Burst Bags Explained: Middle-seal Bulge and End-seal Tear Have Opposite Causes

Burst Bags Fully Explained

Why Middle-seal Burst and End-seal Burst Have Opposite Causes

Cover: bulge or tear at the seal - first tell which seam breaks

Some bags bulge and pop open at the middle seal; others tear open and leak powder at the end seal. Both are "burst bags", but the fix directions are opposite - one needs less heat, the other more. Adjust the wrong way and it gets worse.

01  First: Does It Burst at the Middle or the End?

The manual lists them separately: the middle one is "middle seal weak or bulging", the end one is "horizontal seal weak or tears". Different names, different causes.

Fig.1: middle-seal bulge vs end-seal tear - almost opposite causes

02  Middle-seal Bulge: Usually Too Hot or Too Much Oil

“The middle seal must not leak and its pattern must be clear.”
— VFFS manual - middle seal adjustment

•  Too hot: the middle seal softens and thins the film, then bulges open - lower the middle-seal temp;

•  Too much silicone oil: a thick oil film stops the faces sticking, so the pattern does not press in;

•  Over-inflation: the bag is too full of gas and the middle seal, under the most pressure, goes first.

✔  For a middle-seal bulge, go cooler, use less oil, and check the inflation - do not keep raising the heat.

03  End-seal Tear: Usually Too Cold or Uneven Jaws

•  Too cold: the end film never fuses, the edge tears open and leaks powder - raise the end-seal temp;

•  Front/rear jaws uneven or misaligned: one side presses hard, the other loose - re-adjust the jaws;

•  Dirty jaws: film scraps insulate the heat, so it still will not seal firmly.

⚠  Cut power and let the jaws cool before cleaning; scrub film scraps with a wire brush dipped in silicone oil.

04  The Key Dial: Sponge on Top or Bottom

“The front/rear pressure sponges decide inflation vs venting: to inflate the bag, mount them on the UPPER surface of the jaw base; to vent, mount them on the LOWER surface.”
— VFFS manual - end-seal notes

Fig.2: sponge position sets whether the bag inflates or vents

Final Thoughts

When a bag bursts, first look at which seam broke. A middle bulge goes cooler; an end tear goes hotter. Then check whether the sponges are mounted the right way. Get the direction right and you fix it in one go.

Bags keep bursting no matter what?  Send a photo of the broken seal plus your product and temperature, and we will tell you middle vs end. Reply "manual" to download the manual PDF.