(Aug. 11, 2008) — Plastics are tremendously versatile. Replacing traditional materials, they offer many new design opportunities through their ease of processing, fabrication and decorating. Design innovations are appearing in many markets from cars to packaging, PCs to phones, and domestic appliances to medical devices.
However, along with the aesthetic benefits come the commercial pressures on manufacturers to reduce the costs of tool sampling, product development time and unit prices and, at the same time, to optimize product quality to avoid expensive warranty and recall risks.
Although these demands can be addressed through using finite element analysis combined with rapid prototyping, rapid tooling and flow analysis, enabling more “right first time” products and eliminating the need for expensive and time-consuming tool modifications, it is a fact that around 70 percent of plastics fail before their design lifetime. The result is expensive recalls, warranty claims, retooling, and the most damaging consequence in an ever competitive market — loss of brand credibility.
So where is the issue? All too often, product failure due to poor material selection is the single most common error made by designers or engineers of plastics products. Moreover, it appears to be caused by a lack of awareness or understanding of the material's properties.
This is not surprising given that there are more than 90 generic classes of plastic, which can be broken down into about 1,000 sub-generic modifications and finally more than 50,000 commercial grades from a host of materials suppliers. The result is a vast array of trade names, generic nomenclature and a plethora of all-too-often incomplete, inconsistent data with insufficient standardization. This is further compounded by suppliers' trade literature, often extolling a material's advantages and masking disadvantages — all of which can confuse and frustrate even the most experienced plastics experts.
However, the human causes of failure can be largely avoided through considering, or being aware of, the following five key areas:
* Material selection — the right generic plastic for the application.
* The effects of time, temperature and stress/strain — generate equivalent long-term data.
* Environmental stress cracking.
* Service environment and possible synergistic effects.
* Processing effects.
At the outset of material selection, the basics of polymer structure and properties should be considered. For example, for most plastic products, the most common material choice is thermoplastics, which are either amorphous or semi-crystalline.
Amorphous plastics are preferred for applications where transparency, good appearance, high gloss, high dimensional accuracy and stability are required. They are unsuitable for applications involving thermal or mechanical stress cycling, high mechanical abuse or contact with a range of chemical environments.
Semi-crystalline plastics have ordered crystallite structures and can better withstand fatigue than amorphous plastics. They are best used where chemical contact, mechanical abuse and resistance to repeated cyclic loading is required.
A major contributory factor to this is that designers historically are used to working with metals and other materials that exhibit a predictable linear elastic stress/ strain relationship, where the effect of temperature, environment and long-term load (creep) can generally be ignored. Plastics, however, are visco-elastic materials and respond to stress as if they were a combination of elastic solids and viscous fluids. As a result, they exhibit a nonlinear stress/strain relationship and their properties depend greatly on the time under load, temperature, environment and the stress or strain level applied and geometrical features of part design.
Unfortunately, many designers too often assume that “it is just a plastic” and readily use the short-term test data provided by the supplier's data sheets. The main issue we see at Smithers Rapra Technology Ltd. is that many designers are unaware of the long-term properties of plastics.
The information on the data sheets is derived from short-term tests that do not take into account time, temperature and environment. Test pieces are also simple shapes and molded under ideal conditions. This rarely applies to molded products.
Designers are advised that in the short term many polymers can endure strain levels of 200 percent or more. But for long-term performance, the window is massively smaller.
I recommend the following design strains for static stress conditions: amorphous plastics, 0.5 percent strain, and semi-crystalline, 0.8 percent strain.
For cyclic stress conditions: amorphous plastics, 0.3 percent strain, and semi-crystalline plastics, 0.5 percent strain
I am a strong advocate for making provision in the development phase of a product for the generation of real test data at the temperature and environmental conditions that the product expects to see in service.
If reliable long-term performance data is generated and material behavior within a specific service time frame is understood, then appropriate action can be taken to design accordingly to compensate for it.
Environmental stress cracking can be defined as the premature initiation of cracking and embrittlement of a plastic due to the simultaneous action of stress and strain and contact with specific environments (liquid or vapor). It is a failure mechanism that contributes to many industrial and domestic accidents with substantial associated costs.
I challenge designers to ignore the potential of ESC at their peril.
The majority of plastic product failures are due to the cumulative effects of synergies between creep, fatigue, temperature, chemical species, ultraviolet and other environmental factors.
I recommend that the designer who considers all of these factors as part of the design and material selection process has a better chance of avoiding an inevitable product failure that is predetermined from the outset.
Even the best plastic designs with good material selection can fail. But we often find the cause is poor processing due to a blatant disregard for established processing procedures and guidelines provided by material manufacturers. The driving force behind this is often economic — the need to achieve reduced cycle times and higher production yield.
Chris O'Connor is technology manager at Smithers Rapra Technology Ltd. in Shrewsbury, England.