Tension Control and Coating Machine Process Efficiency

The performance of a coating process depends upon a number of process fundamentals. Besides the thickness of the coated film, cosmetic quality is also of importance, and each coating practitioner works to a set standard of tolerance for frequently encountered defects.

Commonly encountered defects include pinholes, where localized regions of the substrate remain uncoated. This is usually due to non-uniformity, but may also be caused by other factors such as bubbles in the coating liquid due to air entrapment. Still other defects, which can arise, are streaks and drips, which can be caused through irregularities on the coating film, and ribbing. The latter refers to the transverse variation of film thickness, giving rise to a regular striated pattern on the coated film.

The control and prevention of coating defects comes down to a number of factors with tension control being particularly important. Accurate web tension control is necessary to maintain close coating weight tolerance and to ensure freedom from cross bars, streaks, bubbles and other coating defects. Tighter tolerances and a trend towards thinner coatings, which often have to be, applied on ever-thinner substrates further underlines the need for precise tension control and web handling. Without precise closed loop tension control optimum coating machine speed will be limited, as will quality, thus resulting in high levels of waste.

Given that coating applications are becoming ever-more complex and precise, for example if a customer wants the converter to put down a coating a few thousandths thick, not once but time after time, then tension-sensing load cells (transducers) provide distinct advantages.

In situations that require ultra-precise methods of measurement and control of the web, tension-sensing load cells (transducers) are frequently recommended as part of a tension control system. These ultra-compact devices designed and developed by Montalvo provide repeatable and continuous measurement of ‘actual’ tension; the information relayed to a controller such as the X-3400ce tension controller or DTI-3400ce controller for evaluation and adjustment according to set point.

The DTI-3400ce dancer controller with tension indication was developed to provide a higher standard of tension control by simultaneously processing signals from a dancer roller and a pair of load cells (transducers). Designed for 21st century productivity requirements the DTI-3400ce combines the storage and shock capabilities of a dancer with the accurate, repeatable results of a tension-sensing load cell (transducer) system. Control is provided by ‘Progressive PID circuitry’ which regulates the output to the unwind brake and maintains the position of the dancer arm. This controller offers inertia compensation, sample, hold and ratio splice function, web break detection, and, when combined with Montalvo’s P-3000RE, a cycling range expander with 2, 3 or 4 valves for optimum control of a Montalvo pneumatic friction brake.

The multi-functional nature of this controller, dancer and load cell (transducer) combined, makes this an attractive controller to consider.

Montalvo load cells (transducers) as mentioned earlier can be used with the X-3400ce tension controller. This most sophisticated Montalvo controller also incorporates progressive PID circuitry and can be used on unwind or rewind positions. As an intermediate zone controller, the regulator also receives tachometer input.

Differing technologies, all of which must function as a uniform whole, make up the modern production coater. If tension is improperly controlled it will jeopardize the whole operation. Due in large measure to advances in areas such as electronics and communicative protocols, machine builder and component producers are more able to link the processes into a unified entity. Montalvo’s control modules incorporate the latest industry standard on-board communication protocols, i.e. RS-232, Profibus, DeviceNET and CANopen for optimum machine control management.

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