Static Controls Take Charge
Although it can't be eliminated, static charge build-up can be monitored, neutralized and otherwise controlled.
By Stanley Weitz, President, Electro-Tech Systems Inc.
Each of the vast number of substrates used in today's package printing and converting applications possesses the ability to produce static electricity when brought in contact with the very equipment used to convert it, making it extremely difficult to support increased printing speeds without sacrificing yields.
Cause and effect
Static electricity is caused by unbalanced molecules. Generally, static electricity occurs at a point where two surfaces touch each other or are separated from one another, throwing molecules out of their natural balance. Additionally, large amounts of energy cause electrons to flow, while heat causes the surface transfer of electrons. Insulating materials compound the problem by giving off electrons easily. This is significant since most papers and films are insulators. Therefore, they cannot replenish or compensate for lost or gained electrons. The resulting charge becomes static, causing static electricity.
Two insulators will stick together, causing jams in printing and converting equipment. The static field can also stick tape systems and attract unwanted debris. The attraction of foreign materials, such as lint and dust, can cause friction and binding. This results in poor product quality, rejects, machine downtime and wear.
Built-in static generators
Package printing operations are plagued by static charge build-up from the very start. Static electricity is immediately generated as the web is pulled into the corona treater by idler, infeed nip and pacing rolls.
The print module itself produces numerous contact points, however, the wetting action of the ink tends to hold down any static build-up. As the web passes through tensioners in back-end operations, such as rewinding, slitting or die-cut and stacking operations, it begins to accumulate a static charge.
If more than one web is being run, extreme electrostatic polarization will take place. Now the multi-web setup acts as a single web bonded together by static electricity. Applying static neutralizers serves only to neutralize the outside surfaces. Therefore, static between the webs remains throughout the entire operation.
Static control technologies
Static charge build-up can be monitored, neutralized and otherwise controlled. It cannot be eliminated.
Static-neutralizing equipment can be rendered useless by a number of factors, including varying environmental conditions; different material characteristics, even among the same materials produced by the same manufacturer at different plants; and the fact that when a static-laden material comes into contact with a grounded surface the electrostatic field collapses, preventing ion attraction.
A number of approaches and technologies have been developed to combat the costly effects of electrostatic build up on moving webs.
Humidity Control
The substrate absorbs moisture and becomes dissipative to bleed off static electricity. Humidity levels should never exceed 60 percent. Humidity control is generally used in the converting of some textiles and plastics. It cannot be used for applications where moisture content is a concern, such as paper processing.
The positive aspects of humidity control include its low cost and simple set-up and operation. Conversely, adding humidity to the work environment can adversely affect material characteristics, machinery and the process. For example, increased humidity can lead to premature rusting of converting equipment, while also extending cure rates.
Chemical Treatments
Three types of chemical treatments typically used include applying a permanent coating to the end-product, a temporary application of a spray or mist, and the incorporation of an anti-static agent as part of the end-product's composition.
Chemical treatments can be applied in one place and remain effective throughout production. They all contribute to reduced dust collection and prevent products from clinging together in post-converting and stacking operations. Permanent treatments and anti-static additives can even add to the value of end-products, particularly where the attraction of dust and dirt can detract from the appearance of the package.
On the downside, chemical treatments can be rather expensive and require constant replenishment. Intensifying environmental regulations can impact on their use and lead to the need for reformulation. Additionally, they can affect the performance characteristics of the end-product. Chemical treatments cannot be used on products intended for use as insulators, as they nullify insulation properties.
Ionization
Ionization works by throwing negative or positive ions out into the atmosphere. These ions combine with free electrons to neutralize static. AC ionization bars are the least expensive and generally are adequate for 90 percent of all applications. DC ionization bars are more expensive, but are capable of handling higher speeds. For DC to be effective, the charge must be at a constant level.
Ionization is perhaps the most popular technology used. Ionization bars are quite effective when points of contact and separation are readily identifiable. The difficulty is dealing with the variables that affect static electricity, including variations in the raw substrates being processed. If unmonitored, DC ionization bars can send off too many ions, actually creating a build-up of electrostatic charges on the material. Additionally, dust and dirt can clog ionization bars, impairing operation and even causing failure.
Static Level Monitoring. Static level monitoring and control systems enhance current technologies by addressing situations where charge buildup cannot be eliminated or effectively controlled, or to just monitor the effectiveness of the control system being used. These devices warn of high static-charge levels before a manufacturing process is disrupted or damage occurs to the end product. Figure 1 illustrates the location of static monitoring in rewind operations.
Static-level monitoring systems are usually designed for permanent installation to monitor the charge buildup at a critical site. They monitor on a continuous, real-time basis without the need for an operator or lab technician. Usually, they are installed after the ionization, but under some circumstances locating monitors before and after ionization may be desirable.
The simplest systems contain an analog or digital readout to provide the operator with a continuous indication of the monitored static level. A recorder output may be provided for a permanent record of the static level. Alternately, the system may contain a built-in recorder. Digital data output is accomplished with a suitable data converter.
Automatic fault level detection and indication is one of the most important functions of the static-level monitoring system, particularly in applications where excessive static levels can cause flammable material to ignite or explode.
The static level is continuously and automatically monitored and compared to a preset fault or alarm level. If the static level is exceeded, a fault is declared and stored in an alarm latch. The alarm latch stores the fault event until it is manually reset.
In summary, proper application of currently available technologies for static control, measurement and monitoring assures that virtually any static problem can be effectively controlled.
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- Stanley Weitz