The cavities of the moulds must be realised in a smaller scale with respect to the final dimensions, and this reduction is to be circulated with precision on the basis of the type of material used. An important rule-confirmed by the experience of many years is that of limiting the number of holes for each single injection, especially for the materials with low materials with low specific weights, for the purpose of improving the dosing accuracy of material in the single cavities, the weight of the moulding and the dimensional control.