Refine Big Prints with Post-Processing

Why and how to process FDM and FFF printed parts?

Post-processing is the work that comes after the 3D printing. Objects printed with FFF can be treated and refined in various ways, for example by improving or modifying their surfaces, and then be used as positive or negative forms for molding processes.

Reasons for post-processing

Functional Reasons
UV resistance improvement
Strength improvement
Geometry improvement (adjust tolerances)
Surface quality improvement
Heat stability improvement
Weather resistance
Cosmetic Reasons
Smoothen surfaces and fix imperfections and irregularities associated with the printing process (even out layers, cover seems and bubbles…)
For prototypes: make a plastic part look like the final object it is representing: color, texture…

Abrasive Methods:

The idea with these methods is to improve the quality and the texture of the print by removing material.

Mechanical

Sanding

Sanding 3D printed models is very useful in eliminating layer lines and to achieve smoother surfaces.

This method usually leads to chalkiness and loss of color, therefore it is recommended to follow it with a coat of paint. Sanding can be done manually, using sandpaper, or with the help of a handheld electric sander or sanding block. Always be careful when sanding to avoid the melting of the material due to mechanical forces. You can even cool down the printed object to avoid melting.

Sandblasting

Sandblasting consists of spraying small particles (glass balls, sand, walnut shells…) in order to remove layer lines. It’s quick and gives the object a smooth mate finish. Results are better when sandblasting solid parts. It is not recommended for small or weak objects. Thin walls can melt and get damaged.

Milling

Milling can be useful to create geometries that weren’t possible to print. For example, drilling holes. Parts could also be treated on a 3 or 5 axis milling machine.

Chemical

Dichlormethan or tetrahydrofuran

Chemical processes are based on dissolving the surface of printed parts with organic or inorganic solvents. Parts can be treated using brush or exposing them to the vapor for a specific amount of time (minutes – hours).

It is very important to extreme caution when using dichloromethane or tetrahydrofuran, as they are extremely caustic and carcinogenic. They are primarily dangerous when vaporized, it is recommended to use a respirator. We would recommend further research on the topic to determine if it is worth it for you to smooth your PLA prints.

All trials should be done under very safe conditions and in an opened area.

Additive Methods

With these methods we want to improve the print by adding material.

Epoxy and Polyester Resin

It is only recommended when accurate geometry or sharp edges are not needed. There are many different types of 2 part epoxy resin or polyester resin, each with different material properties and curing properties. Also, there are a wide range of viscosities available.

Epoxy has better sticking and mechanical properties than polyester, and develops no odors during hardening.

These resins may cause reaction and long term damage or allergic reaction when in contact with the skin. Even more important than protection from inhaling is to wear safety clothes that are suited for these chemicals.

Biodur

It is a relatively new polyurethane-based renewable bioplastic. It is easier to use than other resins but does not have the same improving qualities (no UV resistance).

Paint

Any polyester or acrylic paint can help improving the surface of the print. Additives like micro balloons can enforce the filling effect. Paints available on the market are less dangerous and easy to use. Therefore, the mechanic properties are less reliable.

Before painting, parts need to be sanded (coarse surface) or painted with a primer in order to make the paint stick to the object.

Polyester Filler (2K or 1K)

Often used in the car industry 2k filler has better filling properties whereas 1k can be bought in spray cans in every paint store. The 2k composite can be enriched with filling powders according to the spray available gun nozzle.

Tip: thick grooves may become worse as the filler can even out only small uneven faces. It can be painted after drying.

Note for all additive methods: If more than one layer needs to be applied, manual treatment in between layers is crucial (such as sanding or sandblasting).

These methods are based on the principle of adding a layer of plastic to the surface of the print by using a brush or spray gun.

Various viscosities and additives (fiber mats, powders…) improve the filling qualities. They also often improve the properties of the part (strength, UV-resistance). They are toxic during application but harmless after hardening.

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