Injection mold design process description

The runner system, also called the gating system, is the necessary passage for the molten plastic from the nozzle of the injection machine to the mold cavity. The runner system includes the main runner, sub-runner and gate, etc..
1. Main runner
Also called the sprue, it is the channel connecting the injection molding machine nozzle and the sub-runner. It is the first part that the molten plastic flows through after entering the mold.

2. Sub-runner
Also called the runner, according to the mold design, it can be further divided into the first runner and the secondary runner. The runner is a channel connecting the main channel and the gate. It helps to smoothly change the flow direction of the molten plastic. For multi-cavity molds, it also has the function of evenly distributing plastic to each cavity.

3. The gate
It is the narrow passage between the sub-runner and the mold cavity, and is also the shortest and usually thinnest part of the runner system. Its function is to accelerate the plastic melt flow. Besides, high shear rate also makes the plastic flow well due to the shear thinning property of the plastic, and generate the temperature rising effect of viscous heating, which helps to increase the material temperature and reducing the viscosity. At last, the plastic melt is to fill the cavity better.

After molding, the gate is the first part to solidify and seal. This feature not only helps prevent the plastic melt from flowing back during the cooling and solidification process, but also helps slow down the pressure drop in the mold cavity, avoiding defects such as shrinkage and undercut caused by too fast pressure drop. The gate is usually designed to be easy to remove. By degating, the runner system can be completely separated from the plastic part.

4. Cold slug well
The function of the cold slug well is to collect and store the cold plastic wavefront in the initial stage of filling, preventing the cold slag from directly entering the mold cavity, affecting the filling quality or blocking the gate. The cold slug well is normally set at the end of the main runner. When the length of sub-runner is long, a cold slug well should also be set at the end of sub-runner.

1. The cavity layout
·Use the balances layout as much as possible.
·The cavity layout and gate opening should be symmetrical to prevent uneven loading of the mold and the occurrence of mold flash.

·The cavity layout should be as compact as possible to reduce the mold size.

2. The flow guidance
·It can smoothly guide the molten plastic to fill the mold cavity without generating eddy currents and can exhaust the air smoothly.
·Try to avoid direct impact of plastic melt on cores and metal inserts with smaller diameters to prevent core shift or deformation.

3. Heat loss and pressure drop
·The smaller the heat loss and pressure drop, the better.
·The flow path should be short.
·The cross-sectional area of the runner should be large enough.
·It had better to avoid bends of the runner and sudden changes in flow direction like changing direction with an arc angle.
·During the runner process, the surface roughness should be low but not too smooth.
·Multiple gating can reduce pressure drop and injection pressure, but there will be welding line.

4. Flow balance
·When filling multi-cavity mold, the runner should be balanced to try to fill each cavity with plastic at the same time, to ensure the consistency of the quality of each cavity molded product.
·The sub-runner should be arranged in a naturally balanced layout as much as possible.
·If it is impossible to keep natural balance, artificial balancing is used to balance the runner.

5. Scrap material
Under the premise of smooth filling without affecting flow and pressure loss, the runner volume can be reduced to decrease the generation of runner scrap material and recycling costs.

6. Cold slag
On the runner system, design appropriate cold slug wells and flash grooves can store the cold plastic wavefront in the initial stage of filling, preventing the cold slag from directly entering the mold cavity and affecting the filling quality.

7. Venting
The plastic should be guided smoothly to fill the cavity. In the meanwhile, expel the air in the mold cavity in time to avoid burning plastic due to trapped air.

8. Molding quality
·It need to avoid problems such as short shots, burrs, trapped air, weld lines, flow marks, jetting mark, residual stress, warpage, mold core offset, etc..
·When the runner system is long or multiple gating is used, it is necessary to prevent the warpage and deformation of the finished product caused by unbalanced flow, insufficient pressure holding or uneven shrinkage.
·The product has good appearance and properties, and the gate is easy to remove and fix. The gate mark does not damage the appearance and application of the plastic part.

9. Production efficiency
It had better to minimize the required post-processing to shorten the forming cycle and improve production efficiency.

10. Ejection position
The appropriate ejection position should be considered to avoid deformation of the molded parts during demolding.

11. Plastic characteristics
During mold design and injection molding process, if plastics with high viscosity or short L/T ratio, it need to avoid using too long or too small runners.This principle is based on considerations of material fluidity and molding quality.