Because metal is not suitable for molding complex shapes, its application in many parts is limited, which also hinders the reduction of costs. In contrast, the use of long glass fiber reinforced plastic injection molding can overcome many of the above disadvantages. However, the glass fiber may be damaged during the injection molding process and the required strength cannot be obtained.

Material Introduction

Long glass fiber reinforced polypropylene (LGFPP for short) is one of the new varieties that has attracted much attention. As a carrier material for automotive modules, this material can not only effectively improve the rigidity, impact strength, creep resistance and dimensional stability of the product, but also make complex automotive module products. Due to the strength requirements, the previous module carrier is usually made of glass fiber mat reinforced thermoplastic (GMT) or metal sheet with polypropylene as the base material by stamping. Due to the use of press molding, it is difficult to integrate multiple parts. In order to improve rigidity and strength and to obtain a thin molding thickness, reinforcing ribs are also required. In addition, other steps are required to remove the flash and burrs of the molded parts. All of the above factors restrict the reduction of the weight and cost of automotive module products.

In order to make the glass fiber play a good role in improving the strength of plastics, the length of the glass fiber must be greater than its critical length Lo. Relevant data show that when the fiber reinforced plastic with a fiber length less than this critical length is subjected to a certain load, the fiber will be pulled out and the strength of the fiber cannot be fully exerted. The critical length Lo is related to the specific type of plastic. For glass fiber reinforced polypropylene, its Lo is 3.1mm, while the glass fiber length of ordinary short fiber reinforced plastics is generally only 0.2-0.6mm. This shows that the failure mode is mainly that the fiber is pulled out and cannot meet the strength requirements of the module carrier material. Therefore, the development and application of long glass fiber reinforced polypropylene and its injection molding technology is to prepare polypropylene raw materials with a reinforced glass fiber length of about 10mm, and through an improved injection molding process, ensure that the glass fiber length in the product is above 3.1mm.

In 2002, foreign countries successfully developed long glass fiber reinforced polypropylene injection molding technology, and successfully used this technology to produce Mazda 6 front-end modules and door module carriers. This technology includes two aspects:

The first is the material modification of glass fiber reinforced polypropylene, that is, using a polypropylene resin with ultra-low melt viscosity (resin melt flow rate is 300g/10min) so that the glass fiber wrapped in it is subjected to a smaller screw push shear force during the injection molding process to reduce the length loss of the glass fiber, and at the same time adding a polypropylene resin with a high crystalline structure to ensure the strength of the injection molded parts. Through this resin blending modification, the contradiction between material fluidity and product strength is solved. The three mechanical properties of the long glass fiber reinforced polypropylene (LGFPP) after blending modification, namely, the bending modulus, bending strength and impact strength, are equivalent to the same properties of glass fiber mat reinforced polypropylene (GMT), and its fluidity is also 30% higher than that of ordinary glass fiber reinforced polypropylene (FGPP).

The second is the improvement of the injection molding process, that is, by improving the geometric shape of the screw, such as deepening the screw groove, widening the pitch of the screw teeth, optimizing the design of the screw head, and expanding the hot runner, the glass fiber reinforced resin can flow smoothly during the injection process to reduce the high shear force of the resin during the plasticization process, thereby achieving the purpose of reducing the damage to the glass fiber length. Under the condition of using long glass fiber reinforced polypropylene raw materials, the average glass fiber length of the improved low shear force screw injection molded products is 1.7 times the average glass fiber length of the ordinary screw injection molded products.

The characteristics of this long glass fiber reinforced polypropylene injection molding technology are:

Compared with the ordinary process of using a general screw to inject short glass fiber reinforced high melt viscosity polypropylene, due to the smaller shear force on the glass fiber, the glass fiber length in the product is 10 times the glass fiber length obtained by the ordinary process (the glass fiber length of the product obtained by the ordinary process is generally 0.5mm), and the impact strength of the product is increased by 3 times. This material is used in the Mazda 6 front-end module carrier, and the weight is reduced by 9kg.

The addition of ultra-low viscosity components in the resin increases the molding fluidity by 30% compared with ordinary glass fiber reinforced polypropylene and glass fiber reinforced nylon, which can be integrated with a variety of parts and have a thinner molding thickness, thereby reducing manufacturing costs.

The reinforcement of long glass fibers and the addition of high crystalline polypropylene resin make the material's high temperature fatigue strength at 120¡ãC twice that of ordinary glass fiber reinforced polypropylene, and even nearly 17% higher than glass fiber reinforced nylon, which is famous for its heat resistance. Therefore, this new material has the durability and reliability required as a structural part.

The ultra-low viscosity component forms a thick plastic layer on the surface of the product, which can prevent the glass fiber from being exposed to the surface of the product to achieve the effect of beautifying the appearance, and can eliminate the process of treating the surface of ordinary glass fiber reinforced plastics with coatings.

This polypropylene substrate has good recyclability, and even recycled materials have the same physical and mechanical properties as ordinary glass fiber reinforced polypropylene.

As a carrier material for automotive modules, the successful development of long glass fiber reinforced polypropylene has made it not only used in Mazda cars. Recently, the front door module of the new Ford Fiesta model has also been successfully developed by Owens Coring Automotive Company. The door module integrates a variety of functional components, such as door locks, door glass lifters, speakers, anti-theft devices, etc. The carrier material used is DSM's brand StaMax P30YM240 long glass fiber reinforced polypropylene material. In the process of developing the door module, some experts conducted in-depth research on the performance of long glass fiber reinforced polypropylene materials for injection molding, especially the creep resistance of this material. The results show that long glass fiber reinforced polypropylene materials will not produce obvious creep even at a high temperature of 100¡ãC, and have better creep resistance than short glass fiber reinforced polypropylene.

Under high temperature and long-term low load conditions, long glass fiber reinforced polypropylene materials will not deform, and their products can have good dimensional stability, which can be confirmed by the actual measurement results of the dimensions of the front door modules of the new Ford Fiesta models produced in batches. At present, as the modularization of automobile parts has attracted more and more attention and has been applied more and more, long glass fiber reinforced polypropylene will undoubtedly become an ideal module carrier material. For this reason, some people predict that LGFPP material will become a substitute for GMT material as an automobile module application.

Different fiber-reinforced thermoplastic composites based on polypropylene resin, whether GMT, SR-PP or LGFPP, have some common characteristics, namely: compared with metal materials, they have low density, light weight, high specific strength, corrosion resistance, easy molding and other characteristics; compared with thermosetting composite materials SMC and hand-laid fiberglass, they have short molding cycle, good impact toughness, and recyclable utilization. In particular, the renewable characteristics make these materials have broader application prospects today when environmental protection requirements are increasingly stringent.

The modification of PP materials generally includes strengthening and toughening, weathering modification, glass fiber reinforcement modification, flame retardant modification and super toughness modification.

As one of the general plastic materials, PP has excellent comprehensive performance, good chemical stability, good molding and processing performance and relatively low price; however, PP has the disadvantages of low strength, modulus, hardness, poor low temperature impact strength, large molding shrinkage, and easy aging. Therefore, it is modified to adapt to the needs of the product. Each modified PP has a large number of applications in the field of household appliances and automotive fields.

ABS is one of the first plastic materials used in household appliances. Due to the high price of ABS resin, the gradually developed PP modified materials have the advantages of low cost, light weight and good performance; glass fiber reinforced PP can partially replace ABS and PBT resins in household appliances and automotive fields.

Glass fiber reinforced modified PP

1. Generally speaking, the tensile strength of PP material is between 20M~30MPa, the flexural strength is between 25M~50MPa, and the flexural modulus is between 800M~1500MPa. If you want to improve the strength performance of PP, you must use glass fiber to reinforce it. The mechanical properties of PP products reinforced by glass fiber can be doubled or even several times improved. The tensile strength can reach 65MPa~90MPa, the flexural strength can reach 70MPa~120MPa, and the flexural modulus can reach 3000MPa~4500MPa. Such mechanical strength is completely comparable to ABS and reinforced ABS products.

2. Glass fiber reinforced PP is more heat-resistant. The heat-resistant temperature of general ABS and reinforced ABS is between 80¡æ~98¡æ, while the heat-resistant temperature of glass fiber reinforced PP material can reach 135¡æ~145¡æ. It can be used to make axial flow fans and cross-flow fans in refrigeration machines such as refrigerators and air conditioners, and its cost is much lower than that of ABS reinforced products. It can also be used to manufacture the inner barrel, impeller, and pulley of high-speed washing machines to meet their high requirements for mechanical properties, and used in places with high temperature resistance requirements such as rice cooker bases and handles, electronic microwave ovens, etc.

3. The dimensional stability of glass fiber reinforced modified PP is improved, the thermal deformation is reduced, and the shrinkage rate is reduced.

4. The hardness of glass fiber reinforced modified PP is generally improved, and the water absorption performance is reduced.

Development trend and outlook of modified PP

Modified PP has very good application prospects in the home appliance industry. On the one hand, China has become the world's home appliance production center, and has a group of very influential production enterprises, which can actively choose materials; on the other hand, industry competition also prompts enterprises to use materials with more reasonable cost performance. From the perspective of the future development of home appliance technology, the humanization of home appliances will be more prominent, the product variety will be more complete, and traditional home appliances will develop in the direction of miniaturization and large-scale. The application of glass fiber reinforced PP in automotive materials is also constantly expanding, and the continuous emergence of new products has also put forward higher requirements for PP modification. Modified PP will have the following main development trends:

1. High rigidity and high gloss PP. This product will replace PS and ABS in some home appliances, especially small home appliances.

2. Halogen-free flame retardant PP. With the implementation of the EU RoHS directive and the attention paid by various countries to environmental protection, the development prospects of halogen-free flame-retardant PP are broad, and halogen-free intumescent flame retardants have ushered in an opportunity for development.

3. Long glass fiber reinforced PP. Ordinary short glass fiber reinforced PP, due to the short glass fiber contained, is easy to warp, has low impact strength, and is easily deformed when heated. Long glass fiber can overcome the above defects of short glass fiber, and the products have a better surface, higher use temperature, and higher impact strength. They can be used in refrigerators and kitchen appliances with relatively high heat resistance.

4. Glass fiber reinforced flame retardant PP. As people pay more attention to the hazards of fire, the concepts of fire prevention and flame retardancy have been proposed in many household appliances and automotive plastics. Chinese manufacturers have invested in the research of flame retardant reinforced PP materials. Shenzhen Jingqiao Technology's glass fiber reinforced PP halogen-free flame retardant, FR-1610G, was the first to be successfully applied in glass fiber reinforced flame retardant in the household appliance industry and the automotive industry, partially replacing the use of ABS and PBT, and achieving good economic benefits.

5. Antibacterial PP. This plastic is a new functional polymer material with bactericidal and antibacterial properties. It can be used in washing machine liner, refrigerator plastic parts, etc., and has broad prospects.

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