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Injection Molding Process And Molding Cycle

Injection molding process

The advantages of plastic injection moulding are fast production speed and high efficiency; it can realize operation automation, can produce products with a variety of colors and simple or complex shapes; and size can also be from large to small; products are easy to update.

The plastic injection molding process can be roughly divided into the following six stages: mold clamping, injection, pressure holding, cooling, mold opening, and product removal.

By repeating the above process, plastic mould tooling products can be produced periodically in batches. The molding of thermosetting plastics and rubber also involves the same process, but the barrel temperature is lower than that of thermoplastics, and the injection pressure is higher. The mold is heated, and the injection molding materials need to undergo a curing or vulcanization process in the mold after the injection is completed, and then the process of demolding is achieved while it is hot. 

Injection molding cycle

The time required to complete an injection molding process is called the molding cycle, also known as the moulding cycle. The molding cycle directly affects labor productivity and equipment utilization. Therefore, in the production process, the relevant time in the molding cycle should be shortened as much as possible under the premise of ensuring quality.

In the entire molding cycle, injection time and cooling time are the most important, and they both have a decisive influence on the quality of the product. 

(1) Injection time: 

The mold filling time in the injection time is directly inversely proportional to the mold filling rate. The mold filling time in production is generally about 3 to 5 seconds. The holding pressure time in the injection time is the time of pressure on the plastic in the cavity, and it accounts for a large proportion of the entire injection time, generally about 20 to 120 seconds (in terms of extra-thick parts, it can be as high as 5 to 10 minutes).

(2) Holding pressure time: 

The holding pressure time before the melted material at the pouring gate is frozen, has an impact on the accuracy of the product size, while after the melted material at the pouring gate is frozen, it has no effect. The holding time also has the most appropriate value, which is known to depend on the material temperature, mold temperature, and the size of the main runner and pouring gate. If the size of the main runner and pouring gate and the technological conditions are normal, usually the pressure value of the product with the smallest fluctuation range of the shrinkage rate shall prevail. 

(3) Cooling time: 

It is mainly determined by the thickness of the product or the injection moulding material list, the thermal and crystalline properties of the plastics, and the mold temperature. The end of the cooling time should be determined based on the principle of ensuring that no changes are caused when the product is demolded. The cooling time is generally between 30 and 120 seconds. It is not necessary for the cooling time to be too long. It will not only reduce production efficiency, but also cause demoulding difficulties for complex parts, and demoulding stress may even occur when demolding is carried out forcefully. The other time in the molding cycle is related to whether the production process is continuous and automated, or the degree of continuity and automation.

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