Injection molding requirements for runners
As a critical channel connecting the main channel and the gate, the injection molding runner’s cross-sectional shape must be designed to meet the requirements of low melt flow resistance and minimal heat loss, while also balancing processing convenience and material conservation. Common cross-sectional shapes include circular, trapezoidal, U-shaped, and semicircular. Circular cross-sections offer the highest runner efficiency because they minimize specific surface area (surface area to volume ratio), resulting in minimal heat loss during melt flow and uniform flow resistance. These shapes are suitable for most plastics, especially those with higher viscosities such as PC and PMMA. The diameter of a circular runner should be determined based on part size and wall thickness. For small parts, the diameter should be 4-6mm, for medium parts 6-10mm, and for large parts 10-14mm. For example, when producing a small mobile phone case, a 5mm diameter circular runner ensures smooth melt flow without wasting material due to an overly large runner. Trapezoidal runners are easier to process than circular ones. Their side slopes are typically 5°-15°, and their bottom width is 0.6-0.8 times the top width. They are suitable for molds with multiple cavities and symmetrical part layouts. For example, for a small gear part with eight cavities in a mold, a trapezoidal runner (8mm top width, 5mm bottom width, and 6mm depth) can be used to reduce mold processing costs while ensuring even melt distribution.
The length of injection molding runners should be minimized to reduce melt pressure loss and cooling time. Typically, runner lengths should be kept within the range of 50-300mm, depending on the cavity layout. In multi-cavity molds, runner lengths should adhere to the “equal length principle,” meaning that the runners in each cavity should be as equal as possible to ensure that the melt reaches each gate simultaneously, avoiding uneven part dimensions due to variability in filling time. For example, in a four-cavity mold, the four cavities are arranged in a rectangular pattern, with runners extending from the center of the main runner outward . The runner lengths should differ by no more than 5mm to ensure that the melt fills each cavity in the same amount of time. If the cavity layout doesn’t allow for equal runner lengths, the runner diameters should be adjusted to compensate. Longer runners should have a diameter 10%-20% larger than shorter ones. For example, a 200mm long runner should have a diameter of 10mm, while a 150mm long runner should have a diameter of 8mm. This increased runner cross-sectional area reduces pressure loss over long distances.
Injection molding runners require high surface quality and require meticulous machining to reduce melt flow resistance. The inner wall roughness (Ra) should be kept below 1.6μm to avoid turbulence or stagnation during melt flow caused by surface roughness. Milling or grinding should be used to ensure a smooth, scratch-free inner wall. This is especially true for crystalline plastics (such as PE and PP). A smooth runner surface can reduce melt degradation caused by shear overheating. Furthermore, runner corners should be rounded with a radius of 0.5-1 times the runner diameter. For example, for an 8mm diameter runner, a 5mm corner radius is recommended. This prevents eddy currents and pressure loss at the corners, ensuring smooth flow. The transition between the runner, main runner, and gate should be smooth, avoiding steps or sharp corners. The radius of the transition corner should be at least 1mm to prevent localized melt stagnation, which can affect filling efficiency.
The layout of injection molding runners must match the cavity distribution to ensure even distribution of melt pressure and flow within each runner. Common layouts include balanced and unbalanced. A balanced layout requires that each runner have identical length and cross-sectional dimensions, and that the cavities are symmetrically distributed around the sprue. This layout is suitable for high-precision, multi-cavity parts in a single mold. For example, the barrel of a medical syringe utilizes four cavities symmetrically distributed around the sprue, with runner lengths and diameters identical, ensuring dimensional tolerances within ±0.05mm for each part. An unbalanced layout is suitable for molds with a large number of cavities and an asymmetrical layout. Even melt filling is achieved by adjusting runner and gate sizes. For example, in a six-cavity part with two cavities close to the sprue and four cavities farther away, the runners in the closer locations have a diameter of 6mm and a gate size of 1mm, while the runners in the farther locations have a diameter of 8mm and a gate size of 1.2mm. By increasing the runner and gate sizes, all cavities are ensured to be filled simultaneously.
The mold release design of injection molding runners must consider the smooth removal of runner slurry. A cold slug well can be located at the end of the runner, near the gate, to collect cold slugs at the melt front and prevent them from entering the mold cavity and impacting part quality. The cold slug well’s diameter is typically 2-5 mm larger than the runner diameter, and its depth is 1-1.5 times its diameter. For example, for an 8 mm diameter runner, a cold slug well diameter of 10 mm and a depth of 12 mm is sufficient to effectively contain cold slugs. For molds with automatic demolding, runners should be designed with a draft angle. A circular runner’s slope is 0.5°-1°, while a trapezoidal runner’s side slope is sufficient for demolding, eliminating the need for additional draft. Furthermore, the connection between the runner and the cavity should facilitate the separation of slugs from the part. For brittle plastics (such as PS), a weak link (e.g., a reduced diameter) can be designed at the gate-runner junction to allow the slugs to automatically separate from the part during demolding, reducing subsequent processing steps. By rationally designing the demoulding structure of the branch channel, the time for removing the channel condensate can be shortened by 10%-15%, thereby improving production efficiency.
