Decorated plastic parts are everywhere in consumers’ lives. They are an important part of people’s cars, phones, homes and packaging for many of the products consumers purchase. Because the appearance is so important, plastic parts are subject to high standards, not only for the function of the parts but also for the appearance.
Recent postings to the Society of Plastics Engineers’ web-based discussion tool, The Chain, highlighted this issue. The discussion started with a simple question: Is there an accepted industry standard for inspection criteria for cosmetic and functional standards of molded parts in the medical industry? The discussion that followed was wide ranging and contained quite a bit of useful information, as well as some discussion of why it is so important to have agreement on standards with the customer or supplier. For example, one of the issues was a customer’s new quality engineer, who did not have realistic expectations.
This article will focus on critical aspects of the discussion as well as provide some additional guidance. These issues impact both internal costs and customer relationships. How to set visual standards for cosmetic issues and what tools are available to establish these standards are important but not the entire story.
Establish an inspection program
One way to control the quality of the appearance is visual inspection of the parts. The objective is preventing quality issues from reaching the customer once production starts. Visual inspection is not the most reliable quality control method; however, because of the variety of potential issues and the importance of the entire appearance, it is important. While individual aspects can be measured with vision systems, people are still more flexible and more capable of seeing the part in its totality and observing what might be an unexpected anomaly.
There are three parts to ensuring a robust inspection program. The first is clearly defined standards used by both the manufacturer and the customer. The second is an understanding of how best to define and evaluate the appearance of parts. For this it is helpful to understand what tools and standards are available. Last is the need to evaluate and maintain the visual inspection process. In order to have a robust system, all three parts must be in place.
Setting realistic tolerances requires both knowledge and expertise. Unfortunately, many prints and specifications are vague or poorly written. It is important when quoting new business to review the documentation and quote exceptions to unachievable requirements. It is also a good time to recommend appropriate and achievable standards and to begin a dialogue on an agreement between manufacturer and customer.
Care should be taken to avoid immeasurable requirements on prints, such as “no surface defects allowed.” This is especially true when no viewing conditions are defined. A far better route would be to require no visible surface defects when viewed for a specified time under carefully controlled conditions. Clearly a part that has an obvious defect is unacceptable both in manufacturing plants and with customers. The standard, as with color matching, should be for the defect to only be visible under defined conditions that represent the actual use environment. What constitutes acceptable and unacceptable varies with the type of product and how it is used and viewed, so final standards will not be the same for all parts made using the same processes.
Tools for establishing standards
Resources that help in establishing standards can include industry standards and customer-specific standards. If none exist in a specific industry, look to some of the well-established documents from industries using the same production technologies. The processes used to make the parts are often identical and so are all the issues inherent in them. Furthermore, experienced quality and manufacturing engineers are an important resource because of their understanding of customer expectations and the capabilities and limits of the production processes chosen to make the part.
Make sure there is a common language with the customer and across the business. People have a tendency to look at the same defect and call it by different names. In order to communicate about the defect, a common language is critical. If a defect is given a name, it must be one understood by everyone from the inspector through to the final customer. Incorrectly named defects can lead to confusion. Physical limit samples, showing actual defects, are the best communication tool and can be useful for both training and maintaining a consistent standard of inspection. Workmanship standards are important tools for documenting the standards, as are Process Failure Mode Effects Analysis (PFMEA) and Quality Plans. The name of the document is not as important as the content.
There are a number of documents that can be helpful in developing clear standards with customers, and it is useful to look at what other industries are doing as well. One very useful document from the automotive industry is VDA 16 Quality Management in the Automotive Industry, “Decorative Surfaces of Accessories and Functional Parts in the Exterior and Interior Areas of Automobiles.” It covers evaluation conditions, feature definitions and the approach to defects and acceptance criteria. The current edition is the third revision, dated 2016, and it includes more on developing a common coordination between the supplier and the customer. The previous edition – the second revision – has a CD with examples of various defects and is still useful for that reason.
As for molded surfaces, the SPI AQ-103 “Cosmetic Specifications of Injection Molded Parts” 1994 edition is very well constructed and contains information on viewing conditions, acceptance criteria and a glossary of terms for typical cosmetic defects in molded plastic. Since molded defects can read through to decorated parts, it is important to understand this specification.
Many customers will already have their own proprietary documentation that include definitions for what defects are and what constitutes acceptable and unacceptable parts. If there are conflicts between industry and customer specifications, they will need to be resolved with the specific customer.
The Technical Association of the Pulp and Paper Industry produces several useful documents and tools, such as the TAPPI size estimation transparency, which allows for estimating the size of given defects or inclusions.
Some of the most difficult to characterize parts are the high gloss surfaces such as “piano black.” High gloss black finishes are particularly problematic, both because small defects are very visible and the smoothness of the finish is critical. Fortunately, ACT Laboratories has produced a set of Orange Peel Standards that provides a set of visual references on a scale of one to 10 for waviness of the surface.
Setting the standards for quality
Evaluation conditions including position of the part, lighting, distance from the inspector, length of observation and background should be controlled. Of course, good eyesight is an important factor in the choice of inspectors. The typical position would be that in which the part is normally used. Lighting should be about 1,000 lumens at the part, and parts should be held at about 20 to 25 inches from the inspector – approximately arm’s length.
The length of time observing the part should be limited and increase with the size of the part. Five seconds would be typical for a small, quarter-sized part. The best backgrounds do not distract or distort the observation. A low gloss neutral gray or black provides the best results for opaque parts. For parts that are transparent or have transparent sections, a black and a white background are best to identify possible inclusions or visual distortions. Finally, it is best to isolate the inspectors from distractions, such as extraneous light, bright clothing and foreign objects.
Define evaluation zones based on visibility, with immediately visible having the highest standard and those surfaces with obstructed views (not normally visible) a somewhat lower standard. Surfaces that are concealed after manufacturing or that are never seen do not need to be evaluated and should be excluded unless the defect impacts the function of the part.
Of critical importance are both training of inspectors and inspection process monitoring to make sure that the process remains consistent over time. Training should be frequent and use actual samples with and without defects. Evaluate the inspection performance using attribute agreement analysis. These are pass/fail decisions, and the best way to evaluate is to have a representative group of inspectors evaluate 30 visual defects and then repeat the evaluations at least once.
In addition to matching their own conclusions, the inspectors should agree with each other. Inspections should be done using the Kappa, a statistic that estimates the level of agreement in data beyond what would be expected by chance. A Kappa agreement of greater than 0.9 is excellent and 0.7 to 0.9 is good. Minitab provides a useful tool in completing this evaluation. Current best practice involves inspectors reviewing test samples every two weeks to measure and maintain their capability. This is not testing the ability to find a defect but to determine if it is a pass or fail part. More than one set of parts should be used on a rotational basis.
Finally, track and use the information. If a sudden change in yields based on visual inspection is seen, either an increase or decrease, take action. Review the parts and process the data to determine why. It is typical for inspectors’ standards to drift over time. Collecting data, including the location of defects, also can have the additional benefit of providing a tool for process improvement. If quality is fluctuating, make sure actual parts are used in communicating the issue. Pictures and verbal descriptions often are not enough.
Many tools exist to help establish and maintain a successful visual inspection system. Understanding them and using them will lead to improved quality and relationships between the manufacturers and customers of decorated plastic parts.
Paul Uglum has 43 years’ experience in various aspects of plastic materials, plastic decoration, joining and failure analysis. He owns Uglum Consulting LLC working in the areas of plastic decoration and optical bonding. For more information, send comments and questions to [email protected].