A method of making a homogeneous mixture of polyolefin solids and carbon solids without melting the polyolefin solids during the making The method comprises applying acoustic energy at a frequency of from 20 to 100 hertz to a heterogeneous mixture comprising the polyolefin solids and the carbon solids for a period of time sufficient to substantially intermix the polyolefin solids and the carbon solids together while maintaining temperature of the heterogeneous mixture below the melting temperature of the polyolefin solids, thereby making the homogeneous mixture without melting the polyolefin solids.