Commonly said is that you go to the mall to buy pants, pants length generally not just to the ankle, but a few centimeters long, to prevent shrinkage overnight into cropped pants is a concept.

Because in glass fiber reinforced plastics, glass fiber length can only reach a certain length to transfer stress, play the role of strengthening materials.

In addition, even if the optimal length value is found, it is not 100% guaranteed that the effective length of the glass fiber in the product is this value, because the processing equipment and technology have a great impact on the final effective length of the glass fiber.

So, let's talk about how to ensure the effective length of glass fiber in the equipment and process.

2. Factors affecting the effective length of glass fiber in plastics

1. Equipment factor
(1) Screw configuration and barrel configuration
The overall objectives of twin-screw configuration design and barrel configuration suitable for preparing glass fiber plastics are:

① Prevent matrix resin degradation, can make the glass fiber evenly dispersed in the matrix resin;
② Ensure that each fiber is moistened to the maximum extent by the polymer melt;
③ Cut the fibers to the optimum length to ensure that the mixture achieves the maximum reinforcement effect;
④ The volatile matter in the extrusion process can be cleared.

⑴ Screw configuration of glass fiber added:

The spiral groove into which the glass fiber is added and discharged shall be large in lead, so that the polymer fills the spiral groove with enough space for the outflow of the glass fiber. In addition, in order to avoid polymer blockage of the entry port, the short glass fiber can be imported with thread elements, and the long glass fiber can be imported with at least one pair of kneading disc elements.

⑵ Cutting and dispersing of glass fiber:

① The screw configuration design downstream of the glass fiber joining port should be conducive to the change and homogenization of the glass fiber length. Long glass fibers are headless and must be cut to a certain length after the addition of a screw and mixed well with the melt. The mixing section shall consist of distributed mixing elements, or thin kneading discs or toothed mixing elements.
(2) Short glass fiber, mainly rely on polymer melt to wet and disperse the fiber, so the mixing section can be composed of a thin kneading disc or a slot on the screw thread or tooth disk components.
③ The high viscosity polymer or the high glass fiber content (>40%) of the screw configuration than the low viscosity polymer or low glass fiber content of the screw configuration to provide a softer shear force, to ensure that the glass fiber will not break too much.

(2) Setting of exhaust section
Because some glass fiber is pretreated, such as long glass fiber twist fiber is treated by paraffin emulsion impregnation, and untwist fiber is treated by fortifier. At a certain temperature, after mixing the glass fiber with the melt, the impregnating agent and the fortifier on the glass fiber will become the hair component after being subjected to high temperature in the extrusion process, and the exhaust section should be set up to eliminate it. The exhaust section should be located downstream of the glass fiber addition port. The upper part of the exhaust section near the exhaust port should be provided with a sealed screw element, such as a reverse screw element or a reverse kneading block, to prevent particles from being extracted under the action of a vacuum pump.

(3) The last section of the screw (homogenization and pressure building section)
In order to mold the extrusion outlet of the mixture to granulate, a small lead forward thread element should be used to establish extrusion pressure. Between the exhaust port and the last section of the screw, sometimes the tooth plate element is set up to homogenize the fiber and ensure the uniform distribution of the glass fiber.

2. Technological factors
(1) The addition of glass fiber
Long and short glass fibers are added by different methods.

(1) Short glass fiber is generally added with a metering feeding device, but when the length of short glass fiber is greater than 6mm, vibration metering feeding device can be used to add the polymer and the premix of glass fiber together, otherwise it will cause the separation of fiber and resin. In order to increase production, side feeding device can be used to add by side feeding port.
(2) Long glass fiber is easier to join, and no special feeding device is needed. As long as the long glass fiber is hung up, it can be introduced into the feeding port of the twin screw.

(2) The position where the glass fiber is added
In general, the polymer is added at the first (main) feeding port, after it melts, and then the glass fiber is added at the downstream feeding port, that is, the use of subsequent feeding. Because of the subsequent feeding, the glass fiber is added to the melted polymer. After mixing the melt with the fiber, the fiber is wrapped, which plays the role of lubrication and protection, reduces the excessive fracture of the fiber and resin during the mixing process, and is conducive to the dispersion and distribution of the glass fiber in the melt.

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