How to Successfully Manufacture Injection Molding Gears

The two types of gears are fundamentally different. Machined gears are cut to size on a dedicated gear machine designed for a specific machining task; injection molded gears are molded in gear cavities, which are typically machined using wire-cut electrical discharge machines (EDMs). The size of the injection mold cavity ensures that the injection molded gears, which are cooled and shrunk after injection, have the correct dimensional tolerances. Millions of injection molded gears can be produced from a single cavity. The gear-cutting manufacturer has the task of cutting and machining each gear to tolerance. The injection molded gear manufacturer is faced with the task of creating a near-perfect mold cavity for gears and then machining all the gears in that cavity to the required tolerances. This seemingly small but significant difference leads to many other changes. This difference takes shape once the decision is made to use injection molded gears. The design of injection-molded gears leaves no doubt that injection-molded gears must be shaped in the mold cavity. This fact has important consequences. Injection mold cavities and the shaft parts therein are hardly capable of the precise tolerances that mechanical drives can provide. Cavities and gears have the potential to shrink or expand at different rates with changes in humidity and temperature. The strength, hardness and even transmission efficiency of injection molded gears will vary with local conditions. Under loaded conditions, the temperature of the gear tooth surface will increase, which will affect the properties of the plastic. Because of these variables and a number of other factors, it is necessary to customize the design of the gear's teeth. The advantage of injection molded gear design is in the application. Most injection molded gear drives are unique. A gear can be precisely designed to perform its designated function only when meshed with another paired gear. In addition, injection molded gears can be optimally designed and manufactured with little or no tooling considerations. The accuracy of the mold cavities manufactured on wire EDM machines depends on the precision of the computer-aided design. Gear mold cavities are available with tolerances down to the micron level. In fact, conventional hobbing tools are no longer needed, and the diameter pitch or module number is no longer an important technical parameter. The involute base circle becomes the important variable. The pressure angle can be adjusted analogically to balance the strength of the gear teeth in relation to their height as they mesh. Custom-designed gears offer significant improvements and enhancements in performance, quietness, and allowable tolerances over standard gears. Gear Molding Devices After the gear mesh is designed and tolerances are established, the next step requires the fabrication of a molding device. The gear molding unit must be precise, have good thermal stability, hardened slides and surfaces, accurate gear cavity shape, and be designed for high pressure injection molding. The gear cavity itself must be specifically designed for the material selected for the mold. The actual shrinkage of injection molded gears for a given application cannot be predicted precisely due to many factors. The most important of these factors is that injection molded gears do not shrink isotropically in the mold cavity. The shrinkage of the gear body may be closer to the manufacturer's prediction, but because the gear teeth are surrounded by steel, their cooling pattern is different from the macroscopic cooling pattern of the larger gear body. A better method for determining shrinkage is the two-step convergence method. Pre-estimate the shrinkage coefficient of the gear, according to which the injection mold is made and the first batch of gears are processed, the involute tooth profile of the gear prototype is measured precisely to determine the shrinkage rate of each part, and then a new mold cavity is remade according to the measured shrinkage rate, and then finally injection molded gears with qualified geometrical accuracy can be obtained. The shrinkage rate of the involute can only be precisely determined through the inspection of the tooth profile. Certain conditions of uneven shrinkage of the gears can be known by rolling inspection of the gears, but this can sometimes be misleading. Sometimes glass-filled materials can be used to manufacture injection-molded gears, due to the low shrinkage of this material, at which point shrinkage phenomena no longer become a problem in the design of the injection mold. However, this method may also cause new problems. Not filled with glass engineering resins such as nylon and acetal, although there is shrinkage, but can be molded to process a very precise shape. Glass-filled materials, on the other hand, create lap lines at the front convergence of the injection molding stream, which will cause deformation of the gear tooth surface as well as create some localized weak points in the gear. In general, glass-filled gears are more susceptible to wear over their lifetime than equivalent gears that are not filled with glass. Filler materials are usually only used where there is a special need, such as when overweighting of the gear will be a problem. Gear Molding Processes Various molding processes and molding machines vary. Gear molding processes require a high degree of accuracy and repeatability. Generally speaking, high precision gears need to be manufactured with new resins. However, even with new resins, the material must still have the proper dryness, the melting temperature must be precisely controlled and repeatable, and the injection pressure must be precisely controlled. Consideration must also be given to the coordination of the molding device with the control of the molding process. When molding at high temperatures and pressures, the melted plastic must replace the air in the mold cavity. Therefore, it is necessary to have an exhaust port that allows the air to escape without the resin flowing out. If the exhaust port is too small, the air will not escape properly and may cause combustion; if the port is too large, the melted plastic will flow out and form a fringe on the part. It is recommended that injection molded gear users visit an injection molded gear processing plant before finalizing a contract. A general look at the condition of the molding equipment, the cleanliness of the plant, inspection capabilities and staffing will help to properly evaluate the plant's potential for successful molding processing and control. For example, it is very difficult to manufacture precision injection molded gears in an environment without temperature control. It is extremely difficult to process precision injection molded gears at 90% humidity and 100?Fahrenheit. Inspection of Injection Molding Gears Over the years, gear inspection techniques have been improved to accurately measure most of the errors produced in gear cutting and machining. Scanning measurements of involute tooth profiles usually detect only a few teeth in a revolution. Metal gears are machined on gear machines such as rolling and interpolating machines, and the tooth profile is essentially the same for each tooth. Injection molded gears, on the other hand, can have large individual errors at a certain position on any tooth face of the gear. What's more, the mold injection process can introduce many different types of errors than traditional machining. Since any injection molded gear has to shrink, and the involute tooth profile is a target tooth shape, it is not a given value. Whether radial pitch, modulus, base pitch, pressure angle, or any other involute parameter is used to control the gear geometry, these parameters are variables for the actual part being machined. For these varying parameters, it is necessary to set realistic tolerances. The only way to be able to determine that an injection molded gear is dimensionally acceptable (located in the tolerance zone) is to scan and measure the involute tooth profile to determine the actual physical geometry of the gear. However, there may be cases where the dimensions of an injection-molded gear are completely out of tolerance, but the results of a comprehensive rolling inspection still pass. For example, the profile of a gear whose involute base circle has deviated far from the specified value. The measured gear has 64 teeth, the measured gear also has 64 teeth, the rolling inspection of the gear at the same time the number of meshing teeth is more, the measurement results are almost no comprehensive deviation of a tooth. The base circle of this gear is very small although it appears to be large. Because of the gear's thinned tooth thickness, good specifications can be achieved in rolling inspection. And once these injection-molded gear parts are supplied to the user, they fail immediately when meshed with correctly sized metal gears. In order to prevent such errors, technical specifications must be developed for each of the dimensional variables of the gear's labeled tolerances. One such specification, recognized by AGMA (American Gear Manufacturers Association), is the recently completed Injection Molded Gear Inspection Guide. Recommended Technical Parameters for Injection Molding Gears In the AGMA system, the base circle geometry of the gear is used as the basic control parameter. Indirect gear parameters such as diameter pitch and pressure angle are used as working data and as a reference frame during conventional analysis. Gear rolling inspection can be considered the best way to ensure consistent quality of injection molded gears in series production. It not only expresses the combined Total Combined Error (TCE) or One-Tooth Combined Error (TTE) of the gear, but also determines whether the actual center distance of the measured gear meshing with the measuring gear lies within the specified positive and negative tolerance bands. This provides an easy way to ensure consistency in the day-to-day production of injection molded gears. Statistical analysis of the rolling inspection results for a sample batch of gears can determine if the overall shape and absolute dimensions of the gears are within the tolerance band. Rolling inspection for injection molded gears is more like establishing a rolling inspection qualification verification and should ensure that injection molded gears produced on a daily basis conform to this verification. The future of injection molding gears is quite promising. Materials continue to improve dramatically, injection molding machinery is becoming more and more sophisticated, and inspection equipment has been able to measure these unique injection molded gears with high accuracy. In the future, injection molded gears can be expected to replace metal gears in lighter duty transmission applications. Manufacturers are continuing to look for applications and areas of use where metal gears cannot be used, but where plastic gears are useful. In order to enter these new potential applications, every step must be implemented correctly, tapping into every advantage of injection molded gears. The result will be a new generation of power transmission products with superior performance.