Processing technology and material analysis of plastic gears

The application of plastic power gears has been further expanded. In some large-size applications, plastic gears are often used to replace metal gears, such as plastic washer drives, which change the size of the gears. Plastic gears are also used in many other applications, such as ventilation and ventilation systems for ventilation and air conditioning systems (HVAC), valve transmissions in mobile installations, automatic sweepers in public restrooms, and control-stable dynamic spirals for small aircraft. Instruments, snails in the military field, and controls.

Large size, high strength plastic gear

This is an important reason for the development of plastic gears due to the advantages of plastic gear shaping and the ability to form larger, higher precision and higher strength. Early plastic gear development trends were generally spur gears with a span of less than 1 inch and a transmission capacity of no more than 0.25 horsepower. Gears can now be made in many different configurations, with a transmission power of 2 horsepower and a diameter range of 4-6 inches.

How to design a gear configuration, to maximize transmission power while minimizing transmission errors and noise, also faces many challenges. This imposes high machining accuracy requirements on the concentricity, tooth profile and other characteristics of the gear. Some helical gears may require complex forming motions to make the final product, and other gears require core teeth to reduce shrinkage in thicker sections. Although many molding experts have used 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 match this high precision. product.

Difficulties in control

The tolerances allowed for high-precision gears are generally difficult to describe as "good" as described by the American Plastics Industry Association (SPI). But today most molding experts use the latest molding machines with machining control units to control the precision of the forming temperature, the injection pressure and other variables on a complex window to shape the precision gears. Some gear forming experts use more advanced methods in which they place temperature and pressure sensors in the cavity to improve the consistency and repeatability of the molding.

Manufacturers of precision gears also need to use specialized inspection equipment such as a two-toothed side roll detector for controlling gear quality, a computer-controlled detector for evaluating gear tooth faces and other features. But having the right equipment is just the beginning. The molders who are trying to enter the precision gear industry must also adjust their molding environment to ensure that the gears they produce are as uniform as possible for each injection and every cavity. Since the behavior of mechanics is often a decisive factor in the production of precision gears, they must focus on the training and control of the employees.

Since the size of the gear is easily affected by the seasonal temperature change, even if the door is opened to allow the temperature fluctuation of a forklift to affect the dimensional accuracy of the gear, the mold manufacturer needs to strictly control the environmental conditions of the forming zone. Other factors to consider include a stable power supply, suitable drying equipment to control polymer temperature and humidity, and a cooling unit with constant airflow. In some cases, the use of automation technology, through a repeated action, the gear is removed from the molding position and placed on the transfer unit to achieve the same cooling method.

Important molding cooling steps

The processing of high-precision parts requires more attention to detail and the measurement techniques required to achieve accurate measurement levels. This tool must ensure that the 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 symmetry cooling of the gears and the consistency between the cavities.

The molds of precision gears generally do not exceed 4 cavities. Since the first generation of molds produced only one gear, there are few specific instructions, and gear inserts are often used to reduce the cost of secondary cutting.

The precision gear should be injected from a gate at the center of the gear. Multiple gates tend to form weld lines, changing pressure distribution and shrinkage, affecting gear tolerances. For glass reinforced materials, since the fibers are radially arranged along the weld line, the eccentric "collision" of the radius is liable to occur when multiple gates are used.

A molding expert can control the deformation at the cogging, and obtain a controllable, consistent, uniform shrinkage product based on good equipment, molding design, material stretching ability and processing conditions. At the time of molding, it is required to precisely control the temperature of the molding surface, the injection pressure, and the cooling process. Other important factors include wall thickness, gate size and location, filler type, amount and direction, flow rate, and forming internal stress.

The most common plastic gears are spur gears, cylindrical worm gears and helical gears. Almost all metal-made gears can be made of plastic. Gears are often formed by splitting the cavity. When the helical gear is machined, the gear or the gear ring that forms the tooth must be rotated during injection, so attention should be paid to the details.

The worm gear generates less noise than the straight teeth. After molding, it is unscrewed from the cavity or removed by multiple sliding mechanisms. If a sliding mechanism is used, high precision must be used to avoid significant seams on the gears.

New process and new resin

More advanced plastic gear shaping methods are being developed. For example, the secondary injection molding method, by designing an elastic body between the axle and the teeth, makes the gear run quieter, and when the gear suddenly stops running, it can absorb vibration better and avoid tooth damage. The axles can be remolded with other materials, and composites with better flexibility or higher value and better self-lubricating properties can be selected. At the same time, gas-assisted method and injection compression molding method were studied as a method to improve the quality of gear teeth, the overall accuracy of gears, and reduce internal stress.

In addition to the gear itself, the molder needs to pay attention to the design of the gear. The position of the gear shafts in the structure must be linearly arranged to ensure that the gears run in line, even in the case of load and temperature changes, so the dimensional stability and accuracy of the structure is very important. In consideration of this factor, a material having a certain rigidity should be formed using a material such as a glass fiber reinforced material or a mineral-filled polymer.

Nowadays, in the field of precision gear manufacturing, the emergence of a series of engineering thermoplastics gives processors more choices than ever before. The most commonly used materials such as acetal, PBT and polyamide can produce excellent fatigue resistance, wear resistance, smoothness, high tangential stress strength, and can withstand vibration loads such as reciprocating motor operation. Gear equipment. For the crystalline polymer, it must be molded at a sufficiently high temperature to ensure sufficient crystallization of the material. Otherwise, the temperature of the material rises above the molding temperature during use, and the material undergoes secondary crystallization to cause a change in the gear size.

As an important gear manufacturing material, acetal has been widely used in automobiles, appliances, 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 °C. It has excellent lubricity compared to metals and other plastic materials.

PBT polyester can produce a very smooth surface, without filling modification, its maximum working temperature can reach 150 °C, and the glass fiber reinforced product can reach 170 °C. It works well compared to products of acetal, other types of plastics, and metallic materials, and is often used in the construction of gears.

Polyamide materials, which have good toughness and durability compared to other plastic and metal materials, are commonly used in applications such as turbine drive designs and gear frames. When the polyamide gear is not filled, the operating temperature can reach 150 °C, and the glass fiber reinforced product can reach the working temperature up to 175 °C. However, polyamides have the characteristics of moisture absorption or lubricant 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 reaching into demanding applications, automotive and other end uses.

Precision gears made of liquid crystal polymer (LCP) are dimensionally stable. It can withstand temperatures up to 220 ° C, with high chemical resistance and low molding shrinkage. A molded gear having a tooth thickness of about 0.066 mm has been produced using this material, which is equivalent to 2/3 of the diameter of a human hair.

Thermoplastic elastomers make gears run quieter, making gears more flexible and absorbing shock loads. For example, a low-power, high-speed gear made of a thermoplastic elastomer of a copolyester type allows for some deviation in operation while reducing sufficient running noise while ensuring sufficient dimensional stability and hardness. An example of such an application is a gear used in a curtain drive.

Materials such as polyethylene, polypropylene and ultra high molecular weight polyethylene have also been used in gear production in relatively low temperature, corrosive chemical environments or high wear environments. Other polymeric materials are also considered, but are subject to many stringent limitations in gear applications, such as polycarbonate lubricity, chemical resistance and fatigue resistance; ABS and LDPE materials generally do not meet the lubrication requirements of precision gears. Performance, fatigue resistance, dimensional stability, and heat and creep resistance. Most of such polymers are used in the field of conventional, low-load or low-speed gears.

Advantages of using plastic gears

Compared to plastic gears of the same size, metal gears perform well and have dimensional stability when temperature and humidity change. But compared to metallic materials, plastics have many advantages in terms of cost, design, processing and performance.

The inherent design freedom of plastic molding guarantees more efficient gear manufacturing than metal forming. Plastics can be used to shape internal gears, gear sets, worm gears, etc., which is difficult to mold at a reasonable price. Plastic gear applications are wider than metal gears, so they drive gears toward higher loads and more power. Plastic gears are also an important material for low-quiet operation requirements, which requires high precision, new tooth profile and materials with excellent lubricity or flexibility.

Plastic-made gears generally do not require secondary machining, so a 50% to 90% reduction in cost is guaranteed relative to stamped and machined metal gears. Plastic gears are lighter and more inert than metal gears and can be used in environments where metal gears are susceptible 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 impact loads, and can better disperse local load changes caused by shaft skew and misalignment. The inherent lubricity of many plastics makes them ideal gears for printers, toys and other low-load operating mechanisms, not including lubricants. In addition to operating in a dry environment, the gears can be lubricated with grease or oil.

Material enhancement

In the description of gears and structural materials, the important role of fibers and fillers in the properties of resin materials should be considered. For example, when an acetal copolymer is filled with a 25% short glass fiber (2 mm or smaller) filler, its tensile strength is increased by 2 times at a high temperature and the hardness is increased by 3 times. Long glass (10 mm or smaller) fillers increase strength, creep resistance, dimensional stability, toughness, hardness, wear properties, and more. Long glass fiber reinforced materials are becoming an attractive candidate for large gear and structural applications because of the required hardness and good controlled thermal expansion.