2.3 Product defects
What would you say are common defects found in products? You might answer the question by asking what is meant by a defect. A working definition might be:
a defect is a feature of a product or component that inhibits or prevents the correct operation of that product or component.
The term feature is here used in a neutral way to denote any characteristic that does not normally appear in a specification, such as included in an engineering drawing. Variations in surface texture often appear on plastic mouldings or cast metals as a result of the manufacturing process, but designed into the product.
The definition of features that are also defects is useful because it allows us to examine those features that can affect product function in a little more detail.
The very form of the bathtub curve suggests there are at least two types of defect: primary defects, and defects that develop with time. Primary defects are already present before sale and lead to rejection or functional failure. The second type causes failures after considerable use. (See Table 4.)
Table 4: Types of product defect
|Primary defects||Secondary defects|
|Design: geometry||dimensions (overfit, underfit) stress raisers||cracks from stress raisers|
|Design: materials||poor materials||degradation|
|Manufacture||poor moulding, casting etc.||distortion over time|
|Assembly||poor fit||wear of parts in contact|
|poor welding etc.|
|Finishing||surface blemishes||degradation on surface|
2.3.1 Primary features
Primary features that are often defects in components would include:
sharp corners in stressed areas;
internal voids or inclusions that overstress the product when it is loaded;
deviations from proscribed dimensions so that the component doesn't fit with others during assembly, perhaps caused by the sinking of surfaces of castings or mouldings;
poor quality material, perhaps caused by contamination;
cracks in components.
Any such features can occur in combination with one another, and may also interact: a void near a sharp corner would be a serious defect in a product where the stress is at the corner. Not all such features may be defects by themselves. Whether a feature can be regarded as a defect depends on the product specification.
2.3.2 Secondary features
Features of the second kind, which occur after considerable use, are:
wear of parts that move against one another;
cracks that develop over time or cycles of use;
corrosion by contact with the environment;
change in dimensions with time;
material changes with time.
Like primary features, they will become defects if they lead to fracture or failure, or rejection for aesthetic reasons. The distinction between product features and defects is closely related to the product function, its environment, and usage.
Some severe defects are caused by stress concentration at sharp corners, or from cracks, or from impurities or voids within materials. When products are loaded as part of their normal function, the stress at such defects is raised. These points, called stress raisers, cause the local stress to rise above the intrinsic strength of the material. Cracks are initiated at such points and can either grow rapidly, producing a sudden fracture, or can grow step wise in a manner known as fatigue.
The sketches in Figure 14 show a polysulphone sight tube in outline. It is a part of medical apparatus for controlling airflow to patients. The central part of the sight tube is transparent so a metal float in the centre section can indicate the airflow within (see Box 8).
Which of the features shown are potential defects, and how could they make the product defective? By discussing their possible source and origin, indicate possible ways to eliminate them.
Sight tubes are made by injection moulding, which involves using granules of virgin polymer fed into an externally heated barrel. The moulding screw that fits inside the barrel rotates at a regular rate to homogenise the molten polymer, which is injected under pressure at regular intervals into a heated metal tool, where the polymer solidifies to create the product shape.
Three of the features in the sight tube shown in Figure 14 can be regarded as defects if they make it harder to see the interior flow indicator, so they would affect its function. Another reason for classifying the first three as defects is that, being visible to the user, they may be judged as outside the specification as expressed on an engineering drawing of the product. The final feature, sink marks below the clear central section of the tube, could only be classified as defects if they affected the flow of air through the tube. They are not as visible as the previous features, so might be acceptable to the customer. On the other hand, they would not appear on any engineering drawing, so are strictly outside the specification.
The remedial measures would be as follows:
Foreign particles. The remedy depends on the source of the contamination. The virgin granules could be contaminated, so it would be essential to ensure the granules were completely clean before being fed into the barrel. The feed to the barrel should thus be guarded from the environment. Another source might be wear debris from the machine itself; this could be prevented from entering the product by putting a mesh ahead of the molten stream.
Particles of unmelted granules. They show the granules are not being homogenised completely into the melt. An obvious remedy is to ensure the barrel heaters are working correctly, and ensure the screw is turning fast enough to mix the material.
Scratches. These defects could be created in several ways. They may be due to handling after the solid product has been removed from the tool, so handling would need examining. Gloves might be needed. A more troublesome source could be scratching of the tool itself, in which case the scratches will be identical from component to component. Polishing the metal tool is the remedy.
Sinking of the inner tube. This could be created by under-pressurisation of the melt, so the working conditions of the process might need re-examination if the features are regarded as defects.