What's the best adhesive for plastic? It depends on what kind of plastic you're talking about. The term "plastic" covers a wide range of synthetic materials, from translucent acrylics to tough polycarbonates to lightweight polystyrene. A glue that works great on one will melt another. The best way to pick a plastic glue, then, is to look at the specific materials you're trying to bond.
Adhesives for Acrylic and Polycarbonate
Acrylic and polycarbonate plastics are hard and relatively inflexible; they are also resistant to bonding with most glues. These plastics bond best with toughened two-part epoxies. Epoxies that cure to a Shore D hardness of 70 to 75 work best in such applications. These epoxies typically mix in a 1-to-1 ratio of resin to hardener and should be applied in equal amounts to both sides of the bonding area.
Adhesives for Expanded Polystyrene
Expanded polystyrene (e.g. styrofoam) is notoriously difficult to glue; most solvent-based adhesives simply melt the foam. To bond this foam, polyurethane-based foaming adhesives work the best. The adhesive should exhibit a tensile strength of at least 17.7 Kilogram of pressure per square inch (PSI) and a shear strength of 48 PSI when fully cured.
Adhesives for Polystyrene
Polystyrene is the plastic typically used for models and other vacuum-moulded items. This is the plastic that "plastic cement" is actually made for. These cements are made of polystyrene dissolved in toluene, a fast-vaporising solvent. Plastic cement will weld two pieces of relatively porous plastic in a few moments.
Adhesives for PVC
Gluing PVC pipe or furniture frames is best accomplished with either a phenolic or nitrile rubber cement. These cements can be specified in a number of densities, depending on how much working time is required before the adhesive sets up. For working with PVC plumbing, it's a good idea to prepare the surfaces with a PVC primer, which provides a better gripping surface for the cement.