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.
