The most common extrusion technology is Fused Filament Fabrication (FFF). It is more commonly known as Fused Deposition Modelling (FDM), a term trademarked by Stratasys.
- FFF Printer Characteristics
- Common Problems with FFF Printing
- Dimensional Accuracy
- Benefits & Limitations
Material extrusion 3D printing pushes a string of solid thermoplastic material (also known as filament) through a heated nozzle. The heated nozzle melts the material as it deposits it on the build platform.
The FFF 3D printer lays down the melted plastic and, as it cools, it solidifies to form a solid part. The melting head is guided around the build platform by the specific layer of the CAD design being built. As each layer is completed, the build plate moves down and the next layer is deposited.
Thus the part is built layer-by-layer.
FFF Printer Characteristics
Most FFF 3d printers have many parameters that can be adjusted. Adjusting these parameters will affect build speed, nozzle temperature and extrusion speed.
At a basic level, nozzle diameter and layer height, define the resolution of the FFF printed part. A smaller nozzle diameter and lower layer height generally allow for a smoother surface texture and results in a higher level of detail.
Desktop FFF 3D printers, such as the Zortrax, typically boast a build volume of 200 mm³ to 300 mm³.
Larger format FFF 3D printers, such as the 3D Platform, boast a build volume from 0.5 m³ to 1.05 m³.
In many cases, designers can break down parts that are larger than the build volume, for assembly after printing.
Common Problems with FFF 3D Printing
Warping is the result of differential cooling of the part. When different sections cool at different rates, sections pull on the surrounding areas, resulting in warping or distortion of the FFF part.
A heated bed and good bed adhesion ensure the part is properly anchored, limiting the likelihood of warping and distortion.
Another term used for layer adhesion is ‘bonding’. As filament is extruded, it is important that each layer bonds well with the previous layer.
The downward force of the nozzle, and the partial re-melting of the previous layer ensures good interlayer adhesion (or bonding). The implication of this necessary downward force means, that the filament is deposited in an oval, resulting in small ‘valleys’ between each layer.
These ‘valleys’ create a stress concentration where cracks can form when the part is subjected to loads. This is also the cause of the rough layered appearance of FFF printed parts.
In many cases, FFF printed parts require support structures to ensure a successful print. A support structure is a low volume lattice structure that is removed from the part after printing.
A support is required whenever any overhanging feature exceeds 45° relative to the ground plane. Although it is possible to build overhangs that are less than 45°, the angled surface will be of a lower quality and integrity. If a quick Fit & Form print is needed, overhangs lower than 45° can be used. However, when surface finish, part strength and integrity are requirements, it’s best to use scaffolds.
New layers cannot be deposited onto thin air. In these cases, a support scaffold is required to initiate the build layer.
The downside to support scaffolds is the effect they have on surface finish and the requirement for post processing of the part.
When orienting a part on the build platform for printing, one can minimise the need for support scaffolding on cosmetic surfaces and thus improve finish and reduce post processing requirements.
Newer FFF 3D printers boast ‘dissolvable’ support structures. These machines have dual extrusion heads that allow for multi-material parts. Typically the ‘dissolvable’ material is PVA or HIPS. Be aware that dissolvable supports generally increase the cost of the part and the time taken to complete the build.
FFF parts are not generally printed solid. This saves on material and decreases build times.
Typically an internal, low density structure, known as an infill is used to strengthen the part.
FFF printers have a parameter, known as ‘infill percentage’ that can be varied. The amount of infill percentage used depends on the end use application of the part. For high strength parts, they can be printed 80% solid.
The cooling of the thermoplastic, at different rates, results in part distortion, shrinkage and warping.
This distortion is more apparent on larger parts or on thinner details.
There are a few techniques that can be use to minimise these effects.
Filament is usually supplied on spools, with filament thickness ranging from 1.5 mm to 3 mm in diameter. They are also often available in a range of colours.
FFF thermoplastics are some of the lowest cost 3D printing materials. There are high performance FFF filaments that are more costly.
Benefits and limitations of FFF 3D printing
Due to the ease of operation, the low material and printer cost, FFF is popular for producing custom thermoplastic parts.
The main limitations of FFF centre around the anisotropic nature of built parts — parts that are fundamentally weaker in one direction.
The designer needs to consider the application of a part and how the build direction will impact performance.