How to select the best method for bonding plastics

Edited by the Engineeringtalk editorial team May 1, 2006

Joining Plastics 2006 (25th to 26th April in London) provided an expert forum on the latest technologies and methods to help manufacturers choose the optimal way forwards in bonding polymers.

Delegates discussed how to select the best method for bonding plastics at the Joining Plastics 2006 conference, 25th to 26th April in London, organised by Rapra Technology and NPL.

Bonding experts had a frequent complaint that joining is not considered until the product design is complete and materials have been specified.

Sometimes clients don't know which plastics they are working with as components are purchased ready-made.

This leads to problems in selecting adhesives or the appropriate welding technology.

Factors to be considered in the joining method selection include the joint shape, polymer type, process capability, production volume, the strength and seal required, longevity and any standards specified for the product.

Different types of adhesives are required depending on the substrates to be bonded, as discussed by Ashland, Henkel Loctite and Dow Corning at the conference.

Joint accessibility, application methods and cure times are further considerations in manufacturing.

TWI and NPL have helped to develop an adhesive selection protocol, available online at www.adhesivestoolkit.com.

Plastics welding techniques include infra-red, resistance, hot plate, ultrasonic, vibration, heated tool and laser welding.

Advantages and limitations of each method were highlighted, with presentations from FFR Ultrasonics, Aachen University, Gentex, Branson Ultrasonics and Prolas.

DSM has developed engineering plastics suitable for laser welding, while BASF and Treffert have specialised additives.

Many plastics require a pretreatment step before bonding for both surface activation and to remove contaminants such as mould release agents (one cause of failure of bonding).

Polypropylene, polyethylene, fluoroplastics, silicone rubber, acetal and Peek are difficult to bond.

The process selection again depends on the polymer, joint design and process capability.

Techniques include plasma treatment, corona discharge and chemical primers.

Ralph Dahm of Loughborough University described detailed studies on fluoroplastics, concluding that sodium in liquid ammonia and sodium naphthalenide are the most effective for pretreatment.

Joint testing is a key area for research as most effective methods are destructive.

Joining of composite materials was also discussed at the conference, particularly the growing area of thermoplastic composites.

Geoff Turvey (Lancaster University) covered the strength of bolted plates and Ali Yousefpour (NRC Canada) discussed induction welding in aerospace applications.

Ewan Kellar (TWI) described a technique for composite to metal bonding.

Joining Plastics 2006 provided an expert forum on the latest technologies and methods to help manufacturers choose the optimal way forwards in bonding polymers.