Runner and Gating Design Handbook

Frontmatter

1. Overview of Runners, Gates, and Gate Positioning

Abstract

In many cases, the mold design dictates the gating position, although ideally, the optimum gate position should be determined based on part requirements and afterwards the mold design selected to provide for the desired gate position. Available gating positions, and gate designs, are significantly influenced by whether the runner travels along the primary parting plane of the mold (the parting plane where the part forming cavity is defined) or whether it does not travel along this plane.

John P. Beaumont

2. Rheology and Melt Flow in an Injection Mold

Abstract

This chapter will present both a foundation in melt rheology and flow of plastics in a mold. Rheology is a reasonably well established scientific field, but the science of how polymer melts flow in a mold is not so well understood. The rheology of polymer melts is quite complex and can include influences of shear, temperature, and pressure, converging and diverging flow, elastic effects, tensile viscosities, etc. As this is not a book on rheology, this chapter only provides an introductory overview of some of the issues that most molders and mold designers should understand. This foundation also ties into a number of the later chapters.

John P. Beaumont

3. Filling and Packing Effects on Material and Molded Part

Abstract

The flow of thermoplastics through an injection mold and its relationship to the molded part is quite complex. This chapter focuses on the development of melt conditions within a part-forming cavity and their relationship to the molded part. This will help the reader establish an optimum gating and molding strategy.

John P. Beaumont

4. Gate Positioning and Molding Strategies

Abstract

Gate positioning is critical to the successful injection molding of plastic parts. In addition, it is important to establish a basic strategy related not only to gate position but to part design, mold design, and processing. This chapter first outlines some important considerations in the positioning of gates. This is followed by 14 design and process strategies, which will help assure a successful molding operation.

John P. Beaumont

5. The Melt Delivery System

Abstract

The following list itemizes the basic design objectives for a runner system:

• Deliver melt to desired gating location(s) on the part
• Minimize excess material

John P. Beaumont

6. Filling, Melt, and Product Variations Developed in Multi-Cavity Molds

Abstract

Chapter 6 presents on the development and the negative impact of shear-induced melt and mold filling imbalances created in the industries 1st and 2nd generation runners. Despite the dated hydraulic based design of 2nd generation runners (often still referred to as “naturally balanced”), they are still the most commonly used in industry today. Newer 3rd generation rheologically designed runners will be presented in Chapter 7.

John P. Beaumont

7. Managing Shear-Induced Melt Variations for Successful Molding

Abstract

Chapter 6 revealed what were once simply regarded as mysterious anomalies that occurred during injection molding. These anomalies are now known to be the result of shear-induced melt variations, which have been hidden in nearly every mold since the beginning of the modern plastic industry. Chapter 6 debunks the concept of the “naturally balanced” runner. Chapter 7 reveals the solution. The gremlin that negates the ability of a simple geometrically balanced runner to be “naturally balanced” is revealed to be non-symmetrical melt conditions developed as the melt travels through a mold.

John P. Beaumont

8. Cold Runner Molds

Abstract

For thermoplastic materials, a cold runner mold refers to a mold in which the runner is cooled, solidified, and ejected with the molded part(s) during each molding cycle. Approximately 70% of molds today are cold runner molds.

John P. Beaumont

9. Hot Runner Molds

Abstract

A significant percentage of molds built today utilize hot runners. Though hot runners have many advantages over cold runners, they also create many challenges and are not the best fit for many applications. This chapter will point out both the advantages and the disadvantages that the molder should be aware of.

John P. Beaumont

10. Hot Runner Flow Channel Design

Abstract

The flow channel design of a hot runner is much more critical than that of a cold runner. A hot runner channel needs to be free flowing and ideally does not have any locations where the polymer will not be freely flushed with each shot. Additionally, any problems with the runner that might require relocation, resizing, or reshaping are relatively simple and inexpensive with a cold runner. Similar actions in a hot runner are much more complex and expensive. Modification in runner layout would require the replacement of the entire runner system. In addition, because the cold runner is ejected after every cycle, it eliminates any concerns of stagnant flow that must be addressed in hot runners.

John P. Beaumont

11. Hot Runner Drops, Nozzles, and Gates

Abstract

The hot drop delivers the melt to a gate, which is the hot runner’s interface to the part-forming cavity. The critical challenge at the gate is to allow for the plastic part to freeze on its outlet side and provide for the plastic material to remain molten on its inlet side. The distance between inlet and outlet side can be less than a millimeter.

John P. Beaumont

12. Thermal Issues of Hot Runner Systems

Abstract

Other than insulated hot runner systems, hot runners require their own heating systems. This includes heaters, means to distribute the heat, heater controllers, and thermocouples to feed temperature information from the heated system to the temperature controllers.

John P. Beaumont

13. The Mechanics and Operation of Hot Runners

Abstract

Hot runner assemblies range from relatively simple insulated systems to the more sophisticated externally heated systems. The focus of this chapter is on the dominating externally heated designs. Satisfactory operation of these systems requires precise engineering design, machining, assembly, and proper operating procedure. One of the most critical issues concerning assembly is the potential of system leakage. Leakage of a hot runner system can lead to catastrophic failure that can put a mold out of operation for weeks.

John P. Beaumont

14. Process of Designing and Selecting a Runner System (Gate and Runner) – A Summary

Abstract

Following is a summary of the key factors that should be considered when designing a runner system. Most of these issues have been discussed in more detail throughout this text.

John P. Beaumont

15. Troubleshooting

Abstract

This chapter provides three different approaches to troubleshooting injection molding and includes contributions from John Bozzelli and Dave Hoffman. Section 15.1 presents a flow grouping mold diagnostics method developed for troubleshooting new and existing molds. This method is particularly useful in diagnosing cavity-to-variations in multi-cavity molds.

John P. Beaumont

Backmatter