Gate is a section of material flow channel with a very short length and a very narrow cross section between runner and cavity. Cross-sectional shape and size of gate have a greater impact on the flow of melt, filling of mold, compactness of product, and dimensional accuracy of product. If cross section is too small, melt flow resistance will be large, which will cause material to heat up, decompose and turn yellow, melt will be difficult to fill; if gate cross section is too large, mold filling is good, but exhaust is difficult, melt cooling is slow, and molding cycle is affected. There are many types of gate shapes, which can be selected according to shape and size of product.

1). Direct gate. Direct gate is also called center gate, sprue gate, as shown in Figure 1. It is characterized by short runners, symmetrical material flow distribution, low resistance, low pressure loss, and easy mold filling. It is mostly used in single-cavity mold structure, more suitable for high viscosity, large and deep round products.

 

1. Main runner 2. Cold slug well
Figure 1 Direct gate

2). Side gate. As shown in Figure 2, side gate is generally opened on parting surface, and material is fed from edge of product. It can be fed at one point or at multiple points at the same time. Its cross-section is generally rectangular or approximately rectangular, with a length c of 0.5-3mm, a width b of 1.5-5mm, and a gate depth a of 0.52mm.

 

Figure 2 Side gate

3). Fan gate. As shown in Figure 3, fan gates are also a type of side gates, which are often used to form thin-sheet products with a larger width (lateral dimension). Gate gradually widens along feeding direction, and depth gradually decreases. Plastic melt enters cavity through gate step with a length of about 1mm. When melt passes through fan gate, it gets more even distribution in transverse direction, which can reduce internal stress of product and possibility of air being brought into it. Section size of sector gate: length L is 1.3 to 2mm, width B is 5 to 38mm, and depth h is 0.25 to 1.6mm.

 

Figure 3 Fan gate

4). Point gate. As shown in Figure 4, a point gate is a gate with a very small size. It is suitable for plastics with low viscosity and viscosity sensitive to shear rate. Its diameter d is 0.3～2mm (usually 0.5～1.8mm), depending on nature of plastic and quality of product. Gate length L is 0.5～2mm (common 0.8~1.2mm).

 

Figure 4 Point gate

5). Latent gate. Also called tunnel gate, as shown in Figure 5, latent gate is a variant form of point gate, which has advantage of point gate. Mainly used for molds and molding elastomer materials for small parts with multiple cavities.

 

Figure 5 Latent gate

6). Disc gate. As shown in Figure 6, disc gate is mainly used for cylindrical products with holes in the middle, expands feed along inside of product. This type of gate can feed evenly, flow rate of material is approximately equal on entire circumference, air is easily discharged in sequence, and there is no welding seam. This type of gate can still be regarded as a rectangular gate, and its typical size is 0.25 to 1.6 mm deep and step length is about 1 mm.

 

Figure 6 Disc gate

7). Ring gate. As shown in Figure 7, ring gate is also a gate for expanding feed along entire circumference of product, and is used for tubular plastic products with cores installed on both sides.

 

Figure 7 Ring gate

8). Flat seam gate. As shown in Figure 8, when forming a large area flat product (such as a sheet), a flat slot gate can be used. Depth of flat slot gate is 0.25~0.65mm, width is 1/4 of width of cavity on gate side to full width of this side, and gate step length is about 0.65mm.

 

Figure 8 Flat seam gate

9). Spoke gate. Spoke gate is a deformation of disc gate, as shown in Figure 9. This structure reduces cold material and makes it easy to remove gate, but it increases weld line of product. Cross-sectional size of spoke gate is selected with reference to cross-sectional size of sector gate.

 

Figure 9 Spoke gate

10). Gate for ear protection. It is also called adjusting chip gate or tap gate, as shown in Figure 10. It is dedicated to products with high transparency and no internal stress. Width b of ear protector is usually equal to diameter of shunt, length L is 1.5 times width b, thickness is about 90% of thickness of product at entrance. Thickness of gate is same as that of lugs, width is 1.5～3.5mm, and length of gate is less than 1.5mm (generally 1mm). When width of product is greater than 300mm, multiple gates and multiple ear guards can be used.

 

1. Product 2. Ear protector 3. Main runner 4. Manifold 5. Gate
Figure 10 Ear guard gate

 

Opening position of gate has a great influence on quality of plastic products. When determining position of gate, a comprehensive consideration should be given to factors such as flow of materials, filling sequence, cooling and replenishment. When choosing gate opening position, following issues should be paid attention to:
1) Selection of gate position should be conducive to melt flow and feeding.
2) It should be conducive to discharge of gas in cavity.
3) Gate should be minimized under condition that melt fills cavity.
4) Plastic products should be avoided to produce weld marks.
5) When wall thickness of product is different, gate should be set at wall thickness position to reduce generation of bubbles.
6) Gate position should avoid core and insert, otherwise material flow with higher pressure will cause them to move or deform.
7) Gate position should consider impact of polymer orientation on performance of plastic products.