Solid freeform fabrication is a process for manufacturing three-dimensional objects, for example, prototype parts, models and working tools. Solid freeform fabrication is an additive process in which an object, which is described by electronic data, is automatically built, usually layer-by-layer, from base materials.
Several principal forms of solid freeform fabrication involve a liquid ejection process. There are two main types of solid freeform fabrication that use liquid-ejection: binder-jetting systems and bulk-jetting systems.
Binder-jetting systems create objects by ejecting a binder onto a flat bed of powdered build material. Each powder layer may be dispensed or spread as a dry powder or a slurry. Wherever the binder is selectively ejected into the powder layer, the powder is bound into a cross section or layer of the object being formed.
Bulk-jetting systems generate objects by ejecting a solidifiable build material and a solidifiable support material onto a platform. The support material, which is temporary in nature, is dispensed to enable overhangs in the object and can be of the same or different material from the object.
In both cases, fabrication is typically performed layer-by-layer, with each layer representing another cross section of the final desired object. Adjacent layers are adhered to one another in a predetermined pattern to build up the desired object.
In addition to selectively forming each layer of the desired object, solid freeform fabrication systems can provide a color or color pattern on each layer of the object. For example, inkjet technology can be employed in which a number of differently colored inks are selectively ejected from the nozzles of a liquid ejection apparatus and blended on the build material to provide a full spectrum of colors. In binder-jetting systems, the binder may be colored such that the functions of binding and coloring are integrated. In bulk-jetting systems, the build material may be colored. On each individual layer of the object, conventional two-dimensional multi-pass color techniques and half-toning algorithms can be used to hide defects and achieve a broad range of desired color hues in the completed object.
One of the on-going deficiencies of the solid freeform fabrication techniques described above is inaccurate color reproduction in fabricated objects. In particular, colorants (dyes or pigments) soak below the surface of the object as it is fabricated, causing the apparent colors of the object to become faded as more air and build material come between the colorant and an observer. Additionally, when two or more different colorants are used in adjacent portions of an object, bleeding occurs as one colorant invades into another colorant and the border between the colorants becomes ragged.
In some previous solid freeform fabrication systems that employ ink-jet technology to add color to the object being fabricated, color has been added by printing a desired color throughout each layer of the object being fabricated. In other solid freeform fabrication systems a desired apparent coloring in an object is obtained by varying the color of successive layers that are nested inwardly from a surface of the object.