Quality & process optimization: Moisture measurement in the gypsum fiberboard production

<p>The purpose of online moisture measurements at fermacell® – James Hardie:</p>

• To produce high quality boards through improved moisture control during the process.
• A system to track moisture per gypsum board from production to customer.
• Saving gas by monitoring moisture before and after drying.

James Hardie has been renowned for its innovations in the construction industry for more than 130 years. fermacell®, a division of James Hardie, is responsible for extremely versatile and high-quality gypsum fibreboards for interior applications such as walls, ceilings, and floors.

<p><strong>Production process for gypsum fibreboards</strong></p>

There are no substances harmful to health in fermacell® gypsum fibreboards. The boards contain no glue, which eliminates any odour nuisance and allows the homogeneous board structure to "breathe" better.

The boards are impregnated to make them water-repellent and fire-resistant. In the final stage of production, the boards are impregnated with a water-repellent agent. Before this can happen, the residual moisture content in the boards must be between 1.2% and 2%. If the moisture content is below 1.2%, there is a risk that the boards will absorb moisture from the air and swell. Gas consumption costs will also increase drastically with lower moisture content. If the moisture content exceeds 2%, the boards may become dimensionally unstable during drying due to shrinkage. The target moisture content for an optimum production process is between 1.5 and 1.9%.

Gypsum fibreboards are optimally dried to achieve this target using a board dryer in combination with an in-line ACO moisture measurement system.

<p>Process integration of moisture measurement</p>

ACO supplies a capacitive moisture measurement system that can measure up to 150 mm into the product, i.e. right into the core of the fibreboard. The DMMS system can connect and process up to 16 sensors in 1 Wago PLC. All values can be easily read and processed via ProfiNet. The installation of the sensors is an important point of attention.

The measurement is taken directly after the sanding machine. Plates of different thicknesses (10, 12.5, 15 and 18 mm) are transported over the sensor.

To avoid obstructions and to provide a continuous moisture content output the gypsum fibreboards are measured directly online with the roller track. This enables the sensor to make continuous moisture measurements along the entire length of the board, without being actively involved in the gypsum fibreboard production process. A unique mounting facility with special rollers has reduced the impact of vibrations on the track, ensures the constant distance of the moisture measurement to the fibreboards and reduces the wear of the mounting plates and the fibreboards. Pressurised air also ensures that the rollers and sensor remain clean, reducing maintenance.

To ensure all sensors are correctly installed and operating correctly, a Pertinax plate checks their raw values. This quick check reduces maintenance time.

For the different thicknesses of the fibreboards, four curves are stored in the system. This data is read in via Profinet and displayed as moisture in the boards. Although the distance between the plates and the sensor is similar for all three sensors, they are all calibrated using the same curve, which reduces calibration time.

<p>Laboratory moisture measurement</p>

At fermacell®, the same DMMS moisture measuring system as the online moisture measurement is used in the lab, but in a stationary offline configuration. Normally, the boards had to be dried for 24 hours in the laboratory to get the moisture values. Now the moisture values are easily available within seconds. The measuring system is calibrated according to company instructions and regularly checked using a Pertinax test plate.

A sticker on the board, coming from the production, can be scanned with a QR handheld scanner to select the correct calibration curve/board thickness. With the Profinet communication, the QR code selects the right thickness of the boards and provides an accurate moisture content. Values from the laboratory must match online moisture values. This provides an additional quality control check on the boards.

These values are recorded and stored in the Ethernet for quality and improvement purposes.

<p>Advantages of online moisture measurements</p>

To prevent boards from drying for too long, which is both costly and detrimental to quality, additional online moisture measurements have been mounted before and after the dryer and before storage. This enables more effective monitoring and control of the drying process and quality control, resulting in better quality boards and significant savings in gas consumption each month.

How does a moisture measurement work?

ACO moisture sensors work with the capacitive measurement principle. The signal transmission between the evaluation unit (DIGISYS) and the higher-level control system is carried out either via Profibus DP or analogue standard signals. For each moisture sensor the measured value is transferred to the process control system (PLC) to detect the moisture distribution ratio within the gypsum board width.

The calibration of the moisture measuring system was carried out during commissioning directly in the production process with an easy manageable calibration software. The calibration data is stored in the evaluation unit (DIGISYS). If a sensor has to be replaced, no recalibration is necessary. Up to 16 moisture sensors can be connected to one evaluation unit. Each moisture sensor is able to store and use 16 different material curves. The penetration depth of the moisture sensor is up to 150mm, depending on material and installation situation. The moisture sensor electronics is installed in a robust "golden" stainless steel housing, which can be mounted flexibly using the supplied clamping ring.

Read a detailed article about: How does a moisture meausrement work?

Process integration of the moisture measurement: How to integrate a moisture measurement in the production process for gypsum fibre boards!

One of the most important aspects in the realization process of a moisture measurement is the correct installation position of the moisture sensors. In this special case an installation possibility was sought that would allow the moisture to be measured in the gypsum boards of varying thickness without much effort. On one hand the flow of the system can`t be disturbed in a negative way and on the other hand the best measuring accuracy should be achieved. Out of that two reasons it were decided to measure the gypsum fibrebords inline directly within the transport roller conveyor. This allows the sensor to perform a continuous moisture measurement over the entire length of the plate without actively intervening the production process of the gypsum fibre boards.

Process monitoring and process control: How a moisture measurement has to be integrated in a production process for gypsum fibre boards.

The first test moisture measurements showed that the floor dryer don`t dry homogeneously. One sensor alone is not able to detect differences in the moisture content of the 12 gypsum fibre boards. To be able to detect and evaluate the distribution of residual moisture over the complete gypsum boards with a width of 2.5 m, it is necessary to install at least three moisture sensors dispersion over the entire board width.

In order to control the gypsum dryer, the measured moisture values of the gypsum fibre boards are averaged from the 12 individual floors. With this average moisture value of the gypsum fibre boards the dryer can be controlled. Prior to this, the boards were monitored by means of a hand-held moisture meter "Bindicator", but the measured values were not very accurate. Through the used hand held moisture meter there was a tendentially high percentage of poor quality fibre gypsum boards which were also returned by customers. After equipping the board line with an ACO moisture measurement the quality returns due to incorrect moisture content quickly decreased and production is now at a more consistent quality.