US2172651A

US2172651A - Machine for the continuous extru

Info

Publication number: US2172651A
Authority: US
Publication date: 1939-09-12
Anticipated expiration: 1956-09-12

Description

Sept. 12, 1939. P bu s 2,172,651

MACHINE FOR THE CONTINUOUS EXTRUSION OF METAL Filed May 3, 19-37 4 Sheets-Sheet 1 Fig. 1. 3

p 2, 1939. P. DUNSHEATH 2,172,651

MACHINE FOR THE CONTINUOUS EXTRUSION OF METAL I Filed May 3, 1937 4 Sheets-Sheet 2 Fig. I!

15a 15d [5c 15b 15a 10 l/ 5 I y Sept. 12, 1939. P. DYUNSHEATH 2,172.651

MACHINE FOR THE CONTINUOUS EXTRI ISION OF METAL Filed May 3, 19-37 4 Sheets-Sheet 3 p 2, 1939; I P. DUNSHEATH ,1

MACHINE FOR THE CONTINUOUS EXTRUSION 0F METAL Filed May 3, 19-37 4 Sheets-Sheet 4 1 Fig. '5.

MYM

Patented Sept. 12, 1939 UNITED STATES PATENT OFFICE MACHINE FOR THE CONTINUOUS EXTRU- SION F METAL Application May 3, 1937, Serial No. 140,545 In Great Britain May 11, 1936 30 Claims.

This invention relates to the extrusion of metal by a method employing a rotating screw-threaded impelling device by which the metal is driven forward into an extrusion chamber and from this through an extrusion orifice formed by a die or between an inner and an outer die. The impelling device may comprise two longitudinally extending concentric members of circular crosssection which have on their adjacent faces prom jections which are adapted to engage metal placed in the space between the members. One or both of these sets of projections are screw threads. They work over each other with small clearance. The metal is fed into one end of the. space between the two members in the molten state and is then cooled so that it acquires sumcient rigidity to co-operate with the projections. One or both of the two members is or are driven in continuous rotation. The relative rotary- 20 movement of the two members, combined with continuous feeding of molten metal, causes the solidified metal to be driven forward into and through the extrusion chamber which is located at the delivery end of the two members. An ex- 25 ample of apparatus of this kind is described in the specification of Patent No. 2,051,688.

The nature of the metal which can be extruded in this apparatus is limited to some extent by the form of the apparatus since the metal must 30 have a reasonable degree of plasticity at a temperature at which the working parts have the necessary strength and freedom of movement. The apparatus has hitherto been worked with lead and lead alloys.

It has been found that the process of working as hitherto carried out in such machines frequently results in the segregation of impurities or other constituents of the metal (which may be and generally are only present in very small o quantities) to form layers within the structure of the extrudedbody. Such segregation while apparently not producing deterioration of the mechanical properties of the body is to be avoided if possible.

The causes and procedure of this-segregation are not clearly understood, but the result appears to be favoured, if not, actually caused, by the regularity of the movement of the metal within the machine, which movement is mainly a 50 sliding of the pasty or plastic metal under high pressure over the rigid and symmetrical metal surfaces of the impelling members and the extrusion chamber. The segregated layers generally lie parallel with the surfaces over which 55 the metal has moved or are formed and placed so as to indicate a definite relationship with these surfaces.

It has been recognized that segregation of impurities may occur in metal presses of the ram type and that such segregation and its attendant 5 disadvantages can be avoided or at least minimized by modifying the surfaces of the passages, through which the metal flows from the container to the extrusions die, in such a manner as to produce a disturbance of the regularity of the pathtaken by the metal suflicient to prevent a segregation taking place or (if it has occurred) to break it up before the metal comes near to the extrusion die where the structure of the metal is finally fixed. As indicated in the preceding paragraph, it is believed that segregation in extrusion machines of the rotating screw thread impeller type takes place within the impelling device itself before the metal reaches the extrusion chamber in which the known means for counteracting segregation can be applied. It is the principal object of this invention to provide for such a machine a modified form of screw thread impelling device in which the occurrence of segregation is prevented or at least reduced to a harmless degree, without any reduction in out-put of the impelling device. In our improved form of construction of impelling device, the depths of the grooves of the threads or other channels in one of the two members constituting the device increases progressively as the delivery end of the device is approached. This progressive increase in depth may extend from the rear end to the front end of the member or from some intermediate point. The progressive increase as I the delivery end of the device is approached in the depth of the grooves of the threads or other channels in one of the members constituting the impelling device may advantageously be accompanied by a progressive decrease in the depth of the grooves of the threads or other channels in the other of the two members. The progressive increase in depth of the grooves of the threads or other channels in one of the members causes a folding up of the metal in the channels of that 5 member and a gradual transfer of metal from the channels of the other member to those of the first member and consequently a mixing of the metal passing forward along the channels but does not reduce the output 'of the impelling de- 5 vice. On the contrary, such an increase in depth of the channels in the stationary member has been found to give an unexpected increase in output. Further modifications to the impelling device may be made in order to increase the mixing effect without appreciable reduction in output and these will be described with reference to the accompanying drawings wherein,

Figures 1 and 1 together show a longitudinal section of parts of an extrusion machine embodying the invention,

Figure 1a is a fragmental longitudinal section on an enlarged scale of the front end of the impelling device,

Figure 2 is a cross-section on an enlarged scale, and taken on the line lI-Il in Figure 1 of the outer of the two concentric members constituting the impelling device, V

Figure 3 is a development of the forward end of the inner concentric member,

Figure 4 is an end view on an enlarged scale of the forward end of the inner concentric member,

Figure 5 is a fragmental longitudinal section showing a modified form of front end support for the inner concentric member,

' Figure 6 is a fragmental cross-section on the line VI-V'I in Figure 5, and

Figure 7 is a cross-section on the line VII-VII in Figure 6.

It must be understood that the machine shown is, as -regards the construction and arrangement of its essential parts, such as, for instance, thegeneral form of the impelling device, the number and pitch and shape of the threads or other projections on'the impelling members and the method of drive, merely one example of machine to which the invention has been applied. The way in which it may be applied to other machines of the screw thread impeller type will become clear after reading the description of the way in which the invention has been applied to this machine. It is also to be understood that Figure l, as regards the general construction of the machine, is to some extent diagrammatic. It shows the relative disposition of the important parts of the machine but is not intended to indicate the details of their construction, for the'present invention is concerned only'with the formation of the parts of the machine with which the plastic metal is in contact.

The construction and operation of the machine illustrated will first bedescribed and the various modifications that have been made to the 1m:-

. pelling device will be described subsequently. Re-,

ferring first of all to Figure 1, it will be observed that the rotary part of the screw thread impelling device is constituted by the outer tubular member I which is supported in bearings in the frame 2 and driven by the wheel 3. At the inlet ported at its front end by members I2 extendins between it and the wall of the extrusion chamher 9. At its front end the member Ill carries an inner die or pointf I! which co-operates with an outer die H to form an annular extrusion orifice. The lower parts of the frame and covers and other parts below the outer concentric member have been omitted as their inclusion is unnecessary for an understanding of the present invention. On similar grounds the means for adjusting the outer die, and all heating and cooling devices have been omitted.

In the operation of the machine, molten metal is continuously fed into the chamber I and flows into the space between the two members I and [0 where it is cooled sufliciently to become plastic and to be engaged by the threads l5 and ll, of

the stationary member II and the reaction of these threads assists to force it forward. As a result of these actions, the metal is delivered into the extrusion chamber 9' and forced out between the dies I3 and M. The inner concentric member I0 and the die I! are tubular to permit of the passage of a core, such as an electric cable, when the machine is to be employed for applying a metal sheath to such a body.

The description will now be given of the various modifications that have been made to the impelling device to prevent segregation taking place therein. In accordance with the invention, the depth of the grooves of the threads I6 in the member Ill increases progressively, from a point, indicated by the line A--A, intermediate the length of the threaded portion, as the delivery end is approached. This increase in depth of the grooves in the threads of the member III is accompanied by a progressive decrease in depth of the grooves of the threads in the rotating member I. It will be seen that this arrangement results in a gradual transposition of the plastic metal in an inward direction. This movement superposed on those which normally occur in the screw thread impelling action causes a mixing of the metal. This modification m the depth of the grooves on the channels does not involve an increase in the clearance between the tops of the threads of the two members and does not, therefore, reduce the effectiveness of the impelling device. The progressive increase in depth of the threads of the stationary part of the impelling'devices, although accompanied by a diminution in depth of the threads of the rotating part, may still provide a path for the plastic metal that increases progressively in cross-sectional area as the front end is approached. This increase appears to favor the impelling action.

Instead of, but preferably in addition to, imparting a progressive thread depth variation, continuous in the same sense, to one of the impelling members, the depth of the grooves of the threads l5 or I or other channels in one or both of the members may be made to vary periodically along the length of the member. As shown in Figure 1 the depth of the groove of the thread in each start of the four start thread in the driver member i increases from a minimum to a maximum and then decreases to a minimum again once during each length of lead of the thread. The variation is thus a periodic wave but the w'aves in adjacent threads are not in phase with one another. This periodic variation in depth may be obtained by cuttingeach start of the thread whilst rotating the driver about an axis displacedfromtheaxislloftheboreofthe driver, as shown in Figure 2, the groove lIa being cut whilst the driver is rotated on axis "a, the groove I5b whilst the driver is on axis "b and so on. Alternatively a similar effect may be obtained by rotating the driver about its axis and cutting the grooves to form the thread whilst the tool is reciprocated towards and away from the axis of rotation so as to vary the depth of the groove periodically. This variation is clearly shown in the rear part of the driver member I but is not apparent at the front end because of the gradual diminution in depth previously described. This periodic variation in depth may be applied as shown to all the channels in the driver member or only to certain of the channels. This periodic variation in the depth of the grooves along which the stream of metal passes causes the metal to be forced at the shallow places to move out of the one member into a channel in the other concentric member, so that this additional movement is superposed on those produced by the continuous thread depth variations previously described. Thus a more thorough mixing of the metal results.

In applying a modification of the channels as above described it appears important that the modification should be made in those channels where cooling takes place, since it appears probable that segregation is most likely to occur as the cooling proceeds.

Alternatively or in addition a. mixing of the plastic metal may be effected by providing a local increase in the radial clearance between the projections on the two members. By a local increase in clearance is meant a clearance formed by providing a transverse groove or shallow depression in the wall separating the adjacent channels in one or other of the impelling members, or by equivalent means. ance is not to be confused with a gradual progressive increase in clearance as the front end is approached produced by a gradual dying away of the projections as described in the specification of prior Patent No. Re. 18,710. A progressive increase in clearance of this kind, while it produces a disturbance in the flow of the metal, so seriously curtails the output of the'machine as to be undesirable in present day practice. As shown in the drawings, the walls separating adjacent channels in the inner member. III are mutilated at intervals along the length of the member by cutting grooves I8 therein. The grooves may be either circumferential or longitudinal grooves but the former (shown in Figure I) have been found to be more efficient than the latter. These grooves are generally narrow, as

shown. They may extend to the bottom of the channels or may extend for a part only of the depth of the separating wall. By this means the metal traveling along one channel is at one or more points allowed to slip through into the succeeding channel and mix with the metal there. This movement naturally produces a disturbance of the regularity of flow as well as a mixing between the two streams of metal. It is obvious that the mutilation of the wall by the narrow grooves must affect a comparatively small part of the walls of the channels, so that the general course of the impelling action is not broken up.

A little to the rear .of the delivery end of the impelling device it is preferred to provide a more extensive local increase in clearance by cutting or otherwise forming atransverse groove 26 of considerable width in each of the threads II or other ribs on the stationary part of the impelling Such a local increase in clear device. Preferably this groove is of tapered form, having a maximum depth at or near its leading edge 21 and zero depth at its rear edge as shown in Figures 3 and 4. From Figure 3 it wfll be seen that the width of each of the grooves is approximately equal to half the length of one complete turn of the thread in which it is cut. Preferably also the grooves do not extend to the bottom of the channels. This local increase in clearance allow a considerable amount of metal to be carto intermingle before leaving the impelling device to enter the extrusion chamber. It has been found that such grooves in the channel walls,

I especially the wide groove near the front end of the impelling device, are of considerable benefit in that they have the effect of equalising the pressure in adjacent channels, thus ensuring stability of operation, by which term is meant the ability of the machine to continue for a considerable period to produce a tube of uniform wall thickness, and increasing the continuity of action, that is to say, it increases the period of time during which the machine will continue to extrude without a falling on in output which, for reasons not fully understood, sometimes occurs in machines as hitherto constructed.

At the delivery end-of the impelling device a supporting means are generally required for the inner member. These are disposed at intervals around the periphery of the inner member and extend between this member and the wall of the extrusion chamber. In the machine shown in Figure 1 they are integral with the inner member l0 and project outwards as shown at l2 and are constituted by the end parts of the threads 1.6 or other walls between the channels in the inner member I, the pitch of the threads l6 being increased in the region of the front end to permit of them being broadened without reducing the width of the channels, as shown clearly in Figures 3 and 4. In the former case they are constituted by the end parts of the threads I6 or other walls between the channels in the inner member ID. The pitch of the threads I6 is increased in the region of the front end to permit of them being broadened without reducing the width of the channels, as shown clearly in Figures 3 and 4.

The side walls of the end parts of the threads forming the supports l2 have corrugated surfaces, the corrugations extending to the front face 2| of the support and being inclined at an angle to the general direction of the stream. These may be formed by providing the walls with grooves 22 as shown in Figures 1, ,4 5 and 6 or by providing ribs on the walls. I'hese corrugated surfaces give to the path of the stream which is in each other and thereby produce something in the nature of a diffusion of the meeting streams which assists in counteracting any tendency to segregation that may be caused at the front extremity of the impelling device. In cases where the supports project inwards from the wall of the chamber as shown at in Figures 5, 6 and '7, similar modifications may be made to the lateral walls thereof.

The form of the stream may also be diversified as it leaves the channel in the propelling member by forming a local recess running along in the base of the channel. Such a recess will give extra depth to the channel over a narrow width and accordingly will disturb the transverse form of the stream.

It will be appreciated that although a machine has been described that has an impelling device incorporating several modifications in addition to the modification which comprises imparting to the grooves of the threads or other channels of the one member a progressive increase in depth as the delivery end is approached, the latter modification may be employed by itself or in combinationwith any one (or more) of the other modifications.

What I claim as my invention is:

1. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which co-operate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members being helical ribs separated by a helically extending channel of which the depth varies periodically, thereby producing a disturbance of the regularity of the paths taken by the plastic metal without interference with the general lines of these paths.

2. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which co-operate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members being formed bycutting at least two helical channels in the surface thereof, one of said helical channels having its axis displaced in one direction from the said common axis of rotation which is also the axis of the said surface and the other of said channels having its axis displaced in another direction from the said common axis oLrotatipn thereby forming channels of which the depth varies periodically alon the length' thereof.

3. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which co-operate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members being helical ribs separated by a helically extending channel of which the depth increases progressively as the delivery end of the device is approached.

4. In a. machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the adjacentsurfaces of said members which co-operate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members comprising helical ribs separated by a channel of which the depth increases progressively as the delivery end of the device is approached and the projections on the other of said members comprising ribs separated by a channel of which the depth decreases progressively as the delivery end of the device is approached.

5. In a machine for the extrusion of plastic metal, comprising two members relatively rotatable about a common axis and projections on the adjacent surfaces of said members which cooperate to form a device for impelling plastic metal between said members, the provision of a local increase in the clearance between the said co-operating projections, thereby producing a disturbance of the regularity of the paths taken by the plastic metal without interference with the general lines of these paths.

6. In a machine for the extrusion of plastic metal, two members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which co-operate to form a device for impelling plastic metal between said members, the projections on one of said members comprising a rib from which portions have been locally removed, thereby-producing a disturbance of the regularity of the paths taken by the plastic metal without interference with the general lines of these paths.

7. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which co-operate to form a device for impelling plastic metal between said members, the projections on one of said members comprising helical ribs in which at intervals along the length thereof are narrow circumferentially extending slots for producing a disturbance of the regularity of the paths taken by the plastic metal without interference with the general lines of these paths.

8. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members, means for driving one of said members in rotation about its axis, means for anchoring the other of said members, projections on the adjacent surfaces of said members which co-operate to form a device for impelling plastic metal between said members, and a pre-forming chamber in which the several streams of plastic metal are caused to intermingle before leaving the impelling device, said chamber being located between the inlet and delivery ends of said impelling device andformed in the stationary member by providing a local increase in the clearance between the projections on the stationary member and those on the rotating member.

9. In a machine for the extrusion of plastic metal, two relatively rotatable and longitudinally extending, concentric members, projections on the adjacent surfaces of said members which cooperate to form a. screw thread device for impelling plastic metal between said members, an extrusion chamber at the delivery end of said device, supports extending between the inner of said concentric members and the wall of said chamber, said supports having corrugated side walls, the corrugations extending at an acute angle to the general direction of the stream of plastic metal flowing therepast.

10. In a metal extrusion machine as specified in claim 9, the disposition of the corrugations on the-one side wall of a support at an angle to those on the opposite side wall. of that support.

11. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which cooperate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members comprising ribs separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the device is approached.

12. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members, means for driving one of said members in rotation about its axis, means foranchoring the other of said members against rotation, projections on the adjacent surfaces of said members which co-operate to form a screw thread device for impelling plastic metal between said members, the projections on said anchored member comprising ribs separated by a channel of which the depth increases progressively from a point between its ends, as the delivery end of the device is approached.

13. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which co-operate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members comprising ribs separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the device is approached, and the projections on the other of said members comprising ribs separated by a channel of which the depth decreases progressively, from a point between its ends as the delivery end of the device is approached.

14. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which co-operate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members comprising helical ribs separated by .a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the device is approached, and the projections on the other of said members comprising ribs separated by a. channel of which the depth decreases progressively, from a point between its ends as the delivery end of the device is approached.

15. In a machine for the extrusion of plastic metal, two concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which cooperate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members comprising ribs separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the device is approach and the projections on one of said members being ribs separated by a channel of which the depth varies periodically along a, length of the channel.

16. In a machine for the extrusion of. plastic metal, two concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of'said members which co-operate to form a screw thread device for impelling plastic metal between sa d Embers the projections on one of said members comprising ribs separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the device is approached and the projections on the other of said members being ribs separated by a'channel of which the depth varies periodically along a length of the channel.

17. In a machine for the extrusion of plastic metal, two concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which 00- operate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members comprising ribs separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the device is approached and the projections on one of said members comprising helical ribs separated by a channelof which the depth varies periodically along a length of the channel and also gradually alters, from a point between its ends, as the delivery end of the device is approached.

18. In a machine for the extrusion of plastic metal, two concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which cooperate to form a screw thread device for impelling plastic metal between said members, the projections of one of said members comprising ribs separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the device is approached and the projections on the other of said members comprising ribs separated by a channel of which the depth varies periodically along a length of the channel and gradually decreases, from a point between the ends of the channels, as the delivery end of the device is approached.

19. In a machine for the extrusion of plastic metal, two concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which cooperate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members comprising helical ribs separated by a channel of which the depth increases progressively from a point between the ends of the channel, as the delivery end of the device is. approached, the projections on the other of said members comprising ribs separated by a channel of which they depth decreases progressively from a point between the ends of the channel, as the delivery end of the device is approached and the bottom surface of the channels of one of said members being also of undulatory form.

20. In a machine for the extrusion of plastic metal, two concentric members relatively rotatable about a common axis, projections on the adjacent surfaces of said members which co-operate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members comprising helical ribs separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the device is approached, and the projections on the other of said' members comprising ribs separated by a channel of which the depth decreases progressively, from a point between its ends, as the delivery end of the device is approached and of which the bottom surface is also of undulatory tween its ends, as the delivery end of the machine proached, and the projections on the other of said members comprising ribs separated by a channel which is in the form of a helix whose axis is displaced from the said common axis of rotation and of which the depth varies periodically along the length thereof.

22. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the said members which cooperate to form a screw thread device for impelling plastic metal between said members, the projections on the first of said members comprising ribs separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the device is approached and the projections on the second of said members being formed by cutting in the surface thereof adjacent the first said member at least two helical channels, one of said channels having its axis displaced in one direction from the said common axis of rotation which is also the axis of rotation of the said surface and the other of said channels having its axis displaced in another direction from the said common axis of rotation, whereby the depth of each of the eccentric channels varies periodically along the length thereof.

23. In a machine for the extrusion of plastic metal, comprising two concentric members relatively rotatable about a common axis, ribs on one of said members separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the machine is approached and projections on the other of said members co-operating with said ribs to form a screw thread device for impelling plastic metal between said members, the provision of a local increase in the clearance between the said ribs and the said projections.

24. In. a machine for the extrusion of plastic metal, two concentric members relatively rotatable about a common axis, projections in the form of ribs on one of said members separated by a channel of which the depth increases progressively, from a point between its ends as at intervals along the length thereof narrow circumi'erentially extending slots therein;

25. In a machine for the extrusion of plastic metal, two concentric membera'means for driv- I ing one of said members in rotation about its axis,

means for anchoring the other of said members,

projections in the form of ribs on one-of said.

members separated by a channel of which the depth increases progressively, from a point be- -is' approached, projections on the other of said members co-cperatins with the ribs on the firs member to form a screw thread device for impelling plastic metal between said members, and a pre-forming chamber in which the several streams of plastic metal intermingle before leaving the impelling device, said chamber being located between the inlet and delivery ends of the impelling device and formed in the anchored member by providing a local increase in the clearance between the projections on the anchored member and those on the driven member.

26. In a machine for the extrusion of plastic metal, two concentric members, means for driving one of said members in rotation about its axis, means for anchoring the other of said members against rotation, projections in'the form of ribs on one of said members separated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the machine is approached, projections on the other of said members co-operating with the ribs on the first member to form ascrew thread device for impelling plastic metal between said members, the projections on the anchored member being ribs in each of which is a transverse groove of considerable width, thereby to provide between the inlet and delivery ends of the impelling device a pre-forming chamber in which the several streams of plastic metal intermingle before leavarated by a channel of which the depth increases progressively, from a point between its ends, as the delivery end of the machine is approached, projections on the other of said member co-opcrating with the ribs on the first member to form a screw thread device for impelling plastic metal between said members, the projections on the anchored member being ribs of helical form in each of which is a transverse groove having a width approximately equal to half the length of one complete turn of the rib and having a maximum depth at its leading edge, thereby to provide near the delivery end of the impelling device a pre-forming chamber in which the several streams of plastic metal intermingle beforeleaving the impelling device.

28. In a machine for the extrusion of plastic metal, two longitudinally extending concentric members relatively rotatable about a common axis, projections on the said members which cooperate to form a screw thread device for impelling plastic metal between said members, the projections on one of said members comprising helical ribs separated by a channel of which the depth increases progressively from a point between its ends as the delivery end of the device is approached; an extrusionchamber on the delivery end of the impelling device; supports extending between the inner of said concentric members and the wall of said chamber, said supports having corrugated side walls, the corrugations extending at an acute angle to the general direction of the stream of plastic metal flowing therepast.

29. In a metal extrusion machine as specified in claim 28, the disposition of the corrugations on 'the' one sidewall of a support at an angle to those on the opposite side wall of that support.

.30. In a machinefor' the extrusion of plastic metal, two longitudinally extending concentric members; means for driving the outer or said members in rotation about its axis; means for anchoring the other of said members against rotation; helical ribs on said anchored member separated by a channel of which the depth in- .creases from a point between its ends, as the delivery end of the machine is approached, said helical ribs each having at intervals along its length narrow circumferentially extending slots and near its delivery end a wide transverse groove having a maximum depth at its leading edge; helical ribs on said driven member separated by a channel of which the depth varies periodically along a length thereof and also decreases, from a a point between its ends, as the delivery end of the machine is approached, said helical ribs on said concentric members co-operating to form a screw thread device for impelling plastic metal between said members,.in which segregation of the plastic metal within the device is substantially inhibited.

PERCY DUNSHEATH.