Abstract:
A plasticizing barrel for a plastics processing machine includes a body having a hollow cylindrical configuration and defining an axis; and a coating formed on an inner wall surface of the body and including at least one groove extending in axial direction of the body. The coating, e.g., a wear-resisting protective layer, may be produced through centrifugal casting, whereby the grooves are formed by projections of a negative mold inserted in the hollow-cylindrical body.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims the priority of German Patent Application, Serial No. 102 06 169.6-16, filed Feb. 14, 2002, pursuant to 35 U.S.C. 119(a)–(d), the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates, in general, to a plasticizing barrel, and to a method of making such a plasticizing barrel. 
   Plasticizing barrels are used to plasticize granular plastic material in an injection assembly or an extruder by means of a screw for a subsequent injection process. As the inner wall of the plasticizing barrel is subject to great stress during plasticizing, it has been commonly proposed to line the inner wall with a wear-resisting coating. The application of such a wear-resisting coating may be implemented by a so-called centrifugal casting process in which metallic powder is introduced into a rotating hot hollow-cylindrical body, pressed against the inner barrel wall as a consequence of centrifugal forces, and melted there. After a cool-down period, the thus produced layer on the inside wall exhibits a fairly good surface quality and requires only little finishing works. 
   When more powerful plasticizing units are involved, barrels with grooves are required whereby the grooves extend almost over the entire length of the plasticizing barrel. These barrels do not have a wear-resisting protective layer; rather, barrel blanks are formed in “soft state” with the grooves, and subsequently the inner barrel wall is hardened by a nitriding process. 
   It is also known to place so-called grooved bushings in the feed zone of plasticizing units for improving the transport of introduced raw plastic material. These grooved bushings are normally also coated for wear-resistance, e.g., through a high-temperature isostatic pressing process. The wear-resisting protective layer of the grooved bushing can be provided either directly into the plasticizing barrel, or the grooved bushing can be produced separately and subsequently placed into the plasticizing barrel. This process is applicable only for relatively short grooves. 
   It would therefore be desirable and advantageous to provide for a plastics processing machine an improved plasticizing barrel which obviates prior art shortcomings and which includes an inner coating and exhibits on its inner wall a groove or grooves. 
   It would also be desirable and advantageous to provide an improved method of making such a plasticizing barrel. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a plasticizing barrel for a plastics processing machine includes a hollow cylindrical body, and a coating which lines an inner wall surface of the body and is formed with at least one groove extending in axial direction of the body. 
   The present invention resolves prior art problems by forming the groove or grooves completely in the inner coating itself rather than in the hollow cylindrical body. Suitably, the provision of the groove in the inside coating can be realized by a centrifugal casting process, whereby an axially extending negative mold, formed with axial projections, is placed in the hollow-cylindrical body, and, after carrying out the centrifugal casting process, is removed again. According to an alternative process, the centrifugal casting process may also be carried out in two stages by initially applying a smooth layer on the inner wall of the hollow cylindrical body through centrifugal casting in the absence of a negative mold, and subsequently carrying out the centrifugal casting process with the negative mold. The negative mold may be held in axial centered disposition in the hollow-cylindrical body. However, this centering step may be omitted altogether, when the initial smooth layer is applied at very exact dimensions so that the negative mold is already in centered disposition, when placed into the hollow-cylindrical body. 
   A smoothest possible surface of the areas between the grooves may be realized when the layer has a thickness which is smaller than the height of the projections in radial direction. In the two-stage process described above, the smooth inner layer may be honed in the regions in which the projections are situated so as to realize smooth groove bases in a simple manner. Of course, the entire inner coating may be honed as well. 
   According to another aspect of the present invention, a negative mold for use in a method of making a plasticizing barrel for a plastics processing machine, may include a base body formed with axial projections and defined by a diameter which is smaller than an end inner diameter of the coated plasticizing barrel. The base body may be cylindrical, and the projections may be part of a cage. 
   According to another feature of the present invention, the projections may be coated with an adhesion-reducing layer and/or wear-reducing layer to facilitate a removal of the negative form. As an alternative, it is, of course, also conceivable to so configure the projections in the direction in which the negative mold is removed from the coated hollow-cylindrical body as to have increasing width in circumferential direction and/or increasing height in radial direction. 
   According to another feature of the present invention, the projections may have a helical configuration. As a consequence, the negative mold can then be withdrawn from the hollow-cylindrical body through a twisting motion. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
       FIG. 1  is a schematic, cross sectional view of a plasticizing barrel according to the present invention; 
       FIG. 2  is a schematic, cross sectional view of a negative mold according to the present invention; 
       FIG. 3  is a schematic, cross sectional view of the hollow-cylindrical body with inserted negative mold for making the plasticizing barrel of  FIG. 1 ; 
       FIG. 4  is a schematic, cross sectional view of the hollow-cylindrical body with inserted negative mold and introduction of material into the hollow-cylindrical body at a process stage before centrifugal casting; 
       FIG. 5  is a schematic, cross sectional view of the hollow-cylindrical body with inserted negative mold after centrifugal casting; 
       FIG. 6  is a schematic, cross sectional view of a hollow-cylindrical body with negative mold in an intermediate step of a variation of a method according to the present invention 
       FIG. 7   a  is a plan view of a cage for making a plasticizing barrel according to the present invention; 
       FIG. 7   b  is a fragmentary, perspective view of the cage of  FIG. 7   a;    
       FIG. 8  is a cutaway, sectional view of the negative mold with projections lined with another layer; 
       FIGS. 9   a,    9   b  show schematic views of modified configurations of projections of the negative mold; and 
       FIG. 10  shows a schematic view of another variation of a negative mold. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. 
   Turning now to the drawing, and in particular to  FIG. 1 , there is shown a schematic, cross sectional view of a plasticizing barrel according to the present invention, generally designated by reference numeral  1  and including a hollow-cylindrical body  10  having an inner wall lined with a coating  12 , e.g. a wear-resisting protective layer, which is formed with grooves  14 . 
   Manufacture of the plasticizing barrel  1  according to the present invention involves the use of a negative mold, as shown in  FIG. 2  and generally designated by reference numeral  20 . The negative mold  20  has a cylindrical base body  16  which is formed with radial projections  18  about its perimeter. The projections  18  extend in axial direction along the surface of the base body  16  and may jut out in straight configuration, i.e. in axis-parallel relationship to the base body  16 , or may have a helical configuration, i.e. extending around the surface of the base body  16 , as shown in  FIG. 10 . As indicated in  FIG. 2 , the base body  16  has a diameter which is smaller than an inner diameter D i  of the finished plasticizing barrel  1 . The outer diameter of the negative mold  20 , i.e. the diameter of the inner circle which connects the projection and outer surface is D i +2*h N , wherein h N  is the height of the groove to be provided. B N  represents the width of the projections  18 , i.e. width of the grooves to be provided. 
   The negative mold  20 , shown in  FIG. 2 , is placed into the hollow-cylindrical body  10  which has an inner diameter which is greater than the inner diameter D i  of the finished plasticizing barrel  1  by twice the thickness of the finished coating  12 , as shown in  FIG. 3 . Subsequently, metal powder  22  or other liquid melt is introduced into the intermediate spaces between the projections  18 , as shown in  FIG. 4 . The hollow-cylindrical body  10  with negative mold  20  and metal powder  22  is then subjected to a centrifugal casting process, i.e. the hollow-cylindrical body  10  with negative mold  20  and metal powder  22  are heated and caused to carry out an axial rotation. As a result, the metal powder  22  melts and lines the inner wall surface of the hollow-cylindrical body  10  and migrates also into gaps  19  between the projections  18  and the inner wall surface, as shown in  FIG. 5  to form the coating  12 . Subsequently, the negative mold  20  is removed from the hollow-cylindrical body  10  by being pulled out, when straight projections  18  are involved, or by twisting, when the projections  18  have a helical configuration. As a result of the removal of the negative mold  20 , grooves  14  are formed in the area where the projections  18  have previously been situated, as shown in  FIG. 1 . If need be, the inner contour of the plasticizing barrel  1  may be finished by honing (precision grinding). 
   Of course, the metal powder  22  or liquid melt may also be introduced during the centrifuging process. 
   An alternative to the afore-described manufacturing process of the plasticizing barrel  1  involves the application of a two-step process to make the coating  12 . As shown in  FIG. 6 , the inner wall surface of the hollow-cylindrical body  10  is lined initially with a coating  11 , e.g. a wear-resisting protective layer, of constant thickness, preferably, through centrifugal casting, with the coating  11  having an inner diameter of D i +2h N . Subsequently, the negative mold  20  or a cage carrying the projections  18 , is inserted in the interior of the hollow-cylindrical body  10 . An example of a suitable cage is shown in  FIGS. 7   a  and  7   b  and involves the arrangement of two end rings  30  (only one is shown here) between which the projections  18  are secured. Thereafter, metal powder  22  is introduced into the intermediate spaces between the projections of the negative mold (or centrally into the hollow space of the cage), and the hollow-cylindrical body  10  with coating  11  and the negative mold  20  are subjected to a further centrifugal casting process to form the coating  12  with an inner diameter D i  and with the grooves  14  in a manner described in conjunction with the previous embodiment. Suitably, the areas of the initial coating  11  where the projections  18  are situated are honed, before the negative mold  20  or the cage are placed into the hollow-cylindrical body  10  so that the finished plasticizing barrel  1  has very smooth groove bases. 
   In this variation of the process according to the present invention, the initial coating  11  is thick enough to allow the projections  18  to automatically bear against the coating  11  so that the need for a separate centering of the negative mold  20  or the cage is eliminated. 
   The negative mold  20  or cage may be used repeatedly. After a number of uses, finishing works, such as polishing, coating, may be suitable. 
   Since the diameter of the base body  16  of the negative mold  20  is smaller than the inner diameter D i  of the finished plasticizing barrel  1 , i.e. the groove height h N  is smaller than the height of the projections  18 , the surface of the coating  12  between the grooves  14  is ensured to have a very even and smooth configuration as a consequence of the centrifugal forces. 
   Depending on the stress the plasticizing barrel  1  is subjected, various alloys may be used as protective layer. These alloys are known to the person skilled in the art so that a detailed description thereof is omitted for the sake of simplicity. 
   As shown in  FIG. 8 , the projections  18  of the negative mold or cage may be coated with an adhesion-reducing layer and/or wear-reducing layer, as indicated by reference numeral  23 . As an alternative, it is, of course, also conceivable to configure the projections  18  in the direction in which the negative mold  20  is removed from the coated hollow-cylindrical body  16 , indicated by arrow  24 , so as to have increasing width in circumferential direction, as shown in  FIG. 9   a,  and/or increasing height in radial direction, as shown in  FIG. 9   b.    
   Although, centrifugal casting is a currently preferred process to make a plasticizing barrel  1  according to the present invention, other forming process may also be applicable, without departing from the spirit of the present invention. Other forming processes may involve, however, also different dimensioning of the negative mold. 
   While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
   What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and their equivalents: