Overview of HP Jet Fusion Secondary Post-Processes
Secondary post-processing is an optional step in the end-to-end workflow for parts printed with Multi Jet Fusion technology.
While the raw part—once it has been suitably cleaned by bead blasting or water jet blasting—can be used as is, without further treatment, the specific application of the final part may require processing to fulfill technical requirements, such as a colored finish or a smooth tactile surface.
The techniques included in this step have been classified as providing cosmetics or reducing surface roughness; however, some may provide both attributes. For example, electroplating gives color and finish to the part but also acts as a functional coating, minimizing surface roughness.
In addition to many available cosmetic finishes, post-machining can improve part quality by improving geometric tolerances and localized surface finish. Typical machining tolerances are tighter than 3D printing, and milled surfaces are glossy and jet-black.
Cosmetics
The cosmetics category can be subdivided into techniques that provide color uniformity in the raw part, such as dyeing and graphite blasting, and those that provide coloring different from that offered by the natural part. However, since some techniques (dyeing, for example) give both uniformity and coloring, this distinction has not been made.
The cosmetic category includes five key techniques: Dyeing, painting, graphite blasting, smooth blasting, and electroplating.
Dyeing involves immersing the part in a hot dye for a prescribed period so that the dye penetrates the part completely until a specific color or color homogeneity across the entire part surface is achieved.
Manual Dyeing
Manual dyeing is an inexpensive, efficient technique using a dye bath, vat, and hotplate. The dye mix is made up in the dye bath and then conditioned. It is then heated to 80-100 °C, and the parts are immersed in the mix for approximately 8 minutes. The dyeing time will depend on the color of the raw part and the desired color intensity. The newly dyed parts are then transferred to the rinse bath containing water heated to 60 °C to remove any excess dye from the parts and avoid staining. The parts are then left to dry naturally or by placing them in a drying oven at 50 °C.
Automatic Dyeing Solutions
Automatic dyeing solutions are a convenient and efficient alternative that requires less operator intervention. The principle is identical, but these solutions offer specific programs for mixing, conditioning, dyeing, part rinsing, dye disposal, and cleaning. In some automatic dyeing machines, the mix can be reused 5 to 10 times, and stirring features ensure the dye solution is always homogeneous, achieving better results.
Dyes are available in powder and liquid format. The concentration of the dye mix depends on the dye specifications and the desired color intensity, with approximately seven g/L for powder dyes and a ratio of 0.1 L dye to 10 L water for liquid dyes. The dyeing temperature also varies according to the format used: 100 °C for powder dyes and 80 °C for liquid dyes.
For the best results when dyeing, parts must have been thoroughly cleaned to ensure they are completely free of any unfused material. Parts must move freely within the dye mix, so the dye bath should be adequate for the dyed volume. Small, fragile parts should be placed in a mesh bag to protect them from the effects of stirring in the dye mix.
If stains appear on the parts due to scratch marks or where the dye has become too concentrated—even after rinsing—such marks can be removed by bead-blasting or by smoothing blasting.
Furthermore, in parts prone to warpage, such as large, thin, and flat parts more delicate than 2 mm, there may be increased warpage due to thermal shock and tension caused by the technique. Due to minimal water absorption, there can be a slight impact on tensile strength, elongation at break, and tensile modulus in dyed parts.
