Saving weight without risk

The big issue in the wood based panel industry is saving money and making cost-efficient panels that meet all standards with the minimum quantity of material. But up to what point can the critical limit of material be approached and at what point do you go below this limit?

"Up to now it has not been possible to answer these questions exactly," said Ernst Greten, managing director of GreCon. "Even laboratory measurements cannot give exact information."

However, this manufacturer of measuring devices believes it has found the solution with the X-ray scanner HPS 5000 (fig 1): "This system will be standard in a few years' time," predicted Mr Greten.

"Globally, all companies in the wood panel production sector reach their technological processing limits due to board quality, which has been extremely variable in recent times," he added.

"In discussions during the 8th European Wood Based Panel Symposium in Hanover, Germany in October 2012, it was felt that the industry needed consistent monitoring and appropriate instruments to take measurements, while adhering to existing quality standards.

Verifiably good quality over the entire board, with lowest possible use of raw material, is what GreCon promises with its new HPS 5000.

However, this only explains part of what the measuring system can do, says GreCon, especially as the HPS can do a lot more than just accurate measurement; it says 'Weight Watcher' would explain the HPS' capabilities better.

But first things first.

The density of panels correlates strictly with internal bond and bending strength.

A main challenge for the press operator is securing these quality standards while optimising material usage, so he must take into account the raw density of the board every time. If he does not want to risk an entire batch going to the shredder, he must not go below the minimum density. On the other hand, he needs to produce economically, saving raw material.

In particleboard, MDF and OSB production plants, the weight of the mat or board is normally measured at different positions: Behind the forming station and behind the press, where weigh scales are installed.

Today's high line speeds of up to 90m/min, and the very low weight of thin boards, are a challenge. The results of such devices vary due to dirt and high tare, leading to higher measuring inaccuracy, says GreCon. Also, these systems can only be calibrated during maintenance stops. The exact weight is then usually determined in the laboratory, but this is only done every four hours in normal operation.

The new HPS monitors the weight and material distribution of all panels with high resolution right after the press (figure 2).

Five HPS 5000 systems from GreCon have been installed throughout Europe and GreCon claims the panel manufacturers chose the system for several reasons.

Firstly, they can now carry out a final control of each panel by scanning each mm2 of the board, with up to 13 million measuring values.

Also, the 'in-line-laboratory' application gives confidence to operators, allowing them to perform laboratory measurements in seconds, right after the press (figure 3).

Furthermore, the user can utilise the software not only to monitor the process, but also to analyse longitudinal and lateral density distribution (figure 4).

As underweight zones can cause serious problems in the subsequent processes (eg coating), these areas are observed separately. The variation inside each panel can only be identified because of the high precision, says GreCon. The different density zones are marked clearly by a colour spectrum, from red (high density) to purple (low density).

Figure 5 shows the horizontal density distribution of an MDF panel. The purple zones show the areas in which the raw density was up to 5% below the target values; in the red areas the value is higher by 5%. This illustration gives operators and technologists a detailed overview of the quality of each panel.

Ernst Greten explains that, in his experience, some press operators are cautious and some are brave, as shown by long-term measurements recorded in running plants.

Figure 6 shows that only 0.13kg/m² makes the difference between the 'brave' press operator, who approaches the minimum value closely, and the 'cautious' operator.

"This value, which is small at first glance, implies a saving potential of one to two million euro per year, depending on the size of the plant," said Mr GreCon. "Considering the small margins in the panel industry, this is valuable," he added.

"Anyone wishing to reach the ideal line has to do so in blind flight nowadays," emphasised Mr Greten. "Our system makes a blind man see!"

The layout of the pilot system is shown in figure 2. The X-ray mat scanner Dieffensor, from GreCon, records the weight per unit area of the mat. This information is used not only to protect the steel belt from metal and non-metal foreign bodies, but also to display the material distribution after the pre-press.

After press and cross-cut saw, the panels pass the HPS 5000, which gives the first 100% information and final control of each panel. Both systems, Dieffensor and HPS, control the forming quality, since the information from both systems is included in the process control system of the plant. The objective is to control the process automatically to the ideal line - just as the cruise control of a car maintains a constant value, explained Mr Greten.

Thus, the measuring device can be included in an automatic control loop to manipulate the forming quality considerably. Appropriate solutions for MDF lines, as well as for particleboard lines, are available.

The potential of the new HPS technology was proved in an MDF reference plant, says GreCon.

First, the actual distribution variation in longitudinal and lateral direction was analysed. Then the HPS was integrated in the automatic control of adjustment tables installed beneath a scalping unit. Material distribution was then analysed again after final optimisation.

The comparison between both crossprofiles - in each case collected over two hours - showed a significant improvement (figure 7). In addition, it turned out that not only the amplitude of the cross-profile could be significantly reduced, but also the variations within the cross section could be reduced.

Not only was the lateral material distribution optimised, but the longitudinal trending also showed significant improvement.

This can be seen in figure 8, showing the average weight per unit area of the produced panel within the two-hour period.

"The decreased variation in the panel weight is striking," said Mr Greten. "In those two hours, material costs could be reduced by 1.6%."

Recent experiences
GreCon says that just recently a new HPSsystem was installed in Turkey. During installation and starting of the mat scanner, a GreCon technologist was on site. That technologist said the set-up of the measuring device was realised smoothly without disturbing the production process and the first measurements showed interesting results.

Great optimisation potential was recognised, which can now be implemented by the customer's own resources and little effort (longitudinal distribution of the mat). In the short run, additional saving potentials will be achieved when further complementary measures are taken (lateral distribution of the mat), said Mr Greten.

Summing up, one obvious answer to the money-saving issue in the wood based panel industry is to find out more about what is actually happening inside the produced panel, says GreCon.

By using a high-precision measuring scale, one can look into the mat and take appropriate measures to improve production, reduce raw material and save costs.

Ernst Greten is sure that the HPS will, sooner or later, be an integral part of any modern panel production plant.