Long fiber-reinforced thermoplastics (LFRTs) are being used for high-performance injection molding applications. Although LFRT technology provides good strength, stiffness and impact properties, the processing of this material plays an important role in determining what performance the final part can achieve.
In order to successfully shape LFRT, it is necessary to understand some of their unique characteristics. Understanding the differences between LFRT and conventional 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 traditional chopped, short glass fiber reinforced composites lies in the length of the fiber. In LFRT, the length of the fiber is the same as the length of the pellet. This is because most LFRTs are produced by pultrusion rather than sheer compounding. In LFRT manufacture, a continuous tow of glass fiber roving is first drawn into a die to be coated and impregnated with resin. After coming out of the die, the continuous strip of reinforcing plastic is chopped or pelleted, usually Cut to length of 10 ~ 12mm. In contrast, conventional short glass fiber composites contain only chopped strands of 3 to 4 mm in length, which are further reduced in length to less than 2 mm in shear-type extruders.
LFRT is usually prepared by a pultrusion process, impregnating continuous glass fiber bundles with resin, and then cutting them into long pellets. Glass fiber length equal to the length of the pellet.
The fiber length in LFRT pellets helps to improve the mechanical properties of LFRT - increased impact resistance or toughness while maintaining stiffness. As long as the fibers retain their length during the forming process, they form an "internal skeleton" that provides superb mechanical properties. However, a poor molding process can turn long fiber products into short fiber materials. If the length of the fiber is compromised during the forming process, it is not possible to obtain the level of performance required.
Figure before and after thermal decomposition of injection molded parts. The light color is the internal skeleton formed by the long fibers after the resin burns off, and the skeleton retains the shape of the part. In order to maintain the fiber length during LFRT molding, there are three important aspects to consider: injection molding machine, part and mold design and processing conditions.
01 Equipment precautions
A frequently asked question about LFRT processing is whether it is possible for us to shape these materials using existing injection molding equipment. In the vast majority of cases, equipment for forming staple fiber composites can also be used to shape LFRTs. While typical staple fiber molding equipment is satisfactory for most LFRT parts and products, some modifications to the equipment may be better to help maintain the fiber length.
A universal screw with a typical "feed-compression-metering" section is ideally suited for this process, and fiber destructive shear can be reduced by reducing the compression ratio of the metering section. The metering section compression ratio of about 2: 1 is the best for LFRT products. Manufacture of screws, barrels and other components from special metal alloys is not necessary because LFRT wear out is not as large as conventional chopped glass fiber reinforced thermoplastics.
Another piece of equipment that may benefit from the design review is the nozzle tip. Some thermoplastics are easier to tip with an inverted conical nozzle that creates a high degree of shear as the material is injected into the mold cavity. However, this nozzle tip can significantly reduce the fiber length of the long fiber composite. It is therefore recommended to use a 100% "free-flow" slotted nozzle tip / valve assembly that allows easy access of long fibers through the nozzle. In addition, the diameter of the nozzle and gate holes should be 5.5mm (0.250in) or more in loose size and have no sharp edges. It is important to understand how the material flows through the injection molding equipment and where it is determined that shearing will break the fiber.
02 Parts and mold design
Good part and mold design also helps to keep the fiber length of the LFRT. Eliminating sharp corners around part of the edge, including ribs, bosses, and other features, avoids unnecessary stress in the molded part and reduces fiber wear. Parts should be uniform wall thickness nominal wall design. Larger changes in wall thickness result in inconsistent filling and unwanted fiber orientation in the part. Where thicker or thinner is necessary, abrupt changes in wall thickness should be avoided to avoid the formation of high-shear areas that may damage the fibers and become a source of stress concentration. Usually try to open the gate in the thicker wall, and flow to the thin part, the filling end is kept in the thin part. Common good plastic design guidelines suggest that keeping the wall thickness below 4 mm (0.160 in) will promote good, uniform flow and reduce the possibility of depressions and voids. For LFRT complexes, 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 breaking of the fiber of the material after entering the mold is increased.
Parts are just one aspect of the design and it is also important to consider how the material enters the mold. When runners and gates lead the material into the cavity, a significant amount of fiber failure can occur in these areas without proper design.
When designing a mold that is used to mold LFRT compounds, a full fillet runner is the best, with a minimum diameter of 5.5mm (0.250in). In addition to full-round runner, any other form of runner will have sharp corners, they will increase the stress during the molding process to undermine 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, locate the gate along an edge that does not obstruct the flow of material into the cavity. Sprue on the part surface will require 90 ° rotation to prevent rupture of the fiber and reduce mechanical properties. Finally, pay attention to the location of the weld lines and how they affect the area under load (or stress) when the part is used. The fusion line should be moved to the area where the stress level is expected to be lower by a reasonable layout of the gate.
Computer Filling Analysis can help determine where these fusible links will be located. Structural finite element analysis (FEA) can be used to compare the location of high stress with the location of the confluence line as determined during the filling analysis. It should be noted that these components and mold designs are just suggestions. There are many examples of components that have thin walls, varying wall thicknesses and delicate or fine features that achieve good performance with LFRT composites. However, the farther away from these proposals, the more time and effort is devoted to ensuring the full benefits of long fiber technology.
03 Processing design
Processing conditions are the key to LFRT success. As long as the correct processing conditions are used, it is possible to use conventional injection molding machines and properly prepared molds for the prepared LFRT components. In other words, even with the proper equipment and mold design, the fiber length can suffer if poor processing conditions are used. This requires understanding the conditions that the fiber will encounter during the forming process and identifying the area that will cause excessive shearing of the fiber.
First, monitor back pressure. High back pressure introduces significant shear forces on the material, which reduces the fiber length. Considering that starting from zero backpressure and increasing it only to allow the screw to retract evenly during feeding, a back pressure of 1.5-2.5 bar (20-50 psi) is usually sufficient to achieve a consistent feed.
High screw speed also has adverse effects. The faster the screw rotates, the more likely it is for the solid and unmelted material to enter the compression section of the screw causing fiber damage. Similar to the recommendations for backpressure, try to keep the rotational speed at the lowest level required to stably fill the screw. In forming LFRT complexes, screw speeds of 30-70 r / min are common.
During injection molding, melting occurs through two interacting factors: shear and heat. Because the goal is to protect the length of the fiber in LFRT by reducing shear, more heat will be needed. Depending on the resin system, the temperature at which the LFRT composite is processed is typically 10 to 30 ° C higher than conventional molding compounds.
However, before simply increasing the cylinder temperature completely, pay attention to the reversal of the barrel temperature distribution. Typically, the barrel temperature rises as the material moves from the hopper to the nozzle, but for LFRT it is recommended to have a higher temperature at the hopper. The inverted temperature distribution softens and melts the LFRT pellets prior to entering the high-shear screw compression section, thereby facilitating fiber length retention.
The final note about processing involves the use of back-up material. Grinding a molded part or nozzle usually results in a lower fiber length, so the addition of back-up material can affect the overall fiber length. In order not to significantly reduce the mechanical properties, it is recommended to return the maximum amount of material is 5%. A higher amount of recycled material will have a negative impact on mechanical properties such as impact strength.