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The determination of density distribution in ceramic compacts using autoradiography
.. pwder Techndtogy. 16 (1977) 107 - 122 0 Ekevier Sequoia SA, Lausanne - R&ted
The Determina tion of Density
107 in the Netherlands
Distributions
in Ceramic
Compacts
Using Autoradiography*
H_ M_ MACLEOD UKAEA.
windscale.
Cumbria (Gt. Britain)
K. MARSHALL
Industrial Pharmacy
Linit. Uniuersity of Bmdford.
Bmdford
(Gt. Britain)
(Received March 17.1976: in revised form May 7,1976)
SUM&MARY
The density distribution within ceramic compacts has been examined by contact autoradiography of longitudinal compact sections. This technique has been used to relate changes in compaction conditions to characteristic density patterns_ The variables examined included applied compaction pressure, compact size. feed particle size and lubrication conditionsIn cylindrical compacts of uranium dioxide prepared by single-ended pressure application, the following characteristics could be distinguished: (a) There were three distinct density regions comprising an inner axial region and an outer
peripheral region. each of high density. separated by an annulus of lower density- Changes in the axial region were closely related to applied compaction pressure whilst effects at the periphery were more dependent on lubrication conditions. (b) There was a region of hi& density on the longitudinal axis of the compact in a position remote from the moving punch but coincident with a region of low density at the periphery of the compact. Explanations for the occurrence of these density patterns are offered in terms of die wall frictional effects and relative volume changes.
*Paper presented at the Second International Conference on the Compaction and Consolidation of Particulate Matter, Brighton, England, September 2 - 4, 1975.
1. INTRODUCTION
The densification of powders is an operation of common interest to a wide variety of industries, and many techniques for densification have been developed and optimised to suit particular material properties and product requirements. For the fabrication of precise shapes from powders, the technique most widely employed is that of compressing the powder between loaded punches in a die of appropriate cross-section. Unlike a fluid, which transmits changes in pressure at a point uniformly and instantaneously throughout its mass, a powder subjected to an external force exhibits a characteristic resistance to relative movement, between particles_ When a powder bed of infinite extent is subjected to a static load, the transmitted pressure decreases uniformly as distance from the pressure source increasesIf the powder is confined in a die, however, the pressure is no longer transmitted uniformly through the mass but conforms to a stress pattern imposed by die-wall restraint. As a result of imposed stresses, density gradients are set up within the powder compact which hawe a significant effect on its physical and mechanical proper%ies_ In the processing of ceramic or metallic parts, nonuniform density distribution in the green compact affects the shrinkage behaviour during subsequent sintering treatment and gives rise to undesirable distortion in the sintered product. The ability to control and predict the extent of density variations in compacts, therefore, is a valuable asset to any process employing powder densification.
108
Dlc
FD t
t
FL
applied force is transmitted through the powder to the lower punch where it is measured as transmitted force, FL. The force balance may be expressed as
Wall
FA =FL i-F,
F~
Ez
Fin. l_ Forces operating on a powder under cornpression.
If &, is considered as loss due to friction and FL as force available for densification [2], then FD incorporateslosses duebothtodiewaU friction, fw, and interparticle friction, fi. Each component may be expressed in terms of radial force, & . Where pw is the coefficient of die-wall friction and pr is the coefficient of int.22-particlefriction: &
2. SOURCES
OF STRESS
PATTERNS
The principal source of stress rearrangement in die compaction is die-wall friction, which depends on several variables relating both to the material being compacted and the die set being used. As a result of an externally applied force, FA , several reaction forces are induced in a powder mass confined in a die. Figure 1 shows schematically the forces induced in a cylindrical compact being compressed from one end_ If the material being compressed were a perfect fluid, force would be transmitted isostatically and:
F, =FL =FR
(1)
In a particulate system, however, friction is set up at the material-die wall interface as a result of induced radial force, Fa , whose magnitude relative to applied force, FA, will vary continuously throughout the densification process. In his treatment of radial forces, Long [11 assumed that perfectly elastic conditions obtained up to the material yield point and expressed the reIationship between & and FA as a function of the Poisson ratio, V, of the materid: FR = -f-
l-v
FA
Wheu the elastic limit was exceeded, the relationship between Fa and FA would depend on material yield properties and state of compaction. A proportion of the applied force is transmitted to the die body and appears as diewall reaction force, Fu . The n3naizJer of the
(3)
= (fw + fr)
(4)
= FRI~W + FRPI
(5)
= % (!k- +
(‘5)
t=I)
and expression (3) may be rewritten: FA =FL +&(Pw
+‘I)
(7)
The extent of die-wall and inter-particle frictional forces can be estimated, using these simplified relationships, from external measurements but information obtained by a static balance of forces in this way represents values averaged over the compact. Since Fa and pw :.ave been shown to vary over the compact length [3], the measurements are of limited value. Forces are resolved in terms of their axial and radial components, giving no information about stress trajectories, so that correlation with observed compact properties and, in particular, with internal compact structure is difficult.
3. STUDIES TIONS
OF INTERNAL
STRESS
DISTRIBU-
The basis for much of the -work elucidating stress distributions in compacts was observation of the progr-ssive rezurangement of powder in the die as applied force was increased. The methods adopted ranged from the forming of identifiable layers in the die by a tamp ing technique before compaction [4] to the incorporation in the compact of a soft wire grid whose progressive deformation during compaction could be followed by radiography 153. Train [S] examined in detail the distribution of pressure within a compact by placing maganin wire resistance gauges at uniformly
Fig_ 3. Size distribution by optical microscopy.
Fig_ 2. Relationship between observed pressure distribution and density distribution in compacts (after Train [S] )_
spaced stations in the powder before compaction_ Pressure reactions measured at a range of applied pressures corresponded closely to density levels determined by subsequent accurate sectioning of the compacts on a lathe (Fig. 2). The work reported here examines a technique for studying density and density distribution in compacts of powdered materials possessing natural or induced radioactivity. Autoradiography is widely employed for examining material microstructure in the fields of biology and medicine and is capable of precise, quantitative application. The presence of a radioisotope alters neither the physical nor chemical properties of a material and enables the concentration of material at a point to be accurately determined. Well-defined techniques exist for labelling inert compounds with a wide range of radioisotopes, so that the technique is not limited to the materials described here_ Since die-wall friction is the largest single contributor to density distribution, the variables examined were limited to those affecting die-wall friction. The effects of feed size,
of uranium dioxide
powder
length/diameter ratio, applied force and the presence or absence of lubricant on density distribution were examined in compacts produced by single-ended compaction of powder in a cylindrical die. 4. EXPERIMZNTAL
A_Prepamtionofpressfeed The material used exclusively for t-his study was BNFL ceramic grade uranium dioxide of natural enrichment_ The particle size distribution of the powder was determined by a standard optical microscopy technique 171. The projected uea diameter, d,, of the particles is plotted as a weight distribution in Fig. 3, yielding a median aggregate diameter of 6.5 pm for the distribution_ Seer was determined by a three-point, continuous flow technique, using nitrogen adsorbate in argon carrier gas, and found to be 3-l m* g-l _ Three types of feed were examined during the study and their physical properties are presented in Table 1. (i) Powderfeed Uranium dioxide from the bulk batch was sieved to remove coarse lumps and “fines”. _ Aggregates in the size range 600 - 90 pm were used for compaction_ Powder feed was com_pacted with and without die-wall lubrication_
110 TABLE
1
Chamcterisetion
of uranium
Size range wrn)
PP
(hII% mm2)
16S0 - 1000 IOOO850 850710 TlO350 350210 t210 2317 331.7 231.7 331-i 393.4 370-i 293.4 370.7 293.4 3i0.i 293-4 370.7
2680 - 1000 IOOO600 600210 <2IO 1680 - 1000 1680 - 1000 lOGO600 IOOO600 6QO210 600210 <210 C210
-
600-
90
dioxide
die feeds
Poured density (kg m-s)
Tapped density (kg m-s)
2500 2340 2330
2680
Granules contain 4% by weight methylmethacrylate
2260 2170
25iO 2660
2i30
3.97 4.10 4.20 4.20 3.85 3.91 4.03 4.00 4-06 4.10 3.94 4.12
Dry “pm-slugged” granular feed
3030 2840 2600 2640 2560 2540 2540 2190 2160
2770 2680 2620 2620 2860 2810 2730 2750 2i10 26S0 2790 2670
l-760
2000
5.50
Powder
2520
2100
2550 2800
formed from the powder feed described in (i) with Iength to diameter ratios not exceeding 0.5 at a range of pre-slugging pressures (P,)_ No lubrication was employed during compaction. Compacts were broken down manually with a steel pestle on a coarse screen of approximately 3 mm aperture. The resulting granules were classified by sieving into discrete size ranges of 1680 - 1000 pm, 1000 - 600 pm, 600 - 210 pm and a final fraction containing all material passing through a 210 pm aperture_ Compacts were formed from dry granular feed with and without diewall lubrication. were
(iii] Granuh-
Conditions
3.94 4.09 4.14 3-77 4-11 3-97
2580
(ii) Dry granular feed Compacts
L'=(i)= L’tlt(initlal)
feed with binder
Granuies containing binder/lubricant were formed by a wet-mixing technique followed by screen granuiation. Uranium dioxide powder graded as described in (i) above was blended with 4% by weight of ICI ceramic medium in a Winkworth type I2 Z-blade mixer and granules formed by passing through an Erweka type FGS Screen granulator with a stainless steel mesh of 1680 pm aperture. After air drying at 80 “C for 1 hour, the resulting granules were sieved to remove dust and to classify the
feed
product into the size ranges 1680 - 1000 ym, 1000 - 850 pm. 850 - 710 pm, 710 - 350 Mm. 350 - 210 Mm and a final fraction containing all material passing through a 210 pm aperture. No adc!!ftional lubrication was employed during compaction of this material. B. Tooling,
instrumentation
and press
were compacted in a cylindrical die of 19.05 mm diam. with maximum depth of fill about 140 mm. The stationary lower purch extended into the die bore approximately 12 mm, while the upper punch was 155 mm long to permit a wide variation in compact length. All working surfaces were diainond lapped parallel to the longitudinal die axis to give a nominal surface finish of 50 nm. The original figure for diametral punch clearance in the die was 0.10 + 0.03 mm, and this increased over the reported compaction work to approxima*Yely 0.15 mm. Compaction was carried out using a Denison T42B tensile testing machine with a maximum force capability in compression of 5 X lo5 N. The ram was hydraulic&y operated and ram speed was infinitely variable from zero to 3.8 mm set-l _ E’iezoelectric force transducers were empioyed to measure directly the applied force FA Materials
111
Fig_ 4. Arrangement
(b) ejection_
of press-took and force-transducers during compaction and ejection cycles_ (a) Compaction,
and die reaction L$, during the compaction cycle. The force required for ejection FE was similarly measured during ejection. The transducers used were Kistler type 907B load washers with a measuring range of 4 X lo5 N and a maximum resolution of 0.02 N. The output from the load washers was fed via Vibrometer type TA-3/C calibrated charge amplifiers to a Servoscribe type RE 520.20 twin channel potentiometric recorder. The arrangement of loadwashers during compaction and ejection cycles is shown schemat’.caRy in Fig. 4. C- Compaction procedure Before compaction began, the die bore was cleaned and polished by compacting laboratory tissue paper at a pressure of 154 MN rne2. Traces of material from the previous compaction were completely removed by this technique and the polished surface of the die bore restored_ When die-wall lubrication was required, the tissue was moistened with a solution of 10% w/v stearic acid in trichloroethylene before compaction. This left a thin, uniform film of lubricant on the die wall. When no die-wall lubrication was required, the tissue was moistened with pure trichloroethylene. The upper and lower punches were cleaned by hand with a tissue soaked in trichloroethylene and were not luuricated in any way. A p-weighed .?harge of the material to be compacted was placed in the die and the die gently tapped by hand to settle the material and to dislodge particles of material adhering to the bore_ The upper punch was inserted, the die assembly was placed on the lower
press platen and the moving ram brought to within a few mm of contact. The measurement circuits were energised and force applied to a predetermined level at a measured speed. Dwell time at maximum force was 15 seconds, after which the ram was retracted and the force recorders checked for return to zero before grounding. The compact was ejected from the die in the same direction as the applied force, FA , and the “top” and “bottom” of the compact relative to compaction were carefully identiEed with a steel scriber after ejection_ The force required for ejection, FE, was recorded throughout the ejection cycle.
D_ Compact evaluation Compacts were examined visually during the ejection process and afterwards for visible pressing faults such as lamination, capping and surface scoring. The length and mean diameter of the compact was measure d with a micrometer, the compact was weighed and a mean compact density pm was determined. Where the fragility of compacts produced at low compaction pressures precluded measurement with a micrometer, compact dimensions were determined using a Cambridge Universal Measuring microsccpe. Results for the three types of press feed examined under varying lubrication conditions are presented in Tables 2 - 4. E. Section prepanztion For contact autoradiography a flat longitudinal section through the compact diameter was prepared- Before preparing a section, it
112 TABLE2 Compaction results-umniumdioxidepowderfeed
PA
43
pEC->
Mass
(MNm-')
(MNmm2)
(MNmm2)
(g)
Length (mm)
Diameter (mm)
290.04 283.09 276.14 286.64 279.54 279-38 288.34 279-14 235.98 232-43 235.98 220-23 235.98 23598 229.03 242.93
124.02 11259 167.72 155.37 129.27 X46-87 89-11 62-86 71.66 77.53 71.66 6286 94.36 106.56 122.32 53.90
35.98 38-46 41.67 43.71 65.50 63.12 25-79 14.36 21.62 24-40 17.45 22-08 23.47 29.34 54.83 61.47
30.0 30-O 35.0 35-O 40.0 40.0 25.0 25.0 25.0 25.0 30.0 30.0 35.0 35-O 40.0 40.0
17-78 18.80 21-84 21-59 25.15 25.65 16.00 16.51 15.75 15.49 18.54 18.03 19-30 22-10 26.16 26.42
19.22 19-22 19-22 19.22 19.22 19.22 19.22 19.22 19.22 19.22 19.22 19.22 19.22 19-22 19.22 19.22
349.34 370.35 366.95 380.85 363.40 34595 345.95 359.85 461.22
206.18 171.27 293-44 294.83 185.17 300.54 262.08 286-49 279.54
90.81 55.91 209.58 153.67 66.41 79.38 129.27 143.32 106.56
30.0 25.0 10.0 35.0 25.0 30.0 35.0 40.0 35.0
19.81 16.00 26.92 23.37 16.12 19.83 2863 27-06 20.89
19.23 19.23 19.23 19.23 19.23 19.23 19.23 19.23 i9.25
was necessary to support and strengthen the
compact by embedding it in epoxy resin The mounting medium used was Ciba Araldite MY750 resin with HY951 hardener_ To prevent impregnation of open pores by the resin, the outer surface of the compact was sealed with adhesive celh.zIose fii before mountingThe mounted specimen was clamped to the work arm of a low-speed, diamond-impregnated slitting wheel revolving at approximately 60 rpm and cut along a selected plane using water as coolant/grinding fluid. The surfaces so formed were ground on successively fmer grades of silicon carbide grit paper from 65 pm to 25 pm on a conventional metallographic polishing machine, using water as lubricant. The specimen was examined for scratches and washed with water before changing grades of paper, the duration of grinding on each grade being 7 - 10 minutesF_ Compact
examination
Extensive trials with X-ray film, photo-
HID
Density
Lubricant
(kg mm3)
Relative (a)
0.925 0.978 1.136 1.123 1.309 l-335 0.832 0.859 0.819 C-806 0.965 0.938 l-004 1.150 1.361 1.375
5820 5500 5520 5590 5485 5370 5390 5220 5470 5560 5580 5740 6250 5460 5270 5220
52.9 50.0 50.2 50.8 49.8 48.8 49.0 47.5 49.7 50.5 50.7 52.2 5C8 49.6 47.9 47.5
Wall
1.021 0.832 l-401 1.216 O-838 1.031 l-229 1.407 1.085
5220 5390 5120 5160 5340 5210 5100 5090 5757
47-5 49.0 46.5 46.9 48.5 47.4 46-4 46.3 52.3
Unlubricated
lubricant
graphic film and special nuclear emulsions indicated that fine grained X-ray film gave optimum results for recording the &radiation from U02 [S] . The radiation was of sufficient energy, hc-wearer,to expose both emulsion layers when conventional double-coated X-ray film was employed. A special sample of ultrafine grained, singhz-coated X-ray film, designated Kodak-R X-ray film, was obtained from Kodak Research Laboratories and used exclusively for the quantitative studies reported_ The examin ation technique adopted was simple contact autoradiography. The fihn was placed, emulsion side upwards in a light&&&t box on stiff card. The prepared specimen section was placed in contact with the emulsion and retained in place with 100 g weight. fhe exposure box was stored in the dark during the 24 hour period of exposure, remote from likely sources of vibration_ Exposed film was subjected to a standard developing and fixing treatment recommended by Kodak Ltd and air-dried before evaluation.
PD
43(m)
61.76 90.60 47,32 78,43 23.80 40,33 20471
96.14 101.31 61.47 39,64 104.86 116.37 163,82 122.32 101.31 61.43 79.07 42.32 42A3 62.82 27,BS 22870 26,26 26.79 33.86
77.22 lS4,44 164,44 164A4 77,22 77,22 77,22
164,32 167,37 167.37 X7,37 167,37 160,77 nSO0 237,68 237.08 167a37 160,77 167.37 79,38 70,91 77.99 79,38 76,33 7S,33 79,38 234,29 66.91 36,98 28.73 7.72 16.37 9.11 10,so 14,3S 6.26 2,78 SA7 4.94 14,36
20.23 24,40 11,12 6,SS 22.70 37,37
11502 27,63 8.20 16.70 3.42 6.03 2,22 231,7 231,7 231.7 231.7 293,4 370,7 231,7 293,4 370.7 231.7 293,4 370,7 293,4 370.7 293,4 370,7 231,7 231,7 293.4 370,7
308.9 308,9 308.9 30&O
308,9
308,9 308.9
PP (MN me2) (MNmw2) (MNmw2) (MNmw2)
PA
1680 - 1000 1000 - 600 600. 210 t210 16f30~1000 1630 - 1000 1630 - 1000 1630 ’ 1000 1680 - 1000 1680 - 1000 1680 - 1000 1680 - 1000 1000. 600 1000. 600 600. 210 600. 210 1000 - 600 600. 210 (210 (210
2f300~1680 2800~1680 1680.1000 1680.1000 1000. 710 710. 210 (210
Sizerange (pm)
Compactionrcsull8-urnnium dioxidodry gronulnrfeed
TABLE3
40.0 40.0 40.0 30.0 40.0 40.0 GO.0 60.0 60.0 60.0 30.0 40.0 GO.0 60.0 40.0 40.0 26.8 26.8 26.0 2G,O
39.60 47.60 27.20 41,30 29.80 61.73 16.40
(g)
26,70 26,91 27.18 19,14 24,64 26.72 28,SS 31,12 30094 32861 l&69 26,lO 33.43 32,33 27806 26,76 l&24 l&44 17,40 l&21
29,17 28.24 1952 24.67 24,30 42,115 12.36
(mm)
19.18 19818 19,18 19,18 19,18 19.18 19,18 19,18 19,18 19.18 19.18 19,18 19.16 19,lG 19816 19,lG 19.15 19,lG 19,13 19,18
19.15 19018 19,18 19,18 19,lS 19.18 19,lS
(mm)
1.340 1.3Gl 1,417 0,998 1,279 1,341 1,489 1.623 1,613 1,696 0,974 1.309 1.746 1.688 1,413 1,397 0,9G2 0,903 0,910 0,793
1.623 1.472 1.018 1,231 1.269 2,200 0,646
MllfJE Length Diamotcr H/D
6390 5340 6090 6420 6640 6380 6060 SS60 SG90 6320 6660 GS20 Ii190 6370 6130 6190 6100 6060 6000 G690
4700 6820 G860 6820 4260 4260 4330 49.0 48.6 46,3 49,3 s1,3 48,9 66.1 60.G SO.8 48.4 SO.5 so.2 47.2 48,8 46.6 47,2 49.4 46.9 46,4 61,7
42.7 62.9 63.3 62,9 38.7 30.7 39.4
(kg rnD3) Relntivo(%)
Density
Walllubricant
Unlubricated
Lubricant
E
114 TABLE
4
Compaction
results-uranium
PA
pD
(MN m-* )
(MN m-*)
231.66 231.66 231-66 231.66 231.66 231.66 463.32 463.32 463.32 163.32 463.32 463.32 401.82 401.82 4os.s1 401.82 489-18 42279 401.82 398.33
99.56
98.13 98-21 96-63 9’7.94 98.11 178.32 176.80 177.20 1’13.69 178.41 Iii-90 110.46 112.51 139.i6 139.76 L65_97 157-24 181.69 176.45
dioxide granular feed with binder Size range
Mass
Length
Diameter
(MN m-*)
(pm)
(fz)
(mm)
(mm)
20.76 18.34 21.69 19.44 22.ii 21.38 32.18 30.47 32.13 30-78 31.16 30.90 20.79 20.i9 24.11 25.16 33.54 35-64 41.93 41.23
1680-1000 lOOO850 850710 710350 350210 c210 1680-1000 lOOO850 850710 710350 350210 t210 PlO210 710210 ilO210 710210 7x0210 710210 710210 7PO210
35.5 35.1 35.1 34.7 35-l 35.1 35.3 35.3 35.1 34.8 35.2 35.0 25-O 25.0 30.0 30.0 35.0 35-O 40.0 40.0
21-30 21.06 21.01 20.73 21-04 21.03 19.53 19.51 19.39 19-17 19.45 19.28 13.49 13.40 16.04 16.14 18-71 1x73 21.73 21-68
19.23 19.23 19.23 19.23 19.23 19.23 19.26 19.26 19.26 19-26 19.26 19.26 19.25 19.25 19.26 19.26 19.26 19-26 19.26 19.26
PEt-)
For quantitative autoradiography. the Kodak-R ffirn was calibrated by exposing under standard conditions a series of uranium dioxide compacts prepared to cover the range of density variation expected. Seven discs, with length to diameter ratios in the range 0.31 to 0.38 according to the level of appiied pressure, were prepared by compacting graded powder feed described in A(i) with stearic acid die-wail lubricant. Relative densities, deduced from mean compact densities determined by the geometric technique described in D, ranged from 43.00 to 52.01%. The film was exposed in contact with the top surface of each compact and a graph constructed relating the optical density of the resulting autoradiogram to the mean relative density of the compact (Fig. 5). This calibration graph was used for all evaluations of density carried out on the test compacts, allowance being made for the presence of binder in the case of internally lubricated specimensG. Analysis
of automdiograms
density distribution of autoradiagrams was evaluated using a Joyce-Loebl Chromoscan 5121 reflectance densitometerA standard scanning aperture of rectangular The
optical
H/D
l-108 1.095 1.093 1.078 1.094 1.094 1.014 1.013 1.007 0.995 l-010 1.001 0.701 0.696 0.833 0.838 0.971 0.972 1.128 1.126
Density (kg rnm3)
Re!ative (%)
5740 5740 5750 5760 5740 5750 6200 6210 6210 6230 6210 6230 6368 6410 6420 6380 6421 6414 6318 6333
54-4 54.4 54.5 54.5 54-4 54.5 58-7 58.8 58.8 59.0 58.8 59.0 60.3 60.7 60.8 60.4 60.8 60-7 59.8 60.0
Fig. 5_ Relationship between mean compact density and optical density of autoradiograms from UO2 standards (Kodak R, X-ray film).
shape was employed, of length 1.0 mm and width 0.5 mm. Signal gain was constant at 50% of amplifier output and pen amplification was varied to suit autoradiogram conditions. A typical trace of optical density from a longitudinal specimen section is shown schematically in Fig_ 6. Optical density values were recorded at regular intervals along the scan from a reference point, a, at the top of the compact. Be cause of edge effects, point a was placed 3 mm from the edge of the autoradiogram, point (i). and a similar limitation was placed on the final value, point h_ To eliminate the effects on film density of possible variations
115
Fig. 6. Evaluation
of optical density of autoradiograms.
Fig. 7. Schematic position of densitometer autoradiogram of compact.
scans on
Fig. 8. Schematic densitometer sca~ns of compact ing position of density readings.
show-
in processing or esposure conditions, the optical density of each autoradiogram was measured relative to that of the film base, which was taken as zero for comparative purposes_ The method of building up a density cliagram of the compact cross-section is shown schematically in Fig. 7. The optical density of the autoradiogram was determined along 5 parallel scans es shown, with scan 1 centred on the longitudinal axis and the others separated by 3 mm from each other. The recorded optical density traces were of the form shown in Fig- 8. The displacement from the baseline was measured at fixed intervals along the trace and the value of optic-al density related to compact density using the calibration curve (Pig. 5). The observed density figure was entered on a grid plan of the compact cross-section with points ccrresponding to the points measured on the recording chart (Fig. 9). Points with the same density value were joined to form a series of density contours on the compact cross-se&ion and the pattern so formed used to analyse compaction behaviour. 5. PRESSURE
RELATIONSHIPS
Ohsewed values of applied force, FA, and axial die-wall reaction. FD f were tabulated and
Fig. 9. Grid reference system superimposed tudinal section of compact_
on iongi-
the force transmit+ted through the compact to the lower punch, FL, deduced by difference: FL = FA -FD
@I
Values were expressed as pressures and graphs of PA uersLtsPL plotted for a range of compaction conditions with values of applied pressure of 15 - 400 MN me2 (Figs 10 - 12). Straight lines were fitted to the results by a least-squares regression analysis_ Relevant parameters of the regression equations are shown in Table 5. In most cases correlation coefficients of O-99 or better were obtained, although, in common with results reported by Nelson and co-workers [9J ) more scatter was apparent at high values of applied pressure. The value of the regression coefficient was dependent on compact weight and lubrication conditions. The proportion of applied pressure transmitted to the lower punch decreased as compact weight increased with constant diameter. For each condition of lubrication examined, the relationship between PL and PA was described by
(9)
100
200 PL
0
(10) where H is compact length, D is compact diameter and M is compact weight. These relationships are in agreement with those observed by Nelson and co-workers [9] for the densification of pharmaceutical materials. Results are plotted in Fig. 13. The relationship between final compact density Pm and applied pressure, PA, was found to be a complex one- As expected, the compact density increased as the ratio of transmitted pres==, PL, over applied pressure, PA, incred, ix. the lower the frictional losses the more efficient b ecame the work of compaction. At low values of H/D ratio in the range 0.3 - 0.4, a simple relationship was oh served of the form: = K3pm
as pIott&
in Fig- 14_
2a3 PL
Fig. 10. Relationship between applied pressure and pressure transmitted to the lower punch during uranium dioxide compaction (combined internal-diewall lubricant).
InP,
143
(HKm-2)
W
(iiui~
Fig_ 11. Relationship between applied pressure and pressure transmitted to the iower punch during uranium dioxide compaction (die-wall lubrication).
As frictiona! forces increased owing to increase in sample height or to inferior conditions of lubrication, +he relationship corresponded to:
I+
A
=K.&j&
Results are plotted in Fig. 15. The size of the feed particle had little effect on either pressure transmission through the compact (Fig. 16) #Jr final compact density (Fig. 17 j. Conditions of lubrication had a significant effect on pressure transmission throughout the compact. as is apparent from results in Figs- 10 - 12. The use of an internal binder/lubricant gave optimum results for pressure transmission. Die-wall lubricant, although present in much smaller quantity, gave only slightly inferior results, while severe frictional problems were encountered with unlubricated material_
_I
--
-_
_-
_
_-
_ _-
: --
_
TyBLE5_:-. PA =tlPL+@
25
Wall lubrication 1.3974
.
A
:
_
_ _
. - _ Nolubtkation
lkgreesof
-.
freedom
(vi)
Correlation
- coefficient
(C)
Standard deviation
25 22 25 24
0.9975 0.9959 0.9910 0.9976
2190 2.609 3.757 2.012
2.3585 1.9229
4.4765 4.6158 x x lO-3 lo-”
11 12
0.9990 0.9938
3.210 1.142
3.7419 4.3652
-7.4512x X0-* -4.2014 x 1O-2
I1 13
0.9846 0.9905
5.065 4.306
17 16 17 18
0.9992 0.9991 0.4966 0.9981
1.311 1.413 2.875 1.960
1.4246 1.5718 1.6273 1.9010
Internal/wall lubrication
B-
2.786 x 1O-2 --1.8957 x~O-~ 4-674 ~-lo-~ -2.2931x 1O-2
l-61911.6710 1.9437.
35 40 25 30 35 40
117-
.
_.
Lubrication condiiions
::
t
7.226 .7.912 1.705 1.194
x x x x
lo-* 1O-2 10-l 10-l
Fig. 13. Relationship between pressure ratio pi/P~ and uranium diorside compact H/D ratio, for varying Iubrkation conditions_
ratio, lubrication conditions and to a lesser extenton compaction prassure_Forconstant compactionconditionsitwas observedthatr OY 0
IcD @df= 9
Fig. 12. Relationship between applied pressure and pressure transmitted to the lower punch during uranium dioxide compaction (unlubricated).
The maximum pressure required for l$e ejection of each compact from the die; Ps , was recorcied and is shown in Tables 2 - 4. The value of PC varied directly with die-wall frictional _ forces and was dependent on compact H/D
-_
-
-
m.
lg) 30 35 40
-_ ._-
regrekodqf PA on Pt (Figs_ IO -_12). Parameters for linear regression of tbe form
hranetersofth&s~tight&
Mass
--
-.
-
--_ ._
-
_
-
_
-
:_ .
_
_. _
7
-_
The relationship is plotted for specified compaction conditions in Fig. 18. The profile of the plot of PE versus time was similar for all conditions examined, and is shown schematically in Fig. 19. As pressure was applied, it rose steeply to a maximum at point A, which corresponde@ to the first compact movement. This was the recorded value of Pe and defined “static” ejection pressure_
(-
I 20
-n 40
42
44
RclDtnr
4b Casmtr
58
50
52
POrML-
(w)
Kg. IT. Effect of particle size of feed on compact relative density_
&)
Fig_ 14. Re!ationship between applied pressure and elative density for uranium dioxide calibration specimens (compacr H/D ratio 0.31 - 0.38).
,,I 45
101 25 50
55
Fig_ 15. Relationship between p-ure ratio PL/PA and relative density of uranium dioxide compacts (compact H/D ratios In excess of 0.8).
After movement was initiated, the pressure fell to a new value which remained almost constant throughout the ejection process until appro ximately half the compact was ejected from the die at point B_ From this point the
z-
I 40
-@I Fig. 18. Relationship between ejection mass of uranium dioxide compacts.
Fig. 19. Schematic time_ Fig_ 16. Effect of partick size of feed on the pressure ratio PI/PA for a range of compact weights.
I
30
profile of ejection
45
pressure and
pressure with
ejection pressure decreased steadily to point C, corresponding to final compact ejection, where it fell abruptly to zero. The values of A, B and C were dependent on compact H./D, lubrication conditions and compaction pressure. Point B, which defines dynamic ejection pressure, was additionally dependent on rate of pressure application.
50
Fig. 20_ Density distribution patterns for uranium dioxide compacts of varying H/D ratio. Die-wail lubrication employed.
6. DENSITY
Fig. 21_ Density distribution patterns for uranium dioxide compacts of varying -ratio. No lubricant employed.
3A’iTERNS
The patterns of density distribution were found to be sensitive to changes in compact H/D ratio. Detectable density gradients were observed in a compact of H/D ratio 0.8, the lowest value examined, prepared at an applied pressure of 80 MN mm2 in the presence of diewall lubricant. As compact H/D ratio increased, the range of observed density variation increased and areas of high and low density in the compact became more clearly separated. Density patterns observed for a range of compact H/D ratios are presented in Fig_ 20. Compaction pressures were all within the range 75 - 80 MN mm2 and die-wall lubrication was employed. As die-wall frictional forces increased, the extent and intensity of density variation within the compact increased. Figure 21 shows the density patterns observed for compacts of varying H/D ratio prepared without lubricant. Compaction pmssures varied from 157 to 160 MN me2. Compacts produced without lubricant at pressures of 75 - 80 MN mS2 proved to
s -
4ol-62
n/D
= I-lza
hlN.m-2
%-
390-3
IUD
- I-l26
hsil-~
Fig. 22_ Density distribution patterns for uranium dioxide compacts containing combined intemal-diewall lubricant.
be too friable to provide suitable sections for autoradiography. The most uniformly dense compacts were obtained using the combined binder/internal lubricant system. There was little evidence of significant variation in density in compacts with H/D ratios of l-13, despite the use of compaction pressures up to 400 MN mm2_ The density patterns obtained for the two largest compacts examined are shown in Fig. 22. The variation of compaction pressure examined in this work was relatively wider than that of any other parameter, and hence it had the most significant effect on density distribu-
Fig. 23. Density patterns for uranium dioxide compacts prepared at a range of compaction pressures. Die-wall lubricant employed_
tion in compacts. Whereas the principal effect of increased frictional forces, induced either by inferior lubrication conditions or by increase in compact H/D ratio, was to intensify density gradients, increase in compaction pressure produced distinct changes in density pattern. The effect of compaction pressure on density cik ttibution in compacts is shown in Fig. 23 for
compacts prepared using die-wall lubricant and a sttdard compact weight.
7_ DISCUSSION
The density patterns observed had several features in common. Each compact exhibited three density zones consisting of an inner axial region and an outer peripheral zone separated by an armulus of lower density. The inner zone structure was more dependent on applied pressure than the outer zone, which was markedly affected by lubrication conditions. In the outer density zone the maximum density was observed adjacent to the moving
punch. All compacts showed a discontinuity in the outer zone pattern at a distance fmm the stationary punch dependent on lubrication conditions but independent of applied compaction pressure and compact H/D ratio_ In the case of die-wall lubrication this distance was approximately 0.2 H, and for unlubricated compacts increased to 0.3 H. The discontinuity is interpreted as marking the limit of material movement at the die wall. Analysis of the density patterns observed by Train [S] in the compaction of magnesium carbonate show a similar phenomenon. In the case of magnesium carbonate the corresponding distance from the stationary punch was approximately 0.5 H in the lubricated state and 0.67 H for tiubricated compa&. The inference from these observations is that more slip occurred at the die wall in the case of uranium dioxide than in the case of magnesium carbonate, an unexpected result in view of the more abrasive nature of the former. An explanation for increased movement on the part of uranium dioxide may be found, however, from consideration of relative volume changes during compaction. The average relative volume, Vxa,, at die fill was found to be 2.66 for magnesium carbonate and 5.5 for uranium dioxide. In undergoing densification to an arbitrary relative density of 60% i.e. to a relative volume of 1.67, the ratio of Vrci, to V,,,,, is 3.3 in the case of uranium dioxide and 1.59 in the case of magnesium carbonate. For a cylinder:
v, =
V observed K
= H/w,
where H is compact height and H, is derived compact height for V, = 1. Then the relative movement of an element of powder in similar compacts of uranium dioxide and magnesium carbonate is defined by: Movement of uranium dioxide Movement of magnesium carbonate
3.3 C-S 1.59 2.07
During compaction, therefore, uranium dioxide undergoes twice as much movement as magnesium carbonate to achieve equal relative density. In the axial zone, the maximum density was observed in the lower half of the compact at a
121
Fig. 24. Schematic development of compact density pattern= Arrows indicate direction of compacting forces.
point close to the stationary punch. Its exact position was dependent on compaction conditions, but in general its centre coincided with the discontinuity in the peripheral density zone. Minimum density was observed adjacent to the stationary punch. A second area of low density appeared on the compact axis close to the moving punch, but its extant and position relative to the moving punch varied with compaction conditions. The density structure observed in the axial zone was also observed experimentally by Kamm and co-workers [Xl] and by Train [6], and predicted mathematically by Shaler [ll] . In this work, however, a second region of high density appeared ~~1 the compact axis as frictional forces increased_ Its position varied with compaction conditions but was always between the primary high density region and the moving punch. The maximum density observed for this secondary zone never exceeded that of the primary region under the compaction conditions examined. The density patterns observed by Train [S] for magnesium carbonate compacts showed
similar axial and peripheral density zones. However, in the case of magnesium carbonate, the zones were more widely separated, and in particular the peripheral zone was Iess extensive than that observed for uranium dioxide compacts (Fig. 2). Interaction of axial and peripheraI density zones was observed in uranium dioxide compacts at low relative densities (Figs. 20 - 23). As a result, outward movement of material from the axial zone was restricted and axial density gradients were intensified. The explanation of the development of the density pattern advanced by Train [S] falls short of explaining tbe results obtained in this work. The pressure-bulb concept of Boussinesq 1121 describes conditions existing briefly at avery early stage of compaction (Fig. 24a). Immediately material begins to move at the die wall, adjacent to the upper punch it dilates and expands away from the die wall. As frictional forces develop and the shear strength of the material is exceeded, densification begins, and the process is repeated continuously in successive elements to the limit of material movement. The extent of material movement is governed by applied compaction pressure and by the resistance to inward movement produced by the densification and consequent tendency to outward movement of material at the axis of the compact (Fig. 24b). Material on the compact axis is driven down into the unconsolidated material below it, exhibiting a hemispherical pressure front as predicted by Boussinesq [12] (Fig. 24~). This process persists until the reaction from tbe lower punch exceeds the radial reaction from the die wall. Material from the centre begins to move outwards at the point of minimum radial pressure, which coincides approximately with the limit of material movement at the die wall (Fig_ 24d)_ This material has undergone less consolidation than material in the lower comers of the compact because of the predominance of the axial pressure component in the pressure front at this stage. The change of direction of powder movement induces locally high shearing forces and contributes to the formation of a high density area centred on this point (Fig. 24e)_ The low density areas observed on the compact axis adjacent to the moving punch and in the peripheral density zone adjacent to the stationary punch correspond to areas where negligible powder movement has occurred and where shear forces are minimal.
122
Development of the secondary pattern in the axial density zone m;?y be attributed to a mechanism similar to that causing formation of the primary high density area. In this case the outward material movement and consequent densification is initiated when the axial reaction from the area of primary high density exceeds the radial pressure at any point between it and the moving punch (Fig. 24f). REFERENCES 1 W. hI_ Long. Powder bIetall_. 6 (1960) 73. 2 P_ J_ James, Powder h:et.?ll_ Irk., 4 (2) (1972)
145
3 D. Train and J. k Hersey, Ind. Chem.. 38 (1962) 77. 4 M. Yu. Bal’shin. Vestn. Metalloprom.. 18 (1938) 124. 5 R P. Seelig and J. Wulff, Trans. AIME, 171 (1946) 516. 6 D. Train. Trans- Inst Chem. Eng. London. 35 (1957) 258. 7 B.S. 3406. Part IV (1961). S H_ M. Macleod and IC_ M&hall. unpublished work_ 9 E. Nelson, S. M. Naqvi, L. W_ Busse and T. Higuchi, J_ Am Pharm. AssSci. Ed. 43 (1954) 596_ 10 R Kamm. M_ A_ Steinberg and I Wulff. AIME. 180 (1949) 694. 11 A. J. Shaler, Trans. AIME, 171 (1947) 521. 12 J. Boussinesq. Memoires couronnes et memoires des savan‘ts i&angers, B-els. 1876_
The Determina tion of Density
107 in the Netherlands
Distributions
in Ceramic
Compacts
Using Autoradiography*
H_ M_ MACLEOD UKAEA.
windscale.
Cumbria (Gt. Britain)
K. MARSHALL
Industrial Pharmacy
Linit. Uniuersity of Bmdford.
Bmdford
(Gt. Britain)
(Received March 17.1976: in revised form May 7,1976)
SUM&MARY
The density distribution within ceramic compacts has been examined by contact autoradiography of longitudinal compact sections. This technique has been used to relate changes in compaction conditions to characteristic density patterns_ The variables examined included applied compaction pressure, compact size. feed particle size and lubrication conditionsIn cylindrical compacts of uranium dioxide prepared by single-ended pressure application, the following characteristics could be distinguished: (a) There were three distinct density regions comprising an inner axial region and an outer
peripheral region. each of high density. separated by an annulus of lower density- Changes in the axial region were closely related to applied compaction pressure whilst effects at the periphery were more dependent on lubrication conditions. (b) There was a region of hi& density on the longitudinal axis of the compact in a position remote from the moving punch but coincident with a region of low density at the periphery of the compact. Explanations for the occurrence of these density patterns are offered in terms of die wall frictional effects and relative volume changes.
*Paper presented at the Second International Conference on the Compaction and Consolidation of Particulate Matter, Brighton, England, September 2 - 4, 1975.
1. INTRODUCTION
The densification of powders is an operation of common interest to a wide variety of industries, and many techniques for densification have been developed and optimised to suit particular material properties and product requirements. For the fabrication of precise shapes from powders, the technique most widely employed is that of compressing the powder between loaded punches in a die of appropriate cross-section. Unlike a fluid, which transmits changes in pressure at a point uniformly and instantaneously throughout its mass, a powder subjected to an external force exhibits a characteristic resistance to relative movement, between particles_ When a powder bed of infinite extent is subjected to a static load, the transmitted pressure decreases uniformly as distance from the pressure source increasesIf the powder is confined in a die, however, the pressure is no longer transmitted uniformly through the mass but conforms to a stress pattern imposed by die-wall restraint. As a result of imposed stresses, density gradients are set up within the powder compact which hawe a significant effect on its physical and mechanical proper%ies_ In the processing of ceramic or metallic parts, nonuniform density distribution in the green compact affects the shrinkage behaviour during subsequent sintering treatment and gives rise to undesirable distortion in the sintered product. The ability to control and predict the extent of density variations in compacts, therefore, is a valuable asset to any process employing powder densification.
108
Dlc
FD t
t
FL
applied force is transmitted through the powder to the lower punch where it is measured as transmitted force, FL. The force balance may be expressed as
Wall
FA =FL i-F,
F~
Ez
Fin. l_ Forces operating on a powder under cornpression.
If &, is considered as loss due to friction and FL as force available for densification [2], then FD incorporateslosses duebothtodiewaU friction, fw, and interparticle friction, fi. Each component may be expressed in terms of radial force, & . Where pw is the coefficient of die-wall friction and pr is the coefficient of int.22-particlefriction: &
2. SOURCES
OF STRESS
PATTERNS
The principal source of stress rearrangement in die compaction is die-wall friction, which depends on several variables relating both to the material being compacted and the die set being used. As a result of an externally applied force, FA , several reaction forces are induced in a powder mass confined in a die. Figure 1 shows schematically the forces induced in a cylindrical compact being compressed from one end_ If the material being compressed were a perfect fluid, force would be transmitted isostatically and:
F, =FL =FR
(1)
In a particulate system, however, friction is set up at the material-die wall interface as a result of induced radial force, Fa , whose magnitude relative to applied force, FA, will vary continuously throughout the densification process. In his treatment of radial forces, Long [11 assumed that perfectly elastic conditions obtained up to the material yield point and expressed the reIationship between & and FA as a function of the Poisson ratio, V, of the materid: FR = -f-
l-v
FA
Wheu the elastic limit was exceeded, the relationship between Fa and FA would depend on material yield properties and state of compaction. A proportion of the applied force is transmitted to the die body and appears as diewall reaction force, Fu . The n3naizJer of the
(3)
= (fw + fr)
(4)
= FRI~W + FRPI
(5)
= % (!k- +
(‘5)
t=I)
and expression (3) may be rewritten: FA =FL +&(Pw
+‘I)
(7)
The extent of die-wall and inter-particle frictional forces can be estimated, using these simplified relationships, from external measurements but information obtained by a static balance of forces in this way represents values averaged over the compact. Since Fa and pw :.ave been shown to vary over the compact length [3], the measurements are of limited value. Forces are resolved in terms of their axial and radial components, giving no information about stress trajectories, so that correlation with observed compact properties and, in particular, with internal compact structure is difficult.
3. STUDIES TIONS
OF INTERNAL
STRESS
DISTRIBU-
The basis for much of the -work elucidating stress distributions in compacts was observation of the progr-ssive rezurangement of powder in the die as applied force was increased. The methods adopted ranged from the forming of identifiable layers in the die by a tamp ing technique before compaction [4] to the incorporation in the compact of a soft wire grid whose progressive deformation during compaction could be followed by radiography 153. Train [S] examined in detail the distribution of pressure within a compact by placing maganin wire resistance gauges at uniformly
Fig_ 3. Size distribution by optical microscopy.
Fig_ 2. Relationship between observed pressure distribution and density distribution in compacts (after Train [S] )_
spaced stations in the powder before compaction_ Pressure reactions measured at a range of applied pressures corresponded closely to density levels determined by subsequent accurate sectioning of the compacts on a lathe (Fig. 2). The work reported here examines a technique for studying density and density distribution in compacts of powdered materials possessing natural or induced radioactivity. Autoradiography is widely employed for examining material microstructure in the fields of biology and medicine and is capable of precise, quantitative application. The presence of a radioisotope alters neither the physical nor chemical properties of a material and enables the concentration of material at a point to be accurately determined. Well-defined techniques exist for labelling inert compounds with a wide range of radioisotopes, so that the technique is not limited to the materials described here_ Since die-wall friction is the largest single contributor to density distribution, the variables examined were limited to those affecting die-wall friction. The effects of feed size,
of uranium dioxide
powder
length/diameter ratio, applied force and the presence or absence of lubricant on density distribution were examined in compacts produced by single-ended compaction of powder in a cylindrical die. 4. EXPERIMZNTAL
A_Prepamtionofpressfeed The material used exclusively for t-his study was BNFL ceramic grade uranium dioxide of natural enrichment_ The particle size distribution of the powder was determined by a standard optical microscopy technique 171. The projected uea diameter, d,, of the particles is plotted as a weight distribution in Fig. 3, yielding a median aggregate diameter of 6.5 pm for the distribution_ Seer was determined by a three-point, continuous flow technique, using nitrogen adsorbate in argon carrier gas, and found to be 3-l m* g-l _ Three types of feed were examined during the study and their physical properties are presented in Table 1. (i) Powderfeed Uranium dioxide from the bulk batch was sieved to remove coarse lumps and “fines”. _ Aggregates in the size range 600 - 90 pm were used for compaction_ Powder feed was com_pacted with and without die-wall lubrication_
110 TABLE
1
Chamcterisetion
of uranium
Size range wrn)
PP
(hII% mm2)
16S0 - 1000 IOOO850 850710 TlO350 350210 t210 2317 331.7 231.7 331-i 393.4 370-i 293.4 370.7 293.4 3i0.i 293-4 370.7
2680 - 1000 IOOO600 600210 <2IO 1680 - 1000 1680 - 1000 lOGO600 IOOO600 6QO210 600210 <210 C210
-
600-
90
dioxide
die feeds
Poured density (kg m-s)
Tapped density (kg m-s)
2500 2340 2330
2680
Granules contain 4% by weight methylmethacrylate
2260 2170
25iO 2660
2i30
3.97 4.10 4.20 4.20 3.85 3.91 4.03 4.00 4-06 4.10 3.94 4.12
Dry “pm-slugged” granular feed
3030 2840 2600 2640 2560 2540 2540 2190 2160
2770 2680 2620 2620 2860 2810 2730 2750 2i10 26S0 2790 2670
l-760
2000
5.50
Powder
2520
2100
2550 2800
formed from the powder feed described in (i) with Iength to diameter ratios not exceeding 0.5 at a range of pre-slugging pressures (P,)_ No lubrication was employed during compaction. Compacts were broken down manually with a steel pestle on a coarse screen of approximately 3 mm aperture. The resulting granules were classified by sieving into discrete size ranges of 1680 - 1000 pm, 1000 - 600 pm, 600 - 210 pm and a final fraction containing all material passing through a 210 pm aperture_ Compacts were formed from dry granular feed with and without diewall lubrication. were
(iii] Granuh-
Conditions
3.94 4.09 4.14 3-77 4-11 3-97
2580
(ii) Dry granular feed Compacts
L'=(i)= L’tlt(initlal)
feed with binder
Granuies containing binder/lubricant were formed by a wet-mixing technique followed by screen granuiation. Uranium dioxide powder graded as described in (i) above was blended with 4% by weight of ICI ceramic medium in a Winkworth type I2 Z-blade mixer and granules formed by passing through an Erweka type FGS Screen granulator with a stainless steel mesh of 1680 pm aperture. After air drying at 80 “C for 1 hour, the resulting granules were sieved to remove dust and to classify the
feed
product into the size ranges 1680 - 1000 ym, 1000 - 850 pm. 850 - 710 pm, 710 - 350 Mm. 350 - 210 Mm and a final fraction containing all material passing through a 210 pm aperture. No adc!!ftional lubrication was employed during compaction of this material. B. Tooling,
instrumentation
and press
were compacted in a cylindrical die of 19.05 mm diam. with maximum depth of fill about 140 mm. The stationary lower purch extended into the die bore approximately 12 mm, while the upper punch was 155 mm long to permit a wide variation in compact length. All working surfaces were diainond lapped parallel to the longitudinal die axis to give a nominal surface finish of 50 nm. The original figure for diametral punch clearance in the die was 0.10 + 0.03 mm, and this increased over the reported compaction work to approxima*Yely 0.15 mm. Compaction was carried out using a Denison T42B tensile testing machine with a maximum force capability in compression of 5 X lo5 N. The ram was hydraulic&y operated and ram speed was infinitely variable from zero to 3.8 mm set-l _ E’iezoelectric force transducers were empioyed to measure directly the applied force FA Materials
111
Fig_ 4. Arrangement
(b) ejection_
of press-took and force-transducers during compaction and ejection cycles_ (a) Compaction,
and die reaction L$, during the compaction cycle. The force required for ejection FE was similarly measured during ejection. The transducers used were Kistler type 907B load washers with a measuring range of 4 X lo5 N and a maximum resolution of 0.02 N. The output from the load washers was fed via Vibrometer type TA-3/C calibrated charge amplifiers to a Servoscribe type RE 520.20 twin channel potentiometric recorder. The arrangement of loadwashers during compaction and ejection cycles is shown schemat’.caRy in Fig. 4. C- Compaction procedure Before compaction began, the die bore was cleaned and polished by compacting laboratory tissue paper at a pressure of 154 MN rne2. Traces of material from the previous compaction were completely removed by this technique and the polished surface of the die bore restored_ When die-wall lubrication was required, the tissue was moistened with a solution of 10% w/v stearic acid in trichloroethylene before compaction. This left a thin, uniform film of lubricant on the die wall. When no die-wall lubrication was required, the tissue was moistened with pure trichloroethylene. The upper and lower punches were cleaned by hand with a tissue soaked in trichloroethylene and were not luuricated in any way. A p-weighed .?harge of the material to be compacted was placed in the die and the die gently tapped by hand to settle the material and to dislodge particles of material adhering to the bore_ The upper punch was inserted, the die assembly was placed on the lower
press platen and the moving ram brought to within a few mm of contact. The measurement circuits were energised and force applied to a predetermined level at a measured speed. Dwell time at maximum force was 15 seconds, after which the ram was retracted and the force recorders checked for return to zero before grounding. The compact was ejected from the die in the same direction as the applied force, FA , and the “top” and “bottom” of the compact relative to compaction were carefully identiEed with a steel scriber after ejection_ The force required for ejection, FE, was recorded throughout the ejection cycle.
D_ Compact evaluation Compacts were examined visually during the ejection process and afterwards for visible pressing faults such as lamination, capping and surface scoring. The length and mean diameter of the compact was measure d with a micrometer, the compact was weighed and a mean compact density pm was determined. Where the fragility of compacts produced at low compaction pressures precluded measurement with a micrometer, compact dimensions were determined using a Cambridge Universal Measuring microsccpe. Results for the three types of press feed examined under varying lubrication conditions are presented in Tables 2 - 4. E. Section prepanztion For contact autoradiography a flat longitudinal section through the compact diameter was prepared- Before preparing a section, it
112 TABLE2 Compaction results-umniumdioxidepowderfeed
PA
43
pEC->
Mass
(MNm-')
(MNmm2)
(MNmm2)
(g)
Length (mm)
Diameter (mm)
290.04 283.09 276.14 286.64 279.54 279-38 288.34 279-14 235.98 232-43 235.98 220-23 235.98 23598 229.03 242.93
124.02 11259 167.72 155.37 129.27 X46-87 89-11 62-86 71.66 77.53 71.66 6286 94.36 106.56 122.32 53.90
35.98 38-46 41.67 43.71 65.50 63.12 25-79 14.36 21.62 24-40 17.45 22-08 23.47 29.34 54.83 61.47
30.0 30-O 35.0 35-O 40.0 40.0 25.0 25.0 25.0 25.0 30.0 30.0 35.0 35-O 40.0 40.0
17-78 18.80 21-84 21-59 25.15 25.65 16.00 16.51 15.75 15.49 18.54 18.03 19-30 22-10 26.16 26.42
19.22 19-22 19-22 19.22 19.22 19.22 19.22 19.22 19.22 19.22 19.22 19.22 19.22 19-22 19.22 19.22
349.34 370.35 366.95 380.85 363.40 34595 345.95 359.85 461.22
206.18 171.27 293-44 294.83 185.17 300.54 262.08 286-49 279.54
90.81 55.91 209.58 153.67 66.41 79.38 129.27 143.32 106.56
30.0 25.0 10.0 35.0 25.0 30.0 35.0 40.0 35.0
19.81 16.00 26.92 23.37 16.12 19.83 2863 27-06 20.89
19.23 19.23 19.23 19.23 19.23 19.23 19.23 19.23 i9.25
was necessary to support and strengthen the
compact by embedding it in epoxy resin The mounting medium used was Ciba Araldite MY750 resin with HY951 hardener_ To prevent impregnation of open pores by the resin, the outer surface of the compact was sealed with adhesive celh.zIose fii before mountingThe mounted specimen was clamped to the work arm of a low-speed, diamond-impregnated slitting wheel revolving at approximately 60 rpm and cut along a selected plane using water as coolant/grinding fluid. The surfaces so formed were ground on successively fmer grades of silicon carbide grit paper from 65 pm to 25 pm on a conventional metallographic polishing machine, using water as lubricant. The specimen was examined for scratches and washed with water before changing grades of paper, the duration of grinding on each grade being 7 - 10 minutesF_ Compact
examination
Extensive trials with X-ray film, photo-
HID
Density
Lubricant
(kg mm3)
Relative (a)
0.925 0.978 1.136 1.123 1.309 l-335 0.832 0.859 0.819 C-806 0.965 0.938 l-004 1.150 1.361 1.375
5820 5500 5520 5590 5485 5370 5390 5220 5470 5560 5580 5740 6250 5460 5270 5220
52.9 50.0 50.2 50.8 49.8 48.8 49.0 47.5 49.7 50.5 50.7 52.2 5C8 49.6 47.9 47.5
Wall
1.021 0.832 l-401 1.216 O-838 1.031 l-229 1.407 1.085
5220 5390 5120 5160 5340 5210 5100 5090 5757
47-5 49.0 46.5 46.9 48.5 47.4 46-4 46.3 52.3
Unlubricated
lubricant
graphic film and special nuclear emulsions indicated that fine grained X-ray film gave optimum results for recording the &radiation from U02 [S] . The radiation was of sufficient energy, hc-wearer,to expose both emulsion layers when conventional double-coated X-ray film was employed. A special sample of ultrafine grained, singhz-coated X-ray film, designated Kodak-R X-ray film, was obtained from Kodak Research Laboratories and used exclusively for the quantitative studies reported_ The examin ation technique adopted was simple contact autoradiography. The fihn was placed, emulsion side upwards in a light&&&t box on stiff card. The prepared specimen section was placed in contact with the emulsion and retained in place with 100 g weight. fhe exposure box was stored in the dark during the 24 hour period of exposure, remote from likely sources of vibration_ Exposed film was subjected to a standard developing and fixing treatment recommended by Kodak Ltd and air-dried before evaluation.
PD
43(m)
61.76 90.60 47,32 78,43 23.80 40,33 20471
96.14 101.31 61.47 39,64 104.86 116.37 163,82 122.32 101.31 61.43 79.07 42.32 42A3 62.82 27,BS 22870 26,26 26.79 33.86
77.22 lS4,44 164,44 164A4 77,22 77,22 77,22
164,32 167,37 167.37 X7,37 167,37 160,77 nSO0 237,68 237.08 167a37 160,77 167.37 79,38 70,91 77.99 79,38 76,33 7S,33 79,38 234,29 66.91 36,98 28.73 7.72 16.37 9.11 10,so 14,3S 6.26 2,78 SA7 4.94 14,36
20.23 24,40 11,12 6,SS 22.70 37,37
11502 27,63 8.20 16.70 3.42 6.03 2,22 231,7 231,7 231.7 231.7 293,4 370,7 231,7 293,4 370.7 231.7 293,4 370,7 293,4 370.7 293,4 370,7 231,7 231,7 293.4 370,7
308.9 308,9 308.9 30&O
308,9
308,9 308.9
PP (MN me2) (MNmw2) (MNmw2) (MNmw2)
PA
1680 - 1000 1000 - 600 600. 210 t210 16f30~1000 1630 - 1000 1630 - 1000 1630 ’ 1000 1680 - 1000 1680 - 1000 1680 - 1000 1680 - 1000 1000. 600 1000. 600 600. 210 600. 210 1000 - 600 600. 210 (210 (210
2f300~1680 2800~1680 1680.1000 1680.1000 1000. 710 710. 210 (210
Sizerange (pm)
Compactionrcsull8-urnnium dioxidodry gronulnrfeed
TABLE3
40.0 40.0 40.0 30.0 40.0 40.0 GO.0 60.0 60.0 60.0 30.0 40.0 GO.0 60.0 40.0 40.0 26.8 26.8 26.0 2G,O
39.60 47.60 27.20 41,30 29.80 61.73 16.40
(g)
26,70 26,91 27.18 19,14 24,64 26.72 28,SS 31,12 30094 32861 l&69 26,lO 33.43 32,33 27806 26,76 l&24 l&44 17,40 l&21
29,17 28.24 1952 24.67 24,30 42,115 12.36
(mm)
19.18 19818 19,18 19,18 19,18 19.18 19,18 19,18 19,18 19.18 19.18 19,18 19.16 19,lG 19816 19,lG 19.15 19,lG 19,13 19,18
19.15 19018 19,18 19,18 19,lS 19.18 19,lS
(mm)
1.340 1.3Gl 1,417 0,998 1,279 1,341 1,489 1.623 1,613 1,696 0,974 1.309 1.746 1.688 1,413 1,397 0,9G2 0,903 0,910 0,793
1.623 1.472 1.018 1,231 1.269 2,200 0,646
MllfJE Length Diamotcr H/D
6390 5340 6090 6420 6640 6380 6060 SS60 SG90 6320 6660 GS20 Ii190 6370 6130 6190 6100 6060 6000 G690
4700 6820 G860 6820 4260 4260 4330 49.0 48.6 46,3 49,3 s1,3 48,9 66.1 60.G SO.8 48.4 SO.5 so.2 47.2 48,8 46.6 47,2 49.4 46.9 46,4 61,7
42.7 62.9 63.3 62,9 38.7 30.7 39.4
(kg rnD3) Relntivo(%)
Density
Walllubricant
Unlubricated
Lubricant
E
114 TABLE
4
Compaction
results-uranium
PA
pD
(MN m-* )
(MN m-*)
231.66 231.66 231-66 231.66 231.66 231.66 463.32 463.32 463.32 163.32 463.32 463.32 401.82 401.82 4os.s1 401.82 489-18 42279 401.82 398.33
99.56
98.13 98-21 96-63 9’7.94 98.11 178.32 176.80 177.20 1’13.69 178.41 Iii-90 110.46 112.51 139.i6 139.76 L65_97 157-24 181.69 176.45
dioxide granular feed with binder Size range
Mass
Length
Diameter
(MN m-*)
(pm)
(fz)
(mm)
(mm)
20.76 18.34 21.69 19.44 22.ii 21.38 32.18 30.47 32.13 30-78 31.16 30.90 20.79 20.i9 24.11 25.16 33.54 35-64 41.93 41.23
1680-1000 lOOO850 850710 710350 350210 c210 1680-1000 lOOO850 850710 710350 350210 t210 PlO210 710210 ilO210 710210 7x0210 710210 710210 7PO210
35.5 35.1 35.1 34.7 35-l 35.1 35.3 35.3 35.1 34.8 35.2 35.0 25-O 25.0 30.0 30.0 35.0 35-O 40.0 40.0
21-30 21.06 21.01 20.73 21-04 21.03 19.53 19.51 19.39 19-17 19.45 19.28 13.49 13.40 16.04 16.14 18-71 1x73 21.73 21-68
19.23 19.23 19.23 19.23 19.23 19.23 19.26 19.26 19.26 19-26 19.26 19.26 19.25 19.25 19.26 19.26 19.26 19-26 19.26 19.26
PEt-)
For quantitative autoradiography. the Kodak-R ffirn was calibrated by exposing under standard conditions a series of uranium dioxide compacts prepared to cover the range of density variation expected. Seven discs, with length to diameter ratios in the range 0.31 to 0.38 according to the level of appiied pressure, were prepared by compacting graded powder feed described in A(i) with stearic acid die-wail lubricant. Relative densities, deduced from mean compact densities determined by the geometric technique described in D, ranged from 43.00 to 52.01%. The film was exposed in contact with the top surface of each compact and a graph constructed relating the optical density of the resulting autoradiogram to the mean relative density of the compact (Fig. 5). This calibration graph was used for all evaluations of density carried out on the test compacts, allowance being made for the presence of binder in the case of internally lubricated specimensG. Analysis
of automdiograms
density distribution of autoradiagrams was evaluated using a Joyce-Loebl Chromoscan 5121 reflectance densitometerA standard scanning aperture of rectangular The
optical
H/D
l-108 1.095 1.093 1.078 1.094 1.094 1.014 1.013 1.007 0.995 l-010 1.001 0.701 0.696 0.833 0.838 0.971 0.972 1.128 1.126
Density (kg rnm3)
Re!ative (%)
5740 5740 5750 5760 5740 5750 6200 6210 6210 6230 6210 6230 6368 6410 6420 6380 6421 6414 6318 6333
54-4 54.4 54.5 54.5 54-4 54.5 58-7 58.8 58.8 59.0 58.8 59.0 60.3 60.7 60.8 60.4 60.8 60-7 59.8 60.0
Fig. 5_ Relationship between mean compact density and optical density of autoradiograms from UO2 standards (Kodak R, X-ray film).
shape was employed, of length 1.0 mm and width 0.5 mm. Signal gain was constant at 50% of amplifier output and pen amplification was varied to suit autoradiogram conditions. A typical trace of optical density from a longitudinal specimen section is shown schematically in Fig_ 6. Optical density values were recorded at regular intervals along the scan from a reference point, a, at the top of the compact. Be cause of edge effects, point a was placed 3 mm from the edge of the autoradiogram, point (i). and a similar limitation was placed on the final value, point h_ To eliminate the effects on film density of possible variations
115
Fig. 6. Evaluation
of optical density of autoradiograms.
Fig. 7. Schematic position of densitometer autoradiogram of compact.
scans on
Fig. 8. Schematic densitometer sca~ns of compact ing position of density readings.
show-
in processing or esposure conditions, the optical density of each autoradiogram was measured relative to that of the film base, which was taken as zero for comparative purposes_ The method of building up a density cliagram of the compact cross-section is shown schematically in Fig. 7. The optical density of the autoradiogram was determined along 5 parallel scans es shown, with scan 1 centred on the longitudinal axis and the others separated by 3 mm from each other. The recorded optical density traces were of the form shown in Fig- 8. The displacement from the baseline was measured at fixed intervals along the trace and the value of optic-al density related to compact density using the calibration curve (Pig. 5). The observed density figure was entered on a grid plan of the compact cross-section with points ccrresponding to the points measured on the recording chart (Fig. 9). Points with the same density value were joined to form a series of density contours on the compact cross-se&ion and the pattern so formed used to analyse compaction behaviour. 5. PRESSURE
RELATIONSHIPS
Ohsewed values of applied force, FA, and axial die-wall reaction. FD f were tabulated and
Fig. 9. Grid reference system superimposed tudinal section of compact_
on iongi-
the force transmit+ted through the compact to the lower punch, FL, deduced by difference: FL = FA -FD
@I
Values were expressed as pressures and graphs of PA uersLtsPL plotted for a range of compaction conditions with values of applied pressure of 15 - 400 MN me2 (Figs 10 - 12). Straight lines were fitted to the results by a least-squares regression analysis_ Relevant parameters of the regression equations are shown in Table 5. In most cases correlation coefficients of O-99 or better were obtained, although, in common with results reported by Nelson and co-workers [9J ) more scatter was apparent at high values of applied pressure. The value of the regression coefficient was dependent on compact weight and lubrication conditions. The proportion of applied pressure transmitted to the lower punch decreased as compact weight increased with constant diameter. For each condition of lubrication examined, the relationship between PL and PA was described by
(9)
100
200 PL
0
(10) where H is compact length, D is compact diameter and M is compact weight. These relationships are in agreement with those observed by Nelson and co-workers [9] for the densification of pharmaceutical materials. Results are plotted in Fig. 13. The relationship between final compact density Pm and applied pressure, PA, was found to be a complex one- As expected, the compact density increased as the ratio of transmitted pres==, PL, over applied pressure, PA, incred, ix. the lower the frictional losses the more efficient b ecame the work of compaction. At low values of H/D ratio in the range 0.3 - 0.4, a simple relationship was oh served of the form: = K3pm
as pIott&
in Fig- 14_
2a3 PL
Fig. 10. Relationship between applied pressure and pressure transmitted to the lower punch during uranium dioxide compaction (combined internal-diewall lubricant).
InP,
143
(HKm-2)
W
(iiui~
Fig_ 11. Relationship between applied pressure and pressure transmitted to the iower punch during uranium dioxide compaction (die-wall lubrication).
As frictiona! forces increased owing to increase in sample height or to inferior conditions of lubrication, +he relationship corresponded to:
I+
A
=K.&j&
Results are plotted in Fig. 15. The size of the feed particle had little effect on either pressure transmission through the compact (Fig. 16) #Jr final compact density (Fig. 17 j. Conditions of lubrication had a significant effect on pressure transmission throughout the compact. as is apparent from results in Figs- 10 - 12. The use of an internal binder/lubricant gave optimum results for pressure transmission. Die-wall lubricant, although present in much smaller quantity, gave only slightly inferior results, while severe frictional problems were encountered with unlubricated material_
_I
--
-_
_-
_
_-
_ _-
: --
_
TyBLE5_:-. PA =tlPL+@
25
Wall lubrication 1.3974
.
A
:
_
_ _
. - _ Nolubtkation
lkgreesof
-.
freedom
(vi)
Correlation
- coefficient
(C)
Standard deviation
25 22 25 24
0.9975 0.9959 0.9910 0.9976
2190 2.609 3.757 2.012
2.3585 1.9229
4.4765 4.6158 x x lO-3 lo-”
11 12
0.9990 0.9938
3.210 1.142
3.7419 4.3652
-7.4512x X0-* -4.2014 x 1O-2
I1 13
0.9846 0.9905
5.065 4.306
17 16 17 18
0.9992 0.9991 0.4966 0.9981
1.311 1.413 2.875 1.960
1.4246 1.5718 1.6273 1.9010
Internal/wall lubrication
B-
2.786 x 1O-2 --1.8957 x~O-~ 4-674 ~-lo-~ -2.2931x 1O-2
l-61911.6710 1.9437.
35 40 25 30 35 40
117-
.
_.
Lubrication condiiions
::
t
7.226 .7.912 1.705 1.194
x x x x
lo-* 1O-2 10-l 10-l
Fig. 13. Relationship between pressure ratio pi/P~ and uranium diorside compact H/D ratio, for varying Iubrkation conditions_
ratio, lubrication conditions and to a lesser extenton compaction prassure_Forconstant compactionconditionsitwas observedthatr OY 0
IcD @df= 9
Fig. 12. Relationship between applied pressure and pressure transmitted to the lower punch during uranium dioxide compaction (unlubricated).
The maximum pressure required for l$e ejection of each compact from the die; Ps , was recorcied and is shown in Tables 2 - 4. The value of PC varied directly with die-wall frictional _ forces and was dependent on compact H/D
-_
-
-
m.
lg) 30 35 40
-_ ._-
regrekodqf PA on Pt (Figs_ IO -_12). Parameters for linear regression of tbe form
hranetersofth&s~tight&
Mass
--
-.
-
--_ ._
-
_
-
_
-
:_ .
_
_. _
7
-_
The relationship is plotted for specified compaction conditions in Fig. 18. The profile of the plot of PE versus time was similar for all conditions examined, and is shown schematically in Fig. 19. As pressure was applied, it rose steeply to a maximum at point A, which corresponde@ to the first compact movement. This was the recorded value of Pe and defined “static” ejection pressure_
(-
I 20
-n 40
42
44
RclDtnr
4b Casmtr
58
50
52
POrML-
(w)
Kg. IT. Effect of particle size of feed on compact relative density_
&)
Fig_ 14. Re!ationship between applied pressure and elative density for uranium dioxide calibration specimens (compacr H/D ratio 0.31 - 0.38).
,,I 45
101 25 50
55
Fig_ 15. Relationship between p-ure ratio PL/PA and relative density of uranium dioxide compacts (compact H/D ratios In excess of 0.8).
After movement was initiated, the pressure fell to a new value which remained almost constant throughout the ejection process until appro ximately half the compact was ejected from the die at point B_ From this point the
z-
I 40
-@I Fig. 18. Relationship between ejection mass of uranium dioxide compacts.
Fig. 19. Schematic time_ Fig_ 16. Effect of partick size of feed on the pressure ratio PI/PA for a range of compact weights.
I
30
profile of ejection
45
pressure and
pressure with
ejection pressure decreased steadily to point C, corresponding to final compact ejection, where it fell abruptly to zero. The values of A, B and C were dependent on compact H./D, lubrication conditions and compaction pressure. Point B, which defines dynamic ejection pressure, was additionally dependent on rate of pressure application.
50
Fig. 20_ Density distribution patterns for uranium dioxide compacts of varying H/D ratio. Die-wail lubrication employed.
6. DENSITY
Fig. 21_ Density distribution patterns for uranium dioxide compacts of varying -ratio. No lubricant employed.
3A’iTERNS
The patterns of density distribution were found to be sensitive to changes in compact H/D ratio. Detectable density gradients were observed in a compact of H/D ratio 0.8, the lowest value examined, prepared at an applied pressure of 80 MN mm2 in the presence of diewall lubricant. As compact H/D ratio increased, the range of observed density variation increased and areas of high and low density in the compact became more clearly separated. Density patterns observed for a range of compact H/D ratios are presented in Fig_ 20. Compaction pressures were all within the range 75 - 80 MN mm2 and die-wall lubrication was employed. As die-wall frictional forces increased, the extent and intensity of density variation within the compact increased. Figure 21 shows the density patterns observed for compacts of varying H/D ratio prepared without lubricant. Compaction pmssures varied from 157 to 160 MN me2. Compacts produced without lubricant at pressures of 75 - 80 MN mS2 proved to
s -
4ol-62
n/D
= I-lza
hlN.m-2
%-
390-3
IUD
- I-l26
hsil-~
Fig. 22_ Density distribution patterns for uranium dioxide compacts containing combined intemal-diewall lubricant.
be too friable to provide suitable sections for autoradiography. The most uniformly dense compacts were obtained using the combined binder/internal lubricant system. There was little evidence of significant variation in density in compacts with H/D ratios of l-13, despite the use of compaction pressures up to 400 MN mm2_ The density patterns obtained for the two largest compacts examined are shown in Fig. 22. The variation of compaction pressure examined in this work was relatively wider than that of any other parameter, and hence it had the most significant effect on density distribu-
Fig. 23. Density patterns for uranium dioxide compacts prepared at a range of compaction pressures. Die-wall lubricant employed_
tion in compacts. Whereas the principal effect of increased frictional forces, induced either by inferior lubrication conditions or by increase in compact H/D ratio, was to intensify density gradients, increase in compaction pressure produced distinct changes in density pattern. The effect of compaction pressure on density cik ttibution in compacts is shown in Fig. 23 for
compacts prepared using die-wall lubricant and a sttdard compact weight.
7_ DISCUSSION
The density patterns observed had several features in common. Each compact exhibited three density zones consisting of an inner axial region and an outer peripheral zone separated by an armulus of lower density. The inner zone structure was more dependent on applied pressure than the outer zone, which was markedly affected by lubrication conditions. In the outer density zone the maximum density was observed adjacent to the moving
punch. All compacts showed a discontinuity in the outer zone pattern at a distance fmm the stationary punch dependent on lubrication conditions but independent of applied compaction pressure and compact H/D ratio_ In the case of die-wall lubrication this distance was approximately 0.2 H, and for unlubricated compacts increased to 0.3 H. The discontinuity is interpreted as marking the limit of material movement at the die wall. Analysis of the density patterns observed by Train [S] in the compaction of magnesium carbonate show a similar phenomenon. In the case of magnesium carbonate the corresponding distance from the stationary punch was approximately 0.5 H in the lubricated state and 0.67 H for tiubricated compa&. The inference from these observations is that more slip occurred at the die wall in the case of uranium dioxide than in the case of magnesium carbonate, an unexpected result in view of the more abrasive nature of the former. An explanation for increased movement on the part of uranium dioxide may be found, however, from consideration of relative volume changes during compaction. The average relative volume, Vxa,, at die fill was found to be 2.66 for magnesium carbonate and 5.5 for uranium dioxide. In undergoing densification to an arbitrary relative density of 60% i.e. to a relative volume of 1.67, the ratio of Vrci, to V,,,,, is 3.3 in the case of uranium dioxide and 1.59 in the case of magnesium carbonate. For a cylinder:
v, =
V observed K
= H/w,
where H is compact height and H, is derived compact height for V, = 1. Then the relative movement of an element of powder in similar compacts of uranium dioxide and magnesium carbonate is defined by: Movement of uranium dioxide Movement of magnesium carbonate
3.3 C-S 1.59 2.07
During compaction, therefore, uranium dioxide undergoes twice as much movement as magnesium carbonate to achieve equal relative density. In the axial zone, the maximum density was observed in the lower half of the compact at a
121
Fig. 24. Schematic development of compact density pattern= Arrows indicate direction of compacting forces.
point close to the stationary punch. Its exact position was dependent on compaction conditions, but in general its centre coincided with the discontinuity in the peripheral density zone. Minimum density was observed adjacent to the stationary punch. A second area of low density appeared on the compact axis close to the moving punch, but its extant and position relative to the moving punch varied with compaction conditions. The density structure observed in the axial zone was also observed experimentally by Kamm and co-workers [Xl] and by Train [6], and predicted mathematically by Shaler [ll] . In this work, however, a second region of high density appeared ~~1 the compact axis as frictional forces increased_ Its position varied with compaction conditions but was always between the primary high density region and the moving punch. The maximum density observed for this secondary zone never exceeded that of the primary region under the compaction conditions examined. The density patterns observed by Train [S] for magnesium carbonate compacts showed
similar axial and peripheral density zones. However, in the case of magnesium carbonate, the zones were more widely separated, and in particular the peripheral zone was Iess extensive than that observed for uranium dioxide compacts (Fig. 2). Interaction of axial and peripheraI density zones was observed in uranium dioxide compacts at low relative densities (Figs. 20 - 23). As a result, outward movement of material from the axial zone was restricted and axial density gradients were intensified. The explanation of the development of the density pattern advanced by Train [S] falls short of explaining tbe results obtained in this work. The pressure-bulb concept of Boussinesq 1121 describes conditions existing briefly at avery early stage of compaction (Fig. 24a). Immediately material begins to move at the die wall, adjacent to the upper punch it dilates and expands away from the die wall. As frictional forces develop and the shear strength of the material is exceeded, densification begins, and the process is repeated continuously in successive elements to the limit of material movement. The extent of material movement is governed by applied compaction pressure and by the resistance to inward movement produced by the densification and consequent tendency to outward movement of material at the axis of the compact (Fig. 24b). Material on the compact axis is driven down into the unconsolidated material below it, exhibiting a hemispherical pressure front as predicted by Boussinesq [12] (Fig. 24~). This process persists until the reaction from tbe lower punch exceeds the radial reaction from the die wall. Material from the centre begins to move outwards at the point of minimum radial pressure, which coincides approximately with the limit of material movement at the die wall (Fig_ 24d)_ This material has undergone less consolidation than material in the lower comers of the compact because of the predominance of the axial pressure component in the pressure front at this stage. The change of direction of powder movement induces locally high shearing forces and contributes to the formation of a high density area centred on this point (Fig. 24e)_ The low density areas observed on the compact axis adjacent to the moving punch and in the peripheral density zone adjacent to the stationary punch correspond to areas where negligible powder movement has occurred and where shear forces are minimal.
122
Development of the secondary pattern in the axial density zone m;?y be attributed to a mechanism similar to that causing formation of the primary high density area. In this case the outward material movement and consequent densification is initiated when the axial reaction from the area of primary high density exceeds the radial pressure at any point between it and the moving punch (Fig. 24f). REFERENCES 1 W. hI_ Long. Powder bIetall_. 6 (1960) 73. 2 P_ J_ James, Powder h:et.?ll_ Irk., 4 (2) (1972)
145
3 D. Train and J. k Hersey, Ind. Chem.. 38 (1962) 77. 4 M. Yu. Bal’shin. Vestn. Metalloprom.. 18 (1938) 124. 5 R P. Seelig and J. Wulff, Trans. AIME, 171 (1946) 516. 6 D. Train. Trans- Inst Chem. Eng. London. 35 (1957) 258. 7 B.S. 3406. Part IV (1961). S H_ M. Macleod and IC_ M&hall. unpublished work_ 9 E. Nelson, S. M. Naqvi, L. W_ Busse and T. Higuchi, J_ Am Pharm. AssSci. Ed. 43 (1954) 596_ 10 R Kamm. M_ A_ Steinberg and I Wulff. AIME. 180 (1949) 694. 11 A. J. Shaler, Trans. AIME, 171 (1947) 521. 12 J. Boussinesq. Memoires couronnes et memoires des savan‘ts i&angers, B-els. 1876_