Long fiber reinforced thermoplastics ( LFRT ) are being used in injection molding applications with high mechanical properties. Although LFRT technology can provide good strength, stiffness and impact properties, the processing method of this material can determine the performance of the final part. Play an important role.
In order to successfully shape the LFRT, it is necessary to understand some of their unique characteristics. Understanding the differences between LFRT and conventionally reinforced thermoplastics has driven the development of equipment, design and processing technologies to maximize the value and potential of LFRT.
The difference between LFRT and conventional chopped, short glass fiber reinforced composites is the length of the fibers. In LFRT, the length of the fibers is the same as the length of the pellets. This is due to the fact that most LFRTs are produced by a pultrusion process rather than a shear type compounding.
In the manufacture of LFRT, the continuous strands of glass fiber roving are first drawn into a die for coating and impregnating the resin. After exiting the die, the continuous reinforced plastic strip is chopped or granulated, usually Cut to a length of 10~12mm. In contrast, conventional short glass composites contain only chopped fibers 3 to 4 mm long, and their length is further reduced to less than 2 mm in shear extruders.
The fiber length in the LFRT pellets helps to improve the mechanical properties of the LFRT - increased impact resistance or toughness while maintaining stiffness. As long as the fibers remain lengthy during the forming process, they form an "internal skeleton" that provides superior mechanical properties. However, a poor molding process turns long fiber products into short fiber materials. If the length of the fiber is damaged during the forming process, it is impossible to obtain the required level of performance.
In order to maintain fiber length during LFRT molding, there are three important aspects to consider: injection molding machines, part and mold design, and processing conditions.
First, equipment considerations
One question that is often asked about LFRT processing is whether it is possible to use existing injection molding equipment to mold these materials. In the vast majority of cases, equipment for forming short fiber composites can also be used to form LFRT. While typical short fiber forming equipment is satisfactory for most LFRT parts and products, some modifications to the equipment can help to maintain fiber length.
A universal screw with a typical "feed-compression-metering" section is well suited for this process and can reduce fiber-damaged shear by reducing the compression ratio of the metering section. A metering section compression ratio of approximately 2:1 is optimal for LFRT products. It is not necessary to make screws, barrels and other components from special metal alloys because the wear of LFRT is not as large as conventional chopped glass fiber reinforced thermoplastics.
Another device that may benefit from the design review is the nozzle tip. Some thermoplastic materials are easier to machine with a reverse tapered nozzle tip that creates a high degree of shear when the material is injected into the mold cavity. However, such nozzle tips can significantly reduce the fiber length of long fiber composites. It is therefore recommended to use a 100% "free-flow" design of the slotted nozzle tip/valve assembly that allows long fibers to easily enter the component through the nozzle.
In addition, the nozzle and gate holes should have a loose diameter of 5.5 mm (0.250 in) or more and have no sharp edges. It is important to understand how the material flows through the injection molding equipment and determine where the shear will break the fiber.
Second, parts and mold design
Good part and mold design can also be beneficial for maintaining the fiber length of the LFRT. Eliminating sharp corners around portions of the edges, including ribs, bosses, and other features, avoids unnecessary stress in the molded part and reduces fiber wear.
Components shall be of nominal wall design with uniform wall thickness. Large variations in wall thickness can result in inconsistent packing and unwanted fiber orientation in the part. Where thick or thinner must be avoided, sudden changes in wall thickness are avoided to avoid the formation of high shear areas that can damage the fibers and become a source of stress concentration. It is common to try to open the gate in a thicker wall and to a thin portion so that the filling end is kept in a thin portion.
The general good plastic design principle suggests that maintaining a wall thickness below 4 mm (0.160 in) will promote good uniform flow and reduce the likelihood of dents and voids. For LFRT composites, the optimum wall thickness is typically around 3 mm (0.120 in) and the minimum thickness is 2 mm (0.080 in). When the wall thickness is less than 2 mm, the probability of fiber breakage of the material after entering the mold increases.
Parts are just one aspect of the design, and it is important to consider how the material enters the mold. When the runners and gates direct the material into the cavity, a large amount of fiber damage can occur in these areas if not properly designed.
When designing a mold for forming a LFRT composite, the full fillet flow path is optimal, with a minimum diameter of 5.5 mm (0.250 in). In addition to the full rounded flow path, any other form of flow path will have sharp corners that will increase stress during the forming process and disrupt the glass fiber reinforcement. Hot runner systems with open runners are acceptable.
The minimum thickness of the gate should be 2mm (0.080in). If possible, position the gate along an edge that does not obstruct the flow of material into the cavity. The gate of the component surface will need to be rotated 90° to prevent fiber breakage and mechanical properties.
Finally, pay attention to the location of the weld lines and know how they affect the area of ​​the load (or stress) that the part is subjected to. The fusion line should be moved to an area where the stress level is expected to be low through a reasonable layout of the gate.
Computer-filled analysis can help determine where these weld lines will be positioned. Structural finite element analysis (FEA) can be used to compare the location of high stresses with the location of the merged line determined in the filling analysis.
It should be noted that these parts and mold designs are only suggestions. There are many examples of components that have thin walls, wall thickness variations, and delicate or fine features that achieve good performance with LFRT composites. However, the further away from these recommendations, the more time and effort it takes to ensure the full benefits of long fiber technology.
Third, processing conditions
Processing conditions are the key to the success of LFRT. As long as the correct processing conditions are used, it is possible to use a universal injection molding machine and a properly designed mold to prepare a LFRT part. In other words, even with proper equipment and mold design, fiber length can be compromised if poor processing conditions are used. This requires an understanding of what the fiber will encounter during the molding process and identifying areas that cause excessive fiber shear.
First, monitor the back pressure. The high back pressure introduces a large shear force on the material which will reduce the fiber length. Considering starting from zero back pressure and only increasing it to allow the screw to retreat evenly during the feed, a back pressure of 1.5 to 2.5 bar (20 to 50 psi) is usually sufficient to achieve consistent feed.
High screw speeds also have an adverse effect. The faster the screw rotates, the more likely the solid and unmelted material will enter the screw compression section causing fiber damage. Similar to the recommendations for back pressure, the speed should be kept as low as possible to stabilize the filling screw. A screw speed of 30 to 70 r/min is common when molding LFRT composites.
During the injection molding process, melting occurs through two factors that work together: shear and heat. Because the goal is to protect the length of the fiber by reducing shear in the LFRT, more heat will be needed. Depending on the resin system, the temperature at which the LFRT composite is processed will typically be 10 to 30 ° C higher than conventional molding compounds.
However, before simply increasing the barrel temperature, pay attention to the inverse of the barrel temperature distribution. Normally, the barrel temperature rises as the material moves from the hopper to the nozzle; however, for LFRT, the temperature at the hopper is recommended to be higher. The reverse temperature profile softens and melts the LFRT pellets prior to entering the high shear screw compression section, thereby facilitating fiber length retention.
The last note on processing involves the use of recycled materials. Grinding a molded part or nozzle typically results in a lower fiber length, so the addition of recycled material can affect the overall fiber length. In order not to significantly reduce the mechanical properties, it is recommended that the maximum amount of recycled material is 5%. Higher regrind levels can have a negative impact on mechanical properties such as impact strength.
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