Basics of 3D Printing

What is 3D Printing?

3D printing is a desktop fabrication or additive manufacturing process. 3D models are created on a computer using modelling software and sent to 3D printer to be printed. The result is a physical representation of the object created on the computer.

3D Printing File Types

There are many 3D modelling file types but the most used for 3D printing are STL and OBJ. OBJ files can contain appearance information such as color and type of filament. STL files are more basic and can be used most 3D printing software as it is non-proprietary.

Types of 3D Printing

There are several types of 3D printers but of those SLS (Selective Laser Sintering), SLA (Stereolithograhpy ), and FDM (Fused Deposition Modelling) are the most common.

SLS

In powder-bed SLS, a layer of powdered material is carefully laid down by a leveler or roller on the build tray. A laser then sinters the cross-section of the part. Subsequently, the tray drops another 0.002 to 0.004 in. (0.05 to 0.10 mm) and the process repeats. Similar to SLA, layer thickness varies based on laser strength, material, or tolerance desired. A variation of SLS is called the cladding process. One way to clad is via powder jet, whereby a print head sprays powder to form layers as a laser sinters each layer. The powder-jet cladding process may be used to fix broken or damaged parts. Users can even align the grain structure of the new material with that of the existing part, an important consideration for corrosion resistance. Often times, cladded parts require post machining.

SLA

SLA is a photopolymerization process in which a build tray is submerged 0.002 to 0.006 in. (0.05 to 0.15mm) in a basin of liquid photosensitive material. This depth can vary based on laser strength, material, or tolerance desired. A UV laser (not lamp) solidifies one slice of the part onto the build tray. The tray then submerges, another 0.002 to 0.006 in., and the laser solidifies the next slice of the part. The thickness of the layers can affect the quality of print and tolerances. An industry average tolerance is around 3.9 × 10-3in. (0.1 mm). The laser travels the entire path of the part’s cross-section as it builds up each layer, so speed becomes an important consideration.

FDM

FDM printers use two kinds of materials, a modeling material, which constitutes the finished object, and a support material, which acts as a scaffolding to support the object as it's being printed.

During printing, these materials take the form of plastic threads, or filaments, which are unwound from a coil and fed through an extrusion nozzle. The nozzle melts the filaments and extrudes them onto a base, sometimes called a build platform or table. Both the nozzle and the base are controlled by a computer that translates the dimensions of an object into X, Y and Z coordinates for the nozzle and base to follow during printing.

In a typical FDM system, the extrusion nozzle moves over the build platform horizontally and vertically, "drawing" a cross section of an object onto the platform. This thin layer of plastic cools and hardens, immediately binding to the layer beneath it. Once a layer is completed, the base is lowered — usually by about one-sixteenth of an inch — to make room for the next layer of plastic.

Printing time depends on the size of the object being manufactured. Small objects — just a few cubic inches — and tall, thin objects print quickly, while larger, more geometrically complex objects take longer to print. Compared to other 3D printing methods, such as stereolithography (SLA) or selective laser sintering (SLS), FDM is a fairly slow process.

Once an object comes off the FDM printer, its support materials are removed either by soaking the object in a water and detergent solution or, in the case of thermoplastic supports, snapping the support material off by hand. Objects may also be sanded, milled, painted or plated to improve their function and appearance.

Common Misconceptions or 3D Printing Pitfalls.

3D printing is not a fast process. The more detail and parts in a 3D print greatly increase the total time to print the file. Print quality also plays a role in the length of time to print the object.
Not every print will be 100% successful. Many prints will print without issue but there will be times tinkering and troubleshooting will be required to get an object to print properly. Changing print speed and layout of the object are the primary ways to resolve print issues.
Once an object finishes printing it does not necessarily mean it is finished. Depending on the end result you are looking for, the object you print may need refinement using sandpaper, files or other shaping tools.
The finished part may not be the exact same size as specified on your computer. Because we are dealing with a material that is changing temperatures there will be some retraction or shrinking of material.

Materials Used in 3D Printing

The material used in FDM printing is called filament. There are hundreds of different types of filament and all is not created equal. Every FDM printer can print in PLA. PLA is a plant starch based plastic and is very forgiving for beginners. ABS is another common type of filament but requires special care to print with and also requires you printer to have a heated bed. There are also many exotic filaments that require special care when printing or when storing. The type of filament you can use depends on the capabilities of your 3D printer.

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