Final answer:
The main disadvantage of having a 3 mm Al Equivalent is the higher cost, attributed to complex manufacturing processes and the superior qualities of materials like aluminosilicate glass and boron fibers.
Step-by-step explanation:
When considering the disadvantage of having a 3 mm Al Equivalent, which refers to a material that has the same x-ray attenuation as 3 mm of aluminum, the main disadvantage is likely to be higher cost. Materials like aluminosilicate glass and boron fibers have specific properties that can increase manufacturing expenses. Aluminosilicate glass is more difficult to melt and shape compared to borosilicate glass, but it offers excellent thermal resistance and durability. On the other hand, boron fibers, which are stronger and lighter than metallic aluminum, require time-consuming and costly production methods due to the lack of good polymer precursors for elemental boron or boron nitride. Therefore, the cost is often a prohibiting factor, especially when other, less costly materials, like graphite fibers, can fulfill similar functions without the need for the particular advantages boron offers, such as greater resistance to oxidation.
In this context, while increased weight and decreased flexibility might also be true for some Al Equivalent materials, these attributes are not directly mentioned in the references provided, and therefore, the most correct answer based on the given information is higher cost.