Process for packaging substances in the molten state, which substances are sticky at room temperature or treatment temperature, of the type in which the sticky substance is cast into a tray-mould lined beforehand with a film of plastic material which is non-sticky at room temperature or treatment temperature and compatible in the fluid state with said sticky substance, comprising the following stages: a) providing a plurality of tray-moulds apt to receive the cast of a sticky substance at the molten state, said tray-moulds having a plurality of through-holes passing through at least part of their walls and being arranged in adjacent subsequent rows to form a continuous tray conveyor belt; b) rapidly lining the internal walls of the tray-moulds of one or more trays with a first thin and easily formable film of said non-sticky plastic material, causing it to perfectly adhere to the walls of the mould by connecting the lower external part of the moulds to a vacuum source; c) casting into the thus-lined tray-moulds, under pressure and with a non-turbulent flow, a predetermined amount of sticky substance at the molten state; d) allowing the cooling of the free surface of the sticky substance, during the step-like progress of the conveyor belt, until obtaining the stabilisation of said surface; e) covering said free surface of the sticky substance with a non-sticky material compatible in the fluid state with said substance; and f) welding said non-sticky material to said first film; characterised in that it furthermore comprises, in the above-said stage b), to modulate the degree of vacuum during the opening transient of the connection of the lower external part of the mould to a source of vacuum by the additional steps of: g) detecting the time necessary to reach a first set degree of vacuum, starting from the beginning of the above-said opening transient of the connection to the source of vacuum; h) detecting the value of the degree of vacuum in the lower part of the mould, after a set period of time from the closing of the connection to the source of vacuum; i) giving consent to the start of the above-said stage c) only if the time measured in step g) falls within a set range of acceptability and the degree of vacuum measured in stage h) is below a set maximum value. a) providing a plurality of tray-moulds apt to receive the cast of a sticky substance at the molten state, said tray-moulds having a plurality of through-holes passing through at least part of their walls and being arranged in adjacent subsequent rows to form a continuous tray conveyor belt; b) rapidly lining the internal walls of the tray-moulds of one or more trays with a first thin and easily formable film of said non-sticky plastic material, causing it to perfectly adhere to the walls of the mould by connecting the lower external part of the moulds to a vacuum source; c) casting into the thus-lined tray-moulds, under pressure and with a non-turbulent flow, a predetermined amount of sticky substance at the molten state; d) allowing the cooling of the free surface of the sticky substance, during the step-like progress of the conveyor belt, until obtaining the stabilisation of said surface; e) covering said free surface of the sticky substance with a non-sticky material compatible in the fluid state with said substance; and f) welding said non-sticky material to said first film; g) detecting the time necessary to reach a first set degree of vacuum, starting from the beginning of the above-said opening transient of the connection to the source of vacuum; h) detecting the value of the degree of vacuum in the lower part of the mould, after a set period of time from the closing of the connection to the source of vacuum; i) giving consent to the start of the above-said stage c) only if the time measured in step g) falls within a set range of acceptability and the degree of vacuum measured in stage h) is below a set maximum value.