What is FDM 3D Printing and How Does it Work？

Fused Deposition Modeling (FDM) is one of the most common 3D printing technologies today. This technology uses filament made of thermoplastic material as the raw material. This article explains the basic principle of FDM 3D printing, describes how the technology works, the complete process flow step by step, and material selection.

What is FDM 3D Printing?

Fused Deposition Modeling (FDM) uses filament made of thermoplastic material as the raw material. The filament is heated and melted, then the liquefied material is extruded through a fine nozzle on the print head. After being extruded, the material is deposited onto the build platform or the previously solidified layer. When the temperature drops below the melting point, it begins to solidify. The final part is formed by stacking the material layer by layer.

How Does Fused Deposition Modeling Work？

Before describing how a fused deposition modeling 3D printer works, we can first imagine this scene:

You hold a tube of heated toothpaste. Inside the tube, the toothpaste is in a liquid state, but as soon as you squeeze it out, it solidifies immediately. Then you hold the tube upside down and squeeze the toothpaste onto the table while moving your hand horizontally, just like writing with a Chinese brush. After finishing the work on the first layer on the table, you lift the toothpaste tube a little higher and continue squeezing onto the second plane. At this time, the newly extruded toothpaste will stick to the previous toothpaste, and the earlier extruded toothpaste will solidify to provide support for the later extruded toothpaste. Finally, you keep repeating the above process until you squeeze out the shape you want. This is actually the basic idea of FDM and is also the working principle of the newly appeared 3D printing pens on the market.

Based on this basic idea, engineers first process the raw material into a round filament with a specific diameter. Then the filament-shaped raw material is gradually fed into the hot end through the filament feeding mechanism. In the hot end, the material is heated and melted. Below the hot end is the print head, and the bottom of the print head has a fine nozzle (the diameter is generally 0.2 to 0.6 mm). The pressure formed by the extrusion of the following filament pushes out the liquid material in the molten state.

Due to the requirements of the process, before the 3D printer starts working, it is generally necessary to first set basic information such as the layer spacing and the width of the paths. Then the slicing engine slices the three-dimensional model and generates the printing path. Next, under the control of the host software and the printer, the printing nozzle performs planar movement in the X-axis and Y-axis directions according to the horizontal layer data, while the vertical movement in the Z-axis direction is completed by the build platform. At the same time, the filament is fed to the nozzle by the feeding component. After being heated and melted, the heating temperature is generally set a few degrees above the melting point of the raw material. In this way, when the material is extruded from the nozzle and bonds to the build platform, it will cool and solidify quickly. The printed material quickly fuses with the previous layer. When each layer section is completed, the build platform descends by the height of one layer thickness, and the printer continues to print the next layer. This step is repeated until the entire design model is completed.

The key to the FDM process is to keep the temperature of the molten raw material extruded from the nozzle just above the solidification point, usually controlled at about 1°C higher than the solidification point. If the temperature is too high, it will cause problems such as low accuracy of the printed model and model deformation. However, if the temperature is too low or unstable, it will easily cause the nozzle to become clogged and the print to fail.

Fused Deposition Modeling Process

The process flow of fused deposition modeling can be divided into 5 steps: Create 3D model >> Convert to STL file >> Slicing >> Printing >> Post processing.

1. Create a 3D Model

Generally, designers draw the required three-dimensional digital model through computer-aided design software according to the product requirements. The design software commonly used in design mainly includes Pro/Engineering, Solidworks, MDT, AutoCAD, UG, etc.

2. Convert the Model to STL Format

Generally, there are many irregular curved surfaces on the surface of a well-designed model. Before printing, these curved surfaces on the model must be approximated and fitted. The most common method at present is to convert and save in STL format. The STL format is a file format designed by the American company 3D Systems for 3D printing equipment. It uses a series of connected small triangular planes to approximate the curved surfaces, thereby obtaining a three-dimensional approximate model file that can be quickly printed. Most common CAD design software has the function of exporting STL format files, such as Pro/Engineering, Solidworks, MDT, AutoCAD, UG, etc.

3. Slice the Model and Add Supports

Because 3D printing decomposes the model, then manufactures it layer by layer according to the layer sections, and finally assembles them in a loop. Therefore, the three-dimensional model in STL format must first be sliced and converted into a layer model that the 3D printing equipment can process. Currently, various 3D printing equipment on the market come with their own slicing processing software. After completing the basic parameter settings, the software can automatically calculate the section information of the model.

4. Start Printing

According to the FDM printing principle introduced in the previous section, it can be imagined that, in some large-span structures, the system must add support parts to the product. Otherwise, when the upper layer section enlarges sharply compared to the lower layer section, part of the later printed upper layer section will be suspended (or hanging in the air), resulting in partial collapse or deformation of the section, which seriously affects the forming accuracy of the printed model. Therefore, the final printed model generally includes two aspects: the support part and the solid part. The slicing software will automatically calculate and decide whether to add supports for it according to the different shapes of the model to be printed.

At the same time, supports have another important purpose: to establish the base layer. That is, before formal printing, a base layer is first printed on the build platform, and then the model is printed on this base layer. This can make the bottom of the printed model flatter and also make the finished model easier to peel off. Therefore, a key step in performing FDM printing is making supports. A good base layer can provide an accurate reference plane for the entire printing process, thereby ensuring the accuracy and quality of the printed model.

5. Support Removal and Post-processing

For models made by FDM, the post-processing mainly involves peeling off the supports of the model and sanding the outer surface. First, the support part of the solid model needs to be removed, and then the outer surface of the solid model is sanded so that the accuracy and surface roughness of the final model meet the requirements.

However, according to actual production experience, the supports of models produced by FDM technology on complex and fine structures are difficult to completely remove without affecting the model. It is easy to damage the surface of the prototype, which will have a considerable impact on the quality of the model’s surface. To address this problem, the 3D printing giant Stratasys developed a water-soluble support material in 1999. By rinsing the printed model with a solution to dissolve the support material without damaging the solid model, this problem was effectively solved. However, at present, FDM printing equipment independently developed in China cannot achieve this yet, and the post-processing of printed models is still a relatively complicated process.

Materials Used in Fused Deposition Modeling

At present, the most common filament materials mainly include ABS, PLA, artificial rubber, wax, and polyester thermoplastics. Some equipment needs to use two materials: one is the modeling material used to print the solid part; the other is the support material used to deposit cavity or cantilever parts.

Compared with other 3D printing technologies, the range of raw materials available for FDM printing is relatively wide. When selecting materials for the model solid part, the following factors mainly need to be considered:

• Viscosity: The lower the viscosity, the smaller the resistance, which helps form and makes it less likely to clog the nozzle.
• Melting Point: The closer the melting point temperature is to room temperature, the smaller the printing power consumption, and it is beneficial to improve the mechanical life of the machine, reduce thermal stress, and thus improve printing accuracy.
• Adhesion: The adhesion of the material will determine the connection strength between the layers of the printed item.
• Shrinkage: The smaller the shrinkage rate of the material, the more guaranteed the accuracy of the printed item.

For support materials, the FDM process mainly has the following requirements:

• According to the different solid materials, the support material must be able to withstand a certain high temperature accordingly.
• There should be no wetting between the support material and the solid material to facilitate post-processing.
• Like the solid material, it needs good fluidity.
• It is best to have features such as water solubility or acid solubility.
• A lower melting temperature is preferable.

Typical Equipment

The materials supplied for FDM printing are generally thermoplastics, such as wax, ABS, PC, nylon, etc. Standard printing materials are generally made from filament, and the material cost is typically low. The unit price of domestic ABS or PLA per kilogram is mostly within 100 RMB(about 15 dollars). Moreover, compared with printing equipment that uses powder and liquid materials, filament is cleaner, easier to replace and store, and the printing process will not form powder or liquid pollution.

There are many fused deposition 3D printers available on the market, particularly desktop printers designed for everyday consumers. It is almost the world of Fused Deposition Modeling. The most well-known ones, such as MakerBot’s Thing-O-Matic, Replicator series printers, and 3D Systems’ Cube printer, are all entry-level 3D printers that use FDM technology.

Tolerance and Capacities

At Getzshape, our 3D printing services cover four main technologies, which are SLA, SLS, SLM, and FDM. Our FDM 3D printing tolerance and capacities are listed below.

Finishes for FDM Printed Components

• Sanding: A basic and important surface finishing step for FDM printed parts. It uses sandpaper with different grits to remove the visible layer lines and make the surface smoother.
• Painting: First, the surface is sanded to make it smoother. Then primer is applied to help the paint stick better. After that, color paint is sprayed or brushed on. Painting can hide layer lines completely, give the part a nice color, and make the surface look smooth. It is widely used for display models, prototypes, and final products.
• Polishing: Making the surface of FDM parts smooth and shiny. After basic sanding, special polishing compounds or tools are used to rub the surface. This process removes small scratches and makes the part look glossy. Polishing works well on materials like PLA and ABS.
• Vapor Smoothing: A special chemical finishing method, mainly used for ABS parts. The printed part is placed in a closed container with solvent vapor. The vapor slightly melts the surface of the part, causing the layer lines to flow together and become smooth. After the process, the part has a very smooth, glossy surface without visible layers.