Final answer:
As materials cool, they contract and can experience thermal stress which might cause damage; hence, it is essential not to move parts as they cool through the paste range to prevent this risk.
Step-by-step explanation:
Thermal Stresses During Cooling
Parts must not be moved as they cool through the paste range because they can experience thermal stress due to differential contraction. This means that different parts of an object may contract at different rates when cooling, leading to internal tensions that could deform or damage the material. For instance, glass cooking pans can crack if cooled rapidly or unevenly, which creates such stresses. Pyrex® is less likely to crack because it has a small coefficient of thermal expansion, thus it doesn't experience significant differential contraction. Similarly, railroad tracks and roadways need expansion joints to accommodate temperature changes, as they can buckle due to thermal stress on hot days. Moreover, biological cells can burst when frozen because water expands when it turns into ice, altering taste and texture. Overall, preventing movement during the cooling process can be crucial in avoiding damage caused by thermal stress.