Several Principles Of Injection Mold Gating System Design

1. Composition of the gating system

The common runner system (Runner System), also known as the runner system, or the gating system, is the necessary passage for the molten plastic to travel from the nozzle of the injection machine to the cavity. The runner system includes a primary runner, a sub-runner, a gate, and the like.

1. Main channel

Also known as main runner, sprue or vertical runner, it refers to the runner from the part where the nozzle of the injection machine is in contact with the main runner bush of the mold to the runner. This part is the first part the molten plastic flows through after entering the mold.

2. Shunt channel

Also known as runner or secondary runner. With the mold design, it can be further divided into the first runner (First Runner) and the second runner (Secondary Runner). The runner is the transition area between the main runner and the gate, which can make the flow direction of the molten plastic change smoothly; for multi-cavity molds, it has the function of evenly distributing the plastic to each cavity.

3. Gate

Also known as the feed port, it is the narrow opening between the runner and the mold cavity, and it is also the shortest and thinnest part. Its function is to use the compressed flow surface to accelerate the plastic, and the high shear rate can make the plastic flow well (due to the shear thinning characteristics of the plastic); the heating effect of viscous heating can also increase the temperature of the material and reduce the viscosity. effect.

After the molding is completed, the gate is the first to solidify and seal, which has the function of preventing plastic backflow and preventing the pressure of the mold cavity from dropping too fast, causing the molded product to shrink and sag. After molding, it is easy to cut off to separate the runner system and the plastic parts.

4. Cold well

Also known as cold slug. The purpose is to store and replenish the cold plastic wave front in the initial stage of filling to prevent the cold material from directly entering the mold cavity, affecting the filling quality or blocking the gate. The cold slug well is usually set at the end of the main channel. When the length of the branch runner is long, the cold slug well should also be opened at the end.

2. Basic principles of gating system design

1. Consideration of Cavity Layout

1) Try to use Balances Layout;

2) The arrangement of the mold cavity and the opening of the gate should be symmetrical to prevent the uneven load of the mold and the problem of the overflow of the mold;

3) The cavity arrangement is as compact as possible to reduce the size of the mold.

2. Considerations for Flow Guidance

1) It can smoothly guide the molten plastic to fill the mold cavity without generating eddy current, and it can be exhausted smoothly;

2) Try to avoid the frontal impact of plastic melt on cores and metal inserts with smaller diameters to prevent Core Shift or deformation.

3. Consideration of heat loss and pressure drop

1) The smaller the heat loss and pressure drop, the better;

2) The process should be short;

3) The cross-sectional area of the runner should be large enough;

4) Try to avoid bending of the runner and sudden change of flow direction (change direction with arc angle);

5) The surface roughness should be low when the runner is processed;

6) Multi-point pouring can reduce the pressure drop and the required injection pressure, but there will be suture problems.

4. Consideration of flow balance

1) When filling a mold with multiple cavities (Multi-Cavity), the runners should be balanced, and try to fill each cavity with plastic at the same time to ensure the consistency of the quality of the molded products of each cavity;

2) The runner should be arranged in a natural balanced layout as far as possible (Naturally-Balanced Layout);

3) When the natural balance cannot be achieved, use the artificial balance method to balance the flow channel.

5. Waste Considerations

On the premise of smooth filling without affecting the flow and pressure loss, the volume of the runner (length or cross-sectional area) is reduced to reduce the waste generation and recycling costs of the runner.

6. Consideration of cold material

Appropriate cold slug wells and overflow troughs are designed on the runner system to supplement the cold plastic wave front in the initial stage of filling and prevent the cold slug from directly entering the mold cavity and affecting the filling quality.

7. Exhaust Considerations

The plastic should be smoothly guided to fill the mold cavity, and the air in the mold cavity should be able to escape smoothly to avoid the problem of encapsulation burning.

8. Consideration of the quality of molded products

1) Avoid problems such as short shots, burrs, encapsulation, sutures, flow marks, jets, residual stress, warpage deformation, mold core offset, etc.;

2) When the runner system has a long process or multiple gating, the problem of warpage and deformation of the finished product caused by unbalanced flow, insufficient pressure holding or uneven shrinkage should be prevented;

3) The appearance of the product is good, it is convenient to remove and trim the gate, and the gate mark (Gate Mark) does not damage the appearance and application of the plastic part.

9. Production efficiency considerations

Minimize the required post-processing to shorten the forming cycle and improve production efficiency.

10. Consideration of ejection points

Appropriate ejection position needs to be considered to avoid mold release deformation.

11. Considerations for using plastics

For plastics with high viscosity or short L/t, avoid using too long or too small sized runners.