Alexei Volkov
Heat sealing is a process that joins thermoplastic films, typically less than 0.5 mm thick, by applying heat and pressure. It is commonly used in the packaging industry to seal bags, films, and containers, as well as in medical device packaging and some electronic devices. The two main types of heat sealing are hot-bar welding and impulse welding. Different types of plastics have varying heat seal temperatures, so it is important to consider the melting point of the plastic and use the appropriate temperature and sealing method to avoid overcooking the film. Heat sealing machines also come in various sizes and types, each suited to different projects and materials.
| Characteristics | Values |
|---|---|
| Purpose | To join thermoplastic films, typically less than 0.5 mm thick |
| Types of heat sealing | Hot-bar welding, impulse welding |
| Types of heat sealers | L-bar, I-bar, constant heat sealers, continuous motion sealers |
| Factors affecting heat sealing | Heat, pressure, dwell time, cooldown time, seal time, thickness of the material |
| Plastics used for heat sealing | Polyethylene, polypropylene, polystyrene, polyvinyl chloride, LDPE, LLDPE, PLA |
| Other materials used for heat sealing | Aluminium foil, silicone-coated polyethylene, polypropylene |
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What You'll Learn
Choosing the right heat sealer
When choosing the right heat sealer, there are several factors to consider. Firstly, it is crucial to understand the different types of heat sealers available. The two main types are impulse heat and direct heat sealers. Impulse sealers are ideal for thermoplastic materials such as polyethylene (PE) or polypropylene (PP), which require lower sealing temperatures. They require no warm-up time and are energy-efficient as they only consume power when the jaw is lowered. On the other hand, direct heat sealers maintain constant heat in both jaws, allowing for better heat penetration and making them suitable for thicker materials like coated aluminum foil or waxed paper.
The second important consideration is the material you plan to seal. Different plastics have varying melting points, and some may not be mendable when heat is applied. Thermo-reactive plastics, such as polyethylene, polypropylene, and polystyrene, change shape and form at high temperatures. Therefore, understanding the sealing temperature range of your packaging film is vital. Additionally, consider the thickness and width of the material, as this will impact the type of sealer and heat settings required.
Size and portability are also essential factors in choosing a heat sealer. The size of the sealer should correspond to the dimensions of the material you need to seal. For example, a smaller sealer may be more portable and suitable for individual projects, while larger sealers are typically used for mid-level sealing. The model number usually indicates the size of the sealer in millimetres.
Lastly, other factors to consider include energy usage, durability, and price. Impulse sealers, for instance, can help extend the life of sealing wires and tape by operating at lower heat levels. Constant heat sealers, on the other hand, are known for their durability and cost-effectiveness for high-volume projects. It is important to weigh these factors based on your specific requirements to make an informed decision when choosing the right heat sealer.
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Selecting the right plastic film
Type of Product
Consider the nature of the product you are wrapping. Select a film thickness, width, and length that correspond to the size and weight of your products. Think about the smoothness of the items and whether they have sharp corners that could tear the film. For instance, bubble wrap, made from two welded PE plastic films, is ideal for protecting fragile goods during shipping. The bubble size can vary depending on the film thickness and the weight of the object.
Environment
Take into account the environment in which the sealed product will be stored or transported. If your product requires protection from moisture, opt for polyethylene (PE) films, which offer excellent moisture resistance. On the other hand, polypropylene (PP) films are better suited for higher temperature and pressure environments, providing superior sealing performance in such conditions.
Heat Sealing Equipment
The type of heat-sealing equipment you use will also influence your choice of film. Some heat sealers require specific types of film, so ensure compatibility between your equipment and the film. Additionally, consider the sealing temperature and pressure requirements of your heat sealer, as different films have varying sealing parameters.
Special Requirements
If your product has special requirements, choose a film with specific properties. For instance, some films offer resistance to chemicals, UV rays, or heat. If you need to package confidential items, select a film that can create an ultra-intense heat seal to prevent the sealed contents from being seen.
Sustainability
Lastly, consider the environmental impact of your choice. More and more plastic films are recyclable or biodegradable. Opting for these sustainable options can reduce the environmental footprint of your operations.
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Understanding melting points
The melting point of plastic is a crucial factor to consider when heat-sealing plastic film. Understanding the melting point can help determine the most suitable technique for shaping the plastic and ensure optimal results in the heat-sealing process.
Plastic does not have a fixed melting point; it varies depending on the type of plastic. Thermoplastics, such as polyethylene (PE) and polypropylene (PP), melt at lower temperatures, making them ideal for techniques like injection moulding and extrusion. On the other hand, plastics like polycarbonate and nylon have higher melting points and are more heat-resistant.
The melting point of plastic is the temperature at which it transitions from a solid to a liquid state. When plastic reaches its melting point, it becomes soft and pliable, allowing it to be shaped as desired. This property is essential for manufacturers and processors when selecting the appropriate plastic for different applications.
The Vicat softening point and DSC melting point are two methods used to measure the melting point of a plastic film. These methods determine the temperature at which a polymer begins to soften, but they yield different results. Additionally, the softening point should not be confused with the seal initiation temperature (SIT), which is the temperature at which the plastic begins to seal.
When heat-sealing plastic film, it is important to avoid using too much heat, as this can lead to overcooking the film. Plastics with lower melting points and SITs typically require more cooling time, more pressure, and less dwell time to create a strong seal. Understanding the melting point of the specific plastic film being used is crucial for achieving optimal sealing results.
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Heat, pressure and dwell time
Heat, pressure, and dwell time are the three interdependent elements of heat sealing plastic film. To make a heat seal, the film must be melted together and then allowed to cool down enough to become "crystalline", or solid.
The heat sealing process can be performed using a laboratory heat sealer or a VFFS machine. The sealing temperature and dwell time have a significant influence on the seal strength, while the sealing pressure has a minimal effect. Theller (1989) and Meka and Stehling (1994) concluded that the heat sealing variables' effects on sealing properties are primarily controlled by sealing temperatures and dwell time rather than sealing pressure.
Films with a low melting point seal better with more cooling time, more pressure, and less dwell time, resulting in faster cycle speeds. The seal strength of the sealant layer compared to the paper-based material's strength is an important consideration. At sealing temperatures greater than 120°C, unacceptable tearing failures were observed with both sealing tools.
The minimum dwell time required for sufficient sealing can be estimated based on heat conduction time and reptation time. The dwell time can be lowered from the standard 1 second to 0.5 seconds to imitate a fast-moving manufacturing environment. During the hot peeling process, the seal tester machine is set to a delay time of 0.1 seconds and a peeling rate of 200 mm/s.
It is important to note that the use of too much heat can lead to overcooking the film. The melting point and seal initiation temperature of the film should be considered, as some films may require more cooling time, more pressure, and less dwell time for effective sealing.
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Different types of heat sealing
Heat sealing is a process that joins thermoplastic films, typically less than 0.5 mm thick. It is commonly used in the packaging industry to seal bags, films, and containers made from thermoplastics. Heat sealing is also used in the medical device and electronics industries.
There are several different types of heat sealing:
Impulse Heat Sealing
Impulse heat sealers are one of the most common types of heat sealers. They are versatile, cost-effective, and easy to use. They come in various sizes and designs, making them suitable for both small-scale operations and industrial production lines. Impulse heat sealers have one or two heating elements covered by a layer of Teflon tape that prevents the polymer from sticking to the sealer. The two elements are pressed together over the seal, and a microswitch engages to apply heat.
Constant Heat Sealing
Constant heat sealers, also known as hot bar sealers or direct heat sealers, are designed for packaging materials that require higher temperatures for sealing, such as thick laminates or foil bags. They maintain a consistent temperature throughout the sealing process and are equipped with heated jaws or bars that apply steady heat and pressure to create a secure seal. Constant heat sealers are durable and often cost-effective for high-volume projects and higher-grade materials.
Vacuum Heat Sealing
Vacuum sealers are specialized heat sealers used primarily in the food industry to preserve the freshness of perishable goods and extend their shelf life. They remove air from the packaging before sealing, creating a vacuum environment that inhibits the growth of bacteria, mould, and other contaminants. Vacuum sealers are available in various designs, including handheld models for domestic use and larger, automated machines for commercial and industrial applications.
Band Heat Sealing
Band sealers, also known as continuous sealers, are commonly used in high-volume packaging operations. They utilize a conveyor belt system to feed packages through the sealing mechanism, allowing for efficient continuous sealing without manual intervention.
Clamshell Heat Sealing
Clamshell sealers are designed specifically for retail packaging. They create strong, airtight seals that ensure product integrity and safety.
The choice of heat sealer depends on various factors, including the material type, thickness, desired production levels, size and portability, energy usage, volume, durability, and price. It is important to consider the different melting points of each plastic type and use heat seal-approved packaging to ensure safety and the integrity of the seal.
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Frequently asked questions
Heat sealing is used to join thermoplastic films, typically less than 0.5 mm thick. It is most commonly used in the packaging industry for sealing bags, films, and containers.
There are two main types of heat sealing: hot-bar welding and impulse welding.
Most thermoplastic materials can be heat sealed. This includes polyethylene, polypropylene, PVC, polystyrene, and polyvinyl chloride.
The temperature depends on the type of plastic and its thickness. For example, polyethylene has a heat-sealing temperature of 140 degrees Fahrenheit, while polypropylene seals at roughly 248–253 degrees Fahrenheit.
It is important to consider the melting point of the plastic and to ensure that the film is in a relaxed state while cooling down to avoid gaps and pinholes. Additionally, the temperature cool-down and seal time are critical factors, with thicker bags requiring more heat and a longer seal and cool-down period.
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