Introduction to the Working Principle of Ball Screws

This article introduces the working principle of ball screws, highlighting their high mechanical efficiency and load capacity, which have led to widespread adoption in all-electric servo-driven injection molding machines. It compares ball screw design philosophies for machine tools and injection molding machines, noting that injection units experience loads hundreds to thousands of times greater. Key design priorities for high-load ball screws—such as uniform ball contact pressure, optimized lubrication, and enhanced durability—are discussed, with reference to Ningbo Superior's specialized solutions.

The screw and barrel are the most critical components of injection molding machines, operating under high temperature and pressure. Wear enlarges the clearance between the screw flight and barrel, reducing melting and pumping capacity, causing product quality degradation, lower productivity, and higher energy consumption. The screw is more susceptible to damage than the barrel.

This section examines the key parameters of the venting section in vented extruder screws. Venting effectiveness depends primarily on venting section length L, melt residence time, shear rate, and the fill factor F (the ratio of melt cross-sectional area to channel area). To ensure good performance, the venting channel should be partially filled; experiments suggest L ≥ 3D, F ≤ 0.5, and a shear intensity K > 100 for optimal degassing. For screws with L/D ratios of 24–30, the venting section length is typically 4D, and its channel depth is 2.5–6 times that of the first metering section. Design verification must include fill factor, shear intensity, and screw strength.

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This section discusses the determination of channel depths H₁ and H₂ in venting screws, with emphasis on the pump ratio Ω (Ω = H₂/H₁). The pump ratio directly influences the risk of vent flooding and extrusion stability. A theoretical optimum Ω of 1.5 is derived for Newtonian fluids, while for non-Newtonian polymers like polyethylene an Ω of 1.75 yields maximum die pressure. In practice, most designs adopt Ω values between 1.5 and 2.0. The article also clarifies that the concept of a "second compression ratio" is invalid for venting screws, as the venting section is not fully filled.

This section outlines the functional characteristics of venting screws in extrusion. It identifies three main sources of gases in raw materials—entrained air, adsorbed moisture, and internal volatiles—and describes their detrimental effects on product quality and properties. While conventional methods rely on pre-drying or feed-throat venting, these approaches increase costs, risk contamination, and are often insufficient for high-speed extrusion. The text concludes that vented extruders offer superior performance in effectively removing these gases.

This article analyzes the economics of extruder screw design, focusing on energy consumption, service life, and manufacturability. It argues that design choices—such as specific energy, screw speed, L/D ratio, and mixing element complexity—must be systematically evaluated from an economic perspective rather than purely technical one.