The gate is where the material flows into the mold. It is the opening between the runner and the mold cavity. The type and location of the gate determines whether the molded product can be created satisfactorily with great quality.In the design of a plastic injection mold, according to the structural forms and characteristics of the gate, the commonly used types are shown as follows.
Pros: The plastic melt directly enters the cavity through the larger end of the sprue, so it features a smaller resistance to flow, a shorter runner and a longer compensation time. Such a gate is able to maintain a great melt flow status. The melt flows from the central portion of the cavity bottom to the parting surface, which facilitates ventilation; with a compact structure and an even stress on the injection molding machine, this gate design is able to minimize the projected area of the molded product and the gating system on the parting surface.
Cons: There is a high residual stress around the gate, which tends to cause the plastic injection molded product to warp or deform. At the same time, the gate is large and it is not easy to remove the gate mark, which is also very large and therefore affects product appearance. As a result, this gate type is mostly used for the injection molding of large or medium sized products, or products that feature long runners / deep cavities, or cylindrical or shell-shaped products, especially suitable for high viscosity plastics such as polycarbonate and polysulfone. In addition, this gate type is only applicable to single cavity molds.
2. Side Gate
In foreign countries, the side gate is referred to as the standard gate. The side gate is generally designed on the parting surface. The plastic melt fills the mold cavity from the inside or the outside, the cross-section is mostly rectangular (flat groove), and the shear rate of the melt and the freezing time of the gate can be adjusted by changing the width and thickness of the gate. The location of the gate can be selected according to the shape of the plastic injection molded product, and is convenient for processing and trimming, so it is broadly applied.
Pros: The small cross section of the gate can reduce the consumption of melt in the gating system, and it is easy to remove the gate, leaving an unobvious mark on the product. The gate is suitable for plastic injection molded products of various shapes, but not suitable for slender barrel-shaped ones.
Cons: The plastic injection molded product and the gate cannot separate from each other automatically, there are weld marks, and the injection pressure loss is huge, which is unfavorable for the ventilation of deep cavity molded products.
The point gate is especially suitable for cylindrical, shell-shaped and box-shaped plastic products. For larger flat plastic products, multiple point gates can be designed to reduce warping and deformation; for thin-walled plastic products, the shear rate near the gate is too high, leading to high residual stress, so the product is easy to crack. It is recommended to locally increase the thickness at the gate location.
Pros: There is little limitation on gate location, the gate mark is rather small, and the gate can be automatically removed when the mold opens, which is convenient for automatic operation.
Cons: The injection pressure is relatively high. In most cases, a three-plate mold structure must be applied. The mold is relatively more complicated, with a longer molding cycle.
4. Submarine Gate
Pros: The submarine gate (see Figure 7) is more flexible, because the melt is allowed to enter from both the inside and outside surfaces of a plastic product. The runner is created on the parting surface, and the gate is designed under the parting surface, with the molten plastic entering the cavity obliquely. Since the injector mechanisms are separately designed on the plastic product and the runner, the gate is automatically removed when the mold opens, and the cold slug will automatically fall off. At the same time, the mold structure is simpler than the three-plate mold, which greatly improves production efficiency while reducing production cost.
Cons: It is not suitable for plastics that are too tough (such as PA) or too brittle (such as PS), since the former is not easy to cut, while the latter cracks and blocks the gate easily.
5. Banana Gate
The banana gate is in fact a kind of circular-shaped submarine gate structure. Usually used for two-plate molds, the gate allows the melt to flow in directly from the parting surface, and the two inserts created will be pieced together. The runner and gate are both designed on the insert.
Pros: The melt can be fed in from under the product, showing the features of the point gate, but with a smaller gate mark. The gate can be removed automatically when ejected, thus able to facilitate automation. It will facilitate ejection if the head of the ejector pin is tapered and an ejection mechanism is designed in the runner.
Cons: The shape is more complicated, and it is necessary to electrode process the gate.
6. Fan Gate
As shown in Figure 3, the fan gate is usually designed on the parting surface, and the molten plastic is fed in from the outer side of the cavity. The gate is gradually widened along the feeding direction, while the thickness is gradually reduced. The plastic melt front entering the cavity from the gate is relatively flat, which helps reduce warping and deformation, so it is quite suitable for production of wide plastic sheets.