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What Are the Injection Molding System Components?

The gating system refers to the portion of the plastic that enters the mold cavity from the nozzle, including the main runner, cold slug well, sprue and gate, etc.The gating system, also known as the runner system, is a set of feed channels that guide the plastic melt from the injection machine nozzle to the mold cavity. It is usually composed of main runners, sprues, gates, and cold slug wells. It directly affects the molding quality and production efficiency of plastic products.

Injection molding main runner

It is a passage in the precision injection molding that connects the injection molding machine nozzle to the sprue or mold cavity. The top of the main runner is concave to fit the nozzle. The inlet diameter of the main runner should be slightly larger than the nozzle diameter (0.8mm) to avoid overflow and prevent blockages due to inaccurate connection. The inlet diameter is determined by the size of the product and is usually between4-8mm. The diameter of the main runner should expand inward at an angle of 3° to 5° to facilitate the removal of runner debris.

Injection molding cold slug well

It is an empty cavity located at the end of the main runner, used to collect cold material generated between the nozzle tip's two injections to prevent blockages in the sprue or gate. If cold material enters the mold cavity, internal stresses are easy to produce. The diameter of the cold slug well is approximately 8-10mm, and the depth is 6mm. To facilitate demolding, its bottom is often supported by a demolding rod. The top of the demolding rod should be designed with a zigzag hook or a sunken groove to smoothly remove the runner debris during demolding.

Injection molding runner

It is the channel connecting the sprue and each cavity in the multi-slot mold. In order to make the molten material fill each cavity at an equal speed, the arrangement of the runners on the injection molding should be symmetrical and equidistant. The shape and size of the cross-section of the runner have an impact on the flow of the plastic melt, the ease of product demoulding and mold manufacturing. If the flow of the same amount of material is concerned, the flow channel with a circular cross-section has the least resistance. But because the specific surface of the cylindrical runner is small, it is unfavorable to the cooling of the debris in the runner, and the runner must be opened on the two halves of the mold, which is labor-intensive and difficult to align. Therefore, trapezoidal or semicircular cross-section runners are often used, and they are opened on half of the mold with ejector pins. The surface of the runner must be polished to reduce flow resistance and provide faster filling speed. The size of the runner depends on the type of plastic, the size and thickness of the product. For most thermoplastics, the cross-sectional width of the runner does not exceed 8mm, the extra-large one can reach 10-12mm, and the extra-small one can reach 2-3mm. Under the premise of meeting the needs, the cross-sectional area should be reduced as much as possible to increase the debris in the shunt channel and prolong the cooling time.

Injection molding gate


It is a channel connecting the main channel (or runner) and 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. So it is the part with the smallest cross-sectional area in the entire runner system. The shape and size of the gate have a great influence on the quality of the product.

The function of the gate is:

  • Control the material flow rate;

  • During the injection, the backflow can be prevented due to the early solidification of the melt stored in this part;

  • Make the passing melt subject to strong shear to increase the temperature, thereby reducing the apparent viscosity to improve fluidity;

  • It is convenient to separate the product from the runner system.

The design of the shape, size and position of the gate depends on the nature of the plastic, the size and structure of the product. Generally, the cross-sectional shape of the gate is rectangular or circular, and the cross-sectional area should be small and the length should be short. This is not only based on the above-mentioned effects, but also because it is easier to enlarge a small gate, but it is difficult to shrink a large gate. The gate position should generally be selected at the place where the injection molding product is the thickest without affecting the appearance.

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