The relationship between PFMEA and the Control Plan

The essential relationship is straightforward: the Control Plan turns the measures for addressing risks identified in PFMEA into specific controls. During a manufacturing process audit, we verify how the controls described in the Control Plan are implemented on the production line and how they address the risks identified in PFMEA.

Consider the following points to establish an effective link between PFMEA and the Control Plan:

1. Use the same process flow diagram

PFMEA and the Control Plan should use the same established manufacturing process flow diagram. List the activities in each operation to ensure that the information is complete and can be developed and analysed as required. In practice, some PFMEAs and Control Plans omit work steps or product and process characteristics that require control. This makes manufacturing process control incomplete and less effective.

2. Match each PFMEA risk to controls in the Control Plan

Risks identified in PFMEA need to be addressed through the controls specified in the Control Plan. Without this link, the process cannot be operated optimally using the PFMEA and Control Plan approach.

For example, an injection moulding process may have an identified risk of incorrect injection pressure, leading to a short shot — an incompletely filled moulded part. The PFMEA has identified and analysed this risk. When designing the production line, the risk must be addressed through the machine or line design. The Control Plan records the approach under “Evaluation/Measurement Technique” and “Control Method”. These fields should explain how injection pressure will be controlled to the specified requirements to prevent short shots.

This example illustrates why PFMEA and the Control Plan must be linked so that risks are addressed appropriately and effectively.

Effective risk control helps the manufacturing process achieve its intended overall performance objectives within TPM.

3. Control special and critical characteristics

When preparing PFMEA and the Control Plan, use customer requirements — including drawings, related documents, instructions and customer-specific requirements (CSRs) — together with the organization’s own manufacturing experience to identify the special and critical characteristics to be managed. Establish controls for these characteristics. In principle, identify the characteristics and analyse their risks in PFMEA first, then transfer them into the Control Plan for ongoing control.

The information in PFMEA and the Control Plan must therefore be consistent.

4. Set an appropriate sampling frequency

The sampling frequency stated in the Control Plan must reflect the risk identified in PFMEA.

For example, consider a vehicle headlamp assembly and adjustment process that depends on operator skills. The shape and position of the light beam are safety requirements and special product characteristics. The adjustment operator’s knowledge and skills are important process factors. The process operates over three shifts, but the Control Plan requires sampling only at the start and end of the day.

This frequency does not adequately address the PFMEA risk arising from differences in operators’ knowledge and adjustment skills. Sampling by shift is needed in this example to detect defects or process variation associated with each shift’s team.

Sampling frequency should therefore be appropriate and sufficiently sensitive to support risk detection and reduction. Although this is often overlooked, aligning frequency with risk can make a substantial difference.

5. Set an appropriate sample size

Smaller dimensional shifts generally require more samples to detect. Suppose a tool breaks and causes a large dimensional change: relatively few samples may reveal the shift. If the tool wears slowly and causes only small dimensional changes, a much larger sample may be needed to detect them.

The nature of each process determines the risks to identify and analyse and the responses to develop. Linking PFMEA and the Control Plan helps organizations design and operate processes effectively. PFMEA is like a guiding light that reveals hazards on the road; the Control Plan is like an experienced driver’s responses to those hazards, helping the vehicle reach its destination safely.

During an IATF 16949 audit, auditors commonly examine PFMEA and the Control Plan to verify the relationship between them. Organizations should therefore check this alignment themselves.

With the guiding light and the experienced driver working together, the manufacturing journey can become more effective.