How to successfully manufacture injection gears
Compared with machined gears, injection gears have almost nothing in common except for the use of involute conjugate to transmit motion. These two types of gears are essentially different. Machining gears are cut to a specified size on a special gear machine designed for specific processing tasks; injection-molded gears are injection molded in a gear cavity, which is usually processed by wire-cut electrical discharge machining machines (EDMs) . The size of the cavity of the injection gear can ensure that the injection gear that has been cooled and shrunk after injection has the correct dimensional tolerance. Millions of injection gears can be processed with one cavity.
The task of gear cutting manufacturers is to cut every gear according to tolerance requirements. The task faced by injection gear manufacturers is to create a nearly perfect gear cavity, and then use this cavity to machine all gears that meet the tolerance requirements. This seemingly small but significant difference led to many other changes. This difference is already formed once the decision to use injection-molded gears is made.
Design of injection gear
There is no doubt that the injection gear must be formed in the mold cavity. This fact has important consequences. Note: It is difficult for the mold cavity and the shaft parts in it to have the precise tolerances that the mechanical transmission mechanism can provide. The cavity and gear may shrink or expand at different rates with changes in humidity and temperature. With the different local conditions, the strength, hardness and even transmission efficiency of the injection gear will change. Under load conditions, the temperature of the surface of the gear teeth will increase, which will affect the characteristics of the plastic. Because of these variables and other factors, it is necessary to customize the design of the gear teeth.
The advantage of injection gear design is reflected in the application. Most injection gear drives are unique. A gear can be precisely designed to perform its designated function only when it meshes with another paired gear. In addition, the optimal design and manufacture of injection gears hardly need to consider tool factors.
The accuracy of the cavity made by the wire-cut EDM machine depends on the accuracy of the computer-aided design. The tolerance of the gear cavity can reach the micron level. In fact, traditional hobs are no longer needed, and the diametral pitch or modulus is no longer an important technical parameter. The base circle of the involute becomes an important variable. The pressure angle can be adjusted in an analog manner to balance the relationship between the strength and the height when the gear teeth are engaged. Compared with standard gears, custom-designed gears have greatly improved performance, quietness and allowable tolerances.
Gear injection moulding device
After gear meshing design and tolerances are established, the next step is to make an injection molding device. Gear injection molding device must be precise, with good thermal stability, hardened sliding sleeve and surface, precise gear cavity shape, and design high pressure injection mold. The gear cavity itself must be specially designed according to the selected mold material.
For injection-molded gears in specific applications, due to many factors, the actual shrinkage cannot be accurately predicted. The most important factor is that the shrinkage of the injection wheel in the cavity is not isotropic. The shrinkage of the gear body may be closer to the manufacturer’s prediction, but because the gear teeth are surrounded by steel, the cooling pattern is different from the macroscopic cooling pattern of the larger gear body.
A better way to determine the amount of shrinkage is the two-step approach. Pre-estimate the shrinkage factor of the gear, and then make an injection mold and process the first batch of gears. The involute tooth profile of the gear sample is accurately measured to determine the shrinkage rate of each part, and then based on the measured shrinkage rate Re-make a new cavity, and finally an injection gear with qualified geometric accuracy can be obtained. Only by detecting the tooth profile can the involute shrinkage rate be accurately determined. By detecting the rolling of the gear, it is possible to know some conditions of the uneven shrinkage of the gear, but it can sometimes cause misleading.
Sometimes glass-filled materials can be used to make injection-molded gears. Because this material has a low shrinkage rate, shrinkage is no longer a problem in injection mold design. But this method may also cause new problems. Engineering resins that are not filled with glass, such as nylon and acetal, have shrinkage, but they can be injection molded into very precise shapes. The glass filling material will produce lap lines at the junction in front of the injection flow, which will cause the surface of the gear teeth to deform and produce some local weak points on the gear. Generally speaking, glass-filled gears are more prone to wear during their lifespan than equivalent gears that are not filled with glass. Filling materials are usually only used in occasions with special needs, such as when overweight gears will become a problem.
Gear injection molding process
Various injection molding processes and injection molding machinery are not the same. Gear injection molding technology requires high accuracy and repeatability. Generally speaking, high-precision gears need to be made of new resin. But even if the new resin is used, the material must still have a suitable degree of dryness, its melting temperature must be accurately controlled and repeatable, and the injection pressure must also be accurately controlled. It is also necessary to consider the coordination of the injection molding device and the process control of the injection molding process.
When injection molding is performed under high temperature and high pressure, the molten plastic must replace the air in the cavity. Therefore, it is necessary to set up an exhaust port that allows air to escape without the resin flowing out. If the exhaust port is too small, the gas will not be discharged smoothly, which may cause combustion; if the exhaust port is too large, the molten plastic will flow out and form flash on the part.
It is recommended that users of injection gears visit the injection gear processing plant before finally signing the contract. A general survey of the injection molding equipment, the cleanliness of the factory, testing capabilities and staffing, etc. will help to correctly evaluate whether the factory has the potential for successful injection molding processing and control. For example, it is difficult to manufacture precision injection gears in an environment without temperature control. It is extremely difficult to process precision injection gears under the conditions of 90% humidity and 100°Fahrenheit temperature.
Inspection of injection gear
Over the years, gear detection technology has been continuously improved to accurately measure most of the errors in gear cutting processes. Scanning measurement of involute tooth profile usually only detects a few teeth in a circle. Metal gears are processed on gear machine tools such as hobbing and slotting, and the tooth profile of each tooth is basically the same. The injection gear may have a large individual error at a certain position on any tooth surface of the gear. What’s more, the injection molding process may also introduce many types of errors that are different from traditional machining.
Since any injection gear has to shrink, the involute tooth profile is a target tooth profile, not a given value. Whether using diametral pitch, modulus, base section, pressure angle or any other involute parameters to control the gear geometry, these parameters are variables for the actual machined parts. For these changing parameters, it is necessary to set practicable tolerances.
The only way to determine that the injection gear is qualified (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 the following situations: the size of the injection gear has completely exceeded the tolerance requirements, but the rolling comprehensive test results are still qualified. For example, the tooth profile detection result of a gear, the involute base circle has deviated far from the specified value. The measured gear has 64 teeth, and the measuring gear used also has 64 teeth. In the rolling detection, the gear meshes with more teeth at the same time, and there is almost no comprehensive deviation of one tooth in the measurement result. Although this kind of gear looks larger, the base circle is small. Due to the reduced gear tooth thickness, good technical indicators can be achieved in rolling detection. Once these injection-molded gear parts are provided to users, they will fail immediately when they mesh with metal gears of the correct size.
In order to prevent this kind of error, it is necessary to formulate technical specifications for each dimensional variable marked with tolerances of the gear. One of the technical specifications approved by the AGMA (American Gear Manufacturers Association) is the recently completed “Guidelines for Inspection of Injection Molded Gears.”
Suggested technical parameters for injection gears
In the AGMA system, the basic circle geometric parameters of the gear are used as the basic control parameters. Indirect gear parameters such as diametral pitch and pressure angle are used as working data, which are used as reference benchmarks in traditional analysis.
Gear rolling detection can be considered as the best way to ensure the quality consistency of injection molded gears in mass production. It not only expresses the total comprehensive error (TCE) or one-tooth comprehensive error (TTE) of the gear, but also determines whether the actual center distance between the measured gear and the measuring gear is within the specified positive and negative tolerance zone. This provides an easy way to ensure the consistency of the daily production of injection-molded gears. For the statistical analysis of a batch of sample gear rolling test results, it can be determined whether the overall shape and absolute size of the gear are within the tolerance zone. Rolling inspection is more like establishing a rolling inspection qualification verification for injection-molded gears, and it should be ensured that the injection-molded gears produced every day meet this verification.
The development prospects of injection gears are quite optimistic. The materials continue to be greatly improved, the injection molding machinery is becoming more and more sophisticated, and the testing equipment has been able to perform high-precision measurements on these unique injection-molded gears. In the future, it can be expected to replace metal gears with injection-molded gears in lighter-load transmission applications. Manufacturers are continuing to search for those occasions and fields where metal gears cannot be used, but plastic gears are useful.
In order to enter these new potential application areas, every step must be implemented correctly and every advantage of injection-molded gears must be tapped. As a result, a new generation of power transmission products with excellent performance will be developed.