Large injection mold design

Author: Xi Yongsheng / Edited by Wang Chun
Publisher:
Publish Date: 1998-05-01
Features: To meet the needs of readers engaged in mold design and manufacturing, we have organized and edited a series of books on mold design and manufacturing, including "Practical Injection Mold Design," "Large-Scale Injection Mold Design," "Precision Injection Mold Design," and others. This series introduces readers to the problems and key points in the design, manufacturing, and processing of plastic injection molds in three forms: general, large-scale, and precision. The series is organized in order from small and medium-sized ordinary molds to large and precision molds, so readers can start with the "introductory" level of plastic injection mold design and gradually delve deeper, improving their mold design skills. In terms of content, we require authors to combine theory with practical production, striving to make this series both informative and rational. "Large-Scale Injection Mold Design" is published as the second book in the series of plastic mold design. The author, based on their extensive experience in the design, research, and application of plastic molds, has written this book. Given the current situation in China where most plastic product manufacturers rely on imports for large-scale plastic molds, and the scarcity of relevant information in China, we believe the design theories and methods presented in this book are highly enlightening and readers to reference. The publication of this book aims to broaden the horizons of technical personnel engaged in mold production, mold manufacturing workers, and students of mold majors in universities and colleges, while improving their mold design skills. We sincerely hope that readers will provide valuable feedback on this book. China Light Industry Press, June 16, 1996
Excerpt: Chapter 1 Introduction In recent years, with the rapid advancement of the plastic industry, the mold design and manufacturing industry has also undergone fundamental changes. The proportion of high-efficiency, automated, large-scale, ultra-small, high-precision, and high-lifetime molds in the total mold output has been increasing steadily, such as molds for TV casings, washing machine drums (single and double), refrigerator and air conditioner parts, bathtubs, barrels, and pallets. People are paying attention to the development of new technologies and processes for the design and manufacturing of large-scale plastic molds. Since the design and manufacturing methods for small and medium-sized molds are no longer fully applicable to large-scale molds, technical personnel engaged in large-scale plastic mold design urgently need information and data on large-scale plastic mold design. Therefore, this book provides a detailed discussion on the key points, steps, methods, calculations, and mold life extension of large-scale plastic injection mold design from both theoretical and practical perspectives.
1. Criteria for Classifying Large-Scale Plastic Injection Molds
Typically, injection molding machines are classified as large, medium, and small. The main parameters for injection molding machines are their maximum injection volume or maximum clamping force. However, there is no uniform standard for classifying large-scale molds. The author believes that molds used in injection molding machines with a clamping force of 4.9 MN (500 tons) or above, or with an injection volume of 1 kg or more, are considered large-scale plastic injection molds. Examples of large-scale molds are shown in Table 1-1.
2. Steps and Characteristics of Large-Scale Plastic Injection Mold Design
(1) Stiffness Requirements for Stress-Loaded Parts
In the design of small-scale molds, the stress-bearing parts of the cavity are primarily checked for strength. However, this is not the case for large-scale molds. Because molds that meet the strength requirements may still experience significant deformation under high pressure. The larger the mold size, the greater the elastic deformation, making it more prone to overflow. Therefore, the stress-bearing parts of large-scale molds should be checked based on stiffness.
(2) Composite Requirements for Forming Parts
In large-scale molds, composite structures are generally used for the convenience of mechanical processing, grinding, polishing, and heat treatment. The larger the mold, the larger the cavity size, and the higher the requirement for composite structures. In some cases, from a stiffness perspective, or when the composite structure cannot meet the stiffness requirements, an integral structure may be used instead.
(3) Reliability Requirements for Mold Release
The rationality of the mold release mechanism is one of the important indicators of design quality. The requirements for the mold release mechanism of injection molds are:
(1) Can the mold release mechanism accurately and reliably remove the molded part?
(2) The mold release mechanism itself should be flexible, with sufficient stiffness and stability for the pushing parts.
(3) The pushing parts should be set at the center of gravity of the mold release force and have a sufficiently large contact area with the molded part.
(4) The structure should be simple and easy to operate.
(5) When designing the mold release mechanism, the mold release force must be accurately calculated to obtain reliable data, ensuring the strength and stiffness requirements of the stress-bearing parts.
(6) The weight of the pushing plate is large, and relying solely on a few push rods and return rods is insufficient. Additional guidance mechanisms must be provided to ensure smooth movement of the mold release mechanism.
(4) Requirements for Cavity Venting
Due to the large volume of the feeding system and cavity in large-scale plastic injection molds, a significant amount of air accumulates. Additionally, the molten plastic injected into the cavity produces a lot of steam, and the gases released from the slight decomposition of the molten material also accumulate in the cavity. Therefore, the issue of venting is particularly important in the design of large-scale molds. The design of the venting system for large-scale molds is completely different from that of small and medium-sized molds, and will be discussed in later chapters.
Furthermore, when designing large-scale molds, heating and cooling systems must be installed simultaneously. Because before the initial injection of large-scale molds, the molds must be preheated to a certain mold temperature before injection can begin. After production for a period of time, the mold temperature rises, and it is necessary to ensure that the mold temperature does not become too high, requiring cooling. In developed countries, large-scale mold temperature regulation systems are (constant-temperature automatic control).