Injection position of injection screw
The injection molding screw’s ejection position is a key process parameter determining part quality. This refers to the screw’s position at the front end during the injection phase, directly impacting the melt fill rate, pressure retention, and part weight stability. The ejection position should be determined based on the part’s volume, the runner system’s capacity, and the plastic’s melting characteristics. If the position is too forward (screw stroke too short), the melt will be insufficiently filled, resulting in part defects. If the position is too far back (screw stroke too long), the melt may be overfilled, creating flash or increasing mold cavity pressure.
The basic calculation for injection position is based on the total volume of the part and runner. This volume must be converted to screw advance distance using the formula: S = (V × 4) / (π × D²), where S is the injection position (mm, measured from the maximum screw retraction position), V is the total volume of the part and runner (cm³), and D is the screw diameter (mm). For example, if the total volume of the part and runner is 100 cm³ and the screw diameter is 50 mm, substituting this into the formula yields S = (100 × 4 × 1000) / (3.14 × 50²) ≈ 50.96 mm, meaning an injection position of 50-51 mm is appropriate. When setting the injection position, allow a 5%-10% margin to compensate for plastic compression and metering errors to ensure adequate filling. Furthermore, for thin-walled parts, the injection position may need to be appropriately reduced (increasing the stroke) to increase filling speed.
The injection position must be coordinated with parameters such as injection pressure and injection speed to form a reasonable injection process curve. During the initial injection phase, the screw rapidly advances from the metering position to the injection position. At this point, the injection position must be adjusted to match the melt flow length. For parts with long runners, the injection position should be set to allow the melt to fill the cavity at a higher pressure. For parts with short runners, the injection position can be appropriately reduced to lower the cavity pressure. For example, when the injection pressure is set to 120 MPa, the injection position needs to be increased accordingly to ensure sufficient melt volume to overcome flow resistance. If the injection speed is increased, the injection position can be appropriately decreased to avoid overfilling the cavity. By optimizing multiple parameters, smooth melt filling and dense part formation can be achieved.
Different plastic materials have different sensitivities to the injection position, requiring targeted adjustments to suit the material’s characteristics. Crystalline plastics (such as PP and PA) experience significant volume changes in the molten state, requiring a greater margin for the injection position, such as a 10% increase over the calculated value, to account for shrinkage during cooling. Amorphous plastics (such as PC and PS) have better volume stability, and the injection position can be set according to the calculated value, with a margin of less than 5%. For reinforced plastics with added glass fiber, due to the poor melt fluidity, the injection position needs to be appropriately increased to ensure that the melt has sufficient pressure to fill every corner of the cavity, while avoiding uneven fiber distribution due to insufficient filling. In addition, for plastics with higher viscosities (such as PVC), the injection position needs to be coordinated with a lower injection speed to prevent the melt from generating excessive shear heat during screw advancement.
Stability control of the injection position is key to ensuring consistent part weight, requiring precise control through equipment accuracy and process monitoring. The sealing performance of the screw’s check ring significantly impacts the stability of the injection position. If the check ring wears, causing melt leakage, the actual injection volume will be less than the set value, manifesting as unstable injection position. In this case, the check ring must be replaced promptly to ensure the sealing gap is less than 0.01mm. During production, the weight of the plastic parts must be regularly checked. When the weight fluctuates by more than ±0.5%, the injection position must be recalibrated. This can be corrected by adjusting the zero point of the electronic scale or the screw’s back pressure. For precision plastic parts, it is recommended to use an injection molding machine with closed-loop control, which provides real-time feedback on the screw position and automatically adjusts it to keep the injection position repeatability within ±0.1mm, thereby ensuring consistent size and weight for each molded part.
The debugging of the injection position should follow the principle of coarse to fine, and gradually optimize to the optimal state. In the initial stage of mold trial, the injection position can be set to 120% of the calculated value, and the filling of the plastic part can be observed. If flash appears, the injection position can be reduced. If there is material shortage, the injection position can be increased. The adjustment amount is 2-5mm each time. When the plastic part is basically full, it can be further optimized by adjusting the holding parameters. At this time, the injection position can be fine-tuned to ensure a smooth transition of the pressure curve in the holding stage to avoid internal stress in the plastic part caused by sudden pressure changes. For multi-cavity molds, it is necessary to ensure that each cavity is filled evenly. The melt filling amount of each cavity can be consistent by adjusting the runner balance or individually setting the injection position segment corresponding to each cavity. By scientifically setting and fine-tuning the injection position, the performance of the injection molding machine can be fully utilized to produce high-quality plastic parts.
