Laser printing of soluble toner for rapid manufacturing.

Banerjee, Soumya ; Wimpenny, David

1. INTRODUCTION

Standard laser printers work on the principle of electrophotography, where fine polymeric toner powder is picked up and precisely deposited on a substrate, usually paper, using electrostatic forces. Laser printing offers several key advantages for layer manufacturing applications; it can process dry powders at high speed, offers excellent resolution and it is capable of forming functionally graded structures. Once the toner has been deposited it is fused to form a solid layer, typically by contact with heated rollers or via infrared radiant heating. Laser printing is, in principle, highly adaptable to a range of toner materials based on thermoplastics, cermics and metallic powders. The potential application of laser printing in layer manufacturing has been recognized by several researchers (Bynum.K.D 1992, Grenada 2001, Kumar 2000, 2003 ,2004, Cormier 2002 and Banerjee 2006,2007).

At least two fully functional toners are required to enable complex 3D objects with overhanging features to be formed produce. One should be tough toner for the build material and the other toner should be support material which can be easily removed from the finished object. This paper, aims to study the feasibility of developing soluble toner support material.

2. TONER MATERIAL DEVELOPMENT

2.1 Selection of Toner Materials

Toner can be of two types; either mono-component or dual component. Mono-component toner is very popular for desktop applications whereas dual component toner is more common for industrial printers. Dual component toner is generally comprised of magnetic carrier particles (30 to 300 [micro]m) and fine toner particles (5 to 15 [micro]m) see Figure 1. Dual component systems have an advantage that the carrier particles "do most of the work" in terms of electrostatic transfer and this makes them more flexible in terms of toner formulation. In addition dual component printers offer much high printing speeds, increased layer thickness and better resolution than mono-component machines. Research was carried out at DeMontfort University to check the feasibility of developing a soluble dual component toner material using

P400 support material, an acrylic based copolymer employed as a soluble support material in the FDM process.

[FIGURE 1 OMITTED]

2.2 Powder Grinding Trial

Initial trials were carried out to check if P400 support material can be readily processed to produce fine powder. Short lengths (6mm long) of P400 filament were loaded into a Retsch PM100 impact mill and milled for 2 hours. The powder produced was then sieved using a Retsch AS200 sieve shaker (with mesh size of 20, 36, 50 and 100 micron).It was found that and 30% of the material was below 20 micron. See Figures 2 and 3 below showing the P400 before and after milling respectively.

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To reduce the grinding time, further samples were processed using a more efficient industrial scale Hosokawa 100 AFG mill. After grinding the particle size was checked by laser diffraction and it was found that approximately 60 % of 'P400 support material' was within 20 micron particle size after grinding (see Figure 4 below).

[FIGURE 4 OMITTED]

2.3 Infrared Sintering trial

The ability to sinter the powder material using an infrared radiant source was then evaluated. Loose P400 powder was deposited manually into a mould (tensile test specimen 100mm long x 2mm thick) bone. The mould was then placed under infrared radiated heater (average wavelength of 0.78 to 2[micro]m) at a distance of 100 mm from the source of radiation. Particles are joined together and formed continuous layers with 2mm thickness approximately (see Figure 5. A little shrinkage was observed after sintering.

[FIGURE 5 OMITTED]

2.4 Printing trials

An office dual toner laser printer, Ricoh 7000, was selected for the printing trial (see Figure 6 below). P400 powder was surface coated by mechanical mixing using fumed silica (HDK20TX) with a particle size between 0.17 to 0.25[micro]m. This flow/charge control agent is normally used for developing negatively charged toner. The surface coated P400 support powder was mixed with ferrite carrier to give a developer with a 10% toner concentration.

[FIGURE 6 OMITTED]

The standard black cartridge of the printer was filled with the toner (developer) formulation and test prints produced (see Figure 7). It was noted that deposition of powder particle was not homogeneous throughout the printed areas , with patches of greater and less deposition.

[FIGURE 7 OMITTED]

3. DISCUSSION

These initial trials indicate that P400 soluble acrylic material can be processed to produce a suitable dual component toner formulation which can be successfully printed and fused. However, there are clearly aspects which require further development. During printing toner particles were prematurely dislodged from the surface of the carrier. This was primarily due to weak adhesive force between P400 support and carrier particles. This adhesive force needs to be strong enough to hold toner particles until they are deposited selectively on OPC (Organophotoconductor) drum of the printer. It should be possible to reduce this problem by fine tuning of the carrier particle size, shape, surface texture as well as applying the most appropriate polymeric coating onto its surface.

The dielectric constant of powdered materials is strongly influenced by moisture content in the powder and the humidity level in the surrounding environment. This problem is more acute for materials which are hygroscopic in nature, such as water soluble material like P400. Care must be taken while making the full functional toner to reduce the absorption of moisture. This could be achieved by using silica gel/dehumidifier or similar device to control the humidity and moisture content of the operating environment.

4. REFERENCES

Bynum. K.D. (1992) Automated manufacturing system using thin sections, US Patent 5088047.

Grenda E.P. (2001) Apparatus of fabricating 3 dimensional objects by means of electrophotography, ionography or similar process, US patent 6206672.

Kumar A.V. (2000). Solid free form fabrication using powder deposition, US patent 6066285.

Kumar A. V. and Dutta A. (2003) Investigation of an electrophotography based rapid prototyping technology. Rapid Prototyping Journal, Vol 9,Nos 2, pp 95-103. Emerald, ISSN1 355-2546

Kumar A. V. and Dutta A. (2004) Electrophotographic printing of part and binder powders, Rapid Prototyping Journal, Vol 10, Nos 1, pp 7-13.

Cormier D. (2002). An investigation of selective colouring with 3-D Laser printing. Journal of manufacturing Processes, vol 4, No2, ASME, ISSN: 0022-1817

Banerjee S. and Wimpenny D. (2006) Laser printing of polymeric materials, 17th Solid Freeform Fabrication Conference, University of Texas.

Banerjee S. and Wimpenny D. (2007) Rapid Manufacturing of Thermoplastic Parts by Laser Printing, International Conference on Polymers & Mould Innovations, Belgium.