The gate of a plastic mold refers to a thin and short passage connecting the runner and the strong flow, which is the entrance to the resin injection cavity. The shape, quantity, size, and position of the gate in the mold can have a significant impact on the quality of plastic parts. So the selection of the gate is one of the key points in plastic mold design. Below, we will introduce the gate from several aspects.

1、 The main functions of the gate are:

1. After the mold cavity is filled, the melt first condenses at the gate to prevent it from flowing back.

2. Easy to cut off gate tailings.

3. For multi cavity molds, it is used to control the position of fusion marks.

2、 Type of gate

Gates are generally divided into two types: non restrictive gates and restrictive gates. Restrictive gates are further divided into three series: side gates, point gates, and disk ring gates.

2.1 Non restrictive gate.

Non restrictive gates, also known as straight gates (as shown in Figure 1). Its characteristic is that the plastic melt directly flows into the mold cavity, with low pressure loss and fast feeding speed, making it easier to shape and suitable for various plastics. It has the advantages of good pressure transmission, strong pressure retention and contraction effect, simple and compact mold structure, and convenient manufacturing.

But it is difficult to remove the gate, and the gate marks are obvious; The slow condensation of heat near the gate is prone to generating significant internal stress, as well as causing shrinkage pits or surface indentations. Suitable for large plastic parts, thick walled plastic parts, etc.

2.2 Restriction gate.

The cavity is connected to the splitter channel by a channel with a short distance and a small cross-section at one end. This channel is called a restrictive gate, which can limit the thickness and rapid solidification of the gate. The main types of restricted gates are:

2.2.1 Point gate.

A point gate is a circular gate with a particularly small cross-sectional size (as shown in Figure 2).

The characteristics of point gates are:

1. Small restriction on gate position;

2. After removing the gate, the remaining traces are small and do not affect the appearance of the plastic part;

3. The gate can automatically break during mold opening, which is conducive to automated operation;

4. The stress caused by the replenishment of the gate attachment is small.

The disadvantages are: 1. The pressure loss is large, and the mold must adopt a three plate mold structure. The mold structure is complex, and there must be a sequential parting mechanism. It can also be applied to a two plate mold structure without flow channels.

2.2.2 Latent gate.

The latent gate evolved from the point gate, with its runner opening on the parting surface. The gate penetrates below the parting surface and enters the mold cavity diagonally. In addition to the characteristics of the point gate, the feeding gate of the latent gate is generally hidden on the inner surface or side of the plastic part, which does not affect the appearance of the plastic part. The plastic part and runner are respectively equipped with ejection mechanisms, and the gate is automatically cut off when the mold is opened, and the runner condensate automatically falls off.

2.2.3 Side gate

The side gate, also known as the edge gate, is generally set on the parting surface and feeds from the outer surface of the mold cavity (plastic part) (as shown in Figure 5). Side gate is a typical rectangular cross-section gate that can conveniently adjust the shear rate and gate closure time during mold filling, hence it is also known as a standard gate.

The characteristic of the side gate is its simple cross-sectional shape, convenient processing, and the ability to accurately process the gate size; Flexible selection of gate positions to improve mold filling conditions; Corrections can be made without removing the mold from the injection molding machine; Easy to remove the gate, with small marks. The side gate is particularly suitable for two plate multi cavity molds.

However, plastic parts are prone to defects such as welding marks, keyholes, and depressions, resulting in large injection pressure loss and poor exhaust for shell shaped plastic parts.

2.2.4 Overlapping gate

Overlap gate, also known as overlap gate, is basically the same as side gate, but the gate is not on the side of the cavity, but on one side of the cavity (as shown in Figure 6). It is a typical impact gate that can effectively prevent the spray flow of plastic melt. If the forming conditions are not suitable, surface pits will be generated at the gate. Cutting the gate is quite difficult, as it will leave obvious gate marks on the surface of the plastic part.

2.2.5 Fan gate

A fan-shaped gate is a gradually expanding gate, which is a variant form of side gate and is commonly used to shape wide plate-shaped plastic parts (as shown in Figure 7).

The gate gradually widens along the feeding direction and the thickness gradually decreases to the thinnest. The plastic melt is evenly distributed in the width direction, which can reduce the internal stress of the plastic part and reduce warping deformation; The cavity has good exhaust capacity to avoid surrounding air. But it is difficult to cut the gate and the marks are obvious.

2.2.6 Flat Gate

Also known as sheet gate, it is a variant form of side gate and is commonly used to shape large flat plastic parts (as shown in Figure 8). The distribution channel of the gate is parallel to the side of the mold cavity, called a parallel channel, and its length can be greater than or equal to the width of the plastic part.

The plastic melt is first evenly distributed in the parallel flow channel, and then flows in parallel at a lower linear speed, evenly entering the mold cavity. Therefore, the internal stress of the plastic part is small, reducing the warping deformation caused by stress orientation, and the mold cavity has good exhaust. But the workload of gate cutting is large, and the marks are obvious.

2.2.7 Disc gate

Disk shaped gates are used for cylindrical plastic parts with larger inner holes or plastic parts with larger square inner holes (as shown in Figure 9). The gate is located around the entire inner hole.

The plastic melt enters the mold cavity at approximately the same speed around the inner hole, and the plastic part will not produce fusion marks. The core is subjected to uniform force, and the air is discharged in sequence. For our products, disc gate applications are rare.

2.2.8 Circular gate

The circular gate is located on the outer side concentric with the cylindrical cavity, that is, gates are set around the cavity, so it can be called an outer circular gate, and its gate position corresponds exactly to the inner disc gate (as shown in Figure 10). Suitable for thin-walled long tube plastic parts.

Due to the uniform entry of plastic melt around the core into the mold cavity, uniform mold filling, good exhaust effect, and no fusion marks on the plastic part. But it is difficult to remove the gate and leave obvious gate marks on the outer side of the plastic part. Circular gate is commonly used for small, multi cavity molds.

2.2.9 Spoke gate and claw gate

The application range of spoke gate is similar to that of disc gate, and plastic parts with rectangular inner holes are also suitable. It changes the entire peripheral feeding into several small arcs or straight feeding, so it can be regarded as an inner gate. This gate is easy to cut, with less solidification in the flow channel, and the upper part of the core is positioned to increase the stability of the core. However, the welding marks on the plastic parts affect the strength and appearance quality of the plastic parts (as shown in Figure 11).

Claw gate is a variant form of spoke gate, which opens a flow channel on the conical section of the core. Mainly used for long tubular plastic parts with smaller inner holes or plastic parts with high coaxiality requirements.

3、 Gate position selection

The location and quantity of gates often determine the appearance quality and performance of a product. Therefore, when selecting the location and quantity of gates, the following points should be followed:

1. The gate should be located in a position that allows all corners of the mold cavity to be filled simultaneously;

2. The plastic injected into the mold cavity should maintain a uniform and stable flow rate at all stages of the injection molding process.

3. The gate should be set at the thicker part of the product wall to allow the melt to flow into the thin section from the thick section, in order to facilitate material replenishment and ensure complete mold filling.

4. The selection of gate position should minimize the plastic filling process to reduce pressure loss.

5. The position of the gate should be selected in a location that is conducive to eliminating gas from the mold cavity;

6. The gate should not allow the molten material to rush directly into the mold cavity, otherwise it will generate vortex flow and leave swirling marks on the plastic part, especially narrow gates are more prone to such defects;

7. Consideration should be given to the possibility of welding marks, bubbles, dents, voids, insufficient glue injection, and spraying of materials;

8. The position of the gate should be selected in a location that can avoid the formation of fusion lines on the surface of the product. When the generation of fusion lines cannot be avoided, the selection of gate position should consider whether the location where the fusion lines are generated is appropriate;

9. The selection of the gate position should prevent the formation of seam lines on the plastic surface, especially in circular or cylindrical plastic parts. A cold well should be added at the melting point of the gate surface

10. The setting of the gate should avoid causing melt fracture.

11. When the projection area of the product is large, avoid setting up a gate on one side to prevent uneven injection force

12. The gate should be set in a location that does not affect the appearance of the product

13. Do not install gates in the parts of the product that bear bending or impact loads. Generally, the strength near the gate of the product is the worst;

14. The gate position of an injection mold with a slender core should be far from the formed core to prevent deformation caused by material flow;

15. When forming large or flat plastic parts, compound gates can be used to prevent warping, deformation, and material shortage;

16. Efforts should be made to make the operation of the water outlet easy, preferably automatic;

4、 Commonly used gate types in production.

In production, the following types of gates are used according to product structure, appearance requirements, and automation requirements:

1. Straight gate.

In our commonly used plastic turnover boxes, due to their large structural dimensions and no special requirements for appearance gates, we use straight gates with simple gate structures and easy processing, but it is difficult to remove the material handle. As shown in Figure 12

2. Side gate.

For partially transparent components, since gate positions are not allowed on both sides, a side gate type is adopted (as shown in Figure 13), but it cannot meet the requirements of automated production and requires manual trimming of the material handle.

3. Latent gate

The latent gate is the most commonly used gate type in our injection mold. Most of our functional components, switch fixing frames, etc. use the latent gate type with the gate on the outside as shown in Figure 14, while most product shells use the latent gate type with the gate on the inside as shown in Figure 15.

This type of gate can automatically drop materials, meet automation requirements, and leave no gate marks on the surface of the product.

The commonly used gates are the above, but there are also some special types of gate applications depending on product requirements. Overall, when determining the gate type of a mold, it is necessary to fully consider various requirements. In a set of molds, either a single gate or a combination of different gates can be used, with the ultimate goal of producing qualified products.