Technical Documents - Documentos Técnicos Home

PREPARING A CONTROL CHART

Do not try to predict film or print quality solely from process control results. A good monitoring program uses control charts and transparency or print evaluation in decision-making. A control chart is essential to monitor the processes as a means to separate the process results from those of the printer. Compensating for process deviations with the printer is not likely to result in optimum print quality.

FIRST , THERE ARE SOME TERMS RELATING TO CONTROL STRIPS AND PROCESS MONITORING THAT YOU SHOULD KNOW

It helps to know some of the terms associated with control strips and control plots. These are some of the most commonly used terms. You'll find them used frequently in the description of process monitoring in this page .

Control Strip - These are pre-exposed strips that you can process to check your process. They provide a standard against which you can monitor your process.

Reference Strip - Included in each box of control strips is a reference strip of the same code number, specially exposed and processed by munufacturer . Use it to obtain readings from which you calculate the aim values for your batch (code number) of control strips.

Correction Factors- Correction factors for each code are given in the instruction sheet supplied with each box of control strips. Add or subtract these correction factors from the reference strip densities to obtain your aim values

Aim Values - Aim values are the standard to which you compare your process control-strip readings. The aim values are the values obtained by adjusting the reference-strip densities with the correction factors.

Tolerances and Limits-. Remember that correction factors are different for each code-number batch.

Table 1 :The tolerances and limits listed below apply to the control strips used to monitor your process when using KODAK EKTACOLOR RA Chemicals.

Action Limits-The action limits are "early-warning'' limits They alert you to the need to correct a condition that may cause your process to drift out of control. When any point exceeds the action limit, it is still all right to process production, but you should check for the cause of the shift and correct it before the process drifts out of control. The action limits also define an operating range for the process. It is not necessary to keep the process at the aim line for all density values as long as it remains within the "aim zone" (between the tipper and lower action limits),

Control Limits-The control limit is the largest shift in the control- strip density values that you can tolerate without a potentially adverse affect on production . When any point exceeds the control limit, stop production until you find the cause of the shift and can correct it.

To prepare a control chart using the KODAK Process Record Form, No. Y-55 :

1. Use a separate chart for each processing machine.

2. Record the reference strip code number and the aim values in the appropriate places. Use colored pencils to identify the red, green, and blue densities when recording the aim values.

3. Draw horizontal lines to represent the control and action limits.

4. Record the process and machine identification.

5. When plotting the control values, record the date and time that the control strip was processed and note any chemical or mechanical changes made as a result of the plots.

6. When changing to a new control strip code number, note it on the chart, with the date, and record the new aim values.

7. Plot control strip values against the aim values. Connect the points to give a continuous graph.

When control values plot within the action limits, no adjustments need to be made. If a control value plots outside the action limits, take corrective action. If a control value plots outside the control limits, stop processing customer work until the cause is determined and corrective action brings the plot back within the control limits.

HOW DO I PLOT THE CONTROL STRIP DENSITY READINGS? :Make up a chart using KODAK Process Record Form, Y-55, or similar graph paper. First draw in the tolerance limits given in Table 1 (above) and as shown in the example on the right. Use black for the action limits, and red for the control limits as shown in the example. This chart becomes a running record of your process. It enables you to tell, at a glance, the condition of your process and any changes in it. Next, calculate your aim values by following steps 1 and 2 below. Fill in these aim values in the spaces on the left margin of the Y-55 form. To read the strip, use a precision electronic densitometer , equipped with heatabsorbing glass and status A filters. For the most precise measurement, be sure that you do not move the strip when making the density readings. A step-by-step procedure for starting out is as follows:

1. Set up aim values by using the reference strip supplied with each box of control strips. Be sure that you allow the reference strip to warm up to room temperature for about fifteen minutes before you read it. If you read the strip while it is cold you will get readings that are lower than normal, especially in the green higher densities; this will cause your control strips to plot higher than normal. Measure the red, green, and blue densities in the center of the black patch (BP), the high density patch (HD), the low density patch (LD), and the white patch (Dmin).

If you have several boxes of strips with the same code number, read the patches on the reference strips from six to eight boxes and average the readings. The correction factors will be the same for a particular code number. Each batch is identified by a code number on the label of the control strip and on the reference and control strips.

2. Apply the correction factors from the instruction sheet packed with each box of control strips to calculate the aim values for each density plot. These corrected density values are the aim values for your process for that batch of control strips.

The corrected readings of the black patch are the BP aim values. The corrected readings of the darker neutral patch are the HD aim values. The corrected readings of the low density neutral patch are the LID aim values. Subtract the adjusted LD values from the adjusted HD values to obtain the corrected aim values for HD - LD. The corrected red, green, and blue readings of the unexposed area are the D-min aim values.

To assist you in finding the cause of a control problem, keep records of:

• Machine speed
• Solution temperatures
• Agitation
• Processing solution filter replacement
• Squeegee adjustments
• Parts (racks, gears, pumps, lines) maintenance
• Operators
• Processing solution mixes
• Replenishment rates
• Film types, sizes, and amounts
• Bleach regeneration schedule
• Silver recovery procedures
• Solution adjustments, (i.e., pH, chemical additions, etc)
• Analytical results of mixes 

NOTE: Narrowing the control or action limits recommended by Kodak may cause false out-of-control signals. This can increase process variability and can adversely affect product quality. 

<< Back - Home - Next >>