Presentation on theme: "On Counter-Charges in Development Rollers for Electrophotography"— Presentation transcript:
1On Counter-Charges in Development Rollers for Electrophotography NIP22NIP-22 Denver, September. 2006On Counter-Charges in Development Rollers for ElectrophotographyInan Chen and Ming-Kai TseQuality Engineering Associates (QEA), Inc.Burlington, MA, USA
2Counter-Charges in Development Rollers for Electrophotography NIP22Counter-Charges in Development Rollers for ElectrophotographyLatent images developed by moving Charged Toners- Extensively studied.Counter-charges : Little attention- Reside in carrier beads (2-component development),or development rollers (Single-Component Dev.)Objectives:Quantitative analyses of roles of counter-chargesin Toner-charging and Toner-deposition in SCDRequirements for ideal roller coating materials,and characterization method for SCD rollersIn electrophotography, latent images are developed by moving charged toners from their supplier to the photoreceptors. Extensive works on the measurements and analyses of toner motion have been reported in the literature.In contrast, little attention has been paid to the motion of counter-charges that reside in the carrier beads in two-component development, or in the development rollers in single-component development (SCD).The objective of this paper is to quantitatively analyze the roles of counter-charges in the toner-charging and toner-deposition steps in SCD, and determine the requirements for ideal roller coating materials, and characterization method for Roller characterization.
3Single-Component Development (SCD) NIP22Single-Component Development (SCD)1. Development Rolls:Conductive elastomer coreSemi-insulator CoatingMBDev.RollPRVB2. Toner Charging atMetering Blade (MB):Charges supplied to toner,Counter-charges to Roll coating3. Toner deposition:Charged toners move to PR,Counter charges impede toner motion,must be removed (neutralized)to improve deposition efficiency1. In SCD, the development rolls consist of a conductive elastomer core surrounded by a semi-insulator coating layer.2. Toners are charged as they pass through the metering blade. At the same time, counter-charges are induced in the roller coating layer.3. In the deposition step, toners move toward the PR, counter-charges impede this toner motion, and must be removed, or neutralized in order to improve the deposition efficiency.
4Single-component Development NIP22Single-component DevelopmentInduction at charging, andNeutralization at deposition of Counter ChargesCharge injection and transportin Semi-insulator Coating layerCharge-Transport ModelNon-Ohmic natureApplied and reported :Roller charging of PR (NIP21)Electrostatic toner transfer (NIP16, 20, ICIS’06)Liquid development (J. App. Phy. 80, 6796)Counter-charges in SCD (This talk)This Induction and Neutralization of counter chargesare achieved by charge injection and transport in the semi-insulator coating layer.We have developed a charge-transport model, which considers the non-Ohmic nature of charge injection and transport in the semi-insulating overcoat layer.The model has been applied to Roller charging of PR’s, Electrostatic transfer of toners to media, and Liquid development, and reported at previous conferences and journal.Today, we’ll apply the model to investigate the induction and neutralization of counter-charges in SCD.
5Charge Transport Model NIP22Charge Transport ModelSemi-insulators characterized by 3 parameters1. Densities of mobile charges, qp(y, t), qn(y, t),Initial (intrinsic) value: qi = qp(y, 0) = –qn(y, 0)2. Charge mobility: m(E) - field dependent3. Charge injection strength sInjection currents from boundary at yJi = sE(y), E(y) = field at y ( = 0 or L)Continuity eq. q(y, t)/t = – (mqE)/yPoisson’s eq. E(y, t)/y = (qp + qn)/eResults for SCD charging and depositionsp, sn– – – – – –yLIn this model, the charge transport in semi-insulating layer is characterized by 3 parameters:1. Density of mobile positive and negative charges, qp and qn, having the initial, or intrinsic value: qi;2. The charge mobility m, generally field dependent. The product of m and qi is the conductivity s.And the third is the charge injection strength s. The injection current Ji from the layer boundary at y is given by the product of s and the field E at y.The continuity equation and Poisson’s equation are used to calculate the time and spatial variations of q and E.The numerical results for SCD charging and deposition steps are discussed in the following slides.
6Toner Charging in SCD (1) NIP22Toner Charging in SCD (1)–VBMetering Blade– – Toner – – QTRoll-CoatingQRyRyTLRsToner CoatingThickness: LT LRPermittivity: eT eRBias voltage VBToner charge density:QT(t) = [VB – QR(t)DR – UR(t)]/(DT/2 + DR) (D = L/e = 1/C)QR(t) = Interface charge densityUR(t) = 0LR dy0y’(qP + qN)dy’/eRTransport equations, calculate QR(t), UR(t) QT(t)In the toner charging step, the charging nip is represented by this layer geometry.A bias voltage VB is applied across the toner and the coating layers.The toner charge density QT can be expressed as a function of the bias voltage VB, the interface charge density QR, and the integral of positive and negative charge densities in the roll-coating layer UR.The counter-charges contribute to the interface charge QR and the integral UR.The Charge transport equations are used to calculate QR and UR as functions of time, which in turn, give the time dependence of toner charge density QT.} Counter-charges
7Toner Charging in SCD (2) NIP22Toner Charging in SCD (2)Growth of toner charge QT(t)with timeDependence onInjection Strength svery significant att 100toHigh speed printing-- short charging time:high s importantThis figure shows numerical examples of the growth of toner charge with time, and its dependence on the strength s of charge injection from the bias into the roller coating.It can be seen that the s dependence is very significant at times of about 100 time units. Here, the unit of time is normalized to the layer thickness, charge mobility and the bias voltage, and has a typical value of 10 msec.This suggests that as the print speed increases, and the charging time becomes shorter it becomes more important to have a good injection from the bias electrode.Units:to = LR2/moVB 10 msecso = moqo = eo/to 3x10–11 S/cmqo= eoVB/LR 3x10–6 C/cm3
8Toner Charging in SCD (3) NIP22Toner Charging in SCD (3)Dependence on mobility mp, mnin RCFor QT< 0Smaller pos mobility mphas significant effect (A, B, C)Insensitive to neg mn (A, D)Build-up of counter-chargemostly from injection ofpos charge from VB,not from depletion ofneg charge in coating layerThis figure shows the effects of charge mobilities, mp and mn on the growth of toner charge density.Assuming the toner charge is negative and the counter-charge is positive, then, when the positive charge mobility mp is decreased, as in curves A, to B, and to C, it has significant effect on the charging rate.On the other hand, the charging is rather insensitive to the negative charge mobility mn, as we can see by comparing curves A and D, which differs in the negative charge mobility.This indicates that the build-up of counter-charges comes mostly from the injection of positive charge from the bias, and not from the depletion of negative charge in the coating layer.UnitsMobility: mo 10–5 cm2/ VsTime: to = LR2/moVB 10–2 secChg density: qo= eoVB/LR2 3x10–6 C/cm3
9Toner Deposition in SCD (1) NIP22Toner Deposition in SCD (1)Fields and Voltages in layersPhotoreceptor: EP, VPToner-layer: ET(y), VTRoller coating: ER(y), VRBias voltage: -VB = VP + VT + VRGauss’ theorem relates charges QP, QR, QT to E’sField in toner layer:ET(y, t) = ET0 + (QT/e)(y/LT) (detail in Proc. paper)= func.[VB, QP, QT, QR(t), UR(t), L’s, e’s]Injection & transport of Counter-charges in RCcontribute to QR(t), UR(t)QTNow, for toner-deposition:This figure shows a layer geometry of the development nip, consisting of the photoreceptor, the toner and the roller coating layers.The applied bias voltage VB is equal to the sum of the three layer voltages.The Gauss theorem relates the layer charges to the fields.Then, the field in the toner layer ET can be expressed as a function of the bias voltage VB; the charge densities QP, QT and QR; the integral of mobile charge densities in the coating layer UR, and the layer thickness and permittivity.The injection and transport of counter charges contribute to QR and UR.
10Toner Deposition in SCD (2) NIP22Toner Deposition in SCD (2)Negative toner deposition: ET(y, t) > 0Demarcation line at y = YDET > 0 for y < YDET < 0 for y > YDET(YD) = ET0 + (QT/eT)(YD/LT) = 0Deposition efficiency:YD/LT = – eTET0/QT= func.[VB, QP, QT, QR(t), UR(t), L’s, e’s] (in proc. paper)QR(t), UR(t) from Transport Eqs.Time evolution of Deposition efficiency YD/LT–LPPRRoll Coating (RC)–VB– – Toner – QTQPLTyQRYDFor negatively charged toners to be deposited on the PR, a positive ET is required.If a demarcation line at y = YD separates the toner layer with positive ET(yT) from that with negative ET(yT),then ET at YD is = 0, and the ratio of YD to the total toner thickness LT cancan represent the efficiency of deposition.We can calculate the contribution of counter-charges to QR and UR as functions of time from the charge transport equations, and examine the time evolution of the deposition efficiency YD/LT.
11Toner Deposition in SCD (3) NIP22Toner Deposition in SCD (3)Deposition efficiency YD/LT vs. timeDependence on strength sof injection into RC from VBSignificant effectsdue to small s,in time 10 < t <100Time unit:to = LR2/moVB 10 msecThis figure shows examples of calculated deposition efficiency. In particular, it shows the dependence on the strength of charge injection into the coating layer from the bias.We can see a significant effect of reduced injection s in the time ranges from 10 to 100 time units.Here, the normalized time unit, as mentioned before, has a value of the order of 10 msec, in typical applications.
12pos and neg charges in SCD roller-coating NIP22Toner Deposition in SCD (4)Charge mobility (mP, mN) dependence of YD/LT (QT< 0)Neg. mn reduced (A B C) Significant decreasePos. mp reduced (A D) No effectsNeutralize Counter-chargerequires negative chargeinjection and transport opposite to polarityrequired at chargingThis figure shows the dependence of deposition efficiency (of negative toners) on the charge mobilities.We can see that when the negative mobility mn is reduced, as in curves A to B to C, there is a significant decrease in the deposition efficiency. But, there is little effect if the positive mobility is small, as in curve DThis suggests that the neutralization of counter-charges requires the injection and transport of negative charges from the bias. This polarity is opposite to that required at the charging step.Therefore, we can conclude that to achieve efficient charging and deposition, it requires a good injection and transport for both positive and negative charges in the SCD roller coating.For efficient charging & deposition, it requires good injection (s) and transport (m) for bothpos and neg charges in SCD roller-coating
13Summary and Conclusions (1) NIP22Summary and Conclusions (1)In SCD, Counter Charges in semi-insulator coatingInduced at toner Charging, andNeutralized at toner Deposition stepsAnalyses: Charge-Transport modelGood bi-polar charge injection and transporte.g., for negative toners,Pos. charge inject. & transport for ChargingNeg. charge inject. & transport for DepositionProcess time > 100 to (to = LR2/mVB)High speed printing requires high mobility m (+ and –)Dev. Roller performance can’t be evaluated properlywith closed-circuit resistance measurementsIn summary, we have shown that counter-charges in the semi-insulator coating layer must be efficiently induced at the charging step, and efficiently neutralized at the deposition step.Our analyses based on charge-transport model indicate that this requires good bi-polar charge injection and transport.For example, if the toner charge is negative, positive charge injection is required for charging, and negative charge injection for deposition.In addition, we find that the process takes more than 100 time units. The definition of time unit suggests that for high speed printing with short process time, high mobilities m, for both positive and negative charges are very desirable.Because of these complex requirements, the performance of roller coatings cannot be evaluated properly with the conventional method of closed-circuit resistance measurements.
14Summary and Conclusions (2) NIP22Summary and Conclusions (2)Alternative evaluation method:Electrostatic Charge Decay (ECD) technique(NIP-11, 15, 16, 17; ICIS’02; JHC-00, 02, 05)Open-circuit voltage decay- simulating actual process in ElectrophotographyField applied by Corona charging- Scan and map large area,efficient, non-destructiveApplied to transfer belts, paper,charge rolls, dev- rolls, PRConsistently predict device performanceWe have developed an alternative evaluation method known as ECD, introduced at previous conferences.This technique is based on measurements of open-circuit voltage decay, simulating the actual process in EP.
16Thank you for your attention Please visit Exhibition Booth #210 NIP22NIP-22 Denver, September. 2006Thank you for your attention Please visit Exhibition Booth #210Inan Chen and Ming-Kai TseQuality Engineering Associates (QEA), Inc.Burlington, MA, USA