by Paul Uglum, president, Uglum Consulting, LLC
An article in the July/August 2022 issue of Plastics Decorating described a structured method and steps that can be taken to reduce the risk in adding new processes or upgrading current processes. Even when following these steps, the result sometimes is less than satisfying. Failure to meet the targeted production yields, excessive rework and customer dissatisfaction are but a few of the undesirable outcomes.
A closer look shows a common thread of overlooked conditions or underestimated risks, which impact process success. Many of these arise from a failure to understand product and process requirements and take the steps to ensure these requirements are met. Almost all are preventable or their impact can be greatly reduced.
The failure to achieve targeted performance often is the result of failure to address one of three major root causes: placement of the line, understanding the surrounding environment or properly managing people movement and training. These problems most often show up as defects resulting from contamination, or less often, improperly cured materials.
The surrounding environment
In broad terms, the surrounding environment includes climate and conditions of the region in which the plant is located, the quality and stability of the utilities, the condition and design of the building, and processes and facilities that surround the actual decorated process installation.
Dry, hot climates significantly increase the risk of particulate contamination. In many regions, there can be seasonal issues due to weather conditions and the cleanliness of the surrounding areas. Climate control and air filtration can reduce the impact of, but not eliminate, these environmental conditions. Additional steps, such as part cleaning and manufacturing in an enclosed clean area with increased environmental controls, may be needed.
The in-plant environment is characterized by the location and proximity to activities that generate contaminates or negatively impact environmental controls. Utilities including the water quality, cleanliness of the pressurized air and stability of the electrical supply also can have a significant impact.
It is a good idea to evaluate the proposed location for risks and the level of dust and dirt exposure expected prior to finalizing the location choice.
Placement of the line
Obviously, a greenfield approach to plant design gives the best opportunity for success. In most cases, a new line will need to be placed in the available space in the plant or an addition will be needed to the existing facility. The choice of location for the line will have an oversized impact on the chances of success. Placement where there is significant foot traffic, near doors or loading docks, or next to processes that produce contaminants will greatly increase the chance of contamination issues.
All decorating processes should be placed in the cleanest possible environment. The cost per square foot for cleanrooms is extremely expensive, as is the cost of operating them. In most cases, some level of “whiteroom,” an area with some level of cleanroom features and control, will be adequate. Dust and dirt problems easily can cause 5% or more yield loss – and in the case of high gloss applications, it can be greater than 50% in some cases.
In choosing a location, it is not always possible to avoid the voice of the executive, who either does not understand the process or is constrained by business objectives and cost limits, and overrules the design team’s location choice. This can result in the placement of the line in a totally inappropriate location. At a minimum, if this occurs, designs should include features to reduce the exposure to contamination and ensure only clean parts enter the decoration process.
In designing a clean area, create an easy-to-clean work environment. Keep the number of horizontal surfaces to a minimum and use sloped surfaces to prevent objects from being placed on them or contamination accumulating on them. Use smooth surfaces with rounded edges for easy cleaning. The use of air locks and tack mats at the entry points for people and materials also is helpful.
The distance the part needs to travel and the time spent from molding to decorating should be kept to a minimum, and parts need to be protected from the environment during transport. All containers and transport racks need to be clean and maintained.
Construction always is dirty, to some degree, and can impact both the new line and current processes. Parts and processes can be exposed to elevated levels of aerosols, fibers and particulates. Additional actions should be taken to reduce the risk of contamination during construction.
Start up
Assuming the line is installed correctly and working properly, the primary problem often is contamination. It is of critical importance to understand what form of contaminates exist in the environment and the source. Tools, such as surface energy measurement, particle measurement, optical microscopy and chemical analysis, can be useful in identifying and characterizing contaminants.
Right after installation, it is a good idea to make sure all of the water (including humidification of makeup air) meets the required standards and are not contributing contaminants. Air lines should be checked to make sure that they can meet the required pressure at the cubic foot per minute usage and are free of oil contaminants. Deionizers need to be checked regularly to make sure they are working correctly.
Remember, when parts need either UV or thermal cure, the important parameters are not the output, temperature of the oven or energy of the UV source, but instead is the dose or temperature at the part surface. Under-cured parts may look fine but ultimately will fail in the field.
Contamination
The level of cleanliness required for each process and material set varies. For example, a low-gloss coating generally will be less sensitive to particulate contamination than a high-gloss coating. Metallic thermal transfers and high-gloss in-mold decoration also are sensitive to particulate contamination.
There are many sources of contamination, and it’s surprising that they are not better understood. Contamination exposure can take several pathways: dirt and fibers from people’s clothes; behavior and movement; sweat and cosmetics; and contact with contaminated surfaces.
Process- and packaging-generated contamination also may be significant. Often, particulates and fiber are the most common in-plant contaminates, followed by aerosols. Some contamination risks can be reduced by the process design and others require more direct intervention.
Maintaining a positive air pressure with filtered air surrounding the decorating line, controlling static charge using humidification and anti-static tools, and eliminating contaminate sources are the minimum starting points. A clean environment is important not only in plastic decoration but both before and after the decorating process.
Dirt is attracted to surfaces in several ways and all need to be controlled. The primary ways parts acquire dirt are by static charge on the parts to be decorated; gravity, for particles larger than 5 to 10 microns in the air; and physical contact due to part or air movement. Static is one of the most important to control since plastic parts develop a static charge as they cool. They also can acquire a charge as they are being handled and moved. It is important to remove the charge during the molding process and control it during storage and movement.
Sources of good information about cleanliness include VDA-19.2, which covers technical cleanliness, and ISO 14644.9, Surface Cleanliness by Particle Concentration. Although these focus on assembly and larger particles (in the case of VDA-19.2) and cleanrooms (in the case of ISO 14644.9), much of the content is directly applicable to decorative part manufacturing as well.
Characterizing contaminates
To control and eliminate contaminants, it is important to be able to measure them. Fortunately, several tools exist that can aid in this process. Particles 25 micron or larger are visible to the naked eye. Smaller particles require additional effort to identify and measure.
Small particles and fibers, less than 5 to 10 microns, remain suspended in air until they touch a surface or are attracted by static charge. Large particles and fibers, larger than 5 to 10 microns, fall out of still air and are found on surfaces.
Dust in the air can be reduced by pressurizing the work area, filtering incoming air and avoiding dust-producing materials and processes. Raising the ambient humidity also can help by reducing static charge. In all cases, the first step is to deionize plastic parts as soon as possible after removing them from the mold and every time after they are handled.
Tools such as hand-held static meters are useful in determining if parts have a static charge and if the antistatic bars and blow off are effective in reducing or eliminating the charge on parts.
Instruments for surface partial detection and characterization and real-time partial deposition monitoring are sold by companies such as SentrySciences. They can be useful in characterizing a site prior to installation, monitoring a line during operation to identify spikes in the rate of contamination exposure and in problem solving.
There also are very low-cost methods that provide a visual indication of contaminates. Particles larger than 25 microns are visible to the naked eye but often are overlooked. Using a UV-LED flashlight will cause many fibers to fluoresce, making them much more visible. Placing a flat, clean, high-gloss plaque in the work area for a period of time and then shining a bright 1,000-lumen light at a low angle is a good way to see the relative degree of the problem.
Removal strategy
Preventing the generation of dirt always is better than removing the contamination. When cleaning is attempted, the contaminant should be removed from the part and surrounding area – not just moved from one place to another.
Dirt and fibers are not always easily removed. Traditional cleaning methods like IPA wipes can remove dirt but also can transfer contaminates from one part to another. Due to the influence of van der Waals molecular forces, particles smaller than 20 microns are difficult to remove by blown air alone. This includes removal with anti-static air. Particles in this size range are best removed by a combination of deionized air and mechanical force. Linear brushes, ostrich feather brushes or a CO2 cleaning method that imparts some mechanical force all can be effective for this particle size when combined with deionization.
In all cases, the parts should be periodically measured after cleaning to determine if the cleaning method is effective.
The human portion
When people understand why they are performing an activity, they are far more likely to do it correctly and not look for shortcuts. Make sure all personnel understand how to properly clean parts, maintain a clean working environment and handle parts. Basic rules and qualifications need to be clearly defined and training materials in place.
People often are the largest source of contaminates. Keeping the number of people and amount of movement to a minimum has a significant impact.
Clothing choices have a major impact on cleanliness, both by generating contaminants and by transferring them. The amount of fiber produced can be reduced significantly by using appropriate textiles. If specific clothing is only worn in the clean area, the amount of contamination will be decreased. If the type of clothing worn in the clean area is different than worn in non-controlled areas, people are more aware of special rules and requirements.
Regular cleaning using HEPA vacuums, wet mopping or methods that do not spread contaminants around the decorating process also are critical to success, as are preventive maintenance processes designed so they do not contaminate the area.
Closing
It is important to understand just how much prevention and cleaning are necessary, as not all processes are equally sensitive. It is important to focus on what actually adds value to the process. When adding or upgrading a decorating process, take time to evaluate the site prior to the final location selection. Understand what the process is sensitive to and make sure the proper tools are in the plant to allow both process control and problem solving.
Paul Uglum has 43 years of 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].
