More plastics are now available to the designer and engineer than at any previous stage in the history of industry. Today there are over 90 generic plastics and around 1000 sub-generic modifications with 50 thousand commercial grades available from over 500 manufacturers.
The short history of plastic development and proven usage has meant for the designer and engineer that for critical engineering applications there has never been enough time to fully explore service life and problems that might occur during the use of plastics. There has always been the question of vulnerability to failure and the ramifications of potential litigation. To some degree this situation has improved, as the portfolio of successful plastic designs has grown in demanding engineering applications. However, 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 and 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 and manufacturer reputation.
Considering that approximately 70% of plastic products fail prematurely, failures have been poorly reported since the owners of failed products are naturally generally reluctant to publicise the fact. Failure investigations of such cases tend not to be disseminated due to client confidentiality agreements and for this reason the activity is predominately covert. As a consequence the potential benefits such as 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.
It is clear from the extent of plastic and rubber failure investigations conducted by Smithers Rapra that limited dissemination of plastic and rubber failure knowledge within the public domain has resulted in a continual cycle of plastic and rubber failure incidents from all industrial sectors. The lessons of good plastic and rubber product design are not being learnt even in 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 product; 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 due to 50 years dealing with a diverse clientele providing technical services aimed at problem solving and in particular failure diagnosis.
Failure is a practical problem with a product and 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 and consequently mechanical failure is usually a primary concern. However 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 and processors) continually communicate in order to take advantage of the valuable knowledge and experience of all. Key to successful design is that all aspects of the performance, production, assembly and ultimate use of the part are considered. Furthermore all parties promote building reliability and safety into the product.
In order to reduce the likelihood of product failure all parties within the design process must have the ability to imagine 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/ failure modes and avoidance.
Plastic product failure is commonly associated with human error or weakness and is typically associated with the factors shown in figure 1.0
Human Causes of Failure (%)
In an attempt to reduce the incidence of plastic product failure we must react to the fact that they are typically due to human error, misunderstanding and ignorance of plastic materials and associated processes and that the material or process is usually not at fault.
It is hoped that the following information will provide some insight into complexity of plastics design and plastic failure modes.
Poor Material Selection / Substitution
Failures arising from incorrect material selection and 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 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 feasible and efficient method of manufacture in relation part size and geometry need to be assessed. In terms of economics the material cost, cycle times and part price need to be considered.
Two common reasons for improper material selection are that 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 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. However, 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, use of metal inserts, undercuts, holes, threads, shrinkage, 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 plastic materials and 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 and lack of radii, lack of understanding of the creep mechanism due to plastic visco-elasticity, environmental compatibility, draft, placement of ribs and injection gates.
A significant number of plastic parts fail due to sharp corners / insufficient radius. Sharp corners create stress concentrations resulting in locally high stresses and strains. Since plastics are notch sensitive the stress concentration will promote crack initiation and ultimately fracture. They also impede material flow and ejection form the tool.
A significant number of failures can be attributed to excessive wall thickness and abrupt geometrical change. A pre-requisite is that uniform wall thickness is maintained since this keeps sink marks, voids, warpage, and moulded-in stress to a minimum.
Designers and engineers must be fully conversant with the visco-elastic nature of plastics and their creep, creep rupture, stress relaxation and fatigue mechanisms.
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 and embrittlement of a plastic can occur due to the simultaneous action of stress and strain and contact with specific chemical environments (liquid or vapour)
Design failure may also be attributed to reduced safety factors due to cost pressures and 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. Often the problem can be traced 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.
Typical processing faults are given in Table 1.0. Many of these faults can generally be overcome by attention to processing variables such as temperature, shear rates, cooling times and pressure.
Table 1.0 Processing faults
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).
Self-contamination (e.g., part-melted granules).
Poor material homogeneity
Poor weld lines and spider lines
Residual stress
Molecular orientation
Development of low or excessive crystallinity
Abnormal crystalline texture
Insufficient packing
Scorching
Jetting
Flashing
Abnormal spatial and size distribution of phases in composites
Mis-use / Abuse
Plastic product failure due to mis-use may result from a disregard for manufacturer installation instructions and failure to heed warnings. Failure may also occur due to simply using a product beyond its recommended service life, for function it was not intended or simply due to malicious attack.
Plastic Failure Modes
The main failure modes of plastics can be classed as mechanical, thermal, radiation, chemical and electrical as shown in Table 2.0. Classification of failure mode by mechanism shows that mechanical failure is the predominant mechanism although it is often the end result of many other failure modes.
From Smithers Rapra's experience we have found that the vast majority of plastic product failures are due to the cumulative effects of synergies between creep, fatigue, temperature, chemical species, UV and other environmental factors.
Table 2.0 Plastic Failure Modes Mechanisms
Mechanical Modes
Deformation and 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 and 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 and water treeing
Synergistic Modes
Weathering - effects due to photo and thermo-oxidation, temperature cycling, erosion by rain and wind-borne particles and chemical elements in the environment
Smithers Rapra have undertaken over 5000 failure investigations of which a significant number can be attributed to embrittlement and / or brittle fracture resulting from slow degradation or deterioration processes. From Figure 2.0 it can be seen that ESC, fatigue, notched static rupture, thermal degradation, UV degradation and chemical attack fall into this category, even when the material was reported to be ductile.
This article is free to republish provided the resource information remains intact.
Contact Dr Chris O'Connor is the Technology Manager at Smithers Rapra.Tel. 01939 252423. Email coconnor@rapra.net
http://www.rapralimited.org
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