Published: 31 October, 2019
Today’s designers and engineers now have more access to a plethora of polymers than at any other stage in history. There are currently more than 90 generic plastics and around 1,000 sub-generic modifications with 50,000 commercial grades available from over 500 manufacturers. The same holds true for elastomers with thousands of propriety manufacturer grades available.
However, despite the advances in polymer manufacturing and design technology, it is a fact that around 70% of products fail before their design lifetime.
With such an expanse of polymer grades and classes, several studies clearly demonstrate that failure caused by poor material selection is the single most common error made by today’s designers and engineers.
Often, incomplete and inconsistent data with insufficient standardisation, can sometimes confuse and frustrate even the best polymer engineer. The situation is further complicated by the competitive and positive marketing of material suppliers, which sometimes makes it difficult to identify material limitations and disadvantages. The consequence of uncertainty and poor decision-making can result in costly litigation, expensive recalls, warranty claims, re-tooling and, in an ever-competitive, climate-conscious market – loss of brand credibility.
Ignorance and improper material selection
Failures arising from incorrect material and grade selection are perennial problems in the polymer industry and are commonly associated with human error.
There are two common reasons for improper material selection. The first is that the material selector may have limited polymer 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 the manufacturing site is remotely based.
From the outset of material selection, the basics of polymer structure and properties should be considered. Polymers are broken down into three main groups: thermoset, thermoplastic (amorphous and semi-crystalline) and elastomers (rubbers). In addition, there are the polymer composites combining a smaller number of generic polymers with an array of structural material elements such as carbon fibre, glass fibre etc.
In order to reduce the likelihood of the product not performing properly, all parties within the design process must have the ability to imagine how their designed polymer part could fail. This can only be achieved if the product design team has a complete understanding of material characteristics, specific material limitations, processing, and failure modes.
Such knowledge requires a judicious approach and thorough consideration of the requirements of the application in terms of mechanical, thermal, environmental, chemical, electrical and optical properties. Production factors, such as the most feasible and efficient method of manufacture in relation to the part, size, and geometry need to be assessed. In terms of economics, the material cost, cycle times and price per part also need to be evaluated.
Rules and guidance
While there are no absolute rules pertaining to polymer product design, some general principles and guidelines do exist, particularly between amorphous and semi-crystalline thermoplastics, thermosets and elastomers and their varying processing techniques. These are readily available from material suppliers.
The basic rules apply to fillets, radii, wall thickness, ribs, bosses, taper, holes, draft, use of metal inserts, undercuts, holes, threads, shrinkage, and dimensional tolerance. Design rules which apply to secondary joining and assembly processes (welding, mechanical fastening and adhesive/solvent welding) need to be carefully evaluated too.
The designer and engineer should be aware that due to the diverse range of polymer materials and properties the design criteria will change from material to material as well as application to application. When considering the design and development of an engineering component the designer must understand that in general polymers.
- visco-elastic – time, temperature & stress dependent
- will creep and ultimately experience creep rupture
- susceptible to stress relaxation
- are susceptible to cyclic strain damage – fatigue
- strain sensitive
- undergo material transitions with temperature affecting short and long term physical and mechanical properties
- notch sensitive
- susceptible to:
- Chemical attack
- thermal degradation
- environmental stress cracking (ESC)
- ageing via physical and chemical processes – oxidation
- forms of radiation, inclusive of UV.
Most product failure is caused by cumulative effects due to the synergies between creep, fatigue, temperature, chemical species, UV and other environmental factors.
Therefore, in order to avoid failure, it is imperative that the designer and engineer understand that when subjected to static low stress/strain, the product will succumb to brittle failure in time. Notably, premature initiation of cracking and embrittlement of a polymer can occur due to the simultaneous action of stress and strain and contact with specific chemical environments.
Polymers and sustainability
Polymers are tremendously versatile group of materials, but they have their limitations. For the designer and engineer, it is a practical necessity to understand their fundamental nature, to reduce the likelihood of product failure and impact on the environment.
There is 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 mechanic/physical properties of which seemingly small differences can have a dramatic effect on the material itself and its performance.
While there has always been the question of vulnerability to failure and the ramifications of potential litigation, to some degree this situation has improved for more established designs. However, for new innovative applications which push the boundaries of material performance, the problem remains. Hence the need for robust qualification and fitness for purpose studies to evidence performance.
Today, where the license to operate is closely monitored by society, questioning the viability of a polymer product and its reliability, are tantamount to secure acceptance in a world where particularly plastics are often demonised.
At DNV we are committed to working closely with our customers, providing “right first time” polymer engineering solutions.