How to seal mylar bags
Short answer: mylar bags are heat sealed. An impulse sealer is the right tool and costs about as much as a pizza. A hair straightener or a household iron works in a pinch. Seal about half an inch below the top edge, above the zipper, with a clean, flat, wrinkle-free strip of film. If the seal peels apart when you tug it, it was too cold or too fast.
What you're actually sealing
A mylar bag is a laminate: polyester on the outside for strength and print, a metallised layer that blocks light and oxygen, and a polyethylene layer on the inside that melts. The heat seal works by melting that inner layer against itself. That's why the two inside surfaces have to touch cleanly, and why any product dust or crease in the seal area leaves a gap.
The zipper is for the customer. The heat seal is what keeps the bag airtight and tamper-evident until they open it. Bags with a zipper still get heat sealed above it.
Method 1: impulse sealer (what to use if you're doing more than a handful)
Set the timer or heat dial to a medium setting. On most tabletop sealers that's 3 to 5 out of 8. Thicker film needs a higher number.
Fill the bag, leaving at least two inches of clear film at the top.
Push out extra air with your hand, or press the bag flat on the table.
Lay the top of the bag across the sealing bar, flat and straight, about half an inch from the edge.
Press the arm down until the light goes off, then hold for another second while it cools.
Tug the seal. It should not peel.
Run a couple of test seals on an empty bag first. If the seal looks wrinkled or milky, the setting is too high. If it peels, too low.
Method 2: hair straightener
A flat iron at medium heat does the same job as an impulse sealer, just slower.
Heat it to around 300 to 350°F (150 to 175°C). Most straighteners are there on their middle setting.
Fill the bag and flatten it as above.
Clamp the top half inch of the bag between the plates and hold for two to three seconds.
Move along in overlapping sections until the whole width is sealed.
Tug to test.
Keep the plates clean. Any residue transfers onto the film and looks bad on a printed bag.
Method 3: household iron
Works, but it's the fiddliest.
Set the iron to a medium setting, no steam.
Lay the top of the bag flat on a hard surface and put a piece of parchment paper or a thin cloth over the seal area.
Press the iron down for three to five seconds, moving slowly along the width.
Let it cool, then tug.
The parchment matters. Bare iron on mylar melts the outer layer and leaves a shiny scar.
Can you vacuum seal mylar bags?
Only some of them. Home vacuum sealers rely on a textured bag to pull the air out, and most mylar bags are smooth, so the machine can't draw a vacuum. Two workarounds:
Use a chamber vacuum sealer, which doesn't need textured bags.
Push the air out by hand, leave a small gap in the seal, suck the last of the air out with a straw, and finish the seal. Not pretty, but it works for small runs.
For cannabis, a full vacuum is usually unnecessary. Pressing the air out and adding a humidity pack does more for freshness than a hard vacuum, and it doesn't crush the flower.
Mistakes that make bags fail
Sealing over the zipper. The seal has to be above the zipper track, in clear film.
Product in the seal. Kief or crumbs in the seal area leave channels. Wipe the inside of the top edge before sealing.
Wrinkles. Any fold across the seal line is a leak. Keep the film flat and straight.
Overfilling. Less than two inches of clear film at the top makes a clean seal almost impossible.
Too hot. A seal that looks bubbled or has burned through is weaker than one that looks flat.
Not testing. Pull on a seal from every batch. It takes a second and saves a returned order.
If you're sealing hundreds a day
Dispensaries packaging house flower and brands running small batches usually hit the same wall: a hand sealer is fine for fifty bags and miserable for five hundred. A foot-pedal impulse sealer with a 12-inch bar is the usual next step, and it's still cheap. A continuous band sealer is the step after that.
Two things to plan for at that scale:
Seal position. Keep it consistent, because it's where your tear notch and your label line up. Printed bags are designed around a specific seal height. Ours are on custom mylar bags.
Child resistance. A CR zipper is a separate spec from the heat seal. Certified CR bags still need the heat seal for tamper evidence in most states. If your product crosses state lines, the CBD mylar bags page covers the labelling side.
Odd shapes seal the same way; the zipper still runs straight across the top. That's on die cut mylar bags if you're curious how that works.
Quick answers
Can you seal mylar bags with a hair straightener? Yes. Medium heat, two to three seconds per section, above the zipper.
Do I need to heat seal if the bag has a zipper? For anything you're selling, yes. The zipper is resealable; the heat seal is the airtight, tamper-evident one.
What temperature seals mylar? Roughly 300 to 375°F (150 to 190°C) depending on film thickness. Start in the middle and test.
How do I know it's sealed? Tug it. A good seal stretches the film before it gives. A bad one peels apart cleanly.
Can you reseal a mylar bag after opening? With the zipper, yes. To heat seal it again you need enough clear film left above the zipper, which is why we leave headroom in the first place.
