US3041665A - Cross-head for inhibitor extrusion - Google Patents

Description

July 3, 1962 Filed May 24, 1960 E. J. BOWERS ETAL CROSS-HEAD FOR INHIBITOR EXTRUSION 2 Sheets-Sheet 1 zalL r- '-I-l ,2; \2m

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r lo cv N o r9, 3, lo LL INVENTORS. EMERY J. BOWERS RODNEY D. SUTHERLAND ATTORNEYS.

July 3, 1962 E. J. BOWERS ETAL 3,041,665

:moss-HEAD FOR INHIBITOR ExTRusIoN Filed May 24, 1960 2 Sheets-Sheet 2l INVENTORS. EMERY J. BOWERS RODNEY D, SUTHERLAND '1% 7 wivgb ATTORNEYS.

States 3,041,665 CROSS-HEAD FOR INHIBITOR EXTRUSION Emery J. Bowers, China Lake, and Rodney D. Sutherland,

Pomona, Calif., assignors to the United States of America as represented by the Secretary of the Navy Filed May Z4, 1960, Ser. No. 31,490 4 Claims. (Cl. 18-13) (Granted under Title v35, U.S. Code (1952), sec. 266) Unite i arent tinuously coating rod-like articles by depositing a coatl ing of material on such articles continuously as the articles are drawn through the cross-head.

Devices of the general nature of the cross-head means which is the subject of the present invention are old and well known in the art. It has 'been common practice to coat a rod -or wire with coating or insulating material by drawing the rod or wire through an enclosing mandrel to a zone `wherein coating material under pressure is forced about the rod or wire as it emergesfrom the mandrel and to lfeed the coated article through a die to thereby control the thickness of the coating as well as the outer Idiameter of the coated article. However, these prior dewces have certain disadvantages which result in imperfect coatings and especially where 4great accuracy is required. One of the primary disadvantages of the prior art devices is that the coating had a pronounced tendency to neck down or taper at the ends of short articles fed through cross-heads of the general type -herein discussed. In addition, prior art `devices have not been adequate for the application of coating material to propellant grains because of the unique problems involved. u

In order to apply viscous thermoplastic coating materials to propellant grains, it is necessary to heat the coating material to a predetermined temperature to assure proper ow and bonding characteristics. Steam passages are provided in the cross-head and die to maintain the coating material at the proper temperature. However, since the propellant grain is slowly transmitted through the cross-head and die, the grain is heated which results in reduced bending strength and increased surface friction coefcient. Since the friction coeiicient of the grain is increased, it is necessary to provide minimum surface` contact between the grain and the mandrel so that minimum trictional drag is obtained. In addition, since the propellant grain is fed to the mandrel by an external roller conveyer system, it is necessary that the mandrel have an external opening which is `greater than the diameter of the grain to compensate for misalignment between the conveyed propellant grain and the opening in the mandrel.

The method by which minimum surface contact is o'btained and the mandrel external opening is of suiiicient size to compenate for misalignment, is by vcontinuously tapering the mandrel passage from a first diameter, which is approximately the same size as the propellant grain, to second diameter, which is somewhat largerthan the iirst diameter. The first `diameter section is adjacent the die, wherein the coating is applied to the' grain, and is the section which supports thevlargest weight of the propellant grain. The second.l diameter section is adjacent the conveyer system and provides suflicient tolerance to compensate for misalignment.

As previously noted, the propellant grain is heated,

which results in reduced bending strength and asa consequence the propellantgrain bends, asa result of its weight-within the mandrel and is in contact with the lower surface of the taper. It is critical that the permitted propellant grain lbend, which is determined lby the length and taper of the mandrel, does not exceed the point where permanent set would be introduced into the grain since the grain, as finally coated, would not be longitudinally aligned.

Therefore, the present invention provides a mandrel that has a tapered opening extending the entire length of the mandrel. The primary function of the taper iS twofold: (l) It provides a minimum diameter and a corresponding minimum surface contact at the end adjacent the die. This permits the propellant grain to be laterally moved within the mandrel with minimum force yapplied thereto. (2). It provides a maximum diameter at the end adjacent the conveyer system. This compensates for misalignment or" the feed of the conveyer system. However, the taper is such that permanent set will not be introduced into the heated propellant grain.

An additional lfunction of the taper is the provision of A an air space between the grain' and mandrel surface. This air space reduces the heat transfer from the mandrel to the grain which is highly desirable since reduced grain temperature results in reduced frictional coefcient and increased bending strength. It should be noted that the size of mandrel openingadjacent the conveyer, may be increased or decreased by changing the length of the mandrel. The maximum length is limited; however, since if too long, the propellant `grain would he within the mandrel for too great a time and would heat to its critical temperature. The minimum mandrel length is determined `'by the size of opening adjacent the conveyer which is necessary to compensate for the particular alignment characteristics of the 'conveyer employed. j

Another unique feature `of the present invention is the provision yof means for cooling the propellant grain should the grain become stationary within the mandrel. If the grain were to become stationary within the mandrel, for any of a plurality of yfailure reasons, the heat of the crosshead, :die and mandrel would be transmitted to the grain which would result in increasing the grain temv perature to the ignition point with corresponding eX- plosion. This is the case even though the heating steam is turned oif as soon as the grain becomes stationary. In order to prevent the explosion of the grain, cooling liquid is transmitted through the steam cavities as soon as the grain becomes stationary. A solenoid devicefis employed which shuts oit the steam and turns on the cooling liquid upon the actuation of a single mechanism.

The present novel extrusion coating cross-headovercomes the prior vart diiiiculty of tapering the coating at the ends `of articles being coated. While this tendency of prior art devices is relatively unimportant in coating long articles since a small tapered end can be cut off without great `relative loss, it becomes important when short individually separated articles arey fed through such cross-heads for coating. The novel crosshead which is the subject matter of this invention includes a land which is attached to the die and serves'to outline the die opening size, the land being of a specially critical' thickness which has beenY found to inhibit the tendency of the coating material -to taper at the ends of the articles being coated.V It has been found ythat if the land thickness is too small, there are resulting pulsations or ripples in the coated surface. lfthe land thicknessl is too great there is a resulting excessive necking downor taper at the end of the individual grain. characteristics are not known; however, it is believed that when `the land thickness is too great, there is vincreased land drag which forces the coating material off the endV of The exact reasons for these' V,accompanying drawingsv wherein:

,3 Y the grain. The critical thickness is that land Vtl-rickness whichV results in a smooth coating surface and minimum Q necking. It has been found, for use with viscous thermoplastic materials, that Va land thickness of ,approximately v Mr inch is critical.

Other features of the present invention include: (l) means for easily and accurately adjusting the relative longitudinal position of the mandrel with relation to kthe die jto closely control the volume flow of the coating material, (2) cti-setting the die with relation to the mandrel 'to provide a coating of uniform thickness and (3) means for accurately and'easily reproducing the position of the .die with relation to the cross-head.

tinuous extrusion type coating means fwhichis fed propellant grains by a conveyer means wherein the mandrel .compensates for conveyermisalignment and preventspermarient set from being induced in the propellant grains.

. l' of the crosshead of FIG. 1 with lthe article to be coated removed, .being taken along the line 3-3 of FIG. l, in the direction of the arrows.

Referring now tothe drawings, wherein like reference numerals have been appended to like parts throughout, attention is i'lrst directed to FIG. 1 for a description of the general nature of `the typeof device with which the present invention is concerned. In devices of this type, articles A, to be coated, Iare fed by means of conveyer B through an encompassing mandrel C to a coating zone D Where the articles emerge from the mandrel to be coated by'rnate'rial fed under pressure through an inlet passage E, the Vcoated `articles then moving through a die F which shapes and controls the size of the coatedarticles.

In Vthe speciiic embodiment of such devices whichV forms the subject matter of the present invention a crosshead casing 2 issupported .(by means not shown) in sealed relationship with inlet passage D serving` as a source of supply of coating Vmaterial'. The casing 2 is open at both ends to provide a longitudinal charnber Y having a cylindrical portion at one end serving to closely Aturther object of this invention is to provide a continuous extrusion type coating means for propellantV grains wherein the propellant grains lare insulated vfrom the mandrel by an air space.V Y Y,

A -further object of this invention is to VVrenderit nec,- essary to provide only aV small forceto move the propellant grain longitudinally through the mandrel.

A further object of this invention is to provide continuous extrusion type coating means wherein the mandrel hasa continuously tapering opening extending the entire length ofwthemandrel.

A"still further object of lthis invention is to provide a continuous extrusion type coating means for propellant grains which prevents overheating ofthe propellant grain whenthe grain becomes stationary within the extrusion coating means. fr y t t A still further object of this invention is to provide a continuous Vextrusion cross-headV type coating. means .wherein coating thickness i-s closely and reproducibly coneasily 4resettable Vmeansfor adjusting Vthe outlet die eccenft'rieallyof the longitudinal axis whereby concentric coatings may be obtained despite pressure differentials v coating zone of ther cross-head. Stillanother object of this invention is Vthe, provision ofextrusion coating cross-head means for precisely forming relatively thin, concentric coatings of burning inhibiting plastic material on the external Vsurfaces ofv solid propellant rocketv grains.

These and` other objectsand many of the attendant advantages of the present invention will become'apparent .Y

as the same becomes betterunderstood from-the follow- ,ing detailed description taken in conjunction with the FIG. 1 is a sectional view through an extrusion coating Y cross-head according -to the present invention, being'taken i FIG. 2 is.V a new taken along manner-eef FIG.` 1, in the .direction of-.the arrows, withY the article to be Vcoated'removedand with Yportionsbroken away to show thevorienytation and operationof 4the Vdie eccentricity indi- 7 eating-devices at ,the `outlet accommodatethe after or entry end of the mandrel C and an enlarged chamber D at the other end to provide a coating Yzone around lthe forward/end of the mandrel, the chamber. or zone C being in communication with inlet passage E.

, The mandrel C is adjustably positioned within the cross-head'casing 2 by means of a mandrel retainer body 4 in the form of an annulus. xedly positioned on the after end of cross-head casing 2, as by screws 6. The retainer'body is cut away intern-ally at its after end to provide an annular ledge 8. The ledge 8 is provided 'with an anti-friction bearing ring10 (of a material such as Teflon) to serve as a support and bearing for the mandrel adjustment'ring 12.V The mandrel adjustment ring 12 is of generally annular form, having a stepped outer diameter having a smaller portion 13 of a diameter to be rotatably and slidably` accommodated Within the retainer body 4 and having a larger diameter 14 adapted to be engaged by the shoulder 8 of the retainer body 4 for rotation on the bearing ring 1,0. The larger diameter '14.has uan lannular notch therein for accommodating the radially inner edge Vof a split annular retainer ring 16. Retainer ring 16, engaging the notch in the mandrel adjustment ring 12,V isV attached by a plurality of cap screws 17, `to the yafter end of the mandrel retainer body` nally screw-threaded as at 1'8 for cooperation with mating screw threads 20 on the after end of the mandrel C 'whereby rotation ofl the ring (with the mandrel xed against rotary motion) is eiiective to adjust Ythe longitudinal position of Vthe mandrel in thecross-head. MandrelV a Vernier rtype reading of mandreladjust'ment.

.As VVbestdepicted in FIG. l, mandrel C has Aa continuously tapered cavity4 27 extending'the entire length thereof. At the vend adjacentdie F, the ldianieter of the Y tapered cavity is approximately the'same as the diameter o f the'V propellant` grain.` At the'other end of the tapered cavity, adjacentconveyor B, the diameter is larger than the'propellant grain. The propellant grain is heated in thefcavity and as a result thefrictional coeiiicientLof-the surface is increased and the bending strength is decreased. Therefore the propellant grain will bend and contact the lower internal surface of the mandrel. The taper is such that the propellant grain will not bend to the extent where permanent set is induced therein. It has been found that a taper of approximately 1/8 inch per foot prevents permanent setA from being induced in propellant grains of the type employed. The diameter of the tapered cavity at the end adjacent conveyor B is determined by the taper length of the mandrel and is selected to compensate for maximum conveyor misalignrnent. it should alone be noted that the greatest weight of propellant grain is located at a relatively small surface area near the end of the mandrel adjacent the die which minimizes the force necessary to propel the grain through the mandrel cavity.

j The die F is positioned on the forward end of the cross-head by means of an adapter plate 26 which is suitably attached to the cross-head casing, as by a plurality of screws .1.8, as shown. Adapter plate 26 is in the form of an annulus having an inwardly directed flange 30 deiining an opening of somewhat greater size than the coated articles to be handled and providing an internal chamber 32 of a depth of longitudinally position the die F between the forward wall of the cross-head casing and said flange 30. The chamber 32 is of inner circumference which is somewhat greater than the outer circumference of the die F whereby a slight degree of transverse motion of the die may be effected, relative to the longitudinal axis of the cross-head. The chamber size and thus the degree of possible lateral motion of the die are further determined by the provision of a ledge 34 formed on the forward edge of the cross-head casing 2, the ledge providing a bearing and support for the ater end of the die and the side wall thereof, aligned with the inner wall of chamber 32 serving to limit lateral movement of the die. I

Means are provided for adjusting the concentricity or eccentricity of the die F relative to the mandrel and cro/sshead casing and for indicating said relative position, the best showing of such means being in FIG. 2 of the drawing. The adapter plate is provided, a four equiangularly disposed points with screw-threaded radial bores in which externally, and internally threaded bushings 36 are inserted, as shown in FIG. 2. Adjustment screws 3S are threaded through bushings 36 and are of a length to contact the periphery of the die F as at 40 -whereby adjustment of the eccentricity of the die F may be effected by the settings of said screws 3S.

Means are also provided for accurately indicating a given position of the die F so ythat it may be reproduced without resorting to trial and enror methods. Such means comprise a pair of displacement indicators i2 held in opposed equally oft-center positions relative to the die by means of a bracket support 44 ysuitably attached to the casing 2 by screw means not shown. The bracket and casing have aligned radial bores in which .bushings 46 are positioned to permit passage of the sensing arms 48 of the indicators 42 through the bushings into contact with the outer periphery of the die F. Thus, as may be readily seen in FIG. 2 of the drawings, the set screws SS'may be adjusted to control the lateral `adjustment of the die F, the readings on the indicators 42 indicating the adjusted position. It is therefore possible to reset the die (after cleaning or change of materials) to a this art, to provide a die with a portion, generally termed a land, to define the peripheral outline of the extruded article and the land may be either an integral part of the die or a detachable element. In the illustrative example, as depicted best in FIG. l, the land 50 is formed as an integral piece of an externallyA screw-threaded 'annulus 52 which is threaded into the forward portion of the die body as shown, such an arrangement allowing interchangeable use of different size lands. It has been found,

for use with viscous thermoplastic materials, that a land thickness of approximately 1A inch is critical and results lin substantially overcoming the tendency for tapering of the coating and provides a smooth uniform coating. It has been found that a land having a thickness greater vthan 1A inch, 1/2 inch for example, results in a taper that is too great and consequently unsatisfactory. Whereas a land having a thickness less than .1A inch, 1/8 inch for example, results in a pulsating or rippling surface which is likewise unsatisfactory.

Passages 56 are provided in the cross-head casing 2, and similar passages 54 are provided in the die -F whereby hot tiuids, steam for example, may be circulated about the areas through which coating uids pass, in order to maintain proper condition of such uids when thermoplastics are used..

In operation, articles to be externally coated arepfed by means yof conveyer B, shown in broken lines for simplicity, into the enlarged aft end of the cross-head through the` large tapered portion of mandrel C, and coating material is fed under pressure through the lateral inlet F to the coating zone D, and maintained at ideal coating temperatures by diuids passing through passages 54 and 5 6. The article is pushed through the mandrel and vcoating zone past the die F, which has an opening larger than the article, as defined by the land Sil andthe result is an extrusion of a coating of material aboutjthe article. The feed characteristics of the coating may be accurately preset by rotation of the adjustment ring 12 which controls the longitudinal position of the mandrel C, to adesired position as indicated by indicia on the ring 12, and on retainer ring 16. Moreover, the arrangement is such that adjustments may be made, 'if desired, while the device is in operation. As mentioned hereinabove, it has been discovered that the concentricity of the coating requires an eccentric positioning of the die F relative to the mandrel and screwsv 3S are adjustable to provide such eocentricity, the displacement indicators 42 yserving to give reproducible evidence of the proper position'of die F for a given set of conditions.

If for some reason the propellant grain should become stationary within the mandrel, a cooling medium, cold water for example, is provided to prevent the heat of the cross-head, die and mandrel from being transmitted to the propellant grain which Would result in increasing the grain temperature to the ignition point with correspondg ing explosion. When the grain becomes stationary, valve actuatmg device 62 is actuated in a direction that causes desired, reproducible position by adjusting the known In order to provide automatic control, propellant grain`r motion responsive device 72 senses whether or not the gram 1s 1n motion. When in motion, the output signal from device 72 is applied to the input of valve actuating device 62 which opens valve 70 andk closes valve 66.

However, when the grains become stationary, the output signal from device 72 causes actuating device 62 to close valve 70 and open valve 66. An override switch 74 is provided to disconnect motion responsive device 72 so that the operation of valve actuating vdevice 62 may be manual.

the articles to be coated will ignite at a predetermined temperaturer and the articles to be coated are Y passed through a mandrel having an opening and into a coating chamber which is connected to a source of coating material and, first means forrheating said coating material and mandrel the improvement comprising second means for'eooling said coating material andmandrel, -means responsive to the motion of said articles for rendering said first means operative -when said articles are moving through said mandrel and rendering said second means operative when said articles are stationary in said mandrel, thereby preventing said articles from reaching said predetermined temperature. Y i

2. vAn extrusion coating' device wherein the articles to be coated will ignite at latpredeterroined temperature comprisingy a cross-head casing having passages therein,

v a mandrel disposed in said cross-head` casing andV having an opening therein'for receiving said articles, a die having passages therein and connected'to said cross-head casing and disposed adjacent one end of said mandrel, Va chamber formed between said die and mandrel, a source of coating material incommunication `with said chamber, a source Y of heatingmedium, a source of, cooling medium, means` responsive to the motion yof said articles for connecting saidheating medium to said passages when said articles are moving through said mandrel -and connecting` said cooling medium to said passages when saidparticles are stationary insaid mandrel thereby preventing Vsaid articles fromreaching said predetermined temperature.u

3, An extrusion coating device wherein propellant grains tobe coated will ignite at`a predetermined temperature comprising a cross-'head casing havingy passages therein, a mandrel disposed in said cross-head casing and having ,an` opening therein for'receiving said'propellant grains,'a die having passagestherein andconnected to .saidfcross-head casing anddisposed adjacent onerend of saidmandrel, a chamber formed between Ysaid die and Y means responsive to the motion of said propellant grains f i 1 perature and the bending strength ofthe grains decrease with increasing temperature comprising a cross-head casing having paSSagestheTein, a mandrel ydisposed in said cross-head casing and having an opening extending the entire length thereof, a die having passages therein andv connected to said cross-head casing and disposed adjacent one'end of said mandrelrforming a chamber between said die and said mandrel, means for feed-ing said propellant grain through the opening at the'other end of said mandrel, a source of viscous thermoplastic material commnnicating with said chamber, said die having a radially mandrel, a source of coating materialin communication Y 'With's'aid chamber, a sourceofhot fluid, a-source, of cold Huid, a first conduit interconnecting said source ofrhot inwardly directed land dening the external outline of the coated articles, said land having -a thickness in the order of '1A inch whereby thecoated surface is smooth and there is minimum necking Idown of the coating at the end of the coated grains, said opening of said mandrel being continuously tapered by increasing diameter from said Vone end -to said other end, said taperbeing approximately 1A; inch pervfoot thereby preventing permanent set from being induced in said propellant grain when it is heated and bent in contact with the lower. surface of said tapered opening, a source of heating medium, a

source of cooling medium, means responsive to the motion Y of said grains for connectingV said heatingmedium to said passages' when said propellant grains are moving through said mandrel andfconnecting said cooling mediumto said passages when said propellant grains are stationary in said mandrel thereby Vpreventing said propellant grains from reaching said predetermined temperature.

Y 1,984,038 shaw et al, Dec. 11, 1934 2,591,508 Brown Apr. 1, 1952 2,653,348 Elgin et al. Sept. 29, 1953 2,732,588 Myers Jan. 3l, 1956 2,760,230 Van Riper Aug. 28, 1956 2,832,879 Van Riper Apr. 29, 1958 i i FOREIGN PATENTS 1 41,2709l kFrance f r Ian. 8,1932

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