Feature Article

By: C. O’Connor,
Smithers Rapra, Shawbury, UK

The development of plastics and their associated processing techniques has been a phenomenal episode in the history of materials science. With large-scale development taking place only within the past 60 years, the use of plastics in product design and manufacture has grown at a rate unrivalled by conventional materials. Because a wide spectrum of properties is available, plastics have become one of the most utilised materials in the world today. The range and types of plastics now available to designers and engineers are greater than at any previous stage in the history of the polymer industry. There are more than 90 generic plastics and approximately 1000 sub-generic modifications with 50  000 commercial grades available from more than 500 manufacturers. Obviously only a limited number of sub-generic materials are used in the medical device industry.

 

The short history of plastic development and proven usage has meant that for critical engineering applications there has never been enough time to fully explore service life and problems that may occur whilst utilising a polymer material. There has always been the scenario of vulnerability to failure affecting brand image and the ramifications of potential litigation. This situation has improved as the portfolio of successful plastic designs has grown in demanding engineering applications. For new innovative applications that push the boundaries of material performance the problem remains.

 

Designing to ensure plastic product reliability is critical because of the increasing importance of

Product liability can be the most damaging with settlements and penalties in the order of thousands or even millions of euro, particularly when failure has resulted in personal injury or death. In addition to litigation financial costs, critical employees are distracted from normal duties and there is a loss of product perception, brand credibility and manufacturer reputation.

Considering the number of plastic products that fail prematurely, failure is poorly reported because the owners of failed products are naturally reluctant to publicise the fact. Failure investigations are unlikely to be disseminated because of client confidentiality agreements and for this reason the activity is predominately covert. As a consequence, the potential benefits of learning from the mistakes and misfortunes of others, and identifying priorities for research and critical issues in product development are far from being fully exploited.

 

Failure is a practical problem with a product and implies that it no longer fulfils its function. Frequently, the ability to withstand mechanical stress or strain (and thereby store or absorb mechanical energy) is the most important criteria for a product in service; consequently mechanical failure is usually a primary concern. Failure may also be attributed to loss of attractive appearance or shrinkage.

 

To avert product failure it is critical that at all stages of the design process there must be a concurrent engineering approach to product development. This ensures that from inception of the project until final high volume manufacture all parties involved (industrial design, product engineers, plastic expert, tooling designers/engineers, processors and marketing) continually communicate to take advantage of the valuable knowledge and experience of all. Essential to the success of a design is that all aspects of performance, production, assembly and ultimate use of the part are considered. All parties involved with development should focus on building reliability and safety into the product.

 

To reduce the likelihood of product failure, all parties within the design process should continually focus on how their designed plastic part could fail. This can only be achieved if the product design team has a good appreciation of plastics material selection, product design, processing and specific material weaknesses, and fault and failure modes and avoidance.

Plastic product failure is commonly associated with human error or weakness and is typically associated with one or more of the following:

In an attempt to reduce the incidence of plastic product failure, it should be accepted that failures are caused by human error, and misunderstanding and ignorance of plastic materials and associated processes. The material or process is usually not at fault. The following information will provide some insight into the complexity of plastics design and plastic failure modes.

 

Failures arising from incorrect material selection and grade selection are perennial problems. To perform plastic material selection successfully a complete understanding of plastic material characteristics, specific material limitations and failure modes is required. Good material selection requires a judicious approach and careful consideration of application requirements in terms of mechanical, thermal, environmental, chemical, electrical and optical properties. Production factors such as a feasible and an efficient method of manufacture in relation to part size and geometry need to be assessed. In terms of economics, the material cost, cycle times and part price need to be factors.

 

Two common reasons for poor material selection are: the material selector has limited plastics knowledge and expertise and is unfamiliar with the material selection process. Alternatively, a suitable material has been specified but not used. Materials substitutions most commonly occur when the customer is unable to enforce quality procurement specifications, particularly if a manufacturing site is remote. Common problems encountered include:

 

There are no absolute rules pertaining to plastic product design. Some general principles and guidelines are well established particularly between amorphous and semi-crystalline thermoplastics and thermosets and the various processing techniques. These are readily available from material suppliers.

 

The basic rules apply to fillets, radii, wall thickness, ribs, bosses, taper, holes, draft angle, use of metal inserts, undercuts, holes, threads, shrinkage and dimensional tolerance. Design rules that apply to secondary joining and assembly processes (welding, mechanical fastening and adhesive/solvent welding) also need to be carefully evaluated. The designer and engineer should be aware that because of the diverse range of plastic materials and properties, the design criteria will change from material to material as well as application to application.

 

Common design errors are related to abrupt geometrical changes, excessive wall thickness, sharp corners, lack of radii, insufficient draft angle for ejection, placement of ribs and injection gates, limited understanding of the creep mechanism as a result of plastic visco-elasticity and environmental compatibility. Many plastic parts fail because of sharp corners/
insufficient radius. Sharp corners create stress concentrations and result in locally high points of stress and strains. Plastics are notch sensitive; the stress concentration will promote crack initiation and ultimately fracture, they also impede material flow and ejection from the tool. A significant number of failures can be attributed to excessive wall thickness and abrupt geometrical change. A prerequisite is that uniform wall thickness is maintained; this keeps sink marks, voids, warpage and moulded-in stress to a minimum.

 

Designers and engineers should be fully conversant with the visco-elastic nature of plastics and their creep, creep rupture, stress relaxation and fatigue mechanisms. Visco-plastic materials respond to stress as if they were a combination of elastic solids and viscous fluids. A nonlinear, stress–strain relationship is exhibited and their properties depend on the time under load, temperature, environment and the stress or strain level applied. An example of visco-elasticity can be seen with Silly Putty, a class of silicone polymer marketed as a toy for children. When pulled apart quickly, it breaks in a brittle manner; when pulled slowly apart the material behaves in a ductile manner and can be stretched almost indefinitely. Decreasing the temperature of the material decreases the stretching rate at which it becomes brittle. It is imperative that the designer and engineer understand that

Design failure may also be attributed to reduced safety factors as a result of cost pressures and the use of plastics in demanding applications and taking them to their design limits where on occasion they are exceeded.

 

Many in-service failures are the result of poor processing. The problem can often be traced to a disregard for established
processing procedures and guidelines provided by material manufacturers. The driving force behind this is often economic, that is, the need to achieve reduced cycle times and higher production yield. Typical processing faults can be overcome by attention to processing variables such as temperature, shear rates, cooling times and pressure. Common faults include:

 

When analysing the critical failure modes of plastics, they can be divided into the following categories: mechanical, thermal, radiation, chemical and electrical (Table I). Classification of failure mode by mechanism shows that mechanical failure is the predominant mechanism although it is often preceded by one or more of the other classifications. The vast majority of plastic product failures are the result of cumulative effects of synergies between creep, fatigue, temperature, chemical species, ultraviolet light and other environmental factors.

 

TABLE I: Classification of failure mode by mechanism.
Mechanical modes	Deformation and distortion as a result of creep and stress relaxation, yielding, crazing brittle fracture because of creep rupture (static fatigue), notched creep rupture, fatigue (slow crack growth from cyclic loading), high energy impact, wear and abrasion
Thermal modes	Thermal fatigue Degradation: Thermo-oxidation Dimensional instability Shrinkage Combustion Additive extraction
Chemical modes	Solvation, swelling, dimensional instability and additive extraction Oxidation Acid induced stress corrosion cracking Hydrolysis (water, acid or alkali) Halogenation Environmental stress cracking Biodegradation
Radiation modes	Photo-oxidative degradation (ultraviolet light) Ionising radiation (gamma radiation, X-rays)
Electrical modes	Electrostatic build-up, arcing, tracking, electrical and water treeing
Synergistic modes	Weathering effects as a result of photo and thermo-oxidation, temperature cycling, erosion by rain and wind-borne particles and chemical elements in the environment

Plastics are tremendously versatile materials, but they have their limitations. For the designer and engineer it is a practical necessity to understand their fundamental nature, limitations and failure modes to reduce the likelihood of product failure. There is at times a fine line between good product design, correct material selection and failure that can be easily crossed if expert knowledge is not used. Attention must be paid to the many variables that can influence plastic properties; seemingly small differences in these can have a dramatic effect on plastic and product performance. 

 

is Director of Consultancy and Litigation Services at Smithers Rapra Technology Ltd Shawbury, Shrewsbury SY4 4NR, UK
tel. +44 1939 250 383, e-mail: info@rapra.net