Long fiber reinforced thermoplastics (LFRT) are being used for high mechanical performance injection molding applications. Although LFRT technology can provide good strength, stiffness and impact properties, processing method of this material plays an important role in determining how final part will perform.
In order to successfully shape LFRTs, it is necessary to understand some of their unique characteristics. Understanding differences between LFRT and conventionally reinforced thermoplastics drives development of equipment, design and processing techniques to maximize value and potential of LFRT.
Difference between LFRT and conventional chopped, short glass fiber reinforced composites is length of fibers. In LFRT, length of fibers is same as length of pellets. This is due to fact that most LFRTs are produced by a pultrusion process rather than shear type compounding.
In LFRT manufacturing, continuous strands of fiberglass roving are first drawn into a die for coating and resin impregnation. After exiting die, this continuous strip of reinforced plastic is chopped or pelletized, usually to a length of 10-12mm. In contrast, traditional short glass fiber compounds contain only 3-4 mm long chopped fibers, which are further reduced to typically less than 2 mm in length in a shear extruder.
Fiber length in LFRT pellets contributes to improved mechanical properties of LFRT – increased impact resistance or toughness while maintaining stiffness. As long as fibers maintain their length during molding process, they form an "inner skeleton" that provides ultra-high mechanical properties. However, a poor molding process can turn a long-fiber product into a short-fiber material. If length of fibers is compromised during forming process, it is not possible to obtain required level of properties.
To maintain fiber length during LFRT molding, there are three important aspects to consider: injection molding machine, part and mold design, and processing conditions.

A question that is often asked about LFRT processing is whether it is possible for us to mold these materials using existing injection molding equipment. In vast majority of cases, same equipment used to form short fiber composites can also be used to form LFRT. While typical short fiber forming equipment is adequate for most LFRT components and products, some modifications to equipment can better help maintain fiber length.
A general-purpose screw with a typical "feed-compression-metering" section is well suited for this process, and fiber-destructive shear can be reduced by reducing compression ratio in metering section. A metering section compression ratio of approximately 2:1 is optimal for LFRT products. It is not necessary to manufacture screws, barrels and other components from special metal alloys because LFRTs do not wear as much as traditional chopped glass fiber reinforced thermoplastics.
Another piece of equipment that might benefit from a design review is nozzle tip. Some thermoplastic materials are easier to process with a reverse tapered nozzle tip, which creates a high degree of shear as material is injected into mold cavity. However, such nozzle tips significantly reduce fiber length of long-fiber composites. A slotted nozzle tip/valve assembly with a 100% "free flow" design is therefore recommended, which allows long fibers to pass easily through nozzle into part.
In addition, nozzle and gate holes should have a loose dimension of 5.5mm (0.250in) or more in diameter and be free of sharp edges. It is important to understand how material flows through injection molding equipment and to identify where shear can break fibers.

 

Figure: Three-piece screw tip and ring valve with "100% free flow" design to minimize long fiber breakage

Good part and mold design can also go a long way in maintaining fiber length in LFRT. Elimination of sharp corners around part edges, including rib lines, bosses, and other features, avoids unnecessary stress in molded parts and reduces fiber wear.
Parts shall be of nominal wall design with uniform wall thickness. Large variations in wall thickness can lead to inconsistent filling and unwanted fiber orientation in part. Where it must be thicker or thinner, sudden changes in wall thickness are avoided to avoid creating areas of high shear that could damage fibers and become a source of stress concentrations. Usually try to gate in thicker wall and flow to thinner part, keeping end of fill in thinner part.
General principles of good plastic design suggest that keeping wall thickness below 4mm (0.160in) will promote good uniform flow and reduce likelihood of sinking and voids. For LFRT composites, optimum wall thickness is usually around 3mm (0.120in), with a minimum thickness of 2mm (0.080in). When wall thickness is less than 2mm, probability of fiber breakage increases after material enters mold.
Parts are only one aspect of design, it is also important to consider how material will enter mold. When runners and gates guide material into cavity, without proper design, a lot of fiber damage can occur in these areas.
When designing a mold for molding LFRT compounds, a fully rounded runner is optimal, with a minimum diameter of 5.5mm (0.250in). Runners of any type other than full radius runners will have sharp corners, which can increase stress during molding process and destroy glass fiber reinforcement. Hot runner systems with open sprues are acceptable.
Gate should have a minimum thickness of 2mm (0.080in). If possible, locate gate along an edge that does not impede flow of material into cavity. Gates on the surface of part will need to be rotated 90° to prevent initiation of fiber breakage that would degrade mechanical properties.
Finally, be aware of location of weld lines and know how they affect areas that are subject to load (or stress) while part is in use. Fusion line should be moved to areas where stress levels are expected to be lower by proper placement of gates.
Computer mold filling analysis can help determine where these weld lines will be located. Structural finite element analysis (FEA) can be used to compare location of high stress with location of junction line determined in mold filling analysis.
It should be noted that these parts and mold designs are only suggestions. There are many examples of parts with thin walls, varying wall thicknesses, and delicate or fine features that have achieved good performance with LFRT compounds. However, the farther one departs from these recommendations, the more time and effort will be required to ensure that full benefits of long fiber technology are realized.

Processing conditions are critical to success of LFRT. As long as correct processing conditions are employed, it is possible to make good LFRT parts using general purpose injection molding machines and properly designed molds. In other words, even with proper equipment and mold design, fiber length can be compromised if poor processing conditions are employed. This requires understanding what fiber will encounter during forming process and identifying areas that will cause excessive fiber shear.
First, monitor backpressure. High back pressure introduces huge shear force on material, which will reduce fiber length. Consider starting with zero back pressure and increasing it only so that screw retracts evenly during feeding, using a back pressure of 1.5 to 2.5 bar (20 to 50 psi) is usually sufficient for consistent feeding.
High screw speeds also have adverse effects. The faster screw rotates, the more solid and unmelted material is likely to enter compression section of screw and cause fiber damage. Similar to recommendations for back pressure, try to keep rotational speed at minimum required for a stable filling screw. When molding LFRT compounds, a screw speed of 30~70r/min is common.
During injection molding, melting occurs through two factors acting together: shear and heat. Since aim is to preserve length of fiber by reducing shear in LFRT, more heat will be required. Depending on resin system, processing temperatures for LFRT compounds are typically 10-30℃ higher than for conventional molding compounds.
However, before simply increasing barrel temperature across board, attention should be paid to inversion of barrel temperature distribution. Normally, barrel temperature rises as material moves from hopper to nozzle; but for LFRT, higher temperatures at hopper are recommended. Inverting temperature profile will soften and melt LFRT pellets before entering compression section of high shear screw, thus favoring fiber length retention.
A final note about processing concerns utilization of regrind. Grinding formed parts or nozzles generally results in lower fiber lengths, so addition of regrind affects overall fiber length. In order not to significantly reduce mechanical properties, it is recommended that maximum amount of recycled materials be 5%. Higher regrind usage can have a negative impact on mechanical properties such as impact strength.