Practical 3D Printing: Surface Finishing

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Surface Finishing

08.27

00:00	Depending on the application, some prints will be ready to use straight out of the printer, after cleaning and curing of course.
00:07	In some cases though, we might want to use some post processes to achieve a particular finish for the part.
00:13	To start with, let's discuss removing support structures.
00:16	We did mention this briefly at the end of the printing module, but it's a post process that's involved with surface finishing, so it deserves a bit of a mention here.
00:25	Generally, the supports will be able to be broken off by hand, but we can also use pliers, flush cuts, or other sharp tools like hobby knives for more stubborn areas.
00:35	The issue here is that the supports leave witness marks, which are small bumps or indents on the surface of the part.
00:42	If we look at our velocity stack example, after removing the supports, we can see these marks on the base of the flange and around the bottom of the outside of the part.
00:51	In some cases, this might not be an issue.
00:54	In other cases, like at the base of the flange, we want to make this surface perfectly smooth for functional reasons, and in other cases it might just be for aesthetic reasons.
01:04	We'll park this discussion here for now and come back to discuss how we can smooth the surfaces in just a moment.
01:10	Now, if you've ever searched SLA 3D printing online, you've likely noticed that most of the results involve printing miniatures or figurines.
01:18	The nature of SLA printing lends itself to this due to the prints being of relatively high detail and high quality, with a smooth surface finish, and also being easy to finish with paints.
01:30	We can take advantage of the same attributes for our automotive applications, whether that's for interior or exterior trims, or even preparation as moulds for composite parts.
01:40	As you'd expect though, the process for finishing plastic parts doesn't vary greatly depending on what manufacturing process is used to make it, even between thermoplastics and thermosets.
01:52	With that said, what we discussed in the FDM section of the course doesn't completely transfer to SLA prints, simply due to the nature of the printed parts, and this includes the reason for using the different processes.
02:04	Let's elaborate on that to clear things up a bit.
02:07	Processes like acetone vapour smoothing or application of high build primers aren't used for SLA prints.
02:14	In fact, for parts with fine details, this would be something we'd want to avoid as it would take away from what's so great about SLA printing.
02:22	In saying that, we can still spray the surface of the prints with a regular primer to seal, particularly for the likes of composite moulds before applying release agents.
02:32	The raw surface finish of SLA prints is relatively smooth, even in areas with support witness marks.
02:39	But we can still remove these marks and any layer lines if we have them.
02:44	We don't need to start sanding with coarse sandpaper.
02:47	Taking our velocity stack as an example, we'll typically jump straight to a fine grit like 220, or even higher, working up to something like 1000 grit.
02:56	For our velocity stack mounting surface, it's as simple as holding the sandpaper on a flat workbench and then moving the part over it, constantly checking the surface until it's nice and flat.
03:07	Higher grits like 3000, as well as incorporating wet sanding, will provide a smooth and glossy finish.
03:13	Remember to wear a mask when sanding to avoid breathing the dust.
03:16	And after sanding, we can use a clean rag with IPA to remove the sanding dust.
03:22	It's also possible to polish SLA prints and this is particularly useful when printing transparent resins for lenses.
03:29	Prior to polishing, we'll wet sand up to 10,000 grit and then we can start polishing using a compound designed for plastic like Meguiar's Plastic X for example, and a buffing pad or even a polishing machine.
03:42	Afterwards we can wipe off the remaining compound with a dry microfiber cloth and then wipe down with mineral oil which helps the part keep its shine preventing the surface from drying out.
03:53	It should be mentioned that the clarity of these clear resins can vary between manufacturers and also different manufacturers' offerings, so the resin itself is the first thing to consider if clarity is important to you.
04:07	Some are also very sensitive to post -curing conditions and tend to turn yellow, so do some research on the specific product you're looking at before purchasing.
04:16	On the topic of clear resins, one popular use case is the likes of tail lights, or other coloured lenses.
04:23	While it's possible to purchase transparent colour resins, we can also dye resins to achieve certain colours that we can't purchase.
04:31	This is best kept to printing with clear resins for aesthetic purposes, as the dyes tend to take away from the mechanical properties of other resins.
04:40	This is done using as little as 10ml of alcohol ink dye into 1 litre of resin, and then mixing thoroughly.
04:48	Post-dyeing prints is also an option, however this works best on clear elastomer prints.
04:53	The process involves mixing 10-20ml of the same alcohol ink into 1 litre of IPA, or an equivalent of 100-1 or 100-2 mix, depending on how dark you want the colour to be.
05:07	After we've cleaned the excess resin off as we discussed in the previous module, we can transfer the clean and dry part into the dye and IPA solution, leaving it for 10 minutes before removing and drying.
05:19	Following this, it's back to the normal post-curing process.
05:23	Let's now move on to painting, which is a bit more straightforward, and likely something that most of us are more familiar with.
05:29	Brush painting with acrylic paints is the go-to for the figurines that we mentioned earlier.
05:34	This could be useful for detailed interior parts like buttons, switches, or other trim pieces.
05:40	However, it's more likely that we'll be working with larger surfaces, so naturally, for efficiency's sake, we'll turn to spray painting.
05:48	Most general purpose enamel spray paint, although it can be more efficient to just print with the resin colour that we want in the first place.
05:55	It's also common to use hobby model paints, like lacquer spray paints from Tamiya, for more specific surface finishes that can't be achieved straight out of the printer.
06:05	Some paints are best applied directly to the plastic surface, some over primers, and some over other paints, so it's really best to follow the paint supplier's recommendations here for the best results.
06:17	Finally, on the topic of spray paints, UV resistant clear coats are great for protecting thermosets from degradation in sunlight.
06:24	Past these basic coatings and protections, we can ceramic coat and even electroplate our SLA parts.
06:31	Both of these surface finishes enhance a number of physical performance properties, including abrasion or wear resistance, corrosion chemical and thermal resistance, as well as impact strength and hardness.
06:42	We won't dive into the details of how this works here, as it's not something that's typically done without extra professional equipment, but it's still good to know that it's an option.
06:51	There are many companies offering these services for plastic parts that will complete the entire process.
06:57	We just need to deliver or send them a clean part and communicate our application and requirements.
07:03	They'll then complete the processes such as media blasting, etching, cleaning, coating and curing, and send the part back.
07:10	Although we're just brushing past this in this course, electroplating and ceramic coating is a solid means of bridging the gap between plastics and metals, so it's easy to use.
07:19	It's definitely worth investigating further if you're interested.
07:22	Before we get too carried away with what ifs, we've covered a lot of different finishing techniques for SLA prints in this module, so let's recap the important points to remember.
07:32	Although SLA prints have a relatively good surface finish, there are still ways we can improve them, especially for those witness marks from the support structures.
07:41	After removing the supports with tools like flush cuts, we can sand the surface smooth with anywhere from 220 grit for a smooth finish all the way up to 10,000 grit and polishing for a glossy finish or more clarity in transparent parts.
07:56	Following sanding, we might want to prime and paint the parts, or alternatively we could use dyes during the manufacturing process, either in the liquid resin or after printing and before curing.
08:08	Other coatings can be applied to SLA prints from more simple UV resistant clear coats that naturally help prevent degradation in sunlight, all the way to ceramic coatings, or electroplating which can significantly enhance the surface properties of the parts.