Ensuring Quality: Post-Processing in 3D printing
Generally, we do not consider post-processing as part of 3D printing. This is because the additive manufacturing market shifts from prototyping to finished part production instantly. The importance of the design and feel of 3D printed products is increasing and that is where post-processing 3D printed parts come in place.
What is post-processing in 3D printing?
Parts manufactured with 3D printing technologies usually require post-production treatment known as post-processing, which is an essential step in 3D printing.
Post-processing in 3D printing is a process that must be performed on a printed part to improve the prototype. It is considered as a final touch to refine the parts that are the output of a 3D printer. The choices for post-processing 3D printed parts include removing support or excess material, washing and curing, sanding or polishing a model to painting or coloring.
Cost of post-processing 3D printed parts
Post-processing is often costly, especially when it’s done by hand. However, manual post-processing, being labor-intensive, isn’t scalable. It also becomes unsustainable in large series production. Luckily, the recent development of varied post-processing is usually automated so it brings down the prices.
So, what are the various post-processing techniques available?
Although not all stages are necessary for all projects, there are five primary steps in post-processing:
a) Taking off the support material
Printing models rely on FDM or other material jetting technologies, but support structures that delay the relying features are needed. These support structures are printed using similar material with which the model itself is printed. But when the machine allows for several materials to be printed, the special support material is frequently used. Nonetheless, whenever a support structure is required, there’ll be some post-processing involved.
There are two sorts of support material: soluble and insoluble (usually the latter is the same material the model is printed with).
When using soluble support material, there’s a lower risk of damaging the model. The support structures are often dissolved in water or with a chemical called Limonene. Samples of soluble materials are HIPS (used as a support with ABS material) and PVA (used as a support with PLA material).
Insoluble material is comparatively strong and can only be removed using tools like knives or pliers. This has to be done carefully and there exists the danger of damaging the model or accidentally removing small features.
b) Powder removal (SLS and Powder Bed Fusion)
Models printed using powder bed fusion (SLS, etc) are fabricated using plastic or metal powders. Residues of powder can hold close or remain within the model, for objects without holes it is quite an easy process. The model is brushed which removes the majority of the powder which is then sandblasted to remove the powder which the brush has missed. For models having holes or more complex internal channels, the powder has to be removed otherwise it could get stuck inside the model resulting in a heavier model.
c) Washing (SLA and Photopolymerisation)
Parts that are printed with SLA or other photopolymerization have to be cleaned with isopropyl alcohol. SLA parts that are printed with a support need sanding on that location to even the surface.
Sometimes small repairs are needed to fill small holes or cracks or maybe to connect parts that are printed separately.
Filler like Putty is used to close the cracks or surface defects within the printed object. It works by mixing the compound with a hardener and applied on the surface using a scraper. Many fillers usually get cured within few minutes.
b) Glueing and welding
This technique is used when separately printed parts have to be attached. ABS prints are welded or glued together using acetone. There are many choices of glue that can be used to join other 3D printed parts like PLA using either super glue or epoxy. Cyanoacrylate (Super Glue) like Akfix 705 is an effective option to join parts instantly within 10 seconds and give high strength.
3. Surface Finishing
After the washing, cleaning, removing support or excess material, and curing, different processes are available to make the model look nicer aesthetically. This is often relevant when the models are geared towards consumer markets.
Layer lines or touch-points where the support structure was attached to the model are removed by carefully sanding the surface of the model, using sanding paper with varying grit: from low to high for finishing. In addition to being time-consuming, hand sanding can produce uneven results which can be avoided with automated polishing.
b) Vapor or Chemical Smoothing.
Sometimes chemicals don’t smoothen the model surface. During such a situation vapour is used to react with the outer layer of the object. The layer lines are melted away with the vapour, leaving a smooth outer layer and a glossy appearance to the model.
For models printed with ABS the acetone is usually used, or the agent Tetrahydrofuran (THF).
The problem with this system is that it can’t be controlled. Most often, small features are melted off that are ought to remain. Also, the vapors are often harmful when inhaled. This will be avoided using closed chemical cleaning machines.
c) Resin Coating
Resin coating is an optional procedure that people use for the following reasons:-
- Fill the gap and smoothen the rough surface
- Join 2 parts with high strength
- Add more strength & protective layer to the 3d printed part
Usually, this method is not recommended due to its 24hr of curing time. However, if your part is structurally weak and needs reinforcement then the epoxy coating is a good method.
In some cases, 3D models are often printed using colored material and multi-colored prints are often made. But one can also choose coloring during the post-processing phase.
Coating and painting
Parts that require coloring would ideally be printed using white material. Before the model is painted a layer of primer is typically applied. Painting is usually done manually employing a brush or spray. Some machines make the spraying of parts automatic.
To get a professional surface finish, sanding & priming is done alternatively before you do spray paint. Follow the below steps in sequence for optimum result:-
- 120 grit sanding
- Primer (2 coats)
- 220, 320, 400 grit sanding
- Primer (2 coats)
- 500, 600 grit sanding
- Spray Paint (2 coats)
500 to 600 grit is good enough finishing for consumer-grade products. However, if you are using 3d printed parts as a mold to produce parts like carbon fiber then you will have to sand till 1200 grit sanding paper.
Where am I able to choose post-processing?
So, post-processing is turning into an integral part of 3D printing. The approach is getting more automated as specific post-processing devices are created, making it more scalable than before.
You have the choice to use special post-processing services, but increasingly print services are providing post-processing services to their customers, offering them a one-stop-shop solution.