Injection Molding Shot Weight Repeatability Calculator

Summarize short-term shot or part-weight variation with mean, sample standard deviation, range and CV.

Formula and method

Sample SD = √[Σ(x − mean)² / (n − 1)]. CV (%) = sample SD / mean × 100. Range = maximum − minimum.

Inputs and units

Use one consistent basis: complete shot, one cavity, or a cavity-numbered part. Keep scale resolution and post-mold conditioning time constant.

Worked example

Weights 10.02, 10.01, 10.03, 10.00 and 10.02 g give a sample CV of about 0.11%.

When to use the study

Use for consecutive samples collected under one stable setup to screen short-term consistency.

How to interpret the result

CV normalizes variation by the mean and supports like-for-like comparison. Review the run sequence and range as well as the summary.

Assumptions and limitations

CV is not Cp/Cpk, a control limit or proof of process stability. A five-shot worked example is illustrative; formal validation normally needs a justified sampling plan.

Validation guidance

Preserve machine, mold, material lot, melt/mold temperature, cycle state, units, instrument and sample order with the result. Repeat after any material, machine or tooling change.

FAQ

Is a low CV the same as a capable process?

No. Capability also needs justified specifications, a stable process and an adequate sampling plan.

Related workflow

Process Validation & Stability hub · Scientific molding validation workflow · Weight measurement guide · Gate seal and pressure-drop setup

Sources

PlasticsToday: Scientific Molding, Part 1 · M. Holland: Monitoring Pressure Loss · BASF: Injection-Molding Problems · NIST: Measures of Scale · NIST: Coefficient of Variation · PMC: Filling Imbalance in Multi-cavity Molds · Hingtung: Cavity Balance Study