If a part has been produced satisfactorily and it goes wrong, something has changed. The principle is not to change conditions immediately. The cause should be identified and rectified.

Never adjust one condition to compensate for a failure or change in another condition. For example, if melt temperature has increased, don’t reduce speeds or pressures or mould temperature to compensate. Identify problem, that is to say reduce melt temperature to what it was before, by checking and replacing thermocouples, etc.
 

	Ask questions:
	what has changed ? what is fault? when did it start? how often does it occur? where is fault? is the fault randomly situated or always in same place? etc...
	Identify causes of defects:
	test, observe, conclude and study ‘history of faults’ injection speed: test screw speed: test back pressure: test melt temperature: test etc...
	machine: check heating of cylinder mould temperature control material cushion locking of mould etc...