In your message dated Thursday 26, September 1996 you wrote :
> TechNetters, here is another question about SPC.
> 
> Is anyone using SPC at the board image printing operation?  Our people 
> here have been, at times and in an undisciplined way, been taking energy 
> data from the printer.  The gauge is put at 9 points in a grid and energy 
> delivered thus measured.  (These 9 measurements are made at approximately 
> the same time, at the start of a shift, say.)  To me, this approach 
> seems to lend itself to SPC.  Presumably, the measurements would be done 
> on a regular schedule.
> 
> Anyone care to comment or correspond?   Lou Hart  Quality Assurance
> Compunetics 412-858-6117

Hello Lou

This check does not need to be done very often. Once the characteristics of the 
unit have been plotted and the step wedge difference in the corners and the 
centre of the exposure area have been established, they are not going to change 
significantly in a short time. You might want to recheck every 3 months but we 
never find any change in the distribution.

All that needs to be monitored daily is the exposure step achieved for the light 
units set, and the time it takes to achieve those light units. Time will 
increase as the lamps age and although the step may still be within limits, the 
image quality will deteriorate as the time increases.

Daily plot the exposure step, and the time in seconds. Keep the step within 
recommended limits, and set a maximum time at which to change the lamp(s). This 
should be not more than 50% longer than new lamps ideally, but you could stretch 
them for longer depending on the type of work you are doing.
 
Paul Gould
EMail [log in to unmask]

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