Long glass fibre reinforced polypropylene (PP) injection molding

In automotive engineering, the use of long glass fiber-reinforced polypropylene is increasingly used in dashboards, car front and bottom parts. Because of its low density, low price, and the advantages of recyclability, polypropylene is gradually replacing engineering plastics and metals in these applications. However, when long glass fiber-reinforced polypropylene increases the elastic modulus and impact resistance of the material, the polypropylene material merely satisfies the mechanical properties.
These parts are prepared by injection molding or molding of glass-reinforced polypropylene materials. In the molding process, the raw materials are made of semi-finished glass mat reinforced polypropylene (GMT). Typical GMT molded parts have excellent mechanical properties because of the length of the fibers and their co-directionality, but the production of GMT-enhanced products is very complex and the price of the semi-finished products becomes expensive.
Thanks to recent technological advances, it is now possible to directly mix polypropylene and glass fibers in the molding process. These advances have been made in processing technology. In spite of this, there are still major drawbacks when compared with the injection molding process. In most cases, parts need to be reworked. As a rule, stamped parts can only be stamped with subsequent stampings, which creates processing waste and therefore increases costs.
Parts made by injection molding with long fiberglass-reinforced polypropylene usually require the processing of long glass fiber particles with the aid of a plastic modification unit. Based on an alternative to particle processing, Krauss-Maffei introduced a new injection molding compounding system. This system can directly manufacture injection molded parts from polypropylene and glass fiber raw materials in one step. This injection molding compounding device consists of a twin screw extruder and an injection molding machine. The extruder melts the polypropylene and mixes it with the glass. The melt passes through a buffer into an injection unit called the injection chamber where it is injected into the mold. Direct mixing in the injection molding process eliminates the need for semi-finished products that have previously passed through.
Next, we compare the technology of injection molding machines and injection molding with economic standards to help plastics manufacturers decide which of the two methods is more suitable for their production tasks.
Characteristics of glass-reinforced thermoplastics Good fiber adhesion is critical to the mechanical properties of the parts. GMT can produce some stronger force and impact resistance than directly processed molding compounds or long fiber particles.
If it is necessary to use the force generated by the fiber, it must be longer than the critical fiber length Lc. Correspondingly, the critical length Lc of the sheet made of polypropylene and glass fiber ranges from 1.3 to 3.1 mm, and the special coupling requires 0.9 mm. .
The ratio of the existing fiber length to the critical fiber length can be used to infer the quality of the fiber coupled to the masterbatch. If the actual fiber length in the part exceeds the critical fiber length, ie, exceeds Lc, the fiber may be destroyed if it is short. At the critical length of the fiber, fiber detachment occurs, that is, there is a problem with the fiber-masterbatch surface, and the usual fiber length in the compound is 0.2-0.6 mm.
Strictly speaking, the length of these reinforcing fibers is not so important for the design, and the mechanical properties are more important for the design of the parts such as strength, rigidity and impact resistance. Although they are also one of the functions of fiber length. However, their relationship is rather complicated. The length of individual analysis fibers is by far only an empirical parameter, and only some trend information can be obtained.
Fiber Length in Parts When processing long fiberglass-reinforced polypropylene, the combination of the longest fibers and the parts is very important because it gives rise to the mechanical properties of the compounds, but there is no way to avoid fiber damage. Because the use of mechanical forces during compounding and injection molding can damage the fiber and make it shorter, the most severe damage to the fiber occurs when the melt containing the fiber is injected into the mold, and reasonable mold design can be reduced. The degree of fiber shortening, the process of melting and processing also affects the length of the fiber. In this respect, there is a big difference between injection molding machine and injection molding.
The initial fiber length using an injection molding machine is determined by the size of the fiber particles (standard is 10 to 25 mm), and manufacturers of long glass fibers provide coating systems. In this type of system, the fibers gradually aggregate into a bundle in the melt, which allows uniform injection of the fibers. In the case of coated particles, the fibers were co-extruded with the masterbatch. The smelting process of the injection molding machine needs to decompose the clusters of fibers and infiltrate the single fibers with the masterbatch. (To be continued)