Bag SealersStop and think for a moment how many things you use or consume in your daily life that are sealed in plastic: warranty papers, service and instruction manuals for home appliances; ready to eat meals, seafood, meat and processed foods. Plastic bags, envelopes and pouches are inexpensive, durable and ideal for packaging thousands of commodities we use every day. A sealing machine is the device used to seal a plastic bag or pouch. Sealing machines are available in manually operated tabletop models for lower volume applications and fully automated high speed machines for mass production runs. No matter their size and speed, sealing machines all work on the same basis. Two flat surfaces press the plastic bag or pouch together while an electric current is passed through a resistive wire, causing it to heat and melt the plastic together. The amount of current and length of time the resistive wire carries current is adjustable. The operator sets the sealing machine to avoid melting the plastic to the point where it burns through, and to accommodate different thicknesses and compositions of plastic. The resistive wire is known as the sealer wire. Resistive wire has the characteristic of heating up when electrical current is passed through it. Unlike copper, gold and other highly conductive metals, resistance wire is specially alloyed to pose resistance to electrical current, making it ideal for use as a heating element in kilns, ovens, hair dryers, toasters, sealing machines and other appliances. |
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Other Considerations
While each type of resistance wire has different characteristics, the amount of heat generated is always dependent upon the overall resistance of the sealer wire.The thinner the wire, the greater resistance it poses and the greater heat is produced for a given current flow.
In addition, as the length of wire increases the resistance and heating effect increase. Further, as the wire heats, it become even more resistive and hotter. This last effect is known as the "resistivity factor."
| Specification | Nichrome 60 | Kanthal A-1 | Kanthal D |
| Max operating temperature | 1150°C (2100°F) | 1400°C (2550°F) | 1300°C (2370°F) |
| Common Applications | Tubular heating elements as suspended coils | Furnace applications | Appliances |
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Choosing and Replacing Sealer Wire
If the sealing wire in your machine needs replacement you can use any of the three types of resistive wire described here. Nichrome is usually a lower cost option. Many sealing machines used for poly bags and pouches use 18 AWG wire which, in Nichrome, costs as little as 16 cents per foot. It is important to replace the sealing wire with the same gauge wire.
Note that all resistive wire expands (stretches) as it heats. This characteristic is known as the "coefficient of thermal expansion," or CTE. Virtually every substance expands when heated because the atoms making up the substance become more active and move farther apart. Nichrome 18 AWG has a coefficient of 14 x 10-6 parts per million for each Centigrade degree of temperature change. As the Nichrome wire heats from room temperature to as hot as 2100°F you'll observe significant expansion and lengthening of the wire. In most cases sealing machines put the resistive wire under mechanical tension using a spring that takes up the "slack" in the wire as it heats up. Be sure you replace the wire using whatever tensioning mechanism your machine has implemented.
Likewise, look closely at the electrical connections that supply current to the sealing wire. In many cases the resistive wire is connected to eye-hooks which in turn are connected to the power supply. Take care to route the resistive wire following the same path of the wire you're replacing.