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Gluing metal and plastic

Gluing metal and plastic
Sally Sinai of ITW Devcon puts forward a convincing argument for confidently choosing methacrylate adhesives to perform a wide range of bonding applications in place of the traditional use of thermal joining (welding, brazing and soldering) processes or mechanical fastening.

Methacrylate adhesives have been used in a number of industries to bond a variety of metals, plastics, composites and dissimilar substrates for many years, and more and more companies are changing over from traditional fixing methods. But what are the risks attached and can you be sure of their long-term performance?

Traditional jointing methods are undisputedly well-proven. Having said that, the bonding of metals has now been successfully undertaken for many years. Thermal joining is relatively low-cost but the two chief disadvantages are firstly that stress is set up in the components (with the resultant risk of corrosion), and secondly metals can only be joined to metals.  In the case of rivets and other mechanical fastenings, when these are used the substrates have to be drilled and once again there are two down-sides: stress is set up around the holes and the aesthetic appearance is compromised due to the protrusion of the fastening. 

When adhesives are used the problem of stress is eliminated, smooth surface contours can be achieved and the finish is aesthetically more pleasing as the surfaces do not discolour. Weld-spatter is avoided and the surface will have no unsightly protrusions. The bond strength is higher, there is no need to drill holes or specially prepare the substrates. What's more, adhesives will bond dissimilar materials, like steel to ABS for example - and in most cases without the use of primers. The process becomes lower cost and the joints can normally be bonded more quickly.

What about the in-use performance of methacrylate adhesives? Devcon's methacrylate adhesives (MMAs) provide tough flexible bonds for aluminium, stainless steel, exotic and coated metals as well as hard-to-bond plastics, composites and dissimilar substrates. MMAs offer superior load-bonding strength, are tough and flexible, can be rapid-curing, are resistant to weather, humidity and wide temperature ranges. They offer high tensile, impact, peel and shear strengths, high fatigue resistance, UV resistance and can be formulated to gap-fill. The exact performance of each MMA is determined at the design and development stage by the formulating chemist and product manager.

Selecting the right adhesive
So how do you ensure you choose the optimum adhesive? The choice of adhesive is dependent upon many factors especially the substrates and end use application. Consequently it is important to liaise with your adhesive supplier at an early stage in the project. The cost of getting the choice of adhesive wrong could be high but provided your supplier has the required knowledge and formulating expertise, it will take next to no time to select the most appropriate grade of MMA for your application.

The selection depends on several factors, chief of which are substrate type, joint design, production process and in-use durability requirements. One also has to consider the service temperature encountered by the end-user together with potential exposure to any specific chemicals.

With regard to the user's production process, it is important to take into account the adhesive working time, fixture time, rheology (sag and flow characteristics) and part-handling after assembly. Devcon not only has experienced technical personnel to advise on the correct approach, but site visits are often the best way to fully understand the process and customer's specific requirements.

Three Devcon products in particular have proved to be very popular with a large number of existing customers.  These three adhesives, each with its own unique properties, all offer the combined benefits of epoxy and polyurethane products:

  • Metal Welder I: This two-part MMA product offers the end user the ability to bond ferrous and non-ferrous metals and other difficult substrates without priming and with only the minimum of surface preparation. It gives high tensile and shear strength and is used when excellent environmental resistance is required
  • Metal Welder II: This is a two-part MMA having a medium fixture time designed for the primerless bonding of dissimilar metals. It has high impact, peel and shear strength with exceptional fatigue-resistance and salt-spray durability.
  • Devweld 530: A rapid-curing, multi-purpose, toughened, load-bearing MMA suitable for bonding a wide range of metals, plastics, ceramics and other difficult substrates. It is resistant to weathering, humidity and wide fluctuations in temperature.

Devweld 530 was chosen by Birmingham Garage and Industrial Doors (BGID) to bond steel sections into the steel frames used for its security doors.  BGID has been producing doors in Rowley Regis, Birmingham for over 30 years during which time it has earned a reputation for supplying quality precision engineered metal doors. The company has a modern state of the art pre-treatment plant and continuous process powder coat plant. A company spokesman reported that the chief reason for selecting Devweld 530 was that it offers excellent load bearing qualities and exceptional shear, peel and impact strengths whilst offering a robust bond which can withstand the high temperatures encountered through the powder-coating booth.

The number of new applications for MMA adhesives continues to expand within the automotive, furniture, rail, signmaking, computer, aircraft and refrigeration industries to name but a few, and staff at Devcon are freely available to help and advise clients make the move from traditional fixing methods to the use of structural adhesive bonding in the most demanding of circumstances.
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