IMM Review: Understanding Injection Mold Design

Understanding Injection Mold Design is the latest in the excellent Hanser "Understanding" series of minitutorial books. This 115-page, fully indexed book contains one appendix, two tables, and 36 technical illustrations that are almost completely free of errors. 

Author Herbert Rees has more than 50 years experience working with molders, moldmakers, product designers, and mold designers in the injection molding field. Rees' intention is to explain what is truly important in a good mold design and to avoid overwhelming the reader with too many specialized details. This purpose is achieved nicely in a compact, well-written, and easy-to-read tutorial. 

In his introductory chapter, the author gives a concise history of injection molding, moldmaking, and mold design. The topic of mold design economics is also discussed. The designer is asked to consider production requirements, product life, and part tolerance when designing the most cost-effective mold. Often an inexpensive mold will meet the needs of the customer, says Rees. 

In the second chapter, Rees writes about starting new in the mold design field. Here the author recounts the obvious advantages of designing molds with the aid of a computer. He also stresses that a good designer must have more than proficiency with a computer; a good designer must keep an open mind, understand what is required of the mold, and seek input from people with moldmaking and molding experience. 

Chapter 3 is a short chapter dealing with the basic aspects of an injection molding machine. Chapter 4 is devoted to the fundamentals of the injection mold. It begins with sections on the mold cavity and core, parting lines, runners, and gates, along with simple but adequate diagrams. Clamping force, projected area, injection pressure, and their relationship to mold strength are explained next. The need for and actions of mold venting are also discussed. Mold cooling is covered extensively, and eight rules of mold and plate cooling are detailed. 

Chapter 4 concludes with coverage of ejection (including nice illustrations of each ejection system), shrinkage, and mold alignment. The entire gamut of alignment devices—from nothing at all to leader pins and bushings, taper locks, wedge locks, taper pins, and combinations of these devices—is reviewed. The strength of each device is covered, along with preferred applications. When combinations are used, the device that should do most of the alignment work is highlighted. 

Chapter 5 describes the steps that need to be taken before starting to design a mold, such as gathering information and preparing documentation. Questions to be considered include whether tolerances are shown on the product drawing, if they agree with the mold quotation, and whether they are reasonable and suited to the function of the part. Rees then recommends considering the cycle time, expected annual production requirements, and machine specifications. Of course the mold must fit in the press, and there must be adequate clamping force and good shot and plasticating capacity. Core pulls, air ejects, valve gates, and other options may be present as the design dictates. 

Once these preliminaries are attended to, Rees details the mold design process, beginning with choosing the mold base, which he calls a shoe. He discusses mold base steels, comparing the advantages of standard prebuilt mold bases against mold bases built in-house. He then describes mold hardware, such as pins, bushings, ejector pins, and sleeves. Mold drawings, the stack layout, and product cross sections are touched upon next. Here undercuts, internal threads, unscrewing components, and other features are illustrated. 

Hot runner, cold runner, and three-plate runner systems are presented with the help of illustrations on runner balancing, sizing, and layout. Other topics include venting, ejection, cooling, and mold alignment. 

Chapter 5 concludes with a discussion of mold layout and assembly drawings, bill of materials, and finishing touches. In a very short Chapter 6, Rees recommends another design review with the molder after the initial sampling. Chapter 7 identifies typical designs, such as molds for containers and cup-shaped products, along with technical products. 

Chapter 8 is a short but important chapter on estimating mold cost, where nothing can take the place of experience, according to Rees. Two widely used methods for cost estimating are explained. 

Chapter 9 describes machining operations. After a brief history of the moldmaking trade, Rees reviews such operations as EDM, jig grinding, CNC machining, and gun drilling. Materials selection is also covered in this chapter, including two excellent tables on mold steels and properties of mold materials. The chapter ends with an explanation of the heat treatment process. 

An appendix lists sources for CAD/CAM programs, fluid flow programs, and stress analysis. 

Understanding Injection Mold Design packs a lot of information into a small volume. I recommend this book for students as well as seasoned professionals, be they designers, moldmakers, or molders.—Reviewer: Brent Borgerson, process engineering manager, Matrix Tooling/Matrix Plastic Products, Wood Dale, IL, [email protected]. 

Understanding Injection Mold Design is one of many books selected for injection molders that are offered through the IMM Book Club and count toward a Bonus Book award. For more information, call Deb Golanty at (303) 321-2322; fax (303) 321-3552; e-mail [email protected], or visit www.immbookclub.com. 

Rees, Herbert. Understanding Injection Mold Design (2001), 115 pp., figures, tables, appendices, index, $34.95.