Molten plastic enters mold cavity from nozzle of injection molding machine through main runner, runner, and gate. Inlet of mold cavity is called gate. To prevent solidified cold slug from nozzle tip from entering mold cavity, a cold slug well should be designed at the end of runner.

Runners are important channels from main runner to gate, serving as flow path for molten plastic injected from injection molding machine nozzle. Runners should be designed for low resistance and to prevent cooling. Typically, runners are designed in trapezoidal or circular shapes.
Common Runner Shapes

 

For multi-cavity molds, runner design is crucial for achieving good dimensional accuracy. Following diagram shows a typical runner design for a multi-cavity mold.

 

Multi-cavity mold runner system

Gate design, including its location, number, geometry, and size, is crucial for production efficiency and dimensional accuracy. Functions of gate are summarized as follows:
1. Controlling volume and direction of molten plastic flowing into mold cavity.
2. Before solidification, sealing molten material within mold cavity and preventing backflow into runner.
3. Generating heat due to viscous dissipation.
4. Facilitating runner cutting and simplifying post-processing of product.
Classification:

 

Non-restricted gates are called direct gates, as shown in figure below. This type of gate simplifies mold design, operation, molding, and reduces shrinkage. However, this type of gate increases molding cycle time and is prone to molding defects such as cracks, warpage, and residual stress.
Direct Gate

 

Due to its small cross-sectional area, restrictive gate is designed for rapid solidification. Advantages of this type of gate are as follows:
1. Reduced residual stress and deformation around gate, resulting in fewer cracks, warpage, and deformation in finished product;
2. Reduced injection pressure within mold cavity, allowing for a larger projected area of finished product;
3. Shortened gate closing time, resulting in a shorter molding cycle;
4. Improved product quality by eliminating post-processing.
Following are six types of restrictive gates:
① Side Gate

 

Side gates are typically 30%-40% of product wall thickness. Their width is approximately three times product wall thickness. Side gates are applicable to almost all plastics. Overlapping gates and spoked gates are variations of side gate designs.
② Fan Gate

 

Fan gates have a wide and flat cross-section, effectively eliminating gate defects and are often used for flat products.
③ Membrane Gate

 

Image above shows a typical membrane gate design. Its gate width matches part width, but its thickness is much smaller. Like a fan gate, it effectively eliminates residual stress and deformation in part.
④ Disc Gate

 

A thin disc gate surrounds a disc-shaped or ring-shaped part to prevent weld lines. A ring gate is a variation of disc gate.
⑤ Needle Gate

 

Needle gates are typically located in the center of part and are often used for multi-point gates. Because gate diameter is typically 0.8-1.2 mm, small cross-sectional area causes high flow resistance. It is recommended to use low-viscosity plastics or high injection pressure to avoid under-filling.
Characteristics of pin gates are as follows:
1. Relatively less stringent in gate location selection.
2. Low residual stress around gate.
3. Easier to achieve gate balance in multi-cavity molds.
4. For products with large projected areas, multiple pin gates effectively eliminate warping.
5. Pin gates are easy to cut off. For three-plate molds, automatic gate cutting is easily achieved, facilitating separation of product and gate.
⑥ Submerged Gate

 

Submerged gates are shown in diagram above. Normally, gate is located on parting surface of mold, but with a submerged gate, only runner is located on parting surface. Gate is usually located on moving or stationary plate of mold, and sometimes on cavity. Although it is not significantly different from a pin gate, one advantage of submerged gates is that they can even be used in two-plate molds. When molded product is ejected, gate automatically falls off.

For multi-cavity molds, achieving gate balancing is crucial for uniform filling of each cavity with molten plastic. As molten plastic flows from inlet to the end of cavity, polymer pressure decreases accordingly. Therefore, gate balancing should optimize length, width, depth of gate.
Proper gate and runner balancing design can prevent molding defects during actual molding process of multi-cavity molds, such as flow marks, shrinkage, underfill, dimensional fluctuations, and weight variations.

Basic factors to consider when positioning gate are: part design, flow characteristics, and final use requirements of product.
Following points should be kept in mind:
1. Large parts requiring multiple gates should have sufficiently tight gates to reduce pressure loss. This minimizes cooling at resin flow front junction, thus providing better weld line strength. Appropriate gate sizes should be selected to ensure a reasonable pressure and velocity for resin filling.
2. Length of gate transition section should be kept as short as possible.
3. A sprue gate helps ensure that incoming fluid flows directly against mold cavity wall or core, thus avoiding swirl marks.
4. To prevent air trapping, resin flow from gate should be directed to venting channels.
5. Gate location should be carefully determined to allow resin to flow from thick-walled sections to thin-walled sections; minimize weld lines; keep it away from impact and stress zones.
6. To minimize swirl marks, radial spots, and gate halo, gate should be at a suitable angle to runner.

 

7. Direct pouring onto decorative surfaces can cause surface defects.