Introduction to the Working Principle of Ball Screws

Injection molding is fundamentally a process of balancing driving force (pressure) and resistance (cavity pressure). During injection, cavity resistance changes continuously as the melt fills the mold, meaning injection pressure is never constant—even at a fixed speed. The article compares two control approaches: variable closed-loop systems, where injection speed dominates and pressure adjusts automatically, and open-loop proportional valve systems, where pressure and speed must be set manually and interact with each other. It concludes that multi-stage pressure settings are valuable for protecting molds and ensuring stable production, particularly in multi-cavity applications.

This article examines the relationship between injection stroke and screw length-to-diameter (L/D) ratio in injection molding machines. Because the screw retracts during plasticization, a longer injection stroke reduces the effective L/D ratio, creating axial temperature gradients that cause inconsistent melt temperature and uneven part quality—especially in multi-cavity molds. Increasing the L/D ratio improves mixing and reduces axial temperature variation. The article recommends limiting injection stroke to approximately four times the diameter of the B screw and concludes that long-stroke, low-L/D designs suit single-cavity products with modest quality requirements, while constrained stroke and higher L/D ensure multi-cavity product consistency.

This article explains the differences between A, B, and C injection-molding screws, which are distinguished primarily by diameter and length-to-diameter ratio. A-type screws offer high pressure and excellent mixing for engineering materials and precision thin-walled parts; B-type screws are the versatile default for over 70% of general-purpose plastics; C-type screws provide high capacity for large, thick-walled components. Three selection rules are provided: keep shot size within 30%–70% of screw capacity, choose fine high-pressure screws for transparent parts, and specify wear-resistant alloys for fiber-reinforced materials.

Screw cleaning is critical to product quality in modified plastics production, yet often overlooked. This article analyzes four common cleaning methods for twin-screw extruders—resin cleaning, disassembly and manual cleaning, flame cleaning, and water-based cleaning—covering their operational principles, key process steps, precautions, and limitations. It provides practical guidance for selecting and combining cleaning methods based on material characteristics, equipment condition, quality requirements, and cost-effectiveness, while emphasizing standardized operations and regular maintenance to extend equipment service life.

For PVC plastic extrusion production lines, feeding method selection depends on material form and process requirements. Three common systems are compared: spring feeders (quick to clean, suitable for frequent color changes but poor with high-filler powders), screw feeders (high flow rate and anti-bridging, ideal for high-filler powders but not for fiber-reinforced materials), and vacuum feeders (dust-free and closed-loop, best for cleanrooms and fiber reinforcement but limited at high output).

Most screw breakages are caused by the reasons mentioned above. Regular inspection of the gearbox, ensuring adequate heating temperature, avoiding material interruptions, strictly following formulas, and preventing foreign objects in raw materials can effectively reduce the risk of screw breakage.

This article cautions against pursuing excessively short injection molding cycles (e.g., 2.8 seconds), outlining 20 improper high-speed practices and their consequences. It covers material degradation, gas entrapment, mold and machine component fatigue, hydraulic system damage, and mechanical failures, emphasizing that aggressive cycle reduction compromises part quality and equipment reliability rather than yielding genuine efficiency gains.