Plastics come in two main varieties: thermosets and thermoplastics.
Thermoset plastics start as two or more separate materials, typically liquids, that are mixed together to form a new solid plastic. Thermosets cannot be reformed after they are combined because a chemical reaction has occurred that creates a new, polymer chain. Examples of thermosets include epoxies, silicones, and polyurethanes. We typically use thermosets as the casting materials. For example, our Clear Casting Plastic is a two part epoxy that is mixed together to form a clear plastic casting.
Thermoplastics can be reformed by heat. The polymeric chains that make up the material disconnect with heat and reform when cooled. This is how we are able to recycle much of our plastic wastes. We are all aware of many thermoplastics in our daily lives such as polypropylenes, polystyrenes, polyvinyl, and nylons. Add heat and the plastic softens so that it can be formed and reformed. Most thermoplastics melt into a sticky, thick polymer that can be squeezed or pressed into shape under different heating conditions. For example, you can take a yogurt container and melt it down to make a new shape by pressing it while keeping it hot.
By varying the composition, you can make thermoplastics with very different properties. You can vary the hardness or how rubbery the plastic is by varying the plasticizer used while making the plastic. The plasticizer acts like a lubricant at the molecular level. If we continue to use the spaghetti analogy, imagine the spaghetti with butter. It will not be sticky and it will slide past each other.
In plastics, a good example is polyvinyl chloride, PVC, which can be hard and almost brittle such as what you use in your homes for pipes and tubing. Or, you can add more plasticizer to the PVC to make it into a rubber such as that used for rubber fishing lures (fake worms).