In the field of mechanical transmission, gears, as core components for transmitting power and motion, have long garnered significant attention. Plastic gears, with their unique performance and advantages, are gradually emerging in numerous applications and becoming an indispensable part of modern industry.
Plastic gears offer many advantages over metal gears. Compared to metal gears, they are lighter, have lower inertia, and operate more quietly. Plastic gears typically require no lubrication, or they can be lubricated internally with lubricants such as PTFE or silicone oil. Plastic gears are generally lower in unit cost than metal gears and can be designed with other assembly features in mind. Furthermore, these gears can be used in many corrosive environments.
The figure on the right shows the PTFE’s lubricating effect on various resins.
The Most Common Plastic Gears Are:
| Spur gears | Straight Bevel |
| Helical gears | Herringbone & Double Helical |
| Worm gears | Worms |
| Spline and Shafting | Bevel Gears |
Material Properties of Plastic Gears
Plastic gears are typically made of engineering plastics, with common materials including polyoxymethylene (POM), nylon (PA), polycarbonate (PC), polyester (PBT), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK). These materials possess many properties not found in metals. Polyoxymethylene offers excellent overall performance, including high strength and stiffness, good wear resistance, and a low coefficient of friction. This makes gears made with it smoother during transmission and reduces energy loss. Polyamide, on the other hand, offers excellent toughness, fatigue resistance, and self-lubrication, making it an excellent choice for applications requiring high toughness. Polycarbonate, with its high strength, high transparency, and excellent dimensional stability, is suitable for applications requiring visibility of transmission status or requiring a specialized appearance.
POM is the most common and important plastic gear material and is generally the preferred material for plastic gears. POM offers excellent physical properties, including wear resistance, fatigue resistance, high stiffness, and good chemical stability, electrical insulation, and dimensional stability. However, due to its high shrinkage and low heat deformation temperature, POM gears are not suitable for applications with high environmental concerns.
Nylon 66 (PA66) and nylon 46 (PA46) offer excellent toughness and durability, and modified PA materials, in particular, offer superior mechanical properties. However, PA’s strong hygroscopicity can cause changes in the performance and dimensions of plastic gears, making PA gears less suitable for use in environments with high humidity.
PC offers excellent impact resistance, high hardness, low shrinkage, low water absorption, and good dimensional stability. However, because PC is not self-lubricating and is susceptible to wear, PC gears are primarily used in toys with short service lives and light loads.
PBT offers high mechanical strength, heat resistance, and corrosion resistance, and its smooth surface provides excellent mechanical transmission performance, but its notched impact strength is relatively low.
PPS offers high hardness, good dimensional stability, fatigue resistance, and chemical resistance, allowing for long-term use at temperatures above 200°C.
PEEK, a semi-crystalline polymer, is a top-tier material for plastic gears. PEEK not only offers high-temperature resistance, high comprehensive mechanical properties, wear resistance, and chemical resistance, but also low water absorption, high toughness, and impact resistance. Due to its high price, PEEK gears are primarily used in aircraft and military applications.
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Advantages & Disadvantages of Plastic Gears
Advantages of Plastic Gears:
First, plastic gears offer a significant advantage in terms of light weight. Compared to metal gears, plastic gears have a much lower density. In weight-sensitive applications such as aerospace and automotive, using plastic gears can effectively reduce overall weight, thereby improving energy efficiency and reducing energy consumption. For example, in drones, transmission systems using plastic gears can reduce weight and increase flight time.
Second, they offer low noise levels. Plastic gears generate much less noise than metal gears during transmission. This is because plastic has a certain degree of elasticity, which acts as a buffer during meshing, reducing noise caused by collisions. Plastic gears are particularly advantageous in applications such as office equipment and household appliances, where noise control is crucial. For example, products like printers and robot vacuums can achieve quieter operation.
Third, they offer strong corrosion resistance. Plastics are resistant to most chemicals, including acids, alkalis, and salts, and are not susceptible to rust and corrosion like metal gears. In harsh working environments, such as chemical production equipment and marine equipment, plastic gears provide more stable operation and extend the life of the equipment.
Furthermore, plastics offer high design flexibility. Through processes like injection molding, gears of various complex shapes can be created to meet the needs of diverse transmission systems. Furthermore, plastic gears can be integrally molded with other plastic components, reducing assembly steps and improving production efficiency.
Disadvantages of Plastic Gears:
Plastic gears offer advantages such as low noise, corrosion resistance, low inertia, low manufacturing cost, and the ability to operate without lubrication. However, their low elastic modulus, low mechanical strength, poor thermal conductivity, and high thermal expansion coefficient limit their use in certain high-load, high-speed, and high-temperature environments.
Common failure modes for plastic gears are fracture and wear (see Figure 8). A fracture near the tooth root is mostly caused by overload or fatigue exceeding the material’s fatigue limit. Fracture or wear near the pitch is mostly due to the material’s poor thermal resistance. The combined effects of temperature rise caused by tooth friction during gear meshing and mechanical loads lead to severe wear of the tooth surface. Excessive center-to-center distance can also lead to fracture or wear near the pitch. Excessive wear leading to tooth thinning is primarily due to a lack of lubrication or adhesive wear between the contact surfaces, such as particles or wear debris.
Gear Manufacturing and Gear Precision:
The manufacturing of plastic gears differs significantly from that of metal gears. Although many current plastic gears borrow design philosophies from metal gears, these philosophies often fail to translate well into plastic gear designs.
- Undercutting: Plastic gears are injection molded, and undercutting does not occur during the manufacturing process. After all, wire EDM is a contour-based machining method, distinct from metal hobbing.
- Module: Metal gears have a complete set of standard modules, established for ease of manufacturing, allowing for the use of a single tool to produce as many different gears as possible. However, with plastic molds, a gear mold can only be used to mold one type of gear, and the manufacturing process is wire EDM. Therefore, the module can be freely set; simply draw the tooth profile and the gear can be manufactured, regardless of whether the module is standard.
- Displacement: Displacement for metal gears was developed from an installation perspective, based on the divisibility of the involute. This concept of displacement leads to other issues, such as strength and undercutting, after displacement. This concept is based on simplifying metal into a rigid body model. However, the various properties of plastic gears are fundamentally different from those of metal. The impact of plastic gear deformation primarily affects assembly, followed by strength.
- Pressure Angle: Currently, the typical pressure angle is 20°, 14.5°, or 22.5°. However, considering the significant deformation of plastic, even if the design pressure angle is 20°, the actual meshing pressure angle of a plastic gear will not be exactly 20°. The influence of deformation on the pressure angle is likely to be significant, making it unnecessary to simply simplify the plastic gear into a rigid body model for analysis. Therefore, considering the actual pressure angle is the most practical consideration.
- Internal Tooth Parameter Calculation: The internal tooth parameters of metal gears are calculated based on tool parameters. However, for plastic gears, tooling issues are not involved and they are also wire-cut. The calculation formulas in this case are fundamentally different from those for metal.
Precise injection molded thermoplastic gears require precise molds. During the gear molding process, alignment of the mold and cavity bore is crucial. Interlocking devices between the molds are recommended to eliminate loose fits in the guide system.
Air-hardened steels are preferred over oil-hardened steels due to their higher dimensional stability during heat treatment. Additionally, steels with higher carbon content (for greater overall hardness) and higher chromium content (for better wear resistance) are recommended for tight tolerance designs.
For better tolerance control, a combination of H-13 or A-2 steel and D-2 steel is recommended for gate cores, mandrels, and other high-wear areas of the component.
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Plastic Mold Cavity Design
The cavity design of plastic gear molds has long been considered a technical challenge in the mold industry.
This is primarily due to two reasons.
First, it’s difficult to accurately determine the shrinkage rate of plastic. During the plastic gear molding process, plastic is transformed from granular solid raw material at high temperatures into a molten plastic liquid, which is then cooled to form a solid plastic gear product. The shrinkage rate of plastic during this process varies, making it difficult to accurately determine the value.
Second, the mold cavity experiences nonlinear shrinkage calculations. For involute small-module plastic gear molds, the mold cavity is effectively a hypothetical gear. This hypothetical gear is neither a displacement gear nor an internal gear. After shrinking, it becomes the desired plastic gear. The shrinkage of this hypothetical gear on its involute tooth profile differs from the isotropic shrinkage of typical plastic parts.
On the gear plane, the shrinkage in the x and y directions differs, representing nonlinear shrinkage, as shown in the Figure on the right. This nonlinear shrinkage significantly increases the difficulty of designing the cavity for involute plastic gear molds.
Faced with this technical challenge, using the conventional isotropic shrinkage method for plastic parts to design mold cavities is unlikely to yield satisfactory results. Based on years of practical experience in plastic gear mold making at TONGDA LINK company, we recommend using the variable module method for theoretical gear mold cavity design, based on accurate estimation of plastic shrinkage.
This method then ensures accurate and reasonable mold cavities through tooth profile modification. The variable module method posits that during each gear manufacturing process, the base diameter, pitch diameter, addendum diameter, and root diameter remain constant, increasing or decreasing proportionally, similar to the radial dimensional variation of simple sleeve-like parts.
For the gear pitch circle, the formula d=mz indicates that it depends solely on the module m and the number of teeth z. For a specific gear, since its number of teeth is fixed, changes in the pitch diameter during manufacturing can be considered changes in the module. This principle indicates that the space enclosed by the plastic gear mold cavity is a hypothetical gear with a constant module and pressure angle, and its groove represents the cavity’s tooth profile.
The module of this hypothetical gear can be calculated using the proportional method, using the formula: m’ = (1 + η%)m. Here, m’ is the module of the mold cavity tooth profile; m is the theoretical module of the designed gear; and η% is the plastic shrinkage rate. Substituting the module m’ into the corresponding gear calculation formula yields the hypothetical gear for the mold cavity.
TONGDA LINK Practice has proven that gear mold cavities designed using the variable module method can effectively address the nonlinear shrinkage problem of involute tooth profiles, as shown in the mold cavity product diagram below.
Gate Design
When molding plastic gears, gate location significantly affects gear accuracy, particularly radial runout. The gate distribution pattern has a significant impact on the overall mechanical properties of the plastic gear. The optimal gate type for injection-molded gears is a disc or diaphragm gate.
The figure on the right shows a mold filling analysis for a very simple gear with single and multiple disc-shaped gates. A disc-shaped gate provides completely uniform flow in the radial direction, without weld lines. This results in uniform shrinkage in all directions. Because this type of gate is often impractical in actual gear production, gates are often placed on the spokes of the gear.
When gates are placed on the spokes, it is best to use multiple gates evenly distributed across the gear. When a single gate is used, the plastic must flow around a central core pin. This creates a thin weld line near the core pin, after which the plastic flow front moves away from the center. This flow pattern results in a high degree of fiber orientation in the radial direction on the side of the gear opposite the gate.
When designing the gates for a plastic gear mold using a three-point gating method, it’s best to have these three points located on the same arc and evenly spaced, as shown in the Figure “gating” on the right above. With three-point balanced gating, the plastic melt flows radially from the gate, forming three weld lines where the flow fronts converge. At the weld lines, the fibers tend to be oriented parallel to the flow fronts. In gears, this results in a radial distribution of fibers at the weld line, while the rest of the gear is randomly distributed. This creates a low-shrinkage area along the weld line. The difference in fiber orientation between the weld line and the rest of the gear is smaller than with a single-gated gear, resulting in higher gear accuracy. The Figure on the right shows the fiber orientation and filling pattern for a single-point eccentric gate and a three-point evenly spaced gate. Multi-gating systems more easily achieve the concentric, uniform flow conditions typically found in disc-gated gears.
Schematic diagram of gating: three-point and one-point gating
Venting Design
Venting is a crucial consideration in plastic mold design, as insufficient venting can trap air within the mold and lead to variations in melt temperature and cavity pressure during part filling. These conditions affect tolerances. The mold should be provided with as many vent holes as possible, especially in the final fill area. For plastic gear molds, tooth venting is crucial. Because most surfaces of gear molds are machined using a grinder, ensuring good surface fit, underfilling can easily occur at the final fill area during glue injection.
Venting grooves are therefore necessary to eliminate trapped air. Typical tooth venting groove configurations are shown in the Figure on the right.
Cooling is Crucial for Tolerance Control During Gear Molding
A uniform temperature must be maintained throughout the mold to allow the material to shrink at a uniform, controlled rate. Uneven shrinkage can lead to variations in dimensional tolerances. Mandrels and deep cores should be treated with particular care, as they are prone to heating. A three-plate mold with a naturally balanced runner system is ideal for achieving tight tolerances in gear molding. While multi-cavity molds are common, multiple cavities are not recommended for gear molds.
Hot runner systems can be used, but they will reduce the mold’s tolerances. The heat required to keep the runners warm will also heat parts of the mold, requiring additional cooling. When choosing a hot runner system for a gear mold, cooling plates must be adequately set up to properly control the mold temperature. Venting is important because insufficient venting can trap air in the mold and can cause differences in melt temperature and cavity pressure when filling the part. These conditions will affect tolerances. The mold should be provided with as many vent holes as possible, especially in the last area to be filled. The demolding system must be designed to ensure that the part is ejected from the mold with minimal deformation.
Core rods, slides, and sideways motion components are common features in many gear molds.
Whenever possible, these features should extend through the part being molded and lock into a retaining seat in the other mold half. This prevents deflection of the feature over time, which can be caused by repeated impacts of the plastic flow front during processing.
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Mold Structure
Since plastic gear injection molding often utilizes point gates, the mold structure typically employs a three-plate design. The Figure below display the actual gear mold.
The working principle of the gear mold is as follows:
After the injection is completed, the movable mold portion, driven by the injection molding machine, begins to separate:
First Parting: Spring 1 acts to separate the stripper plate from plate A. The sprue hooks secure the sprue to the stripper plate, breaking the main runner at the injection point and separating it from the product.
Second Parting: After the mold opens 95mm, the tie rod assembly separates the stripper plate from the main plate, freeing the main runner from the sprue bushing.
Third Parting: The mold continues to open. The tie rod assembly separates plates A and B. After the mold opens to 90mm, the ejector plate begins to move, ejecting the product. During the ejection process, the ejector plate guide pins are used to ensure ejection balance. Spring 2 returns the ejector plate to its original position. A complete set of mold opening and ejection actions is completed. The demoulding system must be designed to ensure that the deformation of the part is minimized when it is ejected from the mold.
Custom Plastic Gear Mold Manufacturing
During the plastic gear molding process, the gear mold is the molding device for the plastic gear and is crucial for ensuring the precision of the plastic gear. A plastic gear mold can be divided into two major parts: the gear cavity and the mold base. The gear cavity, also known as the ring gear, is the most demanding and precise component of the entire gear mold manufacturing process, and is the most crucial aspect of the entire gear mold manufacturing process.
1 Gear Cavity Machining
Gear cavity machining is crucial for the entire plastic gear mold manufacturing process. Because plastic gear molding is a “copy-like” process, the tooth profile of the cavity serves as a deformed template for the gear tooth shape. Therefore, the dimensional tolerance and surface roughness of the cavity must be strictly controlled, and defects such as burrs, eccentricity, and surface scratches must be avoided. Therefore, a rigorous gear cavity machining process must be developed to ensure cavity precision.
There are four main methods for machining gear cavities: wire cutting, electrical discharge machining, electrodeposition, and beryllium copper alloy casting. Each method has its own advantages and disadvantages when machining gear cavities. Wire EDM is typically used for involute spur gears, while EDM is generally used for helical gears. Furthermore, electrodes used for EDM gear cavities can generally be machined using wire EDM. Wire EDM can also be used for helical gear electrodes with a small helix angle (β ≤ 6°), which replace worm gears in meshing with worms.
2 Mold Base Machining
The mold base, also known as the mold base, is the auxiliary molding component of the gear mold. Mold base machining is similar to conventional plastic injection mold manufacturing.
Plastic Gear Injection Molding Process
We know the difficulties in producing plastic gears: the difficulty in accurately determining the plastic shrinkage rate during injection molding and the nonlinear shrinkage of the mold cavity. (The shrinkage of the tooth profile of an involute plastic gear is nonlinear, meaning the shrinkage in the x and y directions varies across the gear plane.) Therefore, the mold cavity tooth profile represents a hypothetical gear that includes shrinkage. Plastic gear injection molding is actually a form of “profiling” that includes shrinkage. The hypothetical gear is first used as a template, and then the involute tooth profile is formed after cooling and shrinkage. This perspective highlights that controlling shrinkage is crucial in the design and manufacture of plastic gears.
Therefore, the entire plastic gear injection molding process is closely related to shrinkage. In other words, only by properly controlling shrinkage can high-precision plastic gears be produced.
In the injection molding process for plastic gears, shrinkage control must consider two aspects: material properties and injection molding parameters. Since polyoxymethylene (POM) is a common material for plastic gears, this article will briefly introduce the injection molding process for plastic gears using POM as an example.
1. Material Properties
The material properties and processing performance of POM can be summarized into six points:
① Rheology: POM behaves as a non-Newtonian fluid in its molten state. Temperature has little effect on the viscosity of a POM melt, so increasing the temperature to increase the fluidity of the POM melt is not recommended.
② Crystallinity: POM generally has a crystallinity of 75% to 80%. When POM melts, it undergoes significant volume changes. Therefore, sufficient holding time during injection molding is essential to compensate for the volume change during solidification, otherwise shrinkage cavities will form.
③ Thermal Stability: POM has poor thermal stability. While ensuring fluidity, the processing temperature should be kept as low as possible. The general temperature range is 180°C to 200°C for homopolyoxymethylene and 170°C to 190°C for copolymers. Therefore, the heating time should be minimized.
④ Hygroscopicity: POM has low hygroscopicity, generally 0.2% to 0.25%.
⑤ Shrinkage: POM has a relatively high shrinkage of 1.5% to 3.5%.
⑥ Other Aspects: POM has a low coefficient of friction, a fast solidification rate, high surface hardness, high rigidity, self-lubrication, resilience, and low demolding stress, allowing for rapid demolding.
2. Injection Molding Parameters
The precision of POM plastic gears is significantly affected by material shrinkage. Therefore, strict control of injection molding process parameters is necessary to minimize shrinkage fluctuations and achieve high-precision gear products.
Key parameters for the POM plastic gear injection molding process include:
① Barrel temperature: generally 170°C to 190°C.
② Injection pressure: generally 40-130 MPa. Injection pressure is dependent on factors such as the plastic melt flow rate, mold gate shape and size, gear design and size, mold temperature, and injection molding machine type.
③ Injection speed: generally between 20 and 80 rpm. To avoid defects caused by premature melt cooling, rapid injection molding is generally used, while slow injection molding is typically used for thick-walled gears.
④ Mold temperature: generally no less than 75°C. The mold temperature can be adjusted slightly higher for thick-walled gears, but should not exceed 120°C. In addition, the mold temperature should be as uniform as possible to prevent gear warping and deformation.
⑤ Holding time: This increases with gear thickness, generally ranging from a few seconds to several minutes.
⑥ Finished product heat treatment: Generally, the product is held in a constant temperature oven at 130°C for 4-8 hours to release internal stresses.
Among the injection molding process parameters mentioned above, the three most important ones are mold temperature, injection pressure, and holding time. These three parameters have a significant impact on the shrinkage rate of plastic gear molding, as shown in the graphs of the relationship between these parameters and molding shrinkage rate in Figures below. [Note: The curves are approximate and may vary for materials with different compositions.] Because the injection molding process is affected by the machine and other external conditions, multiple adjustments are required to find the optimal injection molding parameters.
High-precision plastic gear products require not only precise calculations during the design phase, but also the manufacture of high-precision gear molds and a suitable injection molding process. Based on many years of experience in plastic gear mold design and injection molding, TONGDA LINK Mould Company briefly introduces the design and manufacture of plastic gears, hoping to provide some reference for more plastic gear design and manufacturing engineers.
Applications of Plastic Gears
Plastic gears have a wide range of applications. In the automotive industry, they are used in components such as window lifts, windshield wipers, and air conditioning compressors. The plastic gears in window lifts smoothly transmit the motor’s power to the lift mechanism, enabling window raising and lowering operations with minimal noise, enhancing driving comfort.
Plastic gears are even more ubiquitous in electronics. Examples include smartphone camera focus modules and laptop cooling fans. Plastic gears in camera focus modules precisely control lens movement, enabling fast and accurate focusing. Plastic gears in cooling fans ensure stable fan operation, protecting heat dissipation in electronic devices.
Plastic gears are also found in household appliances such as washing machines, refrigerators, and air conditioners. The precise transmission of plastic gears is essential for controlling the spin speed of washing machines, regulating the refrigerant circulation system of refrigerators, and adjusting the fan speed of air conditioners.
Development Trends of Plastic Gears
With the continuous advancement of technology, plastic gears are also evolving. On the one hand, advances in materials science are continuously improving the performance of plastics. The emergence of new engineering plastics has further enhanced the strength, heat resistance, and other properties of plastic gears, broadening their application range.
On the other hand, manufacturing processes are also constantly innovating. The application of precision injection molding and 3D printing technologies has enabled higher precision manufacturing of plastic gears, enabling them to meet more complex and sophisticated transmission requirements. At the same time, intelligent manufacturing is gradually being applied to the production of plastic gears, improving production efficiency and product quality consistency.
With their unique material properties, significant advantages, wide application areas, and promising development trends, plastic gears are occupying an increasingly important position in modern industry. I believe that in the future, with continued technological innovation, plastic gears will play an even greater role in even more fields, bringing more convenience and surprises to our lives.