Heat sealing is a process that uses heat and pressure to bond two layers of thermoplastic material together, creating an airtight closure. It’s the technology behind sealed food packages, medical pouches, blister packs, and countless other products where keeping air, moisture, or contaminants out matters. The basic principle is simple: when certain plastics are heated past their softening point and pressed together, their molecules intermingle at the contact surface. Once they cool, those layers fuse into a single, continuous bond.
How the Process Works
Every heat seal depends on three variables working together: temperature, pressure, and time. The temperature has to be high enough to soften the plastic’s sealing layer without destroying the material underneath. Pressure ensures the two surfaces make full contact so the softened plastic can flow together and fill any microscopic gaps. And dwell time, the duration the material stays clamped between heated jaws, determines whether the bond has enough time to form completely.
Getting these three variables right is trickier than it sounds, because the optimal combination changes depending on the material, its thickness, and even the geometry of the package. Research on flexible packaging found that folded corners and overlapping layers create pressure differences along the sealing area. For a common multilayer film structure, temperatures below 130°C with low pressure left gaps in two-layer regions, while areas with three or four overlapping layers needed temperatures above 140°C and pressures above 4 bar to seal properly. Push too far in the other direction, past 160°C and 4 bar, and the sealant layer squeezes out entirely, weakening or damaging the seal. The sweet spot varies at every point along the package.
Impulse Sealing vs. Constant Heat Sealing
The two most common types of heat sealers work on the same principle but handle temperature differently.
Impulse sealers only heat up when you activate them. A thin wire element (typically nichrome) receives a burst of electric current, rapidly reaching sealing temperature. The jaws clamp down on the material, the element heats for a set duration, then the current stops and the material cools while still under pressure. This controlled cooling cycle is a key advantage: the seal solidifies under clamping force, which produces a cleaner bond. Impulse sealers don’t need warm-up time, use less power per cycle, and work well with common thermoplastics like polyethylene, polyurethane, and metalized barrier films. They’re the standard choice for sealing bags and pouches.
Constant heat sealers keep their bars at a steady operating temperature at all times. This makes them better suited for coated papers, foil laminates, military-spec barrier bags, and other thick or multi-layer materials that need sustained high heat to bond properly. The trade-off is that they can’t seal materials with low melting points, since the always-hot bars would melt right through thinner films. They also need time to reach operating temperature before use. However, once they’re running, constant heat sealers maintain very precise temperature control and avoid the repeated expansion and contraction that impulse elements experience.
Ultrasonic Sealing: No Heat Required
Not all “sealing” uses heat directly. Ultrasonic sealing generates high-frequency mechanical vibrations through a tool pressed against the plastic. These vibrations cause the plastic molecules to oscillate rapidly, generating friction at the molecular level. The material softens at the contact point, and when held together by an anvil, the two layers bond as the vibration stops.
Because no external heat enters the package, ultrasonic sealing is ideal for products that can’t tolerate temperature spikes: frozen foods, chocolate, cheese, pharmaceuticals, and medical devices. It works with both liquids and solids inside the package, and the lack of thermal energy means less risk of product degradation near the seal area.
Common Seal Defects and Their Causes
When a heat seal fails, it usually traces back to one of the three core variables being off, or to a maintenance issue with the equipment.
- Burned or melted bags happen when the temperature is too high, dwell time is too long, or the protective Teflon tape covering the heating element has worn through. A worn tape cover allows the element to contact the film directly, concentrating heat on a small area. Using a film with a lower melting point than the machine is set for causes the same problem.
- Seals that peel open are typically caused by contamination on the sealing bar (even a thin layer of dust, oil, or moisture interferes with bonding), misaligned jaws that don’t make uniform contact across the full seal width, or insufficient clamping pressure due to mechanical wear.
- Wrinkled seals result from uneven pressure distribution across the sealing bar, jaw misalignment, or soft film materials that buckle under heat. A wrinkled seal isn’t just cosmetic; the folds can create channels where air or moisture leak through.
Most of these problems are preventable with regular cleaning of the sealing surfaces, periodic jaw alignment checks, and matching machine settings to the specific film being used.
How Seal Quality Is Tested
Seal strength is measured by how much force it takes to peel the sealed layers apart. The industry standard test, ASTM F88, uses a tensile testing machine to pull a sealed sample open at a controlled speed of 200 to 300 millimeters per minute. Test specimens are cut to a precise 25-millimeter width, and the machine records both the average peel force across the entire test and the maximum force at the strongest point of the seal.
There are two main peel configurations. A T-peel pulls both sides of the sealed material apart at roughly 90 degrees, while a 180-degree peel bends one layer back flat against itself. These different angles simulate different real-world stresses a package might face. Manufacturers use these results to verify that their sealing parameters produce bonds strong enough for the product’s handling and shelf-life requirements.
Sealing Compostable and Bio-Based Films
As packaging shifts toward sustainable materials, heat sealing has had to adapt. Compostable films made from materials like PLA (a corn-based plastic) and cellulose behave differently from conventional petroleum-based plastics under heat.
PLA begins to seal at around 75 to 85°C, which is lower than many conventional films, and maintains its bond strength across a wide temperature window of about 70°C. This flexibility makes it relatively forgiving to work with. A PLA-and-PBAT blend, by contrast, has a usable sealing window of only about 5°C, meaning the machine has to hit its target temperature much more precisely or the seal will either fail or degrade.
Cellulose-based films sit at the other extreme, requiring temperatures around 180°C with higher pressures (8 N/mm²) and very short dwell times of 0.4 seconds. Coated paper seals best around 135°C with a longer 1.5-second dwell at moderate pressure. Each sustainable material demands its own parameter profile, and the settings that work for one green film won’t necessarily work for another.