Wednesday 15 August 2012


Leveraging Six Sigma with industrial engineering tools in crateless retort production


M. M. BUNCE, L. WANG and B. BIDANDA


Abstract : This paper integrates Six Sigma objectives/concepts and industrial engineering tools within a quality framework, which are used to improve damaged can claims on a crateless retort production system . An overview of the canning industry and crateless retort processing is provided. Three experiments and a series of analysis tools are discussed. The root-cause analysis showed a correlation between can damage and can fill weight using design of experiments (DOE)-based Six Sigma techniques. Manufacturing system improvement projects and the implementation of a closed-loop control system are discussed as the measures used to continuously improve damaged can claims.
Introduction : Six Sigma has been embraced by manufacturing companies not only for its robust tool set but also because of its well-defined application methodology, called DMAIC (define, measure, analyse, improve, and control). Industrial engineering is a discipline that focuses on efficiency, optimiza-tion, and reduction of waste in operations. Leveraging Six Sigma concepts with industrial engineering tools can provide the shop floor engineer with a robust and rich tool set along with well defined objectives.

A process is said to be Six Sigma when six times the square root of the variation is less than the control range. Today, Six Sigma is regarded not only as a process capability equation, but also as a business philosophy for reducing process variation. The term Six Sigma has come to mean a problem-solving methodology that companies must embrace in order to eliminate variation and consequently improve quality in areas that are critical to their customers.

Canneries use a thermal processing manufacturing system, known as retort, in the production of preserved food. Retort processing is defined as heating foods prone to microbial spoilage in hermetically sealed containers to extend their shelf life . Simply put, this means that canned foods are cooked in a sealed can to kill harmful bacteria. Canneries have traditionally used retort processing, but recently manufacturers have developed an alternative, called ‘crateless retort’, that represents a paradigm shift in the process. In traditional systems, cans are conveyed into crates and loaded into a retort vessel. In crateless retort, cans are conveyed directly into a water-filled retort vessel; this water cushion prevents the cans from material handling damage. Crateless retort is desirable because it reduces overall throughput time. However, crateless retort systems need a more complex control system. Its numerous production variables must be closely controlled in order to maintain acceptable quality levels.


To know more about the process see the video on the site:
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The corporate management team identified this improvement project due to a low level of customer satisfaction and unacceptable levels of damaged cans. Based on management experience with different quality improvement approaches, a Six Sigma team was chosen and a project charter was established (see table 1) with a clear goal to reduce damage claims by 50%. The Six Sigma team was inter-departmental, led by trained quality managers who were empowered to make both technical and financial decisions.
2.   Defining the Problem
The first step in the Six Sigma process is to define the effects of the problem and also to commit a project team to focus on customer requirements. A project charter or contract is a useful Six Sigma tool for committing a project team and translating the voice of the customer into project goals.Likewise, supplier input process output customer (SIPOC) charts provide a clear picture of the variables in a process, the boundaries of improvement efforts, and the scope of a project.

A SIPOC diagram typically helps establish the bounds or scope of the problem. It is most common to build a SIPOC chart by working backwards (i.e. by first identifying the customers of the project, then the outputs, etc.). The SIPOC chart for the crateless retort system is shown in table 2. The customer requirement in this application was to eliminate waste from the crateless retort system. The waste comes in the form of damaged can claims that cost the plant money and time to produce. Although it was relatively easy to measure the cost of damaged cans, it is not as easy to measure the clean up efforts that are required to dispose of the damaged cans. Both are considered forms of waste.

In the filling process, cans are depalletized and conveyed into a filling machine in one piece flow. The can is then filled with the edible product. The can filling includes any water or oil that is used to help preservation. The net weight is measured as the weight of the filling in each can. The headspace of the can is measured as the distance between the open can surface and the can filling. FDA specifications guide the net weight and headspace of each can along with the can seam and processing.

The filled metal containers are then sealed with a seaming machine that puts the lid on the can. The seaming machine creates a double seam in which the can lid and body are folded into each other. The quality of the seam is critical because it prevents harmful bacteria from entering the can. Specifications that govern the quality of the seam are frequently audited.

The seamed cans then enter the coding process and are labelled with a manufacturing text-based code, typically printed on the can by an inkjet. This inkjet is important in the auditing process because it contains product information and the exact date, hour and minute that the can has been filled with food. Once the can is coded, it is automatically loaded into the retort vessel by a conveyor. The vessel is filled with a water cushion that prevents the cans from damaging each other. At the proper fill weight and headspace, the cans (which are slightly denser than water) settle to the bottom of the vessel.

Since the retort process uses steam to cook the cans, the water needs to be displaced out of the retort vessel. The water is displaced when the vessel is full or when time and temperature requirements (TTR) between seaming and retort have been reached. Typically, once cans have been seamed, they are required to undergo retort within 1.5 hours. If the cans do not undergo retort within this TTR, then they will be sent to quality control for quarantine. Once the vessel is completely drained of water, the cans are cooked with steam until the retort TTR is reached. It is typical for cans to undergo one and a half hours of thermal processing at 98–106 0 C. The steam used to cook the cans creates an atmospheric pressure inside the retort vessel, and inside the cans. The cooling process begins by displacing the steam with water. TTR also governs the cooling process, which typically lasts 10 minutes before the discharge. Cooling pressure is typically used to create equilibrium of pressure on the inside and outside of the can. The cooling pressure should match the pressure created by the steam in the retort cooking cycle. Without the cooling pressure, the pressure on the outside of the can will drop creating an imbalance because the inside of the can retains the pressure that was created during the retort cooking cycle.

After the retort cooling process, the cans are discharged onto the submerged conveyor system. The water-filled vessel undergoes a vacuum process so that the water from the vessel does not cause the submerged conveyor system to overfill. The water from the vessel is vacuumed into a valve, and the cans are dropped onto the submerged conveyor. The cans continue to cool while they are on the submerged conveyor. The conveyor system brings the cans out of the water where the cans are unscrambled for one piece flow. The cans are then stacked and palletized in preparation for the packaging process. At this point, damaged cans and cans in violation of TTR will be set aside for quality control to review. The quality control team maintains documentation of damaged cans including the product type, processing time, quantity of the claim, and reason for the claim. Cans that meet quality control standards are labelled and packaged for distribution.

3.   Measuring the Problem
The second step in the Six Sigma based DMAIC process is to measure system variability that can create waste. It is convenient to choose reporting metrics that can be easily understood. An engineering economic analysis was performed and to understand the return on investment (ROI) needed to rationalize the benefits of the project. In this case, a count of all damaged can claims was used. The process flow charting discussed previously revealed that a quality control process step measures the number of damaged cans by process date and damage type. This quality control measurement became the basis for several reporting mechanisms. Damaged cans were categorized into the following four types

  • Buckled cans.
  • Bad seams.
  • No inkjet code.
  • TTR not to specification.
The financial impact of damaged cans is presented as the product of the percentage of can damage claims, the annual production volume of cans and the unit cost of each filled can. Three different products are run on the crateless retort line and the damage was computed as follows: Results of the analysis are shown in table 3.
A Pareto analysis of the system shows that the damage claims on product 3 are significantly higher than the other since they account for 63% of the overall claims (see figure 4). The type of damage, also measured in a Pareto analysis, indicated that most of the damage on the product is due to buckling, accounting for 87% of the overall claims (see figure 5). Therefore, the focus of further research was on buckling damage on product 3.

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