What happens to two thermoplastic materials during heat fusion?

During heat fusion, two thermoplastic materials are heated to a point where their temperature reaches the material's melting point. At this temperature, the polymers become soft and pliable, allowing them to flow and intermix. When the materials are brought together, they can bond as they cool down, resulting in a strong joint without the need for adhesives or additional bonding agents.

This process is critical in applications involving thermoplastics, as it enables the creation of strong, uniform joints essential in many manufacturing and assembly operations. The effectiveness of heat fusion lies in the ability of the thermoplastics to be repeatedly softened and reshaped, which adds versatility to their applications.

The other choices do not accurately represent the process of heat fusion. For instance, heating and separating would not lead to a bonding effect, while cooling and shaping imply a solidification process without fusion. Chemically altering the materials would indicate a chemical reaction, which is not characteristic of heat fusion processes in thermoplastics.