Post-processing in 3D printing

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Post-processing is an often overlooked part of the 3D printing process. As the additive manufacturing market moves from prototyping to end part production geared to consumer markets, the look and feel of 3D printed products is becoming increasingly more important. That is where post-processing 3D printed parts comes in.

What is post-processing in 3D printing?

Parts manufactured with 3D printing technologies usually require some degree of post-production treatment. This important step of the 3D printing process is known as post-processing. In short, post-processing in 3D printing refers to any process or task that needs to be performed on a printed part, or any technique used to further enhance the object. Think of it as a finishing touch to treat and refine parts that come out of a 3D printer. The options for post-processing 3D printed parts include removing support or excess material, washing and curing, sanding or polishing a model to painting or colouring.

Costs of post-processing 3D printed parts

Post-processing can be costly, especially when it is done by hand. Manual post-processing is labour intensive and is not scalable. It will also become unsustainable in large series production.

The cost of post-processing can amount to almost one third of the production cost of a 3D printed model. According to the 2018 Wohler’s report, 27% of the total costs of producing a model can be attributed to post-processing related costs, which include the costs of part breakage.

Luckily, the recent development of various post-processing systems means that the task of finishing 3D printed parts can be automated and, as a consequence, it will bring down the costs.

Different companies are developing post-processing equipment to automate the process.  Some of these companies, such as DyeMansion, are focused on post-processing machines only. Others, like Carbon and FormLabs, are 3D printer manufacturers that are adding post-processing systems to work seamlessly with their printing set up.

So, what are the different post-processing techniques available?

We can identify 5 steps in post-processing, although not all steps are required for all projects:

  1. Cleaning
  2. Fixing
  3. Curing or hardening
  4. Surface finishing
  5. Colouring

Also, beware that the post-processing technique can vary depending on the printing process used to create the model.

1. Cleaning

a) Removing support material (FDM and Material Jetting)

When printing models with overhang on FDM or other material jetting technologies, support structures that hold up the overhanging features are needed.

These support structures can be printed using the same material as the one with which the model itself is printed. But when the machine allows for printing with multiple materials, special support material can be used. Nonetheless, every time a support structure is required, there will be some post-processing involved.

There are two types of support material: soluble and insoluble (usually the latter is the same material the model is printed with). 

Insoluble material is relatively strong and can only be removed using tools as knives or pliers. This has to be done carefully and there exists the risk of damaging the model, or inadvertently removing small features.

When using soluble support material, there is a lower risk of damaging the model. The support structures can be dissolved in water or with a chemical called Limonene. Examples of soluble materials are HIPS (used as a support with ABS material) and PVA (used as a support with PLA material).

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 cling to or remain in the model, for example in holes or more complex internal channels inside the model.

Excess powder can be removed manually but automated solutions have arrived on the market that vibrate or rotate to remove access powder. This 3D printing post-processing technique works like some sort of a centrifuge rotating the part in all 3 dimensions.

SFM-AT800S depowdering machine by Solukon Maschinenbau GmbH in collaboration with Siemens

c) Washing (SLA and Photopolymerisation)

Parts that are printed with SLA or other photopolymerisation can be easily cleaned after printing. Two companies added post-processing washing machines that are seamlessly integrated in their print process line up.

FormLabs added The Form Wash which uses isopropyl alcohol (IPA) to clean the parts. Carbon developed the Smart Part Washer machine to do the cleaning.

2. Fixing

Sometimes small repairs are needed to fill small holes or cracks or even to attach together parts that have been printed separately.

a) Filling

When fillers and hardeners are used to repair unwanted holes or cracks in the printed object.

b) Glueing and welding

Used when separately printed parts need to be attached together. ABS prints can be welded or glued together using acetone.

3. Curing

Just like french fries, baking the models after they have been printed enhances the mechanical properties (crunchiness in case of the fries) of the material.

Formlabs and Carbon have added curing using UV light to their printing process (SLA and CLIP respectively, both Photopolymerisation processes). After the model has been printed special curing machines heat the model to bring the part to its optimal mechanical properties. Curing therefore differs from the other post processing options, that it enhances not just the aesthetic characteristics, but the physical quality of the model.

4. 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 especially relevant when the models are geared towards consumer markets.

a) Sanding

Layer lines or touch-points where support structure was attached to the model can be removed by carefully sanding the surface of the model, using sanding paper with varying grit: from low to high for finishing.
Aside from being labour intensive, manual sanding can create inconsistent results. With automated polishing, this can be avoided.

Layer lines are particularly visible on 3D models produced using layering techniques (like FDM).

b) Vapour or Chemical Smoothing.

Sometimes chemicals are used to smoothen the model surface. The vapours react with the outer layer of the object. The layer lines are melt away, leaving a smooth outer layer while giving the model a glossy look.

For models printed with PLA and ABS the acetone is often used, or the chemical agent Tetrahydrofuran (THF).

The problem with this technique is that is cannot be controlled: small features can be melted off that should remain. Also, the vapours can be harmful when inhaled. This can be avoided using closed chemical cleaning machines.

5. Colouring

In some cases, 3D models can be printed using coloured material and with multi-material printing (multi-) coloured prints can be made. But one can also opt for colouring during the post-processing phase.

Coating and painting

Parts that need colouring would ideally be printed using white material. Before the model is painted a layer of primer is usually applied. Painting can be done manually using a brush or spray. There are machines that automate spraying of parts.

Where can I go for post-processing?

So,  post-processing is becoming increasingly an integral part of the 3D printing process. With special post-processing machines being developed the process is becoming automated which makes it more scalable than previously possible.

You have the option to use special post processing services, but conveniently increasingly print services are providing post-processing services to their customers, offering them a one stop shop solution.

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