How to Design Injection Mould Gating System?

Injection mould is a tool to produce plastic products and also to make plastic products have complete structure and precision size, which is mainly used in industrial circle. Gating system of injection mould is also called the runner system. It is a group of feeding channel which lead the plastic melt to the cavity from injection machine nozzle. It usually consist of main runner, sub-runner, gate and cold slag cavity. It is directly related to the quality of plastic products and production efficiency.



Main runner
It is the channel that nozzle of injection machine connect to the sub-runner and the cavity. The top of the main runner is concave, which is convenient to connect with the nozzle. The inlet diameter of the main runner should be slightly larger than the nozzle diameter (o. 8 mm) to avoid the overflow and prevent the block due to the wrong connection. The inlet diameter is determined according to the product size, generally 4-8 mm. The diameter of main runner should expand inwards to 3 to 5 angle, which is convenient for the demoulding of runner slag.

Gate
The gate is also known as the feeding port, which is the narrow channel between
the sub-runner and the mould cavity, and also the shortest and thinnest part. Its function is to use the tightening flow surface to make the plastic to be accelerated. The high shear rate can make the plastic fluidity good (due to the plastic shear thinning characteristics); The heating effect of viscous heating also has the function of raising temperature and reducing viscosity.
After moulding is completed, the gate first curing sealing, which prevent the reverse flow of plastic, and avoid the cavity pressure drop too fast, so as to make the forming products have the function of contracting sinking. After forming, it is convenient to cutting off, to separate runner system and plastic parts.

Cold slag cavity
It is a cavity which located in the end of main runner. It is used for collecting the cold slag that produced in two times of injection of injection nozzle, which prevent the block of the sub-runner and the gate. Once the cold slag is mixed into the cavity, it is easy to produce internal stress in the injection mould products. The diameter of cold slag cavity is about 8 to 10 mm and the depth is 6 mm.

Sub-runner
It is the channel in the multi-groove mould which connect the main runner and each cavity. In order to fill the cavity with equal velocity, the arrange and distribution of the sub-runner in the plastic mould should be symmetrical and equidistant.

The basics of gating system design:
1. The consideration of cavity layout

• Adopt balances layout.
• The cavity layout and gate opening are designed to be symmetrical, so as to prevent the biased load due to the unevenness of the mould, and the problem of supporting mould overflow.
• Cavity layout is as compact as possible to reduce mould size.

2. The considerations of flow guidance

• It can guide the molten plastic to fill the cavity without producing vortex, and can exhaust smoothly.
• Try to avoid the plastic melt directly hit the core and metal inserts, to prevent core shift or deformation.

3. The considerations of heat loss and pressure drop

• It is better that the heat loss and pressure drop is small.
• The distance of flow should be short.
• The flow area should be large enough.
• Avoid winding and sudden change of flow direction (change direction with circular angle)
• The surface roughness should be low when the runner processing.
• Multiple point feeding can reduce the pressure drop and the required ejection pressure,but there will be suture problems.

4. The considerations of flow balance

• When the multi-cavity is filled, the runner should be balanced, to make the plastic filling each cavity at the same time, so as to ensure the quality consistency of the moulding products.
• The sub-runner should adopt the naturally-balanced layout.
• When the naturally-balanced layout is not available,the artificial balance method can be adopted.

5. The consideration of scrap
Under the premise of filling smoothly and no effect to the flow and pressure loss, reduce the flow volume (length or section size) , so as to reduce the production of scrap of the runner and the charge of recycle.

6.The consideration of cold slag
In the runner system, the appropriate cold slag well and overflow tank is designed to prevent the cold slag from entering the cavity directly, which affects the filling quality.

7.The consideration of exhaust 
Guiding the plastic to fill the mould cavity, and make the air in the cavity discharge out smoothly, so as to prevent the charred problems.

8.The consideration of forming products quality

• Avoid producing problems such as short shot, burr, seal, suture line, flow mark, jet-flow, residual stress, warping, deformation, parting core excursion etc.
• When the distance of runner system is long or the gate is multi-gate,the warping deformation which is caused by the unbalanced flow and insufficient pressure or uneven shrinkage should be prevented.

9.The consideration of production efficiency
Reduce the required post-processing, shorten the forming cycle and improve the production efficiency.

10.The consideration of eject point
The appropriate ejection position should be considered in order to avoid deformation of forming product.

11.The consideration of using plastic
High viscosity or L/t relatively short plastic, avoid using too long or too small size of the runner.

A set of high quality mould, not only need to have good processing equipment and skilled mould manufacturing workers, another important factor is to have a good injection mould design, especially for complex mould, mould design quality accounts for more than 80% of the mould quality. A good mould design is: On the premise of meeting customer requirements, to make processing cost low, machining difficulty small and processing time short. To make it, not only need to meet the requirements of the guests, but also know about the injection moulding machine, the structure of mould, processing craft and the processing capacity of the plastic injection mould manufacturer itself.