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September 5, 2002

13 Min Read
Tooling Corner:  Advanced sequencing and protection of valve gate systems

Editor's note: Thomas P. Linehan is manager of engineering at D-M-E Co. in Madison Heights, MI. He is responsible for the development and support of injection molding control products sold by D-M-E, and a registered professional engineer in Michigan.

Tooling_air_pump.jpg

The air-powered hydraulic pump above is used to actuate hydraulic valve gates. This type of pump generates no heat and saves energy by only operating when needed. The accumulator on top provides stored energy to power large valve gate cylinders.

The control of valve gate sequencing used to be limited to timers, but the current and growing trend is to use screw position and even cavity pressure to determine when to open and close the valve gates in a hot runner system. In some cases, a combination of control parameters may be used.

Sequencing systems can also be made to protect valve gate systems from misuse. For example, the accidental application of injection pressure when no valve gates are open can be avoided, as can the exposure of material to high temperatures when mold cooling isn't present.

Time-based Control

For years, valve gate sequencers used panel-mounted timers to program the opening and closing of valve gates. Typically, the start of the sequence was triggered by the injection-forward signal from the press or a signal just preceding it?the high-pressure clamp, for example. Alternatively, a limit switch mounted to the mold or tie bar could sense the closing of the mold and initiate the process.

In the simplest of applications, the valve gates are all opened and closed simultaneously. They are opened at the beginning of injection and closed during pack or hold. This method does not allow for control of balanced fill. However, it does provide better gate vestige than sprue gates. It also allows for earlier implementation of screw rotate since it is not necessary to wait for the gate to freeze. The latter provides for faster cycle times. In this application a single, large air or oil source can operate all valve gates simultaneously.

Tooling_PLC_Maxima.jpg

The D-M-E sequencer cabinet above is shown next to a Milacron Maxima injection molding machine. The photo shows the similarity in technology with the Milacron operator station.

In some cases, the valve gates are all opened at the onset of injection and then closed independently with timers. However, the more typical approach is to open the harder-to-fill cavities at the beginning of injection and the easier-to-fill cavities later during the first stage (fill). Each of the valve gates is then left open for the required time to achieve the desired part weight and size. This is typical of a family mold application, or one that is not adequately balanced by the design of the hot runner system. This requires that each of the cavities has two timers?one that delays the opening of the valve gate and another that controls the time it is open.

Screw Position and Cavity-pressure-based Control

Recent advancements include the use of screw position (shot size) and cavity pressure to control the sequencing of the valve gates. Both of these parameters are considered better than timers because timer settings do not correct for changes in the viscosity of the material and therefore the velocity of injection and fill. Screw position corrects for variation in injection velocity by ensuring that an accurately metered amount of material is injected into a cavity. This assumes that the nonreturn valve ahead of the screw is in good repair and not leaking.

Cavity pressure provides optimum control, if properly implemented. Peak cavity pressure is an accurate indicator of part weight and therefore size. Simply put, if you want to know what is going on in a cavity, look in the cavity, not the countless other upstream input parameters. Virtually all other inputs can be detected by the cavity pressure profile. This is an important consideration when using an injection molding machine that lacks the ability to accurately control the molding process.

Regardless of the approach taken, it is highly recommended that valve gates not be closed until the second stage (pack) or third stage (hold) to make sure that the cavities are properly packed out.

In none of the aforementioned applications is there any feedback from the valve gate control to the molding machine. Additional capability comes from an expanded logic interface between the molding machine and the valve gate control.

Improved Sequencing Control Features

One of the key improvements has been in the flexibility of control. Whereas older systems are built from hardwired timers, the newer ones are built from programmable industrial computers. A good example of this is cascade molding, where multiple valve gates fill a single large part.

In cascade molding, the flow front is guided from one side of the part to the other (left to right for example). This process avoids weldlines otherwise associated with multiple gates in a single part. For example, the first, or leftmost, valve gate opens and as the flow front passes the second valve gate, a pressure sensor to the right of the second valve gate senses the passing flow, and a trigger opens the gate. This same trigger may also close the first (leftmost) valve gate. As the flow front continues to press forward, it passes yet a third valve gate and another pressure sensor triggers the opening of the third valve gate and possibly the closing of the second. Once a sensor to the far right of the part picks up the flow front, the first two valve gates can be reopened to begin symmetrical pack out of the part. The valve gates are then closed after a predetermined time or when the desired peak pressure is detected inside the cavity.

While this kind of operation could be hardwired, it is convenient if it can be reprogrammed for different applications.

The above application has a couple of unique requirements. First, any sensor (analog input) must be able to be assigned to open or close any of the valve gates. This requires that the control software allow for flexibility in configuring the setup.

Second, a feature that allows for reopening a valve gate for pack out requires that a transfer signal be routed from the injection molding machine to the valve gate control. This signal indicates the end of the fill stage and the beginning of packing. Note that if the valve gate controller were part of the molding machine control, this and other interface signals would be inherently available.

Another effective and easy-to-implement setup is to provide cascade control as described previously but with screw position-based control. Upon receiving the high- to low-volume transfer signal from the machine, the valve gates could be opened again for pack out. A benefit of screw position control is that a single sensor can be used to control multiple valve gates.

Only a single analog input is required and it is the only one that needs to be calibrated. However, it is important that the nonreturn valve of the injection unit be in good shape to prevent leakage. Otherwise, screw position does not provide any better control than time.

Table 1: Signals

SIGNAL

SOURCE AND DESTINATION

Signal

Source and Destination

Injection forward

Injection molding machine to valve gate

Injection transfer

Injection molding machine to valve gate

Alarm output

Valve gate to injection molding machine

Inhibit inject

Valve gate to injection molding machine

High oil temperature

Within valve gate

Low oil level

Within valve gate

No air pressure

Within valve gate

No water cooling

Mold to valve gate

Valve gate(s) open

Mold to valve gate

Valve gate(s) closed

Mold to valve gate

Screw position

Sensor to valve gate

Cavity (other) analog sensor

Sensor(s) to valve gate

Additional interface signals between the injection molding machine and mold are required for an advanced valve gate sequencer. This table outlines the signal source and destination.

Improved Protection of the Hot Runner System
Despite numerous improvements in the sealing of hot runner systems (e.g., cold clearance compensation), avoiding potential threats to the hot runner system is a good practice. This is especially true when such threats can be avoided automatically.

With additional signal interface between the molding machine control and valve gate control, new protective measures can be implemented.

For example, it is easy for the industrial computer to determine that no valve gates are open when an injection signal from the molding machine is received. In this case, an "inhibit inject" signal can be sent back to the injection molding machine before significant buildup of pressure within the hot runner system occurs. This can help prevent accidental damage to the hot runner system sealing.

Similarly, the detection of nonfunctioning valve gates is desirable. If a valve gate doesn't open, it doesn't make a part.

Additionally, if a valve gate doesn't open, the material in the related nozzle and manifold passages experiences an extended heat history, leading to degradation and possible crosslinking. If detected immediately, this degradation can be avoided. Additional digital inputs can be used in conjunction with limit switches to detect cycling (or lack thereof) of the valve gate cylinders.

If a valve gate is programmed to be opened, but is not open according to the limit switch sensor, inhibit-inject and alarm signals can be output to the injection molding machine. The valve gate controller can also output an audible signal and display an indication of which of the valve gates is not functioning properly.

Valve gate position can be sensed with one or two limit switches. However, typical system designs, space limitations, and temperature usually dictate that only one switch is used and that it is placed behind the cylinder in the clamp plate.

When the cylinder pulls back, it contacts and closes the limit switch. When forward (closed), the limit switch is open. Also, by having the limit switch in the clamp plate, adequate cooling can be applied to safeguard the switch.

Sensing water temperature can help spare water seals from potential damage. While this could be part of the hot runner temperature control function, it could be part of the valve gate or injection molding machine control as well.

For example, water-cooled gate inserts often have elastomer seals that could be damaged if water cooling isn't present. The preferred embodiment would include the ability to sense the lack of cooling and provide at minimum an alarm and description of the problem. If integrated with the hot runner system, it could also provide shutdown of the hot runner nozzles or the entire hot runner system.

Control software sample screens

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Figure 1. Valve gate settings


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Figure 2. Individual zone setup


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Figure 3. Analog input setup, p. 1


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Figure 4. Analog input setup, p. 2


Additional Signal Interface Required
All of this new functionality and protection requires additional interface signals to the injection molding machine and mold. Table 1 (above) outlines the additional logic required. However, if the valve gate sequencer is part of the injection molding machine control, much of the logic is built into the system.

Sensing valve gate position and the presence of water cooling requires additional connections to the mold.

When the sequencer is not part of the injection molding machine, full implementation of the aforementioned features and protection require the interface presented in Table 1. Implementation requires additional programming and signal inputs in the injection molding machine.

Assuming some may not want to go through the additional time and expense, a means must be provided to shut off undesired features to prevent false alarms. Some of those options are shown in the sidebar, Figure 2.

Older relay-based molding machines can be easily modified to implement the additional signal interface. Newer PLC or industrial computer-based machines require additional inputs and outputs.

However, additional protection may be gained even without an additional injection molding machine interface. For example, the valve gate sequencer can provide an audio and/or visual alarm should no valve gate be open when the injection signal is received.

It should be noted that the added protection also applies to operators and setup personnel. Inhibiting injection and valve gates from opening when safety guards are open can help prevent serious injury. Furthermore, preventing injection when none of the valve gates is open prevents leakage inside the hot runner system and thereby potential leakage outside the hot runner system.

Contact Information

D-M-E Co., Madison Heights, MI
Thomas P. Linehan
(248) 398-6000; www.dme.net

This article was adapted with permission from an article submitted by the author at the 2002 SPE ANTEC.

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