Practical 3D Printing: Fundamental Knowledge

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Fundamental Knowledge

08.04

00:00	This section of the course will be dedicated to our third and final form of plastic additive manufacturing, primarily focusing on SLS or selective laser sintering of thermoplastics.
00:11	We'll still cover all the key knowledge areas as well as the practical skills required to make use of this technology in your automotive projects.
00:19	However, our approach to this section and the following metal 3D printing section will differ from the FDM and SLA sections in that we won't dive into as much detail on the topics like performance metrics of the machines and how to set them up for printing.
00:35	The reason for this comes down to accessibility.
00:38	Although FDM and SLA machines come with a significant price tag, there are still plenty of options in the hobbyist and consumer markets at affordable prices well under a thousand US dollars.
00:51	SLS machines on the other hand generally start at around 10 times this, putting them out of reach of most home enthusiasts and more into the pricepoint only justified by professionals, which of course, some of you watching this course, might be.
01:05	But regardless, these machines are currently still very specialized.
01:09	Machines are being developed to help bridge this gap like the Micron desktop unit from Micronics, which has recently been acquired by Formlabs and are aiming to have an SLS printer on the market at around the $3000 price point.
01:25	Regardless, in the current market it's fair to assume that But that doesn't mean that we can't make use of this technology.
01:34	Luckily there's an ever increasing number of manufacturers who offer these 3D printing services and also FDM and SLA and most other methods for that matter and we'll be discussing these services in a coming module.
01:47	With that out of the way, let's get into the main focus of this module, understanding how SLS technology works.
01:54	The first thing to know is that SLS is a form of powder bed fusion.
01:59	As is SLM metal 3D printing, but again we'll come back to this later in the course.
02:05	Powder bed fusion starts with a build platform covered in a very thin layer of a very fine powder.
02:11	A high powered laser traces the cross section of the part on the first layer, fusing the small particles of the powder onto the build platform.
02:19	Then the build platform moves down slightly and more powder is re-coated over the top before the laser fuses the next layer.
02:26	This process is repeated, gradually building up the solid part layer by layer as is common for most additive manufacturing.
02:34	The re-coating mechanism and feed of new powder differs between machines, but typically some form of roller or scraper is used to move powder onto the build platform from a reservoir or supply chamber.
02:46	In some cases the powder supply will feature a platform that moves up to supply more powder when needed as the build platform moves up.
02:54	There might even be a supply on each side of the build platform so the re-coating mechanism can work back and forth more efficiently.
03:02	The unfused or loose powder remains on the build platform around the part until the print is finished, at which point the part must be cooled before being removed from the build platform.
03:12	The unfused powder can be re-covered and brushed into a reservoir on the machine or sucked up by a powder recovery system which is somewhat like a big vacuum cleaner.
03:21	This can then be cleaned and re-used in another print to reduce waste.
03:25	Any loose powder still in the part can be cleaned off, usually with a brush or compressed air.
03:31	For clarity, the term sintering refers to the process of applying pressure or heat to smaller particles and compacting and forming them together into a larger solid mass.
03:41	In this case, applying heat from the laser to the thermoplastic powder to form a solid plastic part.
03:48	This all happens without melting the material to a liquid.
03:52	Essentially, the material is softened just enough so that it sticks together.
03:56	This is something to keep in mind for our future discussions on SLM or selective laser melting.
04:02	For SLS, the powder used and therefore the material of the finished part is a thermoplastic, similar to our FDM filaments.
04:10	Remember that thermoplastics can be heated to soften or melt and then remoulded and recycled.
04:16	We know from the FDM section of the course that there's a wide range of thermoplastic filaments available to print with.
04:22	In comparison, the number of thermoplastic powders available for SLS is more limited.
04:28	So, at first glance, it might seem questionable as to why we would choose to use SLS, when we could have our own FDM machine and they're much cheaper and easier to work with, while offering a wider range of materials.
04:41	SLS parts distinguish themselves from FDM in that they're more isotropic and aren't prone to layer adhesion issues.
04:49	All other things equal, the parts are significantly stronger, so for functional and end use parts that are subject to load, this could be the determining factor.
04:58	And of course, we're still working with thermoplastics, so the parts are generally stronger and tougher than the SLA thermosets.
05:06	Additionally, a unique property of SLS is that support structures aren't needed.
05:11	The unfuse powder acts as a natural support, holding up any overhangs.
05:16	As you'd expect, this reduces material waste and process time and removes any witness marks or surface imperfections from the supports.
05:24	The key benefit though is that it allows for more design flexibility compared to FDM thermoplastic parts.
05:30	A quick side note here, the price of powder bed fusion services, like SLS, is a function of the material usage.
05:38	So, the more mass we can take out of the part, the less material that's used and the cheaper things get.
05:44	The resulting parts typically have a grainy or matted finish, and while this is generally considered to have a higher level of detail and better quality, it's not necessarily as close to the level of SLA.
05:56	Post processes, like media blasting with sand and water or tumbling, can be used to polish the SLS parts and achieve a smooth surface finish.
06:05	Before wrapping up this module and moving on to discuss the other topics, it's important to note that an alternative to SLS is MJF, which stands for Multi Jet Fusion, which is a patented technology by HP.
06:19	How this works is a little bit harder to understand.
06:22	In basic terms, MJF uses a liquid fusing agent, which is injected into the powder in the cross section of the part using very fine inkjet nozzles.
06:32	Then heat and infrared light are used to fuse the powder, and the fusing agent facilitates this.
06:38	While SLS can use a broader range of materials, MJF almost exclusively uses nylon 11 or 12, which it should be mentioned are great materials for many automotive applications anyway.
06:52	MJF does tend to be the cheaper service though, as the processing is faster and more of the unused material can be recycled.
06:59	Like SLS, MJF doesn't need support structures, which gives it an advantage over FDM, allowing for more flexibility in the design and the print orientation.
07:10	There are more forms of powder bed fusion for plastics, but we've introduced the main ones that you're most likely to run into, so let's recap this module.
07:18	Powder bed fusion technology involves a build platform.
07:21	The build platform is a platform covered in a thin layer of powder.
07:24	In most cases, a laser is used to fuse the powder together in a cross section of the part.
07:29	As the build platform moves down, it's re-coated and the part is built up layer by layer.
07:35	After the part has cooled, the excess unfused powder can be cleaned off the part and any post-process is completed.
07:42	SLS uses thermoplastic powder and doesn't require any supports, leading to more design flexibility and a stronger, more isotropic part for the build.
07:51	With better surface finish than FDM.
07:54	MJF is another alternative form of powder bed fusion that is commonly used to produce nylon parts.