The ball screw (often called a "lead screw") of an injection molding machine is its core component, often referred to as the "heart" of the machine. Its operation is a complex process integrating physics, mechanics, and thermodynamics. Simply put, its core task is to transport, melt, compress, and homogenize solid plastic granules, ultimately injecting the molten plastic into the mold cavity with sufficient pressure and speed. To better understand its operation, we can divide its working cycle into the following stages: A complete working cycle of an injection molding machine ball screw. In a complete injection cycle, the ball screw mainly performs two actions: rotation and axial movement. Its working cycle can be divided into three stages: 1. Rotation (Plasticizing/Metering) Stage Objective: To transport, heat, melt, and homogenize the solid plastic granules in the hopper. Action: The lead screw rotates at high speed inside the barrel but does not move forward (at this time, the injection cylinder at the rear of the lead screw releases pressure, allowing the lead screw to retract due to the reaction force of the plastic during rotation). Operation Process: Feeding and Conveying: Plastic granules fall from the hopper into the barrel. The rotation of the screw, like a screw turning in a nut, uses the inclined plane of the thread to continuously push the plastic granules forward. Compression and Melting: The screw structure is divided into three sections from back to front: the feeding section, the compression section, and the metering section. Feeding Section: The thread depth is relatively deep, mainly used for stable conveying of solid granules. Compression Section: The thread depth gradually decreases. Here, the plastic is strongly compressed and sheared, while the heating coil outside the barrel also heats it. Under the combined action of "shear heat" and "external heating," the solid plastic rapidly melts into a viscous flow state. In fact, more than 80% of the melting heat comes from the shear heat generated by the screw rotation. Metering Section: The thread depth is the shallowest. Its main function is to further homogenize the temperature and composition of the melt, ensuring the uniform quality of the melt stored at the front end. Result: Uniformly molten plastic is pushed to the front of the screw (at the nozzle), and the accumulated pressure (back pressure) pushes the entire screw backward, reserving a fixed amount of molten material for the next injection. 2. Axial Movement (Injection/Holding Pressure) Stage Objective: To inject the molten plastic reserved in the previous stage into the mold cavity at high speed and high pressure. Action: The screw stops rotating and, under the powerful thrust of the injection cylinder, moves forward at high speed as a piston. Operation Process: Injection: The screw advances forward at extremely high speed, injecting the molten plastic reserved in the front through the nozzle, mold runner, and gate into the closed mold cavity. This process needs to be completed in a very short time to ensure that the molten material fills every corner of the cavity simultaneously. Holding Pressure: When the cavity is about to be filled, the injection speed slows down, transitioning to a high-pressure "holding pressure" stage. The screw continues to move forward slowly, using extremely high pressure to replenish the volume vacated by the cooling and shrinkage of the plastic, preventing defects such as shrinkage marks and insufficient material in the product. 3. Reset (Preparing for the Next Cycle) Objective: To prepare the melt for the next injection molding cycle. Action: After the holding pressure is completed, the screw stops axial movement and begins to rotate again (returning to the first stage) for the next plasticizing and metering. At this time, the mold opens, ejects the product, and then closes, awaiting the next injection. Key Design Features of the Ball Screw To accomplish the above complex tasks, the ball screw itself is designed with great precision: Length-to-Diameter Ratio (L/D): The ratio of the ball screw's length to its diameter. A larger L/D ratio results in better plasticizing and more uniform temperature. Common ratios are between 18:1 and 25:1. Compression Ratio: The ratio of the volume of the first threaded groove in the feeding section to the volume of the last threaded groove in the metering section. It determines the degree of plastic compression and is crucial to melting efficiency. Different plastics require different compression ratios. Three-Stage Design: As mentioned above, the feeding section, compression section, and metering section each perform their respective functions, forming the basis for the efficient operation of the lead screw. In summary, you can visualize the operation of an injection molding machine screw as follows: It's like a "meat grinder": as it rotates, it bites, shears, mixes, and conveys materials. It's like a "piston" or "syringe": as it propels forward, it injects the processed "fluid" under high pressure. It's also a "heat generator": through its own rotational shearing, it generates most of the heat needed to melt the plastic. This ingenious combination of "rotational plasticizing" and "axial injection" allows the injection molding machine screw to efficiently and precisely complete the transformation process from solid granules to precision plastic products.

In industrial automation and precision equipment, trapezoidal lead screws are the core transmission mechanism for achieving rotary-to-linear motion, directly affecting the accuracy and stability of the equipment. However, practitioners often suffer from decreased equipment efficiency and shortened lifespan due to a lack of in-depth understanding of the principles and improper selection. This article will break down the motion principle of trapezoidal lead screws and provide a practical selection guide. I. Product Motion Principle and Related Parameters 1. Motion Principle: The trapezoidal lead screw converts rotational motion into linear motion through the meshing of the screw and nut, simultaneously transmitting energy and power. II. Product Features 1. Simple structure, convenient processing and operation, and economical cost; 2. Self-locking function is achieved when the thread helix angle is less than the friction angle; 3. Smooth and stable transmission process; 4. Relatively high frictional resistance, with a transmission efficiency in the range of 0.3~0.7. In self-locking mode, the efficiency is below 0.4; 5. Possesses a certain degree of impact and vibration resistance; 6. Overall load capacity is stronger than that of ordinary rolling screws. III. Selection and Verification Calculations For general force-transmitting screws, the main failure modes are thread surface wear, fracture under tensile stress, shearing, and shearing or bending at the thread root. Therefore, the main dimensions of the screw drive are determined primarily based on wear resistance and strength calculations during design. For transmission screws, the main failure mode is excessive clearance due to wear or deformation leading to decreased motion accuracy. Therefore, the main dimensions of the screw drive should be determined based on thread wear resistance and screw stiffness calculations during design. If the transmission screw also bears a large axial load, its strength needs to be additionally calculated. Long screws (slenderness ratio exceeding 40) that are not manually adjustable may produce lateral vibration; therefore, their critical speed needs to be checked. IV. Usage Precautions 1. Load Considerations: Additional radial loads should be avoided as much as possible, as such loads can easily cause screw malfunction, increased wear, and jamming. 2. Dust Prevention Requirements: Foreign objects must be prevented from entering the thread. If impurities such as iron filings, tin dross, and aluminum shavings are easily generated under operating conditions, a protective cover should be installed to prevent foreign objects from entering the thread and causing abnormal wear or jamming. 3. Slenderness ratio requirement: When the slenderness ratio exceeds a certain range (60 or above), the screw will bend due to its own weight, resulting in radial off-center load on the nut. Depending on the actual operating speed and torque, this may lead to abnormal wear, jamming, shaft end bending, or even breakage. To solve this problem, an anti-runout device can be installed in the middle of the screw for constraint. 4. During installation, attention should be paid to the coaxiality and levelness calibration of the fixed-support installation method; for the fixed-free cantilever structure, attention should be paid to the control of shaft end tolerances and the locking and reinforcement of the head. 5. When installing a trapezoidal thread screw, runout verification must be performed. If suitable measuring equipment is lacking, the screw can be moved by hand along its entire length once or multiple times before installing the driving component. If the force required to move the outer diameter of the shaft is uneven and accompanied by wear marks, it indicates that the lead screw, nut support, and guide rail are not aligned. In this case, first loosen the relevant mounting screws, and then move the lead screw by hand once. If the required force becomes uniform at this time, the corresponding components can be recalibrated. If the force is still uneven, the mounting screws need to be loosened again to determine the location of the calibration error.

The ability of a machine tool lead screw to operate efficiently and without jamming 24 hours a day is primarily due to the synergistic effect of three factors: suitable design and selection, proper lubrication and maintenance, and reasonable operating condition control. Specifically, this can be broken down into the following key dimensions: 1. High-precision structural design and manufacturing process Precision fit of the transmission pair: Ball screws use steel balls as rolling elements. Compared to the surface contact of sliding screws, this is point contact, resulting in an extremely low coefficient of friction (only 1/10 to 1/3 of that of sliding screws). This leads to low frictional resistance and less heat generation during operation, preventing jamming caused by overheating. Preload process eliminates backlash: A double-nut preload structure (such as washer type, variable lead type, or threaded type) eliminates axial backlash between the lead screw and nut, ensuring transmission accuracy and preventing axial movement and jamming during high-speed operation. High-Quality Materials and Heat Treatment: Lead screws and nuts are typically made of high-carbon steel (such as GCr15) or alloy structural steel, treated with quenching, tempering, and grinding to achieve a surface hardness of HRC58~62. This results in strong wear resistance, preventing wear and deformation during long-term operation and maintaining stable fit accuracy. 2. Stable and Reliable Lubrication and Sealing System Continuous and Efficient Lubrication:** Equipped with an automatic lubrication system (such as a grease pump or oil mist lubrication device), it replenishes the lead screw raceway with specialized grease or oil at regular intervals, forming an oil film that reduces direct friction between the steel balls and the raceway, lowering wear and heat generation. Machine tools operating 24 hours a day are generally equipped with intermittent automatic lubrication to prevent insufficient lubrication or grease aging. Excellent Sealing Protection:** Both ends of the lead screw are equipped with dustproof seals, scraper plates, and other components to prevent cutting fluid, metal shavings, and dust from entering the raceway. Impurities entering the raceway are a common cause of lead screw jamming; the sealing system effectively isolates contaminants and keeps the raceway clean. 3. Reasonable Operating Parameters and Load Control Load and Speed ​​Matching: During selection, the rated dynamic and static loads of the lead screw are determined based on the actual load of the machine tool (cutting force, table weight) to ensure that the load does not exceed the rated value during 24-hour operation, avoiding ball bearing deformation and lead screw bending due to overload. Simultaneously, the speed is controlled below the lead screw's critical speed to prevent resonance and vibration during high-speed rotation. Temperature Control: The machine tool is equipped with a cooling system to control the operating temperature of the lead screw and spindle. Heating the lead screw can cause thermal deformation, leading to pitch changes or even jamming. The cooling system can control temperature fluctuations within a minimal range, maintaining transmission stability. 4. Precise Coordination of Drive and Control Systems Rigid Connection between Servo Motor and Lead Screw: Couplings (such as diaphragm couplings and lamellar couplings) are used to achieve a gapless connection between the motor and the lead screw, ensuring smooth power transmission and avoiding transmission jerks caused by loose connections. Precise adjustment of the CNC system: Through a closed-loop or semi-closed-loop control system, the position and speed of the lead screw are monitored in real time, and the motor output torque is dynamically adjusted to compensate for the elastic deformation and temperature deformation of the lead screw, ensuring uniform speed and no impact during operation. Supplement: The crucial role of routine maintenance: Even with reasonable design and operating conditions, regular maintenance is essential for 24/7 uninterrupted operation. For example, regularly cleaning seals, checking the condition of the lubricating grease, detecting lead screw runout and backlash, and promptly replacing aged grease and worn balls can effectively extend the stable operating time of the lead screw.