It’s not the plastic your trying to shrink, it’s the coin. Or use some nail clippers to score the rim of the capsule on opposite sides, careful not to mark the coin, then place it face doing on a cloth and grip the edges of the capsule with your nails, 90 degrees opposite the scores. Place your thumbs on the back of the capsule and try to crack it apart. But I have to ask, you say it’s a silver round, correct? It really doesn’t have numismatic value so why worry about marking the silver?