IES72794B2 - Grinding discs - Google Patents

Description

This invention relates to grinding discs for use in a grinding machine. It relates in particular to two similar discs for use face to face with one another, in concentric relative rotation and close together, for grinding a stream of particles fed centrally into the gap between the discs and passing through said gap in a generally centrifugal manner. Such discs are hereinafter called ’’discs of the type described”.

More particularly still, the invention relates to grinding discs of the type described, for use in comminuting plastics granules with a view to rotational hot moulding of the resulting powder.

In a conventional grinding machine for this purpose there is a fixed disc and a rotating disc. The discs are mounted upright inside a fixed envelope having a bottom outlet through which the ground powder is discharged for collection and subsequent use in moulding. In operation of the machine, the rotating disc is mounted coaxially on the spindle of a motor which can drive it at speeds up to 12,000 rpm, and the fixed disc is secured to the aforesaid envelope and has a central perforation connected to an inlet port for plastics pellets, the port being further connected to a feed hopper (e.g. a 1-ton feed hopper capable of adjustable and continuous pellet feed). The shape of the discs is such that the gap between them is about 4 mm wide in the region of pellet entry into said gap, and only about 35 microns wide in the disc edge region where the ground powder emerges, and said gap has intermediate widths at intermediate locations.

In said conventional machine, the grinding face of each disc is substantially uniformly covered with grinding blades of a triangular cross-section of which the free or exposed apex is sharp, and constitutes a cutting edge. The blades are curved and have a generally radial disposition (the ends of each blade are at respective ends of a respective disc radius).

The plastics raw material commonly supplied for moulding objects has an average particle diameter or size of 5 to 10 mm, and this is found to be too coarse for use in rotational or centrifugal moulding. In fact, a fairly fine plastics powder is desirable.

Conventional grinding machines are expensive to run, and are energy-intensive and inefficient, mainly because of the large size of the motors needed to power them, due in turn to the number of times the plastics material has to travel along the grinding pathway. The conventional machine shears the pellets and produces (a) dust and (b) particles in which one narrow size range predominates. For rotational moulding, however, this is unsatisfactory. What is needed is a range of sizes from about 500 microns down to a dust known as 53fines63, in preferred proportions shown in the table at the end of this specification.

The invention seeks to improve on this situation by providing for use in the conventional machine a grinding disc which will roll the pellets through a number of different cutting operations and grind the material in a number of different directions in a single run to give the required particle size distribution in the product.

The present invention accordingly provides two discs for use face to face with one another, in concentric relative rotation and close together, for grinding a stream of particles fed centrally into the gap between the discs and passing through said gap in a generally centrifugal manner, characterized in that the grinding face of each disc comprises at least three annular bands concentric with the disc's axis of rotation, namely at least two grinding bands each adapted to cooperate with the corresponding respective grinding band of the other disc for grinding the particles, and, disposed between two said bands at least one spreading band adapted to cooperate with the corresponding spreading band of the other disc to disperse the partly ground material around a circular locus defined by said spreading bands, as said material travels from the radially inner grinding band to the radially outer grinding band in the course of being ground.

Preferably the grinding face of each disc comprises three bands, namely two grinding bands spaced apart radially of the disc, and one spreading band which occupies the annular space between the two grinding bands.

Preferably each grinding band comprises a serrated array of approximately radially oriented ridge-like blades closely spaced, each extending across the band from the radially inner edge to the radially outer edge thereof. The blades of one grinding band may diverge by up to about 15° from the radial orientation, or from the orientation of the blades of the other grinding band, as the case may be.

Preferably the or each spreading band comprises a serrated array of approximately circumferentially oriented concentric ridge-like blades, or it may comprise a similar array presented by successive coils of a single, spiral blade disposed like the ridge bordering the groove of a phonograph disc.

Each blade, for preference, is of triangular crosssection having an acute angle (ie a sharp edge) upstanding from the plane of the banded face of the disc.

Preferably the triangular cross-section of the blade is approximately that of a right triangle, having a short side upstanding approximately perpendicularly from the plane of the banded face of the disc, and a hypotenuse side that slopes down to said plane. The short side defines in effect the height of the blade.

Advantageously the slope down of the hypotenuse side of the blade cross-section, in each grinding band, is back from the short side in the trailing direction defined by the intended rotary motion of the disc with respect to the other, similar disc in use.

It is also of advantage if the aforesaid slope, in each spreading band, is back from the short side and down towards the edge of the disc.

The disc is fabricated for preference from high grade tool steel and the blades are formed, again for preference, by milling them into one face of the disc.

Preferably the axial section of the disc incorporates a slope across the bands, of such magnitude that a working arrangement of two similar discs face to face incorporates a gap between the respective radially innermost bands of the discs of approximately 4 mm, and the width of the gap diminishes progressively towards the edges of the discs where it has a final value of about 35 microns. This assembly gives an initial rolling action on the plastics pellets which progresses to a shearing or milling action as the material approaches the disc edges.

All the above statements of invention are intended to apply equally to each member of the pair of discs used for grinding. The fact that, in use, the discs are in rotation one relative to the other is to be understood in the sense that one disc is motor driven and the other (the feed disc) is motionless with respect to the remainder of the machine, other than the motor driven disc.

The feed disc (in use for grinding) has a concentric circular perforation in its central region, through which the material to be ground is fed into a gap provided between the rotating discs.

The invention will be understood in greater detail from the following description of a particular and preferred embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which Fig 1 is a plan or face view of a grinding disc; Fig 2 is a tangential section of the grinding disc of Fig 1, taken along the line II - II in that figure and viewed in the direction of the associated arrows; Fig 3 is a radial section of the grinding disc of Fig 1, taken along the line III - III in that figure and viewed in the direction of the associated arrows; Fig 4 is a detail enlarged from Fig 2, showing a single blade from the inner band; Fig 5 is a detail similar to Fig 4, showing a crosssection of a single blade from the outer band; Fig 6 is a detail enlarged from Fig 3, showing a cross-section of a single blade of the middle or spreading band; Fig 7 is a schematic cross-section of a plastics grinding machine incorporating a pair of grinding discs according to the invention, and Fig 8 is an exploded and enlarged detail of the machine of Fig 7, showing in particular the grinding discs in perspective view, and their respective crosssections.

Referring now to the drawings, a feed disc 10 for grinding (one of a pair of discs for use together in a suitable grinding machine) comprises a flat cylinder of tool steel having a central perforation 2, and steps 3, 4 by which its thickness is reduced at the edge 5. The disc 10 is adapted for mounting in a grinding machine by holes 11 through which bolts or other fixing means can be passed.

The disc 10 comprises a blank face 12 and a grinding face 13 (Figs 2 and 3). Milled into the grinding face 13 are annular bands 14, 15, 16 concentric with the common axis of rotation 17 of the discs in use, namely an inner grinding band 14, a granule-spreading band 15 and an outer grinding band 16.

Figs 2 and 4 show a cross-sectional profile of the blades 17a of the inner band 14, in tangential section, while Figs 3 and 6 show the cross-sectional profile of the blades 17b of the spreading band 15, in radial section, illustrating how the hypotenuse sides of said blades 17b, in section, slope down toward the edge 5. Fig 5, a cross-section of a single blade from the outer grinding band 16, is quite similar to Fig 4, but is included for completeness.

From Figs 4 - 6 it is seen that the short side 18a, b, c of each blade 17a, b, c, in section, makes an internal angle B of 80°“·100°, preferably about 90°, and the hypotenuse side an internal angle A of 20°~40°, preferably about 30°, with the plane of the grinding face 13 of the disc.

Fig 1 shows the blades 17a of the inner annular grinding band 14 as being radially disposed with respect to the disc 10, and those (17c) of the outer annular grinding band 16 as disposed at an angle of 9°-15°, preferably about 12°, to the respective radial direction.

The outer band 16 has an outside diameter of 100-300 mm, preferably about 200 mm, and the inner band 14 an inside diameter of 30-230 mm, preferably about 130 mm. As a result, the width of each of the three bands 14, 15, 16 is about 23 mm. The blades are spaced apart at 2-3 mm centres.

Referring now to Figs 7 and 8, a grinding machine comprises a housing envelope 21 with delivery outlet 22, a feed disc 10 for grinding, secured to the envelope 21 by conventional securing means 23, a pellet feed hopper 24 and feed pipe 25, and a driven grinding disc 20 having a coaxial drive shaft 26 from a motor (not shown) capable of delivering 12,000 rpm to the disc 20 under load.

The driven disc 20 is enclosed in the envelope 21 and secured to the drive shaft 26 which protrudes from said envelope 21 through a snugly-fitting hole. The shaft 26 may alternatively be journalled in bearings in said envelope 21.

The feed pipe 25 communicates with the feed grinding disc 10 through a central hole 2 in the latter. The grinding discs 10, 20 are disposed face to face, but are shaped so that the clearance C between them at the edge of the hole 2 is about 4 mm, and the clearance at the edge region 27 of closest mutual approach of the discs is about 35 microns.

Our research in rotational moulding materials has shown that material with a range of sizes, from 500 microns down to dust known as ’’fines”, is the best material for moulding in terms both of product superiority and moulding cycle efficiency.

The actual proportions of material of different sizes within this range is also important. Shown below are comparative particle size analyses, in which percentages are by weight.

Sieve size required for ideal material achieved with invention 600 0 % 0 % 500 12.0 50 11.8 S3 425 16.0 10 16.2 go 300 29.0 go 29.4 90 212 23.0 10 22.8 19 150 12.0 IQ 12.2 19 ’’fines18 8.0 IQ 7.6 ββ This invention is giving us the material we require to create the best possible moulds and improves our efficiency on our moulding machines by at least 20 %.

According to the invention we have designed a grinding wheel (or disc) which rolls the pellets past a number of different cutting edges and grinds the material in a number of different directions in one run through the pulverising plant resulting in the material specification that we require. The existing grinding discs (of the type described) for mills of its type simply use a standard cutting wheel and shear the pellet and give us a powder that consists mainly of particles of one size only, and dust, and that consequently does not have the qualities of the product ground by the discs of the invention.

The grinding wheel as shown in the drawings works on the principle of rolling the pellet and cutting it in different directions at the same time. The wheel has three sets of cutting edges with three different respective orientations.

The invention is not limited by or to the details of the specific embodiment shown, many of which can undergo wide variation without departing from the scope of the invention.

Claims (7)

1. A disc for use face to face with another similar disc, in concentric relative rotation and close together, for grinding a stream of particles fed centrally of the disc into the gap between the discs and passing through said gap in a generally centrifugal manner, characterized in that the grinding face of the disc comprises at least three annular bands concentric with the disc's axis of rotation, namely at least two particle grinding bands each adapted to cooperate with the corresponding respective particle grinding band of the other disc in grinding the particles, and, disposed between two said bands, at least one particle spreading band adapted to cooperate with the corresponding particle spreading band of the other disc dispersing the partly ground material around a circular locus defined by the or each said particle spreading band, as said material travels from the radially inner grinding band to the radially outer grinding band in the course of being ground.

2. A disc as claimed in Claim 1 wherein the grinding face comprises three bands, namely two particle grinding bands spaced apart radially of the disc, and one particle spreading band which occupies the annular space between the two particle grinding bands.

3. A disc as claimed in Claim 1 or 2 wherein each particle grinding band comprises a serrated array of approximately radially oriented ridge-like blades closely spaced, each extending across the band from the radially inner edge to the radially outer edge thereof.

4. A disc as claimed in Claim 3, wherein each blade is of triangular cross-section having an acute angle (ie a sharp edge) upstanding from the plane of the 5. Banded face of the disc.

5. A disc as claimed in any preceding claim, wherein the axial section of the disc incorporates a slope across the bands, of such magnitude that a working

6. 10 arrangement of two similar discs face to face incorporates a gap between the respective radially innermost bands of the discs of approximately 4 mm, and the width of the gap diminishes progressively towards the edges of the discs where it has a final

7. 15 value of about 35 microns.