Injection molded parts - Inspection drawing and Inspection plans

[Jeff Patrick]

In a debate and looking for feedback
We mold plastic parts, we have an inspection drawing that we created from the mold designed drawing indicating control dimensions that will be monitored.
We create inspection plans to match this drawing. It has been standard practice that the drawing and the plan match.
If we have a tolerance of ± .010" and we inspect a part at 1st piece inspection and it's on the low end of the allotted tolerance there is a likely hood that the part will shrink out of dimension. (Matl PP). We want to tighten the tolerance on the inspection plan without changing the inspection drawing. Is this bad practice?
Also is there anything I could reference (Juran's, Quality Tech handbook?)

 

[Miner]

You are describing Guard Band tolerances. These are used when you need to guard against accepting bad product due to measurement error. You can use these on your inspection plan and call them guard band tolerances to prevent confusion over the drawing. I recommend using them on the lower limit only if your concern is limited to shrinkage. You may want to limit their use to 1st piece, and use the drawing dimensions after a specified time after the part was molded.

 

[Jeff Patrick]

You are describing Guard Band tolerances. These are used when you need to guard against accepting bad product due to measurement error. You can use these on your inspection plan and call them guard band tolerances to prevent confusion over the drawing. I recommend using them on the lower limit only if your concern is limited to shrinkage. You may want to limit their use to 1st piece, and use the drawing dimensions after a specified time after the part was molded.

Thank you for the response and recommendation. I have not heard of guard tolerances but it's exactly what it is. Tightened tolerance limits.
Much appreciated.

 

[Golfman25]

I assume you have a two step inspection process -- before and after shrinkage. So you want to keep the ultimate tolerances available for the after shrinkage inspection. Like Miner said, some type of tighter tolerance before shrinkage is doable.

 

[Ed-Zhang]

once I have the same question: how to do the FAI, it if the spec is 10+/-0.1（just for instance, and the first production is 9.91, should we proceed
or should we adjust, if adjust, to which level can we continue, 9.94? 9.96? But I didn't get an official answer.
and what my previous company (an CNC manufacture) did is to do 1+3, it mean we inspect 4 parts, if these 4parts all meet the spec (whatever up spec or low spec), we continue...
before this, during NPI or trial run phase, we have made CPK calculate to make sure that the process capability is enough (like >1.33 or >1.67) and also do SPC.