Injection runner form
The injection molding runner is a crucial component of the gating system, connecting the main runner and the gate. Its primary function is to smoothly and evenly distribute the molten plastic from the main runner to the various gates before it enters the mold cavity. The rationality of the runner design directly affects the melt’s flow resistance, pressure loss, cooling rate, and the mold quality and production efficiency of the plastic part. In actual mold design, the appropriate runner configuration must be selected based on factors such as the part’s shape, size, quantity, and the properties of the plastic used to ensure a smooth injection molding process.
Circular runners are the most common type of runner. Characterized by a perfectly circular cross-section, they offer advantages such as low flow resistance and a low specific surface area (surface area to volume ratio). Because a circular cross-section has the smallest circumference, the melt experiences minimal frictional resistance and relatively low pressure loss when flowing through a circular runner given the same flow area. This is particularly important for plastics with poor flow properties, such as polycarbonate and polyoxymethylene, as it ensures smooth filling of the mold cavity with the melt. Furthermore, the smaller specific surface area reduces the contact area between the melt and the runner walls, minimizing heat loss, helping to maintain melt temperature and reducing flow difficulties caused by rapid cooling. However, circular runners also have certain disadvantages. They require semicircular grooves to be machined on both the movable and fixed mold sides, which are then closed to form a circular shape. This requires high mold precision. Inadequate alignment of the semicircular grooves can affect the runner’s cross-sectional shape, increasing flow resistance. Furthermore, circular runners are relatively difficult to manufacture and are slightly more expensive. They are generally suitable for molds for large and medium-sized plastic parts with high melt flow requirements.
Trapezoidal runners are another widely used design. Their cross-section is trapezoidal, typically with the upper base slightly wider than the lower, and the sides formed by inclined straight lines. This type of runner is relatively simple to manufacture, requiring only a trapezoidal groove to be machined on one side of the mold. Unlike circular cross-sections, which require the exacting precision of mold closing, this reduces mold manufacturing complexity and cost. The trapezoidal cross-section has a larger surface area than a circular cross-section, meaning the melt has a larger contact area with the runner walls during flow, leading to faster cooling. For crystalline plastics such as polyethylene and polypropylene, a moderate cooling rate helps control crystallinity and minimize shrinkage and deformation in the part. Furthermore, the inclined sides of the trapezoidal cross-section facilitate the melt’s gradual convergence toward the gate during flow, reducing eddy currents and dead spots during flow and facilitating uniform melt distribution. However, the flow resistance of a trapezoidal cross-section is slightly greater than that of a circular cross-section, so the cross-sectional dimensions should be appropriately increased during design to ensure sufficient melt flow velocity and pressure. Trapezoidal runners are suitable for molds for small and medium-sized plastic parts, and are particularly popular in multi-cavity molds.
A U-shaped runner is a cross-section between a circular and a trapezoidal shape. Its U-shaped cross-section features a semicircular base and two vertical or slightly inclined straight lines. This runner design combines the advantages of both circular and trapezoidal cross-sections: the semicircular base reduces resistance to melt flow, while the straight lines on either side facilitate machining and reduce mold manufacturing costs. The U-shaped cross-section has a surface area between that of a circular and a trapezoidal shape, resulting in a moderate melt cooling rate. This ensures adequate melt fluidity while avoiding problems such as casting caused by excessively slow cooling. U-shaped runners have a wide range of applications and are well-suited to most plastic materials and part sizes. They are often used in mold designs that balance cost and performance. However, when designing the U-shaped cross-section, it is important to consider the ratio between the radius of the semicircular base and the height of the two side edges to ensure smooth melt flow and maintain reasonable pressure loss.
Rectangular runners have a rectangular cross-section, characterized by their simple structure and ease of processing, requiring only a rectangular groove to be milled into the mold. However, because the four corners of a rectangular cross-section are right angles, the melt is prone to forming eddies and stagnation at the corners during flow, increasing flow resistance and pressure loss. This can also lead to uneven melt cooling, impacting part quality. Furthermore, the larger specific surface area of a rectangular cross-section leads to rapid melt cooling, making it prone to flow difficulties and insufficient filling for plastics with poor fluidity or those that are sensitive to temperature. Therefore, rectangular runners are generally only used in simple molds for plastic parts with low molding requirements and small production batches, and are rarely used in molds for high-precision, high-volume production. In practical applications, if a rectangular runner is used, it is usually necessary to increase the cross-sectional dimensions appropriately to compensate for the increased flow resistance, while also strengthening the mold’s temperature control measures to ensure melt temperature stability.
Choosing the appropriate injection molding runner form requires comprehensive consideration of many factors, including the plastic’s fluidity, the part’s structural dimensions, the mold’s processing costs, and precision requirements. During the design process, the runner’s dimensions must be accurately calculated to ensure they meet melt flow and distribution requirements. Generally speaking, the runner’s diameter or cross-sectional dimensions should be determined based on the part’s weight, wall thickness, and number of cavities. Excessively large dimensions will increase the melt’s cooling time and raw material consumption, while too small will result in excessive flow resistance, affecting the filling effect. Furthermore, the runner’s surface roughness must be strictly controlled, typically requiring a smooth surface to reduce frictional resistance during melt flow. By rationally designing the runner’s form and dimensions, the melt’s flow properties can be effectively improved, ensuring uniform filling of each cavity, thereby enhancing the quality of the plastic part and production efficiency.
