Composition of the gating system
The common runner system, also known as 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 primary runner, the sub-runner, and the gate.
1. Main runner
Also known as the sprue or vertical runner, it refers to the runner from the part where the nozzle of the injection machine contacts the main runner bushing of the mold to the sub-runner. This part is the first part that the molten plastic flows through after entering the mold.
2. Sub-runner
Also known as the secondary runner. According to the mold design, it can be further divided into the first sub-runner and the second sub-runner. The sub-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 also has the function of evenly distributing plastic to each cavity.
3. Gate
Also called the feed port, it is the narrow passage between the runner and the mold cavity, and it is also the shortest and thinnest part. Its function is to accelerate the plastic by using the constricted flow surface. The high shear rate can make the plastic flow well (due to the shear thinning characteristics of the plastic); the temperature rise effect of viscous heating also has the effect of increasing the material temperature and reducing the viscosity.
After the molding is completed, the gate is first solidified and sealed, which has the function of preventing the plastic from flowing back and preventing the mold cavity pressure from dropping too quickly, causing the molded product to shrink and sag. After molding, it is convenient to cut off to separate the runner system and the plastic part.
4. Cold well
The purpose is to store the colder 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 well is usually set at the end of the main runner. When the runner is long, a cold well should also be opened at the end.
Basic principles of our gating system design
1. Consideration of cavity layout
1) Balanced layout should be adopted as much as possible;
2) The cavity layout and gate opening should be symmetrical to prevent uneven force on the mold, resulting in eccentric load and overflow;
3) The cavity layout should be as compact as possible to reduce the mold size.
2. Consideration of flow guidance
1) The molten plastic can be smoothly guided to fill the cavity without generating vortex and can be exhausted smoothly;
2) Avoid the plastic melt from directly impacting the core and metal insert with smaller diameter 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 runner bending and sudden change of flow direction (changing direction with arc angle);
5) The surface roughness should be low during runner processing;
6) Multi-point pouring can reduce pressure drop and required injection pressure, but there will be seam line problems.
4. Consideration of flow balance
1) When filling a mold with multiple cavities (Multi-Cavity), the flow channel must be balanced, and the plastic should be filled in each cavity at the same time as much as possible to ensure the quality consistency of the molded products in each cavity;
2) The flow channel should be arranged in a naturally balanced manner (Naturally-Balanced Layout) as much as possible;
3) When natural balance is not possible, the flow channel should be balanced by artificial balancing method.
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