The Importance of Quality Control Testing
Product quality issues cost manufacturers thousands in lost revenue each year. Issues such as bubbles in lamination, fish eyes, haze and adhesive failure can often be avoided or quickly resolved by following simple control procedures before problems lead to significant scrap and production down-time.
Preventing lamination and coating problems before they occur is very often a matter of choosing the correct Quality Control (QC) tests, being diligent in their use, and using properly calibrated instruments. Consistently running these tests allows for a solid baseline to be developed as well as traceability in production. Using properly calibrated instrumentation ensures that the data collected is valid and reproducible. Here we will review the QC tests that are critical throughout the lamination process.
Adhesive Mixing and Weighing
Regardless if its solvent-based, water-based or solventless, a properly mixed adhesive is critical to performance. Always follow the adhesive manufacturer’s recommendations for mix ratio, mixing procedures and dilution. When batch mixing, weigh adhesive using scales that are appropriate for the job. Floor balances are typically used when weighing larger drum-size quantities; however, for smaller quantity products such as co-reactants, a lab bench scale with accuracy in the hundredths is best. When adding the co-reactant, ensure that all contents from the container are emptied when adding as some higher viscosity co-reactants can cling to container sides. When solvent-less adhesives are being used, ensure mix meter dispensing (MMD) pump ratio has been checked using a bench top balance with proper accuracy. Always keep a log of the weights and lot numbers for each run.
It is critical that structure development parameters are monitored and charted daily. Dyne levels of incoming films should be tested and recorded using ASTM D2578. If in-line Corona treatment is used, measure after the film has been treated. Viscosity of the mixed adhesive should be checked periodically using calibrated Zahn cup following ASTM D4212 or Brookfield viscosity using ASTM D789. Gravure cylinder should be kept clean and examined microscopically or volume tested using disposable liquid volume test strips. When running solventless adhesives, monitor adhesive pot-life on rolls, replacing if exceeded. While pot-life is important in water-based and solvent-based adhesives, it is usually at least 8 hours. Lastly, confirm and log measured coating weights at the beginning and end of each run using ASTM F2217.
There are several checks that should be conducted and logged throughout the lamination process. As previously mentioned, film treat levels should also be monitored on the secondary web. It is critical to check and log oven drying temperature and follow recommended adhesive manufacturer’s guidelines for drying. In addition to visual and odor inspection of the laminate, gas chromatography/headspace will confirm ink and adhesive level of drying. Keep log of these results. Always follow manufacturer’s guidelines for nip temperature and check hardness of nip roller using ASTM D2240 to ensure greater than Shore 85 A hardness. Temperatures on nip rolls can be monitored and charted using the readout on heating units and confirmed with a handled IR heat gun. These should also be logged and charted. Finally, bond testing should be run on the laminated structure using ASTM F904. Often bonds are checked off press as well as throughout the cure of the adhesive. These should be logged and charted and be relatively consistent for a given laminated. It’s also important to note the mode of failure within the laminate (i.e. is the adhesive splitting or staying on one or the other substrates). This is important to know should a problem arise later that is inconsistent with normal bond and failure mode.
By being diligent in recording and monitoring the above tests for each laminated structure many potential laminate problems can be averted. Catching shifts in the normal process early can prevent a large laminate recall later. In addition, by charting and analyzing data, one can see normal variability in process that will still yield passing laminate performance. Ranges can then be developed for each of the data. Many of the tests are inexpensive in terms of needed equipment and designed to be run quickly often press side. A small investment in QC can have a major impact on the bottom line.