Structural foam components need to be at least 6mm in thickness for the cellular structure of the core to develop any significant density reduction. The process is divided into two major categories. These are the processes that involve the injection of gas into the polymer melt and those that involve a chemical blowing agent.
Making good bread or cakes involves creating a cellular structure by the decomposition of some of the ingredients - yeast or baking powder into carbon dioxide. In a very similar way a group of chemicals known as azodicarbonomides are used to create a cellular structure in thermoplastics.
With bread or cake making the whole process takes place at atmospheric pressure in a relatively low viscosity fluid. In structural foam moulding the pressures are much higher and the viscosity of a polymer melt are greater too - but the principles are the same so keep it in mind.
The azodicarbonomide (ADC) is in the form of a fine granular powder and is often referred to as a 'blowing agent'.