Why the base of an electric iron is made thick and heavy?
Do you mean the steam iron for flattening cloth? Not exactly sure how the structure is of the internal parts; but the iron is designed to sit or stand on its base, so the center of gravity idea seems to be applied. You generally rest the iron this way to prevent scorching either a garment, rag, or the ironing board.
If you mean the ironing surface itself, it is so that it can hold more heat and thus maintain a more even temp while in use.
It also reduces the chances of getting hot spots
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