What Is Heat Sealability in Plastic Packaging?

Learn what heat sealability means in flexible plastic packaging and why it matters. This guide covers sealing methods, critical factors like temperature and pressure, and how different resins impact seal strength and performance.

Table of Contents

Heat sealability is one of the most critical performance attributes of flexible plastic packaging. Almost every flexible packaging product needs to be formed into a pouch or bag to hold goods. This process relies on heat sealing—melting and bonding the plastic materials to create seams and closures. Without heat sealability, flexible packaging wouldn't exist in its current form.

Why Heat Sealability Matters in Flexible Packaging

Flexible plastic packaging needs to be formed and sealed into bags or pouches to contain products—ranging from meat and snacks to detergents and agricultural supplies. Heat sealing is the most common method to create these seals. Whether forming a pouch from a roll of film or sealing a filled bag, heat allows the plastic layers to fuse together securely.

A weak or improper seal could lead to leaks, contamination, or compromised shelf-life. That's why understanding and optimizing heat sealing conditions is essential for product protection, quality, and safety.

Common Heat Sealing Methods for Plastic Bags

Different plastics require different heat sealing techniques based on their material properties. Here are several widely used sealing methods:

· Manual heat sealing: Often used for small-scale operations or samples.

· High-frequency (HF) sealing: Uses electromagnetic fields to generate heat inside the plastic.

· Hot bar sealing: Heated metal bars press and seal the plastic layers.

· Impulse sealing: Seals are created using short pulses of electricity through a resistance wire.

· Infrared (IR) sealing: Uses radiant heat to melt and seal films.

· Ultrasonic sealing: Uses high-frequency sound waves to generate localized heat.

· Hot air sealing: Applies heated air to melt the seal areas.

The choice of method depends on the type of plastic film, the application, production speed, and equipment.

Key Factors That Affect Heat Seal Performance

Several variables influence how well a plastic film can be sealed. Let's explore the main ones:

Sealing Temperature

The sealing temperature must be above the melting point of the sealing material to ensure effective bonding. However, the appropriate temperature varies based on:

· Film type (single-layer vs. multi-layer)

· Film thickness

· Sealing pressure

· Sealing speed

Rule of thumb:

· Thicker films, slower speeds, and higher pressures usually require lower sealing temperatures.

· Thinner films, faster speeds, and lower pressures require higher temperatures.

Finding the right balance is essential for strong, clean seals without damaging the material.

Sealing Pressure

Pressure helps ensure that the melted plastic layers are in close contact, allowing fusion. Too low a pressure may lead to weak seals, while too high a pressure can squeeze out the molten resin, leading to what’s called “seal thinning” or “cutting-through.”

Ideal sealing pressure depends on:

· Film thickness

· Sealing temperature

· Sealing time

In general, higher temperatures and longer sealing times require less pressure, especially for thin films.

Sealing Time

Sealing time—the duration the heated surfaces are in contact with the film—has a big impact on seal strength.

· Longer sealing time improves bond strength but increases the risk of burning, oxidation, or wrinkling.

· Too short a time may not allow full melting, resulting in poor bonding.

Some materials (like aluminum foil laminates) conduct heat well and require shorter sealing times. Others may need longer times due to ink, adhesive layers, or thicker structures.

Heat Seal Strength of Plastic Resins

The performance of a heat seal also depends on the type of resin used in the seal layer. According to Japanese Industrial Standard (JIS Z1526-1976), a common test condition involves:

· Temperature: 130–140°C

· Pressure: 1 kgf/cm²

· Time: 2–3 seconds

· Material: LDPE-coated film

· Result: Peel strength (seal strength) is measured in kgf per 15mm width

Key Properties of Heat Seal Resins

A. Standard Heat Seal Strength

This refers to the sealing strength of a resin under optimal conditions. For example, sealing a 15μm BOPA (biaxially oriented polyamide) film with various thicknesses of LDPE shows the following:

LDPE Thickness (μm) | Seal Strength (kgf/15mm)

20 | 1.0

30 | 1.5

40 | 2.5

50 | 3.5

60 | 4.2

70 | 5.5

As thickness increases, seal strength rises nearly linearly.

B. Effect of Melt Index (MI) and Density

For LDPE, both melt index (MI) and density affect seal strength:

MI | Density (g/cm³) | Seal Strength (kgf/15mm)

3.7 | 0.923 | 1.25

3.0 | 0.921 | 1.4

7.0 | 0.917 | 1.75

9.5 | 0.917 | 2.0

Higher MI and lower density result in stronger seals.

Some improved LDPE resins (e.g., Mitsui’s VL-LDPE or CS-C) reach seal strengths of 5–6 kgf/15mm at 150°C.

Special Properties of Heat Seal Resins

Low-Temperature Sealability

Low-temperature seal strength refers to a resin's ability to form strong seals at relatively low temperatures. This is critical for:

· High-speed automatic form-fill-seal lines

· Preventing product damage from excessive heat

For example, EVA (ethylene-vinyl acetate) resins show better low-temperature performance as VA content increases:

· Higher VA → lower melting point → better flexibility

· Crystallinity decreases as VA exceeds 25%, making EVA amorphous and more sealable at low temps

Contaminant Resistance (Sealing with Inclusions)

Good resins can still form strong seals even when the sealing surface is contaminated with:

· Dust

· Grease

· Food powder

· Pesticide particles

This property is vital in industries such as:

· Liquid packaging (e.g., sauces or beverages)

· Powdered foods

· Agricultural chemicals

Resins with good contaminant resistance include:

· Surlyn (ionomer resin)

· LLDPE (Linear Low-Density Polyethylene)

· EMAA (Ethylene Methacrylic Acid)

· EEA (Ethylene Ethyl Acrylate)

Final Thoughts

Heat sealing is not a simple one-size-fits-all process. It requires understanding the interplay between:

· Material type