What should be paid attention to in injection molding long fiber reinforced plastic LFRT?

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 method of this material plays an important role in determining what properties the final component can achieve.

In order to successfully shape LFRTs, 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 realize the maximum value and potential of LFRT.

The difference between LFRT and traditional chopped, short glass fiber reinforced composites is the fiber length. In LFRT, the length of the fibers is the same as the length of the pellets. This is because most LFRTs are produced through a pultrusion process rather than a shear compound.

In LFRT manufacturing, a continuous tow of glass fiber roving is first drawn into a die for coating and impregnating resin. After coming out of the die, this continuous reinforced plastic strip is chopped or pelletized, usually Cut to a length of 10 ~ 12mm. In contrast, traditional short glass fiber composites only contain chopped fibers with a length of 3 to 4 mm. In a shear extruder, the length will be further reduced to usually less than 2 mm.

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 maintain their length during the forming 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 fibers is damaged during the forming process, it is not possible to obtain the required level of performance.

To maintain fiber length during LFRT molding, there are three important aspects to consider: injection molding machine, component and mold design, and processing conditions.

First, equipment attention

A frequently asked question about LFRT processing is: Is it possible for us to use existing injection molding equipment to mold these materials. In most cases, the equipment used to form short fiber composites can also be used to form LFRTs. Although typical short fiber molding equipment is satisfactory for most LFRT components 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 by reducing the compression ratio of the metering section, the destructive shear of the fiber can be reduced. A metering segment compression ratio of approximately 2: 1 is optimal for LFRT products. It is not necessary to use special metal alloys to make screws, barrels and other components, as LFRTs are less abraded than traditional chopped glass fiber reinforced thermoplastics.

Another piece of equipment that could benefit from a 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 a slotted 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 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 fibers.

Part and mold design

Good part and mold design can also help to maintain the fiber length of the LFRT. Eliminating sharp corners around some edges, including ribs, bosses, and other features, avoids unnecessary stress in the molded part and reduces fiber wear.

Components shall have a nominal wall design with uniform wall thickness. Large changes in wall thickness can lead to inconsistent filling and unwanted fiber orientation in the part. Where it must be thicker or thinner, abrupt changes in wall thickness should be avoided to avoid the formation of high-shear regions that could damage the fiber and become sources of stress concentration. Usually try to open 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 keeping wall thicknesses below 4mm (0.160in) will promote good and uniform flow and reduce the possibility of dents and voids. For LFRT composites, the optimal wall thickness is usually around 3mm (0.120in), and the smallest thickness is 2mm (0.080in). When the wall thickness is less than 2mm, the probability of fiber breakage of the material after entering the mold increases.

The part is just one aspect of the design, and it is also important to consider how the material enters the mold. When 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 composite, a full-rounded runner is optimal, with a minimum diameter of 5.5mm (0.250in). Except for the full-round flow channel, any other type of flow channel will have sharp corners, which will increase the stress during the molding process and destroy the reinforcing effect of 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 prevent material from flowing into the cavity. The gate on the component surface will need to be rotated by 90 ° to prevent fiber breakage and reduce mechanical properties.

Finally, pay attention to the location of the fusion lines and know how they affect the area where the component is subjected to load (or stress) when in use. The fusion line should be moved to the area where the stress level is expected to be low through the reasonable layout of the gate.

Computer fill 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 with the location of the convergence line determined in the mold filling analysis.

It should be noted that these part and mold designs are only suggestions. There are many examples of components that have thin walls, varying wall thicknesses, and delicate or fine features, and use LFRT composites to achieve good performance. However, the further away from these recommendations, the more time and effort it takes to ensure that the full benefits of long fiber technology are realized.

Processing conditions

Processing conditions are key to the success of LFRT. As long as the correct processing conditions are used, it is possible to prepare LFRT parts using general-purpose injection molding machines and correctly designed molds. In other words, even with proper equipment and mold design, fiber length can be damaged if poor processing conditions are used. This requires understanding what the fibers will encounter during the forming process and identifying areas that will cause excessive shearing of the fibers.

First, monitor back pressure. High back pressure introduces a huge shear force on the material, which will reduce the fiber length. Considering starting from zero back pressure and only increasing it to make the screw return evenly during the feeding process, using a back pressure of 1.5 to 2.5 bar (20 to 50 psi) is usually sufficient to obtain a consistent feed.

High screw speeds also have an adverse effect. The faster the screw rotates, the more likely solid and unmelted material will enter the compression section of the screw and cause fiber damage. Similar to the recommendations for back pressure, try to keep the rotation speed at the lowest level required for a stable filling screw. When molding LFRT composites, screw speeds of 30 to 70 r / min are common.

In the injection molding process, melting occurs through two factors that work together: shear and heat. Since the purpose is to protect the length of the fiber in LFRT by reducing shear, more heat will be required. Depending on the resin system, the temperature for processing LFRT composites is usually 10-30 ° C higher than conventional molding composites.

However, before simply raising the barrel temperature, pay attention to the inversion of the barrel temperature distribution. Generally, when the material moves from the hopper to the nozzle, the barrel temperature rises; but for LFRT, the temperature at the hopper is recommended to be higher. The reverse temperature distribution will soften and melt the LFRT pellets before entering the high shear screw compression section, which is beneficial to maintaining the fiber length.

A final note on processing involves the use of recycled materials. Grinding molded parts or nozzles usually results in lower fiber lengths, so the addition of recycled materials affects the overall fiber length. In order not to significantly reduce the mechanical properties, the maximum recommended amount of recycled materials is 5%. Higher regrind usage will negatively impact mechanical properties such as impact strength.