Alexei Volkov
Plastic gears are toothed wheels made from advanced engineering plastics, used to alter the speed ratio between a motor and its connected components. They are increasingly being used in place of metal gears due to their lightweight design, cost-efficiency, and smooth and quiet operation. The two main categories of plastic materials used for gear manufacture are thermosets and thermoplastics. Thermoplastics, such as nylons, acetals, and polyesters, are commonly used for plastic gears due to their self-lubricating properties and resistance to corrosion and heat. Additives are often introduced to improve the performance of plastic gears, such as mica, carbon powders, and glass beads. Injection molding and hobbing are popular techniques for manufacturing plastic gears, each offering advantages in terms of accuracy, flexibility, and cost. However, plastic gears also face challenges, such as lower strength and stiffness compared to metal gears, and susceptibility to temperature variations.
| Characteristics | Values |
|---|---|
| Plastic gear manufacturing techniques | Hobbing, injection molding, rapid prototyping |
| Plastic gear advantages | Lightweight, smooth and quiet operation, wear and corrosion resistance, low cost, self-lubrication, extended life spans |
| Plastic gear disadvantages | Weaker than metal, inconsistent fit between gear meshing due to heat or moisture, high tooling costs, fluctuating raw material costs |
| Plastic gear applications | Printers, computer memory devices, robots, toys, electronic devices, micromotors, clocks, power tools, automotive components, medical instruments, high-speed and high-torque transmission |
| Plastic gear materials | Nylon, polyacetal, polyphenylene sulfide, polyamides, polyurethanes, phenolics, laminated phenolics, acetals, polyesters, polycarbonates, polytetrafluoroethylene, polyamide resins |
| Additives | Mica, carbon powders, kevlar, glass beads, fibers, graphite, ethylene vinyl acetate, acrylics |
| Gear noise reduction techniques | Two-shot molding, elastomer between hub and teeth, cored teeth, long fiber reinforced plastics, digital acoustic monitoring |
| Gear design considerations | Modulus, fatigue strength, tensile strength, creep resistance, dimensional stability, power, lubrication |
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Nylon plastics
Nylon is a generic name for a group of synthetic polymers first developed by DuPont in 1935. It was the first commercially successful synthetic thermoplastic, first used in nylon-bristled toothbrushes. The name 'nylon' and the prefix 'PA' (polyamide) are used interchangeably. The properties of nylons are often modified by blending with a variety of additives.
There are many types of nylons, typically described by numbers like 6, 6/6, etc. These numbers refer to the number of methyl groups on each side of the nitrogen atoms in their molecular structures. The most common types available in sheet, rod, and tube form are Nylon 6 and Nylon 6/6, which have very similar mechanical, thermal, and electrical properties. Nylon 6 is easy to dye, has greater elasticity, and has a higher impact resistance.
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Acetal plastics
Acetal, also known as polyacetal, polyoxymethylene (POM) or polyformaldehyde, is a semi-crystalline, engineered thermoplastic. It is a popular material for plastic gears, which are toothed wheels used to alter the speed ratio between a motor and its connected components.
Acetal is a good alternative to metals in mechanical gears due to its excellent dimensional stability, machining profile, and ability to handle heat. It is also lightweight, smooth, quiet, wear-resistant, and corrosion-resistant. Acetal plastic gears can be manufactured at a relatively low cost and can be easily reproduced using injection moulding.
Acetal plastic has a melting point that varies by type, but it generally operates effectively between -40°F to 180°F (-40°C to 82°C). It will remain solid until its melting point is reached, at which point it becomes a thin liquid that flows easily.
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Polyacetal
There are six different grades of polyacetal, each addressing various industrial requirements. These grades can provide features such as static dissipation, UV resistance, and enhanced lubricity, while retaining fundamental characteristics like strength, stiffness, and excellent machinability.
The manufacturing process for plastic gears, including those made of polyacetal, typically involves techniques such as hobbing, injection molding, rapid prototyping, and CNC machining. Injection molding, for example, involves injecting molten plastic under pressure into a steel mold with precise dimensions to shape the gear.
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Polyphenylene sulfide
One of the key advantages of PPS is its ability to close the gap in price and performance between standard and advanced polymers. PPS plates, rods, and tubes are highly dimensionally stable, both during machining and operation. This stability makes PPS an excellent choice for applications requiring outstanding bearing and wear performance, as well as exposure to aggressive chemicals and high temperatures. PPS can be used as a lower-cost alternative to materials like PEEK, which may be over-engineered for certain applications, or metal, which may be too heavy.
The heat deflection temperature (HDT) of PPS is impressive, with the plastic capable of withstanding temperatures up to 115°C without deforming under a load of 1.8 MPa (264 PSI). PPS parts can continuously operate at a service temperature of 220°C or 425°F for up to 20,000 hours in air. The material also exhibits higher thermal conductivity than PEEK but falls short when compared to PE and PTFE materials. PPS is inherently flame-resistant and can achieve UL 94 V-0 flammability ratings without requiring additional fillers or additives.
Reinforced PPS grades, such as those with glass-fiber reinforcement, can have a density as high as 1.66 g/cm³, while unfilled PPS typically has a density of around 1.35 g/cm³. PPS offers a lower coefficient of linear thermal expansion (CLTE) than similarly priced engineering plastics like PET and POM, making it a cost-effective alternative in applications requiring dimensional stability at moderate-to-high temperatures. The creep behavior of PPS is also noteworthy, as it can withstand high temperatures for extended periods without deformation.
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Injection molding
The injection molding process involves melting plastic resin and injecting it into a mold, where it cools and hardens into the desired shape. The precision of the process is critical, as inconsistencies in process parameters like time, injection speed, pressure, and shot size can affect the final product. For example, the cooling rate of the product can impact shrinkage and, therefore, precision.
The material used in the injection molding process is an important factor in the success of plastic gear manufacturing. Common materials used for injection molding include nylon, polycarbonate, polyphenylene sulfide, and poly(methyl methacrylate) (PMMA), also known as acrylic. These materials offer advantages such as high strength, impact resistance, and optical clarity.
Nylon, for example, comes in four main grades, each with slightly different mechanical properties. Nylon 11 is used in outdoor applications and has greater resistance to dimensional changes, while Nylon 12 has the lowest melting point of all four grades. Nylon 46 has the highest operating temperature, and Nylon 66 has improved temperature resistance and lower rates of water absorption compared to standard nylon.
Another important consideration in injection molding is the melt flow rate, which is a property that appears on most property data sheets. The melt flow rate is used to gauge how a material will flow during molding, and it is related to the average molecular weight of the polymer. Higher molecular weight materials have improved properties, such as impact resistance and barrier properties.
Overall, injection molding is a critical process in the manufacturing of plastic gears, allowing for the production of complex parts with high precision and strength. The choice of material and attention to process parameters are key factors in the success of injection molding.
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Frequently asked questions
Plastic gears are made from advanced engineering plastics, including nylon, a form of polyacetal and polyamide resin, and polyacetal. Additives such as mica, carbon powders, kevlar, glass beads, and fibres can be added to improve performance.
Plastic gears are lightweight, smooth, quiet, and resistant to wear and corrosion. They are also self-lubricating and have extended lifespans. They are more cost-efficient than metal gears and can be easily manufactured.
Plastic gears are generally weaker than metal gears and cannot withstand the same loads or stresses. They are also susceptible to changes in temperature and moisture, which can cause dimensional shifts and inconsistent fits.
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