Micro electro-discharge machining of ink jet nozzles: optimum selection of material and machining parameters

ELSEVlER

Materials P ess g Techaology

,!ournal of Malelmls Processing Technology 58 (19961 53 66

Micro electro-discharge machining of ink jet nozzles: optimum selection of material and machining parameters D.M. Allen, A. Lecheheb School 01" lmtustrial ,rod Mmm/'acmring Science, Crap!tieM Unir~er.~iO', Bed.lbrd ME43 0.4 L, l'K

Received 9 July 1993: accepted I July 1994

Industrial summary A comprehensive examination of ink jet nozzles manufactured by the micro electro-discharge machining (MEDM) process has been conducted as part of a study to develop an understanding of the effects of MEDM on the hole properties, the issuing jel directionality and stability and the subsequent i:,K jet printer performance. Ink jet orifices were machined into thin foils made of different grades of stainless steel. Optical and scanning electron microscopy of the nozzles were then carried out to assess the effects of the machining variables on the orifice surface integrity. The relationship between the applied energy, the size of the heat affected zone, the foil susceptibility to corrosion and the nozzle stability is discussed. Keywords: Electro-discharge machihing; Ink jet nozzle

1. introduction Ill continuous ink jet printing [!,2] a jet of ink is emitted from a nozzle under pressure and broken into a stream of uniform droplets by the application of ultrasonic vibration. The droplets to be printed are given an electric charge as they break off fi'om the jet and arc deflected, in proportion to their charges, by a unilbrm electric field provided by a pair of deflector plates. This deflection is combined with the motion of the object to be printed to provide the means for placing drops at the correct position in a two-dimensional dot matrix array (Fig. 1). In the continued evolution of ink jet printing, the next generation of machines will need to offer a combination of higher print speed, finer resolution and superior print quality. This will pose new problems in producing small and accurate ink jet orifices. The quality of such orifices is very important as the printer performance is totally governed by the quality of drop formation and directional stability of the jet, which are both controlled primarily by the nozzle characteristics. It is therefore important that a future jet manufacturing process can produce high quality nozzles cost-effectively in high density arrays with reproducible and precise dimensions. This process should be able to produce ink jet orifices of various geometries and in a variety of materials other than those currently used. 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved S S D ! 0924-0 i 36(95)02107-W

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D.M. Allen, A. Lecheheb / Jot~rnal oJ Materials t'rocessing Teclmoh,gy 58 (1996) 53-66

Most of the present-day ink jet apertures are fabricated in such specialist materials as industrial jewels, silicon and electroformed nickel. The limitations of these types of nozzles far outweigh their advantages. Jewel nozzles have low dimensional accuracy and reproducibility. They are manufactured from watch jewels by a four-step process which leads to long fabrication times of approximately 30 rain per hole. Furthermore, they do not lend themselves to being closely spaced. This effectively eliminates them for high quality, high resolution printing applications. Silicon structures [2,3] are thin and fragile and do not provide the rigidity required by industrial continuous ink jet printers. Kuhn et al. [2] report anisotropic etch rates in the (110) and (111) planes of 30 and I ~tm per hour, respectively. Finally, electroformed nozzles have limited thickness and geometry, wear rapidly and are susceptible to corrosion when used with solvent-based inks, and to erosion with inks containing pigments. Their fabrication time is also long, exceeding 5 rain per hole. A manufacturing process that can produce reliable, densely packed and precise ink jet nozzles was introduced in Japan in 1986 [4-6]. It uses micro electro-discharge machining (MEDM) but, unlike conventional EDM, the level of energy used to drill holes is very small, being about one hundredth of a microjoule. The principle of this machining technique stems from the fact that if a high enough electrical potential is applied across two conductors separated by a dielectric, the dielectric will ionize and breakdown, A spark 'lube' carrying an electric current is formed between the two conductors, This spark tube will cause local vaporization or melting of the conductors, The piece to be drifted is one conductor and the tool electrode the other, Therofore during sparking both conductors are eroded, The MEDM machine is equipped with a servecontrol to k ~ p the spark gap constant and prevent an are from forming, The dielectric, is usually a hydrocarbon fluid which is also used to cool the workpiece and to flush away the eroded material debris, The nozzles used in the study described in this paper have been manufactured by one of these MEDM machines, These are equipped with a low impedance relaxation discharge circuit,

2. Experimental procedures Foils 100 lain thick made out of stainless steel alloys, here identified as alloys A, B and C, were fabricated into discs 2 mm in diameter by photochemical machining, Alloy A is a scmi-austenitic steel while B and C are low carbon austenitic stainless steels, Before machining the orifices the A material was precipitation hardened at 482 *C for I h then air cooled (see Appendix A for

materials specifications). The B and C steels were cold rolled to obtain a Vickers hardness of 430 H,,. Hardening of these materials is necessary to prevent the erosion of the ink jet orifice, which in service will be subjected to a shear rate of nearly 3 x 106 S- I . Furthermore, some inks are abrasive because of the pigments present in them. To maintain velocity uniformity from one printer to another and to ensure interchangeability of components, it is necessary to control, among other parameters, the nozzle length. To this end, the authors explored the possibility of achieving the required nozzle length by producing a 2 mm diameter, 1.9 mm deep counterbore in a 2 mm thick plate. This was first made by EDM sinking, then by punching and finally by conventional machining. However, the exercise yielded unsatisfactory results. The accuracy of the resulting nozzle thickness was no better than + 10 tam which corresponded to a velocity uniformity of + 0. I%. Furthermore, the surface finish of the counterbore was poor. It contributed to an unstable direction of the produced jet. Therefore, foils were preferred to counterboring of thicker plates because the tolerance on foil thickness, which, in this case, equals the nozzle length, are comparatively high. This is typically + 1 lam from nominal thus yielding an average velocity uniformity of less than _+0.01%. Furthermore, foils can be lapped to ensure smooth and burr-free orifice inlet and outlet surface faces thus preventing flow separation and turbulence of the emerging jet. To obtain a rigid nozzle structure the foils were electron beam welded to a base plate of similar material 2 mm thick. Alter welding, and belbre micro electro-discharge machining of tile 75 tam diameter holes, the lbils were~ ubjccled to a microhardness test. This was to determine whether any softening had occurred as a result of the welding process, The hardness values were measured using a Leitz Microhardness Indenter, The applied load was 0.98! N. The depth of penetration of the indenter was calculat" . ed at 4,3 lain at 200 H,, and 3.0 tam at 400 H,,. Holes 75 Bm in diameter were then machined into the various foils using increasing levels of discharge energy as shown in Table 1, The MEDM energy is obtained from the formula E = ~C + C')V ~ where V is the electrical potential applied between the two electrodes, and C and C' are the circuit and stray capacitances, respectively, Half of the finished w)zzles were ultrasonically cleaned in acetone then tested for jet directionality and stability. This involved the assembly of a nozzle on to the drop generator shown in Fig. 2, Ink was supplied to the transducer chamber under a pressure of 3 bar to create an ink jet from the nozzle. The resultant jet was stimulated to break into a series of uniform droplets by the application of a modulation voltage to the ultrasonic transducer in the chamber. This was driven by a sinusoidal electrical signal at a frequency of 64 kHz.

D.M. A/h,~:, A. Le~'hchcb / Jour~:ag ~f Magerhds Processing "~]'chnology 58 (I 996) 53.66 Table 1 Experimental conditions

55

Table 2 Conditions and results of corrosion tests of alia3 A

Specimen

Energy (10 ~ J)

Material

Condition tit,)

No~ie

Medium

Phase

Observatio~

Ai A2 A3 A4 BI B2 B3 B4 KI K3 K5 K7

0.01 0.07 0.|5 0.70 0.01 0.07 O. i 5 0.70 0.0 | 0.07 0. ! 5 O.70

A A A A B B B B C C C C

465 465 465 465 430 430 430 430 430 430 43O 430

Al A4 A5 A8 A9 A 13 A i4 A 19 A24

Water-based ink Water-based ink Ethanol-based ink Ethanol-based ink MEK-based ink MEK-based ink Water-based ink Ethanol ink M EK-based ink

Liquid Liquid Liquid Liquid Liquid Liquid Vapour Vapour Vapour

Corros~oll

The experimental arrangement (Fig. 2) for the jet performance study consisted of stroboscopic illumination of the stream of drops, a TV monitor and a microscope camera mounted on an x, y and z micrometer stage. To simplify measurement of jet misdirection and jet stability, the stroboscope light was synchronized with the drop formation fi'equency. This provided the production of stationary pictures of the stream which permitted straightforward measurement of jet misdirection and jet stability. The remaining nozzles were ultrasonically cleaned in acetone then examined using optical and scanning electron microscopy (SEM). The aim was to identify the characteristics of the MEDM process and to assess the effect of the machining variables listed in Table 1. To characterize further the surface topography of the interior wall o1" the hole, nozzle orifices were laser cut into two halves then cleaned of debris in alcoholic aqua regia, Subsequently, optical microscopy and SEM were performed on these sectioned specimens. Corrosion tests to determine the long-term reliability and integrity of the holes were conducted on unused nozzles with three inks at 55 °C for 50 h. Heating was effected by a water bath controlled to +_0.5 °C. In Monitor

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drops

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Corrosion Corrosion Corrosion Corrosion Corrosion Corrosion Corrosion Corrosion

addition, vapour phase corrosion (VPC) tests on unused nozzles were conducted for the same period. This involved positioning the nozzles over evaporating ink solutions where they came into contact with ink vapours. Nozzle codes and corresponding test environments are listed in Table 2.

3. Results and discussion

3.1. Jet perJbrmance Hardness measurements were taken at tl~e locations shown in Fig. 3. Since the foils were originally hardened to a nominal 430 Hv it can be concluded that softening of the lbils has occurred near the edge of the electron beam weld (EBW). More significantly the central area of the tbils retained its initial hardness as anticipated. This was probably helped by the provision of a copper heatsink under the foil during welding. The foil itself was welded to a thicker plate (Fig. 4) which dissipated much of the heat input. Jets issuing from the material A nozzles machined with an energy input of 0.15 x 10 -~6 J, or higher, invariably deflected sideways and in a random manner, whenever the ink supply pressure was raised above 3.5 bar. The random angles of deflection were largest in nozzles machined with the highest energy. In nozzles where the heat input corresponded to an energy of 0.3 x 10 -6 J the jet misdirection increased with ink supply pressure, eventually leading to spray formation at pressures exceeding 4.5 bar. On emerging from the nozzle the jet should transform to a train of droplets under the influence of surface tension forces alone. In general, these forces act symmetrically around the free stream so they cannot deflect the jet. However, asymmetries in the orifice due to the presence of surface depressions, ridges or cracks

¢

Fig. 5. Optical micrograph of material A MEDM hole as manufi~ctured (low heat input).

ness then pierces this layer leading to turbulent flow and subsequently to random jet deflections. In the hope of diagnosing, then eventually curing, the above jet deflection problem, SEM micrographs of the nozzle foils were taken. These are shown in Figs. 5 and 6. They depict the surroundings of the hole and show sensitized regions, such regions being more severe in nozzles machined with high heat input (Fig. 6). These

FiB, 3, Nozzle hardness scan,

have some important consequences on the lluid dynamics, As might be expected, the presence of these defects ing p~ssu~ Or a d ~ a s e i n the fluid viscosity, i,¢, an laminar sublayer; The surface rough-

l~tg, 4, Section through the n o ~ e plate base ( × 62,6) showing the thin foil with the MEDM hole inits midrlle,

Fig. 6. Optical (a) and SEM (b) micrographs of material A MEDM hole as manufactured (high heat input). Note the existence of three

re,ions,

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,hmrmd ~!D"Mlg&'rhgL~' Procc~'s#~g Tcdmohagy 58 {/996)53 06

sensitized zones are also much more pronounced on the nozzle side that was facing the holding fixture during machining. Three different zones are apparent and are characterized as follows: (1) Zone l contains fine cracks and is in the tbrm of a ring immediately around the hole and measuring approximately 35 lain across. (2) Zone 2 seems featureless. It is a ring surrounding zone 1 and is approximately 300 ~Lm wide. It is visible as a bright orange-coloured region when viewed under the microscope. (3) Zone 3 is a 300 ~tm wide ring with its inner edge situated approximately 600 ~tm from the hole. It shows extensive surface debris. Several techniques were employed to analyse the above described regions. It was hoped that when the intbrmation from all the methods used was combined it would be possible to identify fully the composition and origin of these three zones.

3.1.1. Energy dispersire X-ray ariaS,sis (EDX) The analyser was attached to the SEM, The analysis showed these regions to contain the expected elements present in material A, i.e. aluminium, chromium, iron, manganese and nickel. The carbon content of material A was too light an element for the EDX to detect. The analysis suggested that these regions could be one or a combination of the following possibilities: (a) They may simply be heat-ali'ected zones. (b) They could be no more than non-conductive surface contaminants. (c) They might be contaminants composed of light elements which the EDX cannot detect.

3.1,2. Microhar&aess It was possible to check hypothesis (a) by carrying out microhardness scans of the nozzle lbil to determine any signilieant microstructural differences. This time, a Masuzawa Seiki microhardness tester was used to carry out two scans at two different applied loads, 0.981 N and 0,196 N. The indentations were made at 100 lain spacings starting from near the edge of the MEDM hole and working outwards. The results of these tests are plotted on the graph of Fig. 7. They show a marked increase in the hardness values when the indentations were made with the lower weight of 0.196 N. Three extreme hardness values were also obtained with this lower load indicating the apparent existence of three regions across the nozzle foil (zones 2 and 3 and the base material). Attempts at measuring the hardness of zone 1, which resembles a 35 lam wide ring, failed due to its small size. The higher hardness values obtained with the lower applied load may be attributed to the existence of hard diffusion layers in the base material or to the presence of metal oxides films or scales that are

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3. i.3. Laser imdzation mass analyser (LIMA) To explore hypotheses (b) and (c) a more sensitive analysis technique than the EDX analyser or the microhardness survey is needed in order to identify the composition and understand the origin of these surface layers. Fortunately, it was possible to address this problem by the use of the laser ionization mass analyser (LIMA). This was able to define qualitatively the ele° mental composition of the surface of the nozzle including the sensitized regions. To guard against gathering localized data, positive and negative ion spectra were acquired from several points within each zone. Variao lions in composition with depth were investigated by directing multiple laser pulses at the same spot. However, as the LIMA technique removes approximately 0.25 ~tm of material per pulse, the depth resolution of the intbrmation obtained is relatively crude and accurate depth cannot be measured. For comparison purposes nozzle foil made out of titanium and material C and incorporating a MEDM hole each were submitted to the same LIMA analysis. The results are summarized in Table 3 and the associated graphs are shown in Figs. 8~16. The species observed on the base material and zones 3, 2 and ! can be summarized as follows: (a) Base material. The species observed in the base material, that is the surface of the foil outside zone 3. are those of a typical stainless steel alloy A with positive ion spectra such as AI, Cr, Mn, Fe and Ni. The negative ion spectra from the surface of this base material depict the presence of surface oxides such as CrO> AId and AId2 which are characteristic of this material A and are consistent with the microhardness measurements described earlier.

D, M, A IBn, A. L¢cheheb / Journal of Materials Processing Teclmology 58 (1996) 53- 66

~s Table 3 LIMA results Sample

Area

Positive ions

Negative ions

Material A bottom face

Base Zone 3 Zone 2 Zone 1

Na, Na, Na, Na,

K, Cr, Fe, Mn, Ni Cr, Fe, Mn, Ni Cr, Fe, Mn, Ni Cr, Fe, Mn, Ni

CrO2, CrO3, CN, CNO, CI, AIO, AIO2 Cn, CnH, CN, Ci Cn, CnH, CN, Cl, CrO3 Cn, CnH, CN, CI, CrO3

Material A top face

Base Zone 3 Zone 2 Zone l

Na, AI, St, K, Cr, Fe, Mn, Ni Na, AI, K, Cr, Fe, Mn, Ni Na, AI, K, Cr, Fe. Mn. Ni AI. St, K, Cr, Fe, Mn, Ni

CrO2, CrO3, CN, Ci, AIO, AIO2, FeO2 Cn, CnH, CN Cn. CnH, CN Cn. CnH, CN, CI, CrO3

Ti foil

Base Hole edge

Na. K, Ti. TiO Na. K, Tit TiO

CN, small Cn, C! CN, some Cn

Material C foil

Base Hole edge

Na. K. Cr, Fe. Mn. Nit Mo Nat K, Crt Fe, Mn, Ni, Mo

CN, CNO. small CrO2, CrO3 CrO2, CrO3, CN, CNO, CI, MoO3

AI, AI, AI, AI,

St, K, K, K,

iron (Fe) were very weak. At first it was hypothesized that the Na and K peaks are associated with the feature of interest rather than being due to perspiration fron~ handling of the nozzle foil. This is because they are

(b) Zone 3, The positive iot~ spectra from the surface of zone 3 show relatively strong peaks due to sodium (Na) and potassium (K). Aluminium (Ai) and chromium (Cr) were also detected, but peaks due to -

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actedstic it is not unreasonable to assume, e~en a~ this s~age, that this originated from ~he cracking of the dielectric fluid keorosene used in the MEDM machine. It was only after several laser pulses that peaks for volatile chromium oxides CrO, and C r O 3 and evidence of the m:'tal began to be observed, indicating that the hydrocarbon deposit is relatively thick. Optical viewing of the analysis area showed a bright spot in the dark area of zone 3, suggesting that an overlayer was present which was being removed by the laser beam. (c) Zone 2. The results from this region were similar to those of zone 3, but the layer of hydrocarbon-type material is not thought to be as thick. The positive ion spectrum showed relatively strong Na, K and Cr signals. A second laser pulse at the same spot gave a spectrum which was similar to that of the base material. The negative ion spectra from the first laser pulse showed evidence of hydrocarbon-type material diffused in metal. The hydrocarbon features were mainly absent after the second laser shot.

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more intense than those found in the base material and because they are concentrated in an area much smaller than a human fingerprint. However, it could be argued that the relative strong intensity of these peaks in the vicinity of the hole is due to this area being rough and therefore likely to retain more sodium and potassium species from handling perspiration than the base material. This argument is reinforced by examining Table 3 which shows that the titanium and the material C foils also have peaks for sodium and potassium evenly 'spread' across the surfaces. The sodium and potassium are therefore contaminants originating from handling these foils or from rinsing them with water. The negative ion spectra from this same surface of zone 3 showed evidence of an overlayer or altered layer. The spectra had peaks for C,, CN, C3, CNO, C4, Cs, C6, G , Ca, G,, etc. with associated C,,H peaks. These, of course indicate the presence of a hydrocarbon-type material. Although the spectrum is relatively non-char-

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qualitative surface data. Clearly, a method that is more applicable to depth analysis and one that would provide quantitative or quasi-quantitative results would be able to supplement the analyses so far undertaken. To this end, AES was used to ascertain the elemental composition of the surface and sub-surface of zones 1, 2, 3 and the base material. The results are shown in Table 4. This gives compositions (at.°) as a function of analysis area and depth. High oxygen and high chromium to iron ratios were observed at a depth of 15 nm in both zone 3 and the base areas. These results indicate the presence of a mixture of iron and chromium oxides. This, as far as the existence of these oxides is concerned, is in agreement with the results inferred from the LIMA analysis. The aluminium or its oxide is thought to be below the detection level of the Auger (about 2 at.% on these samples). However, the

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(d) Zone I. Analysis of this region gave strong signal~ for Na and K from the first laser pulse and A! and C: in addition to Na and K at the second pulse. It was uot until the fourth laser pulse that a spectrum similar to that of the base material was obtained. The nega:ive ion spectra for this region showed strong peaks ~or CrO., and CrO:~ but much smaller evidence of hydrocarbon-type material as compared to zones 2 and 3. The hydrocarbon contamination is therefore greater: in region 3 but diminishes towards the edge of the hole. i.e. zone I. Nonetheless. there is still evidence of a thick overiayer in zone i evident fi'om examination of the SEM photographs of this region and from the LIMA analysis which showed that a spectrum similar to that of the base material is not obtained in this zone until the fourth laser pulse.

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The A material appears to possess at least a ~aycr of oxide that is porous. The carbon fiom the cracking of the dielectric diffuses through this "open' oxide film and a thick ovcrlayer of hydrocarbon char is then a!lowed to build up.

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The cracks that can be seen in zone 1 appear to be shrinkage-like cracks which indicate that not only is the overlayer of zone 1 relatively thick, but also that the deposit has dried from the liquid state rather than being built up on the metal. This seems to indicate that the origin of zone 1 is the result of a molten and ejected layer which has redeposited around the edge of the hole. The Auger analysis shows that the carbon content of this layer remains very high even al a depth of 450 nm. The carbon, in this case, could originate from two sources; the major percentage is absorbed fi'om the gases |brined by the cracking of the dielectric while the remaining few per cent could have diffused out of the

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Auger results of zone 3 do not completely agree with those obtained by the LIMA which found zone 3 to consist of a thick hydrocarbon layer on top of the chromium oxide film. A plausible explanation for this difference could be the orientation of the sample foil within the AES instrument leading to the depth analysis being carried out at a point with less contamination. More significantly, both techniques showed the presence of high carbon content concentrated in surface layers on zones 1, 2 and 3 rather than the base material. Thus there are three equations to answer. Where did the contamination come from? Why has it built up only around the edge of the hole? Why is it more pronounced on the face of the foil which was in contact with the fixing jig during the MEDM process? The evidence suggests that the contamination is as a result of the cracking of the dielectric fluid kerosene and the existence of a non-protective multilayered oxide scale.

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(b) Fig. 15. LIMA positive ions spectra of 17-7 PH (base material), l::irst laser shot at two different points on the base material.

I},M. :llh'.. A+ Leclwlteh / Jottrmd +!1"Materials t¥ot'(,~'sing 7'('chm)h),tO' 5S ( i 996) 53 66

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base material due to the high temperature of the electrical spark discharge. Carbon diffusion could be a result of the relatively high carbon content of the A stainless steel alloy, The origin and formation of zone I are therefore somewhat different from those of zones 2 and 3, Zone 1 is the result of the redeposition of a molten layer which has absorbed carbon from the cracking of the dielectric and from the base material, respectively. However. zones 2 and 3 are the result of carbon diffusion into the 'open' multilayered oxide, The carbon-rich layer of zone I would be prone to cracking during cooling as it would contract unorc than

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the underlying layer. As a result it would be susceptible to corrosion during service. The Auger analysis shows this layer to contain low chromium and theretbre no protective oxide layer until a depth of several tens of atomic layers. This seems to support the conclusion that corrosion attack would initiate at this region. The formation of region 3 is, as explained earlier, attributed to the diffusion of the carbon from the cracking of the dielectric. This carbon can be observed during machining in the form of a black ring around the rotating electrode and outside the ring formed by the exploding gas bubbles. The featureless nature of zone 2 is the result of the rotation of the electrode and the exploding gas bubbles. These create a swirling flow of the dielectric around the hole thus keeping that region clean and free of debris. The shiny appearance of this region is thought to be attributed to the cavitation erosion that results from the exploding gas bubbles. To shed further light on why the existence of the above three zones is more pronounced on the bottom face of the foil, it was necessary to closely examine how the foil is fixed in the dielectric bath. This examination soon revealed that more than one factor could have contributed to the bottom face of the foil having more pronounced sensitized regions. These factors are as lbllows: (a) The lbil is fixed to a much larger holding fixture. This imposes a resistance to the heat flow to an extent where cooling from the bottom of the lbi! may no longer be effective,

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D, M. A&'~*, A, .',echekeb / ,lot~raaage~,,¢Me~gcr&d,~'t'g'~*~vssie~g'l'~'~'ktwtogy 58 { ~9~26) 53 6¢}

63

asymmetries m ~he veMci~y profite of the je~ arid m~ty indeed cause deflection of the me~m flow as a resuh of flow separation and the subsequent creation of vortices. The material A nozzks machined ~vith ~ower hca~ input as illustrated in Fig. 5 ,,,how tittte evidence of sensitized regions. The dimples, noduks and surface roughness within the hole are also less pronounced. The variation in the state of these nozzles may be the result of variations in the M E D M machining parameters. In operation the jet issuing from these nozzles showed very consistent jet directionality throughout the ink operating supply pressures as the jet did not deflect even when the pressure was set at 4.5 bat'. This supports the earlier hypothesis that jet deflections were due to asymmetries caused by the presence of cracks, dimples and nodules. The SEM micrographs presented m Fig. 19 illustrate the nature o|" the nozzles made out of ma:erials B and C, and machined with high energy input. It appears that the heat input during the M EDM machining of lo~ carbon stainless steel has no deleterious efl'ect on the

Fig. 17. SEM micrographs of material A nozzle Ibil as manulhcturcd: (a) the MEDM hole and tile sensitized region at'otlild the .?,lge; {b) view tff tile rough region a|'ound tile 1"in1of the hole.

(b) The construction of the holding fixture does not allow circulation of the dielectric around the bottom of the foil. (c) The foil and associated holding fixture arrangement is such that the probability of trapped air bubbles beneath the foil is very high. To verify whether the above listed factors are a plausible explanation tbr the three zones being more pronounced on the bottom lace of the lbil, the holding fixture was redesigned to allow easy circulation of the dielectric and better cooling. The foils machined with this improved holding fixture showed comparable ,;ensitized regions on top and bottom faces of the foil. The SEM rnicrographs (Figs. 17 and 18) show the detail of region I described above. A magnified view of this region reveals cracks of the surface around the inlet to the ink jet orifice and dimples and nodules inside the sectioned hole as manufactured. These defects cause

Fig. 18. Close up view of tile sensitized region of Fig. 7: (a) &tail ol a portion of the edge of the hole: (b) magnilied portion of the edge of the hole showing cracks.

D.M. Allen, .4. Lectwheb /.loto'm# of Materials Process[,g Techmdogy 58 (! 996) 53-66

surface topography of the surrounding region of the ink jet orifice, There are no apparent sensitized regions as seen in the material A nozzles. An explanation for this could be because these steels contain only 0.015% carbon and possess a strong adherent and 'closed' chromium oxide layer. On the one hand, this oxide layer is thought to prevent absorption of the carbon from gases f o x e d by hydrocarbon dielectric. On the other hand the original carbon content of the steel is not high enough for it to diffuse out and create a hard, crack-prone region around the hole as observed with the A Steel. As expected, in operation, the nozzles made out of materials B and C did not exhibit any jet misdirection or deflection. 3.2. Corrosion tests

The SEM micrographs presented in Fig. 20 show laser-cut sections through the MEDM holes. The sur-

Fig, 20, SEM micrographs of laser cut secion Ihrough a material A MEDM !lozzle (a} untested and (h) corrosio, leafed by inks and Vl~,

FiB, 19, Microgrtlphs Qf no~le as n~anufactured with high heat input: (a) material B plate and (b) material C,

face of the inside wall of the hole tested lbr corrosion shows similar characteristics to those of an untested hole, i,e. a cratered surface. This suggests, in the first instance, that the nozzle surface is characteristic of the MEDM machining process which erodes the material by a rapid succession of sparks, leaving a cratered surface. Nevertheless, the larger size of the dimples and nodules inside the holes on the tested surface suggests that ink corrosion and VPC had a significant eltL~ct. It is also worth noticing that the extent of the corrosion attack was greatest on nobles machined with higher heat input. It is therefore reasonable to suggest that corrosion is directly related to the MEDM machining variables and that material A nozzles will corrode when used with solvent., waterand ethanol-based inks. Initiation of this corrosion may be associated with the sensitized region around the hole, Figs. 21-24 show SEM micrographs of n, aterial C nozzles machined with low (Figs. 21, 22) and high

D.M. Alh':~, A. Lecheheb /.hmr~ud of Materials t¥~wesx#:g '$~chm~/~s~r 58 ~l~,)!)(~)53 56

Fig. 21. x 750 view of material C nozzle foil machined with low energy input (0.01 |tJ): (a) belbre and (b) after long-term corrosion test in solvent.based ink.

(Figs. 23, 24) levels of energy input. The micrographs depict the condition of the orifices and their surroundings betbre and after corrosion testing. The micrographs in Figs. 21 and 23 were taken at × 750 magnification and show the complete MEDM hole with some of its wall and the surrounding nozzle foil. Those of Figs. 22 and 24 were taken at x 1500 magnification and show part of the edge and wall of the orifice in detail. It can be observed that the size ot" the craters and dimples on the wall of the hole as well as the irregularities of the edge of the orifice increase with the MEDM energy input. However, these nozzles appear to be immune to corrosion attack by the above cited inks and their vapours as neither the sharpness of the edge nor the surface topography of the wall of the hole have been adversely affected by the testing. This further supports the theory that the formation of a sensitized region around the holes is related to high carbon content.

65

Fig, 22. x 1500 view of the nozzle shown in Fig. i1: (a) before and (b) after the long.term corrosion test in solvent-based ink.

4. Condusions This study has found that the 'quality' of llozzles produced by MEDM increases as the energy input decreases for all the materials studied. Jet instability appears to be linked with a high carbon build up around the MEDM hole. This deposit most likely originates fl'om the cracking of the dielectric fluid and fl'om tile rehttively high carbon content of the material from which the nozzle is made. The carbon is thought to diffuse 1hrough the porous multilayered oxide leading to the build up of the thick and crack-prone overlayer around the hole. Ink corrosion tests also show deterioration of nozzle performance with increasing carbon content and MEDM input energy. This was traced to the same carbon-rich deposit which contained cracks and which were shown to increase in size as the corrosion tests progressed.

D,M, Allen, A, Lecheh,eb / Jouenal of Materials Processing Technok,gy 58 f1996) 53,,-66

Fig, 23, Machined with higher energy input (0,7 taJ): (a) before and (b) after long,term corrosion test in solvent,based ink,

Fig, 24. x 1500 view of tile noule shown in Fig. 13: (a) before and (b) after long-term corrosion test in solvent-based ink.

From this work it is clear that low MEDM inpt|t energy and low carbon stainless steels which do not possess porous oxide layers are necessary for high quality nozzle fabrication, Future work should lead to optimized machining times Par shorter than those used in conventional mi. ere-hole making processes such as watch jewel broach• ~ r ' tlhng, ' ing, mlcro-d glass pulling, silicon etching and electro forming,

hon, 0,7% manganese, 0.4'I';, silicon and 1.15% alu-

Appendix A: Materials and their conditions Material C is an austenitic stainless steel containing 16~ 18% chromium, 10= 14% nickel, 0,03% maximum carbon, 2% manganese, I% silicon, 0,04% phosphorus, 0,03% sulphur and 3%~o4% molybdenum. Material B is similar to material C but it does not contain molybdenum. This austenitic stainless steel is used in applications requiring welding. Material A is a semi-austenitic stainless steel containing 17% chromium, 7% nickel, 0.07% maximum car-

minium, Referenc~ [I] R, Mitchell, A. Lecheheb, B, Rock and M. Fox, Ink Jet printing for high volume postal applications, in Jetposte" 93 Seminar. Proe, Ist l~)~ropean C'olff~ dedicated to Posted Technologh,s, Vol, I, Nantes, France (1993), pp, 61-68. [2] L, Kuhn, E, Bassous and R, Lane, Silicon charge electrode array for ink-jet printing, Ib,'EE Trans, buL Applic., IA-25 (10) (1978) 1257~o1260, [3] E, Bassous, it.H, Taub and L, Kuhn, Ink jet printing nozzle arrays etched in silicon, Appl, Phys. Lt.tt,, 31 (2) (1977) 135137, [4] T, Sate, T, Mizutani, K, Yonemochi and K, Kawata, The development of an electrodischarge machine for micro-hole boring, Pr~,ci,~, i~)~g,, 8 (3)(1986) 163 168. [5] T, Masaki, K. Kawata, A, Shibuya and T, Masuzawa, Micro electro-discharge machining,, in Proc, hat. Syrup. Electro-MachOffng ¢ISEM-9), Nagoya (1989), pp. 2629, [6] K, Kagaya, Y, Oishi and K, Yada, Micro electro-discharge machining using water as a working fluid 2: Narrow slit fabrication, Precis, Eng,, 12 (4) (1990) 213 217,