3D printed or additively made parts often need finishing after the build. According to many users, this is a time consuming and sometimes costly part of the overall additive process. A recent survey conducted with support from the Society of Manufacturing Engineers (SME) and PostProcess Technologies, The Annual Additive Post-Printing Survey: Trends Report 2019, published the following statistics about this final step in the additive manufacturing process.
–75% of respondents said that the time it takes to post-process a part is an issue.
— More than 55% use two to three printing technologies, 33% use four, which can become an issue. Respondents would prefer to use fewer processes.
–66% said they have 2 or more challenges with their current post-processing methods.
–The primary difficulties experienced include issues of throughput, scrap rates, and use of human resources.
–Time and consistency are big challenges.
–Respondents report that removal processes, including support, powder, and resin, are the most common ones used.
–51% consider support removal the most time-consuming process. Most of these respondents say they spend more than half of their time on this operation. Additionally, 75% of these respondents say they must also do a surface finishing step after support removal.
–For resin removal, respondents report that a surface finish step is required about 80% of the time.
Common approaches to post-processing
The methods used for post-processing vary depending on the additive technology used (i.e., stereolithography, powder fusion, fused filament fabrication, and so on), and the material used to build the part. Here are the most common processes used.
Parts made with resin-based printers will probably undergo two procedures: rinsing in a solvent bath, then curing in a UV oven. The solvent rinse removes any uncured residues and can open internal cavities. Some vendors of resin-based 3D printers offer washer units to clean parts. Curing in a UV oven helps to give the part its final mechanical properties.
Parts made with plastic filament will usually show layer lines. For parts made from a rigid plastic, like PC or PLA, you can use a one-component or two-component (such as epoxy) spray to smooth over the surface.
For parts made with softer materials like ABS, they can be hand sanded or you can use a solvent to smooth the layers. Depending on the solvent, the finish can be shiny or matt.
Parts made with polymer powders, (for example, through selective laser sintering) typically have less post-processing needed because this additive process eliminates the need for most supports. The main post-processing step is to remove any unused powder stuck to the part. Options here include suction, blowing, sweeping or blasting, or using an automated process such as vibration. In many cases, the unused powder can be recycled.
Parts made from metals often require post-processing. The additive processes include metal laser sintering, powder bed fusion, and electron beam melting. There is a metal process that combines metal with carbon fiber in an extrusion process.
The finish right out of the additive machine tends to be rough. In addition, some parts may need thermal treatment, such as sintering, to remove binder material or to reinforce mechanical properties. Once excess powder is removed and after any thermal treatments, then the part can be machined, polished, or anodized for final finishing.
For material jetting:
Parts made from material jetting processes, such as PolyJet, need little post-processing. Any support material, including on overhangs, can usually be dissolved in a bath and the part rinsed and dried.
For filament-based 3D printing processes:
Parts made from filament-based additive processes (such as FDM and FFF) often have a rough surface finish and the layer lines tend to be visible. A range of post-processing techniques are available for these parts.
These parts usually need to have supports removed. Support material may be soluble or insoluble. Soluble materials, as their category suggests, dissolve easily in a chemical or in water. Soluble materials include HIPS, which is often used with ABS material, and PVA, which is often used with PLA material.
Insoluble materials require more effort. Usually, they are removed by hand or with hand tools. Insoluble materials include PLA, ABS, Nylon, PC, and others. In some cases, removing support materials can leave marks, which could require more post-processing.
Another method of finishing is sanding. It is usually carried out in stages beginning with low grit sandpaper. The drawbacks with sanding include the time it takes and the risk of altering the dimensions of the part.
Another method for FDM/FFF parts is vapor smoothing. Acetone is the most common material used for this technique. It requires an enclosed environment where the parts are exposed to the acetone vapors. The outer layer of the part melts, smoothing out the layer lines and giving the part a glossy look. A drawback is that you must watch this process to ensure that it doesn’t melt too much of the build material. (Acetone is a toxic substance; it is flammable, and it’s best not to breath the fumes.) In other cases, a chemical agent like Tetrahydrofuran (THF) is used.
Painting is another way to finish filament-based parts. Usually, a primer coat is needed as well. Primers and paints can be sprayed or brushed on.
FDM/FFF parts can be polished in post-processing. This may involve a buffing wheel or hand tools.
Electroplating is another method for post-processing parts. In addition to finishing the part, electroplating can enhance the strength of the part.
FDM/FFF parts can also be glued or welded in the finishing processes. In some cases, glue can be used to fix small holes or cracks or to fill them before painting. PLA parts, for example, can be glued with bonding agents such as Anabond. Parts made with ABS can be “welded,” using acetone applied to the mating surfaces and then clamped together to create a chemical bond.