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Working Principle of Injection Molding

2022-07-01 Hits: 12 views

Injection mold is a part that gives shape and size to plastic during molding. Although the structure of the mold may be ever-changing due to the variety and performance of plastics, the shape and structure of plastic products and the type of injection machine, the basic structure is the same. The mold is mainly composed of gating system, molding parts and structural parts. Among them, the gating system and molding parts are the parts that are in direct contact with plastics and change with plastics and products. They are the most complicated and most changeable parts in plastic molds, which require the highest processing smoothness and precision.


The gating system refers to the part of the runner before the plastic enters the cavity from the nozzle, including the main runner, the cold cavity, the shunt runner and the gate. Molding parts refer to all kinds of parts that make up the shape of products, including moving mold, fixed mold and cavity, core, molding rod and exhaust port.


  1. Injection molding step:

When the screw rotates at a high speed to melt raw materials, the rotation speed is critical for melting. If it is too slow, the plastic will not melt completely, and if it is too fast, the plastic will be carbonized. Of course, the barrel is also very important for heating.


  1. Injection molding step:

After the raw material is melted, the melted material is injected into the mold. And the injection pressure provided by the injection molding machine is required to inject the melted raw materials into the mold.

Then it takes some time to press and form.

After that, cool the mold and close the gate of the mold. Finally, the mold is cooled by air and water.


Pick up the product: This is the last step. Some customers need to use robots to pick up products, while others let products fall off automatically after forming.


Mainstream Dao

It is a passage in the mold that connects the injection nozzle of the injection machine to the shunt channel or cavity. The top of the main runner is concave to connect with the nozzle. The diameter of the inlet of the main channel should be slightly larger than the diameter of the nozzle (O.8mm) to avoid material overflow and prevent the blockage caused by the inaccurate connection between the two. The diameter of the inlet depends on the size of the product, generally 4-8mm. The diameter of the main runner should be enlarged inward at an angle of 3 to 5 to facilitate the demoulding of the runner excrements.


It is a cavity at the end of the main runner, which is used to catch the cold material generated between two injections at the end of the nozzle, so as to prevent the blocking of the branch runner or gate. If the cold material is mixed into the cavity, the internal stress will easily occur in the product. The diameter of the cold cavity is about 8-lOmm, and the depth is 6mm. In order to facilitate demoulding, its bottom is often borne by demoulding rod. The top of the demoulding rod should be designed into a zigzag hook shape or a sunken groove, so that the main runner excrements can be pulled out smoothly during demoulding.



It is the channel connecting the main runner and each cavity in the multi-groove mold. In order to make the molten material fill the cavities at the same speed, the arrangement of the runner on the mold should be symmetrical and equidistantly distributed. The shape and size of the runner section have influence on the flow of plastic melt, the difficulty of product demoulding and mold manufacturing. If the flow rate is equal, the resistance of the channel with circular cross section is the smallest. However, because the specific surface of the cylindrical runner is small, it is unfavorable for the cooling of the sprue, and the sprue must be set on the two mold halves, which is labor-intensive and easy to align. Therefore, a trapezoidal or semi-circular cross-section runner is often used, and it is set on a half mold with a demoulding rod. The runner surface must be polished to reduce the flow resistance and provide a faster mold filling speed. The size of the channel depends on the type of plastic, the size and thickness of the product. For most thermoplastics, the cross-section width of the shunt passage is no more than 8m, ranging from 10-12m for extra-large to 2-3m for extra-small ones. On the premise of meeting the needs, the cross-sectional area should be reduced as far as possible, so as to avoid increasing the surplus of the shunt passage and prolonging the cooling time.


Pouring Gate

It is the channel that connects the main runner (or branch runner) with the cavity. The cross-sectional area of the channel can be equal to that of the main channel (or branch channel), but it is usually reduced. Therefore, it is the smallest part of the whole channel system. The shape and size of the gate have great influence on the product quality. The function of the gate is: A, to control the flow rate; B, to prevent backflow due to the premature solidification of the molten material in this part during injection; C, to make the molten material passing through be subjected to strong shear to raise the temperature, thereby reducing the apparent viscosity and improving the fluidity; D, to facilitate the separation of products from the runner system. The design of gate shape, size and location depends on the properties of plastics, the size and structure of products. Generally, the cross-sectional shape of the gate is rectangular or circular, and the cross-sectional area should be small while the length should be short. This is not only based on the above-mentioned effect, but also because it is easy for the small gate to become larger, while it is difficult for the large gate to become smaller. Generally, the gate position should be selected at the thickest part of the product without affecting its appearance. The design of gate size should take into account the properties of plastic melt. Mingyu Mold-A professional injection molding food container mold manufacturer, supplier, company, wholesaler.

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