Our Arro-Tag IM-31 ink marker is a very agressive ink made specifically for marking polyurethane tags permanently. The biggest use for them is in the animal agriculture industry. Farmers use our pens to write on the tags that are then attached to the animals. The ink deeply penetrates the tag and becomes a part of the tag. Through cold, hot, rain, or snow, the mark will always be visible on that tag. You will never get it out.