Sunday 13 December 2020

Rule of Thumb for Polymer Material Selection: Key Properties of Long Fiber Thermoplastics (LFTs)


Rule of Thumb for Polymer Material Selection: Key Properties of Long Fiber Thermoplastics (LFTs)

In this blog post, I present to you another helpful rule of thumb for plastics part design and materialselection.

Long fiber thermoplastics (LFT) – an overview

The pellet size of LFT is between 9 mm and 12 mm. The use of low shear injection moulding enables a 3D network out of long fibers in the part. LFTs are suitable for up-engineering lower cost plastics in order to replace higher cost engineering polymers. Moreover, LFTs are materials for metal replacement and replacement of underperforming polymers.

In the picture above, unfilled Polyamide, 50% short glass fiber PA 6.6 and 50% long glass fiber PA 6.6 are compared to each other with key properties such as HDT, strength, impact, and creep. Long glass fiber PA is outperforming short and unfilled PA in several areas.

When looking at the properties in detail, three of them stick out:

Stiffness: using long fibers will increase the modulus of ductile polymers. Depending on the fiber type (glass, carbon, and natural), different increases are achieved. Furthermore, short term temperature performance will be increased too which is reflected in the higher HDT (heat deflection temperature).  

Strength: increased strength is the result of higher aspect ratio due to longer fiber length. This in turn leads to a higher resistance towards deformation and creep. Fatigue endurance is improved too.  

Toughness: the long fiber network allows transferring impact energy more efficiently between polymer matrices and fibers. Energy dissipation is all over the part and not restricted to an isolated area.  

Additionally, LFTs increase the cyclical fatigue endurance due to the dissipation of stress energy over an extensive part area. The longer fibers decrease crack formation and propagation lowering the overall part failure. 

In addition, properties of LFT are highly process dependent meaning that without taking the process into account no reliable statements about properties are possible. There is always the risk to mill down the fibers when exposed to high screw rotations. Several studies have shown the fiber breakage due to moulding. For creating the 3D network, fiber lengths between min. 1 -5 mm are essential.

Thanks and #findoutaboutplastics



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