A method of forming a shaped article wherein said article comprises thermoplastic polymer parts which are chemically bonded, the method comprising: assembling at least two non chemically bonded parts in a weld cavity of a tool to form an assembly of the at least two parts, wherein the at least two parts are thermoplastic polymer and said weld cavity is adapted to receive the assembly of at least two parts in the form of the shaped article; sealing the weld cavity of the tool; and melting the assembly of at least two parts to chemically bond the at least two parts together to form the shaped article; characterised in that: the shape of the weld cavity is matched to the outer circumference of the assembly of parts such that thermal expansion forces the thermoplastic polymer parts against the weld cavity surface and the pressure leads to full surface bonds between all parts; the tool forming the weld cavity comprises two halves forming a piston which fits into the bore of a bolster component and the plane of the split line between the two piston halves is parallel to the axis of the bore such that the pressure from the thermal expansion of the assembly of parts within the weld cavity is opposed by the surface of the bore preventing the weld cavity split line from opening and therefore preventing leakage of polymer from within the pressurised weld cavity volume; and the two piston components are made from a dissimilar metal with higher coefficient of thermal expansion than the bolster component ensuring that at the welding temperature the piston is larger in diameter than the bore diameter of the bolster component leading to an interference fit which forces the weld cavity split line to close at the weld temperature but which still allows for a clearance gap between piston and bore at room temperature for the tool to be assembled and disassembled. assembling at least two non chemically bonded parts in a weld cavity of a tool to form an assembly of the at least two parts, wherein the at least two parts are thermoplastic polymer and said weld cavity is adapted to receive the assembly of at least two parts in the form of the shaped article; sealing the weld cavity of the tool; and melting the assembly of at least two parts to chemically bond the at least two parts together to form the shaped article; characterised in that: the shape of the weld cavity is matched to the outer circumference of the assembly of parts such that thermal expansion forces the thermoplastic polymer parts against the weld cavity surface and the pressure leads to full surface bonds between all parts; the tool forming the weld cavity comprises two halves forming a piston which fits into the bore of a bolster component and the plane of the split line between the two piston halves is parallel to the axis of the bore such that the pressure from the thermal expansion of the assembly of parts within the weld cavity is opposed by the surface of the bore preventing the weld cavity split line from opening and therefore preventing leakage of polymer from within the pressurised weld cavity volume; and the two piston components are made from a dissimilar metal with higher coefficient of thermal expansion than the bolster component ensuring that at the welding temperature the piston is larger in diameter than the bore diameter of the bolster component leading to an interference fit which forces the weld cavity split line to close at the weld temperature but which still allows for a clearance gap between piston and bore at room temperature for the tool to be assembled and disassembled. the shape of the weld cavity is matched to the outer circumference of the assembly of parts such that thermal expansion forces the thermoplastic polymer parts against the weld cavity surface and the pressure leads to full surface bonds between all parts; the tool forming the weld cavity comprises two halves forming a piston which fits into the bore of a bolster component and the plane of the split line between the two piston halves is parallel to the axis of the bore such that the pressure from the thermal expansion of the assembly of parts within the weld cavity is opposed by the surface of the bore preventing the weld cavity split line from opening and therefore preventing leakage of polymer from within the pressurised weld cavity volume; and the two piston components are made from a dissimilar metal with higher coefficient of thermal expansion than the bolster component ensuring that at the welding temperature the piston is larger in diameter than the bore diameter of the bolster component leading to an interference fit which forces the weld cavity split line to close at the weld temperature but which still allows for a clearance gap between piston and bore at room temperature for the tool to be assembled and disassembled.