By Paul Uglum, president, Uglum Consulting, LLC
What constitutes reliability in decorated plastic parts, and why is it important? A simple way of looking at part reliability is by asking the question: Does the decorated part maintain its original quality (appearance and function) over time? The formal definition of reliability is the probability of performing a specific function without failure under given conditions for a specified period of time.
The standard for decorated plastic is to identify if it outlives the product that it is a part of. In a practical sense, this means there needs to be an understanding of what constitutes a failure and how long the decoration will need to perform. As with most plastic decorating applications, the first step is understanding the environment in which it will operate. If the application is packaging, the intended life most likely is short, but the expectations for consumer products, automotive and medical applications have both higher standards and longer life expectancy.
The discipline for evaluating the appropriateness of a product for its application is product reliability. It often is applied to the finished product but also can be applied to components, such as decorated plastic. When considering reliability, it is important to understand the physics and chemistry of potential failures. The failure modes of each technology vary, but understanding how plastic decoration technologies fail allows designs that are durable for a specific application. Statistical methods of reliability require the study of a population of products with respect to the failure and not to specific problems. These techniques, such as Weibull Life Data Analysis, typically are not applied to decorated plastics but are used for more complex products where field data readily are available. They answer questions such as “How many?” and “How long?” To be useful, accurate field data needs to be available.
THE ENVIRONMENT
One important aspect is defining what “wear out” means for decorated parts. What change in color, appearance or function is allowed? Again, this depends upon the application. Then consider how the product will be used and what it likely will come in contact with over its lifetime. This should include the packaging and shipping of the product to the customer. If it is an outdoor product, what level of sun exposure will it see in Arizona or Florida? Does the surface come into contact with skin? Is it handled? Does it require frequent cleaning or sterilization? All of these environmental exposures should be considered when choosing and developing the decorating technology.
Three of the most common failure modes for inks, paints and coatings are wear due to abrasion, damage due to chemical attack and exposure to light and UV. There are other failure modes, such as adhesion failures between layers or between the decoration and the substrate, which can be associated with the first three. In the case of metallic appearances, the most common failure modes depend upon the technology, with thermal cycling or shock being an important failure mode for plastic plating.
The key to avoiding most failures is understanding the operating environment and knowing where to find information about changes in it. One way to accomplish this is from customer test and validation requirements. The other is learning as much as possible about the actual environments in which the product will be used and how the customers use the product.
CHEMICAL ATTACK
Chemical exposures are an ongoing high risk. They can take the form of chemical attack, softening and plastination or reduced adhesion strength. Chemical exposure has been demonstrated, over time, to be a major driver in plastic field failures (Figure 1). Exposure can occur by transfer from touch or direct application. The sources of exposure include personal care and cleaning products, the formulations of which constantly are changing. Sunscreens tend to aggressively attack polycarbonates and plasticize coatings. Insect repellants, in particular DEET (N,N-Diethyl-meta-toluamide,) act as a solvent and plasticizer for many paints, inks and plastics, and have been known to attack decorated plastic products.
Exposure can occur by transfer from direct physical contact. Air fresheners, which have many different, complex and unregulated formulations, demonstrate a variety of mechanisms of attack. Contact with these can result in failure of both the plastic decoration and the base substrate. Finally, exposures can occur from normal use, such as repetitive cleaning with household cleaning products.
SOURCES OF INFORMATION ABOUT CHEMICALS IN THE ENVIRONMENT
It is important to know where to learn about chemical constituents in consumer products. This knowledge is useful in both designing reliability testing and in understanding field returns.
The Environmental Working Group (EWG) is an environmental organization that specializes in research and advocacy in the areas of toxic chemicals and corporate accountability. As part of its work, the organization identifies the formulas of many personal care and cleaning products. This very useful information is available on its website (www.ewg.org).
HAPPI (Household and Personal Products Industry) magazine has useful information and covers the global personal care, household, industrial and institutional cleaning market.
Product labels can be of varying use since nomenclature can be unclear and relative amounts are not included. Safety Data Sheets can contain some content information. EPA, CDC, FDA and other government agencies also have useful application information.
ABRASION AND MAR
It is important to identify high-contact surfaces, such as those found in personal care products, that are subject to cleaning as well as wear and abrasion – either from use or during storage and shipment.
There are many abrasion-related failure modes. Some are customer dissatisfiers and others can be safety related. Potential failures can include micro scratches in high-gloss parts, mar in coatings and abrasion removal of critical warning labels. No one test characterizes all abrasion exposures, so it is important to know what types of damage are being tested for.
Many sources of abrasive materials are present in the surrounding environment. Dust in offices and households contains fine minerals, such as quartz and feldspar. These have a Mohs hardness rank of 6 to 7 and are hard enough to scratch glass, much harder than even highly cross-linked hard coats. Paper towels contain mineral fillers that can abrade surfaces as well.
LIGHT AND UV
UV or visible light damage is cumulative and depends upon the exposure time and intensity. Most decorating systems are stabilized for UV-A and UV-B exposure. In the case of medical products and related applications, the decorating also must pass exposure to UV-C with wavelengths as low as 222 nm. UV-C wavelengths are very energetic and can break down polymers and fade pigments.
LIFE DEPENDS UPON USAGE
People will use any given product in different ways. No two customers use a product in the same way or at the same rate. Also, each usage parameter – number of cleanings, hours in the sun, environmental chemical exposure – will vary. Usage follows a lognormal statistical distribution, which can be useful in designing testing (Figure 2).
A typical model for field issues is called the bathtub curve, in which the users get some initial failures, then the curve settles down to a low or nonexistent failure rate. This model, which works well for complex systems and electronics, does not represent what happens with decorated plastics (Figure 3). More typically with decorated plastics, there is no infant mortality (if the correct criteria are used for material and process selection), but prior to wear out, there can be spikes in field failures due to isolated exposures to aggressive conditions.
Both usage stress and environmental exposure can vary radically; also, the two stresses can simultaneously occur. For example, if a hand cream softens a coating and it then abrades, the resulting damage can be permanent.
VALIDATION TESTING
An automotive executive once said, “Test specifications are the scar tissue of past mistakes.” This is a reminder that many specifications include, at least in part, tests that are designed to prevent the recurrence of past issues. In other words, they are rear-looking rather than forward looking.
A validated plastic decorated system consists of the materials, decoration and substrate, and the process. Validation testing, if it considers the proposed environment and expected life, should be adequate to ensure a reliable product. Unfortunately, two factors diminish the effectiveness of reliability testing. First, the environment is very dynamic. New and improved products are introduced all of the time – some of these may contain chemistries that are harmful to the decorated surface. Another significant issue is variability in human behavior. A good example of this is the concern about Zika viruses a number of years ago, which resulted in an increased use of insect repellants in some regions. Another good example is the Covid-19 pandemic, which resulted in much more aggressive cleaning and sterilization (sometimes with inappropriate materials). These events cannot be predicted but result in both an increased quantity and frequency of exposure to environmental chemicals.
ACCELERATED TESTING
It obviously is not possible to test parts for long life without some form of accelerated testing. Most accelerated testing depends upon increasing the stress (sometimes by contact or dosage) or increasing the temperature. The rationale for increasing temperature is the Arrhenius model, which predicts the rate of chemical reaction doubles with every 10-degree Celsius temperature increase. All acceleration is only meaningful if it produces the same type of failure as seen at longer times and is best for A to B comparisons. All acceleration methods only are useful if they do not introduce new failure modes. An example of inappropriate acceleration would be increasing the temperature so much that the part melts.
THE COST OF UNRELIABILITY
The greatest risk of an unexpected failure occurs when technologies change or a new technology is introduced to the market (Figure 4). Changes to the decorating process or materials need to be carefully evaluated for what new strengths and, more importantly, weaknesses are introduced. An example of missing a critical weakness in both the design and testing is the introduction of soft-feel paints into automotive interiors. These coatings passed the test protocols that existed at the time but failed once they were introduced into the field. The root cause of the failure was that the coatings were suspectable to hydrolysis and reacted with moisture over time, resulting in chain scission (becoming soft and gummy). A better understanding of the potential failure mode for the chosen chemistry and testing to confirm durability in humid climates would have prevented the failures. Current specifications now include hydrolysis testing, and current coatings now are robust to this failure mode.
A parallel and somewhat more subtle risk occurs when a test for an infrequent exposure uses a material that is not representative of the class of products currently in use in the field. If too much weight is given to infrequent risks, the paint or ink may be compounded in such a way that it is less durable against high-probability failure modes. When testing non-representative materials, there is a risk of missing real failure modes and having a false sense of security.
Field returns provide an important window into potential changes in the environment or risks that were not considered when the initial specifications were written. If the field failures can be tracked to specific batches or manufacturing dates, then the issue most likely is due to a manufacturing error. If the field failures are regional or seasonal and not batch-related, then there likely is an issue with changes in the use environment. In the automotive soft-feel paint example, initial failures occurred in hot, humid climates, giving a clue as to both the cause and mechanism of the problem. Whenever possible, returned parts should be examined to determine the cause of failure. Chemical analysis of returns often will contain clues as to what materials the decorated plastic may have been exposed to.
FINAL COMMENTS
As with design for manufacturing, design for reliability is critically important. This design must begin with an understanding of the customers’ expectations and an understanding of the final use environment. Costs of reliability failures can be high; the earlier in the design sequence that risks are identified and addressed, the better the outcome.
The best course of action to test for reliability is to take the time to look beyond the specifications the customer provides and consider the potential failure modes for the technology that has been chosen. Do not assume the customers’ test specifications are complete or correct. Do not assume their understanding of the potential field exposures is complete or up to date. Careful evaluation at the design stage can eliminate many problems.
For me information, email paul.a.uglum@gmail.com.