Gate of plastic mold refers to a section of short runner connecting sub runner and cavity, which is entrance for resin to be injected into cavity. Shape, quantity, size and location of gates in the mold will have a great impact on quality of plastic parts. Therefore, selection of gate is one of key points in design of plastic mold. Gate will be introduced in several aspects below.

1. After cavity is full, melt will first condense at gate to prevent it from flowing back.
2. Easy to remove gate tailings.
3. For multi-cavity molds, it is used to control position of weld marks.

Gates are generally divided into two types: unrestricted gates and restricted gates. Restrictive gates are divided into three series: side gates, point gates and disc ring gates.

Unrestricted gates are also called straight gates (as shown in Figure 1). Its characteristic is that plastic melt flows directly into cavity, pressure loss is small, feeding speed is fast and molding is easier, it is suitable for all kinds of plastics. It has advantages of good transmission pressure, strong pressure maintaining and shrinking effect, simple and compact mold structure, convenient manufacturing. However, it is difficult to remove gate, marks of gate are obvious; heat concentration near gate is slow and it is easy to produce large internal stress, it is also easy to produce shrinkage pits or surface shrinkage. Suitable for large plastic parts, thick-walled plastic parts, etc.

 

Figure 1 Straight gate type

Cavity and sub runner are connected by a channel with a short distance at one end and a small cross-section. This channel is called a restrictive gate, which limits thickness of gate and rapid solidification. Main types of restricted gates are:

A point gate is a round gate with a very small cross-sectional size (as shown in Figure 2).
Characteristics of point gate are:
1. Small gate position restriction;
2. After removing gate, residual trace is small, and it does not affect appearance of plastic part;
3. Gate can be automatically broken when mold is opened, which is conducive to automatic operation;
4. Stress caused by supplementary material of gate attachment is small.
Weakness is:

Pressure loss is large, mold must adopt a three-plate mold structure. Mold structure is complicated, and it must have a sequential mold splitting mechanism. It can also be applied to a two-plate mold structure without runners.

 

Figure 2 Point gate type

Latent gate evolved from a point gate. Its runner is opened on parting surface. Gate sneaks under parting surface and enters cavity in an oblique direction. In addition to feature of point gate, latent gate is generally located on inner surface or hidden side of plastic part, so it does not affect appearance of plastic part. Plastic part and runner are equipped with an ejection mechanism, gate is automatically cut when mold is opened, and runner condensate automatically falls off.

 

Figure 3 Outside latent gate

 

Figure 4 Inside latent gate              

Side gate, also called edge gate, is generally opened on parting surface and feeds from outer side of cavity (plastic part) (as shown in Figure 5). Side gate is a typical rectangular cross-section gate, which can easily adjust shear rate and gate closing time during mold filling, so it is also called a standard gate. Side gate is characterized by simple gate cross-section shape, convenient processing, and precise processing of gate size; flexible gate position selection to improve mold filling; correction can be made without removing mold from injection molding machine; it is convenient to remove gate and has small traces. Side gate is especially suitable for two-plate multi-cavity molds. However, plastic parts are prone to defects such as weld marks, keyholes, dents, etc., injection pressure loss is large, exhaust gas of shell-shaped plastic parts is poor.

 

Figure 5 Basic type of side gate

Overlapping gates are basically same as side gates, but gate is not on the side of cavity, but on one side of cavity (as shown in Figure 6). It is a typical impact gate, which can effectively prevent jet flow of plastic melt. If forming conditions are improper, surface pits will be generated at gate. It is difficult to remove gate, which will leave obvious gate marks on the surface of plastic part.

 

Figure 8 Basic type of overlapping gate

Fan gate is a gradually expanded gate, a variant of side gate, and is often used to form large-width plate-shaped plastic parts (as shown in Figure 7). Gate gradually widens along feed direction, and the thickness is gradually reduced to the thinnest. Plastic melt is evenly distributed in width direction, which can reduce internal stress of plastic part and reduce warping deformation; cavity has a good exhaust gas and avoids surrounding air. However, gate removal is difficult and marks are obvious.

Also known as sheet gate, it is also a variant form of side gate, which is often used to form large-area flat plastic parts (as shown in Figure 8). Distribution channel of gate is parallel to side of cavity, called parallel channel, and its length can be greater than or equal to width of plastic part. Plastic melt is firstly distributed uniformly in parallel runners, then flows in parallel at a lower linear speed, and enters cavity uniformly. Therefore, internal stress of plastic part is small, warpage deformation caused by orientation is reduced, cavity is well vented. However, gate removal workload is large and traces are obvious.

Disc gates are used for cylindrical plastic parts with larger inner holes or plastic parts with larger square inner holes (as shown in Figure 9). Gate is on entire periphery of inner hole. Plastic melt enters cavity from periphery of inner hole at approximately same speed, plastic part will not produce welding marks, core is uniformly stressed, and air is removed sequentially. For our products, disc gate applications are rare.

 

Figure 7 Basic form of sector gate

 

Figure 8 Basic pattern of flat seam crossing

 

Figure 9 Basic type of disc gate

Annular gate is arranged on outer side concentric with cylindrical cavity, that is, gates are arranged around cavity, so it can be called an outer annular gate, and its gate position exactly corresponds to inner disc gate (As shown in Figure 10). Suitable for thin-walled and long-tube plastic parts. Since melt of plastic part enters cavity evenly around core, mold is evenly filled, exhaust effect is good, plastic part has no welding marks. However, it is difficult to remove gate and leave obvious gate marks on the outside of plastic part. Ring-shaped gates are mostly used for small, multi-cavity molds.

 

Figure 10 Basic form of ring gate

Scope of application of spoke gate is similar to disc gate, and plastic parts with rectangular inner hole are also applicable. It changes entire peripheral feed into a few small arcs or straight feeds, so it can be regarded as an inner side. This gate is easy to cut, runner is less aggregated, upper part of core is positioned to increase stability of core, but weld mark on plastic part affects strength and appearance quality of plastic part (as shown in Figure 11).
Claw gate is a variant form of spoke gate, and a runner is opened on tapered section of core. Mainly used for long tubular plastic parts with small inner holes or plastic parts with high coaxiality requirements.

 

Figure 11 Basic form of spoke gate and claw gate

Location and quantity of gate often determine appearance quality and performance of product, so when choosing location and quantity of gate, following points should be followed:
1. Gate should be located at a position that can fill all corners of cavity at the same time;
2. Plastic injected into cavity should maintain a uniform and stable flow rate at all stages of injection molding process.
3. Gate should be set at thicker part of product wall, so that melt flows from thick section into thin section, so as to facilitate filling and ensure complete filling of mold.
4. Selection of gate position should make plastic filling process the shortest to reduce pressure loss.
5. Position of gate should be selected at a position that is conducive to removing gas in cavity;
6. It is not advisable for gate to rush melt directly into cavity, otherwise it will produce a swirling flow, leaving swirling marks on plastic part, especially narrow gate is more prone to such defects;
7. Possible occurrence of weld marks, bubbles, depressions, vacant positions, insufficient glue and spray materials should be considered;
8. Location of gate should be chosen to avoid fusion pattern on the surface of product. When generation of weld lines cannot be avoided, selection of gate position should consider whether location of weld lines is appropriate;
9. Choice of gate position should prevent production of splicing lines on plastic surface, especially in circular or cylindrical plastic parts, a cold material well should be opened at melt pouring on the surface of gate.
10. Setting of gate should avoid phenomenon of causing melt fracture.
11. When projected area of product is large, avoid opening a gate on one side to prevent uneven injection force.
12. Gate should be set at a position that does not affect appearance of product.
13. Do not set gates in parts that bear bending load or impact load in product. Generally, strength near gate of product is the worst;
14. Gate position of injection mold with a slender core should be far away from molding core, so that molding core is not deformed by impact of material flow;
15. When forming large or flat plastic parts, in order to prevent warping, deformation, and lack of material, multiple gates can be used;
16. Operation of dewatering port should be made as easy as possible, and it is best to operate automatically;

Different plastic raw materials can be used for different gates. Following table shows preferred series of gate types that can be used for different raw materials:

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

In our commonly used plastic turnover boxes, etc., due to their large structural size and no special requirements for appearance gates, etc., straight gates with simple gate structure and easy processing are adopted, but it is difficult to remove material. As shown in Figure 12:

 

Picture Figure 12 Application example of direct gate (turnover box)

In our products, partially transparent parts, such as TZ-C1041, TZ-C1051, T-C1061 lens, etc,  are not allowed to have gate positions on front and back sides, so side gate type is adopted (as shown in Figure 13), but it cannot meet automated production and requires manual trimming of material handle.

 

Figure 13 Application example of side gate Gate position

Latent gate is the most commonly used gate type for our injection molds. Most of our functional parts and switch fixing frames use latent gate type with gate on outside as shown in Figure 14. Product shell adopts latent gate type on inner side as shown in Figure 15. This kind of gate can automatically blank material to meet needs of automation, and there are no gate marks on the surface of product.

 

 

Gates often used in our products are above, but according to different product requirements, there are some special types of gate applications, such as meniscus gate in upper and lower shell molds of TZ-CON01 (as shown in Figure 16). ). Generally speaking, when determining gate type, mold must fully consider requirements of various aspects. In a set of molds, either one gate or a combination of different gates can be used (for example, upper and lower shell molds of TZ-CON01 have both a meniscus gate and an external latent gate, as shown in Figure 17), final goal is to produce qualified products.

 

 

Figure 17 Use of meniscus gate and latent gate