Process control to specification
The success of a process is measured by looking at the output; how many defects are produced and how much the product costs to make. In order to achieve success, control must be exercised ideally during the processing, not just at the end. This is the case of any human activity, from hammering in a nail to operating a manufacturing plant.
Robust control of critical process parameters within a manufacturing process will obtain improvements both in product consistency and in the efficiency of the process.
In a food manufacturing environment, we endeavour to monitor process variables from raw material intake to finished goods. This is to optimise yield and minimise ingredient waste and over-packaging in order to manage product cost.
It is important to start with assessing and defining the critical control parameters which could have a detrimental impact on process performance if not controlled through tracking and monitoring in order to identify source of variation.
Statistical Process Control (SPC)
Not reacting appropriately to the type of variation present in a process can seriously impact customer satisfaction and the amount of variation and defects; therefore increasing costs. Appropriately reacting to the source of variation in a process provides the correct economic balance between overreacting and under-reacting to variation from a process.
Hence, training operators in process control charts is a crucial step and involves trending parameters over time with comparison to their normal operating ranges (statistically determined Action and Warning Limits). The process parameters are then monitored to determine when the process behaves differently to that expected (Out of Control signals). Abnormal events or new trends are instantly recognisable enabling the appropriate action to be taken (process control). Monitoring the process does not improve poor process performance or ensure control. Control is only achieved if the appropriate action is taken in response to the control chart signals. The objective of statistical process control (SPC) is not simply monitoring to ensure conformity to specification but of continuous improvement of the process.
What happens, if we don’t use SPC?
The operations team could fail to properly distinguish between Common and Special Cause Variation which will lead to two possible consequences:
- A) OVERCONTROL: Investigate all incidences of variation as “special cause”
- Waste of resources
- Discouraging: You’ll seldom find a “root cause”
- May make unneeded adjustments…cause more variation!
- UNDERCONTROL: Fail to recognize special cause variation and leave unchecked
- Process variation and control will deteriorate over time
- Benefits of any previous improvement project “come unraveled”
- Time to plan another process improvement project!
IMPLEMENTATION CONSIDERATIONS
The introduction of SPC is as much a behavioural change as a technical one. Therefore, as the Change Agent, one will need to visualise and articulate the changes that need to be made to successfully implement a sustainable and effective SPC program.
In the current state it is possible that staff are adjusting the process when they think that it needs adjusting, NOT when it MUST be adjusted based upon a true signal of change. Intervention without SPC (“tinkering”) can increase variation in the process which can then lead to further intervention thus creating a doom loop. So, behavioural change will be required. Instead of tinkering, staff will need to listen to the voice of the process by letting the control charts tell them when to adjust the process. If this behavioural change is not achieved, SPC will not deliver sustainable performance improvement. It is this behavioural change that opens the gate to true process improvement.
Control charts do not replace the knowledge of local process experts. They just guide staff to make adjustments when really needed. This may be likened to the SPC chart being the traffic lights, whilst the process experts are driving the car; they control the process but the chart provides the signal for intervention when required.
Successful introduction of SPC is a collaboration between a Change Agent, process experts, line staff and area leaders. Consideration should be given to determine the best means of engaging all relevant parties at the earliest opportunity, e.g. line side meetings.
Training in the use of control charts with decision trees and collection of knowledge gained by root cause investigations related to out of control signals will help embed the use of SPC and also support a continuous improvement culture. If used correctly, SPC provides a strong foundation for enhanced process understanding.
It is important to transfer ownership of the SPC activity to the process staff as early as possible. This can be achieved by including process staff in the implementation and involving them in cascaded training sessions. Process staff should be encouraged to contribute to the selection of the parameters to monitor so that these are of meaning to them.
Potential benefits of introducing SPC
Enhancing the Process Understanding Control and Capability
- Improves Process Understanding
- Provides a method for keeping processes in control.
- Leads to predictable processes (risk reduction, flow, quality).
Raising People Engagement and Capability
- Provides employee involvement
- Supports embedding an improvement mindset and culture
Delivering Financial Benefits
- Reduces/eliminates the need for inspection.
- Increases competitive advantage through:
- Improved profitability.
- Reduced product cost.
- Improves Productivity.
- Reduces hidden factory waste.
Example of SPC – Average Quantity System (AQS)
A practical example of process control is the control of giveaway and over-packaging. When operating with a net weight method of measurement, giveaway of goods is unavoidable and can sometime result in a significant additional cost of materials for businesses. Hence process control in pack weight is paramount in order to not increase product cost. Process variability and giveaway can range from 1-6% depending on variably in production.
AQS (Average Quantity System) is a solution to reduce the giveaway by taking into consideration the difficulty to systematically reach an accurate measurement, while providing an assurance to the consumer that goods are in a tolerable range, defined by the AQS requirements. It is applicable for all goods with a ‘constant nominal content’, which means any goods sold by measure (weight, volume, length or area) or count (number of items). Despite increasing the flexibility and compliance of prepacked goods, AQS follows strict regulations and packers/manufacturers need to prove compliance in case of inspection from the NMI (National Measurement Institute). Therefore, it is vital that AQS released products have a robust process control method in place order to be compliant with the required rules and regulations. The logic applied to AQS can be employed across the board to generate other savings, such as product make up giveaway, i.e. protein control, moisture levels etc.
In summary, quality-enhancing measures are only created by informed decisions. If a data management system exists that gives you a picture of the current situation, you can optimise your product and process quality and thus also the company’s success.
You can only repair what you measure.