Holding action

Following a three-year project, Sigmala claims to have achieved a “revolutionary design step forward” for an essential element in slitter rewinder design - knifeholders.

In its new SL50 system, the UK based converting and packaging equipment design consultant has replaced conventional pneumatic controls with electrically powered embedded servomotors.

The result is a system said to require no manual set-up or intervention. Servo controls throughout have enabled Sigmala to introduce a new concept - adaptive slippage compensation (ASC) - to monitor knife wear and adjust the side load automatically to prevent slippage between upper and lower knife blades.

The company says this is not possible with pneumatic knifeholders, as they do not incorporate a sensor to detect the rotation of the upper knife.

Servomotors control each of the three major components of the 50 mm width SL50: traverse unit, upper knifeholder and lower knifeholder. The first two are embedded and incorporate individual microprocessor controls. The lower knife unit uses an AC pancake servomotor for the main drive and a Gates Polychain belt to connect the drive to the knife hub.

The SL50 is the first product to be designed and manufactured by Sigmala under its own name. Previously, the company has operated behind-the-scenes and claims to have been responsible for many design engineering innovations for leading packaging and converting machinery manufacturers.

An encoder built in to the traverse unit that positions and guides the knifeholder along the backplate on the slitting machine can pinpoint knife position to 0.025 mm at 50 mm/sec, it is claimed.

The upper knifeholder, with three embedded servomotors, servodrives and microprocessors, controls the up/down, side load and cant or slitting angle. Up/down is stated to be accurate to 0.025 mm, side load to one Newton and the cant angle to 0.1 degree.

The ASC has been designed to run in “threshold” mode, whereby the point at which slippage occurs then increases the side load automatically to a slightly higher value. According to Sigmala, this ensures the machine can run with the smallest possible side load, reducing wear on both knife blades.

In a perfect slitting world, converters would have total control over any given knife set-up, no matter what materials or speeds they were running. In reality, we know it’s simply not that easy. From operator to operator and shift to shift, knife set-ups vary. With time, most knifeholders wear and loosen, and knife axial and radial (dimension) run-outs worsen, resulting in shorter periods between knife replacements. Slit quality lessens, and the operation becomes dustier.

Not often considered, machine vibration inflicts insidious damage to knife blades. Running at higher production speeds, even low vibration levels can cause cyclical impacting of the top knife to bottom knife, resulting in premature metal fatigue.

The latest generation of knifeholders is designed to ensure slitting with better quality and lower expense. Advances in holder construction and knife side load controls can maximize knife life, no matter what the running speed.

One example is the Dienes DF-145 pneumatic shear knifeholder. This has dual, preloaded linear ball bearings surrounded by a rugged 35 lb steel frame. Claimed benefits include no slip fitting, no keyways and no wearing components. Mounted on dual linear rails and a stiff crossbeam, the unit has no loose fitting parts to be worn down by vibration.

Another recent Dienes development is the DF-150 constant side load pneumatic knifeholder.  Knifeholders typically use mechanical return springs to push the top knife away from the bottom knife when the operator is stopping the slitting process. Air pressure shuts off, and the horizontal cylinder collapses under the return spring force. If the horizontal stroke, starting location distance of the top knife varies from set-up to set-up, the amount of top knife to bottom knife contact side load force will also vary.

The DF-150 removes the return spring force factor. It allows only regulated air pressure to set and control how much mechanical force is developed when contacting the top knife to the bottom knife. This new arrangement is said to make the effects of vibration and run-out on slitter performance negligible. With the return spring not impacting side load force, the quicker responding air cushion acts as an efficient damper to prevent the top knife from leaving the bottom knife.

By enabling converters to control the level and consistency of knife to knife contact force, knife life can be maximised, Dienes states. Such ‘constant side load force’ control “can help converters establish the optimum force level for a given material and reduce knife replacement costs significantly”.