How to Determine the Correct Heat Seal Temperature – Methods & Testing Guide

The core principle of setting heat seal temperature is simple: achieve sufficient seal strength while minimizing thermal damage to the material.
A correct temperature produces a smooth, strong, and undistorted seal. Too low → weak seal. Too high → material embrittlement or shrinkage.

Below is a heat seal temperature testing complete guide – from quick shop‑floor checks to lab‑grade validation.

1. Quick On‑Site Judgement (Visual & Tactile)

2. Hand Peel Test (Quick & Practical)

Manually tear the sealed bag.

• ✅ Acceptable: The tear propagates through the base material (material failure) – the seal interface itself remains intact.

• ❌ Unacceptable: The seal opens cleanly at the interface (interface failure) – indicates incomplete fusion, usually due to low temperature or pressure.

Use this heat seal temperature testing for routine production monitoring. It’s fast but should be backed by periodic lab data.

3. Check for Residue on the Sealing Jaw

During a brief stop, inspect the PTFE (Teflon) coated surface of the heat seal jaw.

• Optimal: Light, uniform film transfer – easy to clean.

• Too high: Molten resin adheres heavily, hard to remove; PTFE cloth may show burn marks.

4. Laboratory Quantitative Methods

4.1 Heat Seal Strength Test (Most Important)

Method:
Cut specimens (typically 15 mm wide) perpendicular to the seal direction. Use a tensile tester with 180° or 90° peel configuration. Record the maximum force.

Criteria:
Report value in N/15mm. Acceptable limits depend on product requirements (e.g., common food packaging often requires >10 N/15mm).

• Low strength → raise temperature.

• If strength meets target but material breaks before the seal peels (brittle fracture) → temperature may be too high.

4.2 Thermal Analysis (for Process Development & Troubleshooting)

DSC (Differential Scanning Calorimetry):
Measures the melting range of the sealant layer material (e.g., CPP, PE).
The optimal heat seal temperature is usually set near the end of the main melting peak. This is the scientific basis for defining process parameters.

4.3 Seal Integrity Test (Leak Test)

Method:
Use a vacuum (negative pressure) or positive pressure seal tester on finished bags.

Interpretation:

• No leakage at standard test pressure → good, continuous seal.

• Leaks concentrated on the seal area → possible uneven temperature or insufficient pressure.

5. Systematic Adjustment Workflow

🔹 Step 1 – Ramp from low temperature

Start 5–10°C below the expected range. Increase in 5–10°C steps. Combine hand peel and strength tests.
The lowest temperature that consistently achieves strength requirements is usually the most economical setting.

🔹 Step 2 – Consider the “Temperature – Pressure – Time” interaction

These three factors work together.

• Low pressure can mimic low temperature (weak seal).

• Excessive dwell time may overheat even at moderate temperature.
  Always adjust all three in a coordinated way.

🔹 Step 3 – Standardize & Record

For every successful batch, document:

• Temperature, pressure, dwell time, line speed

• Ambient temperature and humidity

• Material lot number

Create a standard process card for each material structure.

Practical Summary

• Daily production: Use visual inspection + hand peel test as your first line check.

• Periodic verification: Perform heat seal strength (tensile test) on sampled bags.

• Material change or quality fluctuation: Re‑run the step‑up temperature test.