In addition to quality and cost, time to market is becoming an increasingly important factor in a product's success. A major contributor to product development cycles is the time needed to produce prototypes. Further, in markets where customers are demanding individually tailored components, the cost and time to produce tools or moulds can be considerable.
Rapid Prototyping (RP), also known as Solid Freeform Fabrication (SFF) processes help to overcome these problems In general the processes build complex shapes through additive processes, producing components without the use of tools.
Ink Jet Methods
There are two methods of using ink jet for SFF. The more mature technology is three dimensional printing (3DP) which uses an ink jet to apply a binder to ceramic powder. Direct Ceramic Jet Printing (DCJP) deposits ceramic suspensions directly via an ink jet printing nozzle.
Three Dimensional Printing (3DP)
Three-dimensional printing builds parts in layers. First a layer of loose powder is spread over the machine bed. The component slice is then formed by ink jets which selectively inject a binder into the material to form a green part. The binder can be a refractory material such as colloidal silica or a temporary polymer binder. A piston lowers the part, the next powder layer is spread and the binder is printed again. The procedure is repeated until the part is completed. The part when removed from the bed of unbound powder has a relatively low density, typically 50% theoretical, and is either densified by isostatic pressing and sintered or is infiltrated to give a dense part.
Figure 1. Schematic of a 3DP setup.
A 3-D Printer (Z-Corp)
3DP is able to create parts with complex geometric features such as undercuts, overhangs and internal voids.
3DP has been used to produce several items:
• Ceramic moulds and cores for metal casting in Al2O3, SiO2, ZrO2 and SiC printed with a refractory colloidal silica binder
• Filters for power plants with complex geometries
• Monolithic Al2O3 components with densities of over 99.3% theoretical and average flexural strength of 360MPa
• Sintered Si3N4 parts
• Materials with controlled or graded microstructures. For example Zirconia Toughened Alumina (ZTA) can be made by depositing a ZrO2 slurry onto an Al2O3 bed.
Disadvantages of 3DP
Drawbacks of the technique are anisotropic mechanical properties caused by the weak bonding between the layers and poor control of the dimensions of the grown part due to particle size distribution, wetting of the binder and the processing parameters. Isostatic pressing and sintering reduce anisotropic properties.
Direct Ceramic Jet Printing
Direct Ceramic Jet Printing uses the ceramic powder contained in an ink which passes through a printer nozzle. Since a typical nozzle diameter is around 60 micrometers, the typical droplet diameter is 100 micrometers. Successful DCJP depends on producing a suitable ceramic ink which is essentially a well-dispersed suspension of a fine powder. The dispersion must be stable and free from agglomerates.