to include in any mold used for injection molding of such a basket a number of sharp twists and turns and narrow passageways to form numerous thin-walled areas, tiny crevices, small undercuts, small holes, narrow ridges, narrow indentations and sharp junctions between adjoining surfaces. Because of this highly complicated structure, and because of the high viscosity of the poly(amide-imide) resins in liquid form, persons familiar with poly(amide- imide) plastics considered that it was not worthwhile — if not altogether impossible — to attempt to fabricate bobbin baskets formed of such resins by an injection molding process.

This erroneous conclusion was reached despite the fact referred to above that if such an approach did turn out to be successful, it would provide an entry into the sewing industry comparable to the entry into the field of internal combustion engines that had already been achieved. The erroneous conclusion was reinforced by the fact that it was known that an attempt had already been made to mold an analogous part from a poly(amide-imide) resin — an eyelet

for the thread in a weaving machine. In that case, the attempt failed altogether, and wholly unacceptable wear occurred in the eyelet formed of poly(amide-imide) resin when it was installed and used in a weaving machine. Some of the difficulties involved in molding such a complex structure as a bobbin basket for a lock-stitch sewing machine can be seen from the cross-sectional views provided by FIGS. 3-5 of the drawing. Poly(amide-imide) resins have a relatively high melt viscosity. This factor alone makes injection molding very difficult because it limits flow length for a given wall thickness. (It is believed that the high viscosity of the liquid poly(amide- imide) resin may be a by-product of the very same engineering properties of exceptional mechanical strength, stability and resistance to wear at unusually high temperatures that make this resin so useful a material for bobbin baskets.) In addition to this high melt viscosity, many other factors — such as part geometry, flow direction, and severity of flow path changes — make it impossible to characterize with any degree of certainty at all the relationship between flow length and wall thickness for sections less than .050 inch (1.3 mm.) thick, and these factors are all present in bobbin baskets for lock-stitch sewing machines. The impact of this problem will be clear when it is remembered that the cross-sectional drawings in FIGS. 3- 5 are considerably larger than the actual size of the bobbin basket that is illustrated there. The molding parameters for an actual-sized bobbin basket are extremely troublesome. For one thing, a number of the walls are much // less than the minimum .050" &jff 1.3 mm.) thickness referred to above. For example, portion 22a of side wall 22 (shown ^~ in FIG. 3) is approximately .029" (0.74 mm.) in thickness. Wall portion 22b adjacent needle-receiving slot 48 (shown in FIG. 5) is approximately .027" (0.69 mm.) thick. Side portions 24a and 24b of bottom wall 24 (shown in FIGS. 3

and 4) are approximately .027" (0.69 mm.) thick. If because of inadequate flow of the highly viscous liquid resin through the extremely narrow passage- ways that define these and other thin-walled sections there is incomplete filling of the mold at any one of these points, the resulting bobbin basket would not be usable. In addition, several sharp turns do not conform to the design standard specified by the manufacturer of poly(amide-imide) resins that in any injection molding using these polymers the inside radius of curvature for the junction between any adjoining walls should be at least 1/16" (1.6 mm.).

Applicant has made the surprising discovery that it is possible to produce poly(amide-imide) bobbin baskets by injection molding, but attention must still be paid to the precise form of particular bobbin baskets. Thus it is necessary, with TORLON just as with any other hard resin, to take into account the brittle nature of such resins if one attempts to duplicate a steel bobbin basket exactly, shape for shape and dimension for dimension. The plastic basket will sometimes tend to crack or break in or around its thin walled areas, whereas the steel basket would normally hold up. As a result, the configuration of bobbin baskets molded of poly(amide-imide) or any other hard resins may have to be changed slightly in ways that will be clear to those familiar with industrial sewing machines, so that the basket will continue to fit the machine with which it is specially adapted to be used and at the same time will be able to endure the tremendous pressures, vibrations, and needle impact of a sewing operation.

Applicant\'s surprising discovery that — despite the extremely difficult molding parameters involved — poly(amide-imide) bobbin baskets can in fact be injection molded has exposed the error in the assumption on the part of prior workers in this field that bobbin baskets could not be injection molded from resins of this critically

important type. Thus this discovery makes possible a bobbin basket of outstanding strength, stability and structural integrity that can be manufactured at a very small fraction of the cost of producing the bobbin basket by machining.

Injection Molding Of Bobbin Basket

Other than the surprising discovery that, contrary to the belief of skilled workers in this field, a product having such a complex and difficult structure as a bobbin basket for a lock-stitch sewing machine can indeed be injection molded from poly(amide-imide) resins, the method of producing the bobbin basket of this invention is relatively straightforward. The injection mold that is employed includes a die with an upper molding portion and a lower molding portion or core, as well as various inserts and slides that are used to form certain parts of the basket. The cavity should be well vented at the parting line with vents preferably from 0.002 inch (0.05 mm.) to 0.004 inch (0.10 mm.) deep, and should have a draft of at least 0.5° to 1.5° to facilitate removal of the molded bobbin basket. The removable core is inserted into the cavity of the mold, to cooperate with the cavity and with any inserts and slides that are used, to define the generally basket-shaped molding chamber and the specific holes, passageways, indentations and projections that help shape the particular molded product. In order to minimize flow lengths as much as possible in the injection molding of the bobbin basket of this invention, and thereby minimize the number and extent of "knit lines" within the molded product, the inlet should be located in the center of the cavity, opposite the core.

During injection molding, the amide-imide resinous polymer is injected into the molding cavity at injection molding temperatures and pressures to fill the

cavity and molding chamber, so as to form a thermoplastic, poly(amide-imide) resinous bobbin basket. The basket should be allowed to cool below its plastic deformation temperature to solidify its shape and polymeric orientation. The injection molding temperature (polymer melt temperature) of the polymer is preferably from 630° F. (332° C.) to 670° F. (354° C), which is above the plastic deformation temperature of the amide-imide polymer. The total molding and cooling time ranges from 15 to 30 seconds or somewhat more, depending on the grade of the polymeric resin and the thinness of the various parts of the bobbin basket that make up the special configuration of the basket as determined by the particular type of industrial sewing machine in which the bobbin basket is to be used. After the bobbin basket has cooled below its plastic deformation temperature, the mold is opened and the formed bobbin basket is removed from the mold.

Post-Curing of Molded Basket The cooled molded basket is then post-cured by solid state polymerization, through progressively heating the molded basket, below its heat deflection temperature, to enhance the strength of the product.

In the preferred method of post-curing, the molded bobbin basket is heated in the presence of a circulating gas in an oven for a period of time such that a major portion of the volatiles contained in the injection molded basket are vaporized and removed, while the heat deflection temperature of the polymer is simultaneously increased by an amount from about 15° F. (-9.4° C.) to about 35° F. (1.7° C.) without deformation of the bobbin basket. Post-curing can be carried out by heating the molded basket from an initial temperature to a final temperature with either continuous or stepwise increases in temperature over a period of time, or by heating at a single temperature, for a sufficient time to vaporize and remove the volatiles and increase the heat deflection

temperature of the polymer.

Imidization, cross-linking and chain extension take place during heating. The heating just described greatly improves the engineering properties of the molded bobbin basket. In order to achieve the greatest enhancement of the engineering properties of the bobbin basket of this invention, it is preferred to continuously increase the temperature at which the molded basket is heated — from an initial temperature of 300° F. (149° C.) to 330° F. (166° C.) to a final temperature of 490° F. (254° C.) to 510° F. (266° C.) — during a period of about 40 to 60 hours, including a minimum of at least 24 hours at 500° F (260° C.) .

In this post-curing step the molded basket is heated at a temperature that is about 5° F. (-15° C.) to 25° F. (-3.9° C), and preferably about 5° F. (-15° C.) to 15° F. (-9.4° C), below the increased heat deflection temperature of the polymer, for a period of time such that 7~/ substantial imidization, chain extension and crosslinking take place. As a result of such heating, water and gases continue to be generated and removed, and the molecular weight and heat deflection temperature of the polymer are increased.

The circulating gas in the presence of which post-curing is carried out should flow through and around the molded basket so as to remove water and gases from the polymeric resin. The amount of circulation and the circulation flow pattern should be coordinated to maximize removal of water and the gases without causing substantial variations in temperature. While inert gases, such as nitrogen, can be used, it is preferred that the circulating gas be an oxygen-containing gas, preferably air, because oxygen tends to facilitate cross-linking of the polymer molecules. Post-curing is preferably carried out in a circulating air oven, although it can be carried out in any other suitable apparatus as well.

A more detailed explanation of heat treatment by post-curing is described in Chen U.S. Pat. No. 4,167,620, which is hereby incorporated by reference.

The post-cured bobbin basket of this invention is resistant to thermal shock at temperatures of at least 500°

F. (260° C), and exhibits significantly improved flexural and tensile strengths, tensile elongation and heat deflection temperature properties compared with untreated, molded amide-imide resinous products. The thermoplastic, amide-imide resinous polymer contained in this bobbin basket maintains its shape, dimensional stability and structural integrity under even the highest rotational speeds and the most severe conditions (for example, shock and vibration) at which industrial sewing machines of any type are operated.

Composite Resin

The thermoplastic resin in the bobbin basket of this invention comprises about 50% to about 100% by weight amide-imide resinous polymer. Improved results are obtained if the range is about 70% to about 85%. The preferred proportion is about 75%.

The polymer is preferably reinforced with graphite powder. Suitably, the thermoplastic, amide-imide resinous polymer is mixed with from about 5% to about 30% by weight graphite powder. Improved results are obtained if this range is from about 12% to about 25%. The preferred proportion is about 20%.

The polymer\'s effectiveness is also enhanced if it is mixed with from about 1/2% to about 10% by weight powdered or granular polytetrafluoroethylene (PTFE) .

Improved results are obtained if this range is from about

1% to about 8%. The preferred proportion is about 3%.

The polymer\'s molding characteristics and molecular weight can be controlled to facilitate polymerization with an additional monomer, such as trimellitic acid (TMA) , and can be prepared with the

desired flow properties by the methods described in Hanson U.S. Pat. No. 4,136,085, which is hereby incorporated by reference.

The polymer can be blended with graphite powder, PTFE, or titanium dioxide by the method described in Chen U.S. Pat. No. 4,224,214, which is hereby incorporated by reference.

Engineering Properties of Resin The most preferred amide-imide polymer is reinforced with 20% by weight graphite powder and 3% by weight fluorocarbon, and has the following engineering properties:

2h

Thermal Properties

Deflection Temperature ^• F D648 @ 264 psi 536

Coefficient of Linear Thermal Expansion 14xl0 ^"6 in./in./\'F D696

Flammabilitv 94VO Underwriters 94 Laboratories

Limiting Oxygen Index 45 % D2863

Electrical Properties Dielectric Constant D150

Dissipation Factor

Volume Resistivity Surface Resistivity General Properties Density Hardness "Rockwell" E Water Absorption

Preparation of Amide-imide Polymers

The amide-imide polymers are prepared by reacting an aromatic polycarboxylic acid compound (acyl halide carboxylic acid and/or carboxylic acid esters) having at least three carboxylic acid groups such as trimellitic acid (TMA) , 4-trimellitoyl anhydride halide (4-TMAC) , pyromellitic anhydride, pyromellitic acid, 3,4,3\',4\' benzophenone tetracarboxylic acid or an anhydride thereof, or oxybis benzene dicarboxylic acid or an anhydride thereof.

The amide-imide polymers are preferably prepared by reacting an acyl halide derivative of an aromatic tricarboxylic acid anhydride with a mixture of largely- or wholly-aromatic primary diamines. The resulting products are polyamides wherein the linking groups are predominantly amide groups, although some may be imide groups, and wherein the structure contains free carboxylic acid groups which are capable of further reaction. Such polyamides are moderate molecular weight polymeric compounds having in their molecule units of:

and units of:

and optionally, units of:

wherein the free carboxyl groups are ortho to one amide group, Z is an aromatic moiety containing 1 to 4 benzene rings or lower-alkyl-substituted benzene rings, R _{1f j} and R _j are different and are divalent wholly- or largely-aromatic hydrocarbon radicals. These hydrocarbon radicals may be a divalent aromatic hydrocarbon radical of from 6 to about 10 carbon atoms, or two divalent aromatic hydrocarbon radicals each of from 6 to about 10 carbon atoms joined directly or by stable linkages such as -0-, methylene, -CO-, -S0 ₂ -, -S-; for example, -R\'-O-R\'-, -R\'-CH ₂ -R\'-, -R\'-CO-R\'-, -R\'-S0 ₂ -R\'-and -R\'-S-R\'-. The polyamides are capable of substantially complete imidization through heating, as a result of which they form the polyamide-imide structure having to a substantial extent recurring units of:

and units of:

and, optionally, units of:

wherein one carbonyl group is meta to and one carbonyl group is para to each amide group and wherein Z, R,, _j and R- are defined as above. The copolymers of this invention usually have up to about 50 percent imidization prior to heat treatment, typically about 10 to about 40 percent.

The polyamide-imide copolymers are prepared from an anhydride-containing substance and a mixture of wholly- or partially-aromatic primary diamines. Usefully the anhydride-containing substance is an acyl halide derivative of the anhydride of an aromatic tricarboxylic acid which contains 1 to 4 benzene rings or lower-alkyl-substituted benzene rings and wherein two of the carboxyl groups are ortho to one another. More preferably, the anhydride-containing substance is an acyl halide derivative of an acid anhydride having a single benzene or lower-alkyl-substituted benzene ring and, most preferably, the substance is the acyl chloride derivative of trimellitic acid anhydride (4-TMAC) .

Usefully the mixture of diamines contains two or more, preferably two or three, wholly- or largely-aromatic primary diamines. More particularly, they are wholly- or largely-aromatic primary diamines containing from 6 to about 10 carbon atoms or wholly- or largely-aromatic primary diamines composed of two divalent aromatic moieties of from 6 to about 10 carbon atoms, each moiety containing one primary amine group, and the moieties linked directly or through, for example, a bridging -0-, -S-, -S0 ₂ -, -CO-, or methylene group. When three diamines are used they are preferably selected from the class composed of:

said X being an -0-, -CH ₂ -, or -S0 ₂ - group. More preferably, the mixture of aromatic primary diamines is two-component and is composed of metaphenylenediamine (MPDA) and p,p\'-oxybis(aniline) (OBA) , p,p\'-methylenebis (aniline) (MBA) and p,p\'-oxybis(aniline) , p,p\'-sulfonyIbis(aniline) (SOBA) and p,p\'-oxybis(aniline) , p,p\'-sulfonylbis(aniline) and metaphenylenediamine, or p,p\'-sulfonyIbis(aniline) and p,p\'-methylenebis(aniline) . Most preferably, the mixture of primary aromatic diamines contains meta-phenylenediamine and p,p\'-oxybis(aniline) . The aromatic nature of the diamines provides the excellent thermal properties of the copolymers while the primary amine groups permit the desired imide rings and amide linkages to be formed. When two diamines are used to achieve a polymer usefully combining the properties of both diamines, it is usual to stay within the range of about 10 mole % of the first diamine and 90 mole % of the second diamine to about 90 mole % of the first diamine and 10 mole % of the second diamine. Preferably the range is about a 20 to 80 mole ratio to about an 80 to 20 mole ratio. In the preferred embodiment wherein the acyl chloride of trimellitic acid anhydride is copolymerized with a mixture of p,p\'-oxybis(aniline) and meta-phenylenediamine, the preferred range is from about 30 mole % of the former and about 70 mole % of the latter to about 70 mole % of the former and about 30 mole % of the latter.

While this invention has been described in connection with the best mode presently contemplated by the inventor for carrying out his invention, the preferred embodiments described and shown are for purposes of illustration only, and are not to be construed as

constituting any limitation of the invention. Modifications will be obvious to those skilled in the art, and all modifications that do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.