Fused Deposition Modeling

About

I presented a paper on Fused Deposition Modeling (FDM) in Momentum 2005 (National Level Technical Fest) held at Government Engineering College, Thrissur. This is the reproduction of the entire paper. You can download it in pdf format from here.

Abstract: Fused Deposition Modeling (FDM) is one method to develop rapid prototypes or models. The FDM machine builds the part by extruding a semi-molten filament through a heated nozzle in a prescribed pattern onto a platform. When the first layer is complete, the platform lowers by one layer thickness and the process begins again. The part is easily removed from the platform, supports are removed, and the part is ready. Aside from the basic part design, the user also controls material selection, toolpath, layer thickness and part orientation. The advantages to using FDM include the speed and safety of the machine as well as lower cost compared to SL machines. An additional software package called QuickSlice(TM), serves as an interface between the user and the machine. The FDM process at present uses only plastic materials. The available materials include ABS, medical grade ABS and investment casting wax.

Keywords: Fused Deposition Modeling, rapid prototyping, QuickSlice, Break Away Support System

Rapid prototyping, also known as solid freeform fabrication, is the automatic construction of physical objects with 3D printers, stereo lithography machines and fused deposition modeling systems or selective laser sintering systems. Rapid prototyping is a type of computer-aided manufacturing (CAM) and is one of the components of rapid manufacturing. The first techniques for rapid prototyping became available in the 1980s; traditionally they have been to produce models (prototypes). Nowadays, they are increasingly employed to produce tools or even to manufacture production quality parts in small numbers.

The standard interface between CAD software and rapid prototyping machines is the STL format. The word "rapid" is relative: construction of a model with contemporary machines typically takes 3-72 hours, depending on machine type and model size.

Fused deposition modeling, which is often referred to by its initials FDM, is a type of rapid prototyping or rapid manufacturing (RP) technology commonly used within engineering design. The technology was developed by C. Scott Crump in the late 1980s and was commercialized in 1990. The FDM technology is marketed exclusively by Stratasys Inc.

FDM works on an "additive" principle by laying down material in layers. A plastic filament or metal wire is unwound from a coil and supplies material to an extrusion nozzle which can turn on and off the flow. The nozzle is heated to melt the material and can be moved in both horizontal and vertical directions by a numerically controlled mechanism, directly controlled by a Computer Aided Design software package. The model is built up from layers as the plastic hardens immediately after extrusion from the nozzle.

FDM features the Break Away Support System (BASS), allowing the designer to create models with greater speed and precision. The support tip extrudes a material that supports any overhanging portions of the model’s geometry. When the model is completed, the support is easily broken off leaving behind the final product. A water-soluble material can be used for making temporary supports while manufacturing is in progress, which can simply be washed away after fabrication.

Each CAD file is converted to an STL format. The STL file is read into Stratasys’ slicing software called QuickSlice. QuickSlice breaks the model into individual slices, with each slice representing one layer of material. QuickSlice then generates tool paths to fill the slices. These tool paths form the SML file. After slicing an STL file and creating a Stratasys Modeling Language (SML) file, the SML file is downloaded to the FDM Hardware for modeling.

Several materials are available with different trade-offs between strength and temperature. As well as ABS polymer, the FDM technology can also be used with polycarbonates, polyphenylsulfones and waxes.

With the FDM system, you can take your project from the design stage to concept model to final wax pattern or plastic prototype. The three main parts to the FDM system are:

QuickSlice software is an integral part of the FDM system. It serves as a bridge between your 3-D STL files and the FDM hardware. QuickSlice works with any STL file. It allows you to specify the physical attributes of that STL file and create a Stratasys Modeling Language (SML) file that can be sent directly to the FDM hardware. The SML file provides the instructions necessary for the FDM hardware to build the physical model.

QuickSlice cuts the STL file model into horizontal slices. Each slice is extruded as a single layer of material by the FDM hardware to create a physical model.

QuickSlice can read an STL file in compressed or uncompressed form. The STL file can be in either ASCII or binary file format. Once in QuickSlice, the STL file can be repositioned, rotated, and scaled. It is important for STL files to be relatively free of defect before reading them into QuickSlice. A defective STL file can make the FDM system operator’s job unnecessarily complicated. QuickSlice operation involves the following steps:

Starting from an STL file, the geometry of a part can be read into QuickSlice and sliced at a pre-selected modeling resolution. Each slice consists of one or more curves that follow the shape of the part at that particular height. The specific height of a slice is called a "Z level".

When the FDM hardware builds a physical model, it lays down tracks of modeling material called "roads". These roads follow the shapes of slice curves. In QuickSlice, each slice curve can be assigned attributes such as wall thickness and fill patterns.

Roads are created as toolpaths for the FDM system. Creating roads allows the operator to view where the heads will begin and end each layer of material on the computer screen before sending the file to the FDM System. If the roads need to be changed, the operator can either edit the road values of sets, redistribute slice curves into other sets, or both. Roads then are recreated to see the effect of the changes. Once the roads are satisfactory, the operator can save them by writing the SML file.

There are two main road types – perimeter and fill. A "perimeter’ road follows the shape of a slice curve. The centre of the road is usually offset slightly in from the slice curve to maintain the outer dimension of the original design. The amount of offset depends upon the road width. "Fill" roads fill the solid areas inside the part. The types of fill patterns available are raster, contour, or a combination of both.

A SML file is saved and then sent to the FDM system. A SML file contains a list of instruction for the FDM system to perform the actions necessary to build the 3D model. Included in these instructions are FDM head movements, tip selection, material flow rates, and wait statements.

The FDM is capable of using a variety of inert, nontoxic materials such as wax, ABS plastic and elastomer. Each material comes wound on a spool in the form of a filament approximately 0.07" in diameter, so it is both easy to load and easy to store.

The materials may be stored at normal room temperature. Humidity must be eliminated when using ABS plastics. Exposing the filament material to temperatures outside normal office temperatures may cause the filament to fracture. It is recommended to store ABS plastic spools in dry boxes. These materials also can be stored inside two plastic bags. A bag of desiccant is enclosed with the spool. The plastic bags are placed in a closed cabinet or sealed environment. This is to protect them from dust and moisture. The spools of material are handled with care because sudden or abrupt impacts to material spools may cause the filament to fracture.

This tough plastic is an acrylonitrile-butadiene-styrene based material which produces sturdy prototypes. P400 ABS allows you to make models much more similar to the desired final product than was possible with other modeling materials.

P500 ABS is a medium grade, high impact strength ABS plastic. This tough plastic can be gamma sterilized with negligible discoloration.

Stratasys investment casting wax allows the operator to take an STL file from a CAD model through the Stratasys modeling process and create a wax pattern. Forming and de-waxing a shell mould is done rapidly using normal investment casting procedures. No special burnout is required. The low ash content and low viscosity level of the wax leave a residue-free cavity.

The hardware mainly consists of two parts – the modeler and the printer server.

The modeler with control panel and the working envelop access at the front. Inside the working envelope are the head and the foam foundation which support the part when it is build. The material stock is positioned on the right side of the modeler.

The printer server, a computer with NT platform, is used to download the required file to the modeler.

FDM is the second most widely used rapid prototyping technology, after stereo lithography. A plastic filament, approximately 1/16 inch in diameter, is unwound from a coil (A) and supplies material to an extrusion nozzle (B). Some configurations of the machinery have used plastic pellets fed from a hopper rather than a filament. The nozzle is heated to melt the plastic and has a mechanism which allows the flow of the melted plastic to be controlled. The nozzle is mounted to a mechanical stage (C) which can be moved in horizontal and vertical directions.

As the nozzle is moved over the table (D) in the required geometry, it deposits a thin bead of extruded plastic to form each layer. The plastic hardens immediately after being squirted from the nozzle and bonds to the layer below. The entire system is contained within an oven chamber which is held at a temperature just below the melting point of the plastic. Thus, only a small amount of additional thermal energy needs to be supplied by the extrusion nozzle to cause the plastic to melt. This provides much better control of the process.

Support structures must be designed and fabricated for any overhanging geometries and are later removed in secondary operations. BASS uses a second nozzle to extrude the support material. The supports are designed to prop up the overhanging portions of the part during modeling. The supports detach easily, making the finished product look better with minimal post-modeling finishing.

For best use of the BASS system, the SML file must be made correctly. The layer of support making direct contact with the model must be part of a set designated strictly for supports. ABS and ICW06 have specific release materials associated with them, and it is recommended to make the entire support from the release material. Water-soluble support materials have also become available which can be removed simply by washing them away.

After the model is made, the supports must be detached without removing any of the part. If the support tool path is the same colour as the part tool path, the operator can still determine the difference by visually inspecting the SML file. A support region has wide gaps between the roads, and a part region might have no gap between the roads. A significant advantage in using the BASS system becomes obvious when you take the part out of your FDM machine. Not only is the finished part of better quality than a part made without the BASS system, but the supports come off easily and cleanly. This greatly reduces the time required to manually finish the part.

The model can be polished and finished mechanically. The surfaces of the model are smoothed and the seams are removed with an abrasive such as sandpaper or a flat file or a tool such as a paper cutter. Care must be exercised because the walls of a model are thin and may break if overstressed. A cutter works well in removing seams. Glue may be used to repair any breaks or cracks in the model. A small amount of glue is brushed on each of the two adjoining surfaces. This helps the joining process.

Chemical finishing takes very little physical effort, but is time-consuming, and – as with any chemical use – precautions must be taken to ensure the safety of the user. Solvents such as paint thinner, acetone, heptane, and toluol smooth the surface of the model by chemically dissolving the rough edges, and also bond the layers together.

1. True desktop manufacturing system that can be run in office environment. There is no worry of exposure to toxic fume and chemicals.

4. Material is supplied in spool form which is easy to handle and can be changed in minutes

6. A good variety of material is available including colour ABS and Medical ABS, investment casting wax and elastomer

1. Accuracy is relatively low and is difficult to build parts with complicated details

The accuracy of the FDM hardware and the stability of the materials allow the models to be used as masters for prototype injection molds, producing multiple accurate shapes directly from CAD models.

A common frustration in product assembly occurs when components do not fit together or when they do not fit into the housing planned for them. The FDM hardware produces prototypes to check fit, form, and some function at less cost and in less time than conventional processes.

Investment castings are used in the medical field to make replacement limbs and joints. With the FDM system, the designer eliminates the hand-made master (which was used in the previous system), and goes directly from the CAD design to an investment casting, using the model as the master pattern.

The speed and accuracy of the FDM hardware allows engineers to produce a new part each time the product undergoes a revision. In addition, a full-scale prototype of a proposed part, handed out at the final presentation or included with the proposal package, has strong appeal for the prospective clients.

The FDM process is office-friendly and quiet. FDM is fairly fast for small parts of the order of a few cubic inches, or those that have tall, thin form-factors. It can be very slow for parts with wide cross sections, however. The finish of parts produced with the method have been greatly improved over the years, but aren't quite on a par with stereolithography. The closest competitor to the FDM process is probably three dimensional printing. However, FDM offers greater strength and a wider range of materials than at least the implementations of 3DP from Z Corp. which are most closely comparable. Stratasys is the only western supplier. Similar technology has also been under development in China.