Hydraulic Power Pack for Flexo Machine: Unwind Guiding of Thin Films
A hydraulic power pack for flexo machine guiding supplies the oil that moves the unwind stand sideways, so the film enters the first print deck on the same line from the start of the reel to the end. Krishna Engineering Works builds these units in Ahmedabad and fits them to stack type flexo presses that print thin, stretchy packaging films.
Where does the power pack sit on a flexo press?
The power pack stands on the floor beside the unwind, and its cylinder is fixed between the machine base and the sliding unwind stand. The sensor is mounted on the fixed frame, just after the first idler roller that travels with the stand, so it reads the edge of the web as it leaves the reel.
On a stack press the print decks are arranged one above the other on a common frame, and the web climbs through them over a series of short spans. Nothing inside the stack can move the web sideways. Whatever position the film has when it reaches the first impression cylinder is the position every colour is printed in, which is why the guide belongs at the unwind and not further down the press.
Flexography is used on polyethylene and polypropylene films, paper and woven sacks, so the same press may run a stiff paper one week and a soft film the next. The guide has to cope with both, and the soft film is the harder case.
Why are thin extensible films harder to guide?
Thin films are harder to guide because they run at low tension and stretch when they are pulled sideways. A paper web behaves like a stiff strip: move the reel and the web follows at once. A light film behaves more like a rubber band: move the reel sharply and part of the movement is absorbed as stretch and wrinkles before the web settles.
Three things follow for the hydraulic side.
- The correction must be proportional. The servo valve opens a little for a small error and wider for a large one, so the stand creeps for small errors instead of jumping. A guide that moves in steps will crease a light film.
- The stand must move freely. At low tension the web cannot drag a sticky stand into place. Stiff slides cause the stand to stay put, then break free and overshoot, and the film shows it as a ripple along the edge.
- The sensor must not disturb the edge. A thin film edge flutters easily. An air sensor is set towards the soft end of its 5 to 28 mbar range, and the fork is placed close after a roller where the film is supported.
Extensible films also neck in: the web gets slightly narrower as tension rises. A single edge guide holds one edge true and lets the width change show on the other side. If both margins matter, say so in the enquiry.
How does correction speed relate to line speed?
The guide must be able to move the stand faster than the edge wanders, and the edge wanders faster as the press speeds up. A reel fault is fixed in the reel: a step, a wobble or a taper appears once per turn or once over a certain length of film. Double the line speed and the same fault arrives in half the time.
The oil flow of the pump sets the upper limit. With the published cylinder size, the KEW-17 power pack gives a follow-up speed of 30 mm/sec from 3.4 l/min, and the KEW-30 gives 40 mm/sec from 5.8 l/min. The table shows how different reel faults load that speed.
| Reel fault at the unwind | How the edge moves | Demand on correction speed |
| Whole reel loaded off centre | Constant offset | Very low, corrected once after the splice |
| Tapered or dished reel | Slow steady drift over the reel | Low |
| Core running out of true | Side to side once per turn | Rises with press speed, highest near the core |
| Stepped winding from a stop on the extruder | Sudden shift of a few millimetres | High for a moment |
| Wandering edge from a poor slitter | Irregular movement | Moderate to high |
A wobbling core is the case to watch. As the reel gets smaller it turns faster, so the wobble speeds up towards the end of the reel. A guide that copes easily at full diameter may fall behind in the last few minutes, which is where edge waste on a flexo press often comes from.
More flow is not always the answer. On a very soft film a slower, smoother correction prints better than a fast one, so the sensitivity is turned down and the guide is allowed to average out the quick movements. The flow figure tells you what is available, and the adjuster decides how much of it is used.
Choosing the sensing method for flexo films
Choose the sensing method by the film and the ink system. Both units below use the same oil side.
| Point | Hydro-pneumatic unit (air sensor) | Electro-hydro unit (photocell sensor) |
| Clear and tinted films | Read without special settings, the film blocks the air | Light passes through clear film, so the setting is critical |
| Printed or pigmented films | Read by the physical edge | Read well, the film blocks the beam |
| Solvent based ink area | No electrical signal at the sensor head | Sensor and panel must suit the area |
| Very light film | Jet set soft to avoid lifting the edge | No contact and no air at the edge |
| Signal path | Air pipe to the valve | Cable to the electrical panel board |
The hydro-pneumo web aligner is the usual choice on film presses, because most unprinted film on the unwind is clear or lightly tinted. The electro-hydro web aligner uses a photocell sensor for the edge, an AC motor, an electrical panel board to operate the complete unit, a hydraulic cylinder, an adjuster and an oil pipe.
What is supplied for a flexo press?
The hydro-pneumatic web aligner unit is supplied complete with a 0.5 HP motor, air pipe of 5 metres, oil pipe of 5 metres, one sensor, one adjuster, one air filter, one oil filter and one hydraulic cylinder of 6 inch stroke. Each item is explained on the web aligner unit page.
Power pack units are available in different models from 0.25 HP to 3 HP, and the motor, pump, tank and valve are described part by part on the hydraulic power pack page. Film reels on a stack press are usually modest in weight, so the smaller standard model covers most presses when the stand runs freely.
Fitting the guide to a stack flexo unwind
Fit the guide so that the reel, the first idler and the sensor form one short, stiff group. The order below works on most presses.
- Check that the unwind stand slides freely across its full travel with the heaviest reel loaded. Clean and lubricate the slides or wheels first.
- Mount the cylinder at mid stroke with the stand central, so there is equal travel on both sides.
- Fix the first idler roller to the moving stand, not to the press frame. The web must leave the stand already corrected.
- Mount the sensor on the fixed frame within a short distance after that idler, with the film running through the middle of the gap.
- Place the power pack where the oil pipe and air pipe reach without sharp bends, clear of ink trays and wash up splashes.
- Fill with the stated oil, check motor rotation, and stroke the cylinder end to end several times to clear air.
- Thread the film, set running tension, bring the edge to the sensor and switch to automatic.
- Run at low speed, then at full speed, and set the sensitivity so the stand follows without hunting.
The reel brake matters as much as the guide. Low and steady unwind tension keeps the film flat in the sensor, and an air shaft that grips the core evenly stops the reel from creeping sideways on the shaft during a run.
Guiding inside and after the press
Guiding at the unwind is enough for most stack presses, but long machines need more. A press with an inline laminating or coating station, or a long tunnel dryer before the rewind, gives the web room to drift again after the last print deck.
In that case a tracking roller assembly steers the web in mid machine, and a rewind guide moves the rewind stand so the printed reel builds with flat sides. Two or three guides can share one tank: double and triple power units carry two and three servo valves on the same reservoir. The three guiding positions are compared on the web guiding system page.
The guide holds the web on its line. It does not set colour to colour register, which belongs to the print decks, and it does not remove creases already wound into the reel.
Oil and air care beside a flexo press
Keep ink, solvent and wash up water away from the tank and the sensor. A press room is cleaner than a paper mill, but ink mist and dried ink flakes find their way into anything left open.
| Check | Why it matters on a flexo press | When |
| Sensor ports and gap | Dried ink or film dust in the ports weakens the air signal | Every job change |
| Air filter element | Ink mist loads the element and slows the response | Weekly look, clean when dull |
| Oil level and colour | Low or cloudy oil gives a soft, slow cylinder | Weekly |
| Filler cap and breather | Wash up splashes enter through an open cap | After every wash up |
| Slides of the unwind stand | Film guiding depends on a free stand | Monthly, or when the edge ripples |
| Pipe joints | A weeping joint lets air in and oil out | Monthly |
Use the oil types named for the unit: Servo 32, Enklo 44 or Hydro 37. Spares are kept readily available, and units of other makes that have become slow or unsteady can be sent for repairs and servicing.
Related machines and sources
Presses that print the same films by gravure have a different guiding layout, described on the page for the power pack on a rotogravure printing machine. The other machine pages are listed under power packs by machine. Krishna Engineering Works also builds the press itself, shown on the main site under flexo printing machine.
Krishna Engineering Works has the capacity to supply 100 to 150 units per month and invites enquiries from press builders who buy web aligner units in bulk.
What to send for a flexo press guiding quotation
Send the details below, so the unit, sensor and model can be matched to the press.
- Press type and number of colours, and whether it has an inline station or long dryer
- Film or paper types, thickness range and whether the film is clear
- Web width range and maximum press speed
- Heaviest reel, core size and how the unwind stand is carried
- Running tension, if known, or the brake type
- Ink system: solvent based or water based
- Guide positions wanted: unwind only, or unwind and rewind
- Make and model of any existing guide
Send the enquiry through the contact page.
Frequently Asked Questions
Where is the web guide fitted on a stack flexo press?
The web guide is fitted at the unwind. The cylinder shifts the whole unwind stand, and the sensor sits on the fixed frame just after the first idler roller that moves with the stand. This puts the film on line before the first print deck, which matters because nothing inside the stack can correct sideways position.
Can a hydraulic guide handle thin polyethylene film on a flexo machine?
Yes, when the correction is smooth and the stand moves freely. The servo valve gives small, slow movements for small errors, so the film is not jerked sideways. Low unwind tension, clean slides and a softly set sensor are just as important as the power pack, because thin film shows every sudden movement as a crease.
Does a faster flexo press need a bigger power pack?
Not always. A faster press makes reel faults arrive more quickly, so follow-up speed matters more, and follow-up speed comes from pump flow. If the reels are well wound, the smaller model usually keeps up. If the reels wobble or have stepped edges, compare the 30 mm/sec and 40 mm/sec figures of the two standard models.
Why does the film edge wander more near the end of a reel on a flexo press?
The edge wanders more because a small reel turns faster. Any wobble in the core or the winding repeats once per turn, so the same sideways movement happens more often as the diameter falls. The guide has to work hardest in the last part of the reel, and a marginal guide falls behind there first.
Will the guide correct colour register on a flexo machine?
No. The guide controls the sideways position of the whole web, so the print stays at the same distance from the film edge. Register between colours is set at the print decks. A steady web position does make register easier to hold, because the film is not being pulled across the plates while it prints.
Which sensor suits transparent film on a flexo press unwind?
An air sensor suits transparent film well, because it detects the film by the air it blocks and not by the light it stops. Clear, tinted and printed films give the same kind of signal. A photocell sensor can be used where the film is opaque or pigmented, or where an electrical unit is preferred.