HOTEL 5 ESTRELLAS

This subsection has been added to support issues and discussions arising in §4.6 and §7-9 and is best understood after gaining an appreciation of multilayer transfer challenges.

Upon preliminary consideration, it may seem that using a conductive toner could resolve nearly all of the problems with multilayer transfer described in the literature (as reviewed §3.5). While the challenges detailed in §4.6 may cease to be an issue, depositing conductive toner introduces new challenges depending on the type of development system being used as described below. The comments in this section refer to the intended use of toner materials which are inherently conductive (such as metals).

Discharge in a Two-component Developer

A two-component developer (§3.4.3) is reliant upon attraction between toner and carrier due to tribocharging [103]. Although it is possible to

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tribocharge metals, they discharge very easily due to their inherent conductivity making them impractical to handle in a conventional two- component developer [103]. For that reason, printing of metals using a two-component printer has been achieved by loading it into a polymer matrix toner or encapsulating it within a dielectric layer, which enables it to retain its charge throughout the printing process as has been discussed (§3.2.1.2) and demonstrated [95, 97]. It is noteworthy that the coating must be robust enough to survive hopper/developer mixing and be thick enough to be able to supply an adequate number of electrons to achieve electrostatic adhesion with the carrier (since the Coulomb force is directly proportional to charge, which is directly proportional to the number of electrons available) [103].

Bouncing of Conductive Toners between Charged Plates

Charge induction of conductive particles is one of the most widely implemented means of charging metal particles (including use in the Metal Printing Process §4.4.4). Given the use of toner which is inherently conductive prior to deposition, one could assume that it would continue to be conductive after consolidation. If this were the case, the build surface could be considered to behave effectively like a conductive plate. During the transfer step, this build surface is in close proximity with the surface of the photoreceptor which behaves like a conductor where it has been exposed, so therefore the toner behaviour in that gap (after printing the first monolayer of toner) can be modelled using two conductive plates arranged as shown in Figure 3.28.

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Attempting an electrostatic transfer step requires a potential difference between the photoreceptor and the build surface. The potential difference sets up a field which induction charges conductive particles therein. Since the electrons in the particles are free to transfer out of each particle even with only brisk contact (conduction) with another conductor, unique “bouncing” behaviour is observed as described below.

When a conductive particle is in contact with a charged plate, charge will flow into the particle to exclude the electric field from its interior, thus redistributing electrons according to Gauss’ law for a conductor (Figure 3.10) [108, 227]. Therefore, without mechanically restraining the particle, it will remain in contact only long enough to achieve the same charge as the plate, after which it is repelled away as shown in Figure 3.28 (left). This makes it nearly impossible to attract and hold particles onto consolidated charged layers. After arriving at the second, oppositely charged plate, the particle is not simply neutralized because, opposite charge flows from the second plate into the particle to null any internal electric fields [108]. Having exchanged charge to achieve the same charge as the second plate, it will be repelled back toward the first plate. Even if one of the plates is grounded, the charged plate (and resulting field) will induce charge separation in the grounded plate. This will result in an opposite polarity charge-rich surface on the grounded plate, which will supply charge into the particle to null the internal field (and distribute itself according to Gauss’ law), thereby perpetuating the cycle. This cyclic attraction, contact exchange of electrons, and repulsion, creates an oscillation of the particle between the two plates. Each cycle actually

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completes a circuit by transferring a small amount of charge across the gap (i.e. intermittent current flow) between the two plates [228]. Cho [227] showed that the average electric field on the surface of the charged particle on the plate compared to the electric field between the two plates was 1.65x higher [108]. This reciprocating motion between the two plates has been documented by various researchers, and is often referred to as the “bouncing” problem [54, 103, 108, 181].

Figure 3.28 – Behaviour of conductive powder particles in between conductive and insulative electrode surfaces (After figure 5-13 from [181])

This issue may be avoided by covering one of the plates with an insulative material as shown in Figure 3.28 (right). This is the case for conductive toner printing where the paper acts as the dielectric (as long as humidity is low) and allows transfer of a monolayer of toner [108]. During part of the development of the Metal Printing Process (MPP) (§4.4.4), Sintef deposited wax onto a conductive plate relying upon its insulative and adhesive properties to develop an image using conductive and non-conductive powder [63, 229]. In order for this to solve the transfer problem, as identified in §4.6, it would be necessary to put an

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insulating layer in between each layer of toner (essentially requiring the opposite action as suggested by Honjo [104] as shown in Figure 3.11); which undermines any practical or intended transfer benefits of printing inherently conductive toner layer-on-layer.

Challenges with printing encapsulated conductive particles

In view of the toner bouncing tendency described above, it may seem logical to “retreat” to encapsulating the conductive particles and triboelectrically charging them. While this offers the ability to develop them with a magnetic brush, it does not substantially improve the prospect of electrostatic transfer due to polarization (unless the shell is relatively thick around the core) [103].

Where a contact transfer is used, the conductive core of the particle becomes polarized on the photoreceptor which then repels it away from the substrate as shown by Walker et al. [103] in Figure 3.29.

Figure 3.29 – Electrostatic repulsion created by an encapsulated polarized conductive toner particle during an electrostatic transfer step - After Walker et al. [103]

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The author acknowledges that this principle has been illustrated using simple material classifications and there may be a window of opportunity for materials which are not as conductive as metals, but which are essentially electrostatically dissipating.

Furthermore, both Océ and Delphax have produced devices which use inherently conductive toner (using single component development and ionography respectively) and have dealt with this issue by implementing a thermal transfer and high pressure transfixing step (combining transfer and fixing in the same step) respectively to achieve consistent transfer irrespective of humidity (which enables paper to act as a conductor [54]) [108]. Both thermal input and pressure feature heavily in the more successful early proof of concept attempts as reviewed in §4.4 and analysed in §6.1.

3.4.5.

Fusing

Except in the case of transfixing operations which combine transfer and fixing, toner is fixed to the paper using a fusing means. The most popular method is to use a hot roller, although a range of options are used commercially including fusing by: solvent, radiant heaters, flash, cold rolling, etc. The factors governing fusing physics are typically: temperature, pressure and dwell time in the nip.

Historically, fusing temperatures of 150-180°C were routinely used (and still are for high production machines) as guided by the rule of thumb that fusing temperatures were typically 100°C above the glass transition

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(Tg) temperature of the polymer; however the drive to be more energy efficient has led to development of toners which fuse below 100°C [230].

Typical nip pressures vary between 1-20 kg/cm2 _{(14-142 psi) with the}

most common between 2-3 kg/cm2 _{[231-234]. Typical dwell time in the}

heating nip is 20-50ms for a low-gloss black and white print and 30- 100ms for a high-gloss colour print [230].

Fixing the toner by fusing is prone to two types of defects: cold offset

and hot offset. When the toner is not heated sufficiently to flow, then it

does not adhere well to the paper (or other final receiver) and typically stays on the final imaging member (photoreceptor or transfer roller) [230]. Conversely, when the toner flows too readily (typically due to overheating), hot offset can occur. Hot offset is when the toner particle is liquid enough that it splits into two halves upon exiting the fuser nip, thereby transferring partially onto the fusing roller and partially remaining on the paper [230].

When a transfixing step is used, toner that flows too readily can simultaneously cause hot offset and back transfer (§3.4.4) which can be damaging to the photoreceptor. For this reason printers which use transfixing operations often include a final transfer roller to prevent damaging the photoreceptor; however if overheated toner flow is not sufficiently reduced by the quenching effect of the transfer roller, then a double back transfer can occur fixing toner on the final transfer roller and also on the photoreceptor. A developmental toner-like material used in

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early trials to assess the feasibility of AM by EP is thought to have caused double back transfer (due to fine particles which overheated) (§4.4.5.3).

Historically many fusing rollers were continuously coated with an oil- based release agent to help prevent the toner from adhering to it. More recently, the inclusion of wax inside the toner particle, which is released in the fuser, provides a lubricant which reduces the hot offset tendency of the toner to adhere to the hot roller [230]. Including wax inside the toner has become a far more widely implemented alternative to coating the fusing roller with oil [55]. This is palatable for printing text and images, but is not desirable for AM of 3D parts (unless wax were to be intentionally used as a support material).

It is also important to realize that a satisfactory degree of particle melt for imaging applications (as shown in Figure 3.9) can be relatively low and does not approach full density as known and needed in 3D polymer processing such as injection moulding or AM [235].