- First, identify the variables that have the greatest impact on the process or workflow. These will then become the key control points for the process.
- By finding the key control points we can ensure quality at critical stages in the workflow and reduce the opportunity for defects downstream in the process.
- Due to the physical limitations of the process involved, tolerances must be established for these standards.
|Image 1. Digital Plate Construction|
- Ripped Digital File
- Imaging (mask ablation)
- Processed/Finished Plate
- Image resolution
- Orientation of the image (right reading/wrong reading)
- Linescreen, angle, and dot shape
- Screen configuration (halftones and solids)
- Compensation (bump and printing curves)
- Dimensional accuracy (distortion)
- Registration marks or guides
Image 2. Esko Grapholas visualization tool
- Fidelity of the image (focus adjustment)
- Density of the mask from 3.5 to 4.3
- Stain level of the plate after the device removes carbon mask (Dmin) <0.07
- 50% Density on 0.30 +/- 0.02
Image 3. Out of focus ablated image
Image 4. Transmission densitometer
- Dot formation
|Image 5: Micrometer|
|Image 6: MinDot test with two different screen configurations (stochastic and circular)|
Image 7. Prepress scale where the values that must be measured on the plate for each tone are labeled. Source: FIRST
- Measure and record the lot number, relief, caliper, minimum dot percentage and halftone percentage of at least one color of each job.
- Conduct a weekly "MinDot finder" test when there is a lot change of the photopolymer plate. This weekly test will help establish an appropriate main and back exposure.
- Perform a daily "focus test" and a weekly "stain test" every time there is a lot change This testing will confirm adequate mask ablation. Maintenance for cleaning lens optics and changing parts must be programmed with the manufacturer.
- Conduct a bi-monthly audit of the intensity of the UV exposure lamps. It is recommended to change lamps when the readings fall below 80% of their original power especially for jobs that incorporate special effects in the highlights or solids.
- Weekly measure the Brix degrees or density of the processing solvent and adjust according to the manufacturer's recommendations.
- Review the back exposure at each lot change and adjust accordingly.
- Only back expose plates that will be used in less than half an hour.