Pieces and components made of plastic material lend themselves to various post-processing treatments and finishes aimed at obtaining excellent aesthetics, particular effects or specific technical and functional characteristics.
Industrial additive manufacturing always involves a phase immediately after the production of the piece which takes place in a dedicated department and varies according to the technology used. For technologies that require it, such as stereolithography, the post-extraction phase begins with the removal of the supports, for other technologies it starts by simply cleaning the pieces extracted from the machine. Afterwards, a uniform surface is obtained by using special treatments such as micro-shot peening and sandblasting.
In the specific case of HP and SLS it is possible to use a coloured impregnation treatment, called colour Dyeing, in which the piece absorbs a considerable quantity of specific ink, generally black, so as to obtain a more uniform finish.
During the post extraction phase, operations are also carried out to make subsequent assembly of the pieces easier, if required, through the correct sizing of the critical areas.
Out of all the possible finishes, without doubt painting is the most common post-processing activity and can be used in a multitude of cases. Painting allows you to establish a series of characteristics including the colour, from RAL or Pantone, the level of finish, glossy, matt or embossed, and other specific parameters (fluorescent colour, metallic, etc.).
For a high-quality aesthetic result, it is key to prepare the surface to be painted, the complexity of which will vary depending on the technology used for the production of the piece: some technologies, such as stereolithography, lend themselves more to aesthetic details, other technologies, such as SLS, HP and FDM, on the other hand, are more oriented towards functional aspects and therefore require precise preparation if you intend to proceed with painting.
Part preparation requires smoothing of the piece and possible filling, followed, if necessary, by masking of the areas where paint is not to be applied. A primer is then applied and, finally, the paint.
It is possible to establish the desired level of painting: from a simple coat of spray paint to actual hand painting with precise features.
Smoothing considerably reduces surface roughness of the piece achieving the values required for the project. Since this is a manual process, a loss of quality in terms of detail and shape is possible, especially when the pieces are small: therefore, a high level of manual expertise of those carrying out the process is key. Smoothing can also be used to prepare the substrate for an aesthetic paint finish.
Polishing of thermoplastic materials can be achieved manually, thermally, chemically or by milling with diamond tools.
The activity is crucial when the prototype must be transparent as in the case, for example, of a light guide.
Accurate polishing guarantees that the light output of the prototype corresponds, both in terms of quality and quantity, to that of the mass-produced object.
The natural reduction of roughness by polishing is particularly evident in stereolithography parts where, starting from a transparent resin, a higher level of transparency is reached by first polishing the piece and then applying a special paint that fills the micro imperfections of the surface.
Various industrial treatments carried out on final pieces can be replicated at a prototype level; this is the case of metallising through galvanisation which is generally done with chrome and nickel plating.
Galvanisation, given the complexity of the operation (preparation of the bath, activation, etc.), requires production runs with a high number of pieces and is not ideal for prototyping which instead uses vacuum metallising to obtain the same aesthetic effect. Vacuum metallising allows the piece to be coated with a thin layer of metallic material, making the surface metallised; successful metallising requires good preparation of the substrate since the applied metal layer is extremely thin and therefore not sufficient to cover any imperfections.
Vacuum metallising is extremely practical for prototypes and allows you to even work on small batches or individual pieces. For prototypes made of plastic materials, aluminising is very common, more fragile than nickel and chrome plating, but suitable for prototype use.
Other aesthetic finishes
Aesthetic finishes available for pieces made of plastic material include flocking, soft-touch, screen printing and pad printing, laser-etching, PVD and sputtering.
Flocking is similar to painting and involves the depositing of fibres on the surface of the piece to create a velvet effect.
Similar to flocking is the creation of a “soft touch” surface, that can have both a decorative and practical function by increasing the grip.
Screen printing and pad printing are transfer technologies in which a layer of paint is applied selectively thanks to a screen printing frame (it is used for example to apply logos, symbols or indications on buttons, etc.). Screen printing is used on flat surfaces, whereas pad printing is for curved surfaces.
Laser-etching is a kind of laser engraving or marking. Despite having a similar function to that of screen printing and pad printing, it differs from these since it guarantees a long-lasting result: elements affixed to the component with a paint can fade or fade over time and with continuous use, whereas in the case of laser-etching, being a marking "etched" in the material, long-lasting legibility is ensured (a particularly useful feature in cases where an identification code of the piece is required to ensure traceability).
PVD and sputtering are layering methods similar to metallising that allow you to obtain very bright colours with great aesthetic effect.
Functional post-processing treatments
Pieces and components made of plastic material do not present the typical problems of metal and, usually, do not require post-processing treatments to be functional. However, it is possible to use some treatments to obtain certain characteristics.
For example, for materials such as those used in SLS technology it is possible to carry out a special treatment to make the piece water-repellent: an impregnation is carried out which closes the micro-porosities and interstices of the material making it water resistant.
Other examples are copper plating, which allows the internal part of a plastic piece to be electrically or electromagnetically insulated thanks to the metal coating, and metal encapsulation which, as its name suggests, encapsulates the plastic piece inside a metal structure. Metal encapsulation is a type of galvanisation that usually begins with the application of a layer of copper on which other layers of material (typically nickel) are added, in order to obtain special plastic-based prototypes with greater mechanical strengths depending on the layers of metal added. Encapsulation is possible not only with metals, but also with fibreglass.