nuovo blog
Long fiber reinforced thermoplastics (LFRT) are increasingly being used in injection molding applications where high mechanical performance is required. While LFRT technology can provide excellent strength, stiffness, and impact resistance, the processing method plays a critical role in determining the final performance that a molded part can achieve.
To successfully mold LFRT, it is essential to understand some of the unique characteristics of these materials. Recognizing the differences between LFRT and conventional reinforced thermoplastics has driven the development of specialized equipment, part and mold designs, and processing techniques to maximize the value and performance potential of LFRT.
The key difference between LFRT and conventional short-cut or short glass fiber reinforced compounds is fiber length. In LFRT, the fibers are typically the same length as the pellets. This is because most LFRT materials are manufactured using a pultrusion process rather than shear-based compounding.
During LFRT manufacturing, continuous bundles of untwisted glass fiber rovings are first pulled through a die, where they are coated and impregnated with resin. After exiting the die, the continuous reinforced plastic strand is cut or pelletized, typically into lengths of 10–12 mm. In contrast, conventional short glass fiber compounds contain chopped fibers that are typically 3–4 mm long, and their length can be further reduced to less than 2 mm during processing in a shear-based extruder.
The longer fiber length in LFRT pellets helps improve mechanical performance, particularly impact resistance and toughness, while maintaining stiffness. As long as the fibers retain their length during molding, they form an “internal skeleton” within the material, providing exceptionally high mechanical performance.
However, poor molding practices can effectively turn a long fiber material into a short fiber material. If fiber length is significantly reduced during processing, the desired level of performance cannot be achieved.
To preserve fiber length during LFRT injection molding, three key areas need to be considered: the injection molding machine, part and mold design, and processing conditions.
One of the most frequently asked questions about LFRT processing is whether existing injection molding equipment can be used to mold these materials. In most cases, equipment designed for processing short-fiber reinforced compounds can also be used for LFRT. While standard short-fiber molding equipment is adequate for most LFRT parts and products, certain equipment modifications can help better preserve fiber length during processing.
A general-purpose screw with the typical feed–compression–metering configuration is well suited for this process. Reducing the compression ratio in the metering section can help minimize shear that may damage the fibers.
A metering-section compression ratio of approximately 2:1 is considered optimal for LFRT products. There is generally no need to use special metal alloys for the screw, barrel, or other components, as LFRT causes less wear than conventional short-cut glass fiber reinforced thermoplastics.
Another component that may benefit from a design review is the nozzle tip. Some thermoplastic materials are easier to process with a reverse-taper nozzle tip, which generates a high level of shear as the material is injected into the mold cavity. However, this type of nozzle tip can significantly reduce the fiber length of long fiber reinforced composites.
In addition, the nozzle and gate openings should be generously sized, with a diameter of at least 5.5 mm (0.250 in.), and should have no sharp edges. It is important to understand how the material flows through the injection molding equipment and identify areas where excessive shear may cause fiber breakage.
Proper part and mold design can also play an important role in preserving fiber length in LFRT. Eliminating sharp corners around part features, including ribs, bosses, and other details, can help prevent unnecessary stress in the molded part and reduce fiber damage.
Parts should use a nominal wall thickness design with relatively uniform wall thickness throughout. Large variations in wall thickness can lead to inconsistent filling and undesirable fiber orientation within the part.
Where thicker or thinner sections are necessary, sudden changes in wall thickness should be avoided. Such transitions can create high-shear areas that may damage the fibers and become sources of stress concentration. As a general rule, try to position the gate in the thicker section and allow the material to flow toward the thinner section, so that the end of the fill is located in the thinner area.
General plastic design guidelines recommend keeping wall thickness below 4 mm (0.160 in.) to promote smooth, uniform flow and reduce the likelihood of sink marks and voids. For LFRT compounds, the optimal wall thickness is typically around 3 mm (0.120 in.), with a minimum thickness of approximately 2 mm (0.080 in.).
The part itself is only one aspect of the design. It is equally important to consider how the material enters the mold. Significant fiber damage can occur in the runner and gate areas if these features are not properly designed to guide the material into the cavity.
When designing a mold for LFRT compounds, full-round runners are preferred, with a minimum diameter of 5.5 mm (0.250 in.). Any runner design other than a full-round configuration will have corners that can increase stress during molding and damage the glass fiber reinforcement. Hot runner systems with open gates are acceptable.
The minimum gate thickness should be 2 mm (0.080 in.). Where possible, position the gate along an edge that does not obstruct material flow into the cavity. For gates located on the surface of the part, a 90° turn may be required to prevent fiber breakage and the resulting reduction in mechanical performance.
Finally, attention should be paid to the location of weld lines and how they may affect areas of the part that will be subjected to loads or stresses during service. Proper gate placement should be used to move weld lines into areas where lower stress levels are expected.
Computer-aided mold filling analysis can help determine where these weld lines will be located. Structural finite element analysis (FEA) can then be used to compare areas of high stress with the weld-line locations identified through mold filling analysis.
It should be noted that these part and mold design guidelines are recommendations rather than absolute requirements. There are many examples of parts with thin walls, variations in wall thickness, and intricate or fine features that have achieved good performance using LFRT compounds.
However, the further a design deviates from these recommendations, the more time and effort may be required to ensure that the full benefits of long fiber technology are achieved.
Proper processing conditions are critical to the successful molding of LFRT. With the right processing parameters, high-quality LFRT parts can be produced using a general-purpose injection molding machine and a properly designed mold.
In other words, even with suitable equipment and mold design, poor processing conditions can still result in fiber damage and loss of fiber length. This makes it essential to understand what the fibers will encounter during the molding process and identify areas where excessive shear may occur.
First, back pressure should be carefully monitored. Excessive back pressure introduces high shear forces into the material, which can reduce fiber length. Consider starting with zero back pressure and increasing it only as much as necessary to ensure that the screw retracts consistently during plasticizing.
A back pressure of approximately 1.5–2.5 bar (20–50 psi) is generally sufficient to achieve consistent feeding.
High screw speeds can also have a negative effect. The faster the screw rotates, the greater the likelihood that solid or insufficiently melted material will enter the compression section of the screw, potentially causing fiber damage.
Similar to the recommendation for back pressure, the screw speed should be kept as low as possible while still providing stable and consistent screw recovery. A screw speed of 30–70 rpm is commonly used when molding LFRT compounds.
During injection molding, melting occurs through the combined effects of shear and heat. Since the goal when processing LFRT is to protect fiber length by minimizing shear, more heat is generally required. Depending on the resin system, the processing temperature for LFRT compounds is typically 10–30°C higher than that used for conventional reinforced compounds.
Before simply increasing the barrel temperatures throughout the machine, attention should be paid to the reverse barrel temperature profile. Under normal conditions, barrel temperatures increase as the material moves from the hopper toward the nozzle. For LFRT, however, a higher temperature at the hopper is recommended.
This reverse temperature profile helps soften and melt the LFRT pellets before they enter the high-shear compression section of the screw, which helps preserve fiber length.
The final processing consideration is the use of regrind material. Grinding molded parts or sprues typically results in shorter fiber lengths, so adding regrind can affect the overall fiber length of the material.
From material selection and fiber loading to injection molding recommendations, LFT-G can help you find the right long fiber reinforced thermoplastic solution for your application.
Talk to Our Technical TeamSeguici :
-- ricevi aggiornamenti con gli ultimi argomenti
e-mail
italiano
English
français
Deutsch
русский
español
português
العربية
日本語
한국의
中文