Core Tip: The gate, also known as the feed port, is the channel that connects the split runner to the cavity melt. Whether the gate is properly selected or not is directly related to whether the injected product can be injection molded with good quality. The gate design includes the shape and size of the gate section and the gate location. The determination of the shape and size of the gate section has been mentioned in many textbooks and will not be repeated here. According to the characteristics of different injection molding products, the author compares the differences among various types of gates and discusses the selection methods and principles of the gate location and its structural form. The position of the gate 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. The factors affecting the determination of the gate location include the shape, size, wall thickness, dimensional accuracy, and appearance quality of the product.

One. The type and location of the gate

In the injection mold design, according to the structure and characteristics of the gate, the following 11 kinds of commonly used gate forms.

1. Sprue: This is the main gate, which is non-limiting gate. See Figure 1.

(1) Advantages: The plastic melt enters the cavity directly from the big end of the main flow path, so it has the characteristics of small flow resistance, short flow and long replenishment time. This kind of gate has a good melt flow state, the melt flows from the center of the bottom surface of the cavity to the parting surface, which is beneficial to the exhaust; this gate form minimizes the projected area of the injection molding product and the casting system on the parting surface. The mold structure is compact and the injection molding machine is evenly loaded.

(2) Disadvantages: There is a large residual stress at the feed, which can easily lead to warpage of the injection-molded product. At the same time, the gate is large. It is difficult to remove the traces of the gate and the traces are large, affecting the appearance, so this type of gate is mostly used. Injection molding Large and medium-sized long process, deep cavity, cylindrical or shell-shaped injection molding products, especially suitable for high viscosity plastics such as polycarbonate, polysulfone and so on. In addition, this type of gate is only suitable for single cavity molds.

When designing this type of gate, in order to reduce the gate area at the contact with the injection-molded product and prevent defects such as shrinkage and deformation at the place, the taper angle of the main channel of the taper should be as small as possible (2~4). °) On the other hand, the thickness of the fixed platen and fixed mold base should be reduced as much as possible.

2. Side gates: Foreign side gates are called standard gates, see Figure 2. The side gate is generally set on the parting surface. The plastic melt fills the mold cavity from inside or outside. The cross-sectional shape is mostly rectangular (flat groove). Changing the width and thickness of the gate can adjust the shear rate and pouring of the melt. The freezing time of the mouth. This kind of gate can choose its position according to the shape features of injection molding products, and is convenient for processing and trimming, so its application is more extensive.

(1) Advantages: The gate section is small, which can reduce the consumption of molten material in the pouring system. It is easy to remove the gate and the trace is not obvious. Suitable for injection molding products of various shapes, but should not be used for slender barrel injection products.

(2) Disadvantages: Injection molded products and gates cannot be separated on their own. There are welding marks, and the injection pressure loss is large, which is unfavorable to the exhaust of deep cavity injection molded products.

3. Fan-shaped gates: Fan-shaped gates, as shown in Fig. 3, are generally set on the parting surface and fed from the outside of the cavity. The gate gradually widens along the feeding direction, and the thickness gradually decreases. The wave front of the plastic melt entering the cavity from the sprue is relatively flat, which can reduce warpage and is suitable for forming a plate-shaped plastic product with a relatively large width.

4. Sheet gate: also known as flat slot gate, see Figure 4. The distribution channel of the gate is parallel to the side of the cavity and its length is usually larger than the width of the plastic product.

(1) Advantages: The plastic melt enters the cavity uniformly at a relatively low speed through the gate of the sheet, and the material flow is in parallel flow to avoid warpage and distortion. It is commonly used to form flat large-area thin-walled plastic products.

(2) Disadvantages: It is more difficult to remove the sprue, thereby increasing the production cost of the plastic product.

5. Ear protection gate: The ear protection gate (see Figure 5) is mainly used for high transparent flat plastic products and plastic products with small deformation requirements.

(1)Advantages: The ear protection gate is provided with ear grooves on the side of the cavity. The melt will generate frictional heat on the side of the ear groove through the gate, thus improving the fluidity. After adjusting the direction and speed, the ear protection will be uniform. Smoothly into the cavity, you can avoid jets.

(2) Disadvantages: Gate removal is more difficult and gate marks are larger.

6. Point gates: Point gates are particularly suitable for drum-shaped, shell-shaped and box-shaped plastic products. For larger plate-shaped plastic products, multiple point gates can be set to reduce the warpage; for thin-walled plastic products, the shear rate near the gate is too high, the residual stress is large, and it is easy to crack, and can be locally increased. The wall thickness at the mouth is shown in Figure 6.

(1) Advantages: The position of the gate is limited, the trace of the gate is small, and the gate can be automatically pulled when the mold is opened, which is conducive to automation.

(2) Disadvantages: The injection pressure is large. In most cases, a three-plate mold structure must be used. The mold is relatively complex and the molding cycle is long.

7. Latent gates

(1) Advantages: The location of the submerged gate (see Figure 7) is flexible, and it can be used in both plastic and plastic surfaces. The flow path is set on the parting surface, the gate is submerged under the parting surface, and the melt enters the cavity obliquely. Since the ejection mechanism is provided in the plastic product and the flow channel respectively, the gate is automatically cut off when the mold is opened, and the flow channel aggregate is automatically dropped off. At the same time, its mold structure is simpler than the three-plate mold structure, which greatly improves the production efficiency and reduces the cost.

(2) Disadvantages: It is not suitable for plastics that are too tough (such as PA) or too brittle (such as PS). The former is not easy to cut, the latter is easy to break, and it is easy to clog the gate.

8. Moon-shaped gate: The moon-shaped gate (see Figure 8) is actually a circular-arc-shaped submerged gate structure. It is generally used for two-plate molds. The block inserts are split and the runners and gates are designed on the inserts.

(1)Advantages: It can be glued into the bottom of the product, with the characteristics of the point gate, small traces of the gate, automatic pull-out when ejected, and easy automation. If the head of the ejector pin is made into a conical shape, an ejection mechanism is provided in the flow passage to facilitate ejection. (2) Disadvantages: The shape is more complicated and the gate needs to be processed with electrodes.

9. Ring gate

The gate that takes the form of a circular feed to fill the cavity is called an annular gate, as shown in Figure 9.

(1) Advantages: Feeding is uniform, the flow velocity is roughly the same throughout the circumference, the flow condition is good, the air in the cavity is easy to be eliminated, and weld marks can be avoided. Because the gate is designed on the core, the ring gate is mainly used for forming cylindrical bottomless plastic products.

(2) Disadvantages: The gating system consumes more material, the sprue removal is difficult, and the gate marks are obvious.

10. Umbrella gates: Umbrella gates (see Figure 10) are special forms of ring gates that are mainly used for short and thick tubular plastic products with high quality requirements.

(1) Advantages: uniform feed, no weld marks, good exhaust.

(2) Disadvantages: The removal of the flow channel must be carried out by cutting, which increases the cost.

11. Disc gates: Disc gates (see Figure 11) are actually umbrella gates with a 180° top angle, used for cylindrical plastic articles with larger inner bores, or with larger rectangular bores Plastic products with gates around the entire inner hole.

(1) Advantages: The utility model has the characteristics of an umbrella gate, the plastic melt is injected into the cavity from the periphery of the inner hole in a substantially synchronous manner, the core is subjected to uniform force, welding marks can be avoided, and the exhaust gas is smooth.

(2) Disadvantage: It will leave obvious gate marks on the inner edge of plastic products.

二. Selection of common plastic products into the form of plastic

(1) Axial symmetry products: Products such as gears, blades, etc. have a higher rotational speed in operation, and the surface should be smooth and flat, without bubbles, etc. in order to ensure wear resistance. Such products require that the functional surface (generally the side) cannot have Defects, the bottom surface needs to be leveled, so the gate of such products should be as small as possible and must be concealed. Therefore, it is not suitable to use submerged gates (including submerged ejectors) and other difficult-to-remove gates. It is best to use a moon-shaped gate, or use a three-plate mold to set point gates for good flow characteristics.

(2) Tubular products: The frit should be first filled with the circumference of one end and then filled with the full length of the tube itself. This allows the melt flow front to avoid creating asymmetrical shapes. Common disk gates, umbrella gates, ring gates and submerged gates.

(3) cup-shaped products: like small shells, capacitor cups and the like, the gate should be designed near the base to avoid air pockets. Commonly used point gates or latent gates dive near the base.

(4) Slender products: Gates should be placed longitudinally rather than horizontally, or at the center. Side gates, submerged gates, and moon-shaped gates are commonly used.

(5) Spoke or mesh products: The use of multi-gate form.

(6) Integral articulated products: Keep welding marks away from the hinge points. In any case, it should be avoided that the flow stop part of the melt is designed near the hinge point, and side gates or submerged gates can be used.

(7) Transparent plastic products: In order to avoid snakes and other defects in the products, ear gates are often used.

(8) Electroplating plastic products: The side gates are used together with the plastic products to eject them, and the electroplating hanging points are made on the flow channels. After the electroplating, the gates are removed.

(9) Thin-walled products: Point gates are suitable for thin-walled products. If injected at high speeds, spraying may occur. Set the gate at the thickest part of the product and keep the thickness uniform to avoid distortion.

(10) Flat plastic products: Fan gates or sheet gates are often used. This type of gate feed is even, no weld marks are generated, and the exhaust is good.

(11) Plastic products with metal inserts: Allow the melt to flow around the inserts to minimize the inaccuracy of the inserts. Side gates or submerged gates are often used.

III. Gate Location Selection Principle

The position of the gate is mainly determined based on the geometric shape and technical requirements of the product, and the flow of the melt in the flow path and cavity, filling, feeding and exhausting are determined. Generally should follow the following principles:

1 The gate should be located in the thicker section of the plastic product, so that the molten material flows from the thick section into the thin section to ensure that the mold is completely filled;

2 to minimize the filling process of the molten material, minimize the change of the flow direction, and minimize the energy loss so as to reduce the pressure loss;

3 is conducive to the exclusion of air in the cavity;

4 Gates should not allow the molten material to directly enter the cavity, otherwise it will generate turbulence, leaving spiral marks on the plastic products. Especially the narrow gate is more prone to such defects, preferably from the right cavity The orientation of the wall or coarse and large core enters, changing the flow direction, filling the cavity at a low flow rate smoothly, avoiding the melt fracture phenomenon and eliminating the welding marks on the plastic products;

5 Should prevent weld marks on the surface of plastic products, especially for circular or cylindrical plastic products, should open the cold material well in the melting surface of the sprue;

6 The position of the gate with the slender core injection mold should be far away from the forming core, and the forming core must not be deformed by the material flow.

7 When molding large or flat plastic products, in order to prevent warping deformation and lack of material, multiple gates can be used;

8 Gates should be set up as far as possible without affecting the appearance of plastic products, such as edges and bottoms;

9 When designing a multi-cavity injection mold, consider the balance of the gates in conjunction with the balance of the runners, and try to make the melt fill the cavities at the same time and evenly.

Conclusion: Whether the design of the gate and the choice of location is appropriate or not is directly related to whether the plastic product can be injection molded with good quality and high quality. The form of the gate and the location of the opening not only have a great influence on the molding performance and the molding quality of the plastic product, but also affect the overall structure of the mold. Therefore, a reasonable choice of gate form and opening position is an important part of improving the quality of plastic products. When selecting the form and location of the gate, it is necessary to consider comprehensively the plastic product's structure, process characteristics, and molding quality requirements, and analyze the process characteristics of the plastic raw material, the flow state of the plastic melt in the cavity, and the molding process conditions.