INTERNALLY INSULATED EXTRUSION DIE

Abstract: A die assembly for plastic pelletizing apparatus of the type including a cooling bath into which the plastic is extruded. The die assembly includes a heated main body (10) having first and second oppositely disposed exterior faces (18; 20) connected by a plurality of relatively small extrusion passageways (22). A layer of thermal insulation material (40) overlies the second face (20), and a cover member (44) having an outer surface adapted to be exposed to the cooling bath is positioned over the thermal insulation material and joined to the main body. Wear resistant insert members (52) formed from a material having a higher coefficient of thermal conductivity than the material from which the cover member (44) is made extend from the outer surface of the cover member through the layer of insulation into the main body. Extrusion outlet orifices are formed through the insert members (52) and define continuations of the extrusion passageways (22) of the main body (10). The relationship between the inserts (52), the cover (44), the insulation (40), and the main body (10) produces an increased heat flow from the main body to the outer die face in confined areas about the orifices. This heat flow advantageously reduces the possibility of freeze-off and plugging of the orifices due to premature solidification of the extruded plastic.

Description: The subject invention is directed to the art of extrusion dies and, more particularly to an extrusion die of the type used for extrusion of synthetic resins and plastics. The invention is particularly suited for use in an underwater type, plastic pelletizing apparatus and will be described with reference thereto; however, the invention is capable of broader application and could be adapted for use in other environments and for other purposes. One of the more common apparatuses used for pelletizing plastic material comprises a rotary extruder which extrudes the resin or plastic material in a molten or fluid state through an extrusion die having a multiplicity of small diameter extrusion passages. Typically, the face of the die is submerged in a cooling bath such that the plastic material solidifies just as it is leaving the die. As the material exits from the face of the die, it is cut into pellets or granules by a rotating knife passing over the die face. A continuous and ongoing problem with the foregoing apparatus has been the difficulty of assuring that the material does not solidify during its passage through the die. As is apparent, premature solidification of the material can result in plugging or "freezing-off" of the extrusion passages and/or outlet orifices. In an effort to maintain the material molten during its passage through the die, the dies have typically been designed with internal heating passages through which steam or heated oil has been circulated. In addition, various arrangements have been used to retard the flow of heat from the body of the die to the cooling bath. For example, layers of insulation have been placed over the exposed faces of the die, or between the die face and the main die body. Alternatively, the die passages have been provided with insulating sleeves of ceramic or the like to retard heat flow from the molten plastic to the die body or face. One known extrusion die, which uses a combination of the above approaches to reduce the problem of solidification of the molten material is described in EP-A-0152844 of Montedison S.p.A. In the Montedison die, a layer of thermally insulating material overlies the front (output) face of the die body with nozzle members extending the flow channels of the die body through the layer of thermally insulating material. The nozzle members are in thermally conductive engagement with the die body. A cover plate overlies the layer of thermally insulating material and has openings through which the nozzle members extend. The nozzle members are separated, both physically and thermally, from the cover plate by extensions of the layer of thermally insulating material surrounding the tip of each nozzle member where it passes through the cover plate. While the above mentioned approaches have alleviated the difficulty with "freeze-off" to some extent, it still remains an ongoing problem. To that end, the subject invention provides an extrusion die construction which is relatively simple while operating to significantly reduce "freeze-off" and provide extended surface wear life. In accordance with the present invention there is provided die apparatus for extruding plastic pellets into a cooling bath, the apparatus comprising a die body having passageways extending between oppositely disposed first and second faces of the die body, a thermal insulation layer overlying the second face, means for applying heat to the die body, a cover member covering the thermal insulation layer, insert members extending inwardly from the outer face of the cover member into thermally conductive engagement with the die body and having outer ends lying in a common plane outwardly of the second face; characterized in that the insert members are in thermally conductive engagement with the cover member, by the insert members being made from a wear resistant material having a higher thermal conductivity than that of the cover member and by wear pads mounted in recesses on the outer surface of the cover member, the wear pads being spaced from the insert members and having an outer surface in the same plane as the outer ends of the insert members. The new and improved extrusion die construction of the present invention comprises a heated main die body having a plurality of plastic extrusion passageways extending through the body to an outer face which is provided with a layer of thermal insulation over the outer face and a cover member which overlies and encloses the insulation is joined to the main body. Wear resistant insert members formed from a material having a coefficient of thermal conductivity significantly higher than the material from which the cover is made extend through the cover member and the insulation into the main die body. Each of the insert members includes an extrusion outlet orifice which forms a continuation of an associated extrusion passageway in the main body. The use of the insert members with their relatively higher coefficient of thermal conductivity increases the heat flow from the main body to the outer surface of the assembly in the area closely surrounding the orifices. This permits the temperature of the orifices to be maintained at a higher level to thereby prevent freeze-off without requiring a significant increase in the heat input to the die body. The remainder of the exposed cover member or face is, however, at a relatively lower temperature because of the insulation. The outer end faces of the insert members are located in a common plane which may be coextensive with the outer face of the cover member or outwardly thereof. The wear resistant insert members, together with the wear pads, can thus define the surface over which the knife or knives travel to pelletize the extruded plastic. The insert members are preferably of cylindrical shape with the outlet orifices formed axially therethrough. In addition, the inserts are preferably bonded to both the main body and the cover member, such as by brazing. The principal advantage of the subject invention is the provision of a new and improved extrusion die which is arranged to eliminate the problem of freeze-off while providing a highly wear resistant face. A further advantage resides in the provision of an extrusion die of the general type described that is internally insulated to reduce heat loss to the outer face, but which is provided with means to increase heat flow in the areas about the outlet orifices. Other benefits and advantages of the subject invention will become apparent to those skilled in the art upon a reading and understanding of the following specification. Brief Description of the Drawings The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein: FIGURE 1 is a front elevational view of an extrusion die formed in accordance with a preferred embodiment of the invention; FIGURE 2 is an enlarged view of the portion of FIGURE 1 enclosed by a dash-dot line; and FIGURE 3 is a cross-sectional view taken generally along lines 3-3 of FIGURE 2. Detailed Description of the Preferred Embodiment Referring now to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only and not for the purpose of limiting same, FIGURES 1-3 show an extrusion die assembly comprising a main die body A and a top plate or cover member B which is connected to the forward end face of the main die body A. The subject extrusion die assembly is adapted for use with a conventional extruder apparatus which is not shown inasmuch as it does not comprise a part of the present invention and is not necessary to an appreciation thereof. In using the subject invention in its preferred environment, a synthetic resin or polymeric plastic material in molten or fluid form is extruded from the extruder through main die body A and outwardly through cover or top plate member B. The interior of the die body is maintained at an elevated temperature in a manner subsequently to be described. The outer surface of cover member B is normally disposed in a cooling bath of water or other liquid maintained at a temperature substantially lower than the temperature of the interior of main die body A. As the synthetic resin or polymeric plastic material in a liquid or fluid state is extruded through the main die body A and the cover member B, it is cooled to a solid state as it discharges into the cooling bath. As the solidified material enters the cooling bath, it is cut into pellets by a knife or knives which periodically wipe across the outer surface of cover member B. This general process is well known in the art and is commonly referred to as underwater pelletizing. More particularly, and with reference to FIGURES 1-3, main die body A is defined by a generally cylindrical body 10 formed from any suitable material, eg., stainless steel, and provided about its outer periphery with a plurality of counterbored, bolt receiving openings 12 which allow the body 10 to be suitably connected to the associated extruder (not shown). In addition, a plurality of similarly counterbored openings 16 are formed generally axially through body 10 adjacent the center area thereof to also provide for mounting of the assembly. As best shown in FIGURE 3, body 10 includes a pair of oppositely disposed, generally parallel end faces 18 and 20. End face 18 is arranged to be connected to the discharge end of the extruder to receive the molten or fluid resin or plastic material coming therefrom. As shown, a multiplicity of relatively closely spaced, generally parallel extrusion passageways 22 extend in the axial direction through body 10. In the embodiment under consideration, all of the extrusion passageways 22 are generally identical in configuration and include a tapered inlet end opening 24 with an enlarged, generally cylindrical passageway section 26. The inner end of section 26 is provided with a tapered passageway section 28 leading to the outlet section 30 which extends through face 20. Heating means is provided to maintain the body 10 at an elevated temperature sufficient to prevent solidification of the molten or fluid material during its travel through passageways 22. In the subject embodiment, the heating means comprises a plurality of fluid passages 34 formed transversely of the body intermediate aligned rows of passageways 22. The passages 34 are sealed from passageways 22 and function to conduct a heated fluid such as steam or oil transversely through the die from an inlet 36 to an outlet 38 (see FIGURE 1). It should, of course, be understood that the opposite ends of passages are suitably connected to the inlet and outlet 36, 38 by internal header passages (not shown). In order to prevent excess heat flow from the main body outwardly through end face 20 to the associated cooling bath, the subject die construction includes a layer of suitable thermal insulation material 40 overlying end face 20. Preferably, the insulation 40 substantially completely covers the end face 20 throughout the area under cover member B. As can be appreciated, the particular type of insulation used is not critical to the subject invention and may, for example, comprise a ceramic-type material or any other material which is suitable for high temperature applications and has desirable insulating characteristics. In the embodiment shown, zirconium oxide is preferred and can be sprayed or otherwise applied to surface 40. Additionally, the thickness of the insulation can vary as desired or necessary to provide the needed insulation characteristics. In the illustrated embodiment, the insulation material 40 is sealed from direct contact with the cooling bath by the top plate or cover member B. As best seen in FIGURE 1 and 3, cover member B comprises a generally flat, annular shaped disk member 44 which is positioned concentrically on end face 20 of body 10. The member 44 is preferably positioned in direct contact with insulation 40, and is rigidly and sealingly joined to body 10 such as by circumferential inner and outer weld bead 46 and 48. The body 10 and the cover member 44 are preferably formed from metals having suitable temperature and corrosion resistance as required by the operating environment. For example, precipitation hardening stainless steels have been used successfully in die assemblies of the type under consideration. Of particular importance to the subject invention is the arrangement for connecting the outlet ends of passageways 22 with the outer face 50 of cover member 44. In particular, insert members 52 extend inwardly from outer face 50 through the cover member 44 and the insulation layer 40 into recesses 42 in the main body 10. Each of the inserts 52 preferably has a generally cylindrical configuration with an axially extending centerbore 54. In addition, the insert members 52 are formed from a hard, wear resistant material capable of withstanding the abrasion characteristics of the extruded material and the wear induced by the knives traveling over their exposed ends. Moreover, the insert members must be formed from a material having a coefficient of thermal conductivity substantially greater than that of the cover member 44. In the preferred embodiment, the insert members 52 are formed from sintered tungsten carbide having a coefficient of thermal conductivity of about .19 cal/cm/sec/°C. A typical precipitation hardening stainless steel as used for the cover member 44 has a coefficient of thermal conductivity of about .050 cal/cm/sec/°C. Other materials having suitable characteristics could, of course, also be used. One of the insert members 52 is preferably associated with each of the extrusion passageways 22 and defines the outlet discharge orifice for the associated passageway 22. As best shown in FIGURE 3, the central opening 54 of the inserts 52 is preferably sized to generally correspond to the diameter of section 30 of the extrusion passageways 22. In addition, each of the insert members 52 is desirably permanently bonded such as by brazing or the like to both the body 10 and the cover member 44. By directly connecting the insert members 52 through the insulation layer 40 and into thermal engagement with the body 10, heat flow from the body 10 to the outer face 50 is significantly increased in the area closely surrounding each of the discharge orifices 54. This has the effect of raising the temperature of the outlet orifices and preventing freeze-off or solidification of the plastic material passing therethrough. In addition to the above, it is preferred that the outer end face of each insert 52 lie in a common plane which is preferably in the plane of face 50 or slightly outwardly thereof. The outer ends of the inserts 52 can thus define the surface over which the knives travel in pelletizing the plastic which solidifies as it exits through the outer end of the discharge orifices 54. Preferably, and as best shown in FIGURE 2, the discharge passageway 30 and/or the associated inserts 52 should be laid out such that they are not in circumferentially aligned rows. That is, the wear surfaces provided by the inserts 52 should be relatively evenly distributed at varying radial distances to assure that wear on the knife or knives and the inserts is uniform. The layout of the extrusion passageways 22 is usually such that the pattern cannot be completely uniform about the outer face of the die assembly. In order to provide a more even wear surface to the outer face than can be achieved by the insert members alone, suitable wear pads or buttons 58 (see FIGURES 2 and 3) are positioned in cover plate member 44 at a location so as to provide a more even distribution of wear areas. Preferably, the wear pads 58 are generally cylindrical and are formed from a suitable, wear resistant material. They are positioned in recesses 60 formed in the cover plate 44 such that the outer or exposed surface of each wear pad 58 lies in the same plane as the outer exposed surfaces of the inserts 52. It is important to note, however, that the wear pads 58 should not extend through the cover member 44 or the insulation layer 40. In addition, it is preferably that the wear pads 58 do not engage the inserts 52. The reason for this is that if engagement with the main die body or the inserts were to take place, the wear pads 58 would cause an increased heat flow to the cooling bath with no benefit to the operation of the die assembly. The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims: Die apparatus for extruding plastic pellets into a cooling bath, the apparatus comprising a die body (A) having passageways (22) extending between oppositely disposed first (18) and second (20) faces of the die body, a thermal insulation layer (40) overlying the second face, means (34) for applying heat to the die body, a cover member (44) covering the thermal insulation layer, insert members (52) extending inwardly from the outer face (50) of the cover member into thermally conductive engagement with the die body and having outer ends lying in a common plane outwardly of the second face (20); characterized in that the insert members (52) are in thermally conductive engagement with the cover member (44), by the insert members being made from a wear resistant material having a higher thermal conductivity than that of the cover member (44) and by wear pads (58) mounted in recesses (60) on the outer surface (50) of the cover member (44), the wear pads being spaced from the insert members (52) and having an outer surface in the same plane as the outer ends of the insert members (52). Apparatus according to claim 1 further characterized in that the insert members (52) are joined to the cover member (44) and to the die body (A). Apparatus according to claim 1 further characterized in that the heating means comprises passages (34) adapted to convey a heating fluid through the die body (A). Apparatus according to claim 1 further characterized in that the insert members (52) are formed from sintered carbide. Apparatus according to claim 1 further characterized in that the insert members (52) have an inner end extending into the die body (A). Apparatus according to claim 5 further characterized in that the insert members (52) are bonded to the die body (A). Apparatus according to claim 1 or 6 further characterized in that the insert members (52) define a cylindrical discharge orifice extending axially therethrough. Apparatus according to claim 1 further characterized in that the wear pads (58) are bonded to the cover member (44).

Inventors: WOLFE, JOHN R., JR
Applicants: ROGERS TOOL WORKS, INC.
Application year: 1987
Main classification: B29C47
IPC codes: B29C47, B29B9