Anyone could help me to have a standard computation of tolerance for product weight of plastic component using 3 sigma or related method?
I am not sure I understand your question but if you are trying to determine what tolerance you would need for your current process to achieve +/- 3 sigma capability, try this.
First you must determine what your current standard deviation is. Do a run chart of at least 30 pieces and calculate the mean and standard deviation. Calculate +/- 3 standard deviation from the mean. This will show you the tolerance needed to achieve a 3 sigma process.
You should consider though that this requires perfect centering of the process and allows for no process swing which would be normal over time. You could allow for that swing by adding another 1 1/2 standard deviation to each side of the mean to establish your tolerance.
I hope this helps but feel free to ask again if your question is still unanswered.
Dave