The gate in a plastic mold is a short, narrow runner connecting the runner and the cavity, serving as the entrance for resin injection into the cavity. The shape, number, size, and location of the gate significantly impact the quality of the plastic part. Therefore, gate selection is a key aspect of plastic mold design. The following section introduces gates from several aspects.
I. Main Functions of the Gate:
1. After the cavity is filled, the melt solidifies first at the gate, preventing backflow.
2. Facilitates the removal of gate tail material.
3. For multi-cavity molds, it controls the location of weld lines.
II. Types of Gates
Gates are generally classified into two types: unrestricted gates and restricted gates. Restricted gates are further divided into three series: side gates, point gates, and disc ring gates.
2.1 Unrestricted Gates
Unrestricted gates are also called direct gates (as shown in Figure 1). Its characteristics include direct flow of molten plastic into the mold cavity, low pressure loss, fast feeding speed, and easy molding, making it suitable for various plastics. It offers advantages such as good pressure transmission, strong pressure holding and shrinkage compensation, simple and compact mold structure, and easy manufacturing. However, gate removal is difficult, and gate marks are obvious; heat concentration near the gate leads to slow solidification, easily generating large internal stresses and prone to shrinkage pits or surface concavity. It is suitable for large plastic parts and thick-walled plastic parts.
Figure 1: Direct Gate Type
2.2 Restricted Gate
The mold cavity and runner are connected by a channel with a very short distance at one end and a small cross-section. This channel is called a restricted gate, which can limit the gate thickness and rapid solidification. The main types of restricted gates are:
2.2.1 Point Gate
A point gate is a circular gate with a very small cross-sectional dimension (as shown in Figure 2).
The characteristics of point gates are:
1. Less restriction on gate location;
2. Minimal residue after gate removal, not affecting the appearance of the plastic part;
3. The gate can automatically break off during mold opening, facilitating automated operation;
4. Less stress caused by material replenishment from the gate accessories.
The disadvantages are:
1. Higher pressure loss; the mold must use a three-plate mold structure, resulting in a complex mold structure and requiring a sequential parting mechanism. It can also be applied to two-plate mold structures without runners.
Figure 2. Types of Point Gates
2.2.2 Submerged Gates
Submerged gates evolved from point gates. Their runners are located on the parting surface, and the gate is submerged below the parting surface, entering the cavity at an angle. Besides possessing the characteristics of point gates, submerged gates are generally located on the inner surface or side of the plastic part, thus not affecting its appearance. The plastic part and runners are equipped with ejection mechanisms, automatically cutting off the gate when the mold opens, and the solidified material in the runner automatically falls off.
Figure 3. External Submerged Gate
Figure 4. Internal Submerged Gate
2.2.3 Side Gates
Side gates, also called edge gates, are generally located on the parting surface, entering from the outer side of the cavity (plastic part) (as shown in Figure 5). Side gates are typically rectangular cross-section gates, allowing for convenient adjustment of the shear rate during mold filling and the gate closing time; therefore, they are also called standard gates. Side gates are characterized by their simple cross-sectional shape, ease of machining, and precision machining capabilities. They offer flexible gate placement to improve mold filling. Corrections can be made without removing the mold from the injection molding machine. Gate removal is convenient and leaves minimal traces. Side gates are particularly suitable for two-plate multi-cavity molds. However, they are prone to weld lines, keyholes, and depressions in the molded parts, resulting in significant injection pressure loss and poor venting for shell-shaped parts.
Figure 5: Basic Types of Side Gates
2.2.4 Overlapping Gates
Overlapping gates, also called lapped gates, are essentially the same as side gates, but the gate is not located on the side of the cavity, but on one side of the cavity (as shown in Figure 6). They are typical impact gates, effectively preventing the jetting flow of molten plastic. However, improper molding conditions can cause surface pits at the gate. Removing the gate is difficult and leaves noticeable gate marks on the molded part surface.
Figure 8. Basic Type of Overlapping Gate
2.2.5 Fan-Shaped Gate
The fan-shaped gate is a gradually expanding gate, a variation of the side gate, often used to mold wide, sheet-like plastic parts (as shown in Figure 7). The gate gradually widens along the feed direction, and its thickness gradually decreases to its thinnest point. The molten plastic is evenly distributed in the width direction, reducing internal stress in the plastic part and minimizing warpage; it also provides good cavity venting and avoids trapped air. However, gate removal is difficult, leaving noticeable marks.
2.2.6 Flat Slit Gate
Also known as a sheet gate, this is another variation of the side gate, often used to mold large, flat plastic parts (as shown in Figure 8). The gate's distribution channel is parallel to the side of the cavity, called a parallel runner, whose length can be greater than or equal to the width of the plastic part. The molten plastic is first evenly distributed in the parallel runner, then flows parallel at a lower linear velocity, uniformly entering the cavity. Therefore, the internal stress in the plastic part is low, reducing warpage caused by stress orientation, and cavity venting is good. However, gate removal is labor-intensive and leaves noticeable marks.
2.2.7 Disc Gate
Disc gates are used for cylindrical plastic parts with large inner holes, or plastic parts with large square inner holes (as shown in Figure 9). The gate is located around the entire perimeter of the inner hole. The molten plastic enters the cavity from the perimeter of the inner hole at approximately the same speed, preventing weld lines from forming on the plastic part, ensuring uniform stress on the core, and allowing air to escape sequentially. Disc gates are rarely used for our products.
Figure 7 Basic form of a fan-shaped gate
Figure 8 Basic form of a flat-slit gate
Figure 9 Basic form of a disc gate
2.2.8 Circular Gate
Circular gates are located on the outer side of the cylindrical cavity, concentric with it. Therefore, they are called outer circular gates, and their gate position corresponds exactly to the inner disc gate (as shown in Figure 10). Suitable for thin-walled, long tubular plastic parts. Because the molten plastic enters the cavity evenly around the core, the filling is uniform, venting is good, and there are no weld lines on the part. However, removing the gate is difficult, and obvious gate marks are left on the outside of the part. Circular gates are mostly used in small, multi-cavity molds.
Figure 10: Basic form of circular gate
2.2.9 Spoke gate and claw gate
The spoke gate has a similar application range to the disc gate, and is also suitable for plastic parts with rectangular inner holes. It changes the entire perimeter feed into several small arc or straight feed segments, so it can be considered an inner gate. This type of gate is easy to remove, has less solidified material in the runner, and the upper part of the core is positioned, increasing the stability of the core. However, weld lines on the part affect its strength and appearance quality (as shown in Figure 11).
The claw gate is a variation of the spoke gate, with a runner on the tapered cross-section of the core. It is mainly used for long tubular plastic parts with small inner holes or plastic parts with high coaxiality requirements.
Figure 11. Basic Forms of Spoke-Type and Claw-Type Gates
III. Gate Location Selection
The location and number of gates often determine the appearance quality and performance of the product. Therefore, the following points should be considered when selecting the gate location and number:
1. The gate should be located in a position that allows all corners of the cavity to be filled simultaneously.
2. The plastic injected into the cavity should maintain a uniform and stable flow rate at all stages of the injection molding process.
3. The gate should be located in the thicker part of the product wall, allowing the melt to flow from the thick section to the thin section, facilitating material replenishment and ensuring complete mold filling.
4. The gate location should be chosen to minimize the plastic filling path and reduce pressure loss.
5. The gate location should be chosen to facilitate the removal of gas from the mold cavity;
6. The gate should not allow molten material to flow directly into the mold cavity, otherwise it will create swirls, leaving swirling marks on the plastic part, especially narrow gates are more prone to this defect;
7. Potential weld lines, bubbles, depressions, voids, insufficient injection, and sprue should be considered;
8. The gate location should be chosen to avoid weld lines on the part's surface. When weld lines cannot be avoided, the gate location should be chosen considering whether the location of the weld lines is suitable;
9. The gate location should prevent the creation of seam lines on the plastic surface, especially in ring or cylindrical plastic parts. A cold slug well should be added at the molten material junction on the gate face;
10. The gate design should avoid causing melt fracture.
11. When the product's projected area is large, avoid opening the gate on only one side to prevent uneven injection stress.
12. The gate should be located in a position that does not affect the appearance of the product.
13. Do not place the gate in areas of the product that will bear bending or impact loads. Generally, the strength near the gate is the weakest.
14. For injection molds with slender cores, the gate position should be far from the core to prevent the core from being deformed by the material flow.
15. When molding large or flat plastic parts, a double gate can be used to prevent warping, deformation, and material shortage.
16. The gate removal operation should be made as easy as possible, preferably automated.
IV. Gating Type and Plastic Matching
Different plastic raw materials require different gating types. The table below shows the preferred gating types for different raw materials:
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V. Commonly Used Gating Types in Our Production
In our production, we use the following gating types based on product structure, appearance requirements, and automation needs:
1. Direct Gating
In commonly used plastic turnover boxes, due to their large structural dimensions and lack of special requirements for appearance gating, we use direct gating, which has a simple structure and is easy to process. However, removing the material handle is difficult. As shown in Figure 12:
Figure 12 Example of direct gate application (turnover box)
2. Side gate
In our products, some transparent parts, such as TZ-C1041, TZ-C1051, and T-C1061 lenses, use a side gate type (as shown in Figure 13) because gate positions are not allowed on either the front or back. However, this cannot meet the requirements of automated production and manual trimming of the material is required.
Figure 13 Example of side gate application: Gate position
3. Submerged gate
Submerged gates are the most commonly used gate type in our injection molds. Most of our functional parts, switch fixing frames, etc., use the submerged gate type with the gate on the outside, as shown in Figure 14, while most product shells use the submerged gate type with the gate on the inside, as shown in Figure 15. This type of gate allows for automatic material discharge, meeting automation requirements, and leaves no gate marks on the product surface.
The gates commonly used in our products are those mentioned above. However, depending on the product requirements, there are also some special types of gates used, such as the crescent-shaped gate in the TZ-CON01 upper and lower shell mold (as shown in Figure 16). Generally speaking, when determining the gate type for a mold, all aspects of the requirements must be fully considered. A single mold can use either one type of gate or a combination of different gates (for example, the TZ-CON01 upper and lower shell mold has both a crescent-shaped gate and an external submarine gate, as shown in Figure 17), with the ultimate goal of producing qualified products.
