A new design for the LFT screw was proposed in order to accelerate melting LFT pellets before applying strong shear during the injection molding process. Drawings of the default LFT screw, recommended by the machine manufacturer, and the new LFT screw are provided in Figure 1. First, the pellets’ residence time for melting in the feeding zone was extended by increasing the flight depth. Generally, the residence time (t_R) can be calculated as:

t_R=  V/Q

where V is the volume of the reservoir in the feed zone (m³), and Q is the volumetric flow rate (m³/s), which is the feeding rate. The flight depth was increased from 3.8 mm to 6.8 mm, resulting in a ~79% longer residence time, which allows for longer heating before the plasticization step at the same feeding rate. Second, a longer transition zone was proposed. Polymer is a viscoelastic material, so sudden deformation results in elastic solid-like behavior due to insufficient relaxation time to act like a liquid. By extending the length of the transition zone, gradual deformation of the LFT pellets was designed. Lastly, the blister ring in the default LFT screw was removed, and a mild compression ratio of 2.3 was employed instead. The default LFT screw appears to be designed with no compression until the blister ring, presumably to keep the long fibers, then partially unmelted pellets are melted at a high compression ratio (>4) when passing through the blister ring. This may be beneficial for commodity polymers that have low melting temperatures, but the blister ring seems not to be suitable for Xencor grades as the melting temperature is high and require longer melting time. 

Figure 1. Drawings of (a) the default LFT screw and (b) the new LFT screw.

Figure 2 shows (a) optical microscopic image of glass fiber obtained from the new LFT screw, and (b) fiber length comparison between the default screw and the new screw. Figure 2(a) clearly shows that the new screw yields much longer fibers than the default screw. Figure 2(b) provides a qualitative comparison of fiber length using the default and new LFT screws under various processing conditions. The default LFT screw produces a fiber length (Lp) range between 500 and 900 microns, while the new LFT screw achieves a higher range between 2000 and 2700 microns, resulting in nearly 4 times longer fibers under any conditions.

Figure 2. (a) Optical microscopic image of glass fiber obtained from the new LFT screw, and (b) fiber length comparison between the default screw and the new screw.