As soon as I got home, I fired up the GIMP and started experimenting. It took some time to come up with the effect, but eventually the design started resembling the actual plastic piece:
Below, I cover the main steps of the design process. I have broken down the creation process into six steps:
1. Start up the GIMP and create a new file by choosing File>New from the Window menu. The file size is unimportant since here we are developing a technique that, once mastered, can be applied to any shape. Add a new layer by clicking on the New Layer icon at the bottom of the layer palette and draw or import a shape on it. Mine is shown in the image below. When it's done fill it with light gray.
2. Duplicate the shape layer by right-clicking on its name in the layer palette and choosing the Duplicate Layer from the menu options; then fill the shape with white. Blur the white shape layer by choosing Filters>Blur>Gaussian Blur. When it is done, duplicate the blurred layer once again using the same technique. Move the second duplicate underneath the gray layer and turn its visibility off; this is going to be the shadow that we will add to the finished piece later. Now load the gray shape as a selection by right-clicking on its name in the layer palette and choosing the Alpha to Selection option from the menu. Then invert it by choosing Select>Invert. Next, change to the top white layer and delete the inverted selection by choosing the Clear option from the Edit menu. This trims the blurred shape. Now you should have three layers, similar to the illustration below:
3. In the next step, create a beveled object. Select the gray shape layer and in the filter menu chose Map>Bump Map. From the dialog box of the Bump Map drop-down menu choose the trimmed layer and adjust the Azimuth, Elevation, and Depth sliders to get a smooth beveled object, similar to the image below:
4. Now you can delete the trimmed layer if you want, or just turn its visibility off and drag it out of view. Next we create the actual plastic effect. This is the most important step and the one it took me the longest to figure out, though the process looks simple in retrospect. Create a new layer by clicking on the New Layer icon at the bottom of the Layer palette and move it above the beveled object layer. Fill the new layer with white. From the Filters menu chose Noise>Hurl and fill the entire white layer with noise. The amount of noise depends on the dimensions of the image, but as a starting point use the default value and adjust according to taste. Once the layer is filled with noise, choose Layer>Colours>Desaturate to remove the color values from the noise. Load the beveled object as a selection as you did before and choose Select>Shrink. In the dialog box enter a value of 5 to 8 pixels (jot this value down because you will use it later), then invert and clear the selection by following the steps described when you trimmed the blurred shape. You should have a slightly smaller noise shape above the beveled object. Next, choose Filters>Distorts>Emboss and apply some emboss effect to the noise shape. At this point, you should have a design similar to the following image:
5. Finally, duplicate the beveled layer by right-clicking on its name and choosing the Duplicate Layer option from the menu. Fill the duplicate object with white; this removes the beveled effect. Load the duplicate object as a selection one more time and shrink it as you did earlier by using the same values you applied when trimming the noise texture. Delete the selection by choosing Edit>Clear. This creates a white frame around the edge of the final work. Drag this layer beneath the beveled layer. At this point, the plastic material is basically done; we only have to set the transparency of the various layers. But before do this, create a circle on a new layer by using the elliptical selection tool in the tool box, fill it with black, and make this as the bottom layer. If you want to make the black object look fancier, stroke it with some color, as I did. This black object helps to adjust the proper opacity settings.
6. Now select the beveled layer and set its opacity in the layer palette somewhere between 75 to 80 percent. Next, select the noise layer and set its opacity anywhere between 10 and 15%. At this point you should have a plastic material similar to mine. To make your object look more professional, select the blurred layer you created earlier, turn on its visibility, invert its color to black by choosing Layer>Colours>Invert, then chose Layer>Transform>Offset. In the offset dialog box, set the x and y axis values to about 5 pixels and its layer opacity to 50%. I have placed white text on the top of the artwork just to make it look more complete, but this is not necessary. You can also create a background for the artwork if you want. I have used a standard gradient from the GIMP's gradient library.
That's it -- you're finished. Once you know how to create the plastic effect you can apply it to as many objects as you want, which can include buttons, logos, text, and the like. In the image below, I have created some variations of the plastic material just to give you an idea of the different ways the material can be used.