It is much harder to trim metal parts than plastic parts. Plastic parts can be injected finely without flash, trimming the parting lines is simple and quiet which can be done using a knife because plastic is relatively soft. Casting of metal parts creates flashes around the parting line, trimming the parting lines and flashes are very labour intensive and noisy because metal is hard. When the suspension of the model car is made of metal components, every connecting arm and rod need to be manually trimmed by a file. A push-rod suspension in the Pagani Zonda has to be made with metal parts because plastic parts are too soft to simulate the actuation of the suspension movement. Therefore, the cost of making the model car's suspension in metal is much higher than using plastic. Low cost mass market models will avoid using metal parts other than the body due to cost reason.