Processing technology and material analysis of plastic gear
TIME：2021-5-5
Plastic gears are developing towards larger size, more complex geometry and higher strength. At the same time, the composites filled with high-performance resin and long glass fiber play an important role.
Plastic gears have experienced a change process from new materials to important industrial materials in the past 50 years. Today, they have penetrated into many different application fields, such as automobiles, watches, sewing machines, structural control facilities and missiles, and play the role of transmitting torque and motion forms. In addition to the existing application fields, new and more difficult gear application fields will continue to appear, and this trend is still in-depth development.
The automotive industry has become one of the fastest growing areas of plastic gears, and this successful change is encouraging. Automobile manufacturers are trying to find various auxiliary systems for automobile drive. They need motors and gears rather than power, hydraulic pressure or cables. This change makes plastic gears deeply applied to many application fields, from lifting valve, seat, tracking headlights to brake actuator, electric throttle section, turbine mediation device and so on.
The application of plastic power gear is further expanded. In some application fields with large size requirements, plastic gears are often used to replace metal gears, such as plastic washing machine transmission, which changes the application limit of gears in size. Plastic gears are also used in many other fields, such as vibration damping drivers of ventilation and air conditioning systems (HVAC), valve drives in flow facilities, automatic cleaners in public lounges, power screws for controlling surface stability on small aircraft, screw weight instruments and control devices in military fields.
Large size and high strength plastic gear
Due to the advantages of plastic gear forming and the characteristics of larger, high precision and high strength, this is an important reason for the development of plastic gear. The development trend of early plastic gears was generally spur gears with a span of less than 1 inch and a transmission capacity of no more than 0.25 HP. Now gears can be made into many different structures. The transmission power is generally 2 horsepower and the diameter range is 4-6 inches. It is predicted that by 2010, the forming diameter of plastic gear can reach 18 inches and the transmission capacity can be increased to more than 10 horsepower.
How to design a gear configuration to maximize the transmission power and minimize the transmission error and noise is still facing many problems. This puts forward high machining accuracy requirements for the concentricity, tooth profile and other characteristics of gears. Some helical gears may require complex forming actions to manufacture the final product, while others require core teeth in thicker parts to reduce shrinkage. Although many molding experts use the latest polymer materials, equipment and processing technology to achieve the ability to produce a new generation of plastic gears, a real challenge for all processors is how to cooperate with the manufacture of this whole high-precision product.
Difficulties in control
The allowable tolerance of high-precision gears is generally difficult to describe as "good" as described by the American Plastics Industry Association (SPI). But today, most molding experts use the latest molding machine equipped with processing control unit to control the precision of molding temperature, injection pressure and other variables to form precision gears in a complex window. Some gear forming experts use more advanced methods. They place temperature and pressure sensors in the cavity to improve the consistency and repeatability of forming.
Precision gear manufacturers also need to use professional testing equipment, such as double tooth side rolling detector used to control gear quality, computer-controlled detector to evaluate gear tooth surface and other characteristics. But having the right equipment is only the beginning. Those molding companies trying to enter the precision gear industry must also adjust their molding environment to ensure that the gears they produce are as consistent as possible in each injection and cavity. Since the behavior of technicians is often the decisive factor in the production of precision gears, they must focus on the training of employees and the control of operation process.
Because the size of the gear is easily affected by seasonal temperature changes, and even the temperature fluctuation caused by opening the door to let a forklift pass can affect the dimensional accuracy of the gear, the molding manufacturer needs to strictly control the environmental conditions of the forming area. Other factors to be considered include a stable power supply, suitable drying equipment that can control polymer temperature and humidity, and a cooling unit with constant air flow. In some cases, automation technology is used to remove the gear from the forming position and place it on the transmission unit through a repeated action to achieve the consistency of cooling mode.
Important molding cooling steps
Compared with the requirements of general forming processing, the processing of high-precision parts needs to pay more attention to the details and the measurement technology required to achieve the accurate measurement level. This tool must ensure that the cavity molding temperature and cooling rate are the same for each molding. The most common problem in precision gear machining is how to deal with the problem of gear symmetry cooling and the consistency between die cavities.
The mold of precision gear generally has no more than 4 cavities. Since the first generation of molds produced only one gear, there was little specific description. Tooth inserts were often used to reduce the cost of secondary cutting.
Precision gears should be injected from a gate in the center of the gear. Multiple gates are easy to form fusion lines, change pressure distribution and shrinkage, and affect gear tolerance. For glass fiber reinforced materials, because the fibers are arranged radially along the welding line, it is easy to cause eccentric "collision" of radius when using multiple gates.
A molding expert can control the deformation at the tooth slot and obtain a product with controllable, consistent and uniform shrinkage capacity on the premise of good equipment, molding design, material extension capacity and processing conditions. During molding, it is required to accurately control the temperature, injection pressure and cooling process of the molding surface. Other important factors include wall thickness, gate size and location, filler type, dosage and direction, flow rate and molding internal stress.
The most common plastic gears are straight teeth, cylindrical worm gears and helical gears. Almost all gears made of metal can be made of plastic. Gears are usually formed by split die cavity. During helical gear processing, the gear or gear ring forming teeth must be rotated during injection, so attention should be paid to its details.
The noise generated during worm gear operation is smaller than that of straight teeth. After forming, it is removed by screwing out the cavity or using multiple sliding mechanisms. If a sliding mechanism is used, it must be operated with high accuracy to avoid obvious seam lines on the gear.
New process and new resin
More advanced plastic gear forming methods are being developed. For example, the secondary injection molding method designs an elastomer between the axle and the gear teeth to make the gear run more quietly. When the gear suddenly stops running, it can better absorb the vibration and avoid the damage of the gear teeth. The axle can be remolded with other materials, and composites with better flexibility or higher value and better self-lubricating effect can be selected. At the same time, gas assisted method and injection compression molding method are studied as a method to improve the quality of gear teeth, the overall accuracy of gear and reduce internal stress.
In addition to the gear itself, the shaper also needs to pay attention to the design structure of the gear. The position of the gear shaft in the structure must be arranged linearly to ensure that the gear runs in a straight line, even when the load and temperature change, so the dimensional stability and accuracy of the structure are very important. Considering this factor, the gear structure with certain rigidity should be made of glass fiber reinforced material or mineral filled polymer.
Now, in the field of precision gear manufacturing, the emergence of a series of engineering thermoplastics provides processors with more choices than before. Acetaldehyde, PBT, polyamide and other most commonly used materials can produce gear equipment with excellent fatigue resistance, wear resistance, smoothness, high tangent stress strength resistance and vibration load caused by reciprocating motor operation. The crystalline polymer must be formed at a sufficiently high temperature to ensure the full crystallization of the material. Otherwise, when in use, the material will undergo secondary crystallization due to the temperature rising above the forming temperature, resulting in the change of gear size.
Acetal, as an important gear manufacturing material, has been widely used in automobile, appliance, office equipment and other fields for more than 40 years. Its dimensional stability and high fatigue and chemical resistance can withstand temperatures up to 90 ℃. Compared with metal and other plastic materials, it has excellent lubrication performance.
PBT polyester can produce a very smooth surface without filling modification. Its maximum working temperature can reach 150 ℃, and the working temperature of glass fiber reinforced products can reach 170 ℃. Compared with acetal, other types of plastic and metal products, it works well and is often used in the structure of gears.
Polyamide materials, compared with other plastic materials and metal materials, have good toughness and durability. They are commonly used in turbine drive design, gear frame and other application fields. When polyamide gear is not filled, the operating temperature can reach 150 ℃, and the working temperature of glass fiber reinforced product can reach 175 ℃. However, polyamides have the characteristics of dimensional change caused by moisture absorption or lubricant, so they are not suitable for the field of precision gears.
The high hardness, dimensional stability, fatigue resistance and chemical resistance of polyphenylene sulfide (PPS) can reach 200 ℃. Its application is going deep into the application fields with harsh working conditions, automobile industry and other terminal applications.
The precision gear made of liquid crystal polymer (LCP) has good dimensional stability. It can withstand temperatures up to 220 ℃, has high chemical resistance and low molding shrinkage changes. Using this material, a formed gear with a tooth thickness of about 0.066 mm has been made, which is equivalent to 2 / 3 of the diameter of human hair.
Thermoplastic elasticity can make the gear run quieter, make the gear more flexible, and can well absorb the impact load. For example, a low power and high-speed gear made of copolyester thermoplastic elastomer can allow some deviation during operation and reduce operation noise when sufficient dimensional stability and hardness are guaranteed. An example of such an application is the gear used in the curtain driver.
In relatively low temperature, corrosive chemical environment or high wear environment, polyethylene, polypropylene and ultra-high molecular weight polyethylene have also been used in gear production. Other polymeric materials are also considered, but they are limited by many harsh requirements in gear applications, such as poor lubrication, chemical resistance and fatigue resistance of polycarbonate; ABS and LDPE materials usually can not meet the requirements of lubrication, fatigue resistance, dimensional stability, heat resistance and creep resistance of precision gears. Most of these polymers are used in the field of conventional, low load or low-speed gears.
Advantages of using plastic gears
Compared with plastic gears of the same size, metal gears operate well and have good dimensional stability when temperature and humidity change. However, compared with metal materials, plastics have many advantages in cost, design, processing and performance.
Compared with metal forming, the inherent design freedom of plastic forming ensures more efficient gear manufacturing. Plastic can be used to form internal gears, gear sets, worm gears and other products, which is difficult to form with metal materials at a reasonable price. The application field of plastic gears is wider than that of metal gears, so they promote the development of gears to bear higher load and transmit greater power. Plastic gear is also an important material to meet the requirements of low silent operation, which requires the emergence of materials with high precision, new tooth shape and excellent lubricity or flexibility.
Gears made of plastic generally do not need secondary processing, so compared with metal gears made of stamping parts and machine parts, the cost is reduced by 50% to 90%. Plastic gears are lighter and more inert than metal gears. They can be used in environments where metal gears are prone to corrosion and degradation, such as the control of water meters and chemical equipment.
Compared with metal gears, plastic gears can deflect and deform to absorb the impact load, and can better disperse the local load changes caused by shaft deflection and staggered teeth. The inherent lubrication characteristics of many plastics make them ideal gear materials for printers, toys and other low load operating mechanisms. Lubricants are not included here. In addition to operating in a dry environment, gears can also be lubricated with grease or oil.
Reinforcement of materials
In the description of gear and structural materials, the important role of fiber and filler on the properties of resin materials should be considered. For example, when acetal copolymer is filled with 25% short glass fiber (2mm or less), its tensile strength increases twice and its hardness increases three times at high temperature. The use of long glass fiber (10 mm or less) fillers can improve strength, creep resistance, dimensional stability, toughness, hardness, wear properties and more. Long glass fiber reinforcement is becoming an attractive alternative material in the field of large-size gear and structural applications because it can obtain the required hardness and good controllable thermal expansion performance.
(the article is reproduced from: engineering plastics application official account of WeChat)
