Long fiber reinforced thermoplastics 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 is important for determining how the final part can perform The effect of.
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 conventional chopped, short glass fiber reinforced composites lies in the length of the fibers. In LFRT, the length of the fibers is the same as the length of the pellets. This is because most LFRTs are pultrusion processes rather than shears. Blending to produce.
In LFRT manufacturing, continuous strands of fiberglass rovings are first drawn into a die for coating and impregnation of the resin. After exiting the die, the continuous strip of reinforced plastic is chopped or pelletized, usually Cut to a length of 10~12mm. In contrast, the traditional short glass fiber composite only contains chopped fibers of 3~4mm in length, and its length will be further reduced to 2mm in a shearing extruder.
The fiber length in the LFRT pellets helps to improve the mechanical properties of LFRT - increased impact resistance or toughness while maintaining stiffness. As long as the fibres remain in length during the forming process, they will form an 'internal skeleton' providing super high 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 will not be possible to obtain the required level of performance.
In order to maintain the fiber length during LFRT molding, there are three important aspects to consider: Injection Molding Machine, Parts and Mold Design, and Processing Conditions.
First, equipment precautions
One of the frequently asked questions about LFRT processing is: Is it possible for us 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 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 shear can be reduced by reducing the compression ratio of the metering section. A compression ratio of about 2:1 is used for the metering section. LFRT products are the best. Manufacture of screws with special metal alloys, 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 an inverted tapered nozzle tip, which creates a high degree of shear when the material is injected into the mold cavity However, this nozzle tip significantly reduces the fiber length of the long fiber composite. Therefore, it is recommended to use a 100% 'free flow' design slot nozzle tip/valve assembly that allows long fibers to easily enter the component through the nozzle. .
In addition, the diameter of the nozzle and gate hole should have a loose size of 5.5mm (0.250in) 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 that the shear will break the fiber. local.
Picture: Three-piece screw tip and ring valve with '100% free-flow' design to minimize long fiber breakage
Second, parts and mold design
Good part and mold design are also helpful to maintain the fiber length of the LFRT. Eliminating the corners around the edges (including ribs, bosses and other features) avoids unnecessary stress in the molded part and reduces fiber wear .
Components should have a 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 thick or thin must be avoided, abrupt wall thicknesses should be avoided Change to avoid forming high-shear areas that could damage the fiber and become the source of stress concentration. Usually try to open the gate in a thicker wall and flow to the thin part to keep the filling end in a thin part.
General good design principles for plastics suggest that keeping wall thickness below 4 mm (0.160 in) will promote good uniform flow and reduce the possibility of sinks and voids. For LFRT compounds, the optimal wall thickness is usually 3 mm (0.120 in ) Left and right, the minimum thickness is 2mm (0.080in). When the wall thickness is less than 2mm, the probability of fiber breakage after the material enters the mold increases.
The part is only one aspect of the design. It is also important to consider how the material enters the mold. When the runners and gates guide the material into the cavity, if not properly designed, a lot of fiber damage will occur in these areas.
When designing a mold for forming an LFRT compound, the full-radius runner is optimal, and its minimum diameter is 5.5mm (0.250in). In addition to the full-cornered runner, any other form of runner will have a tip. Corners, they will increase the stress during the molding process and destroy the reinforcing effect of the glass fiber. The hot runner system with open runners is 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 material flow into the cavity. The gate on the surface of the part will require 90° rotation to prevent initiation of fiber breakage. And reduce mechanical performance.
Finally, pay attention to the location of the fusion lines and know how they affect the area where the part (or stress) is applied when the part is used. The fusion line should be moved to the area where the stress level is expected to be lower through reasonable placement 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 convergence lines determined during 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 deviation from these suggestions is more Far, it takes more time and effort 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 general-purpose injection molding machine and a correctly designed mold to produce good LFRT parts. In other words, even with the proper equipment and mold design, if Poor processing conditions, fiber length may also be damaged. This requires understanding of the fiber will be encountered in the molding process, and to determine areas that will cause excessive fiber cut.
First, back pressure is monitored. High back pressure introduces a large shear force on the material, which will reduce the fiber length. Consider starting from zero back pressure and only increase it until the screw is evenly retracted during feeding. Use 1.5 A back pressure of ~2.5 bar (20 to 50 psi) is usually sufficient to achieve consistent feeding.
High screw speed also has a detrimental effect. The faster the screw rotates, the more likely the solid and unmelted material enters the screw compression section and cause fiber damage. Similar to the suggestion for back pressure, try to keep the speed at a minimum required to stably fill the screw. Level. When molding LFRT compounds, screw speeds of 30~70r/min are common.
In the injection molding process, melting occurs through two synergistic factors: Shear and heat. Because the goal is to protect the fiber length by reducing the shear in LFRT, more heat will be needed. According to the resin system, processing The temperature of LFRT compounds is usually 10~30°C higher than that of conventional molded compounds.
However, before simply raising the barrel temperature all the time, pay attention to the reversal of the barrel temperature distribution. Normally, when the material moves from the hopper to the nozzle, the barrel temperature rises; but for the LFRT, the temperature at the hopper is recommended. Higher. The reversed temperature profile will allow the LFRT pellets to soften and melt before entering the high shear screw compression section, which will help maintain fiber length.
The last note about processing concerns the use of recycled materials. Grinding molded parts or nozzles usually results in lower fiber lengths. Therefore, the addition of regrinds can affect the overall fiber length. In order to not significantly reduce the mechanical properties, it is recommended that The maximum amount of regrind is 5%. Higher regrind content will have a negative impact on impact strength and other mechanical properties.