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Screening at a Stroke

Thanks to their temperature and corrosion resistance, PVC products are widespread and practically indispensable in many areas of application. Cost-effecting and flexible for manufacturing, they guarantee the best possible raw material quality as well as precise further processing into high-quality end products. Foreign bodies in PVC powder owing to unclean production of owing to contamination during transportation should be removed before the production process in order not to contaminate batches and plants. Protective screening upstream from silo filling ensures that the desired quality of raw material is made available as well as reducing quality costs, as shown by a current project from screening machine manufacturer, J. Engelsmann AG.

An internationally active specialist in plastic extrusion from South Germany wanted to protect its production plants and end products from contaminated PVC powder. The medium-sized company manufactures plastic profiles for windows and doors for the housing and construction industry as well as for a multitude of applications in the outdoor sector. Most of the products are high-quality registered brands. Manufacturing is done with modern extrusion systems with co-extrusion and multi-strand technology and with automated take-off devices. The PVC for the manufacturing of plastic products is stored in silos, from which it is continually fed into the production process. In order to prevent contamination of the silos and the downstream production systems, the company opted for protective screening upstream from the silo filling. Since no protective screening had previously been used for the screening out of contaminated PVC powder and there was thus no experience, multiple types of machines from a variety of screening technology manufacturers were tested. The company chose the TWS tank lorry sieve from Ludwigshafen-based screening technology specialist J. Engelsmann AG, which committed one or more rental machines to the plastics manufacturer free of charge for the trial phase and the subsequent delivery time. It was, among other things, the individual requirements for the machine solution which tipped the scales in favor of the Engelsmann model. In order to ensure one hundred percent inspection of the raw material deliveries, seven TWS 800 type tank lorry sieves were ordered from Engelsmann for protective screening.

Protective screening as the first quality control

Protective screening occurs between the silo trains and the storage silos. The Engelsmann tank lorry sieve acts as an intermediary here in order to screen out undesirable foreign bodies in the plastic powder. It therefore functions as a quality control immediately after the receipt of goods, before the PVC powder goes into the storage silos or production. For quick connection and disconnection when connecting the machine feed pipes on site, the infeed and outlet of the screening machine are both fitted with coupling halves. The flanged infeed and outlet bends are turnable and can thus be adjusted to the local conditions. Coupled to the feed pipe from the silo lorry, the PVC powder is transferred via the infeed into the control screening machine, where the product comes up against a specially designed discharge cone underneath the infeed pipe, using pneumatic pressure transport. This reduces the pressure before the product lands on the sieve and thus protects the screen cloth.

The TWS 800 model, made from stainless steel, has a sieve ring with a diameter of 800 mm and a screening surface of 0.5 m². The screen aperture of the mesh is between 1 and 5 mm depending of the specifications for the powder delivered. The agitation of the mesh is done with a vibration motor, which leads to the screening out of any foreign bodies. In order to ensure the complete screening out of contaminants, a magnet separator is installed under the screen cloth in order to extract even the finest metal particles. The screened PVC powder is then transferred through the discharge pipe and into the storage silo via the rising feed pipe on site. The TWS 800 is gas-tight to 2 bar and has a throughput of 30 t/h.

Equipped for mobile use in a variety of locations

Since the protective screening machine will see mobile use, there is a chassis under the sieve housing with two lockable fixed casters and two lockable swivel casters. For use in various locations, a connection with an on and off switch as well as a power cable is located on this frame so that the machine can be quickly and easily put into service via a power connection. The properties of plastic in its primary form can make screening more difficult since PVC can become statically charged. This phenomenon can lead to the product sticking to the machine parts. In order to prevent this problem, the screening machine used needs to be earthed. Owing to the use in different location this is done via the respective silo trains. For this purpose, the TWS screening machines are equipped with an 8 meter long earthing cable and a cable drum in order to cover the various distances to the silo vehicles.

Increased productivity and quality

Once an entire vehicle load has been subjected to protective screening, the foreign bodies which remain on the screen cloth are removed and the machine is cleaned. The cover, which is held with quick-release bolts, can be opened without tools so that the sieve ring under it can be removed or changed easily. In addition, all the components which come into contact with the product in the interior are glass-beaded so that the machine can be cleaned quickly and leaving no residue.

Thanks to protective screening with the TWS screening machines, supplied with CE marking, the plastic profile manufacturer is in a position to monitor the quality of all raw material deliveries, carry out assessment of suppliers and make unsatisfactory deliveries of raw materials transparent. The investment has also paid off economically. The improved quality of the PVC powder has lead to reduced production downtimes owing to a reduction in the time required for cleaning and the number of faulty batches of products.

The TWS control screening machine from Engelsmann is available in two different sizes with a sieve diameter of 500 and 800 mm or a screening surface of 0.2 and 0.5 m2. If required, the TWS vibration screener with GMP model can be used in areas with high hygiene requirements and/or, in the ATEX version, in zones where there is a risk of explosion.