Ethan Clarke
Heat sealing is a process that uses heat and pressure to seal plastic sheets or bags. The heat sealer melts the plastic until it co-mingles, then sets it into a single, neat seam of plastic. As the poly material cools, it contracts, tightening the seal. Heat sealing can be done with a heat sealing machine or with a handheld tool such as an iron, hair straightener, or curling iron. The plastic sheets or bags must be made of a material that can be heat sealed, such as polyethylene, polypropylene, or PVC. The process of heat sealing can be used to create a complete seal on poly bags or to fuse together pieces of fabric coated with thermoplastic polyurethane (TPU).
| Characteristics | Values |
|---|---|
| Materials | Poly bags, including polythene, polyethylene, polypropylene, PVC, polyamid, and some thinner materials like PE laminated foil and aluminium foil |
| Tools | Heat sealing machine, curling iron, handheld hair straightener, heat sealer iron, clothes iron, model airplane covering iron, dining room table iron, leather iron, or a heat sealer machine |
| Technique | Apply heat and pressure to melt the material, then allow it to cool and solidify into a crystalline structure; ensure even pressure and avoid overheating |
| Temperature | Low-density polyethylene melts at 320 degrees Fahrenheit; other plastics have lower melting points and require testing |
| Advantages | Cost-effective, time-saving, and versatile; creates a strong seal that is ideal for packaging and manufacturing applications |
| Disadvantages | Requires proper tools and technique to avoid damage; some plastics cannot be heat sealed and require alternative methods, such as cold seal lamination |
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What You'll Learn
Using a heat sealer
To use a heat sealer, start by selecting the appropriate plastic material. Polyethylene or polypropylene bags are ideal, as they stretch well and create strong seals. The bags should be fed into the heat sealer, and the timer should be set according to the material's density. The heat sealer will then melt the plastic until it blends together, forming a single seam. As the seam cools, it will contract and tighten the seal.
It is crucial to consider the heat sealer's settings to achieve a good seal. Avoid using excessive heat, as it can overcook the plastic. Instead, use the right amount of heat and allow for more cooling time, pressure, and less dwell time for a faster cycle.
The type of sealing wire in the heat sealer also affects the seal. Flat sealing wires create stronger and thicker seals, while round wire sealers produce thinner and less visible seals. Additionally, the sealing bar, made of heat-resistant material, should be able to withstand direct heat contact.
By following these steps and adjusting the settings according to the plastic material, you can effectively use a heat sealer to create airtight seals and increase the longevity of your products. You can also use a vacuum sealer or a handheld hair straightener as an alternative to a heat sealer.
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Choosing the right plastic
When choosing the right plastic for heat sealing, there are several factors to consider. Firstly, it is essential to select a plastic that is suitable for heat sealing, as some plastics cannot be heat sealed and require alternative methods, such as cold seal lamination. Examples of plastics that cannot be heat sealed include PET and HDPE.
Common plastics that are suitable for heat sealing include polyethylene, polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC). These plastics are often used in bags and pouches, providing an airtight seal that protects the packaged products. Polyethylene, in particular, is popular due to its low melting point, which allows for quick fusion and a tighter seal upon cooling. Additionally, its low melting point makes it suitable for impulse sealers, which require lower seal temperatures.
Other plastics that can be heat sealed include LDPE, LLDPE, and PLA, but they have very low melting points and can be easily damaged, so testing is recommended before use. Polyvinyl chloride (PVC) is another versatile option, known for its mechanical toughness, chemical and weather resistance, and flame retardant properties. It is more elastic than other materials, making it flexible and easy to form and die-cut.
The choice between impulse and direct heat sealers is also crucial when selecting the right plastic. Impulse sealers require no warm-up time and are suitable for thermoplastic materials like polyethylene and polypropylene, while direct heat sealers maintain constant heat and are better suited for thicker materials like coated aluminum foil and gusset bags.
Finally, the thickness of the plastic and the desired seal type should be considered. For shrink-wrapping, thinner seals are used, while thicker materials require flatter seals that provide extra flexibility to prevent spills. It is important to match the plastic's melting point with the appropriate machine temperature to avoid burning or melting the plastic. Testing is advised for heavy-gauge, multi-layered films to ensure the integrity of the seal.
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Applying heat and pressure
There are various tools that can be used to apply heat and pressure, including a heat sealing machine, a curling iron, a hair straightener, or a clothes iron. It is important to use an appropriate amount of heat to avoid overcooking the plastic. Some plastics have a low melting point and require lower temperatures and more cooling time. For example, low-density polyethylene melts at 320 degrees Fahrenheit, making it ideal for heat sealing.
When using an iron, it is important to ensure that the surface you are working on is heat-resistant and even, to avoid uneven application of heat and pressure. It is also recommended to use a stand or a sheet of heat-resistant material to protect your work surface. Additionally, modifications may be required for certain irons, such as adding a plug adapter or creating a stand to prevent scorching.
Once the plastic is heated and melted, it is important to maintain pressure on the fabric for several seconds while it cools down and solidifies. This can be done by using a rag or a roller to press the fabric together, ensuring that the pieces do not move apart before the plastic fuses into a single layer. This process of applying heat and pressure creates a strong weld that is even stronger than the original fabric.
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Cooling and crystallisation
During the cooling process, the molten plastic undergoes a transformation from a liquid to a solid state. The polymer chains in the plastic, which were free-flowing when melted, now begin to slow down and organise themselves into a more ordered structure. This ordered structure is known as crystallisation, where the polymer chains form microscopic crystals, resulting in a strong and durable seal.
The rate of cooling plays a significant role in the crystallisation process. Rapid cooling can lead to higher seal strength, but it can also cause distortion in the sealed portion. On the other hand, slow cooling can result in a more uniform and aesthetically pleasing seal. The optimal cooling condition for peelable heat sealing of multilayered polyethylene (PE) films was found to be around 80°C, balancing productivity and the appearance of the sealed portion.
Controlling the cooling process is essential to achieving the desired seal strength and quality. The use of cooling bars, for example, can help regulate the temperature and improve the heat seal strength. Additionally, the heat sealing temperature and pressure can be adjusted to influence the crystallisation behaviour and the final seal strength.
The type of plastic also affects the cooling and crystallisation process. Different plastics have varying melting points and sealing initiation temperatures. For instance, films with lower melting points tend to seal better with more cooling time, more pressure, and less dwell time, resulting in faster cycle speeds. Understanding the characteristics of the plastic being heat-sealed is crucial for optimising the cooling and crystallisation process.
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Tools and modifications
The tools and modifications required for heat sealing plastic sheets depend on the scale of the project and the type of plastic being used.
For small-scale projects, a standard clothes iron or curling iron can be used to heat seal plastics. Irons should be set to a temperature suitable for the plastic being used—typically, plastics have a low melting point, so care should be taken not to overheat the iron. Irons with variable temperature settings are ideal. To modify a standard iron for heat sealing, it is important to ensure it has a way to be propped up safely when not in use, as the hot plate can scorch surfaces. This can be achieved by creating a stand from scrap metal or a tin can.
For larger projects, a dining table-sized area is ideal for working on. The surface should be covered with a heat-resistant material, such as MDF, to protect it from the hot iron. Parchment paper can also be placed on top of the MDF to avoid direct contact with the heat-resistant material. It is important to ensure the surface is flat and even, as uneven surfaces can result in uneven pressure and heat being applied to the plastic.
For high-volume heat sealing, an industrial heat-sealing machine is recommended. These machines can handle a variety of poly materials and different bag sizes. Some machines also have attached cutters, which provide a neat finish to the sealed plastic.
When using any heat-sealing method, it is important to ensure the plastic is in a relaxed state while cooling down to avoid gaps and pinholes in the seal.
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Frequently asked questions
A heat sealer, also known as an impulse sealer, uses a resistive heating element to seal and/or cut thin plastic bags or sheets. It uses a timed impulse of heat rather than continuous heat, which would turn the plastic into a gooey mess.
Many plastics can be heat sealed, including polyethylene, polypropylene, PVC, polyamid, LDPE, LLDPE, and PLA. Some plastics with very low melting points, like LDPE, LLDPE, and PLA, can be heat sealed but may be easily damaged, so testing is advised first.
The best way to heat seal plastic sheets is to use a proper heat sealing machine. However, for DIY projects, a regular clothes iron or a model airplane covering iron can also be used. The fabric should be pressed with the iron until the plastic begins to melt, then the iron should be removed, and the fabric should continue to be pressed with a rag or roller until the melted plastic cools and fuses.
The ideal temperature depends on the type of plastic being sealed. Low-density polyethylene, for example, melts at 320 degrees Fahrenheit. It is important to use the correct temperature to avoid overcooking the plastic.
Heat sealing is a cost-effective and time-saving solution for packaging in many manufacturing applications. It can be used on almost any type of polybag and provides a strong seal that can be cut for an attractive finish.
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