A method of modifying a thermoplastic polymer powder. A suspension of thermoplastic polymer powder and reinforcement elements in a liquid is heated within a reaction chamber so that the thermoplastic polymer powder becomes softened and the reinforcement elements migrate into the softened thermoplastic polymer powder to form modified thermoplastic polymer powder. The modified thermoplastic polymer powder is then separated from the liquid. The reinforcement elements have an electromagnetic moment and are subjected to an electromagnetic field as the suspension is heated. The interaction of the electromagnetic field with the electromagnetic moment of the reinforcement elements causes the reinforcement elements to become aligned with the electromagnetic field before they migrate into the powder.