Trimming is the most labour intensive process in die cast model manufacturing. Slots, vents and window openings must be trimmed bit by bit thoroughly by hands because zinc metal die cast can never be cast perfectly, there are always metal flashes and burrs around the opening and they must be removed manually by a metal file. Only plastic injection can be made perfectly with nice and clean opening. Zinc metal is much harder than plastic; the trimming is a slow process which requires a lot of labour. This is the main difference between a plastic model and a die cast model. Low price mass market product avoid making slot, vent and have only a few window openings; the maker normally uses a automatic tumbler with ceramic beads to trim the sharp edges, this process saves a lot of labour cost but it cannot be applied on model car body with fine details.