What are the gear injection molding processes and devices

Update:08-04-2021
Summary:

Gear injection moulding process Various injection mould […]

Gear injection moulding process

Various injection moulding processes and injection moulding machinery are not the same. Gear injection moulding 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 moulding device and the process control of the injection moulding process.

When injection moulding 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 Lianying Plastic Gear Processing Factory before finally signing the contract. A general survey of the mould 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 moulding 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.

Gear injection moulding device

After gear meshing design and tolerances are established, the next step is to make an injection moulding device. Gear injection moulding device must be precise, with good thermal stability, hardened sliding sleeve and surface, precise gear cavity shape, and design high pressure injection mould. The gear cavity itself must be specially designed according to the selected mould material.

For injection-moulded 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 gear 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 coefficient of the gear, and then make an injection mould 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-moulded gears. Because this material has a low shrinkage rate, shrinkage is no longer a problem in injection mould 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 moulded 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.

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