EP0101196B1

EP0101196B1 - Pellet mill die

Info

Publication number: EP0101196B1
Application number: EP83304141A
Authority: EP
Inventors: Thomas John Rowland
Original assignee: PELLET TECHNOLOGY (AUSTRALIA) PTY Ltd
Current assignee: PELLET TECHNOLOGY (AUSTRALIA) PTY. LIMITED
Priority date: 1982-07-19
Filing date: 1983-07-15
Publication date: 1988-03-02
Also published as: EP0101196A2; EP0101196A3; JPS5964316A; ATE32677T1; DE3375767D1; JPH0365243B2; CA1211414A

Abstract

An extrusion die wherein extrusion passageways are provided with a working section having a smooth wall, and a controlled relief section having a rippled wall of similar effective diameter as the working section in order to control pellet expansion and other factors.

Description

This invention relates to the extrusion of mouldable material into pellets or cubes, and more particularly to pellet mill die. The effective length and diameter of a die hole is a carefully selected balance between capacity and pellet quality and wear and strength of the die. A short effective length usually increases capacity and lowers power consumption, whereas a thicker die promotes solid pellet formation at the expense of power and capacity. A larger effective diameter generally promotes capacity at the expense of pellet quality.
US 3,176,631 describes a pelleting machine having in one embodiment a hole which is cylindrical throughout most of its length, from the inner to the outer surface of the pelleting ring. Near its outer end it has a single annular groove, which has two annular sides the lower one of which forms a smaller angle with the wall of the groove than the upper one, so that the hole is substantially self-cleaning. This is not the aim of the present invention.
It is an object of this invention to produce a die which improves the quality of the pellet without decreasing capacity. It is also an object to provide a stronger die without substantial cost increase.
It is a further object of this invention to provide a pellet die hole which has a smooth inlet portion wherein the pellet is formed, and a ridged or rippled outlet portion wherein the diameter of the formed pellet is retained under controlled conditions which reduce friction and minimize heat transfer from the pellet.
These and other objects are obtained in a pellet mill die comprising:

a die member having a multiplicity of die holes constituting extrusion passages extending between the inner and outer faces thereof, each of said die holes comprising:
smooth section having a smooth internal bore extending from said inner face part way through said die hole and
a ridged section extending from, and in axial alignment with, the smooth section towards said outer face, in which the ridged section has an internally ridged bore comprising a plurality of adjacent annular ridges each extending transversely of the axis of the die hole, or a helical ridge of a plurality of complete revolutions, adjacent ridges, or adjacent revolutions of a helical ridge, being separated by a corresponding valley, the ridged section having a minor diameter substantially equal to the diameter of the smooth section.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing, in which:

Fig. 1 shows the cross-section of a typical pellet mill die, having die holes therethrough, according to the present invention;
Fig. 2 is a cross-section of a single die hole to better illustrate the construction thereof;
Fig. 3 is a cross-section of a single die hole showing the details of an alternative embodiment construction.

Referring to Fig. 1, the cross-section of a typical pellet mill die is shown and designated by the reference numeral 1. The die has a plurality of die holes 2 which perforate the die from an inner surface 3 to an outer surface 4.
Each die hole is comprised of: an inlet countersink 5, a working section 6, and a controlled relief section 7. Details of each of these sections are shown in Figs. 2 and 3.
In a typical pellet mill, compression rollers would roll across the inner surface of the die 3 compressing the material to be pelletized through the inlet of each of the die holes. The countersink 5 is provided to facilitate entry of the material and to prevent high stress corners in this heavily loaded area.
The compaction and diameter of the pellet is controlled in the working section 6. This is normally a smooth polished section and the pellet is formed by a combination of friction and heat generated in the walls of the working section.
In conventional dies, once the formed pellet exited from the working section, it was subjected to uncontrolled expansion which often resulted in decreased pellet quality. To accomplish a required length of working section without reducing die thickness which is required to resist the high compaction forces, it became customary in many instances to provide a counter bore relief section. The relief section, however, was ineffectual in controlling the forces of expansion generated in the pellet.
In the present invention, a relief section is provided which is ridged and has an effective diameter close to that of the working section, but because of its ribbed construction, friction is greatly reduced between the pellet and the relief section wall. In addition, material filling the valleys of the ribbed section acts as an insulator to control heat and moisture loss, thus further promoting a durable pellet. The valleys may also be intentionally filled with an antifriction material such a nylon, teflon, or a solid lubricant such as graphite.
Fig. 2 shows a ribbed portion wherein the ribs are concentrically stacked. Fig. 3 shows a threaded ribbed portion. The threads may be any conventional type, such as a square thread or a conventional SAE or Whitworth thread. The ribbed construction may also be formed by an array of small dimples formed in the side wall of the relief section.
In particularly preferred embodiments, the length of the working section is from 1/4" to 2" (6.35 to 50.8 mm) and the length of the relief section is from 1/4" to 1-1/2" (6.35 to 38.1 mm).
Table 1 shows comparative results for a standard die and a die according to this invention. The dies were used to pellet hay in a Century 929-25 pellet mill. In each die the die thickness was 2-1/2" (63.5 mm) and the holes 5/16" (7.94 mm) diameter. In the die used in samples 2 and 3, the holes were tapped with a 3/8" (9.52 mm) Whitworth thread for 1/2" (12.7 mm) depth from the outer face of the die. In sample 5 the thread was tapped in a depth of 1" (25.4 mm).
Other forms of ribbed relief construction will occur to one skilled in the art and we do not wish to be limited in the scope of our invention except as claimed.

Claims

1. A pellet mill die comprising:

a die member having a multiplicity of die holes (2) constituting extrusion passages extending between the inner (3) and outer (4) faces thereof, each of said die holes comprising:

smooth section (6) having a smooth internal bore extending from said inner face part way through said die hole and

a ridged section (7) extending from, and in axial alignment with, the smooth section towards said outer face, characterised in that the ridged section (7) has an internally ridged bore comprising a plurality of adjacent annular ridges each extending transversely of the axis of the die hole, or a helical ridge of a plurality of complete revolutions, adjacent ridges, or adjacent revolutions of a helical ridge, being separated by a corresponding valley, the ridged section having a minor diameter substantially equal to the diameter of the smooth section.

2. A pellet mill die according to Claim 1 characterised in that said ridged section comprises a series of stacked circular concentric ridges.

3. A pellet mill die according to Claim 1 characterised in that said ridged section comprises an internal thread having a minor diameter equal to the diameter of said smooth section.

4. A pellet mill die according to Claim 3 characterised in that said ridged section comprises a standard screw thread.

5. A pellet mill die according to Claim 3 characterised in that said ridged section comprises a square thread.

6. A pellet mill die according to Claim 1 or 2 characterised in that said ridged section has an internal diameter equal to the diameter of said smooth section.

7. A pellet mill die according to any one of the preceding Claims characterised in that material passing through said die is trapped in the valley or valleys of the ridged section.

8. A pellet mill die according to Claim 6 characterised in that said ridged section has the valleys filled with an antifriction material.

9. A pellet mill die according to any one of the preceding Claims characterised in that the length of the smooth section is from 1/4" to 2" (6.35 to 50.8 mm) and the length of the ridged section is from 1/4" to 1-1/2" (6.35 to 38.1 mm).