Technology meets tradition28 June 2005
Headquartered in Monza, on the northern outskirts of Milan, Angelo Cremona has been supplying complete plywood production lines and veneer slicing lines for a very long time.
While plywood is the oldest sector of the panel industry, tracing its history back to the time of the ancient Egyptians, the machinery used in its production has been the subject of continual development.
Cremona says the need to obtain a quality product, while achieving savings in labour and materials, is increasingly felt by all manufacturers of peeled veneer and plywood.
The material costs, the number of workers assigned to the drying process and the need to select the sheets based on grading features have all increased, taking an evermore important share of the final product selling price.
The most logical solution, then, is an automatic system which dries the veneer without breaking it, has the ability to recognize the dried product’s humidity, density and surface defect parameters, and automatically stacks the sheets in bundles determined by these parameters.
Thus Cremona’s latest automated drying system is structured in four areas: sheet preparation and infeed; drying; sheet analysis; and bundle stacking.
The bundles of moist sheets are positioned on roller ways placed on the ground, and placed close to one another in order to obtain optimum use of the dryer width. The bundles are conveyed on platforms and then positioned under the vacuum feeding device which rises to allow the suction pick-up of the sheets and their introduction to the dryer infeed rollers.
The position of the rubber rollers, the possibility of adjusting the suction strength and other selection devices are designed to ensure that the sheets placed on the roller conveyors never overlap, even when they are very moist or have been stacked for some time before drying.
Cremona says the speed of the feeding device ensures productivity is maintained, even with thin or very small sheets.
The variable speed of the roller ways feeding the upper decks, together with front alignment devices, is designed to allow a good infeed rate suited to various working conditions.
To obtain maximum machine utilisation, Cremona says it has integrated within the loader a fast ejection device for the pallets on which wet veneer bundles are normally supplied.
When the veneers have entered the process, a series of rollers conveys the pallets under the connection roller-ways, where a special chain-driven device pushes them to one side and stacks them, if necessary, ready to be picked up and brought to other lines to be re-used.
The drying process must ensure fast water evaporation, conveying inside the dryer without bumping or excessive pressure, and allow physiological wood shrinkage. Cremona’s patented ET Dryer employs bars to do this. The material to be dried is conveyed between two decks of rods moved by chains at the sides of the dryer and towed by two gear motors at the end of the machine.
The small diameter steel rods, connected by strips of steel plate, allow the heat to reach the surface of the sliced veneer and don’t ‘hit’ the sheets, as roller dryers can, during drying. Cremona says this limits the height of possible curling of a sheet during the water evaporation phase.
This kind of conveying is designed to ensure that all sheets fed to the dryer will be conveyed with care throughout and that possible cracks in the veneer will not be increased. Also, the sheets will not be shattered when they are dry as tends to happen in roller dryers, says the company.
Depending on its length, the dryer is divided into two or three areas whose environmental parameters can be set according to need; the temperature and moisture values are automatically controlled by setting the reference measurements.
A moisture circulation system between the various areas ensures a precise distribution to optimise water movement from the inside to the outside of the wood and to obtain a very elastic, dry, peeled veneer which is not fragile.
The dryer is made of modular panels whose thermal insulation ensures that the outer plates of the machine are just a few degrees above room temperature, minimizing the loss of heat inside the dryer and avoiding energy wastage. The radiant batteries inside the machine, whose pipes have an elliptical section, are designed to ensure low load losses and to maximise thermal exchange performance. Flow conveyors divide the air on various decks to ensure constant drying on each deck and in all positions in which the sheets are conveyed.
In a factory, it is normal to find a container near a roller or net dryer where broken veneers are dumped during the drying operation. Cremona says that, with its roller bar dryer, this container is no longer necessary.
For sheet analysis at the dryer outfeed it is necessary to bring all sheets back to only one conveying deck to start the dimensional and qualitative analysis. A series of angle, belt and roller transmissions aligns the sheets and sends them towards a belt conveyor which passes the first measuring device: the moisture and density analyser.
The reading of the residual water content is performed on the entire sheet surface and the data obtained is stored in the virtual directory assigned to each sheet. The analysis of the various readings will then be processed to automatically adjust the dryer speed up or down.
Sheet density values are also analysed and stored in the virtual file. The reading of the two parameters is performed at a speed which ensures the sheet outfeed operates without interrupting the drying process.
Continuing the path, the sheets are aligned at the side, by a belt conveyor which, unlike rollers, does not jolt the sheet, thus preventing fall-out of dried knots.
The veneer is then analysed by a colour camera, which scans the sheets and reads their defects. This can identify open and closed knots, cracks, colour differences and fake thickness due to blind holes, the presence of bark and any other defect which must be considered in describing the overall quality of the sheet. The data read by the camera is added to the virtual directory of each sheet and stored in the PC memory.
After the camera, a vacuum stacker with different stations places the sheets on top of each other to obtain a homogenous bundle.
Where to convey the sheets is decided by the PC which, depending on the parameters chosen by the operator during production line setting, analyses the virtual directories of each sheet and compares the data with the reference setting. The number of sheets, with quality information and general features of each bundle, is thus known, making it possible to define another value which can identify each bundle in order to help in the stacking or composing operations for pressing.
The data processing produces a bar-code, placed on the bundles to report the specific quality, which can also be printed out.
There is only one conveying speed for the whole line after the dryer; speed differences between the single machines are no longer necessary. A device on the angle transmission ensures the necessary space between the sheets so stacking in the stacker is performed smoothly.
To unload the bundles, a powered double carriage, which runs along the vacuum stacker, takes the bundle from the platform and, by means of a special feeding device, positions the pallets on the empty platform.
Two stacking stations on the ground will house the unloaded bundles and the pile of spare pallets, ensuring stocking of the products and giving enough time for the operator to best manage the ejection operation of the bundles already processed.
The advantages claimed for this system are that it is managed by a single operator from a computerised control station; the only other personnel required are the ones who position the sheet bundles on the infeed rollers and remove the dried bundles.
This, says Cremona, both saves labour and means the material choice is performed with parameters no human can assess, and remains constant through a working shift.
Also, it says, the dryer always works at the maximum speed appropriate to the moisture of the material, to give constant high production values. The adjustment of the dryer optimises energy consumption, allowing considerable savings, while the identification of the sheet quality allows optimal use of the material to obtain constant quality and size.
The mechanical features of the resultant panels will also be ensured by the identification of the distinctive parameters of the processed and graded product, says Cremona.