Compared with machined gears,injection-molded gears have little in common except that they use involute conjugation to transmit motion. There are essential differences between these two gears. Machining gear is to cut and machine to the specified size on a special gear machine designed for a specific machining task; Injection-molded gears are injection-molded in gear cavities, which are usually machined by wire-cut electrical discharge machining machines (EDMs). The size of the cavity of the injection gear can ensure that the injection gear which is cooled and contracted after injection molding has correct dimensional tolerance. Millions of injection gears can be processed with one mold cavity.
The task of the gear cutting manufacturer is to cut every gear according to the tolerance requirements. However, the task faced by injection gear manufacturers is to make a nearly perfect gear cavity, and then use this cavity to process all gears that meet the tolerance requirements. This seemingly small but significant difference has led to many other changes. This difference was formed when injection-molded gears were used in Once decide.
Design of Injection Molding Gear
There is no doubt that the injection gear must be formed in the mold cavity. This fact has led to important consequences. It is difficult for injection mold cavity and shaft parts in it to have the precise tolerance provided by mechanical transmission mechanism. The cavity and gear may shrink or expand at different rates with the change of humidity and temperature. With different local conditions, the strength, hardness and even transmission efficiency of injection-molded gears will change. Under the load condition, the temperature of the tooth surface will rise, which will affect the characteristics of plastic. Because of these variables and other factors, it is necessary to customize the gear teeth.
The advantage of injection gear design is reflected in its application. Most injection gear drives are unique. One gear can be precisely designed to complete its designated function only when it meshes with another counterpart gear. In addition, the optimal design and manufacture of injection gears hardly need to consider tool factors.
The precision of die cavity manufactured by WEDM depends on the precision of CAD. The tolerance of gear cavity can reach micron level. In fact, the traditional hob is no longer needed, and the diameter pitch or modulus is no longer an important technical parameter. Involute base circle becomes an important variable. The pressure angle can be adjusted in an analog way to balance the relationship between strength and height when the gear teeth mesh. Compared with the standard gear, the custom-designed gear has been greatly improved in performance, quietness and allowable tolerance.
Gear injection molding device
After gear meshing design and tolerance setting, the next step is to make an injection molding device. The gear injection molding device must be precise, with good thermal stability, hardened sliding sleeve and surface, accurate shape of gear cavity and design of high-pressure injection mold. The gear die cavity itself must be specially designed according to the selected die material.
Due to many factors, it is impossible to accurately predict the actual shrinkage of injection-molded gears in specific applications. The most important factor is that the shrinkage of injection gear in the mold cavity is not isotropic. The shrinkage of the gear body may be close to the manufacturer's prediction, but because the gear teeth are surrounded by steel, its cooling form is different from the macro cooling form of the larger gear body.
A better method to determine the shrinkage is the two-step approach method. The shrinkage coefficient of gears is estimated in advance, and then the injection mold is made and the first batch of gears are processed. Then the involute tooth profile of gear samples is measured precisely to determine the shrinkage rate of each part, and then a new mold cavity is made according to the measured shrinkage rate. Finally, injection gears with qualified geometric accuracy can be obtained. Only by detecting the tooth profile can the shrinkage of involute be accurately determined. Through rolling detection of gears, we can know some conditions of uneven shrinkage of gears, but sometimes it can lead to misleading.
Sometimes, glass filling materials can be used to manufacture injection gears. Because the shrinkage rate of this material is very low, the shrinkage phenomenon is no longer a problem in injection molding design. However, this method may also cause new problems. Engineering resins that are not filled with glass, such as nylon and acetal, can produce very precise shapes by injection molding despite shrinkage. However, the glass filling material will produce lap joint at the junction in front of the injection flow, which will cause the surface deformation of the gear teeth and produce some local weak points on the gear. Generally speaking, glass-filled gears are more likely to wear during their service life than equivalent gears without glass. Filling materials are usually only used for special needs, such as when overweight gears will become a problem.
Gear injection molding process
Various injection molding processes and injection molding machines are different. Gear injection molding process requires high accuracy and repeatability. Generally speaking, high-precision gears need to be manufactured with new resin. However, even if the new resin is used, the material must have proper dryness, its melting temperature must be accurately controlled and repeatable, and the injection pressure must also be accurately controlled. Coordination between injection molding equipment and injection molding process control must also be considered.
When injection molding is carried out at high temperature and high pressure, the air in the mold cavity must be replaced by molten plastic. Therefore, it is necessary to set up an exhaust port that allows air to be discharged without resin flowing out. If the exhaust port is too small and the gas is not discharged smoothly, it may cause combustion; If the vent is too large, the molten plastic will flow out and form a flash on the parts.
It is suggested that injection gear users visit the injection gear processing factory before signing the contract. A general survey of injection molding equipment, cleanliness, inspection capability and staffing of the factory will help to correctly evaluate whether the factory has the potential ability to successfully process and control injection molding. 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 at 90% humidity and 100 degrees Fahrenheit.
Testing of Injection Molding Gear
Over the years, the gear detection technology has been continuously improved to accurately measure most errors in gear cutting. Scanning measurement of involute tooth profile usually only detects a few teeth in one circle. Metal gears are machined on gear machines such as hobbing and slotting, and the tooth profile of each tooth is basically the same. However, injection-molded gears may have large individual errors at a certain position on any tooth surface of the gear. What's more, the injection molding process may introduce many different types of errors from traditional machining.
Because any injection gear has to shrink, the involute tooth profile is a target tooth profile, not a given value. Whether diameter pitch, modulus, base pitch, pressure angle or any other involute parameters are used to control the gear geometry, these parameters are variables for the actual machined parts. For these varied parameters, it is necessary to set practical tolerances.
The only way to determine the size of injection-molded gear is to scan and measure the involute tooth profile and determine the actual physical geometry size of the gear. However, there may be the following situations: the size of injection-molded gear has completely exceeded the tolerance requirements, but the rolling comprehensive test results are still qualified. For example, the tooth profile test result of a gear, its involute base circle has deviated far from the specified value. The tested gear has 64 teeth, and the used measuring gear also has 64 teeth. In rolling detection, there are many teeth engaged at the same time, and there is almost no comprehensive deviation of one tooth in the measurement results. Although this gear looks big, its base circle is very small. Because the tooth thickness of the gear is thinned, good technical indexes can be achieved in rolling detection. Once these injection-molded gear parts are provided to users, they will immediately fail when engaging with metal gears of correct size.
In order to prevent this kind of error, it is necessary to formulate technical specifications for the dimensional variables of each gear marked with tolerance.
Suggested technical parameters of injection gear
In AGMA system, the geometric parameters of the base circle of gears are used as the basic control parameters. Indirect gear parameters, such as diameter pitch and pressure angle, are used as working data, which are used as reference in traditional analysis.
Gear rolling inspection can be considered as the best method to ensure the quality consistency of injection molded gears in mass production. It not only expresses the comprehensive total error (TCE) or one tooth comprehensive error (TTE) of gears, but also determines whether the actual center distance between the measured gear and the measured gear is within the specified positive and negative tolerance zone. This provides a simple method to ensure the consistency of daily production of injection gears. Statistical analysis of rolling test results of a batch of sample gears can determine whether the overall shape and absolute size of gears are within the tolerance zone. Rolling inspection is more like establishing a rolling inspection qualification verification for injection gears, and it should be ensured that the injection gears produced every day meet this verification.
The development prospect of injection gear is quite optimistic. The materials have been greatly improved, and the injection molding machinery has become more and more sophisticated. The testing equipment has been able to measure these unique injection molding gears with high precision. In the future, it can be expected to use injection gears instead of metal gears in light load transmission applications. Manufacturers are looking for places and fields where metal gears can't be used, but plastic gears can be used.
In order to enter these new potential application fields, every step must be carried out correctly, and every advantage of injection-molded gears must be exploited. As a result, a new generation of power transmission products with excellent performance will be developed.
