Plastic

Plastic is a nice example of a material with two different BSDFs mixed together.

• The first BSDF is the plastic’s base color. Set the Refl 0° color to the color of your plastic. The Refl 90° and Nd numbers do not matter and can be left at default because we are using a high roughness value to create a 100% diffuse material. So set roughness to 100.
  
  
• The second BSDF is the shiny “topcoat” of the plastic. The Refl 0° color can be left at default or you can raise it or lower it to have an overall shinier or duller plastic. The Refl 90° color can also be left at default (white) because we want the plastic to be very shiny at the edges, and we do not want any tinting of the reflections.
  
  
• This second BSDF is essentially a mirror, so lower its Roughness to the minimum (0-20). Nd should be set to 3 which is a good value for plastics.
  
  
• The blending weight of the second BSDF was set to 30 so that the shiny BSDF has less influence on the final material. This will create a more realistic plastic.

To make the plastic more or less shiny, there are two possible approaches:

1. You can lower the Refl 90° color, and/ or lower the Nd of the second BSDF to make it less shiny. Conversely, you can raise the Nd to make the plastic shinier, given that the Refl 90º is already set to the maximum value.
2. You can increase the blending weight of the second BSDF so it will have more influence on the final look of the material. 

The basic structure of a plastic material. You can download this sample material here. 

Plastic using two Layers

You can also create a plastic using Additive mode, creating two layers with one BSDF each. Set the top layer (the shiny top coating) to Additive, and adjust the layer’s weight to make it more/ less shiny. This approach will create more vivid-looking plastics which may be preferable in certain cases.

Avoid having more than one layer set to a weight of 100 in Additive mode. A material with three layers, two of which are set to Additive and have a weight of 100, will look unrealistically reflective. And it will also add to the render time. This does not apply to layers in Normal mode. 

Find more plastic examples at the MXM Gallery.