Bag Length Setting Always Off? Time Mode vs Color-Mark Mode Explained (VFFS Guide)

Bag Length Setting Always Off?

A Step-by-Step Guide to Switching Between Time Mode and Color-Mark Mode

You set the bag length at 120, but the bags come out at 125. You follow the manual step by step, and the bags are still sometimes long, sometimes short. This “bag-length tug of war” happens on shop floors almost every day.

The problem is usually not your hands — it is that you have not understood what the machine actually uses to “know” the bag length.  In this article we decode the manual’s bag-length range table and timing table, explain how the two positioning modes switch, and why you should adjust the delay time first — not the bag length — when film pulling is unstable.

1. First Things First: Who Actually Defines the Bag Length?

There is an easy-to-skip sentence in the manual: the packaging bag length is the distance between two color marks on the film.

In other words, bag length is not a number you make up — it is the physical distance between two color marks on the film surface. The machine does not “measure length” to cut the bag; it uses the color-mark photoeye (electric eye) to “see” that distance — it starts timing when it sees the first mark, and when it sees the second mark it knows the position is right and the cutter acts.

✔  Before setting the bag length, do one thing first: align the color-mark photoeye. The manual says: after the film-length setting is done, adjust the color-mark photoeye and line up the cutter position — i.e. when the end-seal jaw halves close, the cutter cuts exactly at the film’s color mark. If the photoeye is not aligned, every adjustment after that is wasted effort.

2. One Table Says It All: The Bag-Length Range Table

The manual divides bag length into three ranges. Each range corresponds to the delay time and action time of three actions: back seal (middle seal), end seal and cutter. These are factory-verified recommended rhythms — align to them first, then fine-tune.

Table 1. Bag-length range table (from the vertical packaging machine instruction manual, timing section)

Three takeaways from this table:

•  The longer the bag, the higher the time values across the board. From 80-130mm to 220-320mm, both delay and action times increase clearly.

•  Cutter times are generally shorter than seal times. The cutter moves fast over a short stroke, so it gets a tighter rhythm.

•  Within the same range, back-seal and end-seal times are basically equal. Both ends must seal in sync for a well-sealed bag.

Why does a longer bag need more time? Because the film travel distance grows, so the rhythm from signal to execution must widen accordingly. If the time values cannot keep up, you get “cut before the film is in position” or “seal jaws not yet closed when the cut happens”.

3. Two Positioning Modes: Color-Mark Mode vs Time Mode

For the same “knowing the bag length” task, the machine can take two paths — depending on whether the film has color marks.

Table 2. Color-mark mode vs time mode

In the normal start-up procedure, this step is on the running screen: select the color-mark photoeye key of “empty bag”, “continuous running” or “motor control”.

⚠  If no color-mark signal is detected, the screen shows “color-mark photoeye fault” and the machine stops automatically — the machine is not broken. It is telling you: the photoeye cannot see what it should see. Go adjust the color-mark photoeye.

4. Film Pulling Unstable? Adjust the Delay Time, Not the Bag Length

This is the most valuable insight today. Many operators, when the bag length is off, immediately start changing the bag-length value over and over — and the bags only get worse.

Here is why: bag length is the “target value”; the delay time is the “execution rhythm”.  Unstable film pulling usually means speed, tension and film material are changing — the film’s actual travel has fallen out of step with the set rhythm. The bag-length number itself is not wrong; the execution rhythm is not keeping up with the film.

“The times set above are not absolute. If factors such as low film temperature or low ambient temperature cause leaking bags, first adjust each sealing temperature, then consider increasing the delay or action times.”
— Vertical packaging machine instruction manual — timing note

So the correct order is:

•  First confirm the photoeye is aligned with the color mark (if not, everything else is wasted);

•  Align the delay and action times to the correct range in the bag-length range table;

•  If still unstable, fine-tune the delay time first so it matches the actual film speed;

•  Change the bag-length number only when the photoeye position or the mark spacing has actually changed.

5. Shop-Floor Quick Reference: Photoeye Drift

Nine times out of ten, bag-length problems end up as “photoeye drift”. The manual’s fault table gives three causes and fixes:

Table 3. Photoeye drift quick reference (from the manual, fault causes & fixes)

✔  Shop-floor self-check: when the color mark is aligned with the photoeye, the indicator light goes out; move it away and the light comes on. One glance tells you whether the photoeye is on target.

Final Thoughts

Next time the bag length is off, do not rush to change the number. Ask three questions first: Does the film have color marks? Is the photoeye aligned? Are the time values aligned to the bag-length range table?  Get the order right and you will solve half the problem.

Next article: the ultimate fix for photoeye (color-mark sensor) drift — sensitivity or position? We will explain it in one article.

Call to Action & Series Note

Still fighting bag-length problems?  Send us your film details (type, thickness, bag length) — our engineers will recommend the correct timing parameters for your machine. Or reply with the keyword “manual” to download the Vertical Packaging Machine Instruction Manual (PDF), ready to print.

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