an improved method is described for thermally bonding contacting pieces of a thermoplastic material provided in a variable volume confined space surrounded by a combustible gas . the combustible gas is ignited and explosion diffused externally of the pieces of thermoplastic material and thus heats the pieces without damage sufficiently to produce interbonding when the heated pieces are compacted in the confined space . the heated pieces are compacted isostatically or non - isostatically using various apparatus which reduce the volume of the confined space . preferably a moveable piston is provided in a cylinder to form the confined space for the pieces . the driving means for the piston can be a combustible gas which is ignited or non - combustible gas under pressure or a resilient means or other mechanical means . the products of the method are in the form of a porous mass composed of the interbonded pieces which are undamaged by the explosion .