Now, this is a good question. If you are familiar with many industrial and simple adhesive glues, you will know that you can easily adhesive glue similar items and surfaces. This is why you need to find a specific adhesive glue to do a job. Since plastics and metals are two dissimilar materials, joining them together can be quite difficult. Since plastics and metal are non-porous, they can be very difficult to bond together. If you are looking to bond plastics to metals, you need the right adhesive which will easily bond both surfaces.
