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Practical Tips for Molding Long Fiber Reinforced Polymers

  Whether glass rovings or short glass fibers, prime fiberglass or precio fibra de carbono are added to the thermoplastic matrix, the purpose is basically to improve the mechanical and structural properties of the polymer. There are many differences between the two main methods of reinforcing thermoplastics for injection molding, from how they are combined with the polymer matrix, to the level of performance they can provide, and one fiber form may be more The other is more suitable, but for the shaper, the main difference between short and long fibers is the degree to which they are processed.



Processing long fiber reinforced thermoplastics

The primary goal of processing long-fiber-reinforced thermoplastics is to maintain fiber length, which is critical for optimizing strength and toughness. Fiber breakage can have a negative impact on the properties of the polymer composite and may ultimately negate the benefits of using glass fibre threads. Improper handling and faulty tooling and component design, or the use of unoptimized processing equipment or setups, can lead to fiber breakage.

Unlike chopped fiber reinforced plastics, long fiber reinforced materials are usually made by pultrusion. The process involves stretching  glass roving  impregnated with thermoplastic resin through a special impregnation die (so that the resin can wrap around and bond the fibers), and then cut the extruded strands into pellets, the fibers in the pellets are typically 12mm The long, full-length features unidirectional fiber reinforcement, and this length is critical for enabling the polymer to efficiently transfer stress to stronger fibers.

When these pellets are used for injection molding, the long fibers are aligned and tightly wound to form an internal skeleton that provides strength and toughness. Compared to short-fiber filled materials, composites reinforced with long fibers, whether fibreglass fibres or carbon fibers, provide higher strength-to-weight ratios, impact toughness, longer cyclic fatigue life, and wider heat resistance and better dimensional stability.

These durable materials offer structural performance comparable to metal, yet are lighter than metal, and are able to take advantage of the processing efficiency benefits of injection molding. 1k carbon fiber cloth is particularly valuable as a metal replacement because they are 70% lighter than steel and lighter than steel. Aluminum is 40% lighter, so long-fiber-reinforced composites can be used to manufacture demanding components in automotive, sporting goods, aerospace, consumer goods and industrial equipment. Typical base resins include polyamide (PA or nylon), polypropylene (PP), rigid thermoplastic polyurethane (ETPU), and high temperature resins such as polyetheretherketone (PEEK), polyphthalamide (PPA), and polyamide. Ether imide (PEI) etc. While any thermoplastic can be reinforced with fibers, only some offer higher performance because they are better reinforced. More precisely, semi-crystalline resins are better reinforced by fibers than amorphous resins, which means that their stiffness and strength are increased even more.


Processing Points of Long Fiber Reinforced Materials
  Compared with unmodified or granular powder-filled resins, molding long fiber reinforced composites has certain requirements on molds, gates, molding equipment, and part design. The processes used to process these materials also differ from those of short fiber reinforced polymers.
As mentioned earlier, maintaining fiber length is the key to success. Factors that can cause fiber length shortening include high pressure and shear from the injection screw, as can sharp corners in the mold and runner system. To maintain fiber length, there are 3 key processing points to be aware of:
  1. Mold material and design
Although long fibers wear less on the mold than short fibers because there are fewer needle-like fiber ends that affect the mold, the same type of mold steel is suitable for both long-fiber and short-fiber reinforced polymers, the most common The first is P20 mold steel, which can withstand more than 100,000 injections continuously. If higher durability is required (above 100,000 injection cycles), H13 chrome molybdenum steel or A9 air hardened steel are better choices. In general, hardened molds are the best choice for processing fiber-reinforced thermoplastics. For worn molds, they can be refurbished using electroplating technology. Aluminum molds can even be used if prototypes must be produced in order to validate the design.
  2. Forming equipment
Long-fiber reinforced thermoplastics can be processed using standard injection molding equipment with only a few non-permanent modifications to preserve fiber length and accommodate higher viscosities. A low pressure or general purpose screw with a non-return ring that allows free flow at the top is recommended. General-purpose nozzles can be used, but nylon nozzles should be avoided because their hourglass shape (designed to prevent drooling) restricts flow, creates shear, and causes fiber abrasion. Another tip to reduce shear is to avoid inverted cone nozzle designs. In general, larger nozzle holes (minimum 5.6 mm) facilitate the passage of viscous fiber-reinforced resins.
A good rule of thumb for any injection machine is to only inject 60-70% of the volume. Too much shot size increases reset time, while too little shot size means the material stays in the barrel for longer, potentially leading to degradation.
  3. Processing conditions
As far as processing is concerned, it is important to address two issues: warpage and creep. In general, long fiber reinforced thermoplastic parts experience less warpage than short strand fiberglass parts because the winding of the filament reduces differential shrinkage, but injection molded long fiber parts still deform, one reason being that the fibers flow along the Orientation alignment, while enhancing part strength, can lead to anisotropy. To prevent warping, alternative gate locations or part designs can be used to avoid excessive fiber alignment in areas that do not require high strength to withstand structural loads.
Component design
  Parts should be designed to protect and maintain fiber length and optimize strength and toughness by aligning fibers in the direction of material flow. Strive for a uniform wall thickness and avoid areas of excessive thickness (greater than 12.7 mm) so that the fibers are aligned in the direction of flow. Random fiber orientation or spheroidization can lead to a reduction in structural strength.
The recommended minimum wall thickness is 1.524 mm, and the optimal wall thickness to promote fiber alignment is 3.175 mm. When the wall thickness is greater than 5.08 mm, the fiber alignment begins to decrease, and the maximum wall thickness is 12.7 mm. Fiber reinforced materials are different from metals, so thickening of the part does not always translate directly into an increase in strength, the fibers do not align through the thickness of the part as the part becomes thicker.
  Avoid designing very long and flat flow lengths without stiffeners as this is more prone to warping. Also, the high viscosity of long fiber reinforced composites can lead to solidification of the material when very long parts have to be filled.
Mechanical properties may be lost when flow fronts just meet without crossing or fibers mixing to form a structural bridge. To avoid this phenomenon, it is important to set up the weld line. If the weld line lacks the added strength and toughness of the fiber and instead relies entirely on the properties of the matrix resin, this is the weak point. Therefore, weld lines should be kept away from critical structural areas. 
Keep the advantage of long fibers
  Successful molding of long fiber reinforced composites requires some modification of design guidelines and processing parameters applicable to non-reinforced resin and short fiber compounds. To get the most out of long fiber reinforcements (which cost more than unfilled materials or fiberglass chopped strands reinforcement because of their high performance), best practices must be followed throughout the process. If long fibers are broken or misaligned due to incorrect handling, die design or equipment setup, their high strength and high toughness benefits will be diminished or even lost.