Even when it’s not a sufficient indicator of material quality, it’s certainly a necessary one. For that reason, engineers working in polymer 3D printing or additive manufacturing would be well served to ensure they have a good understanding of MFI and how it impacts the quality of polymer 3D-printed parts.
What is Melt Flow Index?
Melt Flow Index is a measure of the ease with which a polymer, in a molten state, can be extruded through a specified orifice under standardized conditions of temperature and pressure. It is commonly expressed in grams per ten minutes (g/10 min) and serves as a critical indicator of a polymer's flow characteristics during processing.
MFI and viscosity share an inverse relationship. A polymer with a low MFI has higher viscosity, indicating greater resistance to flow. Conversely, a high MFI corresponds to lower viscosity and better flow properties. The testing procedures for MFI are defined by international standards, such as ASTM D1238 and ISO 1133.
Why MFI Matters for 3D Printing
While most commonly used as a metric in molding, MFI’s applicability to polymer 3D printing is obvious. Melt flow directly influences how well a polymer can be extruded through the printer nozzle, thereby affecting layer adhesion, surface finish and overall print quality.
Understanding MFI can be beneficial to polymer 3D printing in many ways, including:
- Optimizing MFI to ensure a smooth and controlled material flow, reducing the risk of clogs or uneven extrusion.
- Polymers with higher MFI values generally exhibit better extrudability, allowing for faster and more efficient printing processes.
- Better control of the flow ensures precise deposition, resulting in accurate and detailed 3D-printed objects.
MFI is a crucial rheological parameter for 3D printing because it predicts how polymers behave when subjected to the temperatures and pressures of 3D printing processes. As a result, polymers with well-matched rheological properties, guided by MFI, can contribute to a smoother printing process and improved product quality.
How Melt Flow Affects Polymer 3D Printing
Polymers with higher melt flow rates generally exhibit better flowability, aiding in the creation of intricate and precise 3D-printed structures. MFI varies considerably among polymers, making different formulations more or less suitable for various applications.
Take these two types of acrylonitrile butadiene styrene (ABS) filament as examples: Raise3D Premium and Raise3D Hyper Speed, where the latter’s MFI is roughly five times that of the former. Printing speed is the crucial factor here, as pointed out in a recent blog from Ember Prototypes, which found that flow can be a significant factor in reducing part strength at higher speeds as a result of under extrusion or inadequate flow.
Additionally, recent research has shown considerable variation in the MFI for ABS depending on its age and whether the material has been recycled prior to use.
In a separate study, another team of engineers examined the effect of flow rate and print speed on the density and microcellular structure of polylactic acid (PLA) foams in fused filament fabrication (FFF). What they found was that medium levels of flow rate and print speed resulted in foams with the lowest density and highest uniformity in morphology.
Nevertheless, we should not make the mistake of thinking that MFI can serve as the sole determiner of a material’s suitability for 3D printing applications. For example, research involving the MFI of seven commercial PLA grades has indicated that plasticizer type and crystallinity also impact the melt deposition.
The moral is that while MFI is extremely useful, it isn’t the whole story when it comes to evaluating polymer 3D printing materials.
MFI in 3D Printing Materials
Quantifying a polymer’s flowability is essential to evaluating it for additive applications, particularly when they require high speed or high precision. The MFI of a prospective material can be a useful shorthand for its extrudability and layer adhesion, but even when it’s not sufficient as a standard on its own, the mere presence of MFI on a product spec sheet (with the accompanying testing procedure) is a good sign.
We’re still in something of a Wild West when it comes to 3D printing filaments, so it’s important to vet suppliers carefully. If some new vendor’s prices seem too good to be true — and there’s no MFI value — they probably are. In any case, being mindful of MFI can help engineers working with additive manufacturing applications ensure that they produce quality parts consistently.