How to choose the right gate for injection mold

Today we will talk about how to choose the right gate design for injection mold. The gate, also known as the feed gate, is the channel connecting the runner and the cavity melt. Whether the gate is selected properly is directly related to whether the injection molded product can be injected intact and with high quality. The gate position plays a decisive role in the shape of the melt flow front and the effect of the holding pressure, and therefore, also determines the strength and other properties of the injection molded product.
Therefore, we need to know how many types of gates are commonly used in injection molds. Main gate, side gate, fan gate, pin gate, sub-gate and banana gate, which are usually used in cold runner systems, or used with hot runner systems. Regarding hot runner systems, from the perspective of gate type, there are main gates, hot tip gates and valve gates.
It should be noted that the design selection of injection mold gates depends on various factors, such as part design, materials used, mold design and production requirements. How to choose the most suitable gate for our injection molded products? First of all, we should understand their functions.

Basic Concepts of Injection Mold Gates

Injection mold gates are channels for molten plastic to enter the mold cavity. During the injection molding process, molten plastic quickly fills the mold cavity through the gate and forms a product after cooling and solidification. The design of the gate has an important impact on product quality, production efficiency and cost.

The main functions of injection mold gates

1. Control plastic flow: The design of the gate can control the speed and direction of molten plastic entering the mold cavity, ensuring that the plastic can evenly fill the entire cavity, thereby obtaining injection molded parts with precise dimensions and good appearance.
2. Influence on product quality: Reasonable gate design can reduce stress concentration inside the product and improve the mechanical properties and durability of the product. At the same time, the choice of gate position can also reduce defects such as warping and deformation of the product.
3. Improve production efficiency: Optimized gate design can shorten the injection molding cycle and improve production efficiency. For example, the gate design using hot runner technology can achieve continuous production and reduce downtime during the production process.
4. Reduce costs: Reasonable gate design can reduce scrap rate and reduce waste of raw materials. At the same time, reduce post-processing processes such as trimming gate marks, thereby reducing production costs.

Classification and advantages of injection mold gates

Usually, injection mold gates can be divided into two categories: large gates and small gates. The former is also called non-restrictive gates, which refers to direct gates; the latter is also called restrictive gates or internal gates, and commonly used ones include side gates and point gates.

1. Direct gate: The mold is filled with equal flow during injection, and the flow of the pouring system is short. It is often used for injection molding large, thick-walled, long-flow products and some high-viscosity plastics.
2. Side gate: Because it is opened on the main parting surface, the cross-sectional shape is easy to process and adjust. It is often used on multi-cavity molds and can be designed as a two-plate mold. They are usually used for part geometry or design that requires gates on the side of the part instead of the parting line, and the secondary gate is cut off
3. Overlap gate: The side gate is opened on the edge of the end face of the plastic part, which can avoid the phenomenon of melt spraying in large cavities. It is suitable for low-viscosity materials, so that the melt can be injected in an orderly manner during mold filling.
4. Fan gate: It expands from the runner to the cavity in a fan shape, and the depth gradually decreases from deep to shallow. It is suitable for general plastics except for materials with high viscosity. Fan gate is similar to side gate, which is usually used in injection molding process where a large amount of plastic needs to flow into the mold, but it can be used for large-sized, thin-walled parts and flat products, and it can also improve gate marks.
5. Flat seam gate: It evolved from fan gate, and the filling flow is more balanced. Its width is equal to or slightly larger than the cavity width. It is suitable for sheet plastics with requirements for transparency and flatness, and no flow marks on the surface.
6. Point gate: It is a typical restrictive gate. It can be divided into two types according to the applicable position relationship. One is directly connected to the main channel, and the entire point gate becomes a prism, which can only be used for materials with stable temperature. The most commonly used is the point gate with multi-point feeding through the branch channel.
7. Submerged gate: It dives into the side of the parting surface and enters the cavity along the oblique direction. Therefore, when the injection mold is opened, not only can the gate be automatically cut off, but its position can also be set in various hidden places such as the side, end, and back of the product, so that no trace of the gate is left on the outer surface of the product. It is an application form of point gate in special occasions, so it has all the advantages of point gate and has been widely used.
8. Disc gate and circular gate: The feeding along the inner circumference of the plastic part is called disc gate, and the feeding along the outer circumference is called ring gate. It is mainly used for cylindrical products or products with holes in the middle. This can make the feeding uniform, obtain roughly the same flow rate on the entire circumference, and it is also easy to exhaust in sequence without weld seams.
9. Spoke gate: It changes the entire circumference feeding into several small arc feedings. Therefore, it is not only convenient to remove the gate condensate and save materials, but also increases stability because the upper part of the core is positioned. Its scope of application is similar to that of the circular ring gate.
10. Claw gate: The difference between it and the spoke gate is that the runner and gate are not in the same plane, and the top of the core extends into the fixed mold. It is suitable for tubular parts, especially tubular products with small inner holes and high concentricity requirements.
11. Ear-protective gate: It is suitable for products that are difficult to mold or have optical performance requirements, in order to remove the part with residual stress accumulation from the product. It allows shrinkage holes to form near the gate, which can effectively prevent jet flow and improve the internal quality of the product.

Injection mold gate design considerations

The basic factors to be considered in the design of injection mold gate positioning are: part design, flow and product end-use requirements. Appropriate gate balance and runner balance design can avoid molding defects in the actual molding process of multi-cavity injection molds, such as flow marks, shrinkage, under-injection, dimensional fluctuations and weight changes. The planning of the gate is related to factors such as the size, shape, mold structure, injection process conditions and plastic part function of the plastic part. But in terms of the fundamental effect, the gate section should be small and the length should be short, because only the gate can meet the requirements of increasing the flow rate, rapid cooling and closing, facilitating the separation of plastic parts and minimizing the gate marks. Therefore, it is necessary to pay attention to the following issues.

1. Large parts that require multiple gates should have gates that are located close enough to reduce pressure losses. This minimizes cooling at the intersection of the resin flow fronts, providing better weld line strength. The appropriate gate size should be selected to allow the resin to fill at a reasonable pressure and speed.
2. The gate transition section length should be kept as short as possible.
3. Impingement gates will help ensure that the incoming fluid is directly against the cavity wall or core, thereby avoiding swirls.
4. To avoid air entrapment, the resin flow from the gate should direct air to the venting groove.
5. The gate location should be determined so that the resin flows from the thick wall to the thin wall; minimizes the weld line; and is away from the impact and stress areas.
6. To minimize swirls, radial spots and gate white halo, the gate should be at a suitable angle to the flow channel.

Key points of injection mold gate design

The design of injection mold gate is related to factors such as the size, shape, mold structure, injection process conditions and plastic part function of the plastic part. But in terms of basic effect, the gate section should be large and the length should be short, because only in this way can the requirements of increasing the flow rate, rapid cooling and closing, facilitating the separation of plastic parts and minimizing gate marks be met.

The key points of gate design can be summarized as follows:

1. The gate is opened at the thicker part of the plastic part section, so that the molten material flows from the thick material section to the thin section to ensure complete mold filling;
2. The selection of gate position should make the plastic filling process as short as possible to reduce pressure loss;
3. The selection of gate position should be conducive to clearing the air in the cavity;
4. The gate should not allow the molten material to rush directly into the cavity, otherwise a vortex will occur, leaving a spiral mark on the plastic part, especially the narrow gate is more likely to present this defect;
5. The selection of gate position should avoid the occurrence of pin-shaped rings or cylinders on the plastic surface In the plastic parts, a cold well should be added at the molten material pouring point on the gate surface;
6. The gate position of the injection mold with a slender core should be far away from the molding core to prevent the molding core from being deformed by the material flow;
7. When forming large or flat plastic parts, a compound gate can be used to prevent warping, deformation, and material shortage;
8. The gate should be opened as far as possible in a position that does not affect the appearance, such as the edge bottom;
9. The size of the gate depends on the size, shape and function of the plastic part;
10. When planning a multi-cavity injection mold, consider the balance of the gate in combination with the balance of the runner, and try to make the molten material fill evenly.