Sources of failure:

• false calculation of the welding pressure
• machine defect
• machine not calibrated
• chosen welding pressure cannot be maintained (e.g. due to leaky piston linings)

Thus, there must be a certain layer thickness of the residual cast after the welding procedure. It is assumed that the best strength values of the welding seam can be achieved with a melt displacement of about 75 per cent[Ehren04]. Layer thickness values of cast of 0.2 to 0.4 x wall thickness are typical of semi-crystalline thermoplasts[Ehren04].