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Type and selection of injection mold gate

2022-09-13 Hits: 5 views

The gate of the mold refers to a short and thin runner connecting the runner and the cavity, which is the entrance of resin injection into the cavity. The shape, quantity, size and position of gates in the mold will have a great influence on the quality of plastic parts. Therefore, the choice of gate is one of the key points in plastic mold design. The gate is introduced in the following aspects.
I. The main functions of the gate are:
1. After the cavity is full, the melt condenses at the gate to prevent it from flowing back.
2. It is easy to cut off the gate tailings.
3. For multi-cavity mold, it is used to control the position of weld line.
Second, the type of gate
Generally, gates are divided into unrestricted gates and restricted gates. Restrictive gates are divided into three series: side gate, point gate and disk ring gate.

1 unrestricted gate
Unrestricted gate is also called straight gate. It is characterized in that the plastic melt directly flows into the cavity, the pressure loss is small, the feeding speed is fast, the molding is easy, and it is suitable for all kinds of plastics. It has the advantages of good pressure transmission, strong pressure maintaining and feeding effect, simple and compact die structure, convenient manufacturing, etc. However, it is difficult to remove the gate and the gate trace is obvious. The heat concentration and slow condensation near the gate are easy to produce large internal stress, and it is also easy to produce pits or surface dents. Suitable for large plastic parts, thick-walled plastic parts, etc.

2 restricted gate
The cavity and the runner are connected by a channel with a short distance at one end and a small cross section. This channel is called a restricted gate, which can limit the thickness and rapid solidification of the gate. The main types of restricted gates are:
1 point gate
Point gate is a kind of round gate with extremely small cross section.
The characteristics of the point gate are:
1, the gate position limit is small;
2. After the gate is removed, the residual trace is small, which does not affect the appearance of plastic parts;
3. When the mold is opened, the gate can be automatically broken, which is beneficial to automatic operation;
4. The stress caused by gate accessories is small.
Disadvantages are:
1. The pressure loss is large, and the mold must adopt the three-plate mold structure, which is complex and has a sequential parting mechanism. It can also be applied to the two-plate mold structure without runner.
2 latent gate
Latent gate is evolved from point gate. Its branch channel is set on the parting surface, and the gate dives under the parting surface and enters the cavity obliquely. Besides the characteristics of point gate, the entrance gate of latent gate is generally in the hidden place 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 provided with pushing mechanisms, so the gate is automatically cut off when the mold is opened, and the condensate in the runner automatically falls off.

3 side gate
Side gate, also called edge gate, is generally set on the parting surface and fed from the outer side of the cavity (plastic part). Side gate is a typical rectangular cross-section gate, which can conveniently adjust the shear rate and gate closing time during mold filling, so it is also called standard gate. The side gate is characterized by simple gate section shape, convenient processing, and precise processing of gate size. The gate position is flexible to improve the mold filling condition; It can be corrected without removing the mold from the injection molding machine; Gate removal is convenient, with little trace. Side gate is especially suitable for two-plate multi-cavity mold. However, plastic parts are easy to form defects such as weld lines, lock holes, depressions, etc., resulting in large injection pressure loss and poor exhaust of shell-shaped plastic parts.

4 overlapping gate
Overlapping gate, also called overlapping 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. It is a typical impact gate, which can effectively prevent the injection flow of plastic melt. If the forming conditions are improper, surface pits will be produced at the gate. It is difficult to cut off the gate, which will leave obvious gate marks on the surface of plastic parts.

5 sector gate
Fan-shaped gate is a gradually expanding gate, a variant of side gate, which is often used to shape plate-shaped plastic parts with large width. The gate gradually widens along the feeding direction, and the thickness gradually decreases to the thinnest. The uniform distribution of plastic melt in the width direction can reduce the internal stress and warpage of plastic parts. The cavity has good exhaust capacity, avoiding surrounding air. However, it is difficult to cut off the gate and the marks are obvious.
6 Flat-seam gate
Also known as sheet gate, it is also a variant of side gate, which is often used to shape large-area flat plastic parts (as shown in Figure 8). The distribution channel of the gate is parallel to the side of the cavity, which is called 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 runner at first, and then flows in parallel at a lower linear speed, and evenly enters the cavity. Therefore, the internal stress of the plastic part is small, the warping deformation that should be directionally generated is reduced, and the cavity exhaust is good. However, the gate cutting workload is heavy and the marks are obvious.

Second, the gate location selection
The location and quantity of gates often determine the appearance quality and performance of products, so the following points should be followed when selecting the location and quantity of gates:
1. The gate should be located 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 process.
3. The gate should be set at the part with thick wall thickness of the product, so that the melt can flow from the thick section into the thin section, which is beneficial to the feeding and the complete mold filling.
4. The selection of gate position should make the plastic mold filling process shortest, so as to reduce the pressure loss.
5. The location of the gate should be selected in a position that is conducive to the elimination of gas in the cavity;
6. The gate should not make the molten material rush into the cavity, otherwise it will produce swirling flow, leaving a spiral mark on the plastic part, especially the narrow gate is more prone to this kind of defect;
7. Consideration should be given to the possible occurrence of weld lines, bubbles, depressions, false positions, insufficient glue injection and gunning;
8. The location of the gate should be selected at the position that can avoid fusion lines on the surface of the product. When the occurrence of fusion lines cannot be avoided, the selection of gate location should consider whether the location of fusion lines is appropriate;
9, the choice of gate location, should prevent the production of stitching on the plastic surface, especially in the ring or cylindrical plastic parts, should be on the surface of the gate of the melt pouring place and open a cold well.
10. The setting of gate should avoid the phenomenon of melt fracture.
11. When the projected area of the product is large, avoid opening the gate on one side to prevent uneven injection force.
12. The gate should be set at a position that does not affect the appearance of the product.
13. Do not set a gate at the part of the product that bears 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 slender core should be far away from the molding core, so that the molding core will not be deformed by the material flow;
15. When forming large or flat plastic parts, compound gate can be used to prevent warping, deformation and lack of materials;
16, should try to make the nozzle operation easy, it is best to automatic operation;
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