Since the middle of the 20th century the intensive development of production and consumption of plastics can be noticed. In the modern world materials are widely used at every turn of a human life. Gigantic amount of objects around us is made of plastic. New technologies drive the trend of making use of these materials. The scope of plastics is so wide that it is difficult to find a place without it. The huge innovative potential makes that technical as well as decorative systems produced using plastics, replace the outdated constructions made of more expensive and faster consumable materials.
 
The widespread use of new materials generates the growing demand for plastics of stringent parameters which opens broad prospects for further development. High operating temperatures, good sliding and friction properties, high tensile, dielectric and chemical strength make that individual plastics constitute extremely attractive material used by current constructors. Plastics are used in industrial areas such as:

• automotive;
• medical;
• electronics;
• chemical;
• food;
• textile;
• aeronautical

 
Thermoforming is one of the key technologies for production of plastic articles. Thermoforming is a technological process in which a plastic sheet is heated up to a certain temperature, close to the softening point (characteristic of a particular plastic) and then products of certain shapes (die stampings) are formed.
 
The condition necessary to obtain a quality product (die stamping) is the maintenance of certain physicochemical parameters of the raw material i.e. plastic sheets. One of the key parameters in the thermoforming process is the content of water in the raw material.
 
Physical, mechanical and technological parameters of the majority of non-metallic materials depend on water content. At present almost in all branches of industry, architecture, energetics and building, drying and humidification processes are used to change the content of water in materials. That is why, the assessment of water content in hard materials, liquids, such as oil and oil-related and gases is necessary in the industry, science, researches.
 
In the industry, moisture of raw materials and semi-finished products has an enormous influence on the final production quality, productivity and time of work of the machines. In many branches of industry such as: heavy, light, textile, food, chemical etc. main strict technological processes are connected with changes in moisture content of the processed materials.
 
During thermoforming, even small amounts of water in plastic sheets can cause damage to the final product.
 
Why is this happening?
 
If a raw material contains water, which is normally inside the internal structure of the plastic sheet, so in the process of heating to a certain temperature the water changes its state, expands and mechanically damages the material – tearing it apart.
 
In order to remove water from the raw material and prevent its damage, it is heated in driers for many hours before the thermoforming process is started. LEPOLAM company developed a drier which allows for effective drying of plastic sheets in a controlled way.
 
The chamber of the drier includes a cart with holders for hanging plastic sheets. The cart can be entirely moved from the chamber to the outside of the drier, which makes it easier to load and unload the sheets and besides you do not need to waste time for cooling the chamber. Due to vertical position of sheets on the cart the hot air flows perfectly from all sides, which is essential in the drying process, moisture diffuses faster from plastic. The sheets are hung on sticks of the cart with specially designed clamps. The clamps are put on beyond the used area of the material hence there are no material losses. During loading, unloading and drying the sheets do not touch so you do not need to worry about scratches, besides the sheets may have a factory protective film layer on one side (this may prolong the process of drying).
 
In the process of drying the drier uses two systems:
 
The system of hot air drying (the air heated to the temperature setpoint circulates in the closed circuit, flows around the hanging sheets absorbing the moisture).
 
The system of dry air drying (from the main circuit, the second system takes the humid air, passes it through the rotor of the drier where the moisture is left and as dry air it returns to the chamber of the drier. The moisture is removed out of the drier. The entire system works in a closed loop.
 
This technology shortens the drying time and above all considerably reduces electricity consumption.
 
All devices connected with the drying technology are placed on the roof of the drier.
 
All heat emitting devices such as ducts, air heaters, dehumidifier, heat exchanger are covered with an insulating layer in order to minimize the heat loss.
 
In spite of such an innovative solution as the drier- still the question is:

• How much water is in the raw material?
• Does it have to be dried?
• How long does it have to be treated with hot air in the drier?

 
It is worth noticing that these are the questions asked by the entire plastics industry in the world. In order to meet such needs the company LEPOLAM decided to create an innovative device – spot moisture sensor for measuring the water content in plastic sheets. For the implementation of the idea thereof the funds were obtained under the NCBiR project.
 
Preliminary analysis and research show that such a device – the spot water content sensor will allow with high accuracy and repeatability to measure the content of water in plastic sheets.
 
In the future, a comprehensive solution – preliminary detection of water content in the raw material, using the spot sensor will allow to clearly state – how long the material has to be dried which will considerably increase the efficiency of the drier.
 
Very precise moisture control of the used materials has a huge economic impact. It is difficult to evaluate it terms of money, in particular due to the fact that in most cases the saving is achieved not as a result of obtaining knowledge of water content but intentional and effective use of this information.