A device (100, 400) for polymerizing lactams in molds, comprising: a hopper (110, 410) for storing solid lactam (120, 420); a lactam feeding means (130, 430) for receiving the solid lactam (120, 420) from the hopper (110, 410), wherein said feeding means comprises a plasticizing screw (131, 431) for melting the lactam and simultaneously drive the molten lactam in a dosed manner towards a mixing chamber (160, 460); the device being characterized in that in use of the device (100, 400), said plasticizing screw (131, 431), either by means of an extrusion system or by means of an injection system, melts a fraction of the solid lactam placed in the hopper at the end of the plasticizing screw, and doses the molten lactam into the mixing chamber (160, 460) in a continuous way, the device further comprising: a first dosing means (140, 440) for feeding a liquid initiator, said initiator being liquid and stable at room temperature; a second dosing means (150, 450) for feeding a liquid activator, said activator being liquid and stable at room temperature; wherein said mixing chamber (160, 460) is configured to receive in a dosed manner the lactam, the initiator and the activator, from respectively said lactam feeding means (130, 430), said first dosing means (140, 440) and said second dosing means (150, 450), said mixing chamber (160, 460) having three separate inlets for receiving, separately, the lactam, the initiator and the activator, so as to allow said lactam, initiator and activator respectively to flow into the mixing chamber (160, 460), coming into contact with one another for the first time at the moment immediately prior to entering a mold (170, 470) located next to the chamber and so arranged for the polymerization reaction to take place inside the mold. a hopper (110, 410) for storing solid lactam (120, 420); a lactam feeding means (130, 430) for receiving the solid lactam (120, 420) from the hopper (110, 410), wherein said feeding means comprises a plasticizing screw (131, 431) for melting the lactam and simultaneously drive the molten lactam in a dosed manner towards a mixing chamber (160, 460); the device being characterized in that in use of the device (100, 400), said plasticizing screw (131, 431), either by means of an extrusion system or by means of an injection system, melts a fraction of the solid lactam placed in the hopper at the end of the plasticizing screw, and doses the molten lactam into the mixing chamber (160, 460) in a continuous way, the device further comprising: a first dosing means (140, 440) for feeding a liquid initiator, said initiator being liquid and stable at room temperature; a second dosing means (150, 450) for feeding a liquid activator, said activator being liquid and stable at room temperature; wherein said mixing chamber (160, 460) is configured to receive in a dosed manner the lactam, the initiator and the activator, from respectively said lactam feeding means (130, 430), said first dosing means (140, 440) and said second dosing means (150, 450), said mixing chamber (160, 460) having three separate inlets for receiving, separately, the lactam, the initiator and the activator, so as to allow said lactam, initiator and activator respectively to flow into the mixing chamber (160, 460), coming into contact with one another for the first time at the moment immediately prior to entering a mold (170, 470) located next to the chamber and so arranged for the polymerization reaction to take place inside the mold. a first dosing means (140, 440) for feeding a liquid initiator, said initiator being liquid and stable at room temperature; a second dosing means (150, 450) for feeding a liquid activator, said activator being liquid and stable at room temperature; wherein said mixing chamber (160, 460) is configured to receive in a dosed manner the lactam, the initiator and the activator, from respectively said lactam feeding means (130, 430), said first dosing means (140, 440) and said second dosing means (150, 450), said mixing chamber (160, 460) having three separate inlets for receiving, separately, the lactam, the initiator and the activator, so as to allow said lactam, initiator and activator respectively to flow into the mixing chamber (160, 460), coming into contact with one another for the first time at the moment immediately prior to entering a mold (170, 470) located next to the chamber and so arranged for the polymerization reaction to take place inside the mold.