FDM post processing

FDM: Fused Deposition Modelling, an additive manufacturing method that builds an object by melting material layer by layer.

Thermoplastic materials: Materials that become softer when heated and harder when cooled.

Post processing: Any task or process after the 3D printing is completed, such as surface finishing, cleaning or painting

Mechanical strength: Ability to resist the force applied to the object

Soluble: Able to be dissolved e.g. in water

Fused Deposition Modelling (FDM) is a very fast way of prototyping 3D designs but often, especially when supports are used,  the printed part will need post-processing. Support removal and sanding/filing are to be expected but you may also want to heat, prime, paint or apply a finish to your model.

If your finished 3D printed part has a standard plastic support structure, made from the same or a similar material to what the part is made from, you will have to remove them by hand.

The first step is to remove the majority of the support material and this can be done with a small pair of flush cutters to cut and pull the support away. If your part is delicate it is important to do this slowly and carefully. While some people use pliers (like the image below) we have generally found that flush cutters (available in the makerspace) work better.

Unless you are using 'Breakaway' support on a flat surface, you can expect small bits of supports to be left behind and the surfaces where you have pulled and cut the support away from to be rough and slightly damaged. To fix this you need to file or sand these down. We have generally found this is best achieved by engineers files (very small files we have available in the makerspace) to smooth off the rough surfaces, filing from a variety of angles to achieve the best result. Large parts may require larger files or sandpaper (also available in the makerspace).

Please note that these process become incredibly difficult for small partially enclosed spaces/obscured structure and that in these cases, PVA support may save you several hours of frustrating post processing.

If your finished 3D printed part has a soluble support structure, made from a completely material to what the part is made from, removing it becomes generally very easy. In addition, the final finish should be near perfect without any need for filing or sanding required.

Though others exist, the only soluble support material we use in the Makerspace is PVA which dissolves in water. To remove this support material you simply place the model into water and wait.

This process, though very hands off, can be quite slow, especially if your model has a lot of support. If you need to speed up the process you can:

• Use an amount of water significantly larger than the amount of support to be dissolved
• Use warm (not hot) water that you replace every few hours, making sure to wash out all the PVA from the container each time.
• Stir/mix the water regularly
• If you in a rush, you can even start gently pulling off the PVA by hand as it softens in the water.

Be careful not to wash large amounts of non-dissolved PVA down drains as it can lead to blockages. The PVA will dissolve completely given enough time and water changes.

If your model has internal supports, there will need to be a hole for the water to get in and the PVA to get out of. If the hole is too small this can take a very long time (several days or more).

FDM 3D printers build objects layer by layer. The can result in highly visible layer lines (especially when the print settings selected have a large layer height). In addition there can be rough areas from printing errors, unsupported surfaces or support removal. Sanding is a great and simple method to have a smooth surface finish on a 3D printed object if you want it.

It is recommended to start sanding with low-grit sandpaper (around 150 grit, each piece of sand paper will have its grit written on it), and then gradually use higher grit sandpapers to go over the same surfaces until you are happy with the finished surface.
For example, for a very polished surface you could use 200, then 600, 1000, 1600 and finally 2000 grit - but this would be overkill for most makers, who would only need the first one or two steps to get a decent finish.

To avoid heating or release of dust, you can wet your part with water before sanding and then re-wet it every now and then (highly recommended). The key in sanding is to use circular motions and avoiding sanding parallel or perpendicular to the layers.

If your model is PLA and you have already removed all the support, another way to remove layer lines is using heat to soften the outside surface of the model and essentially melt the lines away.

The most commonly used tool for this purpose is a heat gun, set to its lowest heat setting (available in the makerspace). The best, most consistent results are attained by placing the print on a turntable and spinning it slowly to evenly distribute the heat. This also helps prevent over-melting any specific areas for an overall better result. Be careful not to heat any part of the model too much or it may warp or bend.

While it might seem like a hairdryer would be a good alternative to a heat gun, a hairdryer isn’t designed to be hot enough to melt plastic so you’ll be waiting a long time to see any change.

Before painting a 3D printed part, priming/a base coat is required to prepare the surface for painting, as paint will not stick well to bare plastic. If desired, sand the part first (starting with a low grit and then up to 600).

Primer can be bought as a liquid to be applied by brush or as a spray. You may intend to use the primer only for painting on top of, but you can also choose a primer/spray that will form part or all of the final colour of your model. Primer can be bought online or from a range of hobby shops.

If using a spray, spray from approximately from a 15-20 cm distance. Allow the part to dry between each coat and then asses whether it requires an additional coat (usually at least two are needed) . It is advised to this in a well ventilated area and you may want to do it on cardboard or newspaper to avoid marking other surfaces.

Once the priming is complete and the part is dried, you can start painting. You can use either spray paint or a brush. As an example you can use painter’s tape to spray different sections of the part with different colours.

Once you are happy with your finished model, you may want to protect the paint from damage and wear. To do this can apply a transparent top coating, usually as a spray obtained online or from hobby shops.

https://www.3dhubs.com/knowledge-base/post-processing-fdm-printed-parts/#polishing

https://all3dp.com/2/fdm-3d-printing-post-processing-an-overview-for-beginners/

https://all3dp.com/2/pla-smoothing-a-beginner-s-guide/

https://manufactur3dmag.com/8-techniques-for-post-processing-of-fdm-3d-printed-parts/