The key component of the injection device – the screw head
The screw head is a key component in the injection molding machine’s injection unit, connecting the screw and the melt flow path. Located at the front end of the screw, its primary function is to guide the melt into the nozzle during the injection phase, prevent melt backflow during the plasticization phase, and ensure uniform mixing of the melt. The screw head’s structural design directly affects melt conveying efficiency, plasticization quality, and injection stability. Different types of screw heads are suitable for plastics with different properties and molding requirements. For example, standard screw heads are suitable for general plastics, while check ring screw heads effectively prevent backflow of high-viscosity plastics. Therefore, a thorough understanding of the screw head’s structural characteristics, operating principles, and selection principles is crucial to ensuring smooth injection molding production and consistent product quality.
The basic structure of a screw head consists of a connection, a check ring, and a flow guide. Each design must meet different functional requirements. The connection is connected to the front end of the screw via a threaded connection. The thread precision must ensure a secure connection to prevent loosening during high-pressure injection. Trapezoidal or rectangular threads are typically used, reinforced with a locknut. The check ring is the core of the screw head and consists of a check ring, a sealing ring, and a support ring. Its function is to open the flow channel to allow melt flow during injection and to close the channel to prevent melt backflow during pressure holding and plasticization. For example, when the screw advances, melt pressure pushes the check ring backward, opening the flow channel. When the screw stops or retracts, the check ring returns under the action of a spring or melt pressure, tightly fitting the sealing surface of the screw head and blocking the backflow channel. The flow guide is typically designed with a tapered or arc-shaped transition structure to guide the melt smoothly into the nozzle, reducing flow resistance and pressure loss. Its surface should be polished to a roughness of Ra ≤ 0.8μm to prevent melt retention and degradation.
Screw heads are primarily classified by structural type: standard, check ring, and combination types. Different types are suited to different plastic properties and molding conditions. Standard screw heads have a simple structure and lack a dedicated check ring. They rely solely on the gap between the screw and the barrel for sealing. They are suitable for low-viscosity, high-flow plastics (such as PE and PP). Their advantages are compactness and low cost. However, their disadvantages are poor check ring effectiveness, which can easily cause melt backflow and lead to unstable injection volume. Check ring screw heads are the most widely used type. They achieve bidirectional sealing through the opening and closing of the check ring, providing excellent check ring effectiveness. They are suitable for medium- and high-viscosity plastics (such as ABS and PC), effectively reducing backflow and ensuring injection volume accuracy. Combination screw heads integrate multiple functions such as mixing, check ring, and flow diversion. For example, mixing pins or spiral grooves in the check ring enhance melt mixing. They are suitable for plastics requiring high uniformity (such as glass fiber reinforced plastics and masterbatch-colored plastics). Their advantages are high plasticizing quality, but their disadvantages are complex structure, high cost, and difficult maintenance.
The screw head operates based on the synergy of pressure differential and mechanical motion, exhibiting different operating states during different molding stages. During the plasticizing phase, the screw rotates under the drive of the motor, conveying solid plastic to the front end and compacting the melt. During this phase, the screw head’s check ring, acting under melt pressure and spring force, engages the screw head’s sealing surface, preventing melt from flowing back toward the rear end of the screw and ensuring melt accumulation at the front end. When the accumulated melt reaches the set amount, the injection phase begins. The screw, propelled forward by the hydraulic cylinder, instantly increases melt pressure, pushing the check ring backward against the spring force, opening the flow path. The melt flows through the guide section of the screw head into the nozzle and ultimately into the mold cavity. After injection is complete, the holding phase begins, maintaining a certain screw pressure. The check ring closes again under melt pressure, preventing melt from flowing back into the cavity and ensuring adequate shrinkage of the part. When the plasticizing phase resumes, the screw rotates backward, reopening the check ring and allowing new melt to enter the front end, completing the cycle.
The screw head selection should be determined based on the plastic’s properties, product requirements, and molding process to ensure optimal performance. For heat-sensitive plastics (such as PVC and POM), a screw head with excellent anti-return properties, smooth flow paths, and no dead angles should be selected, such as a short anti-return ring screw head. This reduces melt residence time and prevents degradation. For example, when processing PVC, a screw head with a small contact area between the anti-return ring and the sealing surface and a sufficient clearance (0.03-0.05mm) should be selected to prevent backflow and reduce shear heating. For high-viscosity plastics (such as PC and PMMA), a screw head with low flow resistance and flexible anti-return ring opening, such as a large-diameter flow-guiding screw head, should be selected to minimize melt flow pressure loss. For precision plastic parts requiring precise injection volume control (such as electronic connectors), a high-precision anti-return ring screw head should be selected to ensure the sealing reliability of the anti-return ring and keep injection volume deviation within ±0.5%. In addition, the material of the screw head needs to match the characteristics of the plastic. When processing reinforced plastics containing glass fiber, the screw head should be made of wear-resistant material (such as high-speed steel W18Cr4V) and nitrided with a surface hardness ≥HRC60 to improve wear resistance and service life.
Screw head maintenance is crucial to ensuring long-term stable operation. Regular inspection, cleaning, and replacement of vulnerable parts are essential. During daily production, the screw head connection should be checked for looseness and the check ring should be checked for smooth opening and closing. If injection volume fluctuations or pressure abnormalities are detected, the screw head should be disassembled and inspected promptly. Specialized tools should be used during disassembly to avoid damaging the screw head and screw threads. During cleaning, any residual melt and impurities should be thoroughly removed, especially around the contact area between the check ring and the sealing surface to prevent impurities from affecting the sealing effect. Severely worn vulnerable parts such as check rings and sealing rings should be promptly replaced. When replacing, use parts that are identical to the original model to ensure a precise fit. For example, if the sealing surface of a check ring is scratched or wear exceeds 0.1mm, it must be replaced with a new one. Failure to do so will increase melt backflow and affect injection accuracy. Additionally, when installing the screw head, apply high-temperature grease to reduce frictional damage during assembly. Ensure that all components are installed in the correct order to avoid malfunctions caused by incorrect assembly. Through scientific maintenance, the service life of the screw head can be significantly extended and the working stability of the injection device can be ensured.
