What is Heat Sealing?

Heat sealing joins two layers of plastic film by pressing them together and applying heat until the surfaces melt and fuse. As the seal cools it forms a permanent, airtight bond that is usually stronger than the film itself. Most Australian businesses do this on a benchtop impulse sealer with a jaw between 200mm and 1000mm.

Shop heat sealers ↓

The three things every seal needs

A heat seal is a weld, not a glue. Heat takes both film surfaces past their softening point, pressure holds them in contact so the polymer chains from each layer tangle across the join, and the film then has to cool while it is still clamped so the weld sets. Get all three right and the seal is stronger than the film beside it.

On a benchtop impulse sealer you do not dial in a temperature, which surprises most first-time buyers. The dial sets how long current flows through the element when you press down, so it controls energy rather than degrees. Pressure comes from your hand on the arm. Dwell time is the part people skip: after the indicator light goes out the film is still molten, so the jaw has to stay closed for another two or three seconds while the seal cools and hardens.

Impulse sealers and constant heat sealers

An impulse sealer keeps its element cold until you press down. A burst of current heats a thin ribbon for a fraction of a second, the film melts, and the element cools again under the closed jaw. Nothing is hot when the machine is idle, there is no warm-up wait, and it draws power only in that instant. That suits polyethylene, which is why it is the standard machine on an Australian packing bench.

A constant heat sealer, also called a direct heat or bar sealer, holds both jaws at a set temperature all day. It is what you need for thick, laminated or foil-lined pouches that an impulse cannot drive enough heat through, and it needs a warm-up period. QIS stocks hand-operated impulse sealers rather than constant heat or automatic form-fill-seal machinery, so if you are sealing poly bags, layflat tubing or vacuum pouches on a bench, an impulse machine is the right tool.

Which films can be heat sealed?

Only thermoplastics weld to themselves, and some are far more forgiving than others. Polyethylene is the easy one, which is why almost everything you seal on a bench is polyethylene.

Material	Impulse sealer	Notes
LDPE	Yes, easily	The most forgiving film there is. Wide settings window, forgiving of a slightly short dwell.
LLDPE	Yes	Seals much like LDPE and is tougher once sealed.
HDPE	Yes, with care	Stiffer and crinkly, needs a higher setting and a narrower window before it burns through.
Polypropylene and BOPP	Sometimes	Needs more energy than polyethylene. Plain BOPP tends to shrink away from the jaw, which is why most cello style bags close with a self-adhesive lip instead.
Cellophane (regenerated cellulose)	No, unless coated	Uncoated cellulose film does not weld to itself.
Vacuum pouch film	Yes	Multi-layer pouches are built with a polyethylene sealing layer on the inside.
Laminated and foil-lined pouches	No	Too much material to heat through. These need a constant heat machine.
Uncoated paper	No	Paper has nothing to melt. Only PE-coated paper will seal.

Do not run PVC film through an impulse sealer. Overheated PVC gives off corrosive fumes that attack the element and the machine.

How to pick a jaw length

Jaw length is the maximum width of seal the machine can make in one press. QIS sealers run 200mm, 300mm, 400mm, 500mm, 600mm, 800mm and 1000mm. The 200mm to 500mm models are also available with a built-in cutter, which trims the waste film off in the same movement and is genuinely useful if you are cutting bags to length from a roll of layflat tubing. The 600mm, 800mm and 1000mm machines seal only.

The rule is simple: buy a jaw longer than the widest bag you seal, with room to spare. If the bag is wider than the jaw you have to make two overlapping presses, and the overlap is where leaks start. Sealing at the very end of the element also wears that spot out first. If you are between sizes, go up.

Diagnosing a bad seal

Almost every seal failure traces back to one of the three variables or to a worn consumable. This is what to look at before you decide the machine is faulty.

Symptom	Likely cause	What to change
Seal peels apart with light pressure	Not enough energy, or the jaw was released while the film was still molten	Move the dial up one step, then hold the jaw closed for two to three seconds after the light goes out
Film cuts through along the seal line	Too much energy, or the PTFE cover has worn through onto the element	Turn the dial down and check the cover tape
Seal holds in the middle, open at one or both ends	Bag wider than the jaw, or the film was creased when the jaw came down	Move up a jaw length, and smooth the film flat before pressing
Seal looks perfect but the pack leaks	Product, powder, grease or moisture caught in the seal area	Leave a clean margin at the top of the bag and fill below it
Seal is wavy, wrinkled or brittle	Too much heat for that film, or the film was under tension	Lower the setting and let the film sit slack in the jaw
No light, no seal at all	Broken element, or cover tape burnt onto the element	Fit a new element, PTFE cover and backing strip

Elements and PTFE tape are consumables

An impulse sealer is a simple machine with parts that wear out. The heating element is a thin ribbon that expands and contracts every time you press down, so it eventually fatigues and breaks. Over it sits a PTFE cover tape, the slippery strip that stops molten film sticking to the element, and the opposite jaw carries a silicone backing strip that gives the seal something to press against. QIS repair kits include all three, matched to the jaw length of your machine.

Change the cover tape as soon as it discolours, scores or starts collecting melted film. A worn cover is the most common reason a sealer suddenly begins burning through bags, and a strip of tape is far quicker to fit than a new machine.

Heat sealing, food packing and the Australian rules

A heat seal does more than close a bag. Because the film is welded, the pack cannot be opened and closed again without leaving obvious damage, so a heat seal is what makes a poly bag tamper evident. That matters for food, pharmacy and anything despatched through a courier network.

Australia has no pre-market approval regime for food contact materials, so there is no such thing as an official food grade certificate for a bag or a sealer here. The obligation sits in the FSANZ Food Standards Code instead. Standard 3.2.2 requires packaging to be fit for its intended use and not likely to cause food contamination, and Standard 1.4.1 sets maximum levels for contaminants that could migrate into food. A properly welded seal on a food grade polyethylene bag is part of how a business meets the first of those, so a weak seal is a food safety issue rather than just a packing annoyance.

Practically, the seal check belongs in your process, not only in your machine settings. Pull-test the first bag of every run, watch for the symptoms in the table above, and keep a spare repair kit on the shelf so a failed element does not stop a day of despatch.

Related guides

The film matters more than the machine. Start with what is LDPE, the film that seals most easily, then what is layflat tubing if you want to make your own bag lengths, and what is BOPP for the films that do not seal as readily. Thickness is quoted in microns here, explained in what is a micron.

If you need a closure you can open again rather than a welded one, see press seal vs zip lock vs resealable bags. To buy, the range sits on heat sealer machines and the spares on teflon tape and elements.

Not sure which jaw length suits your bags? Contact our Brisbane team on 1800 555 343 and we will match a machine to the widest bag you seal.