Plastic melt enters mold cavity from nozzle of injection moulding machine through main flow channel, runner and gate. In order to prevent solidified cold material at nozzle end from entering mold cavity, a cold material well should be designed at the end of flow channel.

Runner is an important channel from main flow channel to gate, and is flow channel of molten plastic ejected from nozzle of injection moulding machine. Runner should be designed to have low resistance and prevent cooling. Usually, runner is designed as a trapezoid or a circle.

 

Common runner shapes

For multi cavity mold, in order to obtain good dimensional accuracy, design of flow channel is very important. Flow channel design of a typical multi-cavity mold is shown below.

 

Multi cavity mold runner

Design of gate system, such as position, number, geometry and size, is very important for production efficiency and dimensional accuracy. Function of gate can be summarized as follows:
1.Control volume and direction of plastic melt flowing into mold cavity
2. Before curing, seal melt in mold cavity and prevent melt from flowing back to flow channel.
3. Generated by heat due to viscous dissipation
4. Easy to cut off runner, simplifying post-processing of products
Classification:

 

Non-restrictive gate is called a straight gate. As shown in figure below, mold of this type of gate has a simple design, easy operation, easy molding, and reduced shrinkage. However, gate molding cycle becomes longer, and molding defects such as cracks, warpage and residual stress are prone to occur.

 

Direct gate

Because of cross-sectional area of this gate, restricted gate is designed to solidify quickly. Advantages of this form of gate are as follows:
1. Reduced residual stress and deformation around gate, thereby reducing cracks, warpage and deformation of product;
2. As injection pressure in mold cavity is reduced, a larger product projection area is allowed;
3. Because gate closing time is shortened, molding cycle is shortened;
4. Improved product quality by eliminating post-processing.
Here are 6 types of restricted gates

 

Side gate

Thickness of side gate is usually 30% to -40% of wall thickness of product. At the same time, its width is about 3 times wall thickness of product. Side gate can be applied to almost all plastics. Superposition gate and spoke gate are deformation designs of side gate.

 

Fan gate

Fan gate has a wide and flat cross section, which effectively eliminates defects of gate, and is often used for flat products.

 

Film gate

Picture above is a typical film gate design. Gate width is same as product width, but thickness is much smaller. It can effectively eliminate residual stress and deformation of product like fan gate.

 

Disc gate

A thin disc-shaped gate surrounds disc-shaped or ring-shaped product to avoid formation of weld marks. Ring-shaped gate is a deformation of disc-shaped gate.

 

Pin gate

Pin gate is usually located in the middle of product, and is often used for multi-point gates. Because gate diameter is usually 0.8- -1.2mm, small cross-sectional area causes high flow resistance, it is recommended to use low viscosity plastic or high injection pressure to avoid under-injection.
Characteristics of pin gate are as follows:
1.Choice of gate position is relatively not strict
2.Low residual stress around gate
3. For multi cavity mold, it is easy to achieve gate balance
4. For products with large projection area, multi-point pin gate can effectively eliminate product warpage.
5. Pin gate is easy to cut. For three plate mold, automatic gate cutting is easy to achieve, and it is easy to separate product from gate.

 

Sub gate

Sub gate is shown in figure above. Generally, gate is located on parting surface of mold, but sub gate is only runner on parting surface. Gate is usually located on moving or static plate of mold, and sometimes on cavity, Although it is not much different from a pin gate, one advantage of a sub gate is that it can even be applied to two-plate molds. When molded product is ejected, gate automatically falls off.

For multi cavity mold, each mold cavity is evenly filled with plastic melt, and it is important to achieve gate balance. As plastic melt flows from inlet to end of mold cavity, polymer pressure decreases accordingly. Therefore, gate balance should optimize length, width and depth of gate.
Appropriate gate balance and runner balance design can avoid forming defects such as flow marks, shrinkage, under-injection, dimensional fluctuations and weight changes in actual molding process of multi cavity mold.

Basic factors to be considered for gate positioning are: part design, flow, and end-use requirements of product.
Following points should be kept in mind:
1. Large parts that require multiple gates should have gates close enough to reduce pressure loss. Doing so minimizes cooling at junction of resin's leading edges of flow, which provides better fusion line strength. Proper gate size should be selected so that resin is filled with a reasonable pressure and speed.
2. Length of gate transition should be kept as short as possible.
3. Impingement gate will help to ensure that incoming fluid directly faces cavity wall or core, thereby avoiding swirling.
4. To avoid entrainment, resin flow from gate should direct air to exhaust slot.
5. Position of gate should be determined so that resin flows from thick wall to thin wall; minimize fusion line; and keep it away from impact and stress area.
6. To minimize swirls, radiation spots, and gate whitening, gate should be at an appropriate angle to runner.

 

7. Pouring directly onto decorative surfaces can cause surface defects.