How to Document an Injection-Moulding Process Signature for Meaningful Comparisons

What a process signature describes

A process signature is a time-based record of one or more signals under an identified manufacturing condition. For energy, it may show power versus time over a cycle; other variables, such as pressure or temperature, require suitable instruments and connections.

To interpret a signature, the cycle start, duration, settings, material, mould and machine conditions must be known. A curve alone does not identify the product reference or verify product quality.

• Machine, mould, material, product reference, active cavities and order or period.
• Cycle signal and timestamps for consistent segmentation.
• Electrical supply included in the measurement and power or energy actually recorded.
• Measured process variables, their units and data sources.
• Inspections and acceptance criteria when quality is part of the objective.

Reference recording: consecutive cycles under stable conditions

First document a reference condition after thermal and operational stabilisation. Preserve a sequence of consecutive cycles and record the interval between cycles, integrated energy per cycle and the relevant variables that are actually available.

An initial practical data set might contain approximately twenty stable cycles to explore process behaviour; this is not a universally sufficient sample size for every process or conclusion. Variability, transients and the effect size of interest determine whether more observations are required.

If the material, number of active cavities, mould, auxiliary load or thermal regime changes, mark the transition. Do not combine these observations into one comparable operating condition without technical justification.

Relating process phases to measured signals

Reliable PLC markers or operating signals can be used to align the beginning of each cycle and describe phases such as injection, holding pressure, plasticising and cooling, depending on the instrumented variables and actual machine sequence.

Power measured during one phase does not, on its own, establish the cause of a defect. To investigate changes, compare time-aligned traces and retain inspection results and settings that can test the hypothesis.

Energy per cycle is calculated by integrating power over the defined interval. Peak power is a different measure and must not be confused with total energy consumption.

Comparing two injection-moulding conditions with proper context

After an adjustment: what should remain comparable?

• Documented product reference, mould, material and operating conditions.
• Same electrical measurement scope and time-segmentation rules.
• Quality variables verified using equivalent criteria.
• Number of cycles, variability and any operating-regime transitions.
• Explicit record of the adjusted parameter and the time of change.

Differences in energy per cycle, time or pressure must be interpreted alongside observed variation. A change seen only during start-up should not be presented as an improvement sustained under stable operation.

Process engineering applications · SMARTPRO and available variables

Further reading

• How to verify that an improvement is sustained after a trial
• How to distinguish cycle energy from energy outside production
• How to investigate quality variation

Guide developed from KERN IoP process-analysis experience, earlier articles on standardisation and more recent work comparing operating conditions. Sample size must be justified for each application.