Many automotive plastics possess numerous advantages over traditional materials, primarily characterized by their lightweight nature, excellent aesthetic appeal, a variety of practical applications, good physical and chemical properties, ease of processing and molding, energy efficiency, and sustainability.
Engineering Plastics
Engineering plastics, due to their superior comprehensive mechanical properties, are also widely used in the automotive industry. This article mainly introduces Polyamide (PA), Polymethyl Methacrylate (PMMA), Polyoxymethylene (POM), Polyurethane (PU), and Polycarbonate (PC).
1. Polyamide (PA)
PA, commonly known as nylon, comes in many industrial varieties, with PA6, PA66, and PA610 being the most commonly used. Polyamide is easy to print and dye, and it has excellent electrical properties; it is resistant to chemicals and oil corrosion. The mechanical commonality across polyamides is toughness; they all possess high surface hardness, good tensile strength, good impact resistance, fatigue resistance, fold resistance, and stress cracking resistance. The tensile and compressive strength of PA changes with temperature and moisture absorption, so water acts as a plasticizer for PA. When glass fibers are added, its tensile and compressive strength can be increased by about two times, and its temperature resistance also improves accordingly. PA itself has very high wear resistance, so it can work continuously without lubrication.
The disadvantages of PA include poor acid resistance, poor light resistance, and poor contamination resistance. Due to the effects of thermal expansion and water absorption, the dimensional stability of the parts is poor, with a shrinkage rate of 1-2%, and the dimensional changes due to moisture absorption after molding should be noted. The water absorption rate can reach 100%, and it can absorb up to 8% when relatively moisture-saturated. The suitable wall thickness is 2-3.5mm.
Injection Molding Performance: Due to the presence of amide groups, PA (polyamide) has strong and stable water absorption. Therefore, it should be thoroughly dried before injection molding, typically at 120°C for 3-4 hours. PA has low viscosity and fast flow rate. To prevent nozzle drooling, a self-locking nozzle or a nylon-specific nozzle should be used. Additionally, precision of the mold should be taken into account.
Application Range: In the automotive field, PA is mainly used to manufacture hoses (brake hoses, fuel lines), fuel oil filters, air filters, oil filters, water pump casings, water pump impellers, fans, brake fluid reservoirs, power steering fluid tanks, louvers, front headlight housings, and seat belts.
2. Polymethyl Methacrylate (PMMA)
PMMA, commonly known as acrylic or Plexiglas, is resistant to outdoor aging and has excellent light transmittance. Even after 240 hours of accelerated light aging, it can still transmit 92% of sunlight, and after ten years outdoors, the transmittance remains at 89%, with 78.5% of ultraviolet light passing through. It has relatively high mechanical strength, some degree of cold resistance, corrosion resistance, good insulation properties, dimensional stability, and is easy to mold. However, it is somewhat brittle, easily melts in organic solvents, and its surface hardness is not high, making it prone to scratching and fuzzing. It is suitable for plastic products that require a certain level of strength in transparent structural components.
Injection molding performance: PMMA has a moisture absorption rate of 0.3%, and it must be dried before injection molding, typically at around 80°C for 2-4 hours. During injection molding, the melt temperature should be between 240-270°C, and the mold temperature should be controlled at 35-70°C.
Due to its excellent light transmission properties, PMMA is widely used in automotive lighting signs, car door windows, and lamp covers.
3. Polyoxymethylene(POM)
POM, commonly known as acetal or polyacetal and by the trade name Delrin, is characterized by high tensile strength, impact toughness, rigidity, and fatigue endurance. It exhibits excellent creep resistance, dimensional stability, low water absorption, and low coefficient of friction, which contributes to its outstanding wear resistance. Even at high temperatures and in water, it maintains significant rigidity. Its chemical resistance is comparable to that of polyamide (PA), but it is less expensive. Polyoxymethylene is resistant to repeated twisting and has exceptional resilience. It can be used long-term at temperatures ranging from -40 to 100 degrees Celsius. However, POM has its drawbacks: it is susceptible to strong acids, not heat-resistant, and has poor thermal stability. It has a shrinkage rate of 2-3.5%, with an optimal wall thickness of 1.5-2.5mm.
POM has a distinct melting point, melting at 175°C and decomposing at 240°C. The general processing temperature range is narrow, between 190°C and 220°C. POM does not absorb water, so typically, it does not require pre-drying before injection molding. However, for high-quality products, drying at 60°C for 1-2 hours may be beneficial. The temperature should not be too high to prevent discoloration of non-pigmented products. Its acid resistance is poor, making it unsuitable for dyeing with acidic dyes.
Applications: POM is used in the automotive industry for manufacturing dashboard glove compartment accessories, various valves (such as drainage valves, air conditioning valves, etc.), various impellers (water pump impellers, heater blowers, oil pump wheels, etc.), small gears in electrical switches and instruments, various handles, and door pins, among others.
4. Polycarbonate (PC)
PC exhibits outstanding impact toughness and creep resistance. It has good heat resistance and excellent cold resistance, with a brittle temperature reaching -100°C. Its flexural strength is comparable to that of nylon, and it possesses a higher elongation rate and elastic modulus, although its fatigue strength is less than that of nylon 66. PC has a low water absorption rate, low shrinkage, and good dimensional stability. Its wear resistance is comparable to nylon, and it has a certain level of corrosion resistance, but it requires high molding conditions. PC has good weather resistance and can be used for long-term applications at higher temperatures and high loads, but it should not be used in humid heat conditions due to poor solvent resistance and susceptibility to stress cracking and poor fatigue strength. The shrinkage rate is 0.5-0.7%, and the suitable wall thickness is 2-3.5mm. The addition of glass fiber to PC can improve its shrinkage rate, mechanical strength, and temperature resistance. At around 100°C, its rigidity will increase for long-term use, and annealing can improve internal stress.
PC has a distinct melting point, melting at 220°C and decomposing at 350°C. The general processing temperature is between 250°C and 320°C. PC is hygroscopic, and even a small amount of moisture can cause it to decompose at high temperatures, so it must be dried before injection molding. The drying temperature can be set to 120°C for 4-5 hours. PC material has a high melt viscosity, requiring higher pressure during injection molding. If conditions allow, a mold temperature controller can be used to increase the mold temperature during processing to reduce residual stress in the product. PC's shrinkage rate is independent of processing conditions and product wall thickness. Its longitudinal and transverse shrinkage rates are quite close, allowing for the production of highly precise products; its shrinkage rate is 0.5%.
The main synthetic compound PC-ABS is a blend of PC and ABS, typically supplied in pellet form after blending. If the two materials are simply mixed and directly injection molded, the result is poor, showing delamination. PC's advantage is its rigidity and toughness, but it is prone to stress cracking and has high viscosity. ABS's advantage is good flowability, but it has low surface hardness. The blended material PC-ABS retains the advantages of both; it has higher surface hardness, rigidity, and toughness, as well as higher resistance to stress cracking. Its mechanical properties are intermediate between the two materials.
In the automotive field, PC is mainly used in the manufacture of lamp covers, left and right wheel arch guards, instrument panel bodies (PC+ABS), left and right air frame covers, center air frame covers (PC+ABS), and rear bumper cushions.
Car Dashboard
5. Polyurethane(PU)
PU, based on the different polymerization products, is commonly divided into rigid polyurethane and flexible polyurethane. Rigid polyurethane has high strength, good toughness, excellent thermal insulation, effective waterproofing, simple molding processes, and high production efficiency. Flexible polyurethane has excellent elasticity, sound absorption, aging resistance, chemical resistance, and its comprehensive mechanical properties far exceed those of materials like PVC. The polymerization of polyurethane is convenient, as it can not only occur under normal temperature and pressure, but can also be carried out on-site with immediate foaming or through spraying, repairing, and other construction methods.
Polyurethane foam plastics are widely used in automotive interiors and vibration-absorbing components, such as coating materials, rigid polyurethane plastic sheets, polyurethane elastomers, seat soft foam materials, decorative parts, synthetic leather for sofas, and car roof accessories. The most common applications are various soft and rigid polyurethane foam materials, which provide vibration isolation, sound insulation, noise reduction, and thermal insulation. PU can also be made into automotive polyurethane coatings, adhesives, sealants, etc. Automotive PU components include dashboards, rearview mirrors, bumpers, seat cushions, headrests, steering wheels, dashboard vibration dampening pads, pillar decorations, front headliners, window frames, ceiling and side frame decorations, door panels, sun visors, and rear top frame decorations.
6. Special Plastics: Glass Fiber Reinforced Plastics
Glass fiber reinforced plastics are materials that have glass fibers and other additives incorporated into the base plastic, thereby extending the range of applications for the material. Generally speaking, most glass fiber reinforced materials are used in structural parts of products and are considered structural engineering materials, such as PP, ABS, PA66, PA6, PC.