Practical 3D Printing: Fundamental Knowledge

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Fundamental Knowledge

09.57

00:00	This section of the course will be dedicated to resin 3D printing, primarily focusing on the most common form being SLA or stereo lithography, which is used as somewhat of an umbrella term for resin 3D printing.
00:13	Just like in the FDM section of the course, we'll cover all the knowledge areas as well as the practical skills required to make use of this technology in your automotive projects.
00:24	Compared to FDM or FFF, SLA is relatively similar in terms of accessibility, with some very affordable options for consumer grade printers and materials, although it's not as widely and commonly used, and the resin is usually a bit more expensive than the filament.
00:41	SLA and FDM printing uses have some crossover, and similar parts can be printed using either technology.
00:48	However, there's enough difference in the materials and the properties that they generally lend themselves to different applications.
00:55	The point is, we can achieve things that aren't possible with FDM using SLA and vice versa.
01:02	That said, rapid prototyping, tooling, manufacturing aids, and even end-use parts are all possible.
01:09	Where SLA shines though is in printing parts with very fine detail and great surface finish, as well as the mechanical and heat resistance properties of the material, but we'll come back to this soon.
01:21	There are three main forms of SLA commonly used.
01:25	Laser powered SLA, MSLA, which stands for masked stereolithography, but also commonly referred to as LCD, as in liquid crystal display, and DLP, which stands for digital light projection.
01:39	These are all fairly similar though, and the fundamental ideas being much the same, but we'll cover the differences as we go.
01:46	Resin 3D printing falls into the additive manufacturing category known as VAT photopolymerization, which if you're familiar with chemistry, may give you a bit of a clue as to how this process works, but of course we'll explain it anyway.
02:01	The first thing to understand is the material used, being the resin component.
02:05	This is a light activated resin, or photopolymer, that is initially in a liquid form.
02:11	When exposed to light, a chemical reaction occurs which changes the structure of the resin.
02:17	In basic terms, it hardens and solidifies through a process called curing.
02:22	As you'd expect, the resin needs to be kept out of sunlight to remain in liquid form.
02:27	Another key point is that these are thermosetting resins, and the chemical bonds formed when curing provide great mechanical properties and heat resistance, but when the cured material is exposed to excessive heat, it degrades, breaks down, and becomes brittle.
02:42	The alternative thermoplastics from our FDM discussions aren't immune to excessive heat either, tending to soften, though they can be reformed and recycled to some extent.
02:53	To their credit, SLA parts are more isotropic in their mechanical properties compared to FDM, as they aren't as prone to layer to layer adhesion issues due to the chemical bonding between the layers, but they tend to be much more brittle in comparison to FDM parts.
03:10	All of this is to say that it's generally accepted that FDM still produces tougher and stronger parts from thermoplastics than what's possible with photopolymer resins used in SLA.
03:21	Moving on, the term inverted SLA is commonly used because the build platform is upside down and moves upwards during the printing process.
03:30	This allows for much smaller scale consumer grade printers as opposed to the original right side up or non inverted printers.
03:38	The process starts with the build platform lowered into a pool of liquid resin, referred to as the vat.
03:44	From here, a light source beneath directs a laser at the resin and cures the cross section of a thin slice of the part on the build platform.
03:53	Then the platform and the next cross section slice is cured, slowly building up the part layer by layer until it's complete.
04:01	Because each layer is chemically and physically bonded to the next, the differentiation between each layer is much harder to detect and this leads to the amazing surface finish of these parts when compared to those from FDM printing.
04:15	An important part of this process is the flexible release film that forms the bottom of the vat.
04:20	This is a clear film that allows the light to pass through it and cure the resin and is commonly called the FEP sheet or film as an abbreviation of the material that it's made from.
04:31	The top layer of the resin sticks to the film to some degree while curing and as the build platform moves up it must release.
04:39	This is done with minimal force thanks to the film's very low surface tension which goes a long way in ensuring reliability and a smooth surface finish.
04:49	These laser based systems are the traditional approach to SLA and can achieve very accurate results and surface finish, but it's generally a very slow process.
05:00	The technology under the resin vat that makes the print possible is commonly referred to as the print engine and this is a key area of difference between the different technologies.
05:10	DLP uses a light projector reflected through an array of micro mirrors laid out on a semiconductor chip.
05:17	Each micro mirror represents a pixel of the image projected on the resin to cure the cross section for each layer.
05:24	So, that means that the number of these micro mirrors per build area determines the resolution.
05:30	Because the light can be projected over the entire cross section in one instance this is a much faster approach.
05:37	The other alternative that we need to discuss is MSLA or masked stereo lithography.
05:43	This is where the machine has an array of light sources.
05:46	Usually, many LEDs and sections of this are selectively covered or masked.
05:51	The uncovered light will be the cross section of the part that is cured each slice before the build plate moves up and the mask changes for the next slice.
06:00	The reason that the term LCD is somewhat used interchangeably with MSLA is because the majority of these printers on the market use an LCD for masking.
06:11	On that note, most modern consumer grade resin printers use LCD MSLA technology.
06:17	These are a relatively recent development in resin 3D printing and the advancements in LCDs now make it possible to design screens with much smaller pixels leading to more resolution.
06:29	Within these LCD equipped printers, a key differentiating factor is the type of LCD.
06:35	The cheaper and more basic are RGB screens, while monochrome screens are usually seen as a step up.
06:42	Essentially the filters in the RGB screen block some of the light, lowering the amount that makes it to the resin.
06:49	The higher transmission of the light with monochrome screens helps secure the resin almost instantly, offering much faster printing speeds, even when compared to DLP printers.
07:00	The filters in the RGB screen also mean that more light is absorbed into the screen, which causes damage and shortens its lifespan.
07:09	Not only do monochrome screens allow for faster printing and last longer, they generally provide a crisper image for more accuracy, albeit at a higher price point.
07:19	It's worth mentioning here that at the time of filming this course, one of the main players in the film industry, being Formlabs, has just released the Form 4 printer.
07:29	This uses a new LFD or low force display print engine, which is a development of MSLA.
07:36	New units like this allow for very fast printing, with most parts within their size limitations being able to be printed in just a few hours.
07:45	At this stage, these examples make resin 3D printing the fastest of all the additive manufacturing processes.
07:52	Moving on, after the print is complete, the part, and any supports, can be removed from the build platform.
07:59	These supports are required for similar reasons to FDM printing, although since the build platform is inverted, they're more supporting the part from above rather than underneath.
08:09	The supports still essentially hold the part up.
08:13	Inadequate support for the weight of the part can cause it to fall from the build platform, either as a whole piece or in sections.
08:20	Supports are quite an involved subject though, so we'll come back and cover these in more detail.
08:25	After removal, the print needs to be cleaned to remove the excess resin.
08:31	This can be done in water, although isopropyl alcohol, or IPA, is usually recommended to help dissolve the uncured resin.
08:39	Dedicated cleaning stations are also available.
08:42	These basically work like a dishwashing machine, moving the cleaning fluid to agitate and clean the part more efficiently.
08:49	Afterwards, the parts can be exposed to UV light for post-curing, which ensures they are fully cured and achieve their final properties.
08:57	We'll continue to cover more information on SLA resin 3D printing in the following modules, so let's summarise our first look before moving on.
09:06	SLA resin 3D printing involves curing liquid resin with a light source in cross -sectional slices, layer by layer, to build up the part.
09:15	Some printers use a laser for this, and while the accuracy in detail is very good, it's a very slow and inefficient process.
09:23	DLP printers make use of micro-mirrors to speed up the process.
09:28	The number of the mirrors per printable area defines the resolution.
09:32	MSLA printers commonly use LCD screens to mask the light around the cross-section and define the area of resin that's cured, allowing for a step-up again in efficiency.
09:43	After printing, the part can be removed from the printer and washed to get rid of any excess uncured resin, and post-curing can help ensure the part achieves its full properties.