Determine Feed System Layout

Section 4.3.1 provided some common layouts for mold cavities. The feed system must be designed to provide the needed amount of melt flow at the proper melt pressures to each of the cavities. For this reason, a number of feed system layouts have become common including series, branching, radial, hybrid, and custom. Each of these types of feed system layouts is next discussed.

A series layout of cavities can most compactly deliver the polymer melt to many in-line cavities through a single primary runner with many subsequent runners leading to individual cavities. Such a scenario is shown in Figure 6.13. Unfortunately, since the secondary runners branch off at different locations down the length of the primary runner, the pressure drop along the length of the primary runner will cause lower flow rates to be delivered to cavities

Table 6.2: Feed system types and properties

Feed system type Upfront

Two-plate cold runner Lowest Lowest Low Lowest

Three-plate cold runner Low Low Low Low

Insulated runner Moderate Moderate Moderate Moderate

Hot runner High Moderate High High

Stack mold Highest High High Highest

135

further from the sprue. This non-uniform flow can be abated somewhat by reducing the diameters of the secondary runners closer to the sprue as shown by the secondaries off the right primary runner in Figure 6.13. However, such artificial balancing can be difficult to achieve, and does not guarantee consistent part quality associated with different dynamics during the post-filling stages of the injection molding process. For these reasons, the series layout of runner systems is not frequently used in precision applications.

By branching the feed system multiple times, the melt flow to multiple cavities can be naturally balanced as shown in Figure 6.14. Compared to the series layout, the branched layout con-sumes significantly more material while also imposing a high pressure drop from the sprue to the cavities. Another problem with naturally balanced feed systems is the development of

Figure 6.13: Series layout of runner system

Figure 6.14: Branched layout of runner system

6.4 Feed System Analysis

136 6 Feed System Design

melt temperature imbalances associated with the turning of the melt across multiple branches.

This effect has been well document [27, 28], and led to the development of a “Melt Flipper^TM” to assist in correcting flow imbalances in naturally balanced systems with multiple branches.

For all these reasons, molding applications with a high number cavities are increasing utilizing hot runner feed systems to avoid excess material utilization and pressure drops.

Radial layouts of feed systems, in which multiple primary runners emanate from the sprue, are also quite common. The primary benefit of a radial feed system layout is that the flow rates and melt pressures are naturally balanced with only a moderate amount of runner volume.

The number of primary runners that can emanate from the single sprue is somewhat limited due to the large size of the primary runners compared to the base of the sprue. To increase the number of primary runners, a disk cavity, or “diaphragm”, may be located at the base of the sprue. This diaphragm can be used to feed many primary runners as shown in Figure 6.15.

Compared to the branched layout of Figure 6.14, this radial layout has a lower feed system volume and provides more balanced flow. However, longer primary runners and more waste is necessary as the size of the cavities increases.

Mold designers are free to develop the feed system layout to best fit their molding application.

As previously discussed, the primary motivation is to provide balanced flow and minimal pressure drops while consuming the least amount of material. As such, many feed systems utilize a hybrid of branched and radial layouts. One such design is shown in Figure 6.16, which consists of a branched feed system with primary and secondary runners which then feeds four separate radial feed systems, each with four tertiary runners. Compared to the feed system layouts shown in Figure 6.14 and Figure 6.15, the hybrid layout of the feed system design utilizes less material while also providing naturally balanced flow.

Many molding application requirements are best fulfilled by custom feed systems that do not comply with any of the previous feed system layouts. For example, many multi-gated parts require the feed system to deliver melt to different locations across the mold cavity. In such molding applications, there is no reason to adhere to either branched or radial or even naturally balanced layouts. Indeed, the mold designer should purposefully choose a feed

Figure 6.15: Radial layout of runner system

137

system layout and specify dimensions that deliver the desired amount of polymer melt at the desired melt pressures to different portions of the mold cavities.

One example of a custom layout is shown in Figure 6.17. This feed system consists of four primary runners. The two longer primary runners feed the polymer melt via four secondary runners to a relatively large part surrounding the feed system. The two small primary runners closer to the sprue are used to feed smaller mold cavities that provide optional components for assembly with the large molding. These secondary runners may be fitted with rotating shut-offs that can be installed in the mold to change the connectivity of the feed system, and thereby produce different combinations of moldings while the mold is in the molding machine.

Figure 6.16: Hybrid (branched-radial) layout of runner system

Figure 6.17: Custom layout of runner system

6.4 Feed System Analysis

138 6 Feed System Design

The performance of the feed system is ultimately determined by the creativity and care of the mold designer according to the requirements of the molding application. The mold designer has significant freedom in the design of the feed system. However, some general guidelines are as follows:

The total length of the feed system should be as short as possible to minimize material consumption;

The total length of the feed system should be as short as possible to minimize pressure drop through the feed system;

Naturally balanced feed systems provide greater cavity to cavity consistency with respect to melt flow, melt pressure, and molded part quality than artificially balanced designs;

The total number of branches in a feed system should be minimized to avoid excessive runner volume and potential melt temperature imbalances;

To minimize pressure drop for a given feed system volume, the diameters of the feed system are generally largest with the sprue and subsequently become smaller with the primary, secondary, and other runners with decreasing flow rates;

Economic analysis is vital to determine the correct number of mold cavities, the layout of the mold cavities, and the type of feed system; and

Hot runner and three-plate molds should be considered when cavities in a two-plate mold obstruct the desired layout of the feed system.