Final answer:
In straight vacuum forming, the major downside of using deep cavities is poor material distribution, where the plastic sheet may become too thin in the deepest sections, affecting the part's structural integrity.
Step-by-step explanation:
The major disadvantage of straight vacuum forming using deep cavities is C) Poor Material Distribution. When using this method, the plastic sheet is stretched into deep cavities, which can lead to thinning of the material at the deepest points. This uneven material distribution can compromise the structural integrity of the formed part. Factors such as uneven heating, high energy consumption, and long cycle times can also be concerns but do not represent the primary disadvantage which is the risk of inadequate material distribution where it is needed most.