Smoothing 3D objects improves the surface quality and thereby the look of the model. This is of only secondary importance when working with visually impaired students. Nevertheless, it makes sense from time to time to reflect on the option of smoothing. After all, this optimises not only the optical surface quality but also the tactile quality.
Because post-processing smoothing renders the individual layers almost invisible, it is no problem to print with a very low resolution (higher layer heights) of 0.2 to 0.3mm. This can cut the printing time almost in half.
The big benefit is that more support structures can be added. With smoothing, there is no longer any need to worry that the bases will still be noticeable afterwards and will disturb handling of the model.
Sandpaper
The cheapest, but also the most time-consuming method for smoothing PLA prints is sanding with sandpaper.
It is best to use a wet sanding method for sanding plastics. This paper is waterproof and is continuously immersed in water during sanding. The water cools the plastic during sanding. PLA in particular can get hot very quickly when sanding by hand.
You should start with a relatively coarse wet sandpaper (for example, with a 200 grit). Afterwards, the surfaces are smoothed further with a 400 grit. The final step is often sandpaper with a 1000 grit or finer. Sanding should be done in a circular motion, as this will diffuse the sanding marks.
The disadvantage of smoothing with sandpaper is that small breakthroughs, recesses and edges cannot be reached easily with wet sandpaper. For small breakthroughs, you can try using diamond files. They have a similar surface to sandpaper. Diamond files can also be used for wet sanding.
Nevertheless, sanding remains very time-consuming and does not give a very good finish either.
Polishing
After sanding, the surface is smooth but dull. This surface quality is ideal if the model is to be painted.
To achieve a shiny surface, sand with even finer sandpaper of up to about 2000 or 4000 grit.
A special plastic polish can then be applied. It ensures a smooth surface by filling and concealing fine grooves. This gives the surface a very smooth and shiny appearance.
You can also use the polishing compound with an electric hand grinder such as a Dremel. To do this small polishing discs are used onto which some polishing paste is applied.
The machine should be set to a low speed and you should only work with low pressure.
Spraypaint and fast-drying polyurethane
First clean the model of any dust, dirt and hair.
The model is first spraypainted with the colour of your choice. Directly on top of this, a layer of fast-drying polyurethane is sprayed.
The object is now placed in front of a fan so that the paint does not run. The two substances will mix and settle into the small grooves, holes and flaws and fill them in. This technique works best with thin layers of paint.
Several coats are needed, which makes this technique time-consuming, especially at the end, as you have to wait a long time until the model is completely dry.
Example of the spraypaint for plastic
Epoxy resins
Epoxy resins are classically used to glue things together. Models made of PLA can also be glued with epoxy resins. However, epoxy resins can also be used to achieve smoothing. Basically, any epoxy resin with the right consistency is suitable for this. It must be possible to apply it with a brush. The curing time should not be too short, which is why it is better not to use 5-minute epoxy resins.
The XTC-3D resin from SmoothOn is available specifically for smoothing 3D models made of PLA. As with any epoxy, the two components (resin and hardener) must first be mixed together. Equal parts of each liquid should be used as far as possible but a precise ratio is not necessary. The mixture must now be applied to the model within 30 minutes.
Like other epoxy resins, XTC-3D creates a thin glossy protective layer, and does not melt the PLA.
Smoothing other filaments
Smoothing ABS
Models made of ABS are much more complex to print than models made of PLA. However, smoothing with acetone vapour only works with ABS. At this point we would like to point out that we explicitly do NOT recommend this technique for safety reasons.
Acetone dissolves ABS. If you apply it correctly, all visible layer lines are completely and totally removed and a smooth and clean surface is created. The 3D printed part will look more like an injection moulded object, rather than one that just came out of a 3D printer. The results are truly amazing!
The best way to smooth ABS is with an acetone vapour bath (“ABS Acetone Vapour Smoothing”).
Again, the dangers should be highlighted:
Acetone is extremely volatile and the vapours are highly flammable. Therefore, no naked and unprotected ignition sources, no flames or fireplaces should be in the vicinity. Acetone must not be heated under any circumstances! Inhalation of the vapours must be prevented at all costs. Avoid all contact with the skin. When steam-smoothing, children and animals must not be in the same room. For these reasons, we do not recommend Acetone Vapour Smoothing unless you are in a professional environment and have the necessary expertise to ensure safety.
Smoothing with Polymaker PolySmooth
With the Polysher, 3D printed models made from PolySmoothfilament can be smoothed. This filament is as easy to print as PLA.
The surface quality of the printed models is extremely high using this technique (comparable to ABS Acetone Vapour Smoothing).
Liquid containing isopropanol is vaporised in the Polysher. The object placed inside is dissolved by the vapour, causing the layers to “fuse” together. The advantage of this technique is its simplicity and safety. The models are simply placed in the Polysher and the desired duration is then selected using the rotary control. Once the process is complete, the 3D model can be removed.
The smart electronics and an airtight seal mean that this process has advantages over acetone smoothing.
The disadvantage is the relatively high price. The Polysher must be purchased (~350€). In addition, the special filament must be used, which, at €45 per 750g spool, is about twice as expensive as PLA.
