by John Kaverman, Pad Print Pros, LLC
My mission for this article was to write about eliminating contamination, but as I thought about it, I realized that minimizing it is a more realistic expectation. Why? Because contaminants are everywhere: in and on the material, on human operators and equipment, on packaging and in the production environment itself.
While the costs of minimizing contamination can range from the few extra dollars it takes to implement cleaner materials and better techniques within an existing production environment to the costs of building a cleanroom, the potential cost of doing nothing (losing customers) can be even higher. Let’s take a look at some common causes of and remedies for contamination.
The material itself can be a haven for contaminants. Mold release agents, grease, oils, condensation and even some additives can be a problem for decorators. For example, most decorating processes require the material’s surface energy to be above some minimum. Mold release agents are a definite “no-no” because, by nature, they generally lower the material’s surface energy.
Grease and oils, as well as other types of lubricants and even “cleaners” from upstream manufacturing processes, also can cause issues with the initial transfer efficiency of decorations, in addition to problems with the subsequent adhesion and durability of the image. Cleaner-lubricants, such as WD-40, on tools, conveyors and machine accessories easily can cross-contaminate product and should be avoided. Once parts are contaminated, they can sometimes be cleaned using isopropyl, denatured alcohol or “plastic cleaners” that are specially formulated for certain substrates, such as ABS.
An important note: If forced to wipe parts to remove contaminants, be sure to change the wipes frequently to avoid simply cross-contaminating product. I’ve had clients that found it necessary to dispose of wipes as often as every part to avoid simply wiping the contaminants from one part to another.
If cleaning galvanized metal, I recommend using distilled water instead of cleaning solvents, as distilled water does a much better job on galvanized metals. Also, avoid using municipal tap water to clean: It can contain chlorine, as well as fluoride and other additives that – by themselves – can cause problems with adhesion, in addition to concerns with the chemical and mechanical resistance of inks.
Some pretreatment technologies include a “cleaning” component. For example (as I learned at the Society of Plastics Engineers Decorating and Assembly Division TopCon in Tennessee earlier this summer), plasma can remove certain kinds of contaminants at a molecular level.
Failing to adequately acclimate a substrate to the printing environment also can result in contamination, since decorating processes (especially those using petrochemical base inks) don’t like water. When bringing cold parts into a warm production environment or warm parts into a cooler production environment, condensation occurs. Even if it can’t be seen, water is present on the substrate material, and that water will eventually find its way into the ink.
Finally, some additives can cause problems. I was once called in to consult on an appliance panel pad printing application where the client was having all sorts of issues with adhesion, as well as chemical and mechanical resistance. After a lot of investigation, we determined that the issue was the fire retardant additive that had been added to the ABS resin. It turns out that the temperature in the conveyor oven the client was using to dry the printed parts was high enough to cause the additive to leach out of the material, causing loss of adhesion that was not clearly evident until several days after the parts were processed.
A human operator with the personal hygiene of Pigpen (the Peanuts character) might be expected to contaminate parts, but even a well-groomed person can contaminate parts with hair, dead skin cells, cosmetics, oils, perspiration and fibers from clothing.
People should be relatively “clean” when handling parts. Hair should be reasonably restrained to avoid having it fall onto parts. This includes facial hair. Excessive cosmetics and, in extreme cases, even residue from fragrances can contaminate parts, as can moisturizers in hand creams.
Operators and material handlers should their wash hands after handling food and should dry wet hands with lint-free towels or a dryer.
The production environment should be kept at a temperature that is comfortable enough to minimize the chances of people perspiring to the degree where their sweat can contaminate parts, as well as for reasons to be discussed later.
Operators should avoid wearing articles of clothing that shed a lot of fibers. If sweaters and fleece are necessary to keep people warm, consider whether the cost of turning up the heat or providing a disposable over-garment is cheaper than the scrap their clothing potentially introduces to the decorated product. Lab coats aren’t expensive, nor are scrubs. Both are easy to clean and on the low end of the scale for shedding fibers as compared to exposed cotton or other loosely woven fabrics. And, of course, there are always good old disposable garments…but some of those can be hot and uncomfortable.
Dirty equipment is bad. Air filters left unchanged on machines and dryers simply blow dirt around the shop. Bag filters as soon as they are removed, and always take the filters out of the decorating area to clean them.
Spilled ink left to dry can later flake off and cause defects. When a spill occurs, clean it immediately before it migrates.
Plastic shavings from upstream gate removal processes invariably find their way onto screens, clichés and even dies, causing voids in images. Fork trucks and pallet jacks rolling from a dirty warehouse, across a parking lot and into the print room can introduce all kinds of contaminants.
A strong preventive maintenance program should include thorough cleaning, along with proper lubrication schedules and the close inspection of moving parts that, left maladjusted, can produce scrap via metal shavings.
Consider using a sealed system vacuum instead of a broom or dust mop to clean up dirt, dust, shavings and other debris. Brooms and open system vacuums create clouds of tiny particulates that find their way onto everyone and everything in the area.
Packaging is a big contributor to contamination, with cardboard boxes, bins and dividers leading the way. Investigate reusable (cleanable) plastic totes instead of cardboard whenever possible. If cardboard must be used, consider putting a bag over the boxes before they are introduced to the decorating environment. Even if printing parts from bulk that are bagged inside of cardboard boxes, the boxes themselves, if left uncovered, can introduce contaminants to the area.
Reusable containers should be cleaned regularly to ensure they are reasonably clean. If plastic bins or covers are damaged, don’t tape them back together with packaging tape: replace them. Tape on a material handling bin acts as a dirt magnet.
Wood pallets are hard to avoid, but be aware that they potentially bring a lot of contaminants into the production area. Staging palletized materials outside the decorating space and transporting them in with a conveyor (manual roller or mechanized) or on a two-wheeled cart that is dedicated to the decorating area is helpful. Of course, plastic pallets are better, if possible.
The production environment is perhaps the biggest contributor to contamination. Having walls and floors that can be cleaned is helpful. Concrete, cinder block or paneled walls are difficult to clean, whereas drywall painted with a washable paint is easier. Flooring surfaces, such as smooth concrete painted or sealed with polyurethane or laminated or tiled flooring, are easier to clean than wood or carpet.
Stainless steel, metal or laminated work surfaces and shelves are better than wood – especially bare wood or compression board. If wood must be used, I recommend sealing or laminating it with contact paper that can be easily wiped clean.
When cleaning the walls, floors and work/storage surfaces, use a sealed system vacuum instead of a broom for sweeping. If sweeping is necessary, I strongly recommend spraying down a sweeping compound first to act as a magnet for dirt and airborne contaminants, keeping them on the floor and out of the air. Spray sweeping compound can be found at a local big box store. It can be mixed with water and applied using a pump sprayer, such as those used to apply fertilizer or weed killer.
Airflow is important. Laminar airflow (uniform, nondirectional airflow originating at the ceiling) and a slight positive pressure in the production environment definitely helps. Keep doors (especially overhead doors) closed. If laminar airflow capability isn’t available, at least keep machines and materials away from doorways and turbulent airflow/high-traffic areas whenever possible. If turbulent airflow is available, at the very least try to make it positive so contaminants are forced out of the production environment as opposed to being blown in.
As I mentioned before, having controlled temperature and humidity is helpful, as it keeps operators comfortable, enables the materials to acclimate before decorating (to avoid condensation) and generally aids in efficient transfer of inks. If the climate cannot be controlled for the entire production area, consider purchasing equipment that is enclosed and controlled. Weve installed several enclosed pad printing systems with their own climate control and HEPA (high efficiency particulate arrestance) filters. Soft-walled cleanrooms also are a less expensive option to full-blown, hard-walled cleanrooms.
For certain applications a cleanroom might be necessary. For example, if decorating medical devices or pharmaceutical packaging, it likely will be required.
The costs associated with soft- and hard-walled cleanrooms vary depending upon the classification required to meet the customer’s specifications. Cleanrooms are classified according to the number and size of particles permitted per volume of air.
Cleanrooms are another complete article all together. In researching this article, I found a lot of useful information about cleanroom technology online at www.cleanairtechnology.com/cleanroom-classifications-class.php.
Minimizing contamination doesn’t need to be difficult, and the costs associated with implementing most commonsense “clean” practices usually are lower than the cost of contamination-related scrap and rework.
John Kaverman is president of Pad Print Pros LLC, an independent consulting firm specializing in pad printing. Kaverman has 28 years of combined industrial screen and pad printing process experience. He can be reached via email at firstname.lastname@example.org or through his website, www.padprintpros.com.