Design of cold slug well, pull rod and ejection mechanism of pouring system
The design of the gating system’s cold slug well is crucial for preventing cold slug from entering the mold cavity. Its function is to collect cold slug at the melt front, preventing it from forming defects on the part surface or clogging the gate. The size of the cold slug well depends on the runner size and plastic type. Its diameter is typically 2-5mm larger than the branch runner diameter, and its depth is 1-1.5 times the diameter. For example, for an 8mm diameter branch runner, a cold slug well with a diameter of 10mm and a depth of 12mm is sufficient to effectively contain the cold slug. The cold slug well should be located at the end of the main runner or branch runner, maintaining a distance (5-10mm) from the gate to prevent cold slug from entering the mold cavity. For example, for a pinpoint gate, the cold slug well should be located directly in front of the gate, 8mm from the gate, to prevent cold slug from entering the thin-walled cavity. For hot runner molds, the cold slug well can be located at the hot nozzle outlet. Its dimensions are slightly smaller than those of conventional runners (1-2mm larger in diameter and 0.8 times the diameter deep), adapting to the more stable melt temperature of hot runners. The shapes of the cold slug hole include cylindrical, spherical and conical. The spherical cold slug hole has the best cold slug accommodation effect and is suitable for plastics with higher viscosity. The conical cold slug hole is convenient for cooperation with the pulling rod to improve the pulling effect.
The pull rod design must coordinate with the slug well to ensure smooth removal of the runner aggregate. Its structural form should be selected based on the slug well shape and demolding method. Z-shaped pull rods are suitable for cylindrical slug wells. Their Z-shaped head hooks the aggregate in the slug well and pulls the runner aggregate out of the main channel during mold opening. They are suitable for small and medium-sized molds. The pull rod diameter is the same as the slug well diameter (e.g., 10mm), and the Z-shaped head height is 3-5mm, ensuring that the aggregate is hooked without affecting melt flow. Spherical pull rods are used in conjunction with slug wells. The pull rod head is hemispherical and fits into the slug well’s spherical portion, retaining the aggregate by its contraction force. They are suitable for elastic plastics (such as PE and PP). The slug’s diameter is 0.5-1mm smaller than the slug well’s spherical portion, ensuring smooth aggregate removal. The tapered puller is designed for tapered slug wells. It relies on the friction of the tapered surface to pull the slug. It is suitable for high-viscosity plastics (such as PC). The taper matches that of the slug well (typically 5°-10°), and its length is 2-3mm longer than the depth of the slug well to ensure full contact. The puller is made of quenched and tempered 45# steel (HB220-250), with a head surface roughness of Ra ≤ 1.6μm to avoid scratching the slug or hindering demolding.
The ejector mechanism must be designed to work in conjunction with the slug puller and cold slug well to ensure simultaneous and smooth demolding of the part and runner condensate. The type of ejector mechanism should be selected based on the part’s structure. Common options include ejector pins, ejector plates, and ejector tubes. Pin ejection is suitable for small and medium-sized parts. Pins have a diameter of 3-10mm and are located on rigid parts (such as bosses and ribs) to prevent deformation during ejection. For example, a mobile phone case uses six 5mm diameter ejector pins located on the bosses around the case. This ensures uniform force during ejection and keeps deformation within 0.1mm. Plate ejection is suitable for large, thin-walled parts. The contact area between the ejector plate and the part must be at least 50% of the part’s bottom area to ensure uniform force during ejection. For example, a washing machine panel uses an integral ejector plate with a thickness of 10mm, a contact area of 80% with the panel, and a flatness error of ≤0.2mm/m after ejection. The top tube ejection method is suitable for cylindrical plastic parts with holes. The inner diameter of the top tube is 0.5-1mm larger than the hole diameter of the plastic part, and the outer diameter is 0.5-1mm smaller than the outer diameter of the plastic part. For example, plastic bearings are ejected by top tubes. The inner diameter of the top tube is 15mm, the outer diameter is 20mm, and the clearance with the bearing is 0.5mm to ensure smooth ejection.
The coordinated design of the cold slug well, slug puller, and ejector mechanism is crucial for smooth demolding. The correct sequence and positional relationship of these three elements must be ensured. During mold opening, the slug puller first pulls the runner slurry from the main flow channel. The ejector mechanism then activates, simultaneously ejecting both the runner slurry and the part. The time difference between these two actions should be kept within 0.1-0.3 seconds to prevent slug shedding or part deformation caused by uncoordinated movements. The slug puller and ejector pin should be staggered, with a minimum distance of at least 5mm, to prevent motion interference. For example, the ejector pin next to a Z-shaped slug puller should be kept 8mm away from the slug puller to ensure that the ejector pin does not collide with the slug puller during movement. The cold slug well should be positioned to facilitate ejection of the slug by the ejector mechanism. For example, a dedicated ejector pin with a diameter 1-2mm smaller than the cold slug well diameter and 2-3mm longer than other ejector pins should be placed below the cold slug well to ensure that the slug is ejected first, followed by the part. For molds used in automated production, the design of the cold slug hole and the pull rod must ensure that the runner slug can fall off automatically. For example, if an inverted tapered pull rod is used, the slug will fall off the pull rod due to its own weight during ejection and fall into a designated collection device.
The maintenance design of the gating system’s cold slug well, slug puller, and ejector mechanism should facilitate mold cleaning and component replacement to extend mold life. The cold slug well must be easily cleaned to prevent residual slug accumulation. A removable insert can be installed at the bottom of the cold slug well and regularly removed for cleaning. For example, large molds with cold slug wells using inserts should be removed and cleaned every 5,000 mold cycles. The slug puller head should be treated for wear resistance (e.g., quenched to HRC 40-45) to prevent wear and tear that can lead to slug failure after long-term use. For example, quenching the head of a Z-shaped slug puller can extend its service life from 30,000 to 50,000 mold cycles. The ejector mechanism’s guides (such as guide pins and sleeves) require regular lubrication, with grease added every 2,000 mold cycles to ensure smooth ejection. The clearance between the ejector pin and the ejector hole should be regularly checked. If wear exceeds 0.1mm, the ejector pin should be replaced to prevent ejection marks on the part surface. Through reasonable maintenance design, the failure rate of molds can be reduced. For example, a mold uses a detachable cold slug insert to shorten the cleaning time from 2 hours to 30 minutes, thereby improving equipment utilization.
