SPC Tampering and Give-Away Limits on a Bottling Line
A beverage plant fills 500 mL bottles on a 12-head rotary filler. The label declares 500 mL; the legal minimum is 500 mL, and the internal upper limit is 510 mL to control give-away. After a kaizen, the team set the mean fill to 505.0 mL and reduced the standard deviation from 3.0 to 1.6 mL. The control plan uses an Xbar-R chart with subgroups of five bottles every 30 minutes; operators also adjust the filler whenever a single bottle looks low. Over three days, the chart shows two points above the upper control limit on the Xbar chart and one run of eight points below the centerline after a filler-bowl gasket change. The line runs three shifts; only the day-shift lead has been trained on the new chart. Changeover to a different bottle size occurs twice per shift. The plant manager wants to remove the 510 mL internal limit to save give-away cost and let the chart control the process.
1. Assess process capability against the 500-510 mL limits and evaluate the manager's proposal.
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With mean 505.0 mL and standard deviation 1.6 mL, Cp = (510 - 500) / (6 × 1.6) = 10 / 9.6 = 1.04. Cpk = minimum of (505 - 500)/(3 × 1.6) and (510 - 505)/(3 × 1.6), which is also 1.04. The process is centered but not capable against a 1.33 expectation. Assuming normality, each tail is about 3.125 standard deviations from the mean, so roughly 880 ppm would fall below 500 mL and about 880 ppm above 510 mL.
Removing the 510 mL internal limit does not reduce low-fill risk and does not make the process capable. It may allow the mean to drift upward and increase give-away, which is the opposite of the stated cost goal. Control limits are not specification limits; they describe the voice of the process. To reach Cpk 1.33 while centered, the standard deviation must be about 1.25 mL or less. The right response is variation reduction and a validated response plan, not removing a customer or legal guardrail.
2. Interpret the SPC signals and prescribe the correct operational response, including the gasket change and multi-shift training.
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Two points above the Xbar upper control limit are a special-cause signal; investigate the filler heads, product temperature, valve condition, and gasket installation. The run of eight points below the centerline after the gasket change is a sustained process shift, not random variation. Verify that the gasket was installed correctly and recalibrate if needed. If the new level is stable and acceptable, update the standard and recompute control limits from the stable period; do not keep plotting against stale limits indefinitely.
Operators adjusting on a single low bottle is tampering. It increases variation and makes the chart uninterpretable. The response plan should require verification with a rational subgroup and a defined check before any adjustment. Train all three shifts, not just the day-shift lead. Because changeovers occur twice per shift, use short-run SPC or stratify charts by bottle size so changeover variation is not mistaken for special cause. The control plan should name who responds, what they check, and when to stop the line.
3. What control-phase elements would make the improvement stick?
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Build a control plan with standard work for filler setup, gasket changes, and fill-head checks; a visual response guide at the line; and a trained owner for each shift. Include a layered audit that checks whether operators are following the response plan rather than adjusting on individual bottles. Add mistake-proofing where feasible, such as automatic rejection below 500 mL and a fill-volume interlock.
Track capability and control-chart signals monthly, along with give-away ounces and low-fill rejects. Update the PFMEA and control plan after the gasket change, and verify the new limits after a defined stabilization period. Use a 30/60/90-day sustainment review. For change management, explain to operators why single-bottle adjustments hurt them, involve them in setting the response rules, and make the chart part of the daily huddle. The manager's give-away proposal should be rejected in favor of variation reduction and disciplined SPC.