Long fiber reinforced thermoplastics (LFRTs) are being used in injection molding applications with high mechanical properties. While LFRT technology can provide good strength, stiffness and impact properties, method of processing material plays an important role in determining how well final part can perform.
In order to successfully shape LFRTs, it is necessary to understand some of their unique characteristics. Understanding differences between LFRT and conventional reinforced thermoplastics drives development of equipment, design, and processing techniques to maximize value and potential of LFRT.
Difference between LFRT and traditional 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, a continuous strand of glass fiber roving is 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 10-12 mm. In contrast, traditional short glass fiber composites only contain chopped fibers with a length of 3 to 4 mm, which are further reduced to typically less than 2 mm in shear-type extruders.
Fiber length in LFRT pellets helps improve 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 "internal 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 molding process, it may not be possible to achieve desired level of performance.
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 frequently asked question about LFRT processing is whether it is possible for us to use existing injection molding equipment to mold these materials. In vast majority of cases, equipment used to form short fiber composites can also be used to form LFRTs. While typical staple fiber forming equipment is adequate for most LFRT parts and products, some modifications to equipment can better help maintain fiber length.
A general purpose screw with a typical "feed-compress-meter" section is well suited for this process, and fiber-destructive shearing can be reduced by reducing compression ratio of 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, as LFRT does not wear as much as conventional chopped glass fiber reinforced thermoplastics.
Another piece of equipment that may 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 when material is injected into mold cavity. However, such nozzle tips can significantly reduce fiber length of long fiber composites. It is therefore recommended to use a slotted nozzle tip/valve assembly of a 100% "free flow" design, which allows long fibers to easily pass through nozzle into part.
In addition, diameter of nozzle and gate holes should have a loose size of 5.5mm (0.250in) or more and have no sharp edges. It is important to understand how material flows through injection molding equipment and identify where shearing can break fibers.
Figure: Three-piece screw tip and ring valve with a "100% free flow" design to minimize breakage of long fibers

Good part and mold design also goes a long way toward maintaining fiber length in LFRTs. Eliminating sharp corners around some edges, including ribs, bosses, and other features, avoids unnecessary stress in molded parts and reduces fiber wear.
Components shall be of a nominal wall design with uniform wall thickness. Larger variations in wall thickness can lead to inconsistent packing and unwanted fiber orientation in the part. Where it must be thicker or thinner, avoid sudden changes in wall thickness to avoid creating areas of high shear that can damage fibers and become a source of stress concentration. Usually try to gate gate in thicker wall and flow to thin part, keeping fill end in thin part.
General principles of good plastic design suggest that keeping wall thicknesses below 4mm (0.160in) will promote good uniform flow and reduce likelihood of sinks and voids. For LFRT composites, optimal wall thickness is usually around 3mm (0.120in) and minimum thickness is 2mm (0.080in). When wall thickness is less than 2mm, probability of fiber breakage of material after entering mold increases.
Parts are only one aspect of design, and it is also important to consider how material enters 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). Any other form of runner other than a full fillet will have sharp corners that add stress during molding process and destroy fiberglass reinforcement. Hot runner systems with open runners are acceptable.
Gate should have a minimum thickness of 2mm (0.080in). If possible, position gate along an edge that does not impede flow of material into cavity. Gate on part surface will require a 90° turn to prevent fiber breakage that can degrade mechanical properties.
Finally, pay attention to location of weld lines and know how they affect area of part that is under load (or stress) in service. Fusion line should be moved to an area where stress level is expected to be lower by proper gate placement.
Computerized 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 confluence line determined in mold filling analysis.
It should be noted that these part and mold designs are suggestions only. There are many examples of parts with thin walls, varying wall thicknesses, and delicate or fine features that have achieved good performance with LFRT composites. However, the further one deviates from these recommendations, the more time and effort it takes to ensure that full benefits of long-fiber technology are realized.

 

Processing conditions are critical to the success of LFRT. As long as correct processing conditions are used, it is possible to prepare ready-made LFRT parts using a general-purpose injection molding machine and a properly designed mold. 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 fibers will encounter during forming process and identifying areas that will cause excessive fiber shear.
First, monitor back pressure. High back pressure introduces large shear forces on material, which will reduce fiber length. Consider starting with zero back pressure and only increase it until screw returns evenly during feeding process, using a back pressure of 1.5-2.5 bar (20-50 psi) is usually sufficient for consistent feeding.
High screw speeds also have a detrimental effect. The faster screw rotates, the more likely solid and unmelted material will enter compression section of screw and cause fiber damage. Similar to recommendations for back pressure, try to keep rotational speed at the minimum required for a stable filling screw. When molding LFRT compounds, screw speeds of 30 to 70 r/min are common.
During injection molding, melting occurs through two co-acting factors: shear and heat. Since purpose is to preserve length of fiber by reducing shear in LFRT, more heat will be required. Depending on resin system, temperature for processing LFRT compounds is typically 10-30℃ higher than for conventional molding compounds.
However, before simply raising barrel temperature across board, be aware of inversion of barrel temperature distribution. Normally, barrel temperature rises as material moves from hopper to nozzle; however, for LFRT, a higher temperature at hopper is recommended. Inverted temperature profile will soften and melt LFRT pellets before entering compression section of high shear screw, which facilitates fiber length retention.
A final note about processing concerns use of recycled materials. Grinding molded parts or nozzles often results in lower fiber lengths, so addition of regrind can affect the overall fiber length. In order not to significantly reduce mechanical properties, it is recommended that maximum amount of recycled material is 5%. Higher regrind usage negatively affects mechanical properties such as impact strength.