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.
Further damage to the fiber also occurs with reduced dissolution accompanied by decreased resistance to flow. Large cross-zone flow channels are good for fibers, so when working with long glass fiber particles, the screw structure and barrier return valve should be adjusted accordingly.
When the particles are infused, the fibers enter a complete melting process and the mechanical tension on the fibers will continue for a longer period of time. The beginning of plasticization also applied this enormous force, because at that stage, the masterbatch was not completely melted, and some fibers were exposed to great shear forces.
The screw size and stroke specifications also affect the mechanism of fiber damage. The test results show that the larger screw (D=165mm) is less damaging to the fiber than the smaller screw (D=90mm). Longer metering strokes can also have an adverse effect on the remaining fiber length. The dispersed arrangement shows the effect of the long glass fiber particle structure.
Contrary to this, the injection molding compounding is to melt the pure masterbatch without fiber, and the fiber is added into the melt later, so that correspondingly less mechanical tension is absorbed. This method can achieve higher average fiber lengths than smelt mixing in an injection molding machine. Injection molding compounding (IMC) allows the crude fiber to be directly added to the melt without decomposing the fiber. Although the coarse fiber is broken down into short parts under the rotation of the screw, the final fiber still has a long average length. .
The price of raw materials from an economic point of view is very important for fiber-reinforced polypropylene parts. Long glass fiber particles for injection molding are cheaper than GMT semi-finished products. However, manufacturers pay more for buying pellets than buying a single raw material. One advantage of using injection molding compounding for raw material manufacturers is that raw material prices are more economical than using long glass fiber particles, and the proportion of raw materials in the entire component manufacturing cost can also be reduced.
Adding long glass fiber-reinforced polypropylene particles to injection molding machines reduces the degree of intensification of the particles into pellets compared to injection molding. It is possible to process long glass fiber particles by adjusting or replacing plasticizing units in existing injection molding machines. Even if it is difficult to retrofit, you must install a new machine. Injection molding machine processing still requires less investment, and the twin-screw extruder needed for injection molding compounding complicates the equipment.
How to decide in addition to the advantage of fiber length distribution in the parts mentioned above, the injection molding compounding also has potential savings in the raw materials, but this potential effect can only be realized through additional investment. An important criterion in choosing from injection molding machines and injection molding compounding is the weight and volume of the manufactured components. Injection molding compounding has the advantage when the yield is high, because the investment saved when purchasing raw materials will quickly exceed the increased investment in purchasing equipment, so that the investment can be allocated quickly. When the number of parts is small or the yield is low, processing long glass fiber particles on the injection molding machine is a better method because it requires only a small investment.
Injection molding compounding gives manufacturers more flexibility so they can tailor materials to their own needs. They can selectively adjust the ratio of masterbatch and fiber, for example, to make the fiber content of the component meet the desired technical characteristics. When glass fiber particles are processed, this selective adjustment is only possible under certain conditions. Because manufacturers only provide a variety of fiber types for pellets, producers need long glass fiber pellets mixed with unreinforced polypropylene - a work step that provides additional requirements and a raw material supply system.
However, the degree of freedom that the injection molding compounding provides to the manufacturer can also be said to be a mixture of materials that improves the interpretability of the product and the reliability of the manufacturer. Producers must now take responsibility for ensuring quality and ensuring the quality of pellet manufacturing. There are potential opportunities for injection molding compounding to increase the additional prices created in production.
