The gate of the plastic mold refers to a short and thin runner connecting the runner and the cavity, which is the entrance for the resin to be injected into the cavity. The shape, quantity, size and position of the gate in the mold will have a great impact on the quality of the plastic part. Therefore, the selection of the gate is one of the key points in the design of plastic molds. The following introduces the gate through several aspects.
1. The main functions of the gate are:
1. After the cavity is filled, the melt will first condense at the gate to prevent its backflow.
2. It is easy to remove gate tailings.
3. For multi-cavity molds, it is used to control the position of weld lines.
2. Gate type
Gates are generally divided into two types: non-restricted gates and restricted gates. Restricted gates are further divided into three series: side gates, point gates and disk ring gates.
2.1 Unrestricted Gates
Unrestricted gate is also called straight gate (as shown in Figure 1). Its characteristic is that the plastic melt flows directly into the mold cavity, the pressure loss is small, the feeding speed is fast, and the molding is easy, and it is suitable for various plastics. It has the advantages of good pressure transmission, strong pressure holding and feeding, simple and compact mold structure, and convenient manufacture. However, it is difficult to remove the gate, and the traces of the gate are obvious; the heat concentration near the gate is slow to condense, and it is easy to generate large internal stress, and it is also easy to produce shrinkage pits or surface shrinkage. Suitable for large plastic parts, thick-walled plastic parts, etc.
2.2 Restricted gate
The cavity and the runner are connected by a channel with a short distance at one end and a small section. 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
The point gate is a circular gate with a very small cross-sectional size (as shown in Figure 2).
The characteristics of point gate are:
1. The gate position is limited;
2. After removing the gate, the residual traces are small, which does not affect the appearance of the plastic part;
3. The gate can be automatically broken when the mold is opened, which is conducive to automatic operation;
4. The stress caused by feeding the gate accessories is small.
weakness is:
1. The pressure loss is large, the mold must adopt a three-plate mold structure, the mold structure is complex, and there must be a sequential mold parting mechanism, and it can also be applied to a two-plate mold structure without runners.
2.2.2 Latent gate
The latent gate is evolved from the point gate. The runner is opened on the parting surface. The gate sneaks under the parting surface and enters the cavity along the oblique direction. In addition to the characteristics of the point gate, the latent gate has the characteristics of a point gate. , the feed gates are generally hidden on the inner surface or side of the plastic part, so it does not affect the appearance of the plastic part. The plastic part and the runner are respectively equipped with a push-out mechanism. The gate is automatically cut off when the mold is opened, and the flow channel condenses Automatic fall off
2.2.3 Side gate
The side gate is also called the edge gate, which is generally opened on the parting surface and fed from the outer side of the cavity (plastic part) (as shown in Figure 5). The side gate is a typical rectangular cross-section gate, which can easily adjust the shear rate and gate closing time during mold filling, so it is also called a standard gate. The characteristics of the side gate are that the shape of the gate section is simple, the processing is convenient, and the size of the gate can be precisely processed; the position of the gate can be selected flexibly in order to improve the filling condition; it can be corrected without unloading the mold from the injection molding machine; The mouth is convenient and the trace is small. Side gates are especially suitable for two-plate multi-cavity molds. However, plastic parts are prone to defects such as weld marks, keyholes, and depressions, and the injection pressure loss is large, and the exhaust of shell-shaped plastic parts is poor.
2.2.4 Overlapping gates
The overlapping gate is also called the lap gate, which is basically the same as the 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, which can effectively prevent the injection flow of plastic melt. If the forming conditions are not appropriate, surface pits will be generated at the gate. It is difficult to remove the gate, and it will leave obvious gate marks on the surface of the plastic part.
2.2.5 Fan gate
The fan gate is a gradually expanded gate, which is a variation of the side gate, and is often used to form a wide plate-shaped plastic part (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 a good air volume and avoids surrounding air. However, gate removal is difficult and the traces are obvious.
2.2.6 Flat seam gate
Also known as the sheet gate, it is also a variation of the side gate, and is often used to form large-area flat plastic parts (as shown in Figure 8). The distribution channel of the gate is parallel to the side of the 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 evenly distributed in the parallel flow channel first, and then flows in parallel at a lower linear speed, and enters the cavity evenly, so the internal stress of the plastic part is small, which reduces the warping deformation caused by orientation, and the cavity Well vented. However, the gate removal workload is large and the traces are obvious.
2.2.7 Disc gate
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). The gate is on the entire perimeter of the bore. The plastic melt enters the mold cavity at roughly the same speed from the periphery of the inner hole, the plastic parts will not produce weld marks, the core is evenly stressed, and the air is discharged in sequence. Disk gate applications are rare for our products.
2.2.8 Ring gate
The annular gate is set on the outside concentric with the cylindrical cavity, that is, the gate is set around the cavity, so it can be called the outer annular gate, and its gate position corresponds to the inner disc gate (as shown in Figure 10). It is suitable for plastic parts of thin-walled long tubes. Since the melt of the plastic part surrounds the core and enters the cavity evenly, the mold is filled evenly, the exhaust effect is good, and the plastic part has no weld marks. However, it is difficult to remove the gate and leave obvious gate traces on the outside of the plastic part. Ring gates are mostly used for small, multi-cavity molds.
2.2.9 Spoke gate and claw gate
The scope of application of the spoke gate is similar to that of the disc gate, and it is also suitable for plastic parts with a rectangular inner hole. It changes the entire peripheral feed into several small arcs or straight lines, so it can be regarded as the inner side Gate, this kind of gate is easy to cut, the flow channel has less aggregate, and the upper part of the core is positioned to increase the stability of the core, but the weld marks on the plastic part affect the strength and appearance quality of the plastic part (as shown in Figure 11 ).
The claw gate is a variation of the spoke gate, and the runner is opened on the tapered section of the core. It is mainly used for long tubular plastic parts with small inner holes or plastic parts with high coaxiality requirements.
3. Selection of gate location
The position and quantity of the gate often determine the appearance quality and performance of the product, so when selecting the position and quantity of the gate, the following points should be followed:
1. The gate should be set at a position where all corners of the cavity can be filled at the same time;
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 set at the thicker part of the product, so that the melt flows from the thick section into the thin section, so as to facilitate feeding and ensure complete mold filling.
4. The selection of the gate position should make the plastic mold filling process the shortest to reduce pressure loss.
5. The position of the gate should be selected in a position that is conducive to the removal of gas in the cavity;
6. The gate should not make the molten material rush directly into the cavity, otherwise swirl will be generated and swirl marks will be left on the plastic part, especially the narrow gate is more prone to such defects;
7. The possible occurrence of welding marks, air bubbles, depressions, empty positions, insufficient injection and spraying should be considered;
8. The position of the gate should be selected to avoid fusion lines on the surface of the product. When the generation of fusion lines cannot be avoided, the selection of gate position should take into account whether the location of fusion lines is suitable;
9. The location of the gate should be selected to prevent stitching lines on the plastic surface, especially for circular or cylindrical plastic parts, and a cold well should be added at the melting point of the gate surface.
10. The setting of the gate should avoid the phenomenon of melt fracture.
11. When the projected area of the product is large, avoid opening gates on one side to prevent uneven injection force
12. The gate should be set at a place that does not affect the appearance of the product
13. Do not set a gate in the part of the product that is subjected to bending load or impact load. Generally, the strength near the gate of the product is the worst;
14. The gate position of the injection mold with a slender core should be far away from the forming core, so that the forming core will not be deformed by the flow of material;
15. When forming large or flat plastic parts, double gates can be used to prevent warping, deformation and material shortage;
16. Try to make the operation of the water removal port as easy as possible, preferably automatic operation;
4. Matching of gate type and plastic
The applicable gates for different plastic materials are not the same. The following table shows the optimal series of gate types for different materials:
Plastic gate form
Hardness PVC
PE
PP
PC
P.S.
PA
POM
AS
ABS
PMMA
Short dimension plastic
Sprue
point gate
Latent gate
side gate
Overlap Gate
Fan gate
Flat Slot Gate
Disc gate
5. Gate types commonly used in our production
In our production, the following types of gates are used according to product structure, product appearance requirements and automation requirements:
1. Straight sprue
In our commonly used plastic turnover boxes, etc., due to their large structural size, there is no special requirement for appearance gates, etc., so direct gates with simple gate structure and easy processing are used, but it is difficult to remove the material handle. As shown in Figure 12:
Picture Figure 12 Application example of sprue (turnover box)
2. Side gate
In our products, partly transparent parts, such as TZ-C1041, TZ-C1051, T-C1061 lenses, etc., are not allowed to have gate positions on both sides, so the type of side gate is adopted (as shown in Figure 13 shown), but it cannot meet the needs of automated production, and manual trimming is required.
picture
Figure 13 Application example of side gate Gate position
3. Submerged gate
Submerged gate is the most commonly used gate type in our injection molds. Most of our functional parts, switch fixing frames, etc. adopt the latent gate type with the gate on the outside as shown in Figure 14, and most of the The product shells all adopt the latent gate type on the inside as shown in Figure 15. This kind of gate can be automatically blanked to meet the needs of automation, and there is no gate trace on the surface of the product.
picture
picture
The gates commonly used in our products are the above types, but according to different product requirements, there are also some special types of gate applications, such as the meniscus gate in the upper and lower shell molds of TZ-CON01 (as shown in Figure 16 )Wait. Generally speaking, the mold should fully consider the requirements of various aspects when determining the gate type. In a set of molds, either one gate or a combination of different gates can be used (for example, TZ-CON01 upper and lower shell molds have both The crescent-shaped gate, and the latent gate, as shown in Figure 17), the ultimate goal is to produce qualified products.
