Controlled spraying in board manufacture

Misting and overspray were among the problems solved on a particle board production line, increasing product quality and reducing waste by use of the Autojet PWM spray volume control system.

Unilin, a large scale manufacturer of wood-based products, needed to upgrade their spray system on a continuous press line at their plant in Wielsbeke, Belgium.

Among the benefits recorded by Unilin were improvements in product quality and reduction of product waste, while also increasing production speed.

Central to the upgrade was the installation of a state-of-the-art spray system for applying release agent to the conveyor and particle mat.

The new system is based on Pulse Width Modulation, the unique spray technology developed by Spraying Systems Co.

On this project Unilin, part of Mohawk Industries, worked in close collaboration with spray specialists Spraying Systems, and the Belgian development lab of AutoJet Technologies.

To prevent board from sticking to the conveyor or compression belt during production, a thin coating of release agent is applied to the bottom conveyor belt immediately ahead of the first chip strewing position.

The release agent, mixed with water, is also sprayed onto the strewn mat just before it goes under the compression belt.

This spray process is critical.

Too thin a coating could allow the board to stick to the conveyor belt, damaging the product surface finish.

If too much liquid is applied, residual water in the chips will cause the board to burst after the press (Imperfect boards are rejected as scrap).

Moreover, unless the liquid is applied uniformly over the entire surface, heat penetration from the compression belt will be uneven - with the result that the thickness of the manufactured board will vary over the length of the run.

Twelve years before the latest upgrade, Spraying Systems and Unilin had worked together on an earlier project for improving the application of the anti-adherent coating.

On that occasion, a system was designed using spray nozzles that were mounted rather high and used compressed air to atomize the liquid, allowing fine layers of coating to be produced.

The system worked well, though with some limitations.

Transfer efficiency of the spraying system was not very high, causing an odour nuisance and cleaning problems in the production hall.

Also, an opened door could easily disrupt the spray performance and the mechanical nozzles required frequent maintenance.

For the technology available at the time, it was a good solution.

But Unilin knew that a more robust and precise spray system that gave better control and faster production would potentially result in quick payback..

Elimination of compressed air on the application would also reduce on-going costs.

Spraying Systems Belgium knew that their new PWM system could deliver the results Unilin were now seeking.

The PWM system has been specifically developed to apply in a controlled manner a precise and adjustable amount of liquid - and to deliver the liquid at flow rates far lower than would normally be possible without compromising other parameters.

The system works by very rapid on/off cycling of the spray, using an electrically actuated automatic nozzle (the Spraying Systems PulsaJet) that can cycle up to 10,000 times per minute.

Every aspect of the operation is controlled by means of an AutoJet spray controller that can monitor external inputs and is easily programmed to set, maintain or alter flow within a fraction of a second - while keeping all other spray parameters stable.

There is no need, for example, to change liquid pressure.

Flow rate depends on the percentage of time the spray is 'on', which can be linked to a process parameter such as conveyor belt speed.

In this way, the amount of liquid remains equal, per cm2 on the target, regardless of belt speed.

And as liquid pressure stays constant, there are no variations in spray characteristics such as drop size and spray angle.

Consequently the spray pattern is stable while flow rate is extremely flexible.

With detailed knowledge of its process, Unilin was able to specify desired spray profile and flow very precisely.

Coating tests were performed in the Belgian AutoJet Technologies lab, where the choice of nozzle and PWM parameter settings were determined.

This data was implemented on the machine and the control system was integrated into the new machine control.

A fieldbus connection was made between the Rockwell PLC RS Logix and the AutoJet spray controller, allowing PWM flow control through a standardised, user-friendly HMI.

Everything runs automatically.

Having integrated the spraying equipment, and using the new visualisation software, it is now possible to adjust spray flow on the top and bottom layers of the board independently.

An AutoJet 'spray integrity' system was also incorporated for each spray header (three nozzles below and three on top).

Blockages and excessive wear at the nozzle are detected immediately and signalled to the control room, along with the location of the fault.

The solution met the Unilin expectations, installation was quickly completed and commissioning (including training) was carried out in just one day.

From the time it was commissioned the line has been in service 24 hours a day, except for scheduled monthly stops.

Board thickness has been uniform; damage to the surface finish (through sticking) avoided.

Compressed air is no longer used on this line, and the consumption of anti-adherent liquid has also decreased.

Extra spraying, with the resulting cleaning work, has become unnecessary.

The chemicals no longer cause an odour nuisance, because of the reduced levels of misting.

Overall system cost was about one-third that of an alternative solution using rotating discs.

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