TPT July 2013

Article

At this point there were still important issues to be accomplished from a technical point of view. The pipe conditioning was one of them. The pipe had to be heated up homogeneously with extreme accuracy in the three cylinders coordinates: axial, hoop and radial. Other kind of heating methods like radiation or contact were discarded because they were not able to guarantee these specifications. A special oven was designed for that purpose, with high internal air movement speed, allowing close to a hundred renovations per minute. Isolation was very carefully designed to prevent heat loss and noise. Up to this point the prospects of output and homogeneity were fulfilled. The speed of expansion is very important in orientation. This was another important challenge to deal with. Whereas the speed of expansion is quite easy to control using water with air this aspect is more sophisticated. The expansion is executed by communicating tanks of compressed air at higher pressure than the one needed to expand. In the first place, the pipe to expand is at atmospheric pressure, and the tanks are at high pressure. Graph 1 represents the flow that is able to pass through a hole of certain diameters depending on gradient of pressure between two different pressures. As shown in the experimental graph, in order to maintain constant, for example, a certain flow of 3,000 litres per minute, from a deposit of nine bars to the pipe at atmospheric pressure, the passing orifice has to change from a certain diameter to a diameter four times bigger. Thanks to the electronic regulation and fast answer pressure detector and servo valves, this required flow can be maintained during the main part of expansion stage, and a constant speed of expansion is guaranteed. Air chilling was another drastic change in the PVC-O concept. It consists of freezing the pipe from orientation temperature to a temperature below Vicat without water. Following this a research project just for freezing was started. Aspects such as speed of flow, geometry of pipes or even air humidity were considered and the results obtained, as shown in Graph 2, were used to design the final equipment, optimising energy consumption and output.

Graph 2: Results from a simulation process of freezing a pipe internally with air. Map of temperatures

A real machine was built to produce pipes from 160mm to 400mm Ø and tested in regular production with an extrusion line based on a Cincinnati Argos 93. The mixure used was based on PVC resin from Solvin and Naftosafe from Chemson. The following results were obtained:

These huge savings resulted from maintaining constant the mould temperature and avoiding the use of water as fluid for the process. The energy for conditioning is only applied to the plastic to be processed. The equipment is so compact that the length of the whole line for manufacturing up to 400mm Ø can be of 45m and the width is less than 4m. The line is fully automatic, so no extra labour is needed. Other positive features were also implemented such as swiftness in mould changing, an operation that now takes less than one hour for two people. All the technical developments were implemented in a recipe system; in this way the manufacturer only has to choose the pipe he wants to produce and all the parameters change automatically to the new conditions. The global situation with ever increasing prices of raw materials and the standardisation activities in many countries are making PVC-O one of the most attractive investments in the plastic industry. The superb quality of the product, in addition to the important raw material savings that can reach 50 per cent in many cases, and access to more profitable markets for high pressure applications, forecasts a bright future for this material.

Graph 1: Flow passing through some holes of different diameters, and real curve obtained after regulation with a consignee flow of 3,000 litres/min

Molecor Tech Madrid, Spain Email: pilar.sanz@molecor.com

97

www.read-tpt.com

July 2013