Bonding plastic to metals and alloys – factors to consider

Plastics are one of the most versatile materials in manufacturing where they can be shaped and moulded into many different forms. There are many adhesive options for bonding plastic to metals and it is important to consider certain factors that might affect the bond.

Factors to consider 

Type of Plastic

The majority of plastics have enough surface energy to bond well to other substrates including metals. Low-energy surface plastics, Polyethylene and Polypropylene can be more difficult to bond, and will require pre-treatment.

Hybrid plastics such as GFN can have significant problems due to the levels of moisture. In these cases, a combination of pre-treatment and the right adhesive selection is needed.

Type of Metals

The type of metal and alloy can present problems, where the surface can be affected by particulates, chemicals, and oxidisation. This can affect the adhesion on the surface of the material. As a result, it is advisable to lightly abrade aluminium, copper and its alloys before application.

Size of the Material

Every type of metal and plastic expands and contracts at different rates. This means a large area with different expansion and contraction characteristics can cause major stresses between the bonded elements. In some cases where there are variations in temperature, a more flexible adhesive and a thicker application will counteract the different expansion and contraction rates.

Traditionally, design engineers have used mechanical fastenings in manufacturing, however these can create and aggravate stresses and fractures in the material. In recent years, there has been a trend towards weight saving in manufacturing and as fasteners are heavy, adhesives tend to be a more favourable option when bonding plastic to metals.

Adhesives provide an even distribution of stress over the bonded surface and protects the substrate surfaces from rust and corrosion. They also eliminate the need for holes in the case of mechanical fasteners. In addition, adhesives can improve the aesthetics of a product and enable greater flexibility in design.

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