Most injection molds are of the “cold runner” variety (two-plate and three-plate designs), or “hot runner”. The major functional difference between the two types is that the parts produced by the cold runner have a sprue and a runner. These typically must be removed to finish the part.
Molds typically are constructed of aluminum or tempered steel. Financial considerations determine which material is chosen. Steel molds tend to cost more to construct, but this higher expense can be offset by the far greater number of parts a steel mold can endure to produce. Aluminium molds can cost substantially less, and when designed and machined with modern computerized equipment, can be economical for molding dozens or even hundreds of parts.