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Chapter

14

C-MOLD Runner Balancing:

Introduction

Overview

C-MOLD Runner Balancing is a tool designed to automatically balance the runner systems of multi-cavity molds. It balances runner systems by re-sizing some runner diameters, without changing the layout of the runners or the processing conditions.

C-MOLD Runner Balancing uses a balancing technique that is derived from optimization theories; it is robust and efficient. In the optimization strategy, the objective function is the summation of the filling time differences from all the cavities. To minimize the objective function, the equivalent diameters of related runners are chosen to be re-sized within their constraints.

C-MOLD Integration

C-MOLD Runner Balancing is an optional software module which supplements C-MOLD Filling, using an injection molding or reactive molding process. The integration can be summarized as follows. C-MOLD Filling results (or the filling results from C-MOLD Reactive Molding) are passed to C-MOLD Runner Balancing, which performs an analysis and recommends improved sizing of runner diameters (or equivalent diameters) for the system. C-MOLD then automatically performs a new filling analysis, using the re-sized diameters. Next, the balancing analysis executes, using the latest filling results. The runner balancing and filling analyses automatically pass results back and forth through multiple iterations. Upon completion of the process, the recommended runner diameters converge to a balanced runner system design for a given criterion, usually with fewer than ten iterations.

Objectives

Designers and engineers use the results of C-MOLD Runner Balancing to achieve the following objectives:

Features

C-MOLD Runner Balancing incorporates C-MOLD Filling; thus the balancing analysis includes the features of the filling analysis. Additionally, C-MOLD Runner Balancing has the capability to:

Background and Definitions

This section provides background information on runner balancing. Also included are definitions of terms and explanations of assumptions made by C-MOLD Runner Balancing.

Runner

A runner is defined as at least one one-dimensional element that connects two nodes; the nodes are of the following types:

In most cases, the elements within a runner have been assigned the same diameter in the geometric model. However, if the diameters vary, re-sizing will be performed proportionally.

Sprue

In terms of the finite-element mesh, a sprue extends from the polymer entry point to the first junction node. The sprue in a runner system will not be changed by C-MOLD Runner Balancing unless runner diameter limits are specified for it.

Gate Runner Segment

A gate runner segment is defined as the one-dimensional runner element next to the cavity, having a separate diameter from the runner immediately upstream. A gate runner segment will not be changed by C-MOLD Runner Balancing if it is defined separately from the runner next to it.

Tapered Runner

C-MOLD Runner Balancing does not save the angle of a tapered runner. However, a tapered angle will have a minimum increment of one-half degree. The diameter of the starting side is used as the primary variable; the diameter of the ending side will be changed according to the tapered angle and the starting diameter.

It is very important to model tapered runners correctly, as described in the C-MOLD Modeler and Visualizer User's Guide. Keep in mind that tapers work best if they are a single element in length.

Runner System Tree Structure

For C-MOLD Runner Balancing analysis, a runner system must have a tree structure. A tree structure means that there is only one path from the polymer entrance downstream to one gate.

Multi-cavity, Multi-gate, and Mixed Cases

A multi-cavity mold is a mold that contains more than one cavity. The number of gates is equal to the number of cavities. C-MOLD Runner Balancing balances runners only for multi-cavity molds.

A multi-gate cavity means that there are two or more gates into one cavity.

Mixed multi-cavity / multi-gate denotes one mold that contains two or more cavities, and the total number of gates is more than the total number of cavities.

Slug Wells

A runner system has a tree structure in which the entry point is the root, and branches lead away to all the cavities or to portions of one cavity. However, there can be some dead branches in the tree structure. These are the slug wells of a runner system. Careful modeling of the slug wells can produce some special effects. To balance a runner system with a serial layout, it is strongly suggested that slug wells be used in order for C-MOLD Runner Balancing to distinguish between the main runner and the sub-runners, as shown in Figure 14-1.

Figure 14-1 Use of slug wells in a runner system.

A Complete Runner System and Its Simplification

A complete runner system must be supplied for C-MOLD Runner Balancing. A complete runner system, from the modeling point of view, has only one polymer entrance.

A complete runner system does not, however, preclude simplification. In order to reduce computing time, it is strongly recommended that a connector with a multiplicity greater than one be used for symmetrical layout, as described in the C-MOLD Modeler and Visualizer User's Guide.

Constraint on Total Volume of Runner System

One of the important contstraints imposed by C-MOLD Runner Balancing is that the total volume of the complete runner system will not exceed the total volume of the original runner system design. Note that there are cases in which the total volume of the complete runner system myst be increased before balance can be achieved.

Where to Find Further Information

The chapters that follow provide further information on C-MOLD Runner Balancing. They describe the three steps involved in the design cycle and provide examples. The chapter topics are as follows:

Figure 14-2 illustrates the design cycle, using C-MOLD Runner Balancing.

Figure 14-2 C-MOLD Runner Balancing design cycle. This cycle stops when all the design requirements for processability, productivity and part quality are met.



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