Published on 21 January 2013
A solution to a longstanding filtration problem was devised by TWI's Plastics Department for P & S Filtration Ltd.
Liquid filtration units comprising a series of formed metal plates covered by filter cloths are used in many industrial processes to remove impurities. The cloths, made from woven polypropylene-covered fabrics of about 2mm thickness, are joined to polypropylene-based synthetic rubber gaskets. Traditionally, the filter cloths have been sewn to the gaskets, but it is a slow, labour-intensive system and the join is prone to leak through the stitching at operating pressures (up to 14bar).
TWI was asked to find a joining technique to increase the production rate and improve the performance above that of sewn cloths. A number of service criteria had to be met:
- The filter cloth must retain its flexibility.
- The join between the filter cloth and gasket must be ripple free.
- The bonding system must withstand the operating pressure of the filter press.
The need to maintain flexibility precluded joining processes that would heat through the bulk of the material. Consideration was therefore given to welding techniques that could provide localised heating at the joint interface, such as induction heating (implant welding), heated shoe and hot gas welding.
A scheme to hot gas weld the filter cloths to gaskets using a manual technique was successfully demonstrated. To validate the technique, a series of full-scale cloths was welded for application on a working filter press. Results have demonstrated that the hot gas welds withstand normal operating pressures in a working press without leaking.
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