While choosing a glue for attaching two types of plastic is a relatively simple procedure, there are various elements that come into play when you are attaching two materials that are made of different properties, such as metal and plastic.
Why Not Fastenings?
While some constructions would be better suited to using fastenings, this can present problems on some surfaces, such as the time and expense involved in drilling holes to install the fastenings and the potential disadvantage of having weight distributed in one area as opposed to being distributed evenly across the surface through the use of adhesive. This can cause long-term weakness in this fastening area and a potential for unwanted galvanic interactions between the fasteners and the substrates they are joining.
Finding the Compromise
Substrates such as polycarbonates and ABS are much more adaptable when joining to metals, but care should be taken to ensure that any adhesive you use is suitable for both materials. Adhesion is something that can be affected by the weight that a material can bear, as well as something called elastic modulus, which describes how the material bounces back when stress is relieved from it.
It’s important, then, to find an adhesive that bridges the gap between the adhesive properties of both substrates, as well as considering the primary use for the joined materials. Non-structural Items require a bond strength of less than 1,000 psi. Those that bear weight are above this figure.
The Impact on Surface Energy
Plastics come under two categories – those with high surface energy and those with low surface energy. The latter is usually cheaper to create, but fewer adhesives are suitable for them. Strong bonds can be made on surfaces with high surface energy (HSE) as the adhesive flows more readily, creating a very secure bond.
To increase the adhesive effect of low surface energy materials, it’s recommended that you use specialist adhesives such as CT1 which are suitable for some low surface polystyrenes. This is also a suitable metal bonding adhesive, killing two birds with one stone.
One way of improving the bond between two materials is to use a suitable primer that is designed to work with different substrates to give a solid adherence. There are many primers available, so carefully checking their properties will increase your chances of success.