VALVE GATE SEQUENCING USING CAVITY PRESSURE

Art Schubert, RJG, Inc.

Injection molders commonly use valve gates to reduce cycle times, control gate vestiges and limit gate discharge. When in-cavity pressure sensing and DECOUPLED MOLDING^SM are combined with valve gate control, more robust processes are developed, even for processes that would otherwise be impossible.

Shortcomings in Non-Gated Hot Runners

Variation: Injection molds with hot runner systems eliminate runners and the need to dispose of, or reprocess them. But without valve gate sequencing they create a new kind of variation not found in cold runner molds. For example, cavity 1 may fill first on one shot, then cavity 4 fills first on the next shot, then cavity 8 and so on.
At the heart of this variation is the non-Newtonian flow characteristic of polymers known as “multi-process disease”. At the start of filling flow can begin at any one tip. Once flow starts, the shear thinning reduces resistance to flow. Thus the flow favors one path until the dynamic pressure is high enough to force flow in the other paths.

Imbalance: Hot runner molds commonly have consistent cavity imbalance as well as shot-to-shot variation: one set of cavities fills regularly before another set. Molders often attempt to thermally balance the molds, adjusting zone temperatures until all cavities fill simultaneously.

The following examples show some uses of valve gates and cavity pressure sensing to reduce variation and improve balance in hot runner and family molds. These processes are running today using eDART TM based valve gate sequence control from RJG, Inc. or Incoe Corporation

Each example is accompanied by a figure that describes the process. The top portion of each figure plots volume and cavity or injection pressure curves over the filling and packing portion of the cycle.
The traces at the lower edge of the graph show the opening of the valve gates: high is open, low closed. The images at the bottom of each graph represent a view inside the cavity as each event occurs.

Independent Cavity Sequencing

The term “Independent” implies that each cavity has its own controlled valve gate (or gates) and, for best results, cavity pressure sensors. The sensors are commonly located in areas of influence closest to sensitive features on the part. The controller closes the gates at a set pressure at a selected sensor. Figure 1

Open at Volume Levels for Balanced Fill
Use this technique to balance the flow in family molds and as an alternative to thermal balancing of imbalanced hot runner molds. All cavities should finish filling volumetrically near the same time. The valve gates open in sequence with the smaller volume cavities opening last.

We use volumetric screw position (cubic cm) instead of stroke (cm) so that the tool can be transferred to presses with different barrels and keep the same control settings. The bottom portion of figure 1 illustrates how each cavity in a three cavity family tool begins filling at different volumes. Cavity #C (“center”) is about three times the volume of cavities #LS and #RS (left and right sides).

At 0.0 cc cavity #C opens (1) fills partially. Then cavity #RS opens at 16 cc (2) followed by #LS at 19 cc (3). Thus all cavities fill at nearly the same time at (4).

The sudden change in the slope of the volume curve is the change in speed. We set the machine to slow down (at a position) before the cavities are completely filled. This is the fill portion of a DECOUPLED MOLDING^SM process.

We have found that using volume to open the valves has been reasonably robust when viscosity changes. Balancing using volume also works in non-family tools.

Close at Cavity Sensor Levels for Pressure Control
Even though multiple cavities may have balanced filling (i.e. fill at the same time), they may not pack to the same pressure. Using valve gates to close each cavity at a specified cavity pressure can correct this, ensuring consistent packing of all cavities. Figure 1 also illustrates the pack phase of the process. Here, cavities #LS and RS reach their cavity pressure set points first so the controller closes them (5). Cavity #C then reaches its set pressure last and the controller closes it (6) and transfers the machine.

This method requires a constant, low velocity pack stage. This controls the peak cavity pressure, pack rates and static pressure loss more reliably than using fast speeds. In turn this ensures more consistent dimensions.

Pin Pack: Figure 1 shows a pronounced example of the pressure rising suddenly just after the gate for each cavity closes. As the gate pin moves forward it pushes more material into the cavity near the gate. This pin packing phenomenon is common to all valve gate applications.

Alternate Cavity Sequencing

The Alternate Cavity method provides more independent control of the filling of dissimilar cavities and may help to reduce clamp tonnage requirements. Here the controller opens one cavity (or set of cavities), fills and packs, closes the first gate (or gates) and opens another set for a second fill and pack. See Figure 2

Alternate Cavity Process Steps
Figure 2 illustrates a family mold with two similar cavities (#1 and #3) and one that requires much more material (#2). In this example, the two similar cavities fill simultaneously, followed by the larger cavity filling alone. The velocity profile on the press provides a fast fill and slow pack stage for each set of cavities. The gate sequence control closes each gate at set cavity pressure levels. In Figure 2 cavities #1 and #3 open at specific volumes (steps 1 and 2) in order to correct a slight imbalance between them. They fill quickly to a fixed position on the press whereupon the press slows to a controlled velocity pack. At step (3) cavity #1 reaches its closure level followed by cavity #3 at step (4). Once both cavity #1 and #3 close cavity #2 opens. Since #3 closes later it becomes the trigger that opens gate #2 (step 4).

The machine profile is set to speed up at a position for the filling of cavity #2. This must be set late enough to ensure that both cavities #1 and #3 are closed under low velocity before initiating high velocity fill for cavity #2. Being controlled by cavity pressure, the closure time of gates #1 and #3 can move right or left .

After a short time at the slow speed (“gap”) the machine speeds up to fill cavity #2 (5). At the next profile position the machine slows down to pack cavity #2 until it reaches its set pressure. Then the controller closes gate #2.

Below are some important observations:
Extra Large Cushion on First Cavities: When cavities #1 and #3 have completed packing and their gates close there is still something more than 1/3 of the entire shot volume ahead of the screw. Think of this as a very large cushion. In some molds this excess cushion can reduce the ability to control pack flow in the early cavities because of the large volume of compressible melt ahead of the screw.

Differences in Cooling Time: Another fact that can be observed from the graph is that cavities #1 and #3 get much more time to cool than does cavity #2. This may or may not be significant to the parts in other molds.

Importance of Backup Control on Volume: It is imperative that the controller be set to close the gates on the first cavities (and thereby open the gate on the remaining cavities) at a shot volume that matches the speed-up position on the machine. This backup protects the first cavities from being over packed on high speed fill.

Clamp Force Balancing: Depending on the mold configuration the filling and packing of one side of the mold before another can cause the mold to “rock” open as pressure builds in the first cavities. Whenever possible, fill the cavities to balance opening forces on the mold. In our experience we have seen less mold deflection than we had expected.

Sequential Gate Control

Use this process for long parts where the aspect ratio is high and the part cannot fill using one gate alone. With multiple gates in a single cavity opening all gates at the same time will inevitably produce knit (or “weld”) lines as the flow fronts from each cavity contact each other.

Figure 3 shows how Sequential Gate Control of a long part can prevent knit lines and provide better packing control across the entire part. Filling begins at the central gate (#1) only. As the flow front passes the sensor at each gate downstream, the sensor detects the presence of the flow front, causing that controller to open the gate.

Practical Considerations and Details

Planning and Design
Sensor placement: Proper sensor placement is crucial. It depends on part geometry and whether the sensor will be used exclusively for control or whether it will also be used to monitor for part defects.

Choice of control sensor: In valve gated tools any sensor can be used for control because the shutoff is at the cavity.

Ensure sufficient valve force: The pressure in the valve cylinders must be sufficient to close the pin against pressure in the cavity.

Ensure enough flow at pressure: The valve gate must close rapidly in order to prevent overshoot of cavity pressure set points.

Use fast-acting control valves: Slow solenoid valves can cause delay in the opening and closing of gates.

Use an independent power source: Whenever possible use an auxiliary power source to drive the gates.

Ensure exhaust flow: Commonly overlooked is the need to exhaust the air from the back side of the valve piston as it is being moved by pressure on the other side.

Consider thermal design to prevent sticking: One common cause of sticking gates is over-heating of the valve pins, especially during down times. Ensure that tolerances take thermal expansion into account.
Setup and Operation

Start-up testing: After warm-up time, always run a test of the control by checking that the correct gates open and close on the correct signals and that those gates are in the cavities that you expect.

Control with time discouraged: Many early controllers used time to open and close the gates, this is very inconsistent and should be avoided.

Cooling sink effect: It may be necessary to pack to higher pressures than normal to ensure proper packing and remove sink. Figure 4 illustrates this phenomenon. The dashed line is a typical pressure curve with hold pressure while the solid line is the pressure profile with the gate closed.

Summary

These new methods require a focus on details not present with single cavity or simple hot runner molds. It is perilous to ignore these details. Yet if they receive proper attention the results can be dramatic, creating new competitive advantages for technical molders.

Figure 1: Independent Cavity Sequencing of Family Tool with Fill Balanced

Figure 2: Alternate Cavity Sequencing of Family Tool with Fill Balanced

Figure 3: Sequential Gating in Long Part with Pressure Sensors

Figure 4: Comparison of Pressure Decrease During Cooling in Valve Gated and Cold Runner Molds