QC Inspection : Why some Points are NOT Checked on All wood panels or plywood Samples
When it comes to random inspections, the inspector has to check a certain number of pieces. For example, if there are 1,000 pieces of Plywood in the whole batch, he/she has to check 200 pieces (according to the customers’ requested ).
If the inspection was properly prepared, the inspector has certain checkpoints to follow. However, does it mean that these points have to be checked on all samples (in my earlier example, on 200 pieces)?
This is usually not realistic.
Let’s take an extreme example: an inspection of UV prefinished Plywood. Doing a full UV sheen check on 1 piece could take an entire 10 minutes. So it is not realistic to check all Sheens on all samples, in this case.
Very often there are checkpoints that take more than a few seconds per piece. If the inspector checks all these points on the whole sample size, it can easily get the workload from 1 man-day up to 3 man-days. This is quite common.
To drive my point home, here are a couple of examples:
- Checking the sizes and thickness of the UV Plywood — it usually makes sense to measure one piece of plywood, and to have a quick look at the others (“are they roughly the same size?”).
- Abusing a Melamine Plywood for 15 minutes to see if it breaks or gets delamination easily — do this on 200 pieces of samples, and just this one test will take 3,000 minutes, or 50 hours.
So, in my mind, the logic should be as follows:
- What we can check very fast(in the case of your product: visual defects, sizes like lengths and widths and diagonal tolerance ,Moisture content,thickness tolerance is sanded by calibrated sander ,the core construction and how many plies of the inside core …) is checked on the whole sample size.
- What we can’t check very fast is divided in two categories:
2.a What is very important to the buyer (“critical to quality”), doesn’t take a very long time to check, and/or might vary substantially from one sample to the next due to the production process: can be checked on the whole sample size but it might increase the workload substantially.For example ,the face/back wood veneer grade of the plywood like color variation ,mineral streaks ,knot holes …. ,can be various ,so need to check carefully piece by piece .
2.b Other checkpoints: to be checked on a smaller sample size.For examples the manufacturing defects like open splits and repaired holes ,telegraphing …
A few more remarks:
- In the 2.a case, any problem is classified as a defect, just the way a visual defect is counted. It makes sense since all inspection samples were checked for this point .
- In the 2.b case, let’s say a point is checked on 5 pieces and one problem is found on 1 of the 5 samples. The inspector can simply consider this problem as critical, which means the checkpoint is failed. And then the buyer (or the quality manager) decides whether it is critical or not.
- This is a bit crude but that’s the way most inspection firms have been doing for a long time. The alternative would be to inspect all points on all samples, but usually that’s out of the client’s budget.
So ,the inspectors’s QC reports to the clients should be detailed and objectively .
1.How many pieces randomly inspected .
2.HOw many pieces rejected ,and what are the major quality problem to let you reject them .,
3,.How many pieces can meet the quality requirements
I have experienced some QC ,they sent reports to the clients with all bad materials info ,nothing good .One piece of plywood has only one visual defect but not affect the final quality,Then rejected .
The second piece ,with all the major natural defects like pin knots ,then rejected again ,because the customers don’t want pin knots .
The third piece of plywood with mineral streaks ,then reject again .
The fourth piece of plywood with color variation,then rejected again ,as the grading rules not accept color variation .
Means one piece of the plywood has only one visual defect ,then rejected .
We all know that the plywood are made of different wood core veneer and face veneers from different logs ,can not be the uniform color and natural defects or wood grain .BUt can be 90% ok the uniform .
If every piece of plywood rejected by only one or two visual defects ,how you make plywood ?