Long fiber reinforced thermoplastics (LFRT) are being used for injection molding applications with high mechanical properties. Although LFRT technology can provide good strength, stiffness, and impact properties, the processing of this material plays an important role in determining how the final part can perform.
In order to successfully shape LFRT, it is necessary to understand some of their unique features. 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 traditional chopped, short glass fiber reinforced composites lies in the length of the fibers. In LFRT, the length of the fiber is the same as the length of the pellet. This is because most LFRT's are produced by pultrusion rather than shear-type compounding.
In LFRT manufacturing, continuous strands of glass fiber rovings are first drawn into a die for coating and impregnation of the resin. After exiting the die, the continuous strips are chopped or pelletized, usually Cut to a length of 10-12mm. In contrast, traditional short glass fiber composites only contain chopped fibers that are 3 to 4 mm long, and their length is further reduced to typically less than 2 mm in shear-type 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 are kept in length during the molding process, they will form an "internal skeleton" providing superb mechanical properties. However, a bad molding process can turn long fiber products into short fiber materials. If the length of the fiber is compromised during the molding process, it is not possible to obtain the required level of performance.
In order to maintain the fiber length during the LFRT molding process, there are three important aspects to consider: injection molding machine, part and mold design, and processing conditions.
First, equipment precautions
One question that is often asked about LFRT processing is whether we can use existing injection molding equipment to shape these materials. In the vast majority of cases, equipment for forming staple fiber composites can also be used to form LFRTs. Although typical short fiber molding equipment is satisfactory for most LFRT parts and products, some modifications to the equipment can better help maintain fiber length.
A universal screw with a typical "feed-compression-metering" section is very suitable for this process, and fiber-destructive shearing can be reduced by reducing the compression ratio of the metering section. A 2:1 meter segment compression ratio is optimal for LFRT products. The use of special metal alloys for the manufacture of screws, barrels, and other parts is not necessary because LFRT wear is not as large as traditional chopped glass fiber-reinforced thermoplastics.
Another device that may benefit from the design review is the tip of the nozzle. Some thermoplastic materials are easier to machine with a reverse tapered nozzle tip, which creates a high degree of shear as the material is injected into the mold cavity. However, such nozzle tips significantly reduce the fiber length of long fiber composites. It is therefore recommended to use a slotted nozzle tip/valve assembly of 100% "free flow" design that allows long fibers to easily pass through the nozzle into the component.
In addition, the diameter of the nozzle and the gate hole should have a loose size of 5.5 mm (0.250 in) or more, and there is no sharp edge. It is important to understand how the material flows through the injection molding equipment and to determine where the shear will break the fibers.
Second, parts and mold design
Good parts and mold design are also helpful in maintaining the fiber length of LFRT. Eliminating the sharp corners around a portion of the edge (including ribs, bosses and other features) avoids unnecessary stress in the molded part and reduces fiber wear.
Parts shall be of nominal wall design with uniform wall thickness. Larger changes in wall thickness can result in inconsistent packing and unwanted fiber orientation in the part. Where thickness must be thicker or thinner, sudden changes in wall thickness must be avoided to avoid the formation of high-shear areas that can damage the fibers and become the source of stress concentration. It is usually tried to open the gate in the thicker wall and flow to the thin part, keeping the filling end in the thin part.
The general principle of good plastic design suggests that keeping a wall thickness of less than 4 mm (0.160 in) will promote good and uniform flow and reduce the possibility of sinks and voids. For LFRT compounds, the optimal wall thickness is usually 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 the fibers breaking after entering the mold increases.
Parts are only one aspect of the design, and it is also important to consider how the material enters the mold. When the runners and gates guide 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 an LFRT compound, the full-radius runner is optimal with a minimum diameter of 5.5 mm (0.250 in). In addition to the full-round channel, any other form of flow channel will have sharp corners, which will increase the stress during the forming process and destroy the reinforcing effect of the glass fiber. 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 on the surface of the part will need to be rotated 90° to prevent initiation of fiber breakage and degrade mechanical properties.
Finally, pay attention to the location of the fusion lines and how they affect the area where the parts are subjected to load (or stress) when used. The fusion line should be moved to an area where the stress level is expected to be lower by a rational layout of the gate.
Computer filling analysis can help determine where these fusion lines will be located. Structural finite element analysis (FEA) can be used to compare the location of high stress and the location of the confluence line as determined in the filling analysis.
It should be noted that these parts and mold designs are only recommendations. There are many examples of components that have thin walls, wall thickness variations, and fine or fine features that use LFRT compounds to achieve good performance. However, the further from these recommendations, the more time and effort it takes to ensure that the full benefits of long fiber technology are realized.
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 general-purpose injection molding machine and a properly designed mold to prepare LFRT parts. In other words, even with proper equipment and mold design, fiber length may suffer if poor processing conditions are used. This requires understanding what the fiber will encounter during the molding process and identifying areas that will cause excessive fiber shear.
First, monitor the back pressure. High back pressure introduces a large shear force on the material that will reduce the fiber length. Considering starting from zero back pressure and only increasing it until the screw is evenly retracted during the feeding process, using a back pressure of 1.5 to 2.5 bar (20 to 50 psi) is usually sufficient to obtain consistent feeding.
High screw speed also has an adverse effect. The faster the screw rotates, the more likely the solid and unmelted material will enter the screw compression section and cause fiber damage. Similar to the recommendations for back pressure, it should be kept as fast as possible to stabilise the minimum required to fill the screw. When molding LFRT compounds, screw speeds of 30 to 70 r/min are common.
In the injection molding process, melting occurs through two factors that act together: shear and heat. Because the goal is to protect the length of the fiber in the LFRT by reducing shear, more heat will be needed. According to the resin system, the temperature of the processed LFRT compound is usually 10-30° C. higher than that of the conventional molded compound.
However, before simply raising the barrel temperature all the time, pay attention to the reversal of the barrel temperature distribution. Normally, the barrel temperature rises as the material moves from the hopper to the nozzle, but for the LFRT it is recommended that the temperature be higher at the hopper. The reversal of the temperature distribution allows the LFRT pellets to soften and melt before entering the high shear screw compression section, thereby facilitating the retention of the fiber length.