The short answer
A shrink wrap machine does two jobs. First it seals shrink film around the product and cuts it from the roll. That leaves a loose sealed bag. Then it heats the bag so the film shrinks tight.
Two separate machines can do the two jobs, or one machine can do both. A sealer and a shrink tunnel is the common pair. Eastey sells the two as separate machines and as combo units on one frame.
Why shrink film shrinks
Shrink film is stretched warm when it is made, so it pulls back when it is heated again. Kitchen cling wrap and pallet stretch wrap are not made to shrink. A shrink wrap machine only works with shrink film.
Each film material shrinks at a different temperature and by a different amount.
| Film | Shrinks at about | How much it shrinks |
|---|---|---|
| PVC | 250°F | 35% of length, 50% of width |
| Polyethylene | 265°F | 15 to 20% in all directions |
| Polyolefin | 300°F | 64 to 80%, by grade |
The figures are from the Uline shrink film guide. Uline calls these shrink temperatures. They are not machine settings. See shrink film for how to pick a material and gauge.
Step 1: the film goes around the product
Most sealers run centerfold film. That is one sheet folded in half along its length and wound on a roll. The fold is already one closed edge of the package.
The film feeds over the machine, and the product slides between the two layers. On a manual machine the operator does this by hand. On an automatic machine an infeed conveyor carries the product into the film.
The seal should fall at half the package height. So the platform or conveyor is set lower for taller products. Eastey’s manual gives the example of a 6 in. package set 3 in. below the lower seal pads.
Step 2: the sealer seals and cuts
A heated bar presses the two film layers together. It melts them into a seal and cuts the package free from the roll. The bar’s shape sets how many strokes a package takes.
| Sealer | What one stroke does |
|---|---|
| I-bar | Makes one straight seal. The operator moves the package and seals each open edge in a separate step. |
| L-bar | Seals the open side and the trailing end at once. The cut leaves an end seal that becomes the leading edge of the next bag. |
| Side sealer | Seals the side nonstop as the product travels. A separate cross seal splits one package from the next. |
Conflex describes the L-bar as “an ‘L’ shaped sealing system to fully envelope the product in film”. A side sealer has no fixed-length side-seal frame, so no seal bar limits product length. See L-bar sealer vs. side sealer.
The bar gets its heat in one of two ways.
- Impulse (hot wire). The wire heats only during the seal cycle. Seal time is about one second on the Uline H-2378, per its manual.
- Constant heat (hot knife). A temperature controller holds the knife at a set point, so it stays hot between cycles. On Eastey’s Professional Series, magnets hold the head down for 1 to 5 seconds, per its user guide.
Impulse vs. constant-heat seals compares the two.
Step 3: the bag lets air out
A sealed bag traps air. If the air cannot escape, the bag balloons in the heat and seals can blow out. Eastey’s manual says “perforation is required to allow air to vacate when the product passes through the shrink tunnel”.
The holes come from one of two places. Many L-bar sealers have a pin perforator that punches small holes as the film feeds. Or the film is ordered pre-perforated. Perforation covers both.
Step 4: heat shrinks the film
The loose bag now needs heat on all sides. Three heat sources do this job.
| Heat source | How it works |
|---|---|
| Heat gun | The operator aims a handheld gun at the sealed bag. It is the cheapest option, and results depend on the operator. |
| Hooded chamber | The L-bar seal and the shrink happen under one hinged hood. The operator loads, closes, and waits. |
| Conveyor tunnel | A conveyor carries the package through a heated chamber with recirculating air. |
In a tunnel, two settings control the shrink: air temperature and time in the heat. Time in the heat is called dwell. It equals the heated chamber length divided by belt speed. The Sealer Sales CN-4520A has a 31.5 in. chamber. At 10 ft per minute, a package spends about 16 seconds in it.
Uline’s tunnel manual suggests starting at 325°F for polyolefin and 275°F for PVC. It then says to raise temperature and slow the conveyor in small steps until the film shrinks. Clysar’s target is a bag that balloons in the main body of the tunnel and collapses about ¾ of the way through.
Matching the tunnel to the sealer has the dwell and belt speed math.
Step 5: the package cools
The package leaves the heat warm. PAC offers external cooling fans at the tunnel exit to finish the shrink and cool the product. Clysar suggests a fan at the pack-out station so warm packages can go into cartons sooner.
How each machine type does the steps
| Machine | Loading | Sealing | Shrinking |
|---|---|---|---|
| I-bar kit | By hand | Several strokes of a straight bar | Heat gun |
| Hooded chamber | By hand | L-bar under the hood | Same hood, same cycle |
| Manual or semi-automatic L-bar | By hand | One stroke of the L-bar | Separate heat gun or tunnel |
| Sealer and tunnel combo | By hand | One stroke of the L-bar | Tunnel on the same frame |
| Automatic L-bar | Infeed conveyor | The seal head cycles on its own | Separate tunnel |
| Side sealer | Infeed conveyor | Nonstop side seal and a cross seal | Separate tunnel |
Shrink wrap machines lists speeds, prices, and example models for each type.
What an automatic machine adds
An automatic machine does the same steps without an operator stroke. On an automatic L-bar sealer, an infeed conveyor carries products into the film. Photo eyes detect each product and set the bag length. The seal head cycles on its own. A scrap winder collects the trim.
The machine still needs labor. An operator keeps the infeed supplied, changes film rolls, and clears faults.
What the machine does not do
- It does not make an airtight package. The vent holes let air through. Uline notes its I-bar system will “not create an airtight seal”.
- It does not stretch wrap. Stretch film is stretched around a load and needs no heat. See shrink wrap vs. stretch wrap.
- It is not a sleeve labeler. Shrink bands and sleeve labels on bottles use tube film and different tunnels. See shrink tunnels for bottles.
To run a sealer step by step, see how to shrink wrap. To fix a finished package that looks wrong, see troubleshooting.
Common questions
It does two jobs. A sealer closes shrink film around the product and cuts it from the roll, which leaves a loose sealed bag. Then a heat gun, hooded chamber, or shrink tunnel heats the bag, and the film shrinks tight around the product.
Shrink film is stretched warm when it is made, so it pulls back when it is heated again. Uline lists shrink temperatures of about 250°F for PVC, 265°F for polyethylene, and 300°F for polyolefin.
No. An I-bar or L-bar sealer only seals and cuts. Shrinking needs a separate heat gun or shrink tunnel, unless the machine is a hooded chamber or a sealer and tunnel combo.
The holes let trapped air out as the film shrinks. Without them the bag balloons and seals can blow out. Eastey's manual says perforation is required so air can leave the bag in the tunnel.
Don't assume so. The vent holes let air through, and Uline notes its I-bar system will not create an airtight seal. An airtight package needs a film, seal, and process the film supplier has specified and validated.