Why Plastic Products Fail

The development of plastics & their associated processing techniques has been a phenomenal episode in the history of materials science. With large scale development taking place only within the last 60 years, the use of plastics in product design & manufacture has spiraled at a rate unrivalled by conventional materials. With the wide spectrum of properties available, plastics have become one of the most utilised materials in the world today.

The range & types of plastics now available to the designer & engineer are greater than at any previous stage in the history of the polymer industry. There are over 90 generic plastics & around 1000 sub-generic modifications with 50 thousand commercial grades available from over 500 manufacturers.

The short history of plastic development & proven usage has meant that for critical engineering applications there has never been enough time to fully explore service life & problems that might occur whilst utilising a polymer material. There has always been the scenario of vulnerability to failure effecting brand image & 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 pushing the boundaries of material performance the problem remains.

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

• Product liability claims
• Environmental concerns
• Certification in order to become an approved supplier
• An awareness of quality costs

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

Considering that approximately 70% of plastic products fail prematurely, failure is poorly reported as the owners of failed products are naturally reluctant to publicise the fact. Failure investigations are unlikely to be disseminated due to client confidentiality agreements & for this reason the activity is predominately covert. As a consequence the potential benefits of learning from the mistakes & misfortunes of others, identifying priorities for research & critical issues in product development are far from being fully exploited.

It is clear from the extent of plastic & rubber failure investigations conducted by Smithers Rapra that limited dissemination of plastic failure knowledge within the public domain has resulted in a continual cycle of plastics & rubber failure incidents from all industrial sectors. The lessons of good plastic & rubber product design are not being learnt even in the light of the enormous growth in product liability cases that have imposed an entirely new dimension on the consumer product environment. It is now well established in law that manufacturers are liable for injuries resulting from defective products; for injuries from a hazard associated with a product against which the user should have been warned; or for damages caused by misapplication of a product which could have been foreseen by the manufacturer.

It is a practical necessity to understand why plastics fail in order to minimise the failure scenario. Smithers Rapra has acquired this knowledge from over 50 years of supporting a diverse clientele providing technical services aimed at problem solving & in particular failure diagnosis.

Failure is a practical problem with a product & implies that the component 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 criterion in service; consequently mechanical failure is usually a primary concern. Failure may also be attributed to loss of attractive appearance or shrinkage.

In order 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 system ensures that from inception of the project until final high volume manufacture all parties involved (marketing, industrial design, product engineers, plastic expert, tooling designers/engineers & processors) continually communicate in order to take advantage of the valuable knowledge & experience of all. Key to the success of a design is that all aspects of performance, production, assembly & ultimate use of the part are considered. All parties involved with development should focus on building reliability & safety into the product.

In order 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 & specific material weaknesses & fault/ failure modes & avoidance.

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

• Poor material selection / substitution
• Poor design
• Poor processing
• Mis-use & abuse

In an attempt to reduce the incidence of plastic product failure, it should be accepted that failures typically human error, misunderstanding & ignorance of plastic materials & associated processes. The material or process is usually not at fault.

The following information will provide some insight into complexity of plastics design & plastic failure modes.

Poor Material Selection / Substitution

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

Two common reasons for poor material selection are the material selector has limited plastics knowledge & expertise & 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 remotely based. Common problems encountered include:

• Processor simply substituting with a cheaper material.
• Use of the wrong grade of material (incorrect MFI).
• Use of general purpose PS rather than HIPS.
• Homopolymer used instead of copolymer
• Incorrect pigments, fillers, lubricants or plasticisers used.

Poor Design

There are no absolute rules pertaining to plastic product design. Some general principles & guidelines are well established particularly between amorphous & semi-crystalline thermoplastics & thermosets & 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, use of metal inserts, undercuts, holes, threads, shrinkage, dimensional tolerance. Design rules which apply to secondary joining & assembly processes (welding, mechanical fastening & adhesive/solvent welding) need to be carefully evaluated too.

The designer & engineer should be aware that due to the diverse range of plastic materials & properties the design criteria will change form 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, limited understanding of the creep mechanism due to plastic visco-elasticity, environmental compatibility, draft, placement of ribs & injection gates.

A significant number of plastic parts fail due to sharp corners / insufficient radius. Sharp corners create stress concentrations resulting in locally high points of stress & strains. Plastics are notch sensitive; the stress concentration will promote crack initiation & ultimately fracture, they also impede material flow & ejection form the tool.

A significant number of failures can be attributed to excessive wall thickness & abrupt geometrical change. A pre-requisite is that uniform wall thickness is maintained, this keeps sink marks, voids, warpage, & moulded-in stress to a minimum.

Designers & engineers should be fully conversant with the visco-elastic nature of plastics & their creep, creep rupture, stress relaxation & fatigue mechanisms.

Visco-plastic materials respond to stress as if they were a combination of elastic solids & viscous fluids. A non-linear stress-strain relationship is exhibited & their properties depend on the time under load, temperature, environment & the stress or strain level applied. An example of viscoelasticity can be seen with Silly Putty. When pulled apart quickly it breaks in a brittle manner, when pulled slowly apart the material behaves in a ductile manner & can be stretched almost indefinitely. Decreasing the temperature of Silly Putty, decreases the stretching rate at which it becomes brittle. It is imperative that the designer & engineer understand that:

• Plastics will deform under load
• When subjected to static low stress / strain a ductile / brittle transition will occur at some point in time resulting in brittle failure
• Cyclic stressing will result in a ductile / brittle transition resulting in brittle failure at low stress level
• Premature initiation of cracking & embrittlement of a plastic can occur due to the simultaneous action of stress & strain & contact with specific chemical environments (liquid or vapour)

Design failure may also be attributed to reduced safety factors due to cost pressures & the use of plastics is demanding applications taking them to their design limits where on occasion they are exceeded.

Poor Processing

Poor processing, accounts for many in-service failures. The problem can often be traced to a disregard for established processing procedures & guidelines provided by material manufacturers. The driving force behind this is often economic – the need to achieve reduced cycle times & higher production yield.

Typical processing faults can be overcome by attention to processing variables such as temperature, shear rates, cooling times & pressure. Common faults include:

• Use of inappropriate process equipment
• Non-uniform wall thickness
• Short shots, Bubbles, Sink marks
• Post-moulding shrinkage
• Warping / distortion
• Foreign body contamination
• Voids, Cosmetic – discolouration, splay marks
• Degradation(insufficient drying of material, process temperature too high, residence time in the barrel too long, shear heating, too much regrind.
• Self-contamination (e.g., part-melted granules).
• Poor material homogeneity, weld lines & spider lines
• Residual stress
• Molecular orientation
• Development of low or excessive crystallinity
• Abnormal crystalline texture
• Insufficient packing, Scorching, Jetting, Flashing
• Abnormal spatial & size distribution of phases in composites

Plastic Failure Modes

When analysing the key failure modes of plastics they can be broken down into the following categories; mechanical, thermal, radiation, chemical & electrical. Classification of failure mode by mechanism shows that mechanical failure is the predominant mechanism although it is often preceeded by one or more of the other classifications.

Mechanical Modes	Deformation & distortion due to creep & stress relaxation, Yielding, Crazing Brittle Fracture due to Creep rupture (static fatigue), Notched creep rupture, Fatigue (slow crack growth from cyclic loading), High energy impact, Wear & abrasion.
Thermal Modes	Thermal fatigue Degradation – thermo-oxidation Dimensional instability Shrinkage Combustion Additive extraction
Chemical modes	Solvation, Swelling, dimensional instability & additive extraction Oxidation Acid induced stress corrosion cracking (SCC) Hydrolysis (water, acid or alkali) Halogenation Environmental stress cracking (ESC) Biodegradation
Radiation Modes	Photo-oxidative degradation (UV Light) Ionising radiation ( gamma radiation, X rays)
Electrical Modes	Electrostatic build-up, Arcing, tracking, Electrical & water treeing
Synergistic Modes	Weathering – effects due to photo & thermo-oxidation, temperature cycling, erosion by rain & wind-borne particles & chemical elements in the environment

Smithers Rapra’s experience has proven that the vast majority of plastic product failures are due to the cumulative effects of synergies between creep, fatigue, temperature, chemical species, UV & other environmental factors.

Over 5000 failure investigations have been completed & recorded by Smithers Rapra of which a significant number can be attributed to embrittlement & / or brittle fracture resulting from slow degradation or deterioration processes. The chart below highlights that ESC, fatigue, notched static rupture, thermal degradation, UV degradation & chemical attack fall into this category, even when the material was reported to be ductile.

Material / phenomenological causes of failure

In summary

Plastics are tremendously versatile materials but are not without their limitations. For the designer & engineer it is a practical necessity to understand their fundamental nature, limitations & failure modes in order to reduce the likelihood of product failure. There is at times a fine line between good product design, correct material selection & failure which can be easily crossed if expert knowledge is not used. Attention must be paid to the many variables which can influence plastic properties of which seemingly small differences can have a dramatic affect on plastic & product performance.

The Consultancy Centre at Smithers Rapra provides a complete range of services including Plastics & Rubber training courses, materials selection,FEA, mould flow, long term property generation & lifetime prediction services. Contact us for further information.