Extrusion 8-2016

The stress relaxation is obtained by pre-heating the mate- rial gradually and under “free” conditions before the ther- moforming process as the polypropylene tends to distress naturally during heating when it is not being blocked in the chains. For this reason, special pre-heaters have been studied and the market offers different solutions. Since the mid-eighties, WM Wrapping Machinery has per- formed a number of tests developing various alternatives (see ) such as infra-red radiation systems, contact by rollers etc. The best final solution that ensured the greatest benefit was the oven with hot air circulation (see ). A special pre-heather has been developed which consists of an insulated structure and mobile roller located on the inside which enables various meters of foil being treated continuously during the pre-heating phase before being entered into the thermoforming machine. Inside the heating chamber, the air is heated and temperature-con- trolled to a maximum of 130 °C. A centrifugal fan with an internal high performance auger and impeller with re- versed blades, generates an upward airflow. In addition, a special device called "Full air delivery" is installed on the floor, inside the oven, in order to ensure a correct distribu- tion of the hot air flow. The heating takes place through a group of electric heat- controlled resistances and is controlled by a dedicated probe inside the oven. This PH pre-heating unit is totally autonomous, equipped with a control panel and can be installed easily between the reel-unwinding device and the thermoforming unit. The PH oven may be installed on existing machines at Customers factory. (See ) WM is offering the PH 920 and PH 1100 models which are differing in power and maximum size width of the foil, ranging from 920 mm to 1,100 mm respectively. These pre-heaters have evolved significantly over the last few years and the last generation of the PH series are the re- sult of the experiences and tests made over the years at WM factory. As already mentioned before, the insulation element is of high importance ensuring a maximum performance with the highest possible energy savings. It’s im- portant to highlight that the standard ovens chamber insula- tion has a thickness of 80 mm. It is also important to say that the feeding rolls, which consist of a double rollers unit, are aligned in a upper vertical position when entering the material. This allows a quick and easy feed- ing of the foil by the operator, then, one of the two rolls group moves downward in order to position the material with the best possible distribution in the heating chamber (see ). 45 Extrusion 8/2016 WMWrapping Machinery SA Via dei Pioppi 3, 6855 Stabio, Switzerland (CH) www.wm-thermoforming.com Picture 5 THANK YOU FOR YOUR TIME! We enjoyed our contact during the K 2016 – and wish you all the best for the time you save, using the fastest products of the plastics industry! www.graewe.com GRAEWE GmbH Max-Planck-Straße 1-3 D-79395 Neuenburg am Rhein Tel. +49(0)7631/7944-0 11:40