How to improve Long fiber reinforced thermoplastic (LFRT) injection parts quality
How to improve Long fiber reinforced thermoplastic (LFRT) injection parts quality
Long fiber reinforced thermoplastic (LFRT) is being used in injection molding applications with high mechanical properties. Although LFRT technology can provide good strength, stiffness and impact performance, the processing method of this material plays an important role in determining the performance of the final components.
In order to form LFRT successfully, it is necessary to understand their unique characteristics. Understanding the differences between LFRT and conventional reinforced thermoplastic promotes the development of equipment, design and processing technology in order to maximize the value and potential of LFRT.
The difference between LFRT and traditional short cut and short glass fiber reinforced composites is the fiber length. In LFRT, the length of fiber is the same as that of pellet. This is because most lfrts are produced by pultrusion rather than shear blending.
In LFRT manufacturing, the continuous tow of glass fiber roving is first drawn into a die for coating and resin impregnation. After coming out of the die, the continuous reinforced plastic strip is chopped or granulated, usually to a length of 10-12mm. In contrast, the traditional short glass fiber composite only contains 3-4mm long short cut fiber, and its length will be further reduced to less than 2mm in the shear extruder.
The fiber length in the LFRT pellet helps improve the mechanical properties of LFRT - increased impact resistance or toughness while maintaining stiffness. As long as the fibers remain in length during the molding process, they form an "internal skeleton" that provides ultra-high mechanical properties. However, a poor molding process can turn 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 the fiber length during LFRT molding, there are three important aspects to consider: injection molding machine, part and mold design and processing conditions.
One of the frequently asked questions about LFRT processing is whether it is possible for us to use existing injection molding equipment to mold these materials. In most cases, equipment used to form short fiber composites can also be used to form LFRT. Although the typical short fiber molding equipment meets the requirements for most LFRT components and products, some modifications to the equipment can better help maintain the fiber length.
A general screw with a typical "feed compression metering" section is very suitable for this process, and the destructive shear of the fiber can be reduced by reducing the compression ratio of the metering section. A compression ratio of about 2:1 is the best for LFRT products. It is not necessary to make screw, barrel and other parts with special metal alloy, because the wear of LFRT is less than that of traditional short cut glass fiber reinforced thermoplastic.
Another device that may benefit from a design review is the nozzle tip. Some thermoplastic materials are easier to machine with a reverse tapered nozzle tip, which can form a high shear when the material is injected into the mold cavity. However, the tip of the nozzle can significantly reduce the fiber length of the long fiber composite. Therefore, it is recommended to use a 100% "free flow" slotted nozzle tip / valve assembly, which makes it easy for long fibers to enter the components through the nozzle.
In addition, nozzle and gate holes shall have a loose size of 5.5mm (0.250in) or more in diameter and have no sharp edges. It is important to understand how the material flows through the injection molding equipment and to determine where the shear will break the fiber.
Fig.: three piece screw tip and annular valve with "100% free flow" design can minimize the fracture of long fiber
Good part and mold design is also helpful to keep the fiber length of LFRT. Eliminating sharp corners around some edges, including ribs, bosses, and other features, can avoid unnecessary stresses in the molded part and reduce fiber wear.The components shall be of nominal wall design with uniform wall thickness. Large changes in wall thickness can result in inconsistent filling and unwanted fiber orientation in the component. Where it is necessary to be thicker or thinner, sudden changes in wall thickness should be avoided to avoid the formation of high shear areas that may damage fibers and become the source of stress concentration. Usually try to keep the gate in the thick wall and flow to the thin part to keep the filling end in the thin part.
General good plastic design principles suggest that maintaining a wall thickness of less than 4mm (0.160in) will promote good uniform flow and reduce the possibility of dents and voids. For LFRT composites, the optimal wall thickness is usually about 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 fracture increases when the material enters the mold.
Parts are only one aspect of design, and it is also important to consider how materials enter the mold. When the runner and gate guide the material into the cavity, if not designed correctly, a lot of fiber damage will occur in these areas.
When designing a mold to form LFRT composite, the full round runner is the best, with a minimum diameter of 5.5mm (0.250in). In addition to the full round channel, any other channel will have sharp corners, which will increase the stress during the forming process and destroy the reinforcement effect of glass fiber. The hot runner system with open runner is acceptable.
The minimum thickness of the gate should be 2mm (0.080in). If possible, locate the gate along an edge that does not prevent material from entering the cavity. The gate on the surface of the component will need to be rotated 90 ° to prevent fiber fracture and reduce mechanical properties.
Finally, pay attention to the location of the fusion lines and know how they affect the area where the components bear the load (or stress) when they are used. The fusion line shall be moved to an area where the stress level is expected to be low by a reasonable layout of the gates.
Computer filling analysis can help determine where these fusion lines will be located. Finite element analysis (FEA) can be used to compare the location of high stress with the location of confluence line determined in mold filling analysis.
It should be noted that these parts and die designs are only recommendations. There are many examples of components with thin wall, wall thickness variation and delicate or fine features, which have achieved good performance with LFRT composite. However, the further you deviate from these recommendations, the more time and effort will be required to ensure that the full benefits of long fiber technology are realized.
3、 Processing conditions
Processing conditions are the key to the success of LFRT. As long as the correct processing conditions are adopted, it is possible to use a general injection molding machine and a properly designed mold to prepare LFRT components. In other words, even with proper equipment and mold design, fiber length can be damaged if poor processing conditions are used. It is necessary to understand the situation that the fiber will encounter during the forming process and determine the area that will cause excessive shearing of the fiber.
First, monitor the back pressure. The introduction of high back pressure will reduce the fiber length. Considering starting from zero back pressure and only increasing it to make the screw return evenly in the feeding process, a back pressure of 1.5-2.5bar (20-50psi) is usually enough to obtain consistent feeding.
High screw speed also has an adverse effect. The faster the screw rotates, the more likely the solid and unmelted materials will enter into the compression section of the screw and cause fiber damage. Similar to the recommendation for back pressure, try to keep the speed at the minimum required for stable filling screw. The screw speed of 30 ~ 70r / min is common in LFRT composites.
In the process of injection molding, melting occurs through two factors: shear and heat. Because the aim is to protect the fiber length by reducing the shear in the LFRT, more heat will be required. According to the resin system, the processing temperature of LFRT composite is usually 10 ~ 30 ℃ higher than that of conventional molding composite.
However, before simply and comprehensively increasing the barrel temperature, it is necessary to pay attention to the inversion of the barrel temperature distribution. In general, the barrel temperature rises when the material moves from the hopper to the nozzle; however, for LFRT, a higher temperature is recommended at the hopper. The reverse temperature distribution will soften and melt the LFRT particles before they enter the compression section of the high shear screw, which is conducive to the maintenance of fiber length.
The last note on processing relates to the use of recycled materials. Grinding formed parts or nozzles usually results in lower fiber lengths, so the addition of recycled materials can affect the overall fiber length. In order to not obviously reduce the mechanical properties, it is suggested that the maximum amount of recycled material is 5%. Higher recycled material content will have a negative impact on mechanical properties such as impact strength.
