Heat Seal Poly Bags: Films, Sealers and How to Get a Strong Seal

Heat seal poly bags are open-top plastic bags that you fill and then close by melting the mouth shut with a heat sealer. The heat fuses the two film layers into one seam, so the bag is closed without tape, staples or a zipper, and anyone opening it has to cut or tear it. Most are LDPE flat bags or lay-flat tubing, sealed on an impulse sealer for low volume or a band sealer on a packing line.

Below we cover which films seal well, what sealer to use, how to get a strong seal every time, and what to specify when you buy the bags.

Two meanings of “heat seal bag”

Buyers use the phrase in two ways, and it helps to be clear which one you mean when you ask for a quote.

1. Bags you heat seal after filling. Open-top flat bags or tubing that your staff fill and close on a sealer. This is what most people search for.
2. How every poly bag is made. Almost all PE bags, including trash bags, produce bags and mailers, are formed by heat sealing at the factory. The bottom or side seams you see are heat seals.

Why heat seal instead of a zipper or tape?

• Cost. No extra closure material. You pay for film only, and a basic sealer is cheap.
• Tamper evidence. A sealed bag must be cut open, so you can see if it was opened.
• A full-width seal. A good heat seal keeps out dust, moisture and insects across the whole mouth.
• Clean look. A neat, flat edge, which looks better on retail and food packs.
• Any size. From small parts bags to long lay-flat tubing cut to length.

The trade-off: once it is open, it stays open. If the end user needs to reclose the pack, use a zipper bag, or a heat seal above a zipper for both tamper evidence and reclosing.

Which films heat seal well?

Polyethylene is the easiest film to heat seal. It melts at a fairly low temperature and has a wide window between “too cold to seal” and “burned through”.

Film	Melting point (approx.)	How it seals
LDPE	221–239 °F (105–115 °C)	Easiest. Wide sealing window, forgiving on simple impulse sealers. Our default for heat seal bags.
LDPE/LLDPE blends	Similar to LDPE	Seals as easily, with tougher seams and better puncture resistance.
HDPE	248–266 °F (120–130 °C)	Seals well but needs a bit more heat; thin HDPE can burn through if the dwell is too long.
Polypropylene (PP)	320–331 °F (160–166 °C)	Needs more heat than PE. Cast PP seals on a good sealer. Plain oriented PP (OPP) film is hard to seal without a sealable layer.
PA/PE (nylon/poly)	Seals on the PE layer	Used for vacuum bags; the PE inner layer melts and the nylon outer layer carries strength.

One correction to a figure that floats around the web: 320 °F is roughly where polypropylene melts, not LDPE. LDPE melts around 221–239 °F. If you set a sealer for LDPE as if it were PP, you will burn holes in the film.

Thickness also matters. Most hand impulse sealers are rated for film up to about 6 mil (152 µm) total. For thicker bags, gusseted bags (four layers at the gusset) or laminates, you need more heat, a wider seal bar or a constant-heat sealer.

Types of heat sealers

Sealer	How it works	Best for
Impulse sealer (hand or foot)	A wire heats only while the jaw is closed, then cools while the film is still clamped	PE and PP bags, low to medium volume, start-stop work
Constant-heat (hot bar) sealer	Jaws stay hot all the time	Thicker or layered films, coated films
Continuous band sealer	Bag mouth runs between heated bands on a conveyor	Production lines with steady volume
Vacuum sealer	Removes air, then heat seals	PA/PE vacuum bags for meat, cheese, seafood

For most importers and their customers, a tabletop impulse sealer is enough. Its timer is usually adjustable from a fraction of a second to about 1.5 seconds, and you set it by test, not by a fixed number.

How to get a strong heat seal

1. Keep the seal area clean. Powder, grease, liquid or product crumbs in the mouth of the bag are the most common cause of weak seals. Fill with a funnel or fold the mouth back while filling.
2. Lay the film flat. Wrinkles across the seal bar leave channels that leak.
3. Start low and work up. Begin at a low heat or short timer, seal a test bag, then raise one step at a time.
4. Hold until it cools. On an impulse sealer, keep the jaw closed for a moment after the light goes off. The seam sets while it cools under pressure.
5. Test. Pull the two sides of the bag apart at the seam. A good seal tears the film next to the seam, not along the seam.

Troubleshooting

Problem	Likely cause	Fix
Seal peels apart easily	Too little heat, dwell or pressure	Raise heat or time one step; check pressure
Holes or burned seam	Too much heat or dwell	Lower heat or time; check the PTFE cover on the bar
Seal leaks in spots	Wrinkles or dirt in the seal area	Flatten the film; clean the bag mouth
Weak seal at gussets	Four layers need more heat than two	Raise heat slightly or use a wider bar

How poly bags are heat sealed at the factory

When we make bags, the seams are heat seals too. The construction changes how the bag performs:

• Bottom seal: made from tubing, with one seal across the bottom. Strong for heavy loads.
• Star seal: the tube is folded before sealing, so the bottom closes in a star shape with no gaps. This is the common seal on trash can liners because it spreads the load and does not leak at the corners. See our star seal bags on roll.
• Side seal: made from folded film, with seals down both sides. Good for printed bags and flat poly bags. See our side seal block head bags.

Buying tips: what to send us

• Bag type: open-top poly flat bags, or lay-flat tubing on a roll that you cut and seal to any length.
• Size: width x length in inches or cm. Leave about 1 in (2.5 cm) extra length above the product for the seal.
• Thickness: in mil or microns (1 mil = 25.4 µm). Tell us your sealer model so we stay inside its range.
• Material: LDPE is the safe default for heat sealing. Tell us if you need clarity, stiffness or food contact; our food-contact bags are FDA-compliant food-contact material (21 CFR 177.1520).
• Print: gravure up to 8 colors, Pantone matched. Keep the print out of the seal area.

We make nearly all of these to custom size. Lead time is 4–6 weeks for a first printed order and 3–4 weeks for repeats. Our plants work to Japan’s JIS standard and ISO 9001.

FAQ

Can you heat seal any plastic bag?

Most PE and PP bags, yes. LDPE seals easiest. Oriented PP (OPP) and some coated or metallized films need a sealable layer or a different sealer. Paper and most biodegradable bags need their own settings or cannot be heat sealed at all.

What temperature do you heat seal poly bags at?

There is no single number. LDPE melts at about 221–239 °F (105–115 °C), and the sealer setting depends on film thickness, seal bar width and dwell time. Start low, test, and raise step by step.

Can I use a household iron or hair straightener?

For a one-off, it can work with a sheet of baking paper between the iron and the bag. For any business use, buy an impulse sealer. It gives consistent seals and is not expensive.

How thick a bag can an impulse sealer seal?

Typical hand impulse sealers are rated up to about 6 mil (152 µm) total. Check the sealer’s label; thicker or gusseted bags may need a constant-heat sealer.

Are heat-sealed bags airtight?

The seal itself is, if it is clean and complete. PE film still lets small amounts of air and moisture pass over time. For long-term airtight food storage, use PA/PE vacuum bags.

Send us your bag size, thickness and the sealer you use, or a photo of your current bag, and we will match it and send a free sample from stock. Send us your spec.