6 Feed System Design
6.2 Objectives in Feed System Design
6.2.1 Conveying the Polymer Melt from Machine to Cavities
The primary function of the feed system is to convey the polymer melt from the nozzle of the molding machine (where it is plasticized) to the mold cavities (where it will form a desired product). In most molding applications, the polymer melt must traverse portions of both the mold height and the mold width. The traversal of the height and width can be accomplished by two different layouts designs for the feed systems as shown in Figure 6.1. The feed system layout shown at left corresponds to a two-plate mold design. The sprue is used to guide the polymer melt from the nozzle of the molding machine to the parting plane. Runners in the parting plane are then used to guide the polymer melt across the parting plane to one or more mold cavities.
The second layout design, shown at right of Figure 6.1, corresponds to a three-plate or hot runner mold. In this second design, the polymer melt is guided across the width and length dimensions of the mold by runners that are offset to the parting plane. Since the runners are offset from the parting plane, there is significant design freedom with respect to their routing and gating location. However, two sets of sprues are needed for the polymer melt to traverse
1 These three types of feed systems are the most common, though a few other feed system technologies are discussed in Section 13.6.
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the height of the mold. First, a sprue is needed to guide the polymer melt from the nozzle of the molding machine to the plane of the lateral runners. After the melt flows across the runners, a second set of sprues is needed to guide the melt down through a portion of the mold height to the mold cavities.
6.2.2 Impose Minimal Pressure Drop
As the melt propagates through the feed system and cavities, the melt pressure in the injection molding machine will increase. The feed system must be designed so that there is sufficient melt pressure to drive the polymer melt throughout the mold cavities. As shown in Figure 6.2, a feed system with a large flow resistance will incur a substantial pressure drop during the molding process. The flow rate of the polymer melt will begin to decay when the molding machine reaches the maximum allowable injection pressure. If the flow rate decreases substantially before the end of the mold filling process, then a short shot or other defects are likely to occur.
The feed system must be designed to incur an acceptable pressure drop to avoid short shots, extended cycle times, and other defects. The “acceptable” pressure drop through the feed system will depend on the specifics of the molding application, especially the melt pressure required to fill the cavity compared to the melt pressure available from the molding machine.
Down
Across Across
Down
Across
Across Down
Down
Figure 6.1: Two feed system layouts for melt conveyance
Time
Pressure
Time
Flowrate
Pmax
Psprue & runners
Pgate
Pcavity
Figure 6.2: Pressure and flow rate coupling
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For example, a thin wall molding application may use a molding machine with 200 MPa of available melt pressure. If 150 MPa is required to fill the cavity, then the pressure drop through the feed system should not exceed 50 MPa. However, if the same machine was used to mold a part requiring only 100 MPa of pressure, then the feed system could be designed to impose a pressure drop of 100 MPa.
To accurately specify the acceptable pressure drop for the feed system design, the mold designer should contact the molder to obtain the molding machine’s maximum injection pressure. The mold designer should also obtain an estimate of the melt pressure required to fill the cavity through analysis, simulation, prototype molding, or prior experience. If this information is not known, then the mold designer can assume a maximum pressure drop through the feed system of 50 MPa (7,200 psi). While this pressure drop is slightly higher than some industry practices, this specification will result in a steel-safe design with smaller feed system diameters and lower material utilization.
6.2.3 Consume Minimal Material
To achieve the best feed system design, the mold designer should specify the diameters of the feed system to jointly minimize the pressure drop and the feed system volume. These design constraints are represented in Figure 6.3. As the diameters of the various segments of the feed system increase, the pressure drop decreases below the specified maximum. However, increasing the diameters of the feed system also results in an increase in the volume of the feed system, which can be undesirable for both cold and hot runner feed systems.
In cold runner designs, the large size of the feed system can result in extended cycle times as well as excessive waste associated with the molding of the feed system. Some molding applications allow the use of regrind mixed with virgin material. A typical limit on regrind may be 30%, which translates directly to a specification on the maximum volume of the feed system. For example, if a molding application had two cavities totaling 50 cc, then a 30%
regrind specification would limit the volume of the feed system to 15 cc.
In hot runner designs, large feed systems reduce the turn-over of the material in the hot runner.
Low turn-over is undesirable for two reasons. First, long residence times of the polymer melt in the hot runner can cause material degradation which frequently causes black spots and reduced properties of the molded product. Second, large volumes of material in the hot runner
Diameter
Figure 6.3: Coupling between volume and pressure drop
6.2 Objectives in Feed System Design
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system can impede color changes during molding, not only due to the large volume of the plastic melt that needs to be flushed, but also due to the low associated shear stresses along the walls of the feed system. Low shear stresses during purging allow the material to stick to the walls of the hot runner, reducing the removal of old material during color changes.
The maximum volume of polymer melt in a hot runner feed system can be difficult to specify since it is related to the type of material being molded, the need to perform color changes, and the desired pressure drop. Hot runners are being increasingly designed with smaller diameters, such that the material turns over every molded cycle. For example, if a molding application had two cavities totaling 50 cc, then a turn over of the melt with every molding cycle would specify the volume of the feed system to be 50 cc. If a very low pressure drop is desired, then the volume of the feed system may be specified as 100 cc or even 200 cc if degradation and color change issues are not expected. It should be noted, however that unlike a steel-safe designed cold run-ner system, high costs may be incurred to reduce the diameters of a hot runrun-ner system.
6.2.4 Control Flow Rates
Since the primary function of the feed system is to convey the melt from the molding machine to the mold cavities, it is desirable for the feed system to control the amount of polymer melt to each mold cavity. The two most common applications pertain to multi-cavity and multi-gated molds.
In a multi-cavity mold, as shown in Figure 6.1, the molding application may require differ-ent pressure drops in each leg of the feed system to cause the differdiffer-ent mold cavities to fill at the same time. In this example, if the cup required a higher pressure to fill than the lid, then the mold designer could provide a lower pressure drop in the portion of the feed system leading to the cavity for the cup. Such a mold design is known as “artificially balanced”.
In a multi-gated mold, a common objective in the feed system design is to control the polymer melt flowing through the feed system to alter the melt front advancement in a multi-gated mold. For instance, it may be desirable to drive more material through one gate to move a knit-line to a different location. Other common uses include the altering of the mold filling to eliminate a gas trap or avoid over-filling a portion of the mold cavity.
Using different diameters in the feed system can control the flow of the polymer melt, but there are limits as what can be achieved. First, the pressure drop through each leg of the feed system is dependent on the viscosity of the polymer melt. As such, an artificially balanced feed system may not balance the mold filling for different materials and processing conditions.
Second, differently sized feed systems will solidify at different rates and thereby provide different dynamics during the packing stage of the molding process; runner segments with smaller diameters will tend to freeze quickly and reduce the amount of packing to downstream cavities. For these reasons, the mold designer should strive to utilize mold cavities that have similar filling requirements. If family molds or other needs dictate very different flow rates through each gate, then the mold designer may wish to utilize a melt control technology such as Dynamic FeedTMas discussed in Section 13.6.4.
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