AS most readers understand, a product that often appears simple to the user and the designer may tax the manufacturer, and may need to be re-specified to allow it to fit the available manufacturing processes.
The reasons for this can vary but many products are designed without full consideration being given to the details of manufacture.
However, the discipline of systems engineering has been developed to cope with such complex problems. It offers some simple tools for both designers and manufacturers to allow communication of the key issues without a huge investment of resources.
First, the product designer and the other stakeholders must document the requirements for even a simple product. The manufacturer must then review the requirements specification. If the manufacturing stage of the life-cycle has not been considered this is the manufacturer’s opportunity to have his/her say.
The approaches outlined below will help to make sure all of the manufacturing issues have been captured.
Life cycle
Each stage of the product’s life should be considered by the designer in defining the product’s requirements, not just the period after sale. A typical life-cycle would be:
• design and test;
• manufacture and test;
• shipping and delivery;
• installation/set-up;
• use (including idle periods);
• maintenance; and
• disposal.
This forces the designer to explicitly address the needs of the manufacturer as the stakeholder during one part of the product life cycle.
Manufacturing staff should check the requirements specification to ensure that the manufacturing stage has been adequately considered.
Manufacturers may have a standard set of requirements with which they will ask designers to comply. Simple examples may include moulding machine capacity or tool size or weight.
Context diagram
This simple tool forces the designer to think about the product’s requirements without clouding the issues with the design itself.
Simply consider one portion of the product’s life cycle and consider all of the inputs and outputs to the product during this period.
The designer and the manufacturing engineer then need to quantify the constraints that apply to the tool size, material characteristics and production cost. This gives the designer some structured information on the manufacturing requirements for the product.
It is not uncommon for the requirements of production testing to be overlooked during product design. The context diagram should capture these requirements and make sure they are visible to the product designer.
Risk assessment and FMEA
It is usual to consider the various risks and hazards associated with a manufacturing process. However, an analysis of the technical risks associated with product manufacture should also be carried out.
There are several tools available, including Failure Mode and Effects Analysis and Failure Mode Effects and Criticality Analysis. These tools rely on identifying a failure mode and then categorising each failure in terms of likelihood and severity. They are described in detail in IEC60812 Analysis Techniques For System Reliability-Procedure for FMEA.
Identifying how a process will fail is the most time-consuming part of the analysis. This should be done with all stakeholders present. Failure modes can be identified by:
1. Considering each function that the product or process must carry out and determining how each of these functions could fail.
For example, a container could fail to hold a liquid product as a result of a faulty moulding. The faulty moulding could be caused by a short shot or a moulding gate breaking out the wall of the container.
This approach is usually termed ‘top-down’.
2. Start with a standard list of failures and then identify the ultimate consequences.
For example a short shot could cause the container to lose its contents but it could also cause other features not to form correctly, which will lead to other consequences.
This approach is termed ‘bottom-up’.
Whichever process is used, it is useful to use a Fault Tree to capture the failure modes during this meeting. This is described in detail in IEC61025, Fault Tree Analysis (FTA). The key to this process working is the involvement of all stakeholders; including designers and manufacturers.
Each failure then needs to be catergorised in terms of likelihood and severity and a decision made on whether the failure will be detected or some action is required. Actions will usually involve placing some constraint on the product design or production process.
Feeding these constraints back to the designer as de-facto requirements is one way to ensure they are properly considered in the product’s development.
*David Tokell is director of Cubiqs Product and Process 0430 433 672.