What are the common engineering plastics for automobile parts molds?
With the development of automobile lightweight and the increase of automobile production, the amount of plastic used in automobiles is increasing. In recent years, the market demand for energy conservation and consumption reduction has been exerting pressure on the lightweight design of automobiles. In order to better adapt to the increasingly strict market regulation, many auto parts manufacturers have been looking for environment-friendly materials to replace traditional plastics. So let's take a look at which plastics are used in automobile parts molds?
Engineering plastics are also widely used in the automotive field due to their good comprehensive mechanical properties. In this paper, polyamide (PA), polymethacrylate (PMMA), polyformaldehyde (POM), polyamide (PU), polycarbonate (PC) are mainly introduced.
Polyamide, commonly known as nylon, has many kinds of PA used in automobile parts molds, including PA6, PA66 and PA610. PA is easy to print and dye, with excellent electrical properties; It is resistant to chemicals and oil. The common property of machinery is toughness, which has high surface hardness, good tensile strength, good impact resistance, fatigue resistance, folding resistance, and stress cracking resistance. The tensile and compressive strength of PA varies with temperature and moisture absorption, so water is the plasticizer of PA, and its tensile and compressive strength can be increased by about 2 times after adding glass fiber, and its temperature resistance can also be improved accordingly. The abrasion resistance of PA itself is very high, so it can work without lubrication.
Disadvantages of PA: poor acid resistance, poor light resistance and poor pollution resistance. Due to the influence of thermal expansion and water absorption, the dimensional stability of the parts is poor, and the shrinkage rate is 1-2%. Attention should be paid to the dimensional change of moisture absorption after molding. The water absorption rate is 100%, and it can absorb 8% when the relative moisture absorption is saturated. The appropriate wall thickness is 2~3.5mm.
Injection molding performance: PA has strong water absorption and firm water absorption due to the existence of amide group, so it should be fully dried during injection molding, generally at 120 ℃ for 3-4 hours; PA has low viscosity and fast flow speed. In order to prevent the nozzle from flowing, self-locking nozzle or nylon special nozzle shall be used. At the same time, pay attention to the mold precision.
Scope of application: PA is mainly used in the automobile field to manufacture hoses (brake hoses, fuel pipes), combustion oil filters, air filters, oil filters, water pump housings, water pump impellers, fans, brake fluid tanks, power steering fluid tanks, white leaf windows, front headlight housings, and safety belts.
Polymethacrylate, commonly known as polymethyl methacrylate, is widely used in automobile accessory molds. It is resistant to outdoor aging, has excellent light transmittance, and can still penetrate 92% of the sunlight after 240 hours of accelerated aging of light without affecting its transparency. 89% of the sunlight can still pass through the outdoor ten years, and the ultraviolet line reaches 78.5%. High mechanical strength, certain cold resistance, corrosion resistance, good insulation, stable size, easy to form, brittle, easy to melt in organic solvents, insufficient surface hardness, easy to scratch and fuzz. Suitable plastic products: transparent structural parts with certain strength requirements.
Injection molding performance: the moisture absorption rate of PMMA is 0.3%, and it must be dried before injection molding, generally at about 80 ℃ for 2-4 hours; During injection molding, the melt temperature shall be 240~270 ℃, and the mold temperature shall be controlled at 35~70 ℃.
PMMA is widely used on automobile lighting signboards, door glass and lamp glass cover due to its good light transmission performance.
Polyoxymethylene (POM), commonly known as plastic steel, is an indispensable plastic for automobile parts molds. It has high tensile strength, impact toughness, rigidity and fatigue strength, high creep resistance, good dimensional stability, small water absorption, small friction number, and excellent friction and wear resistance. It still has great rigidity in high temperature and water. Its chemical corrosion resistance is the same as PA, but its price is low. It is resistant to repeated twisting and has outstanding resilience. It can be used for a long time between - 40 ℃ and 100 ℃. POM is vulnerable to strong acid corrosion, not resistant to high temperature, and has poor thermal stability. Shrinkage: 2-3.5% Suitable wall thickness: 1.5-2.5mm.
Injection molding performance: POM has obvious melting point. It melts at 175 ℃ and decomposes at 240 ℃. The general processing temperature is 190 ℃~220 ℃, and its processing range is very narrow; POM does not absorb water. Generally, it does not need to be dried during injection molding, but products with high quality requirements can be dried at 60 ℃ for 1-2 hours; The temperature should not be too high, otherwise the non dyed products will change color; It has poor acid resistance and cannot be dyed with acid dyes.
Scope of application: POM is used in automobiles to manufacture glove box accessories for instrument panel, various valves (drainage valves, air conditioner valves, etc.), various impellers (water pump impellers, air heater impellers, oil pump impellers, etc.), various electrical switches, pinions on electrical instruments, various handles, door pins, etc.
Polycarbonate, with outstanding impact toughness and creep resistance, has good heat resistance and cold resistance. The embrittlement temperature is up to - 100 ℃. The bending strength is equivalent to that of nylon, and has high elongation and elastic modulus, but the fatigue strength is less than that of nylon 66. Low water absorption, small shrinkage and good dimensional stability. The abrasion resistance is equivalent to that of nylon, and it has certain corrosion resistance, but the molding conditions are required to be high. It has good weather resistance and can be used under high temperature and high load for a long time, but cannot be used under wet temperature. It has poor solvent resistance, stress cracking and poor fatigue strength. The shrinkage rate is 0.5-0.7%, and the appropriate wall thickness is 2-3.5mm. The shrinkage, mechanical strength and temperature resistance of PC can be improved by adding glass fiber. Long term use of PC at about 100 ℃ will increase its rigidity. Annealing can be used to improve the internal stress.
Injection molding performance: PC has obvious melting point. It melts at 220 ℃ and decomposes at 350 ℃. The general processing temperature is 250 ℃~320 ℃; It absorbs water. A small amount of water can cause it to decompose at high temperature. It must be dried during injection molding. The drying temperature can be 120 ℃ for 4-5 hours; The melt viscosity of PC material is large, so a large pressure is required for injection molding; When PC is processing, if conditions permit, the mold temperature machine can be used to increase the mold temperature to reduce the residual stress of the product; The shrinkage of PC has nothing to do with the processing conditions and the wall thickness of the product. Its longitudinal and transverse shrinkage is relatively close, so it can be processed into products with high precision; Its shrinkage is 0.5%.
Main synthesis PC abs: PC-ABS is a blend of PC and ABS, which is usually supplied in granular shape after blending. If the two materials are simply mixed and directly injected, the effect is very poor, and delamination will occur; The advantages of PC are rigid and tough, while the disadvantages are stress cracking and high viscosity; ABS has the advantage of good fluidity, but low surface hardness; In this way, the blending material PC-ABS retains the advantages of both; PC-ABS has high surface hardness, high rigidity and toughness, and high resistance to stress cracking; Its mechanical performance is between the two.
Scope of application: PC is mainly used in the manufacturing of lamp shades, left and right wheel cover shields, instrument baffle body (PC+ABS), left and right wind frame covers, middle wind frame covers (PC+ABS), and rear bumper buffer pads in automobile accessories molds.
Polyurethane can be divided into hard polyurethane and soft polyurethane according to different polymerization products. Rigid polyurethane has high strength, good toughness, good thermal insulation, good waterproof effect, simple molding process and high production efficiency; Soft polyurethane has good elasticity, sound absorption effect, aging resistance, chemical medium resistance, etc. Its comprehensive mechanical properties far exceed those of PVC and other materials. The polymerization of polyurethane is very convenient. It can not only be polymerized under normal temperature and pressure, but also be directly foamed or sprayed or repaired after on-site polymerization.
Polyurethane foam plastics are widely used in automobile interior decoration and vibration absorbing parts, such as coating materials, polyurethane rigid plastic sheets, polyurethane elastomers, soft foam materials for seats, decorative parts, sofa leather, roof accessories; The most widely used is a variety of soft and hard polyurethane foam materials, which have vibration isolation, sound insulation, noise reduction, thermal insulation; PU can also be made into polyurethane coatings, adhesives, sealants, etc. for vehicles. The PU representative products on automobiles include instrument panel, rearview mirror, bumper, seat cushion, headrest, steering wheel, instrument panel anti vibration pad, pillar trim, front roof lining, window frame frame, ceiling and side roof frame decoration, door lining, sun visor, rear roof frame decoration, etc.
Special plastics Glass fiber reinforced plastics
Glass fiber reinforced plastic is based on the original pure plastic, adding glass fiber and other additives to improve the use of materials. Generally speaking, most of the glass fiber reinforced materials are used in the structural parts of automobile parts molds, which are structural engineering materials; Such as PP, ABS, PA66, PA6, PC, POM.