Long glass fiber reinforced PP injection molding process and injection molding method: Long glass fiber reinforced polypropylene (PP) parts are usually made of injection molding long glass fiber pellets. A new one-step process can compound polypropylene and glass fibers directly to produce injection-molded parts. Two methods have their own characteristics, and which method to adopt should be determined according to characteristics of part production.

 

In automotive engineering, instrument panels, front end parts and underbody elements are increasingly made of glass-fibre-reinforced polypropylene. Polypropylene has characteristics of low density, low material cost, and easy reuse. Therefore, it is gradually replacing engineering plastics and metals in above application fields. However, polypropylene can only meet mechanical specifications if long glass fiber reinforcements enhance elastic modulus and impact strength.
Parts are made of injection or compression moulded glass fibre reinforced PP. In compression molding process, starting material is usually a semi-finished sheet made of glass mat thermoplastic (GMT) reinforced PP. Due to its long fibers and isotropic properties, conventional GMT compression molding usually produces parts with excellent mechanical properties. However, GMT production process is very complicated. Therefore, cost of semi-finished products is relatively high.
Thanks to latest technological developments, PP and glass fibers can now be compounded in-line, followed by direct compression molding. With development of process technology, compression molding has many disadvantages compared with injection molding. In most cases, parts must be reworked. Openings in compression-molded parts are usually only created during downstream stamping process. As a result, scrap is created, which increases the overall cost.

Good bonding between fibers and substrate is critical to mechanical properties of part. GMT provides higher strength and impact strength than direct processing molding compounds and long glass pellets. Due to good bonding of fibers and fiber filaments, filaments are evenly distributed, resulting in a needled felt structure, which has several advantages. However, if flow path is too long during compression molding, these advantages are lost compared to molding compounds injected directly or through long-fiber pellets. Because injection molding can create fiber orientation in part, lack of needling properties can be partially offset by proper design for resulting stresses.
A conclusion is drawn on processing method based on damage of fiber structure in composite material. Fiber structure damage includes fiber breakage, fiber debonding, and fiber pullout. To fully utilize fiber strength, it must be ensured that fiber length is longer than so-called critical fiber length. For fiber/substrate composites composed of PP and glass, corresponding literature values for critical fiber length (LC) range from 1.3 mm to 3.1 mm. With special coupling agents, values up to 0.9 mm can be produced.
Quality of fiber-substrate coupling can be inferred from ratio between actual fiber length and critical fiber length. If actual fiber length of part is greater than critical fiber length, fibers are prone to breakage. Below critical fiber length, fiber pullout may occur. This mainly refers to fracture of fiber/substrate interface, which usually occurs in chopped fiber compounds with fiber lengths typically ranging from 0.2 mm to 0.6 mm.
Strictly speaking, length of remaining reinforcing fibers in fibers is not relevant to design. Mechanical properties such as strength, rigidity, and impact strength are even more important for component design. Although these properties are a function of fiber length, relationship is complex. Therefore, fiber length analysis alone can only achieve current goal, but it is indeed a very practical parameter for obtaining trend information.

 

During processing of long glass fiber reinforced polypropylene, it is necessary to ensure that longest fibers are incorporated into part in order to produce the best mechanical properties in composite material. However, there is currently no reliable way to avoid fiber breakage when mechanical stress is applied, which can cause fiber shortening during compounding and injection molding. The most damage to fibers occurs when fiber-containing melt is injected into mold. However, rational design can reduce magnitude of fiber shortening. At the same time, melting process also greatly affects length of fibers. In this regard, an injection molding machine and an injection molding compounder (IMC) are very different.
When processed by an injection molding machine, initial fiber length is limited by size of pellets (usually 10 mm to 25 mm). Long glass fiber manufacturers offer sheathed and pultruded pellets. In pultruded pellets, fibers are wetted by substrate in a melt tank and then assembled into small bundles. This method allows substrate to soak all fibers evenly. In jacketed pellets, fibers and substrate are coextruded together. Injection molding machine must dissolve clumps of fibers during melting process, then thoroughly wet fibers with substrate.
During melting process, degree of fiber damage decreases as flow resistance decreases. When cross section of flow channel is larger, damage to fiber is smaller. Therefore, when processing long glass fiber pellets, screw configuration and check valve should be modified accordingly.
When pellets are injection molded, fibers undergo the entire melting process. Mechanical stress will remain on fiber for a considerable period of time. Plasticization starts with a lot of force on fibers because substrate is not fully melted at this stage. Some fibers are caught and subjected to enormous shearing forces.
In contrast, injection molding compounders melt neat substrates without fibers. Fibers are added after base material has melted and, therefore, are less subject to mechanical stress. This method causes less damage to fibers than injection molding machine melting and increases average fiber length. Jointless rovings (not chopped strands) can be mixed directly into melt using an injection compounder. Although rotation of screw breaks roving into shorter pieces, final length of fibers is relatively long.

Price of starting material is the key to production of fiber-reinforced PP parts. Compared with GMT semi-finished products, price of long glass fiber pellets for injection molding is lower. However, processors pay more for pellets than when buying individual components. For secondary processors, one of main advantages of injection molding compounders is that starting material price is lower than that of long fiber pellets, and material cost accounts for a lower proportion of part production cost.
Processing glass fiber reinforced polypropylene as pellets on an injection molding machine requires less investment than an injection molding compounder. However, it is also possible to retrofit or replace plasticizing unit on an existing injection molding machine to make it suitable for processing long glass fiber pellets. Even if retrofit is not possible and new machinery must be installed, investment required for injection molding is relatively low. Adding a twin-screw extruder to injection compounder only complicates equipment.

In addition to above-mentioned advantages of uniform distribution of fiber length in the part, injection molding compounders can also save costs in terms of starting materials, but this potential can only be realized with additional investment. Therefore, choice of injection molding machine or injection molding compounder should be determined by weight and output of produced parts. If output is high, injection molding compounder can show its advantages. Reason is that cost savings when purchasing starting material can quickly exceed basic investment in purchasing equipment, so investment is shared within a short period of time. If parts are small and output is low, it is more cost-effective to use injection molding machine for long glass fiber processing, because investment in injection molding machine is smaller.
Injection compounders provide processors with increased production flexibility and ability to tailor materials to specific requirements. Modifications to substrate/fiber/styling system can be made by processor so that fiber content in the part fully complies with various specifications. When processing pellets, this selective modification is only possible under certain conditions because manufacturer only supplies pellets with a specific fiber content. To change fiber content for conventional injection molding, processor must mix long glass fiber pellets with unreinforced polypropylene, a step that places additional demands on machinery and material supply system.
However, as injection molding compounders offer processor a high degree of freedom in terms of material composition, this increases product liability and processor liability. Processors now have to take responsibility for quality assurance, in addition to those previously imposed on pellet manufacturers. However, there are also huge future opportunities. With an injection molding compounder, processors can realize substantial value-add.
In conclusion, long glass fiber reinforced PP parts can be produced by modified injection molding machines and compounding machines, which is better depends on production conditions. Advantages of injection molding compounding are that initial raw material investment is lower, damage to fibers is less, and fibers can be retained longer, but it is more suitable for parts with large output and high mechanical specifications. If part size is small and output is small, it is better to use a conventional injection molding machine.