The plastic that's used in Shrinky Dinks is called polystyrene. You'll see it frequently in cafeterias – it's used for the clear lids that protect sandwiches and other foods. If you dig through your recycling bin, you'll almost certainly come up with recycled plastic No. 6. That's polystyrene, too. In fact, by simply cutting apart these containers, sanding and coloring them, they'll work just like the plastics in Shrinky Dinks.
All plastics are polymers, which are long chains of repeating molecules. Polystyrene, like so many plastics, is made from petroleum-based chemistry.
Polystyrene has some distinct characteristics. At room temperature, it's rigid, lightweight and transparent, making it well-suited for food containers and similar products. When warmed to temperatures above 375 degrees Fahrenheit (190 degrees Celsius), polystyrene melts and can easily be manipulated into many shapes.
To make sheets of plastics like those needed for Shrinky Dinks, manufacturers use an extrusion process. In short, that means they load all of the ingredients for polystyrene into a heated mixing drum, which then forces the pliable, rubbery plastic through a slot die, creating sheets that are 0.09 inches (2.3 millimeters) thick.
While the sheets of 0.09-inch (2.3-millimeter) plastic are still warm, the machine feeds them through a roller that compresses the sheets down to 0.01 inches (0.3 millimeters) in thickness. And here's where the true magic of the process happens. Those rollers are cooled, meaning they quickly chill the freshly compressed plastic, in essence "freezing" the plastic molecules into their stretched and flattened form.
What consumers receive, then, are sheets of polymers just begging to return to their original extruded 0.09-inch (2.3-millimeter) shape. When heated in ovens, that's exactly what happens. Warmed to a more flexible condition, the molecules realign themselves and return to a thicker sheet that's two-thirds smaller. It's as if the polystyrene remembers its original form. That's why some people refer to polystyrene as memory plastic.
Not all plastics shrink evenly, and that's partly because of production conditions. In stretching the plastics, manufacturers can pull them in just one direction (called axially oriented) or two directions (biaxially oriented). When warmed, polymers that are axially oriented shrink unevenly. Those that are biaxially oriented, however, return to their original shape without much distortion. Shrinky Dinks require biaxially oriented polystyrene – otherwise, when shrunk, your cute cartoon characters might take on nightmarish proportions.