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Plastic gear parts injection molding process and material analysis

Plastic gear parts injection molding process and material analysis. Plastic gears are developing toward larger sizes, more complex geometries, and higher strength, with high-performance resins and long-glass fiber-filled composites playing an important role.

Plastic gears have experienced a change 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, etc., and play the role of transmitting torque and motion forms. In addition to the existing application fields, new and more difficult-to-machine gear application fields will continue to appear, and this trend is still developing in depth.

The automotive industry has become the fastest growing field of plastic gears, and this successful change is encouraging. Car manufacturers are working hard to find various auxiliary systems for vehicle drive. They need motors and gears rather than power, hydraulics or cables. This change has led to the penetration of plastic gears into many applications, from liftgates, seats, and tracking headlights to brake actuators, electric throttle sections, turbine adjusters, and more.

The application of plastic power gears has been further broadened. In some application fields with large size requirements, plastic gears are often used to replace metal gears, such as washing machine transmissions using plastic, which changes the application limits of gears in terms of size.

Plastic gears are also used in many other areas, such as vibration-damping drives in ventilation and air conditioning systems (HVAC), valve drives in mobile facilities, automatic sweepers in public restrooms, and power screws used to control surface stability on small aircraft. instruments, snail instruments and control devices in the military field.https://solidcomould.com/product/plastic-office-chair-headrest-mould-4/

Large, high-strength plastic gears

Due to the advantages of plastic gear molding and the ability to mold larger, high-precision and high-strength features, this is an important reason for the development of plastic gears.

How to design a gear configuration that maximizes transmission power while minimizing transmission errors and noise still faces many problems. This places high requirements on the concentricity, tooth shape and other characteristics of the gear.

Some helical gears may require complex forming operations to create the final product, and other gears may require the use of core teeth in thicker sections to reduce shrinkage. While many molding experts have used the latest polymer materials, equipment and processing techniques to achieve the ability to produce a new generation of plastic gears, a real challenge for all processors will be how to manufacture this entire high-precision product.

Difficulties in control

The tolerances allowed for high-precision gears are generally difficult to describe as “good” as described by the Society of the Plastics Industry (SPI). But most molding experts today use the latest molding machines equipped with process control units that control the precision of molding temperature, injection pressure, and other variables on a complex window to mold precision gears. Some gear molding experts use a more advanced approach, placing temperature and pressure sensors in the mold cavity to improve molding consistency and repeatability.

Manufacturers of precision gears also need to use specialized inspection equipment, such as double-tooth flank roll detectors to control gear quality, and computer-controlled detectors to evaluate gear tooth surfaces and other characteristics. But having the right equipment is just the beginning.

Those molders trying to get into the precision gear industry must also adjust their molding environment to ensure that the gears they produce are as consistent as possible, every shot, every cavity. Since the behavior of technicians is often the decisive factor when producing precision gears, they must focus on employee training and control of the operating process.

Since the dimensions of gears are easily affected by seasonal temperature changes, and even temperature fluctuations caused by opening a door to let a forklift pass through can affect the dimensional accuracy of gears, molders need to strictly control the environmental conditions in the molding area.

Other factors to consider include: a stable power supply, suitable drying equipment to control polymer temperature and humidity, and cooling units with constant air flow. In some cases, automation technology is used to remove the gear from the molded position and place it on the transfer unit through a repeated action to achieve a consistent cooling method.

Important molding cooling steps

Compared with the requirements of general molding processing, the processing of high-precision parts requires attention to more details and the measurement technology required to achieve accurate measurement levels. This tool must ensure that the molding temperature and cooling rate in the cavity are the same for each molding. The most common problems in precision gear machining are how to deal with symmetrical gear cooling and consistency between mold cavities.https://doi.org/10.1016/j.polymertesting.2023.107982

Precision gear molds generally have no more than 4 cavities. Since the first generation of molds produced only one gear, with few specific instructions, gear tooth inserts were often used to reduce the cost of secondary cutting.

Precision gears should be injected from a gate located in the center of the gear. Multiple gates can easily form fusion lines, change pressure distribution and shrinkage, and affect gear tolerances. For glass fiber reinforced materials, since the fibers are arranged radially along the welding line, it is easy to cause eccentric “collision” of the radius when using multiple gates.

A molding expert can control the deformation of the tooth grooves and obtain products with controllable, consistent, and uniform shrinkage capabilities based on good equipment, molding design, stretch capabilities of the materials used, and processing conditions. During molding, precise control of the temperature, injection pressure and cooling process of the molding surface is required.

Other important factors include wall thickness, gate size and location, filler type, amount and direction, flow rate and molding internal stress.

The most common plastic gears are spur gears, cylindrical worm gears, and helical gears, and almost all gears made of metal can be made of plastic. Gears are often formed using split mold cavities. When processing helical gears, the gear or the gear ring forming the teeth must be rotated during injection, so attention to detail is required.

The noise generated by the worm gear during operation is smaller than that of spur gears. After molding, it is removed from the mold cavity by unscrewing it or using multiple sliding mechanisms. If a sliding mechanism is used, it must be operated with high precision to avoid obvious seam lines on the gears.

New processes and new resins

More advanced plastic gear forming methods are being developed. For example, the secondary injection molding method makes the gear run quieter by designing an elastomer between the wheel shaft and the gear teeth. When the gear suddenly stops running, it can better absorb vibration and avoid damage to the gear teeth.

The axle can be re-molded with other materials, choosing a more flexible or higher-value, self-lubricating composite. Gas-assisted methods and injection compression molding methods were also studied as a method to improve gear tooth quality, overall gear accuracy, and reduce internal stress.

In addition to the gear itself, molders also need to pay attention to the design structure of the gear. The position of the gear shaft in the structure must be linearly arranged to ensure that the gears run in a straight line, even under load and temperature changes, so the dimensional stability and accuracy of the structure are very important. Taking this factor into account, materials such as glass fiber reinforced materials or mineral-filled polymers should be used to make gear structures with a certain degree of rigidity.

Now, in the field of precision gear manufacturing, the emergence of a series of engineering thermoplastics provides processors with more choices than before. The most commonly used materials, such as acetal, PBT and polyamide, can produce excellent fatigue resistance, wear resistance, smoothness, high tangential stress strength resistance, and can withstand vibration loads caused by reciprocating motor operation, etc. Gear equipment.

Crystalline polymers must be molded at a high enough temperature to ensure full crystallization of the material. Otherwise, when the temperature rises above the molding temperature during use, secondary crystallization of the material will occur, resulting in changes in gear dimensions.

Acetal, as an important gear manufacturing material, is widely used in automobiles, appliances, office equipment and other fields, with a history of more than 40 years. Its dimensional stability and high fatigue and chemical resistance can withstand temperatures up to 90°C and above. It has excellent lubrication properties compared to metal and other plastic materials.

PBT polyester can produce a very smooth surface. Its maximum working temperature can reach 150℃ without filling modification, and the working temperature of glass fiber reinforced products can reach 170℃. It performs well compared to products made from acetal, other types of plastics, and metal materials and is often used in the construction of gears.

Polyamide materials, compared with other plastic materials and metal materials, have good toughness and durability, and are often used in turbine transmission design and gear frames and other applications. The operating temperature of polyamide gears without filling can reach 150°C, and the operating temperature of glass fiber reinforced products can reach 175°C. However, polyamides have the characteristics of absorbing moisture or lubricants and causing dimensional changes, making them unsuitable for use in the field of precision gears.

The high hardness, dimensional stability, fatigue resistance and chemical resistance of polyphenylene sulfide (PPS) can reach 200°C. Its applications are penetrating into applications with demanding working conditions, the automotive industry and other end uses.

Precision gears made of liquid crystal polymer (LCP) have good dimensional stability. It can withstand temperatures up to 220°C, has high chemical resistance and low molding shrinkage changes. This material has been used to create molded gears with a tooth thickness of about 0.066 mm, which is equivalent to 2/3 the diameter of a human hair.

Thermoplastic elastomers can make gears run quieter, make gears more flexible, and can absorb impact loads well. For example, a low-power, high-speed gear made of copolyester thermoplastic elastomer can allow some deviation during operation and reduce operating noise when sufficient dimensional stability and hardness are ensured. An example of such an application is the gears used in curtain actuators.

Materials such as polyethylene, polypropylene and ultra-high molecular weight polyethylene have also been used in gear production where temperatures are relatively low, corrosive chemical environments or high-wear environments. Other polymeric materials were also considered, but were subject to many stringent requirements in gear applications;

Polycarbonate has poor lubrication performance, chemical resistance and fatigue resistance; ABS and LDPE materials usually cannot meet the performance requirements of precision gears such as lubrication performance, fatigue resistance, dimensional stability, heat resistance and creep resistance. Such polymers are mostly used in conventional, low-load or low-speed gear applications.

Advantages of using plastic gears

Metal gears perform well and have good dimensional stability with changes in temperature and humidity compared to plastic gears of the same size. But plastics have many advantages over metal materials in cost, design, processing and performance.

The inherent design freedom of plastic molding ensures more efficient gear manufacturing compared to metal molding. Products such as internal gears, gear sets, worm gears, etc. can be molded from plastic, which are difficult to mold using metal materials at a reasonable price. Plastic gears are used in a wider range of applications than metal gears, so they push the development of gears that can withstand higher loads and transmit more power.

Plastic gears are also an important material that meets low-quiet operation requirements, which requires the emergence of high-precision, new tooth shapes and materials with excellent lubricity or flexibility.

Gears made of plastic generally do not require secondary processing, so compared to stamped parts and machined metal gears, the cost is guaranteed to be reduced by 50% to 90%. Plastic gears are lighter and more inert than metal gears, and 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 effects of impact loads, and can better disperse local load changes caused by shaft deflection and misaligned teeth. The inherent lubricating characteristics of many plastics make them ideal gear materials for printers, toys and other low-load operating mechanisms, lubricants not included here. In addition to operating in a dry environment, gears can also be lubricated with grease or oil.

Material reinforcement

The important role of fibers and fillers on the properties of resin materials should be taken into account in the specification of gear and structural materials. For example, when acetal copolymer is filled with 25% short glass fiber (2mm or smaller) filler, its tensile strength increases by 2 times and the hardness increases by 3 times at high temperatures.

Using 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 for large gear and structural applications because it provides the required stiffness and good controlled thermal expansion properties.

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