Initial Process Studies are important element of Production Part Approval Process (PPAP). These studies shows you manufacturing process is capable to produce parts meeting customer specifications or not.
It is a Statistical Process Study (SPC) conducted to check your process for stability and capability.
In this article, we will see the concept of Initial Process Studies in more detailed.
Let’s dive in…
What Are Initial Process Studies in PPAP?
Initial process studies are methods to check that your manufacturing processes can produced parts consistently within specification limit or not. It will help your company to meet customer requirement before actual mass production begin.
It is more focused on special characteristics, which is important for part fit, function and performance.
- The Initial studies focus on “special characteristics” by the customer or your company.
- You must do the agreement with customer on process capability before PPAP submission.
- The most important point is you should do the Measurement System Analysis (MSA) before doing SPC. To make sure your measurement system is accurate one.
Note: If there is no special characteristics, customers may ask for initial studies from supplier. You can do studies on other characteristics.
Why Initial Process Studies Conducted?
The goal of this studies is to confirm that your manufacturing processes will produce good quality parts consistently.
This includes:
- Understand the process variation.
- Analyze and predict that the process can meet specification in normal operation.
How to conduct Initial Process Studies?
1. Define the scope
Clearly define the purpose of your study. What you are going to evaluate from this study. Which process you need to check and which indices you want to calculate. Example Cp, Cpk or Pp, Ppk.
2. Select Critical Characteristics
Select the special characteristics you want to select for study. This characteristics should be special product or process characteristics which impacting product function or performance.
It is generally align with your customer.
3. Collect Data
Usually take PPAP samples and do the inspection of those samples by checking special characteristics dimension on parts.
The sample size is 10 subgroups of 5 parts each (50 measurements) or as agreed with customer.
4. Perform Statistical Analysis
Once you have the sample inspection data, calculate process capability indices:
- Cp and Cpk: Measure process capability during stable conditions.
- Pp and Ppk: Measure performance in short-term period.
Also use control charts for this data to check process is stable or not.
Initial Process Studies terms
- Cp (Process Capability Index): Measures how your process fit within specification limits, considering the variation tendency.
- Formula: Cp = (USL – LSL) / 6σ
- Cpk (Process Capability Index Adjusted): Measures how well process is centered within specification limits. It represent the location shift of your process.
- This conditions check when the process is stable.
- Formula: Cpk = Min[(USL – μ) / 3σ, (μ – LSL) / 3σ]
- Example: If Cpk > 1.33, your process is capable.
- Pp (Process Performance Index): Measures overall process performance, including variation.
- Formula: Pp = (USL – LSL) / 6σ
- Ppk (Process Performance Index Adjusted): Measures overall performance of the process, which includes all type of variation relative to the mean and specification limits.
- Formula: Ppk = Min[(USL – μ) / 3σ, (μ – LSL) / 3σ]
- It is useful when your process is not completely stable. This study typically done during new product development.
- Example: If Ppk > 1.67, your process is acceptable.
5. Analyze Results
Compare your results with the acceptance criteria as per industry standards.
Usually, the criteria is: Cpk ≥ 1.33 for most of the automotive applications.
After analyze the results you need to decide whether the process is stable, capable, or need improvement.
What will be acceptance Criteria for Initial Studies?
When you conduct initial studies, based on results you might know that, is it acceptable, conditional accepted or need improvement. Refer below acceptance criteria for your reference.
- Cpk/Ppk > 1.67: The process is good to go.
- 1.33 < Cpk/Ppk < 1.67: Process might be acceptable; get customer approval.
- Cpk/Ppk < 1.33: The process needs improvement. Inform to customer and submit a corrective action plan to improve it.
6. Document and Report Findings
Document all study results as a statistical process control study format/template.
Attached this document in your PPAP submission package and submit to customer for approval.
What are the problem you can face in Initial Studies?
Unstable Processes
- If your process shows irregular behavior, find out the root cause and fix the process behaviors. Make the process stable.
- Inform customer about the non-stable process and submit a action plan to make it stable.
Processes With Special Cases
- If your process has one-sided specifications (e.g., only an upper or lower limit) or data that does not follow a normal pattern, you need to take special approval from customer.
- Communicate and discuss with customer to modify tolerance or alternative solution.
What If Acceptance Criteria not met
- Inform customer before the PPAP submission.
- Propose a corrective action plan, which may include 100% inspection until the process is corrected and stable.
Measurement System Errors
- Ensure Measurement System Analysis (MSA) is conducted to check for data accuracy.
- Always do MSA before initial process studies like SPC.
Summary
Initial process studies shows us our process behavior, it indicates that whether your process produced consistently good parts or not.
Your process is stable or not. Once it is stable then you do inspection and find out Cpk/Ppk to check whether your process capability/process performance.
Your PPAP submission and approval done based on initial process study results. If it is acceptable then the customer will give you approval for process studies.
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