By Sudeep Kumar Singh
Fused Deposition Modeling (FDM). the most widely used process among 3D printing family. In this raw material i.e., thermoplastic polymers in continuous filament form are heated to its melting point and then extruded, layer by layer, to create a three-dimensional object.
The Cube home 3D printer. (Image credit: 3D Systems Corporation / cubify.com)
The technology behind FDM was invented in the 1980s by Scott Crump, co-founder and chairman of Stratasys Ltd., a leading manufacturer of 3D printers. Other 3D printing organizations have since adopted similar technologies under different names. The Brooklyn-based company MakerBot (now owned by Stratasys), was founded on a nearly identical technology known as Fused Filament Fabrication (FFF).
How FDM works
FDM printers uses two kinds of materials,
(i) a modeling material, which constitutes the finished object, and
(ii) a support material, which acts as a scaffolding to support the object as it's being printed.
1.First of all, computer-aided design (CAD) model for the object to be created with an FDM printer.
2.Before printing, the CAD file must be converted to a format that a 3D printer can understand — usually .STL format.
3.During printing, these materials take the form of plastic threads, or filaments, which are unwound from a coil and fed through an extrusion nozzle.
4.The nozzle melts the filaments and extrudes them onto a base, sometimes called a build platform or table.
5.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.
6.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.
7.This thin layer of plastic cools and hardens, immediately binding to the layer below. 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.
8.Printing time depends on the size of the object being manufactured. Small objects takes just a few cubic inches, and tall, thin objects print quickly, but 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.
Application
FDM has gained huge application in
(a) automotive (BMW, Hyundai, Lamborghini) to
(b) consumer goods manufacturing (Black and Decker, Dial, Nestle).
(c) used to produce end-use parts — particularly small, detailed parts and specialized manufacturing tools.
(d) Some thermoplastics can even be used in food and drug packaging
(e) popular 3D printing method within the medical industry
These companies use FDM throughout their product development, prototyping and manufacturing processes.
Materials Used
The most common printing material for FDM is acrylonitrile butadiene styrene (ABS), a common thermoplastic that's used to make many consumer products, from LEGO bricks to whitewater canoes. Along with ABS, some FDM machines also print in other thermoplastics, like polycarbonate (PC) or polyetherimide (PEI). Support materials are usually water-soluble wax or brittle thermoplastics, like polyphenylsulfone (PPSF).
Thermoplastics can endure heat, chemicals and mechanical stress, which makes them an ideal material for printing prototypes that must withstand testing. And because FDM can print highly detailed objects, it's also commonly used by engineers that need to test parts for fit and form.
Companies using FDM
While Stratasys is responsible for inventing FDM, it's not the only company profiting from this technology. Over the past two decades, FDM has become the most widely used 3D printing method in the world.
Many companies that manufacture FDM printers also offer a range of 3D printing services to clients, including external 3D modeling and printing.
Working
Here is how the FDM fabrication process works:
I. A spool of thermoplastic filament is first loaded into the printer. Once the nozzle has reached the desired temperature, the filament is fed to the extrusion head and in the nozzle where it melts.
II. The extrusion head is attached to a 3-axis system that allows it to move in the X, Y and Z directions. The melted material is extruded in thin strands and is deposited layer-by-layer in predetermined locations, where it cools and solidifies. Sometimes the cooling of the material is accelerated through the use of cooling fans attached on the extrusion head.
III. To fill an area, multiple passes are required (similar to coloring a rectangle with a marker). When a layer is finished, the build platform moves down (or in other machine setups, the extrusion head moves up) and a new layer is deposited. This process is repeated until the part is complete.
Schematic of a typical FDM printer
Characteristics of FDM
Printer Parameters
Most FDM systems allow the adjustment of several process parameters, including the temperature of both the nozzle and the build platform, the build speed, the layer height and the speed of the cooling fan. These are generally set by the operator, so they should be of little concern to the designer.
What is important from a designer's perspective is build size and layer height:
The available build size of a desktop 3D printer is commonly 200 x 200 x 200 mm, while for industrial machines this can be as big as 1000 x 1000 x 1000 mm. If a desktop machine is preferred (for reducing the cost) a big model can be broken into smaller parts and then assembled.
The typical layer height used in FDM varies between 50 and 400 microns and can be determined upon placing an order. A smaller layer height produces smoother parts and captures curved geometries more accurately, while a larger height produces parts faster and at a lower cost. A layer height of 200 microns is most commonly used.
Warping
Warping is one of the most common defects in FDM. When the extruded material cools during solidification, its dimensions decrease. Different sections of the print cool at different rates, their dimensions also change at different speeds. Differential cooling causes the buildup of internal stresses that pull the underlying layer upwards, causing it to warp, as seen in figure below. From a technology standpoint, warping can be prevented by closer monitoring of the temperature of the FDM system (e.g. of the build platform and the chamber) and by increasing the adhesion between the part and the build platform.
The choices of the designer can also reduce the probability of warping:
- Large flat areas (think of a rectangular box) are more prone to warping and should be avoided when possible.
- Thin protruding features (think of the prongs of a fork) are also prone to warping. In this case, warping can be avoided by adding some sacrificial material at the edge of the thin feature (for example a 200 microns thick rectangle) to increase the area that touches the build platform.
- Sharp corners are warping more often than rounded shapes, so adding fillets to your design is a good practice.
- Different materials are more susceptible to warping: ABS is generally more sensitive to warping compared to PLA or PETG, due to its higher glass transition temperature and relatively high coefficient of thermal expansion.
Schematic showing edge warping of an FDM part
Layer Adhesion
Good adhesion between the deposited layers is very important for an FDM part. When the molten thermoplastic is extruded through the nozzle, it is pressed against the previous layer. The high temperature and the pressure re-melts the surface of the previous layer and enables the bonding of the new layer with the previously printed part.
The bond strength between the different layers is always lower than the base strength of the material.
This means that FDM parts are inherently anisotropic: their strength in the Z-axis is always smaller than their strength in the XY-plane. For this reason, it is important to keep part orientation mind when designing parts for FDM.
For example, tensile test pieces printed horizontally in ABS at 50% infill were compared to test pieces printed vertically and were found to have almost 4 times greater tensile strength in the X,Y print direction compared to the Z direction (17.0 MPa compared to 4.4 Mpa) and elongated almost 10 times more before breaking (4.8% compared to 0.5%).
Moreover, since the molten material is pressed against the previous layer, its shape is deformed to an oval. This means that FDM parts will always have a wavy surface, even for low layer height, and that small features, such as small holes or threads may need to be post processed after printing.
Support Structure
Support structure is essential for creating geometries with overhangs in FDM. The melted thermoplastic cannot be deposited on thin air. For this reason, some geometries require support structure.
Surfaces printed on support will generally be of lower surface quality than the rest of the part. For this reason, it is recommended that the part is designed in such a way to minimize the need for support.
Support is usually printed in the same material as the part. Support materials that dissolve in liquid also exist, but they are used mainly in high-end desktop or industrial FDM 3D printers. Printing on dissolvable supports improves significantly the surface quality of the part, but increases the overall cost of a print, as specialist machine (with dual extrusion) are required and because the cost of the dissolvable material is relatively high.
Infill & Shell Thickness
FDM parts are usually not printed solid to reduce the print time and save material. Instead, the outer perimeter is traced using several passes, called the shell, and the interior is filled with an internal, low-density structure, called the infill.
Infill and shell thickness affect greatly the strength of a part. For desktop FDM printers, the default setting is 25% infill density and 1 mm shell thickness, which is a good compromise between strength and speed for quick prints.
Common FDM Materials
One of the key strengths of FDM is the wide range of available materials. These can range from commodity thermoplastics (such as PLA and ABS) to engineering materials (such as PA, TPU, and PETG) and high-performance thermoplastics (such as PEEK and PEI).
FDM at home
Some FDM printers — like 3D System's Cube, MakerBot's Replicator and Stratasys' Mojo — are designed for use by hobbyists, inventors, do-it-yourselfers and small business owners. They're small, efficient and user-friendly. However, this popular technology is becoming less expensive.
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