Quality Control Technologies
by John Kaverman, president
Pad Print Pros LLC
As US-based companies strive to become leaner and processes for decorating plastic products become increasingly more automated, many decorators are looking for quality control technologies that can fit seamlessly into their existing systems. The determining factors in deciding whether to employ any of these technologies are, of course, cost and return on investment.
For example, in the front end of automated pad printing systems, cameras may be used to find parts as they are randomly spaced on conveyors, communicating their respective locations and orientations to a robot for pickup, orientation and loading. This makes sense if a company is tasked with printing thousands of parts per hour – a human operator cannot possibly orient and load parts accurately at the required rate.
Such was the case in a recent application where cameras were integrated into a “flexible feed” operation with parts conveyed directly from a powder coating line to a pad printing system. As the parts traveled along a conveyor with random spacing and orientations, an optical sensor signaled a high-resolution camera to photograph each individual knob. The camera then communicated each part’s location on the conveyor belt, as well as the part’s orientation, to a multiple-axis robot that picked up the parts and placed them in the correct orientation before loading them onto the pad printing system’s rotary table. All of this was accomplished at the rate of 60 parts per minute.
The parts then were indexed to the pad printer, where graphics were applied on two different planes prior to UV curing. After printing and curing, the parts were indexed under an LED-lit dome, where a second set of high-resolution cameras inspected each for print quality.
The system was tasked with locating, loading, printing, curing and quality-inspecting about a dozen different colors of parts. For different colored parts, separate files were created for both sets of cameras. Using lighting to increase or decrease contrast as required for the various substrate colors, the camera was able to discern and communicate the position of a small, molded feature that was used for orientation.
For the image quality inspection step, the operator could set different inspection fields individually, for both planes, specifying the exact location and dimension of the inspection area. Additionally, the operator had the latitude to increase or decrease the PASS/FAIL tolerances by adjusting, in pixels, the percentage of missing pixels (seen by the camera as contrast between the substrate color and the color of the printed image) within the inspection fields.
The QC camera system included a “heads up” display at the operator interface, with real-time images of each part as it was inspected. If no defects were detected, a green light was shown under the photo in a scrolling, chronological record. If defects were detected, the software indicated exactly where in the inspection field the defect occurred, accompanied by a red light, so the operator could inspect reported defects and troubleshoot the problem more effectively.
Of course, in that example, the parts were always exactly the same shape, just different colors. Setting up such a system for a wide variety of different shapes, sizes and/or a lower volume of parts per hour probably wouldn’t be cost-effective. If that scenario sounds closer to what occurs within your operation, using a camera for print quality still can be beneficial, especially if the print is small in size and a human operator would be hard-pressed to see it easily. In that case, using a camera that simply magnifies the image 10x or 20x for easier viewing can make the operator more efficient at catching potential defects.
In either fully automated, high-throughput or semi-automated, lower-throughput applications, cameras also can evaluate the dimensional tolerance of the print relative to some feature of the part, saving a company from printing a batch of parts off location.
In cost-justifying either solution, weigh the costs of cameras vs. the potential costs of scrap and/or additional inspection by human operators.
Color monitoring systems
Color is another area in which QC is becoming increasingly important as brand owners push more requirements onto suppliers and decorators.
In some cases, the software of post-print cameras can be taught to evaluate printed color vs. a master. More commonly, decorators use small, static-mounted or handheld spectrophotometers to compare the reflected color of the substrate, as well as the applied graphics, to standard. These handy devices can display results in any one of several different ways, depending on their specific color measurement system requirements.
For transmission color measurement, larger, bench-top instruments can be employed. For reflectance or transmission, standards can be easily saved, named, compared and recalled with relative ease, as compared to as recently as a few years ago.
Twenty years ago, spectrophotometers required a 10mm diameter “target” to effectively measure a sample, limiting their use. Today, some instruments can measure areas as small as 1.5mm in diameter, making them useful in measuring the color of smaller sample areas. Translation: I used to use them exclusively in screen printing applications; now I can use them in pad printing applications in which the graphics typically are much smaller by comparison.
Don’t know where to start with regard to color management? No problem. Michigan-based X-Rite (www.xrite.com) offers one-day FOCA (Fundamentals of Color and Appearance) seminars around the country, at a frequency of three or four each month. These seminars require no prior color training and address color control for a variety of plastics-related industries, including automotive, electronics, packaging and coatings. FOCA is a prerequisite to the company’s more in-depth FIQC seminars, where color theory is put into practice using the latest instruments and software. Attendees can participate in one or both, since they run on consecutive days in locations nationwide. While I’ve not attended an X-Rite seminar to date, I’ve attended seminars with two other color measurement companies and found them to be extremely useful and well worth the time.
The quality and color of printed graphics are difficult things to accurately quantify in the absence of the two technologies that I’ve mentioned. No two people can, or will, look at a decorated part and give consistent feedback as to the quality of color of the graphics. We are human beings and, therefore, subjective. Even over time, as we tire or our physiology changes, our visual acuity and color perception vary. Such is not the case for cameras.
So far, we’ve discussed quality control technologies as they relate to pre- and postpress. What about on press?
Programmable, stepper motor-driven pad printing equipment
Where the actual cycle parameters (e.g. pickup and transfer stroke distances, pick-up, transfer, doctoring and part conveyance speeds and locational accuracy) of the pad printing machine are concerned, nothing beats the process control offered by a programmable, stepper motor-driven machine. Unlike most pneumatic machines, in which speeds and distances still are largely mechanical and the consistency of either is subject to fluctuations in air pressure and the volume of air available, stepper motors are programmable, significantly more accurate and entirely consistent.
Thinner metering/viscosity control
For the ink in the machine, some people mistakenly turn first to thinner metering devices in an effort to maintain the consistency of print quality over the course of a production run, shift or day before looking at plant conditions.
Any decorating process works better, and can be much more accurately maintained, in a controlled environment. If the printing environment is subject to constantly changing ambient conditions (temperature and relative humidity), a thinner metering device is unlikely to be of much help.
In fact, regardless of whether the printing environment is controlled, a thinner metering device is of questionable value. Thinner metering does only what the name implies: meters a measured amount of thinner into the ink. Simply adding thinner without first knowing the optimal viscosity for a given ink – and accurately measuring that viscosity on press – achieves nothing of value. What is needed (if it’s really needed, read on) is viscosity control.
Viscosity control involves a method of recirculating the ink, pumping it first from the sealed ink cup to a viscosity measuring device that monitors the resistance of vanes as they rotate in the ink, adding thinner until the resistance is within the range that correlates to the desired viscosity and then pumping it back into the ink cup.
Metering alone simply adds thinner to ink with an unknown viscosity, usually by gravity feeding it on top of the ink within the sealed ink cup, then relying on the doctoring motion of the ink cup and cliché to effectively distribute that thinner consistently – something that is not likely to happen with any level of efficiency.
So, when is viscosity control really necessary? Only with single-component inks and only if tasked with mass producing the same part in a controlled environment with a process that already is carefully controlled for a longer period of time (up to 24 hours) without being required to add ink to the system.
Two-component inks (inks with a catalyst/hardener) change chemically until their pot life has expired. As a result, it is easy to compromise the integrity of two-component inks, especially as they get further along in their operational pot life.
Even single-component inks become compromised over time. Every time the machine doctors old, dried ink that remains in the etch or the surface of the cliché is re-wet with “new” ink, airborne contaminants are invariably introduced. If operators don’t have to add ink to the system, it is probably safe to assume that the inks will maintain transfer characteristics and chemical integrity up for up to 24 hours (equal to three eight-hour shifts). If ink must be added, avoid adding new ink to old. Since that involves purging the entire recirculating viscosity control system and reloading it, it most likely negates the purpose of having viscosity control in the first place.
Many technologies can be employed to ensure quality control. Vision systems can aid in controlling part orientation, image location, print quality and color. Color monitoring systems can ensure that the color of the substrate and printed images remain consistent. Programmable, stepper motor-driven machines can eliminate the potential for variations in cycle parameters, and ink viscosity control can eliminate deteriorating image quality.